Capacitive Proximity Sensors

A proximity sensor is a sensor able to detect the presence of a nearby object without any physical contact. A proximity sensor often emits an electromagnetic field or a beam electromagnetic radiation (infrared for instance), and looks for changes in the field or a return signal. The object being sensed is often referred to as the proximity sensor’s target. Different proximity sensor targets demand different types of proximity sensors. For example, a capacitive photoelectric sensor might be suitable for sensing a plastic target, while an inductive proximity sensor is used to detect a metal target. The maximum distance that a sensor can detect is defined as its “normal range”. Some sensors have adjustments of their normal range or means to report a graduated detection distance. Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between the sensors and the sensed object. Proximity sensors are also used in machine vibration monitoring to measure the variation in distance between a shaft and its support bearing. This is common in large turbines, compressors and motors that use sleeve-type bearings.

  • Capacitive Proximity Sensor: According to physics, two objects in the vicinity of each other have different electrical charges which create an electrostatic field between them. An object is said to be negatively charge if it has an excess of electrons relative to its surroundings; an object is positively charged (+) if it is deficient in electrons with respect to its surroundings. Capacitance refers to the “capacity” of the two objects within some reasonable proximity of each other to hold this charge. A change in the distance between the two objects changes the capacitance. It is this change of capacitance that capacitive sensors use to indicate changes in position of a target.

A capacitive proximity sensor is a type of proximity (non contact) sensor that senses and measures changes in the capacitance charge of a target. With a capacitive sensor, the sensing surface of the sensor’s probe is the electrified plate. The sensor electronics continually changes the voltage on the probe surface (called excitation voltage). The amount of current required change this voltage is measured which indicates the amount of capacitance distance between the probe and target

Capacitive proximity sensors can not only be used to detect metal but also nonmetallic materials such as paper, glass, liquids, and cloth; thus making capacitive proximity switches an excellent solution bulk granular handling machines, fluid level detectors, and package detection applications.

Other than their ability to sense metallic and non-metallic target objects, the advantages to capacitive sensors are: high speed, feature good stability and high resolution; use less power than inductive sensors; and are low cost. The disadvantages of capacitive sensors are: they are affected by temperature and humidity; can be triggered by dust or moisture; they are sensitive to noise; poor linearity; and capacity sensors are not as accurate compared to inductive sensors

  • Inductive: An inductive proximity sensor is particularly used in detecting metal target object. Instead of using capacitance to detect a target distance, the inductive proximity sensor uses an electro-magnetic field to detect a conductive target

Sometimes called and Eddy-Current sensor, an inductive proximity sensor has a sensing coil in the end of the sensor probe. The sensing coil when excited creates an alternating magnetic field which induces small amounts of current in the target object; these currents are called eddy currents. The eddy currents create an opposing magnetic field which resists the field being generated by the sensor probe coil. The interaction of the magnetic fields is dependent on the distance between the sensor probe and the target. As the distance changes, the inductive sensor senses the change in the field interaction and produce a voltage output which is proportional to the change in distance between the probe and target. Typically the target surface should be three timers larger the sensor probe diameter for normal operation.

Inductive proximity sensors are useful in any application requiring the sensing, measurement or monitoring of the position of a conductive target, especially in a dirty environment. Other inductive sensor advantages are: typically inductive sensors are not sensitive to material in the gap between the probe and the target; and these sensors are less expensive than capacitive sensors. Disadvantages are: inductive proximity sensors are not high resolution devices and they are not particularly useful in applications where there us a large gap between the sensor and target

  • Other: There are many different types, sizes and operating specifications for photoelectric, capacitive and inductive proximity sensors making them suitable for the majority of non contact industrial sensing applications. However other types of sensors are offered for specialty or relatively unique applications. Some of these different kind of sensors are:
    • Ultrasonic Sensors: Also sometimes called transceivers, ultrasonic sensors work in a principle similar to radar or sonar. Ultrasonic sensors generate a high frequency radio or sound waves and evaluate the echo receiver back from the target object. Ultrasonic sensors calculate the time interval between sending the signal and receiving the echo to determine the distance of the target from the sensor probe. This technology is useful for measuring wind speed and direction, the fullness of a tank or the speed of an object or particle traveling through air or water. For measuring speed or direction, an ultrasonic sensor uses multiple detectors and calculates the speed from the relative distances to particulates in the air or water. To measure the amount of liquid in a tank, the ultrasonic sensor measures the distance to the surface of the fluid. Other ultrasonic sensor applications include: humidifiers, medical ultrasonography, burglar alarms and non-destructive testing. This technology however is limited by the surface shapes and the density or consistency of the target material. For example foam on the surface of a fluid in a tank would probably distort the ultrasonic reading. Ultrasonics are a great solution for clear object detection, clear label detection and for liquid level measurement, applications that photoelectric sensor struggle with because of target translucence. Target color and/or reflectivity don’t affect ultrasonic sensors which can operate reliably in high-glare environments.
    • Fiber Optic Sensors: Fiber optic sensors are immune to electromagnetic interference and do not conduct electricity so they can be used in places where there is high voltage electricity or inflammable material such as jet fuel. Additionally fiber optic sensors can be designed to withstand high temperatures. A fiber optic sensor sends a light source (either LED or laser) through optical fiber cable and measures properties in the changes of the signal. There are two types of fiber optic sensors:
    • Intrinsic fiber optic sensors – are fiber optic sensors that measure the intensity of light or through the optical cable or light modulation takes place directly in the fiber. Any strain, temperature change or pressure on the optical cable modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber is measured. Temperature can be measured by using a fiber that has evanescent loss that varies with temperature of the optical fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Special fibers like long-period fiber grating (LPG) optical fibers can be used for direction recognition. Intrinsic fiber optic sensors are particularly useful in providing distributed sensing over very large distances
    • Extrinsic fiber optic sensors – measure the light modulation effects on a non-fiber transducer or electronic sensor connected via optical fiber to an optical transmitter. For example a extrinsic fiber optic sensor can used to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are also used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting
    • Piezoelectric Sensors: The word piezoelectricity means electricity resulting from pressure. A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force on a piezoelectric material (notably special crystals or certain ceramic materials) by converting them to an electrical charge. In other words, piezoelectric sensors are electromechanical systems that react to compression of a sensing element that shows almost zero deflection, contraction or compression. This is the reason why piezoelectric sensors are rugged, have an extremely high natural frequency and an excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions.
    • Laser Sensors: Laser sensors are typically used to measure displacement, distance and sometimes for gauging dimensions. Similar to a photoelectric sensor the laser sensor transits a laser beam and very accurately measures the speed of its reflection back to a sensor

There are many other types of specialty sensors for measuring/sensing light, motion, temperature, magnetic fields, gravity, humidity, moisture, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment.

Encoders

An encoder is an electro-mechanical device used to sense and measure the motion of a motor’s rotating shaft and the encoder translates this motion information (velocity, position and acceleration) into useful electrical data. As a motion measurement feedback device or sensor, the encoder divides the motor shaft’s rotation is dividing into a fixed number of increments called counts. These counts are then outputted as either digital or pulse signals by the encoder.

The most popular type of encoder is the optical encoder, which is made up of three basic parts: (1) a rotating disk, (2) a LED light source, and (3) a photo-detector or light sensor. The disk is mounted on the motor’s rotating shaft. The encoder’s disk has a pattern of a number of opaque or transparent sectors or apertures (reference marks) around the circumference of the disk called a code track. The encoder’s light source and photo-detector or light sensor are located on opposite sides of the disk’s code track. As the disk rotates, photo-detector/light sensor senses the presence of light or no light (1 or 0) through the code track which creates a series of pulses. These pulses or digital signals are outputted by the encoder as measurement feedback of the motor’s motion.

There are two general types of optical encoders: incremental and absolute. The difference between two optical encoder types is that an incremental encoder does not keep track of the motor’s motion position. It merely keeps track and counts how far the motor as moved as it creates motion displacement. The only way to determine the absolute position of incremental encoders is to set a sensor at known reference position and then zero the incremental encoder’s counters. A single code track incremental encoder is known as a tachometer encoder because its pulsed signal frequency indicates velocity of motion displacement. However, the output of the single-channel encoder or tachometer does not indicate direction.

To determine direction, a two-channel, or quadrature incremental encoder is required. A quadrature encoder has two photo-detectors and two parallel code tracks on the encoder disk or two output channels (A and B) to sense position. The “B” code track/output channel is positioned 90° out of phase with the “A” code track. These two output channels of the quadrature encoder indicate both position and direction of rotation. If the “A” pulse leads “B”, the disk is rotating in a clockwise direction. If the “B” pulse leads “A” pulse, then the disk is rotating in a counter-clockwise direction. Thus by monitoring both the both the number of pulses and the relative phase of signals “A” and “B”, you can track both the position and direction of rotation of the motion displacement. Some quadrature encoders include a third output channel, called a zero or reference signal, which supplies a single pulse per revolution. This single pulse can be used for precise determination of a reference position.

An absolute encoder keeps track of its position at all times, and provides this position information at start up or as soon as power is applied. An absolute encoder generates a unique word pattern for every position of the shaft. There are generally four to six code tracks on a absolute encoder disk which are commonly coded to generate a binary code, binary-coded decimal (BCD or gray code outputs. Absolute encoders are most commonly used in applications where the device will be inactive for long periods of time, there is risk of power down, or the starting position is unknown.

Human Machine Interface (HMI)

Human Machine Interface or HMI is a software application program that provides graphic based visualized information about an industrial control or process control system’s operational dynamics. Previously called a “MMI” or man-machine interface, a control system’s HMI is an operator interface or graphical user interface (GUI) software that usually resides in a Microsoft Office-based Windows computer, typically located in an office or control room, that communicates real-time to programmable logic controllers (PLC), programmable automation controllers (PAC), motion controllers, distributed control systems (DCS) and or other in-plant specialized computers allowing factory operators, plant engineer and management to interaction with the manufacturing process

HMIs come in a variety of sizes, types and features. In its simplest form, a HMI can be a one-on-one operator interface with a single controller device e.g. PLC, and is used as a replacement for pushbuttons, thumbwheel, toggle switches, pilot lights, alarm annunciators, and panel meters in an operator control panel. The HMI graphics screens or displays along with an operator input device such as a touch screen, keyboard/keypad, mouse, joystick or scroll wheel, not only replaced the operator control panel but also save space, simplifies operator interaction, reduces system costs and provides enhanced operator feed back process information such as historical and or real-time trending and remote messaging. These single controller HMI systems are typically referred to as operator terminal or operator interface terminal (OIT) and have HMI display screen that range from 4.3” to 15”.

Larger HMI systems are usually networked (Ethernet) to multiple controller devices have significantly more real-time functionality than OITs that includes: multiple display terminals with multiple personnel interaction; display animation/simulation; recipe management include recipe storage and downloading to controllers; alarm/event management and logging; statistical process control; historical logging and browsing; math and logic capabilities, report generations; supervisory control and data acquisition; security password levels/management; SQL server, ODBC support and remote web based monitoring and control; email and more. HMI systems are used in a wide range of applications that include: machine monitoring and control; Supervisory Control and Data Acquisition (SCADA); control center monitoring, tracking, and control; building automation and security; electrical substation monitoring; pipeline monitoring and control; transportation control systems; batch process monitoring and control; continuous process monitoring and control; heating, ventilation, and air conditioning; statistical process control; telecommunications; discrete manufacturing and more.

HMI products are offered by vendors as either stand-alone software or the software is bundled or integrated with computers. There is a growing trend toward wireless portable HMI terminal products.

The key to a successful HMI System implementation is a well-grounded definition and understanding of the how the operators will interface with the controlled process. Will the operator be a passive/intuitive user? If so, commands/functions should be simple with an easy-to-comprehend interface. For an expert user, where more sophisticated control definition is desirable, there may be multiple layers or levels for interfacing with equipment.

Inductive Proximity Sensors

A proximity sensor is a sensor able to detect the presence of a nearby object without any physical contact. A proximity sensor often emits an electromagnetic field or a beam electromagnetic radiation (infrared for instance), and looks for changes in the field or a return signal. The object being sensed is often referred to as the proximity sensor’s target. Different proximity sensor targets demand different types of proximity sensors. For example, a capacitive photoelectric sensor might be suitable for sensing a plastic target, while an inductive proximity sensor is used to detect a metal target. The maximum distance that a sensor can detect is defined as its “normal range”. Some sensors have adjustments of their normal range or means to report a graduated detection distance. Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between the sensors and the sensed object. Proximity sensors are also used in machine vibration monitoring to measure the variation in distance between a shaft and its support bearing. This is common in large turbines, compressors and motors that use sleeve-type bearings

  • Capacitive Proximity Sensor: According to physics, two objects in the vicinity of each other have different electrical charges which create an electrostatic field between them. An object is said to be negatively charge if it has an excess of electrons relative to its surroundings; an object is positively charged (+) if it is deficient in electrons with respect to its surroundings. Capacitance refers to the “capacity” of the two objects within some reasonable proximity of each other to hold this charge. A change in the distance between the two objects changes the capacitance. It is this change of capacitance that capacitive sensors use to indicate changes in position of a target.

A capacitive proximity sensor is a type of proximity (non contact) sensor that senses and measures changes in the capacitance charge of a target. With a capacitive sensor, the sensing surface of the sensor’s probe is the electrified plate. The sensor electronics continually changes the voltage on the probe surface (called excitation voltage). The amount of current required change this voltage is measured which indicates the amount of capacitance distance between the probe and target.

Capacitive proximity sensors can not only be used to detect metal but also nonmetallic materials such as paper, glass, liquids, and cloth; thus making capacitive proximity switches an excellent solution bulk granular handling machines, fluid level detectors, and package detection applications

Other than their ability to sense metallic and non-metallic target objects, the advantages to capacitive sensors are: high speed, feature good stability and high resolution; use less power than inductive sensors; and are low cost. The disadvantages of capacitive sensors are: they are affected by temperature and humidity; can be triggered by dust or moisture; they are sensitive to noise; poor linearity; and capacity sensors are not as accurate compared to inductive sensors.

  • Inductive: An inductive proximity sensor is particularly used in detecting metal target object. Instead of using capacitance to detect a target distance, the inductive proximity sensor uses an electro-magnetic field to detect a conductive target

Sometimes called and Eddy-Current sensor, an inductive proximity sensor has a sensing coil in the end of the sensor probe. The sensing coil when excited creates an alternating magnetic field which induces small amounts of current in the target object; these currents are called eddy currents. The eddy currents create an opposing magnetic field which resists the field being generated by the sensor probe coil. The interaction of the magnetic fields is dependent on the distance between the sensor probe and the target. As the distance changes, the inductive sensor senses the change in the field interaction and produce a voltage output which is proportional to the change in distance between the probe and target. Typically the target surface should be three timers larger the sensor probe diameter for normal operation.

Inductive proximity sensors are useful in any application requiring the sensing, measurement or monitoring of the position of a conductive target, especially in a dirty environment. Other inductive sensor advantages are: typically inductive sensors are not sensitive to material in the gap between the probe and the target; and these sensors are less expensive than capacitive sensors. Disadvantages are: inductive proximity sensors are not high resolution devices and they are not particularly useful in applications where there us a large gap between the sensor and target.

  • Other: There are many different types, sizes and operating specifications for photoelectric, capacitive and inductive proximity sensors making them suitable for the majority of non contact industrial sensing applications. However other types of sensors are offered for specialty or relatively unique applications. Some of these different kind of sensors are:
    • Ultrasonic Sensors: Also sometimes called transceivers, ultrasonic sensors work in a principle similar to radar or sonar. Ultrasonic sensors generate a high frequency radio or sound waves and evaluate the echo receiver back from the target object. Ultrasonic sensors calculate the time interval between sending the signal and receiving the echo to determine the distance of the target from the sensor probe. This technology is useful for measuring wind speed and direction, the fullness of a tank or the speed of an object or particle traveling through air or water. For measuring speed or direction, an ultrasonic sensor uses multiple detectors and calculates the speed from the relative distances to particulates in the air or water. To measure the amount of liquid in a tank, the ultrasonic sensor measures the distance to the surface of the fluid. Other ultrasonic sensor applications include: humidifiers, medical ultrasonography, burglar alarms and non-destructive testing. This technology however is limited by the surface shapes and the density or consistency of the target material. For example foam on the surface of a fluid in a tank would probably distort the ultrasonic reading. Ultrasonics are a great solution for clear object detection, clear label detection and for liquid level measurement, applications that photoelectric sensor struggle with because of target translucence. Target color and/or reflectivity don’t affect ultrasonic sensors which can operate reliably in high-glare environments.
    • Fiber Optic Sensors: Fiber optic sensors are immune to electromagnetic interference and do not conduct electricity so they can be used in places where there is high voltage electricity or inflammable material such as jet fuel. Additionally fiber optic sensors can be designed to withstand high temperatures. A fiber optic sensor sends a light source (either LED or laser) through optical fiber cable and measures properties in the changes of the signal. There are two types of fiber optic sensors:
    • Intrinsic fiber optic sensors – are fiber optic sensors that measure the intensity of light or through the optical cable or light modulation takes place directly in the fiber. Any strain, temperature change or pressure on the optical cable modulates the intensity, phase, polarization, wavelength or transit time of light in the fiber is measured. Temperature can be measured by using a fiber that has evanescent loss that varies with temperature of the optical fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber, which alter the polarization of light as a function of voltage or electric field. Special fibers like long-period fiber grating (LPG) optical fibers can be used for direction recognition. Intrinsic fiber optic sensors are particularly useful in providing distributed sensing over very large distances
    • Extrinsic fiber optic sensors – measure the light modulation effects on a non-fiber transducer or electronic sensor connected via optical fiber to an optical transmitter. For example a extrinsic fiber optic sensor can used to measure the internal temperature of electrical transformers, where the extreme electromagnetic fields present make other measurement techniques impossible. Extrinsic sensors are also used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting
    • Piezoelectric Sensors: The word piezoelectricity means electricity resulting from pressure. A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force on a piezoelectric material (notably special crystals or certain ceramic materials) by converting them to an electrical charge. In other words, piezoelectric sensors are electromechanical systems that react to compression of a sensing element that shows almost zero deflection, contraction or compression. This is the reason why piezoelectric sensors are rugged, have an extremely high natural frequency and an excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions.
    • Laser Sensors: Laser sensors are typically used to measure displacement, distance and sometimes for gauging dimensions. Similar to a photoelectric sensor the laser sensor transits a laser beam and very accurately measures the speed of its reflection back to a sensor

There are many other types of specialty sensors for measuring/sensing light, motion, temperature, magnetic fields, gravity, humidity, moisture, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment.

Meters

A panel meter is a digital or analog information display instrument. A panel meters provides a visual operator display of the condition of controlled process (states of inputs or outputs or alarm conditions) for process monitor and control purposes. Panel meters are typically mounted in control panels however wall mount and bench mounted versions are also available. Many panel meters also have serial, Ethernet and field bus communications capabilities for the transfer control information to computers, manufacturing information systems or to websites for remote motoring.

There are four basic types of panel meters:

  • Temperature and process panel meters are the most common types of panel meter. These meters usually accept inputs from temperature sensors such as thermocouples and RTDs as well as process analog signals such as 4-20mA, 0-5VDC and 0-10VDC and provide a digital display of the signal state. Single and multi-input process panel meters are offered. Multi-input panel meters can switch from one input channel to the next by pressing a button on the front of the meter while others will automatically scan through a series of channels.
  • A totalizer is a type of panel meter that provides a summation over time of the input signal. Totalizers are commonly used with pulse inputs to provide a digital count of the number of pulses. This is typically used in flow measurement applications.
  • Large display panel meters are commonly used when the digital display must be visible at a distance.
  • Analog meters are dial-type gauges, like a car’s speedometer, displaying a control condition or signal.

Photoelectric Sensors

A photoelectric sensor, as know as a photo eye, is a device used to detect the distance, absence, or presence of a target by using a light (visible or infrared) transmitter or emitter that transmits light to a receiver. There are three different functional types of photoelectric sensors: through beam, retro-reflective, and direct reflection.

A through beam (as known as opposed) photoelectric sensor consists of two parts: a receiver located within the line-of-sight some distance from the actual light transmitter. An object is detected when the light beam is blocked from getting to the receiver from the transmitter. One advantage of a through beam photoelectric sensors is that it has a long sensing distance because the light beam travel in one direction. However installation cost may be higher than other types of photoelectric sensors because the transmitter and receiver need to be mounted wired and adjusted separately.

A retro-reflective sensor has three parts: a transmitter, receiver and a reflector. Typically the transmitter/emitter and receiver are housed together. The reflector is located remotely in line of sight for the transmitter/emitter. The target object is sensed when in interrupts the light beam between the remote reflector and transmitter/receiver. Advantages to retro-reflective photoelectric sensors include easy installation and the reflector can be fixed to a moving target. Disadvantages are shorter sensing range than a through beam system since the light beam has to travel from the light source to the reflector and back to the receiver, and high-gloss objects can function as reflectors and may cause malfunctions.

In a direct reflection or defuse photoelectric sensor the transmitter/emitter and receiver are house together and the integrated sensor uses the light directed reflected of the remote target object to detect the object. Advantage to direct reflection sensors include easy installation and no reflector is required. Disadvantages are different target objects have different sensing distances due the target’s reflective capacity e.g. type of surface and color.

Programmable Logic Controller (PLC)

A programmable logic controller (PLC) or programmable controller is a specialized digital computer used to control discrete manufacturing processes such as assembly lines, shop floor machinery and related factory mechanical, electrical and electronic equipment. Unlike a computer, a PLC is designed for real-time use/control in rugged, and sometime hostile industrial environments. Typical PLC environment specifications include: minimum operating temperature range 0° to 50°C (32° to 122°F); relative humidity 5% to 95% (non- condensing); high electrical noise, radio interference and vibration immunity. Some PLC models have explosion proof and water resistance ratings

PLCs are typically based on a modular design composed of three basic types of modules: input modules, output modules and a logic module or CPU for solving pre-programmed control logic. Input modules are wire connector to process sensing devices such as limit switches, proximity switches, thermocouples, flow meters etc that provide real-time data about the current state of a process e.g. the current level of a tank, or the desired state an operator wants the process to transition/change to, such as a pushing push button for a motor starter or to start a fan. Output modules are wired to actuation devices such as motors, fan, pilot lights, valves and other device that create changes in the controlled process. Input and output modules are either stacked next to the logic module or inserted into a rack containing a logic module

The exceptions to this modular PLC specification are nano and some micro PLCs. Nano/Micro PLCs are defined as a PLC with less than 32 inputs and outputs (I/O) points. These Nano/Micro PLCs usually feature an “all-in-one” design that integrates the input module, output module and logic module into a single unite that looks like a brick or block.

With all PLCs the logic module contains preprogrammed control logic that provides the choreography between the status of inputs and the desire change in outputs. The logic module is composed of two elements: a processor (CPU) to solve the programmed control logic and memory to store the control program for the processor to use. The CPU processor systemically reads all the input statuses, solves all programmed logic conditions and activates outputs based on the logic program’s outcomes. This systematic routine is called a scan or sweep of the PLC. This scan or sweep is considered deterministic because the actual time it takes to read all inputs, solve logic and update outputs can be calculated in millisecond or microseconds and sometime in nanoseconds. The PLC’s fast processing routine or scan times are the reason a PLC is considered a real time sequential control device.

The logic module is programmed with either a hand held programmer or with programming software that runs on a computer. For many years PLCs have been programmed using relay logic or ladder diagrams (LD) which are schematic diagrams for relay logic circuits or line diagrams made up of a combination of input and or output conditions that generation outcomes. These input and or output conditions often referred to as contacts, are connected in series, parallel, or series-parallel and sometimes combined with timers and counters to obtain the logic required to drive or active the output.

Over the years PLC programming languages have evolved and today five major PLC programming languages have been defined by IEC standard 61131-3 as the primary PLC programming languages. In addition to ladder diagram these programming languages are:

Sequential flow chart (SFC) – a graphical programming language similar to a flow chart that shows the main states of a control system, all the possible changes or steps and transition of state required to create outcomes. SFC is typically used for defining control sequences that are time and or event driven.

Function Block diagram (FBD) –is also a graphic language based on block diagram in which input and output variables are connected to function block by connection lines and the output of a function block create a real outcome or may be connected to another function block command. Function block commands/logic statements are usually predefined and reusable however in some case function blocks may be user defined. Typical function block commands include: Boolean expressions (and, or, xor and not); mathematical (add, subtract, multiple, divide etc); comparative (equal, less than and greater than); bit operations (shift right and left etc); numeric (sine, cosine, tangent, etc) and other related PLC command functions.

Structured Text (ST) – is a textual PLC programming language similar to Pascal. ST has four basic programming elements: Variables – are used to hold data and I/O states and are similar to data register in LD programming; Operators – define logic operations to be performed such as arithmetic, If, Then, Else etc; Expression – is a combination of operator and variables to form logic or control statement; and Control Flow Statements – are used to conditionally execute expressions or statements, to loop over statements, or to jump to another area in the program.

Instruction List (IL) – is also a textual PLC programming language with assembler language features. IL is typically used by hand-held programmers to represent a ladder diagram program. Instruction list uses mnemonic codes as a substitute for traditional ladder diagram.

Like a computer, PLCs are offered with many communications and networking capabilities that include: Ethernet; serial RS232/RS422/RS485; and USB. However the protocols associated with these communication mediums are tend to be deterministic or predictable as to their communication transaction times. Additionally many PLCs offer field bus communications such a DeviceNet and Profibus. Field buses are defined as industrial networking protocols used for real-time distributed control of I/O and other control level smart device.

An outgrowth of programmable logic controllers are programmable automation controllers (PAC). Like a PLC, a PAC is designed for real-time deterministic sequential control of I/O; however PACs have up to four specialized logic processor modules with different control functions that process their respective logic program in parallel with each other and simultaneously update I/O. The specialize processor modules offered for PACs typically include: a PLC logic processor; motion controller; process control module similar to a DCS (Distributed Control System); BASIC programming modules; and other dedicated functionality computing modules.

There are four important considerations in selecting and designing a PLC system:

  • The type of I/O and the number of I/O points required by the application: Many different types of I/O modules are offered that include: a multitude of discrete input and output modules voltage signal and amperage; combination integrated input and output modules; analog, thermocouple and RTD modules; intelligent modules such as high speed counters for KHz and MHz signals, stepper control modules, servo control modules, bar code readers, PID controllers, and other dedicated I/O controllers.
  • Where are the I/O points to be controlled is located: Are the I/O points centralized in a single location or are the I/O points remotely located some distance for primary controller location? Remote I/O is a typically require for I/O points concentrated in blocks over 50 to 250 I/O points in a single remote location over more than 200 feet from the primary control location. Distributed I/O is used for smaller 4 to 20 I/O points blocks where a number of these small I/O blocks are remote located
  • What logic control functionality is required for the application: Is the control logic primarily sequential control or primary process control? It is important to select the logic processor with a programming language that will efficiently support the application. For sequential control usually ladder diagram or instruction list programming language is used. For process control applications a graphical programming language such as sequential flow chart or function block diagram may be suitable. It is important to remember that a PLC is a scan based processor thus the larger the program the longer the scan time.
  • Is the PLC required to communicate to other controller/computer devices and if so what is the networking architecture: Is the PLC is only communication to a HMI or is the PLC part of a factory wide information system? Is application a telemetry SCADA or supervisory control and data acquisition system? How much information must be communicated? The answer to these questions determines the selection logic processor module’s communication ports and if additional networking modules are required.

Type 2 Light Curtains

A light curtain is a presence detecting opto- electronic device used to protect/safeguard personnel in or working in the vicinity of moving or rotating machinery such as punch presses, winders, palletizers and related hazardous equipment. Light curtains are used as an alternative to traditional machinery safe guards such as mechanical or physical barriers. There are two parts to a light curtain: a transmitter and receiver. The transmitter projects an array of parallel infrared light beams to a receiver consisting of photo electric cells. When one or more of the infrared red beams is not received by the receiver, the light curtain send a signal to a controller to lock the machinery in a safe condition.

There are two types of light curtains that comply with IEC 61496 standards: Type 2 and Type 4. However, the only type of light curtain meeting United States OSHA and ANSI standard is Type 4 light curtains.

Type 4 light curtains have two significant safety features that make them a superior choice over Type 2 light curtains:

  • Type 4 light curtains have embedded redundant automatic self checking circuitry that is designed to immediately detect the failure of a single component or single internal fault within a defined response time. If it detects an internal fault, the Type 4 safety light curtain immediately sends a stop signal to the machine safety controller and the light curtain enters a lockout condition. Only after replacement of the failed component, and an appropriate system reset, will the Type 4 light curtain with its associated guarded machine be restored to operating condition. In other words no single point component failure/fault will create an unsafe Type 4 light curtain condition. This is not the case with a Type 2 light curtain.
  • The Type 4 light curtain’s receiver has a narrow ±2.5° optical angle which substantially minimizes the possibility of foreign reflective surface interference (from sunlight, mirror etc) causing the receiver to see light when the transmitter’s infrared beam has actually been broken. This clearly unsafe condition subjects the operator to unwarranted risks and possible injury. A Type 2 light curtain has a wide ±5° optical angle – two times less precise than a Type 4.

The primary advantage to a Type 2 light curtain is that it is 15% to 30% less expensive than a Type 4.

Type 4 Light Curtains

A light curtain is a presence detecting opto-electronic device used to protect/safeguard personnel in or working in the vicinity of moving or rotating machinery such as punch presses, winders, palletizers and related hazardous equipment. Light curtains are used as an alternative to traditional machinery safe guards such as mechanical or physical barriers. There are two parts to a light curtain: a transmitter and receiver. The transmitter projects an array of parallel infrared light beams to a receiver consisting of photo electric cells. When one or more of the infrared red beams is not received by the receiver, the light curtain send a signal to a controller to lock the machinery in a safe condition.

There are two types of light curtains that comply with IEC 61496 standards: Type 2 and Type 4. However, the only type of light curtain meeting United States OSHA and ANSI standard is Type 4 light curtains.

Type 4 light curtains have two significant safety features that make them a superior choice over Type 2 light curtains:

  • Type 4 light curtains have embedded redundant automatic self checking circuitry that is designed to immediately detect the failure of a single component or single internal fault within a defined response time. If it detects an internal fault, the Type 4 safety light curtain immediately sends a stop signal to the machine safety controller and the light curtain enters a lockout condition. Only after replacement of the failed component, and an appropriate system reset, will the Type 4 light curtain with its associated guarded machine be restored to operating condition. In other words no single point component failure/fault will create an unsafe Type 4 light curtain condition. This is not the case with a Type 2 light curtain.
  • The Type 4 light curtain’s receiver has a narrow ±2.5° optical angle which substantially minimizes the possibility of foreign reflective surface interference (from sunlight, mirror etc) causing the receiver to see light when the transmitter’s infrared beam has actually been broken. This clearly unsafe condition subjects the operator to unwarranted risks and possible injury. A Type 2 light curtain has a wide ±5° optical angle – two times less precise than a Type 4

The primary advantage to a Type 2 light curtain is that it is 15% to 30% less expensive than a Type 4.

Vision System Cameras

A vision system camera is an optical sensor that acquires the desired object image. There are two formats that vision system camera are offered in: analog and digital. After acquiring the image, an analog camera transmits a real-time continuously frequency and amplitude variable electronic signal. An analog output device then interprets the video information. Analog cameras are very susceptible to electronic noise, cable length and connector type which affect the analog signal. Thus quality of the video information resolution and frame rate is based on the quality of the analog signal and the interpretation device. Typically, analog cameras are less expensive and less complicated than their digital counterparts, making them cost-effective and simple solutions for common video applications.

A digital camera transits binary data or a stream of one (white) and zero (black) bits as an electronic signal. The light intensity of a pixel corresponds to continuous voltage that is assigns a grayscale value between 0 (black) and 2N-1, where N is the number of bits of the pixel encoding. An output device converts the binary information into video information. There are two key differences unique to digital and not analog cameras types:

  • The digital video signal is exactly the same when it leaves the camera as when it reaches an output device.
  • The video signal can only be interpreted in one way.

These differences eliminate errors in both transmission of the signal and interpretation by an output device. Compared to analog counterparts, digital cameras typically offer higher resolution, higher frame rates, less noise, and more features. The disadvantages are: digital cameras are generally more expensive than analog ones; feature packed digital cameras may be more complicated to set up, even for systems that just require basic capabilities; and in most cases digital camera have shorter cable connection lengths.

Both analog and digital cameras divide an image into rows or fields and then scan each row to form an image by accumulating rows. There are two types of camera scans methods: Interlaced and progressive scan. Images scanned by an interlaced camera are scanned in two separate fields: odd fields (rows 1, 3, 5, etc) and even fields (rows 2, 4, 6, etc). These fields are then integrated or interlaced to produce a full frame. Progressive scan cameras scan each field or row sequentially (rows 1, 2, 3, 4, etc); thus the integration of the rows to produce a full frame image is faster than an interlace scan.

Assuming a frame rate close to human’s vision of 30 frames per second (fps), an interlacing scan camera takes 1/60 of a second to read a field. For most applications, interlaced scanning does not cause a problem. However for high speed or moving applications interlaced cameras can causes ghosting or blurring effects in the resulting image. A progressive scanning camera does not produce this same image result. However, care should be taken when selecting a progressive scan camera because many different camera output signals are offered for digital cameras. Most progressive scan camera are digital but analog version are offered and again care needs to be taken when selecting a digital camera because few monitors are able to display progressive scan images. For this reason capture boards are recommended to produce an analog image for display

Vision system cameras are also classified as area scan or line scan cameras. An area scan camera is similar to a family’s point-and shoot digital camera. The imaging lens of an area scan camera focuses the entire image onto a sensor array and the image is sampled at a pixel level for reconstruction. Area scan camera are an ideal solution for images that are not moving quickly or if an object is not extremely large. Typically an area scan camera has a 4:3 aspect ratio. With a line scan camera the image is scanned a line at a time as the object moves pass the camera and the image is reconstructed with software. The pixels arranged in a line allows for very long arrays. The advantage to long arrays is that large amount of information can be read per camera exposure making it ideal for processing wide objects.

In general an area scan camera is suitable for a wider range of applications, has a faster shutter speed and is easier to set up than a line scan camera. However line scan cameras are better for capturing wide objects. Line scan cameras do require special alignment and possible more complex integration, but only require simple illumination.

Typically a monochrome camera (with its shade of gray) is a better choice over color cameras. Color cameras are monochrome sensors with a matrix of different color filter over the camera lens. These filters degrade over time (up to 30% degradation). To compensate for this problem increase object illumination or lower lens iris setting are required. Additionally if a color camera is required for an application, it is best to purchase a three chip (blue, green and red) color camera. With single chip color camera the color resolution degrades. The color of one object pixel may not quite match the same pixel on a neighboring object. Often the imaging processing software corrects this problem however the accuracy and the consistency of the color may not be quite the same.

Lastly camera sensors are offered in two versions: CCD (charge-coupled device) and CMOS versions (complementary metal-oxide semiconductor). Both image sensors start at the same point in that they convert light into electrons; however each processes this electrical charge differently. With CCD camera sensor, the pixels are recorded on a chip and then one by one sent through the analog to digital converter to build the optical data. CCD sensors use a special process to create the ability to transport pixel charges across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity.

With CMOS camera sensors, each pixel has its own voltage processor circuitry so pixels are read simultaneously and then transmitted as digital information all at once. The CMOS sensor also includes amplifiers, noise-correction, and other digitization circuits. In most CMOS devices, there are several transistors at each pixel processor that amplify and move the charge. These additional transistor electronics increase the design complexity and reduce the area available for light capture. Additionally with each pixel doing its own conversion, uniformity is lower.

Because of these image processing differences, there is some noticeable performance differences between the two camera sensors:

  • CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.
  • Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons hitting the chip hit the transistors instead of the photodiode circuits.
  • CMOS traditionally consumes little power. CCDs are based on a process that consumes lots of power. CCDs can consume as much as 100 times more power than an equivalent CMOS sensor.
  • CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.

Based on these differences, CCDs cameras tend to be used in the majority of vision systems that focus on high-quality images with lots of pixels and excellent light sensitivity. CMOS cameras traditionally have lower quality, lower resolution and lower sensitivity. CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.

Vision System Lighting

When implementing a machine vision system, illumination of an object or an area of interest is critical for successful image acquisition and processing. There are three primary illumination goals or criteria that must all be achieved for successful vision inspection:

  • Maximizing the contrast of the features of inspection object or area of interest.
  • Minimizing the contrast outside the inspection area.
  • Provide the correct measure of inspection robustness

Every vision inspection is different and to illustrate the importance of interrelationship between these three criteria the following basic plastic wrapped package barcode reading application is good example. At first glance it would appear that maximizing the barcode label contrast and minimizing the contrast of the area outside the label would be fairly easy to achieve with direct lighting using a co-axial ring light mounted around the camera sensor. However if the package’s plastic covering the barcode label area is buckled or bubbled, light may be reflected creating graying of barcode contrast. Obliviously this lighting solution (direct lighting) does not achieve the correct measure of inspection robustness.

In addition to environmental ambient lighting conditions such as the availability of natural sunlight, there are four illumination lighting variables the affect every inspection’s contrast criteria and system robustness that must be considered as part of any machine vision system analysis:

  • What is the 3-D spatial relationship among three elements: the desired inspection object or area sample; the area lighting; and the camera? This is also called system geometry. Using geometry to effect contrast changes involves moving the sample, light source, and or camera positions until a suitable configuration is found.
  • What is the shape of the light projected onto the sample? This is referred to as lighting structure or pattern. Contrast changes via structure or the shape of the light projected involve different types of light source form factors and or lighting techniques such as: back lighting; diffuse lighting; directional lighting; and dark field
  • How the light is differentially reflected or absorbed by the object or area sample as well as its immediate background? This object or sample light reflection and absorption impact the light wavelength or color spectrum seen by the camera. The same is true with background
  • What light filters will provide the correction for the object or sample’s reflection and absorption differentially blocking passing spectral wavelengths and or light directions?

Types of lighting sources affect the abovementioned illumination variables. There are five basic types of machine vision light sources: fluorescent; quartz halogen – fiber optics; LED – light emitting diode; metal halide (mercury); xenon; and high pressure sodium. Fluorescent, quartz-halogen, and LED are by far the most widely used lighting types in machine vision, particularly for small to medium scale inspection stations. Metal halide, xenon, and high pressure sodium are more typically used in large scale applications, or in areas requiring a very bright source. Metal halide, also known as mercury lighting, is often used in microscopy because it has many discrete wavelength peaks, which different filters complement. A xenon source is useful for applications requiring a very bright or strobe lighting.

Each of the above light sources possesses different characteristic involving: light intensity; brightness and spectral content; heat output; output stability; application flexibility; operational longevity; and cost effectiveness.

Historically, fluorescent and quartz halogen lighting sources have been most commonly used. In recent years, LED technology has improved in stability, intensity, and cost-effectiveness; however, it is still not as cost-effective for large area lighting deployment, particularly compared with fluorescent sources. However, on the other hand, if application flexibility, output stability, and longevity are important parameters, then LED lighting might be more appropriate. For microscopy applications full light spectrum quartz halogen, xenon or mercury lighting is useful particularly for color imaging. However for black and white microscopy, monochrome LEDs with a CCD camera provide successful operational longevity alternative.

Applications requiring high light intensity, such as high-speed inspections, it is useful to match the light source’s spectral output with the spectral sensitivity of vision camera used. For example, CMOS sensor based cameras are more IR sensitive than their CCD camera, creating a significant sensitivity advantage for infrared LED or infrared tungsten light sources.

Many times, one light source type does not provide a complete illumination solution for all the application variables. In these cases the combination of multiple light source types may provide the best spectral lighting and illumination characteristics.

Lighting techniques also impact object illumination. There are four basic lighting techniques: back lighting; diffuse lighting; bright field; and dark field.

Back lighting involves placing the light source in back of the object or sample area. Back lighting generates instant contrast as it creates dark silhouettes against a bright background. Backlighting is useful for: detecting the presence or absence of holes and gaps; part placement; or for measuring objects.

Diffused lighting is also called is full bright field lighting commonly used on shiny or mixed reflective object samples when even multi-directional lighting is required. Diffused lighting involves using reflected light. There are two primary diffused lighting techniques:

  • Hemispherical dome/cylinder diffused lighting: This diffused lighting techniques uses a reflective dome or bowl with light two light sources placed on the dome edges pointed into the dome and then reflected onto the sample surface. The camera is place in a hole in the center of the dome. Diffuse dome lights are very effective at lighting curved surfaces. There is a variety of domes offered for diffused lighting application. Typically the dome needs to be in close proximity to the object surface
  • On-axis diffused lighting: The simplest way to explain this technique is a light source is aimed at a two-way reflective mirror and the mirror shines the light onto to object surface. A camera is located in the other side of the two-way mirror pointed at the object. On-axis diffused lighting is particularly effective at enhancing angled, textured, or topographic features on relatively flat objects.

Partial bright field lighting, also called directional lighting, is similar to natural sunlight and is the most commonly used technique. The camera sensor is pointed directly over the object and a single light source is mounted to either side of the camera directionally aimed at the object. No reflective devices are used to distribute the light. Partial bright field lighting, due to its directional nature, is a good choice for generating contrast and enhancing topographical detail.

Dark field lighting is similar automobile headlight illumination. Dark field illumination involves two light sources directional pointed at low angles to the object surface. The two light sources are placed at opposite ends of the surface. Light is reflected off the surface and back to the camera sensor. Dark field lighting is useful in identifying small surface imperfection.

AC Drives

AC Drives convert fixed voltage and frequency into variable voltage and frequency to run AC motors. The output of AC drives is defined as V/Hz (“Volts per Hertz). Essentially all AC drives today are PWM (Pulse-Width-Modulated) and share similar power circuit topologies where AC input power is converted (rectified) to DC voltage, filtered, then inverted to variable frequency, variable voltage AC. There are three basic types of AC drives and they are called: Inverters or “V/Hz drives”, open loop or “sensorless” vectors, and closed loop vectors. All AC drives can handle some amount of regenerated energy from the motor, but it varies widely from 20% of its torque rating for Inverters, to 100% of rated torque for AC drives with common busses. Refer to “Regenerative Drives” for more detailed information. AC drives should be used with appropriately rated “inverter-duty” or “vector-duty” AC motors.

Inverter (Open Loop): Open-loop AC Inverters require no motor feedback and are used only to control speed; they do not control torque or position. The inverter’s output voltage always varies linearly with frequency without compensation for motor or load dynamics resulting in a typical 1 to 3% speed regulation. The most notable limitations of the AC inverter are: low dynamic performance on sudden load changes, limited starting torque, and no zero speed holding torque. Thus, inverters are ideal for continuous speed applications such as conveyors, mixers, agitators and other non-dynamic loads. Inverters can also run multiple motors and are often used for variable torque applications such as pumps and fans. Inverters typically offer a limited amount of braking capability that is limited by the amount of regenerative energy the DC bus can handle and the size of the braking resistor. Refer to “Regenerative Drives” for more information. Overall, open-loop AC inverters are generally used to control the speed of peripheral rather than process motors due to their limited performance.

Open Loop Vector: Open-loop AC Vector drives require no motor feedback and can be used to control speed or torque. The output voltage is varied linearly with frequency plus dynamic self-adjustments for the motor and/or load, resulting in typically better than 1% regulation. Open-loop vector drives offer high starting torque and excellent shock load response characteristics. The most notable limitations of the open-loop vector are no zero speed holding torque and single motor control only. Open-loop vector drives are suitable for all general purpose, variable torque and moderate to high performance applications that don’t require exceptional regulation such as process lines, extruders, some winders and unwind stands. Open-loop vector drives typically have braking and regenerative capabilities and are sometimes able to share a DC bus with other vector drives in order to obtain greater motor regeneration benefits. See “Regenerative Drives” to learn more.

Vector (Closed Loop): Closed-loop AC Vector drives require motor feedback and can be used to control speed and torque. The output voltage is varied linearly with frequency plus dynamic self-adjustments for the motor and/or load, resulting in excellent speed regulation to .01%. Vector drives offer high starting torque, excellent shock load response characteristics, full torque to zero speed and a very wide range of control.

The primary limitations of vector drives are: they require motor feedback; they are useful for only single motor control; and a premium vector duty motor may be required to obtain full performance benefits.   Some vector drive brands feature electronic gearing, 4-quadrant operation and a limited form of position control. Vector drives are best for high performance applications such as converting, printing, winding, spindles, lathes and other historically high performance digital DC applications. Vector drives typically have limited braking and regeneration capability and sometimes are able to share a DC bus with other vector drives in order to obtain greater motor regeneration benefits.

PWM Drives: PWM drives, both AC and DC, use transistors as their power devices for their front-end rectifier. The rectifier is unidirectional and cannot be used to return AC current to the power supply.   Instead, regenerated current is sent to the DC bus via transistor switching and back diodes, where the excess voltage is stored in a capacitor for use later by the drive when motoring. If the amount of regen energy exceeds the capacity of the bus capacitor, the bus cap will burn up unless the energy is directed elsewhere.

Most PWM drives include an extra transistor that will turn on when the bus cap reaches its limit allowing current flow to a resistor that will burn off the excess energy as heat. Small horsepower AC and DC drives (10hp and under) often include the resistor in the drive. Larger horsepower drives (greater than 10nhp) supply the resistor loose for mounting outside of the drive due to the significant amount of heat generated.

DC PWM drives are typically only seen in low horsepower units (7.5hp and below) for economic reasons. Regen energy in DC PWM drives is handled as described above with an extra transistor that turns on when excess bus voltage is detected when the bus capacitor is full, to allow current flow to a resistor to bleed off the excess energy. The resistor is sometimes included in the drive and sometimes offered loose to be mounted external to the drive.

PWM AC Drives: In moderate performance variable frequency AC PWM drives (VFD), the regen transistor, capacitor and resistor is typically only large enough to handle braking energy and is not meant for continuous regen applications. Although ratings vary, it is common for these drives to be rated for only 20% regen capability with some tolerance based on the sizing of the resistor.

High performance and higher cost AC Vector PWM drives will often be sized with a slightly higher rated transistor, capacitor and resistor for 30% or 40% stand alone regen capability. These high performance PWM AC Vector drives usually offer an additional feature. This feature is the ability to connect the DC bus of one drive to another drive or multiple drives. This allows one drive to utilize the excess regen energy produced by another drive for its own motoring torque. The net result is no loss of energy and the ability to provide up to 100% regen torque for a drive. These bus sharing systems need to be carefully planned to ensure regen torque overall does not exceed the motoring torque of the system.

AC Motors

An electric motor is an electromechanical device that converts electrical energy into mechanical energy. AC motor is an electric motor driven by alternating current. There are two types of AC motors: asynchronous induction motors; and synchronous motors.

The AC induction motor is the most commonly used AC motor in industrial applications because of its design simplicity, rugged construction, and relatively low manufacturing costs.   The induction motor theory of operation is based electromagnetic induction or the production of electric current across a conductor moving through a magnetic field (Faraday’s law). There are two primary elements to an induction motor: a stationary stator and an inside rotor.

The stator is located inside the motor housing and surrounds the rotor with a small gap between the stator and rotor. The stator has two parts: a stator core and stator wire winding coils also simply called winding.   The stator core is a lamination stack of notched steel plates with “slots” in-between. The stator winding coils (two or more) are wrapped around the notches. The rotor is the rotating element of the induction motor. The rotor consists of rotor bars, which are the current carrying conductors of the rotor, and laminations which function as the magnetic medium.

When AC power is applied to the coils, the stator induces magnetic lines of flux across the air gap into the rotor, creating a magnetic field (No direct electrical connections are made to the rotor). The stator magnetic field rotates proportional to the frequency of the AC power supplied and as this magnetic field passes by the rotor bars an induced current is developed in the rotor and the rotor chases the rotating magnetic field of the stator. The rotor is attached to a motor shaft which is also rotates on set of bearing.

The speed of the stator rotating electromagnetic field is dependent on the frequency of the AC power supply and the number of motor poles or sets of coil windings. This is called the motor’s synchronous speed. The Synchronous Speed = (120 X Frequency of Power Supply) / # of poles. Thus a 4 pole motor running on 60 Hz power has a synchronous speed of 1800 RPM.

However the gap between the stator and the rotor causes the rotation of the rotor to lag behind the stator’s synchronous speed. This rotor lag is called slip. Slip is defined in as a percent of synchronous speed. %Slip = (Synchronous speed – actual speed) / (Synchronous speed) X 100. This slippage is why type of induction motor is called an asynchronous induction motor.

Slippage is what creates torque in the AC motor. As the rotor slips, the rotor bar current frequency increases resulting in a greater rotor magnetic field strength which creates torque producing current and increases the torque of the motor shaft.

A synchronous motor has a slightly different construction than an asynchronous or non-synchronous induction motor. The difference is that there is not gap between the rotor and the stator. Carbon brushes and slip rings on the rotor make contact with the stator. As the electromagnetic field is created by the stator, the brushes and rotor rotate at the stator’s synchronous speed or at a sub multiple of the current supplied to the stator.

Torque and horsepower are two very important characteristics that determine the size of the motor for a particular application. Torque is the motor shaft’s turning effort and is measures in foot pounds or its metric equivalent Newton Meters. An easy way to explain torque is to suppose a grinding wheel with a one-foot crank arm and to turn the crank at a steady rate it takes a force of one-pound. The torque required is one-pound times one-foot equals one-foot-pound. A Newton Meter (NM) is equal to about 0.74 pound-foot. As long as the crank is turned at a stead speed the torque remains the torque same regards regardless of how fast the crank is turned. If the crank is turned twice as fast, the torque remains the same. Amount of torque produced by motors varies with speed.

Horsepower takes into account how fast the crank is turned (speed). Turning the crank more rapidly takes more horsepower than turning the crank slowly. Horsepower is the rate of doing work. By definition, one horsepower equals 33,000 foot-pounds per minute. In other words, to lift a 33,000-pound load one foot in one minute would require one horsepower. Electric motors are rated by horsepower.

The two most common types of AC motors are single-phase and three-phase. Single phase motors use ordinary household single sine wave alternating current with two power wire conductors / poles. Single-phase induction motors are typically used for low horsepower applications and requires separate starting circuitry to provide a rotated field to the motor. The windings within a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction. There are five basic types of single-phase motors:

  • Split Phase Single Phase AC Motors: Split Phase non-synchronous single phase AC motors are low cost and very common in household appliance while also used in some industrial applications. They are of relative constant speed, have excellent starting torque, small frame sizes and good power efficiency.

The Split Phase motor has an additional “auxiliary” or starting winding, sharing the same slots, and in parallel with the main stator winding. The auxiliary winding is shifted in space from the main winding and is typically wound with different wire and a lesser number of turns so that current flow through this winding is nearly in phase with AC power. Current flow through the main winding lags behind the current in the auxiliary winding creating a “split” in the single phase power.

The result of this phase displacement between the main and auxiliary windings in the stator creates a rotating magnetic field that will cause the rotor to begin rotating and results in a high starting torque around 175% of motor rated torque.

When the motor reaches a particular speed (typically about 70% of rated speed), a switch or relay disconnects the auxiliary winding and the motor rotates only on the field created by the main winding. The switching out of the auxiliary winding should occur in just a few seconds after startup.

Reversing of the Split-Phase single phase motor is accomplished by incorporating a switching device that connects power to the main winding in reverse. This switching must occur at standstill or at a speed low enough that the auxiliary winding is still in the circuit. In practicality, reversing should be done at standstill only to preserve the life of the starting auxiliary winding.

Split Phase motors are best in applications that have infrequent start/stops, require a high starting torque and rapid acceleration of a normal load. They are not suited for applications with high inertia loads that can take a long time to accelerate to the speed where the switch removes the auxiliary winding. The result can be overheating or damage of the auxiliary/starting winding. This is also true with frequent starting and stopping of the motor.

When the motor reaches a particular speed (typically about 70% of rated speed), a switch or relay disconnects the auxiliary winding and the motor rotates only on the field created by the main winding.

When applying these motors, the high starting current of the motor and the amount of load to be accelerated must be taken into account when sizing the power wiring.

  • Capacitor Start Single Phase AC Motors: The Capacitor Start (CS) single phase asynchronous motor, like the Split Phase motor, has an additional “auxiliary” starting winding sharing the same slots and in parallel with the main stator winding. However, the Capacitor Start motor also includes an electrolytic capacitor in series with the auxiliary winding.

The purpose of the capacitor in the starting winding is to allow a high starting torque (75%-150% of rated torque) while reducing the starting current (which is typically very high in a Split Phase motor that does not have a capacitor). Thus size selection of the capacitor will affect the amount of starting torque and should be taken into consideration with load and acceleration requirements.

The starting winding and capacitor are switched out of the circuit when the CS motor reaches a particular speed (typically around 70% of rated speed) leaving only the main winding in the circuit.

As with conventional Split Phase motors, frequent starting and stopping, long acceleration times and reversing while in motion are not recommended.

Permanent Split Capacitor Single Phase AC Motors: The Permanent Split Capacitor (PSC) single phase AC motor is similar to the Capacitor Start motor except the AC type capacitor that is in series with the auxiliary winding is rated for continuous duty and it remains in the circuit at all times. Thus, the auxiliary winding is energized at all times.

The capacitor in the PSC motor causes the current in the auxiliary winding to be out of phase with the main winding current. The result is a rotating magnetic field similar to a two-phase AC motor.

Having the capacitor in the circuit all the time results in a smoother, more efficient and quieter motor operation while running. The startup benefits (of high starting torque with lower starting current) of the Capacitor Start motor are retained in the PSC motor. Note however, changes to the capacitor size to increase running benefits will reduce start up benefits and vice versa. Thus it is important to use the capacitor rating recommended by the manufacturer.

Two Capacitor Start Single Phase AC Motors: The Two Capacitor Start/Single Capacitor Run single phase AC motor is a variation of the Permanent Split Capacitor type AC motor. It uses two capacitors for startup and one for running.

The running capacitor, which is an AC type continuous duty capacitor, is also used for start up along with an electrolytic capacitor. The two capacitors are in parallel which results in good starting torque characteristics (75-150% of rated torque). However, two capacitors would not be optimal for normal running conditions, so only one remains in the circuit in running mode.

The PSC and Two Capacitor Start (TCS) /Single Capacitor Run (SCR) motors tend to run hotter under light loads so it is best to run them at or near their rated load. In addition, these motors should only be reversed when the motor is at zero speed.

Although they are more expensive, TCS/SCR motors are often the best choice for applications that require long or frequent starts which can be detrimental to conventional Split Phase or Capacitor Start single phase motors. TCS / SCR motors offer smooth, quiet and efficient operation.

Shaded Pole Single Phase AC Motors:  Shaded pole AC motors are simple and inexpensive motors found in many commercial and industrial low power and low torque applications (typically up to 1/4hp), such as electric fans, microwave ovens and other small appliances. They are made up of a rotor, stator and stator windings like other single phase AC motors and require a method to initiate the rotation of the rotor.

  • Shaded Pole motors, however, don’t have a second auxiliary winding for starting like the capacitor start single phase AC motors. Shaded Pole motors are designed with a stator lamination that allows for “salient” poles to project out radially either from the rotor or the stator. The main windings are then wrapped around the salient poles instead of the slots around the stator. The number of salient poles is equal to the number of poles in the motor.

Each of the salient poles has a copper shorting ring, called a shading coil placed on it that will “shade” a portion of the main windings magnetic field. The main windings are connected to the power supply, while the shading coils are connected to themselves creating a short circuit.

The magnetic field created by the main windings induces a current in the shading coil creating another magnetic field that lags the main winding magnetic field enough to provide starting torque.

Shaded Pole motors are essentially constant speed motors that are very reliable, smooth and quiet. Their disadvantage is relatively low starting and running torque and less efficient when compared to split phase motors, thus they are typically used in light load, unidirectional applications.

Three-phase motors are typically used for higher horsepower applications such as compressor, hydraulic pumps, air conditioning compressors, irrigation pumps and other industrial applications.   Three-phase power is non-dimensional (no peaks or valleys) because it incorporates three sine-wave power curve each offset by 120°. Each of the three-phase motor’s winding and poles are also displace 120°. Thus a three-phase induction motors generates it own rotating magnetic field and torque (no starting circuitry is required). With three-phase motors the stator passes energy to either a short circuit squirrel cage rotor or wound rotor. The squirrel cage rotor is a cylinder mounted on the rotor shaft. Internally it contains longitudinal conductive bars usually made of aluminum or copper set into the cylinder’s grooves and connected at both ends by shorting rings forming a cage-like shape.

Compared to a squirrel cage rotor the wound rotor has more winding turns and windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.   Squirrel cage are less costly than wound rotor motors, general require less maintenance and explosion proof due to the absence of brushes slip rings and brushes which eliminates the risks of sparking.

Inverter or Vector Duty three-phase AC induction Motors  are asynchronous motors with torque producing slip. They are designed specifically for use with AC drives in typically industrial environments. Inverter duty AC motors have rotor bars that are optimized for inverter use, along with larger frame sizes and/or constant speed fans to provide better cooling for variable speed, inverter applications.

Inverter duty motors are also typically designed with tachometer or encoder mounting provisions that are often desired or required with high performance AC drives. Along those same lines, inverter duty motors typically offer larger speed ranges that allow for above base speed operation in the constant horsepower range.

Inverter duty AC motors are often rated with a “base speed” and a “maximum speed” value. The base speed is the synchronous speed of the motor minus slip. For a typical, 4 pole AC inverter duty motor, wound for 60Hz and used with 230VAC, 60Hz power, the synchronous speed is 1800 RPM. The base speed of this motor would be 1750 RPM (a slip of 50 RPM) achieved at maximum supply voltage at 60Hz frequency. The maximum speed rating is the maximum speed the motor can mechanically achieve and is typically 1.5 to 2 times base speed.

The base speed of the motor is affected by the V/Hz design where the optimal power of the motor is achieved if the base speed of the motor is at peak supply voltage.   Although most standard motors for the U.S. market have stators wound for 60Hz power, custom motors can be wound for different frequencies for special applications. For example a stator wound for 20Hz at 230VAC would achieve its base speed and maximum power at 20Hz frequency (at 230V).   As the frequency increases beyond 20Hz up to 60Hz, the motor will increase in speed and its torque will drop off, resulting in constant horsepower to maximum speed.

The constant horsepower range is the range of motor operation above base speed up to the maximum speed of the motor. This range of operation exhibits a drop off in torque with an increase in motor speed resulting in a constant horsepower rating. Operation in the constant horsepower range is easily accomplished when using an AC Drive with a user adjustable output frequency greater than the frequency of the AC power supply.

Again with synchronous AC induction motor, the speed of the rotor matches the speed of the rotating magnetic field in the stator. When the rotor is spinning at the same speed as the stator’s magnetic field (the motor’s “synchronous” speed) the motor’s “slip” is equal to zero.

Synchronous motors can be single phase or three-phase (poly-phase) and run at a constant speed that is dependent on the frequency of the power supply (and the number of poles), thus they are used heavily in timing applications.

The two most common different types of rotors used in induction synchronous motors are the Reluctant Synchronous motor and the Hysterises Synchronous motor. Difference between the two types of synchronous motors follows:

  • Reluctant Synchronous: The Reluctant Synchronous motor has a variation to the typical squirrel cage rotor in that it has areas of high reluctance with fixed salient poles in-between. The result is a motor that starts and accelerates much like a typical induction motor, but as it approaches matching the rotational field speed of the stator, the rotor increases its acceleration speed and based on the position of the salient poles, “snaps” into synchronous speed.

If the inertia of the load (or the load itself) is too great for the rotor to pull in to synchronous speed with its acceleration burst, then the motor will not lock in to synchronous speed and will continue to operate in an uneven and irregular manner. In the same way, if the motor is already running at synchronous speed and the load is increased to an amount greater than the motor’s rating, it will break out of synchronism and again run roughly.

Reluctant synchronous motors can be single phase or poly-phase. Single phase versions have the same winding choices as asynchronous, single phase motors (Split Phase, Capacitor Start, Permanent Capacitor, etc.)

  • Hysteresis Synchronous: The Hysteresis Synchronous motor is uses another variation of the typical squirrel cage rotor. The difference, which results in its unique operating characteristics, is the hardened magnet alloy material that is used to make the rotor.

The Hysteresis rotor poles are induced poles, unlike the fixed salient poles of the Reluctant motor. The Hysteresis rotor poles will shift around the outside of the rotor during acceleration until the rotor speed locks in with the stator’s magnetic field-which can occur at any random angular position of the rotor. The rotor poles then take up a fixed position-which is not in any particular location. Thus, the rotor starts on the hysteresis principle and maintains a fairly constant acceleration until the rotor speed locks in with the stator’s synchronous speed.

As with the Reluctant motor, the load on the Hysteresis motor must not be too great or too inertial so that the rotor is not able to lock into synchronous speed or increased so much that the rotor falls out of synchronous speed. However, because the Hysteresis motor does not need an increase in acceleration of the load in order to lock in the rotor poles (like the Reluctant motor), it is able to lock in more inertial loads.

It is important when sizing synchronous motors that the worst case scenario load conditions be taken into account, so as to be certain that the motor can accelerate the load to synchronous speed and maintain synchronous speed.

In general, synchronous motors are larger and more expensive than asynchronous motors of the same power rating. However in applications where the load needs to be driven at an exact rate of speed a synchronous motor would probably cost less than an asynchronous motor with a speed control.

Connectors

An electrical connector is an electro-mechanical device that joins or physical interfaces two electrical conductors or electrical circuits together. Electrical connectors are used to either join two lengths of flexible electric wire or cable, or connect a wire or cable to an electrical terminal.

Electrical connectors are characterized by their physical design and construction, size, contact resistance, insulation between pins, ruggedness and resistance to vibration, resistance to entry of water or other contaminants, resistance to pressure, reliability, pinout (a cross-reference between the contacts, or pins, and their functions), lifetime (number of connect/disconnect operations before failure), and ease of connecting and disconnecting. Some electrical connectors are mechanical keyed to prevent: prevent insertion in the wrong orientation; connecting the wrong pins to each other; and or have locking mechanisms to ensure that they are fully inserted and cannot work loose or fall out.

Desirable electrical connector properties include: easy visual identifications; rapid assemble typically only requiring a simple tool and connectors should be relatively inexpensive. There are hundreds of types of electrical connectors. The major types of electric connectors follow:

  • Wire terminal is a simple type of electrical connector that connects two or more wires to a single connection point. Wire terminal may have a screw connector or spring pressure connector into which the wire is inserted. A wire nut or screw-on electrical connector to connect two or more wire together is another example of a simple electrical connector.
  • Terminal blocks also called terminal board or strips are a connector that allows more than one electrical circuits to be connected without slicing or physically joining the ends. They are usually used to connect wiring to other equipment within an enclosure. Since terminal blocks are offered in a wide range of wire sizes and terminal quantity making terminal bocks one of the most flexible types of electrical connector available. One of the disadvantages is that the terminals are generally are not very well protected from contact with persons or foreign conducting materials. Terminal blocks typically contain two long copper or aluminum strips, designed to connect different components. These strips create a bus bar, or electricity conductor, for power distribution, which is sent to the connected components.

Terminal blocks are typically offered in plug-in, screw-in or PCB versions. To prepare the wire for connection to the terminal block a short length of insulation is removed or stripped for the end of the wire. Sometimes wire rings or spade terminal lugs (see blade connectors below for more information) are crimped onto the wires for easy connection. Using screw terminal blocks wires are connect to the block by screws. Plug-in terminal blocks contain a male-type terminal at the end of the wire, which is connected to the female port on the block. PCB, or printed circuit board terminal blocks, are commonly found in computers, and are soldered, or fused, to the circuit board.

  • Post connectors are a general type of connector that simply screws or clamps wire to connection posts. Post connectors are frequently used in electronic test equipment and audio equipment.
  • Insulation-displacement connector (IDC) or insulation-piercing connector is an electrical connector designed to be connected to the conductor(s) of an insulated wire or cable by a connection process which forces a selectively sharpened blade or blades through the insulation, bypassing the need to strip the wire of insulation before connecting. When the connection is properly made, the connector blade cold-welds to the wire, making a highly reliable gas-tight connection. Types of IDC connectors include: ribbon cable connections; telephone or network connector called RJ (Registered Jack) connectors; and punch-down blocks designed to connect individual wires into each position in the terminal block with a special punch-down tool.   Punch-down type terminations are used in telephone and network wall sockets, in patch panels and distribution frames, and in telephone equipment such as PBXs.
  • Crimp connectors are solderless connectors typically used to terminate stranded wire. Crimp connector have a multitude of uses such as allowing the wires to be easily terminated to screw terminals, fast-on / quick-disconnect / spade-foot type terminals, wire splices, and more. Crimping the connector to a wire is accomplished with special crimping pliers. A stripped wire end is inserted into a sleeve of the crimp connector and the crimping pliers are used to compress the sleeve against the wire to form a gas tight connection. Crimp connectors provide a reliable, fast and easy connection with superior mechanical connection characteristics. Some types of crimp connectors are: a barrel connector – or oval terminal (sometime called a ring terminals) crimped connection that looks like a washer into which the screw of the screw terminal is inserted; an open barrel connector – also called a “U” or “V” connector because of their terminal shape; blade and spade connectors – type of single wire connection using a flat conductive blade which is inserted into a blade receptacle.
  • Plug and socket connectors are composed of two connectors: a male plug and female receptacle or socket. These connectors are usually multi-wire connectors with male plug having pins (attached to wires) and inserted into a multi-wire mating sockets. Plug and socket connection usually have a pinout diagram to identify the wire or circuit monde connected to each pin. The connection between the mating metal parts must be sufficiently tight to make a good electrical connection and complete the circuit. Plug and socket connectors may be a ‘molded” connector where the plug, socket and connected wired are cover in an insulation material mostly plastic.
  • A jack is a kind of plug and socket connector but the female connector is fixed to a bulkhead or enclosure. A male jack plug with terminated wires is inserted into the jack. There are many types of jacks such as: an Ethernet jack or RJ45 (RJ means Registered Jack) or RJ11 connection used for computer networking; a phone jack for connection of telephones plus some electronic products; a RCA jack commonly used for consumer electronics; a headphone jack; an EIAJ jack designed for consumer appliances requiring less than an 18.0 volt power supply; and many more.

Cordsets

International standards define a cordset as “an assembly consisting of one flexible cable or cord fitted with one non-rewireable plug and one non-rewireable connector”. A power cordset is “intended for the connection of an electrical appliance or equipment to the electrical supply”. An interconnection cordset is “intended for the interconnection of the electrical supply from one electrical appliance or equipment to another”.

In other words a cordset consists of a flexible cable (multi-conductor wiring harness) or cord with electrical connectors on either end: one male and one female. One end of the cord set is attached to a molded electrical plug; the other end typically has a molded electrical receptacle to prevent the possibility of having an exposed live prong or pin which would cause electric shock. The female plug is connected to the piece of equipment or appliance while the male plug connects to the electrical receptacle or outlet.

Cordsets are usually manufactured by specialty manufacturers instead of in-house custom made cable because cordset manufacturers rigorously test the finished product to insure top performance quality and often obtain agency approvals for UL, CSA and CE.

Cordsets are offered in a near limitless combination of connectors, orientations, cable types, number of conductors, jacketing material and other options. Important considerations when specifying a cordset include:

  • Is the cordset AC or DC rated?
  • What length of cord is needed? Cordsets are typically available in 2, 4 or 6 meter lengths. If more than 6 meters is required, and extension cordset will be necessary.
  • What types of connectors are required? Connectors are offered in: male or female version; straight or right angle connectors; number of pins and or sockets required; snap lock or thread coupling nuts; size and other related connector attributes.
  • What type of cable is required? What number of needed conductors? What are the wire gauge and rating specifications? Is shielding required? What kind of jacketing material?
  • Are there are required agency approvals?
  • Are there any other specialty options such as indicator LEDs, coiled retractable cables or reversed key needed?

Most manufacturers offer a matching extension cord to all their cordsets. This makes getting the correct cord length easy. The same questions for cordsets apply to extensions. It is best to choose your cordset first, and then make the appropriate match for an extension.

Cordsets are also used with junction boxes with a single output connection at one end. Junction boxes are often found with 4, 6, 8 or more ports. Junction boxes can come with or without an integral power supply, and have options such as cable type and LEDs. Junction boxes can be an “open-face” style with flush connections or can be a completely sealed box with quick connect or screw type connections.

Additionally cordsets are offered with splitters that split a signal into tow identical copies. A splitter cordset has either male to two female, or female to two male connectors. Splitters cordsets can have a single molded piece or come with branches of varying cord lengths.

Adapters are often used with cordsets to convert one type of connection to another. They are typically a single molded piece. Adapters are numerous and available from most connector and cordset manufacturers for adapting between their own connector offerings and/or other standard connectors.

DC Drives

DC drives convert an AC or DC power supply into a variable DC voltage to run DC electric motors. DC drives provide speed and/or torque control, controlled acceleration/deceleration, protect the motor from excess current and other benefits. There are many different types of DC drives. Some are simple analog devices, while others are digital with programmable parameters.

There are three basic types of DC drives: SCRs (silicon-controlled-rectifier); IGBTs (insulated-gate-bipolar-transistor); or MOSFETs (transistor) power devices. The transistor devices in drives are switched on and off using PWM (pulse-width-modulated) technology. The advantages of using SCRs are their simplicity, robustness, their low cost and their availability in very high power ratings for high horsepower drives applications. The advantages of using transistors are that they can be switched on and off at high frequencies (8-24kHz is typical in drives) resulting in short circuit protection and less audible noise. They also produce near pure direct current to the motor.

DC drives are available with a multitude of features for simple speed control to complex process control to multiple drive system control with communications. However all DC drives can be categorized as having: 1-quandrant; 2-quadrant: or 4-quadrant control. It’s important to understand these categories when choosing a drive for an application.

SCR: Silicon-controlled rectifiers (SCRs) are power devices used in DC drives to rectify AC voltage into DC voltage. SCRs are a diode with a gate (switch). When the gate is pulsed, the SCR turns on and allows current to flow through it unidirectional. The SCR cannot be turned off with another gate pulse; it can only be turned off when the current flow through it drops below a minimum holding current. In DC drives with SCR bridges, this occurs every 8.3msec when the input AC sine wave crosses zero.

SCRs control the average DC voltage out to the motor. Pulsing the SCR gate during different times of the rectified AC voltage results in different motor speeds. Increasing the firing angle (pulsing the gate early in the voltage cycle) results in the SCR conducting current for a longer period of time producing a higher DC voltage output and a higher speed. A small firing angle (pulsing the gate late in the cycle) results in a lower DC voltage output and a slower speed.

The firing angle of the SCRs also determines how “clean” the DC output voltage will be. Quantifying the purity of the DC voltage is described as ‘form factor’ (ff). Form factor represents the amount of AC ripple riding on the DC voltage. The form factor of pure DC is 1.00ff. The closer to 1.00 form factor is, the cleaner the DC and the less power lost to heat.
The cleanest DC from an unfiltered SCR drive is obtained at maximum average voltage output (1.37ff) while the ‘dirtiest’ DC occurs at the lowest average voltage output (as high as 3.00ff). Thus, DC motors with SCR drives as a rule, will run hotter, nosier and wear out brushes faster at low speeds, than at high speeds. Some SCR drives have a filter to smooth out the ripple improving their form factor. Make sure your motor is an “SCR duty motor” which is made to handle the imperfect DC.

PWM with IGBTs or MOSFETs: Insulated Gate Bipolar Transistors (IGBT) and Metal Oxide Semi-conductor Field Effect Transistor (MOSFETS) are transistor power devices. They are turned on (conduct current) by a pulse generated by a PWM (Pulse-Wide-Modulation) chip. The transistor conducts in one direction only and turns off when the pulse goes to zero. The PWM chip in DC drives can generate high frequency switching, typically 16-22kHz, with approximately 55 µsec between pulses.

In PWM DC drives, the average voltage out to the motor is changed by varying the pulse widths, while the peak or amplitude of the pulse width is equal to the bus voltage and is constant. The wider the pulse width results in a higher average voltage output because the transistor is in conduction for a longer time during each pulse.

The constant amplitude of the high frequency pulses results in a clean DC voltage out of the PWM DC drive. Quantifying the purity of the DC voltage is described as ‘form factor’ (ff). Form factor represents the amount of AC ripple riding on the DC voltage. The form factor of pure DC is 1.00ff; the closer to 1.00 form factor is, the cleaner the DC.

The DC from a PWM drive is about 1.05ff regardless of the output voltage (or motor speed). Thus, DC motors with PWM drives will run quieter, cooler and with longer brush wear at all speeds than SCR drives which have a higher form factor. Another benefit to PWM drives is that less power is lost to heat than with an SCR drive, resulting in slightly more power ‘out’ with a PWM drive than an SCR drive with the same power in.

Due to the high frequency switching at bus voltage, PWM DC drives should be used with DC motors that are minimally rated ‘SCR duty’ but preferably ‘PWM duty’. Some DC motors don’t have the insulation needed to handle the high peak pulses of a PWM drive, resulting in “pits” or “holes” in the insulation of the motor.

1-Quandrant Control: Single-quadrant simply means that the drive has the ability to control speed and current flow (torque) in one direction only. The actual direction of rotation is irrelevant to the drive and can be changed by using switches to swap the output leads to the motor armature after the motor has stopped.

When using a single-quadrant drive to control motor speed, the current flow (torque) required to maintain set speed is only available in the direction of rotation. Thus, as long as the motor is ‘powering’, speed control is maintained. If the motor or load has other forces that continuously pull on it, such as a downstream motor or gravity, the single-quadrant drive can not apply a reverse current or reverse torque to maintain the set speed. If the overhauling load is removed, the motor will coast down to the set speed where speed control is regained. Sometimes intermittent overhauling loads with enough recovery time in-between are acceptable applications to the user.

2-Quadrant Control: A DC drive described as “two-quadrant” (2Q), means that it has the ability to control speed in one direction and current flow (torque) in both directions. A two-quadrant control is only available in PWM based drives. The actual direction of rotation is irrelevant to the drive and can be changed by using switches to swap the output leads to the motor armature after the motor has stopped.

Users should select a two-quadrant drive if their application is unidirectional (or can be brought down to a complete stop before switching armature leads to reverse) and they require a reverse torque to counteract overhauling loads that could pull the motor faster than its set speed or require braking of high inertial loads.

Two quadrant drives must be able to handle the excess energy that can be created by a reverse torque that exceeds the motoring or forward torque during operation, or by fast braking. In PWM drives, the excess energy is stored in a bus capacitor for use later. If the regenerated energy exceeds the bus capacitor rating, then the excess energy is bled off through a resistor (sometimes called a “braking resistor”) and lost as heat.

4-Quadrant Control: A DC drive described as “four-quadrant” (4Q), means that it has the ability to control speed and current flow (torque) in both directions simultaneously. Four-quadrant drives are often called “regenerative” or “regen” drives because of their ability to handle regenerated electrical energy from the motor.

Four-quadrant, regenerative drives provide quick, contactor-less reversing that can be initiated on the fly. Reversing occurs without the need to swap armature leads to the motor. When a reverse setpoint is seen by the regen drive, it will drive the motor down to zero then instantly accelerate in the reverse direction to the reverse set point. Regenerative braking is available on regen drives in addition to the typical deceleration and/or coast to stop options. Regen drives are ideal for applications that involve reversing, quick braking, high inertia loads and/or overhauling loads that require a reverse torque to maintain set speed.

Regenerative Drives: Originally the term “regenerative” or “regen” was used to describe an SCR, DC drive’s ability to return excess energy to its AC power source. Today the term is used to describe any electronic drive that has the ability to handle the excess energy created by the motor or load by: returning it to the power source; storing it for later use; or bleeding it off as heat. In DC drives, these drives are also called either two-quadrant (2Q) control or four-quadrant (4Q) control drives. PWM AC and DC drives today also have regen capabilities.

The excess energy described above is commonly created by the motor/load in the following conditions:

  • The drive is maintaining set speed while counteracting with reverse torque, a continuous overhauling load that attempts to drive the motor faster than its set speed.
  • The drive is applying a reverse torque to stop a high inertia load at a rate faster than the load would coast to a stop. This is called “regen braking”.
  • The drive has been given a reverse set point thereby applying a reverse torque to quickly stop the motor and reverse.
  • The drive is controlling a cyclical load that alternates between motoring and regen’ing.
  • An instantaneous event has occurred requiring a large reverse torque to retain set speed.

SCR DC Drives: How the excess energy is handled varies with the technology of the drive and the application. Four-quadrant, regenerative SCR DC drives have two SCR bridges. One accepts the AC power current for motoring torque, the second allows the regen current to flow directly back to the AC power source. SCR DC drives handle regen energy very easily and cost effectively. Typically these drives can regen 100% of their torque rating.

PWM Drives: PWM drives, both AC and DC, use transistors as their power devices for their front-end rectifier. The rectifier is unidirectional and cannot be used to return AC current to the power supply. Instead, regenerated current is sent to the DC bus via transistor switching and back diodes, where the excess voltage is stored in a capacitor for use later by the drive when motoring. If the amount of regen energy exceeds the capacity of the bus capacitor, the bus cap will burn up unless the energy is directed elsewhere. Most PWM drives include an extra transistor that will turn on when the bus cap reaches its limit allowing current flow to a resistor that will burn off the excess energy as heat. Small horsepower AC and DC drives (10hp and under) often include the resistor in the drive. Larger horsepower drives (greater than 10nhp) supply the resistor loose for mounting outside of the drive due to the significant amount of heat generated.

PWM DC Drives: DC PWM drives are typically only seen in low horsepower units (7.5hp and below) for economic reasons. Regen energy in DC PWM drives is handled (as described above in the ‘Regenative Drive’ section) with an extra transistor that turns on when excess bus voltage is detected when the bus capacitor is full, to allow current flow to a resistor to bleed off the excess energy. The resistor is sometimes included in the drive and sometimes offered loose to be mounted external to the drive.

DC Motors

A programmable logic controller (PLC) or programmable controller is a specialized digital computer used to control discrete manufacturing processes such as assembly lines, shop floor machinery and related factory mechanical, electrical and electronic equipment. Unlike a computer, a PLC is designed for real-time use/control in rugged, and sometime hostile industrial environments. Typical PLC environment specifications include: minimum operating temperature range 0° to 50°C (32° to 122°F); relative humidity 5% to 95% (non- condensing); high electrical noise, radio interference and vibration immunity. Some PLC models have explosion proof and water resistance ratings

PLCs are typically based on a modular design composed of three basic types of modules: input modules, output modules and a logic module or CPU for solving pre-programmed control logic. Input modules are wire connector to process sensing devices such as limit switches, proximity switches, thermocouples, flow meters etc that provide real-time data about the current state of a process e.g. the current level of a tank, or the desired state an operator wants the process to transition/change to, such as a pushing push button for a motor starter or to start a fan. Output modules are wired to actuation devices such as motors, fan, pilot lights, valves and other device that create changes in the controlled process. Input and output modules are either stacked next to the logic module or inserted into a rack containing a logic module

The exceptions to this modular PLC specification are nano and some micro PLCs. Nano/Micro PLCs are defined as a PLC with less than 32 inputs and outputs (I/O) points. These Nano/Micro PLCs usually feature an “all-in-one” design that integrates the input module, output module and logic module into a single unite that looks like a brick or block

With all PLCs the logic module contains preprogrammed control logic that provides the choreography between the status of inputs and the desire change in outputs. The logic module is composed of two elements: a processor (CPU) to solve the programmed control logic and memory to store the control program for the processor to use. The CPU processor systemically reads all the input statuses, solves all programmed logic conditions and activates outputs based on the logic program’s outcomes. This systematic routine is called a scan or sweep of the PLC. This scan or sweep is considered deterministic because the actual time it takes to read all inputs, solve logic and update outputs can be calculated in millisecond or microseconds and sometime in nanoseconds. The PLC’s fast processing routine or scan times are the reason a PLC is considered a real time sequential control device

The logic module is programmed with either a hand held programmer or with programming software that runs on a computer. For many years PLCs have been programmed using relay logic or ladder diagrams (LD) which are schematic diagrams for relay logic circuits or line diagrams made up of a combination of input and or output conditions that generation outcomes. These input and or output conditions often referred to as contacts, are connected in series, parallel, or series-parallel and sometimes combined with timers and counters to obtain the logic required to drive or active the output.

Over the years PLC programming languages have evolved and today five major PLC programming languages have been defined by IEC standard 61131-3 as the primary PLC programming languages. In addition to ladder diagram these programming languages are:
Sequential flow chart (SFC) – a graphical programming language similar to a flow chart that shows the main states of a control system, all the possible changes or steps and transition of state required to create outcomes. SFC is typically used for defining control sequences that are time and or event driven

Function Block diagram (FBD) –is also a graphic language based on block diagram in which input and output variables are connected to function block by connection lines and the output of a function block create a real outcome or may be connected to another function block command. Function block commands/logic statements are usually predefined and reusable however in some case function blocks may be user defined. Typical function block commands include: Boolean expressions (and, or, xor and not); mathematical (add, subtract, multiple, divide etc); comparative (equal, less than and greater than); bit operations (shift right and left etc); numeric (sine, cosine, tangent, etc) and other related PLC command functions

Structured Text (ST) – is a textual PLC programming language similar to Pascal. ST has four basic programming elements: Variables – are used to hold data and I/O states and are similar to data register in LD programming; Operators – define logic operations to be performed such as arithmetic, If, Then, Else etc; Expression – is a combination of operator and variables to form logic or control statement; and Control Flow Statements – are used to conditionally execute expressions or statements, to loop over statements, or to jump to another area in the program.

Instruction List (IL) – is also a textual PLC programming language with assembler language features. IL is typically used by hand-held programmers to represent a ladder diagram program. Instruction list uses mnemonic codes as a substitute for traditional ladder diagram

Like a computer, PLCs are offered with many communications and networking capabilities that include: Ethernet; serial RS232/RS422/RS485; and USB. However the protocols associated with these communication mediums are tend to be deterministic or predictable as to their communication transaction times. Additionally many PLCs offer field bus communications such a DeviceNet and Profibus. Field buses are defined as industrial networking protocols used for real-time distributed control of I/O and other control level smart device

An outgrowth of programmable logic controllers are programmable automation controllers (PAC). Like a PLC, a PAC is designed for real-time deterministic sequential control of I/O; however PACs have up to four specialized logic processor modules with different control functions that process their respective logic program in parallel with each other and simultaneously update I/O. The specialize processor modules offered for PACs typically include: a PLC logic processor; motion controller; process control module similar to a DCS (Distributed Control System); BASIC programming modules; and other dedicated functionality computing modules

There are four important considerations in selecting and designing a PLC system:

  • The type of I/O and the number of I/O points required by the application: Many different types of I/O modules are offered that include: a multitude of discrete input and output modules voltage signal and amperage; combination integrated input and output modules; analog, thermocouple and RTD modules; intelligent modules such as high speed counters for KHz and MHz signals, stepper control modules, servo control modules, bar code readers, PID controllers, and other dedicated I/O controllers.
  • Where are the I/O points to be controlled is located: Are the I/O points centralized in a single location or are the I/O points remotely located some distance for primary controller location? Remote I/O is a typically require for I/O points concentrated in blocks over 50 to 250 I/O points in a single remote location over more than 200 feet from the primary control location. Distributed I/O is used for smaller 4 to 20 I/O points blocks where a number of these small I/O blocks are remote located
  • What logic control functionality is required for the application: Is the control logic primarily sequential control or primary process control? It is important to select the logic processor with a programming language that will efficiently support the application. For sequential control usually ladder diagram or instruction list programming language is used. For process control applications a graphical programming language such as sequential flow chart or function block diagram may be suitable. It is important to remember that a PLC is a scan based processor thus the larger the program the longer the scan time.
  • Is the PLC required to communicate to other controller/computer devices and if so what is the networking architecture: Is the PLC is only communication to a HMI or is the PLC part of a factory wide information system? Is application a telemetry SCADA or supervisory control and data acquisition system? How much information must be communicated? The answer to these questions determines the selection logic processor module’s communication ports and if additional networking modules are required.

Gear Reducers

Gearboxes are also known as gearhead, gear reducers and or speed reducers. A gearbox is an enclosed system of assembled gears that is used to change the speed, torque and direction of mechanical energy transmits between two mechanical devices. In factory automation applications, gearboxes are typically used with electric motors. The motor’s shaft is attached to one end of the gearbox (i.e. the motor’s output is the input to the gearbox) and through the internal configuration of gears using the principle of mechanical advantage; the gearbox provides a torque-speed conversion transmission of the high speed motor shaft to a slower but more forceful torque output. This torque-speed conversion is determined by the gear ratio of the gearbox.

Gear ratio consists of two elements: the speed ratio and torque output. Speed ratio is the ratio of the rotational speeds of the first and final gears in a train of gears or of any two meshing gears. Torque output is the multiplication of torque or force achieved by the speed ratio. For example, if a gearbox has a 2:1 speed ratio and the motor shaft speed is 1,000 RPM output, the 2:1 gearbox will reduce the motor shaft’s speed to 500 RPM at the gearbox gearhead. Additionally if this same 1,000RPM motor shaft provided 20-inlbs of torque, the 2:1 gearbox’s torque output at the gearhead would be multiplied to 40 in-lbs times an efficiency rating. If the gearbox has an efficiency rating of 85% then the torque output is 40 in-lbs times .85 = 34 in-lbs. Obliviously the efficiency rating of the gear box in addition to speed ratio and torque output performance characteristics is important selection criteria.

Another important gearbox selection criterion is gear arrangement. The more efficient the gear arrangement, the more energy it will allow to be transmitted and converted into torque, rather than energy lost in heat.

There are five basic types gearing used in gearboxes: spur, helical, bevel, hypoid and worm gear types.

  • Spur gears or straight-cut gears are the simplest type of gear. Spur gears are cylinder disks with straight teeth projected radially on the disk. The edges of each gear tooth straight and are aligned in parallel to the axis of rotation. Spur gears mesh together only on parallel shafts. Spur gearboxes typically: are compact; cost effective; have high gear ratios and torque output. Major disadvantages are spur gearboxes tend to be noisy and prone to wear.
  • Helical gears are a type of spur gear however the leading edges of the teeth are not parallel to the axis of rotation, but are set at curved angle to form a helix segment. Helical gears can be meshed in a parallel orientation or a crossed / non-parallel orientation known as skewed gears. The helical gear’s angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly. Additionally helical gearboxes are very efficient, offer high horsepower and offer smooth quiet operation. Disadvantages of helical gearboxes are a greater degree of sliding friction between meshing teeth that requires special lubrications and a resultant thrust along the axis of the gears.
  • Beveled gears are shaped like a right circular cone with most of its tip cut off. The gear’s teeth are cut on this conical angular surface. Beveled gears are offered with either straight or spiral teeth. Straight bevel gears have straight and tapered teeth and are used in slow speeds applications. Spiral bevel gears have curved and oblique teeth and spiral bevel gearboxes are used in applications requiring high-performance, high speed applications. The shafts of meshed beveled gears can be oriented at different angles to other; thus the major advantages of beveled gearboxes are they feature right angel configurations and they are very durable. However poorly cut beveled gear cut teeth may result in excessive vibration and noisy gearbox operation.
  • A hypoid gear is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear. This gearing arrangement allows the translation of torque ninety degrees. Hypoid gears are similar to spiral bevel gears except that the shaft center lines do not intersect. Hypoid gears combine the rolling action and high tooth pressure of spiral bevels with the sliding action of worm gears. Hypoid gearing can be meshed in parallel or cross orientation. Additionally hypoid gearboxes feature high horsepower with smooth efficient operation. Disadvantages of hypoid gearboxes require special lubricates due to a greater degree of sliding friction between meshing teeth and a resultant thrust along the axis of the gears.
  • Worm gear consists of a threaded cylindrical gear (worm) similar to a screw, that mates with a gearwheel (worm wheel), that transforms the rotor motion between two shafts are right angles to each other. Worm gears have less friction and have longer wear than other types of gears due to the sliding action along the cylindrical gear. Additionally the worm gear cannot turn the worm thus worm gearing feature an inherent braking systems. Advantages to worm gearboxes are high precision, low noise, minimal maintenance and they offer right angle configurations. Disadvantages are worm gear boxes are usually non-reversible and offer low efficiency.

The term planetary gearbox refers to a gear train that consists of one or more outer gears rotating around a central set of gears. A planetary gear train (also called an epicyclic gear trains) consists of a ‘sun gear’ in the center of an outer ‘ring gear’ sometimes called an annulus ring. Between the sun gear and the ring gear are planetary gears. Planetary gears rotate around the sun gear and are meshed with the sun and ring gears. Planetary gearboxes are feature a highly efficient power transmission and are used in applications that require compact size with high power density. Planetary gearbox offer great load distribution among planetary gears. Disadvantages to planetary gearboxes are high bearing loads, and complex designs that are often inaccessible.

All gearboxes basically work the same; in that a gear’s rotational direction is dependent on the input gear’s rotation direction and the orientation of the gears. If the first gear in a gear train is rotating in a clockwise direction, the gear that engages the first gear rotates in a counter clockwise direction. This reciprocal gear rotation continues to the line of multiple gears in the gear train. Additionally the size of the gear and the number of gear teeth are very important in determining shaft speed and output torque – called gear ratio. Simply put gear ratio is the ratio of the speed of input gear or first gear in a gear train to the speed of the output gear or the last gear in a gear train. For example, if an input gear (A) has 20 teeth and the output gear (B) has 40 teeth, gear “A” will travel two complete turns for every complete turn of gear “B”. The gear ratio of these two gears is 1:2 because gear “A” has a rotational speed half of gear “B”. If the reverse condition existed (gear “A” has 40 teeth and gear “B” has 20 teeth) than one turn of gear “A” would create a half a turn or half the speed of gear “B”, then the gear ratio of 2:1. High gear ratios offer more output torque at lower speeds;   lower gear ratios mean higher output speed and less output torque.

The gearbox’s output shaft or gearhead orientation to the motor shaft (gearbox input) is another important consideration. While gearhead orientation is better than another each has different mounting benefits. There are three gearhead orientations offered:

  • In-line shaft gearhead: These gearboxes feature simple direct coupling to machinery. In-line shaft gear boxes are the most commonly used.
  • Parallel shaft gearhead: Parallel shaft gearbox means the motor shaft and gearhead output are in parallel but not in-line with each other resulting in an offset between the two shafts. Parallel gear boxes are usually very compact.
  • Right angle gearhead:  Right angle gearheads have an output shaft that is at a right angle to the motor shaft.

Input motor mounting to the gearhead is an important issue because there are a lot of motors that are not standard in addition to numerous NEMA and IEC frame sizes. While a single manufacturer offered off-the-shelve compatible motors and gearboxes, stock gearheads and stock motors from different manufacturers are not always compatible, even if they list the same mounting standard.

Gearheads do come in output mounting standards such as NEMA 17, 23, 34, 41 and 56 sizes and 56C mount. However, output shaft sizes and dimensions are not always consistent from manufacturer to manufacturer, so mounting dimensions should always be checked before purchasing a gearhead. In order to ensure mounting compatibility when the motor and gearhead manufacturers are different, it is best to purchase a gearhead after a motor has been selected.

In addition to AC and DC electric motors, gearboxes are often used with motion control servo motors. Servo motors inherently have low torque density or torque carrying capabilities and gearboxes are used to increase torque.

A gearmotor a consist of any electric motor, DC brushless, DC brush, AC, servo or stepper, coupled with a gear box also known as a gear reducer and integrated into a single package. An integral gearmotor is an electric motor and a gear reducer sharing the same shaft; however gearmotors can also be manufactured to be a combination of separate components. A gearmotor can significantly reduce complexity of application designs requiring high torque low speed output usually at a lower cost.

In closing, to select the correct gearbox for an application the following factors or selection criteria needs to be considered:

  • Gear ratio: Gear ratio is defined as the ratio of the speed of input gear or first gear in a gear train to the speed of the output gear or the last gear in a gear train.
  • Output torque: Output torque is dependent on gear ratio. The lower the speed the higher the torque.
  • Speed or RPM: Speed is proportional to the gear ratio of the system. The output speed can be determined by dividing the input speed by the gear ratio. T he higher the ratio the lower the output speed will be and vice versa.
  • Gear arrangement:  The more efficient the gear arrangement, i.e. spur, helical, bevel, hypoid, worm or planetary) the more energy that is transmitted and converted into torque, rather than energy lost in heat.
  • Backlash: Backlash is important in applications that involve shaft rotation reversals or load reversals for accurate positioning eg. CNC machines, robotics etc. By definition backlash is a measurement in arc-minutes of the space between the meshing teeth on the gears. Simply put it is how much movement is in the gearhead shaft if the motor shaft is held rigid. It is a worst case measurement of how much input rotation is required to initiate a rotational output. It’s a measurement of how (in the worst case scenario) much input rotation is required to initiate a rotational output.

Gearheads

Gearboxes are also known as gearhead, gear reducers and or speed reducers. A gearbox is an enclosed system of assembled gears that is used to change the speed, torque and direction of mechanical energy transmits between two mechanical devices. In factory automation applications, gearboxes are typically used with electric motors. The motor’s shaft is attached to one end of the gearbox (i.e. the motor’s output is the input to the gearbox) and through the internal configuration of gears using the principle of mechanical advantage; the gearbox provides a torque-speed conversion transmission of the high speed motor shaft to a slower but more forceful torque output. This torque-speed conversion is determined by the gear ratio of the gearbox.

Gear ratio consists of two elements: the speed ratio and torque output. Speed ratio is the ratio of the rotational speeds of the first and final gears in a train of gears or of any two meshing gears. Torque output is the multiplication of torque or force achieved by the speed ratio. For example, if a gearbox has a 2:1 speed ratio and the motor shaft speed is 1,000 RPM output, the 2:1 gearbox will reduce the motor shaft’s speed to 500 RPM at the gearbox gearhead. Additionally if this same 1,000RPM motor shaft provided 20-inlbs of torque, the 2:1 gearbox’s torque output at the gearhead would be multiplied to 40 in-lbs times an efficiency rating. If the gearbox has an efficiency rating of 85% then the torque output is 40 in-lbs times .85 = 34 in-lbs. Obliviously the efficiency rating of the gear box in addition to speed ratio and torque output performance characteristics is important selection criteria.

Another important gearbox selection criterion is gear arrangement. The more efficient the gear arrangement, the more energy it will allow to be transmitted and converted into torque, rather than energy lost in heat.

There are five basic types gearing used in gearboxes: spur, helical, bevel, hypoid and worm gear types.

  • Spur gears or straight-cut gears are the simplest type of gear. Spur gears are cylinder disks with straight teeth projected radially on the disk. The edges of each gear tooth straight and are aligned in parallel to the axis of rotation. Spur gears mesh together only on parallel shafts. Spur gearboxes typically: are compact; cost effective; have high gear ratios and torque output. Major disadvantages are spur gearboxes tend to be noisy and prone to wear.
  • Helical gears are a type of spur gear however the leading edges of the teeth are not parallel to the axis of rotation, but are set at curved angle to form a helix segment. Helical gears can be meshed in a parallel orientation or a crossed / non-parallel orientation known as skewed gears. The helical gear’s angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly. Additionally helical gearboxes are very efficient, offer high horsepower and offer smooth quiet operation. Disadvantages of helical gearboxes are a greater degree of sliding friction between meshing teeth that requires special lubrications and a resultant thrust along the axis of the gears.
  • Beveled gears are shaped like a right circular cone with most of its tip cut off. The gear’s teeth are cut on this conical angular surface. Beveled gears are offered with either straight or spiral teeth. Straight bevel gears have straight and tapered teeth and are used in slow speeds applications. Spiral bevel gears have curved and oblique teeth and spiral bevel gearboxes are used in applications requiring high-performance, high speed applications. The shafts of meshed beveled gears can be oriented at different angles to other; thus the major advantages of beveled gearboxes are they feature right angel configurations and they are very durable. However poorly cut beveled gear cut teeth may result in excessive vibration and noisy gearbox operation.
  • A hypoid gear is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear. This gearing arrangement allows the translation of torque ninety degrees. Hypoid gears are similar to spiral bevel gears except that the shaft center lines do not intersect. Hypoid gears combine the rolling action and high tooth pressure of spiral bevels with the sliding action of worm gears. Hypoid gearing can be meshed in parallel or cross orientation. Additionally hypoid gearboxes feature high horsepower with smooth efficient operation. Disadvantages of hypoid gearboxes require special lubricates due to a greater degree of sliding friction between meshing teeth and a resultant thrust along the axis of the gears.
  • Worm gear consists of a threaded cylindrical gear (worm) similar to a screw, that mates with a gearwheel (worm wheel), that transforms the rotor motion between two shafts are right angles to each other. Worm gears have less friction and have longer wear than other types of gears due to the sliding action along the cylindrical gear. Additionally the worm gear cannot turn the worm thus worm gearing feature an inherent braking systems. Advantages to worm gearboxes are high precision, low noise, minimal maintenance and they offer right angle configurations. Disadvantages are worm gear boxes are usually non-reversible and offer low efficiency.

The term planetary gearbox refers to a gear train that consists of one or more outer gears rotating around a central set of gears. A planetary gear train (also called an epicyclic gear trains) consists of a ‘sun gear’ in the center of an outer ‘ring gear’ sometimes called an annulus ring. Between the sun gear and the ring gear are planetary gears. Planetary gears rotate around the sun gear and are meshed with the sun and ring gears. Planetary gearboxes are feature a highly efficient power transmission and are used in applications that require compact size with high power density. Planetary gearbox offer great load distribution among planetary gears. Disadvantages to planetary gearboxes are high bearing loads, and complex designs that are often inaccessible.

All gearboxes basically work the same; in that a gear’s rotational direction is dependent on the input gear’s rotation direction and the orientation of the gears. If the first gear in a gear train is rotating in a clockwise direction, the gear that engages the first gear rotates in a counter clockwise direction. This reciprocal gear rotation continues to the line of multiple gears in the gear train. Additionally the size of the gear and the number of gear teeth are very important in determining shaft speed and output torque – called gear ratio. Simply put gear ratio is the ratio of the speed of input gear or first gear in a gear train to the speed of the output gear or the last gear in a gear train. For example, if an input gear (A) has 20 teeth and the output gear (B) has 40 teeth, gear “A” will travel two complete turns for every complete turn of gear “B”. The gear ratio of these two gears is 1:2 because gear “A” has a rotational speed half of gear “B”. If the reverse condition existed (gear “A” has 40 teeth and gear “B” has 20 teeth) than one turn of gear “A” would create a half a turn or half the speed of gear “B”, then the gear ratio of 2:1. High gear ratios offer more output torque at lower speeds;   lower gear ratios mean higher output speed and less output torque.

The gearbox’s output shaft or gearhead orientation to the motor shaft (gearbox input) is another important consideration. While gearhead orientation is better than another each has different mounting benefits. There are three gearhead orientations offered:

  • In-line shaft gearhead: These gearboxes feature simple direct coupling to machinery. In-line shaft gear boxes are the most commonly used.
  • Parallel shaft gearhead: Parallel shaft gearbox means the motor shaft and gearhead output are in parallel but not in-line with each other resulting in an offset between the two shafts. Parallel gear boxes are usually very compact.
  • Right angle gearhead:  Right angle gearheads have an output shaft that is at a right angle to the motor shaft.

Input motor mounting to the gearhead is an important issue because there are a lot of motors that are not standard in addition to numerous NEMA and IEC frame sizes. While a single manufacturer offered off-the-shelve compatible motors and gearboxes, stock gearheads and stock motors from different manufacturers are not always compatible, even if they list the same mounting standard.

Gearheads do come in output mounting standards such as NEMA 17, 23, 34, 41 and 56 sizes and 56C mount. However, output shaft sizes and dimensions are not always consistent from manufacturer to manufacturer, so mounting dimensions should always be checked before purchasing a gearhead. In order to ensure mounting compatibility when the motor and gearhead manufacturers are different, it is best to purchase a gearhead after a motor has been selected.

In addition to AC and DC electric motors, gearboxes are often used with motion control servo motors. Servo motors inherently have low torque density or torque carrying capabilities and gearboxes are used to increase torque.

A gearmotor a consist of any electric motor, DC brushless, DC brush, AC, servo or stepper, coupled with a gear box also known as a gear reducer and integrated into a single package. An integral gearmotor is an electric motor and a gear reducer sharing the same shaft; however gearmotors can also be manufactured to be a combination of separate components. A gearmotor can significantly reduce complexity of application designs requiring high torque low speed output usually at a lower cost.

In closing, to select the correct gearbox for an application the following factors or selection criteria needs to be considered:

  • Gear ratio: Gear ratio is defined as the ratio of the speed of input gear or first gear in a gear train to the speed of the output gear or the last gear in a gear train.
  • Output torque: Output torque is dependent on gear ratio. The lower the speed the higher the torque.
  • Speed or RPM: Speed is proportional to the gear ratio of the system. The output speed can be determined by dividing the input speed by the gear ratio. T he higher the ratio the lower the output speed will be and vice versa.
  • Gear arrangement:  The more efficient the gear arrangement, i.e. spur, helical, bevel, hypoid, worm or planetary) the more energy that is transmitted and converted into torque, rather than energy lost in heat.
  • Backlash: Backlash is important in applications that involve shaft rotation reversals or load reversals for accurate positioning eg. CNC machines, robotics etc. By definition backlash is a measurement in arc-minutes of the space between the meshing teeth on the gears. Simply put it is how much movement is in the gearhead shaft if the motor shaft is held rigid. It is a worst case measurement of how much input rotation is required to initiate a rotational output. It’s a measurement of how (in the worst case scenario) much input rotation is required to initiate a rotational output.

Actuators

A linear actuator is a device the converts some type force, usually rotary force e.g. electric and hydraulic motors, into linear movement. Most industrial automation applications use some kind of electric motor, usually servo or stepper motors. There are a myriad of linear actuator choices and getting the most performance for your application means choosing the right actuator. Some of the key questions to evaluate your application requirements are: What is the load (types, size and weight)? What speed and accuracy is required? What duty cycle and equipment life-time? Will the actuator operate in a wash-down, vacuum or other harsh environment? How much space is available? What is the project budget?

There four basic linear actuator technologies: screw technology; belt drive; and rod/rodless style actuators.

Ball & Lead Screws

There four basic linear actuator technologies: screw technology; belt drive; and rod/rodless style actuators.

  • Screw linear actuators are based in a turning screw to generate motion. There are three types of screw actuators: lead screw, ball screw and planetary roller screw. A lead screw actuator uses a threaded nut that moves with respect to a long or lead screw generating linear motion on whatever load on the nut. Lead screw technology is simple, economical and widely used. The actuators tend to be quiet and suffer less from back driving than other alternatives. Because the surfaces of the nut are sliding against the threads of the screw, however, friction, and thus wear, become issues. This reduces lifetime, efficiency, speed, and also performance, because wear on the solid nut may affect accuracy/repeatability. Frictional resistance may even necessitate higher input torques. In general, lead screws are good solutions for cost-driven applications that can tolerate performance trade-offs.

In a ball screw actuator, the motion of the nut around the screw gets an assist from ball bearings, reducing friction, distributing load and increasing lifetime predictability over a lead-screw-type design. Ball screws can take heavy loads and deliver precision positioning. They offer efficiency and thrust capabilities higher than those of a lead screw. Ball screw actuators tend to be more expensive than lead screws. Another other ball screw actuator disadvantage is the bearings can become contaminated, brindled, or even fractured, reducing performance or triggering failure. Additionally the reduced friction of a ball screw makes it easier to drive, but that also makes it easier to back drive, for example in a vertical application. The ball bearings also generate more noise.

A planetary roller screw actuator has a planetary arrangement of threaded rollers surrounding the main threaded shaft. This design increases the surface area that takes the load and giving this screw actuator its name. Planetary roller screws offer the highest possible thrust and lifetime of any screw-type actuator, operating with minimal maintenance and at high efficiency. A trade-off to planetary roller screw actuator is they are expensive and typically are the highest cost of the three lead screw technologies. Additionally, because of the number of screw rollers around the central lead screw in a planetary design, the nut tends to be larger. As with the ball screw, reduced resistance to motion means increased vulnerability to back drive. With some application loss of motor torque can put planetary screw into free fall. In general, the planetary screw is the best solution for applications requiring high thrust force and a long lifetime (in some cases up to 20 years).

  • Belt drive actuators use a belt grooved belt instead of a lead screw. A carriage with the load rides on the linear belt. Belt drive actuator is an excellent choice for application that requires high speed. According to some linear motion experts, with the right combination of motor and gear reduction, a belt drive actuators achieve speed as fast as 200 inches per second (in/s). In comparison, these same experts claim that screw actuator speed range is 10 in/s to 50 in/s. All screw actuator have specification called critical speed. All screw devices have what is called a critical speed. As the screw spins more rapidly, it begins to sag outward in what is called screw whip, causing vibration, premature wear, and, ultimately, failure. This phenomenon depends on screw diameter, lead, and rotational speed. In general the greater the lead screw diameter, the greater the critical speed.

Belt drives are ideal for horizontal applications requiring speed and force. However they are generally not used in vertical applications, as breakage of the belt could put the load into freefall. Additionally belt drive actuators with steel belt offer reasonable lifetimes. Noise or complexity does not tend to be issues for belt drives. However like any belt though, they stretch and require regular tensioning. For stroke lengths, belt sag may also be a concern.

  • Rod and rodless linear actuators have two different designs. In a rod style actuator, the thrust element or rod moves out of the end of the actuator creating motion that pushes the load. In a rodless style, the actuator housing completely surrounds the screw, which moves the load on a platform that rides along the top of the actuator.

The rod style actuator produces more force than rodless actuator, and rod actuators excel at thrust type applications. For example, a rod-style actuator does a fine job of pushing slabs of wood into a carton, but might not necessarily be the best solution for positioning those slabs of wood for a hole-drilling sequence in a drill press. Downsides to the rod style actuator are: most applications require a structure to carry the load; and in horizontal application the projecting rod can sag with a load, compromising rigidity and increasing wear on the bearing elements.

Rodless linear actuators provide excellent load bearing and position guidance characteristics and rodless actuators tend to be less subject to screw whip (critical speed). With rodless actuators, the linear screw is enclosed and can be supported at each end for stability. Rodless actuators also offer smaller footprints because the screws are contained. Rod actuators typically require more real estate.

Rod-style actuators really excel is in dirty environments, such as dusty factory floors, washdown environments, or environments with corrosive materials. The rod style actuator is sealed except for the mounting point of the motor and the exit point of the rod. Rodless actuators require narrow slits the length of the actuator to permit the load-bearing platform to couple to and move with the nut, making them difficult, if not impossible, to seal for wet environments.

A linear motor also called and integrated actuator is a linear actuator integrated with a motor. Integrating the actuator into the motor eliminates the need for a coupling, which eliminates weight, parts, and can become another point of failure. Weight, in particular, becomes an issue for applications in which inertia must be minimized, such as at the end of robot arms. Integrated actuators range from designs in which the screw is positioned within the motor so that as the windings are energized, the lead screw spins, to versions that incorporate drives and controls.

Linear Guides

Linear guides are designed to guide linear motion with minimal friction of an object. Types of linear guides include linear bearings, guide rails and electric cylinders.

Linear bearings are used in applications where a component needs to be moved along a straight line with high accuracy and usually return to its origin with high repeatability. Linear bearings come in a variety of styles and have wide load capacities. Choices for linear bearing styles include air, flat ball cage, ball spline, ball bushing, needle roller cage, cam follower, crossed roller, guide wheel, hydrostatic, plain or journal, linear motion guide, flat roller cage, and linear roller.

  • Air bearings are a very low friction bearing that rides on a cushion of air.
  • A flat ball cage is a non-recirculating or recirculating bearing design where the balls are trapped in a frame that spaces them at equal distance.
  • A ball spline bearing has balls that contact grooves in a matching spline shaft on at least four sides, depending on the number of splines. Ball spline bearings provide nearly friction-free linear motion while simultaneously transmitting torsional loads.
  • Ball bushings are similar to a bushing in construction but have rows of balls to reduce friction and increase stiffness, accuracy, and smoothness.
  • Caged needle bearings are low friction, high load bearings in a composite cage.
  • A cam follower is a wheel on an axle and that wheel runs on bearings.
  • Roller and cross roller bearings are used on linear ways where straightness and accuracy are critical.
  • A guide wheel is a low cost, versatile and rugged alternative to traditional linear bearings.
  • Linear hydrostatic bearings work much like an air bearing. The bearing body rides on a thin film of oil or water that provides for near frictionless movement.
  • Journal bearings consist of a split cylindrical shell of hard, strong metal held in a rigid support and an inner cylindrical part of soft metal, which holds a rotating shaft.
  • A linear motion guide is a linear profiled rail that has a set of mating bearings that travel along the rail.
  • A flat roller cage is similar to crossed roller bearings except that the axes to all of the rollers are parallel instead of crossed.
  • A linear roller is a set of circulating roller bearings that ride on a round or flat guide. A bearing or rail assembly is a complete assembly comprised of the bearing and guide or rail.

Guide rails are a track or rail that is designed to control the movement of an object. Guide rails offer a smooth surface that supports and guides the rolling element of a linear bearing.

Electric cylinders were designed to apply force through an extendable rod. They are essentially thrust producing devices for applications requiring high axial force with the moment and side loads already properly supported. The chief advantage of electric cylinders is the ability to extend into a work area during an operation, and then retract to clear the area for subsequent operations. Electric cylinder technology also provides a simple and unique approach to solving rigid or pivoting linear motion applications.

Tables & Slides

Linear slides are simple linear motion devices composed of a stationary base and a moving carriage. Linear stages are slides with an actual drive mechanism to provide controlled, precise positioning along a linear axis. There are many types of linear slides, such as ball slides, crossed roller slides and dovetail slides.

  • Ball slides are simple linear motion devices comprised of a stationary base with a carriage riding on top. They are sometimes referred to as ball and rod bearing assemblies, or ball bearing slides (ball slide stages are simply ball slides coupled with a drive mechanism).

Ball slides use a bearing system composed of two rows of balls on both sides of the base. Each row is contained between four rods. These two rows of balls are preloaded to eliminate wobble and unwanted play in the system. As the carriage moves, the balls roll along the rods. Ball slides provide smooth, accurate, low friction motion. They are typically very reliable, and because there is no direct sliding contact between the base and carriage, they do not require regular lubrication. The result is a very smooth linear motion, accurately controlled by rotation of the drive mechanism.

  • Crossed roller slides are also simple linear devices that are composed of two rows of rollers with a stationary base and a moving carriage. The rollers are ‘crossed’ 90° in an alternating fashion and are enclosed in the rails, which have a machined ‘V’ groove to support the rollers. They offer a higher load carrying capacity and higher support stiffness for the load than ball slides.
  • Dovetail slides are a little more complex composed of a flaring tenon or saddle (the moving member) and a fixed base into which the saddle interlocks, yet can slide. Dovetail slides are direct contact systems where the sliding area is considered the bearing system of the dovetail slide. Since the surface contact is so large, dovetail slides are ideal for heavy load applications and industrial uses. However, since dovetail slides have direct contact between the base and saddle rather than indirect contact (as with ball bearing slides and roller bearing slides), the force required to move the saddle is greater than other systems resulting in slower acceleration rates.

Important specifications for linear slides include the base length, base height, carriage length, carriage width, and top height. Larger dimension devices can handle greater loads, although size is often a limiting factor since the slide has to fit into the housing or drive system. Other important specifications include rate and distance of linear movement, dynamic load carrying capacity and maximum linear velocity or speed. Lastly for slides with ball, lead of Acme screw another important consideration is the number of leads (inches) per revolution of the screw).

Linear slides are also offered with a number of additional features the increases their functionality. Some of these features include way covers and bellows, wipers and scrapers, stackability, double carriages, and a locking mechanism. Way covers are sometimes referred to simply as bellows, cover the rails/guides for protection from dust and dirt. Wipers or scrapers are for cleaning the surface of the rails/guides while in service. This is a very valuable feature if the slides are to be used in dusty or otherwise compromised environments. Stackability means that the slides may be stacked (one on top of another) to provide linear motion in more than one axis. Double carriages have two sliding platforms for additional work or carrying capacity. Lockable slides have a brake assembly for slowing or stopping the linear motion, or a lock for maintaining the carriage in a specific position. In some cases they may have a clutch for seamless changes in speed.

Linear tables are also called positioning tables and are best suited for applications where the accuracy and repeatability requirements are more important than the axial thrust of the drive train. Linear tables are used for X-Y positioning, and in some models, Z positioning as well. They are well suited for physically larger loads in less precise applications or where moment load support is necessary. Positioning tables can handle heavier, larger loads and have excellent repeatability and position accuracy. Overall, positioning tables are ideal building blocks for complete multi-axis positioning systems.

Servo Drives

To understand difference between servo based motion control versus stepper motor motion control one need to understand open loop control versus close loop feedback control. Open loop control means that there is no performance feedback once an actuator is told (by a control system) to start and complete a task. An automobile’s automatic window button is an example of an open loop control systems. Once the window open button is pressed the window opens all the way unless you hit the button again to stop the window.

A closed loop system means that a performance feedback sensor continually sends performance data back or error correction data to the controller as the actuator is performing its commanded task. The controller compares the feedback with a set point and determines if a correction command need to be sent to the actuator. An automobile cruise control system is an example of close loop control. Once the cruise control is set the car monitor its actual speed against a set point and the car’s speed is either accelerated or decelerated to achieve the desired set point.

A servomechanism or servo system is an automatic device (usually consisting of a group of components) which uses error-sensing feedback to correct the performance of a mechanism. For motion control applications, a servomechanism almost always refers to closed loop control systems for position, velocity and or torque closed loop control. A typical industrial automation motion control servomechanism consists of at least four and possibly five components: a control system; a servo drive / amplifier; servo motor; a feedback device typically an encoder or resolver; and many times a gear train or some positioning device.

A servo drive is a special electronic amplifier used to control power to a servomotor and monitor the servomotor’s feedback to continually adjust for deviation from expected behavior. In other words, the servo drive is a motor control device that outputs electrical signals to a servomotor to induce motion. The servo drive can be built in as part of the servomotor or it can be a separate device.

A servo drive, as known as a servo amplifier, receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically the command signal represents a desired velocity, but can also represent a desired torque or position. A sensor attached to the servo motor reports the motor’s actual status back to the servo drive. The servo drive then compares the actual motor status with the commanded motor status. It then alters the voltage frequency or pulse width to the motor so as to correct for any deviation from the commanded status.

There are many different types of closed loop algorithms for servomechanisms. Generally the PID (Proportional Integral Derivative) control loop algorithm is used for servo motors. The control loop is constantly checking to see if the motor is on the right path and if not, it makes the necessary adjustments.

In a properly configured servo control system, the servo motor rotates at a velocity that very closely approximates the velocity signal being received by the servo drive from the control system. Several parameters, such as stiffness (also known as proportional gain), damping (also known as derivative gain), and feedback gain, can be adjusted to achieve this desired performance. The process of adjusting these parameters is called performance tuning.

Based on the electrical signal from the servo drive or servo amplifier, the servomotor creates motion and determines the precise position of the motors armature and motor shaft. A servo control system has many advantages over and open loop stepper control system. Servo control systems offer: higher intermittent torque; have a higher to inertia ratio; are excellent for high speed position control and velocity control. Disadvantages of servo control system compared with stepper control are: servo control systems are generally more expensive; servo systems cannot be operated as open loop systems; servo systems require tuning of the control loop parameter (can be time consuming); and servo systems with bushed DC motors require more maintenance than stepper motors.

Typical applications for servo control include industrial automation, machine tools, EDM machining, coil winding equipment, medical equipment, press feeders, robotics, assembly and machining equipment, postal sorting machines, material handling equipment, packaging equipment and other types of specialty machines requiring precise control of torque, velocity and position. Their main advantage over traditional DC or AC motors is the addition of motor feedback. This feedback can be used to detect unwanted motion, or to ensure the accuracy of the commanded motion. The feedback is generally provided by an encoder of some sort.

Servo Motors

As mentioned in the servo control discussion, a servomechanism or servo system is an automatic device (usually consisting of a group of components) which uses error-sensing feedback to correct the performance of a mechanism. For motion control applications, a servomechanism almost always refers to closed loop control systems for position, velocity and or torque closed loop control. A typical industrial automation motion control servomechanism consists of at least four and possibly five components: a control system; a servo drive / amplifier; servo motor; a feedback device typically an encoder or resolver; and many times a gear train or some positioning device.

After receiving the control command from the servo drive, as known as a servo amplifier, the servomotor (also called a servo) creates motion and determines the precise position of the motors armature and motor shaft. Servo motors have three wires, two which provide power and one which provides a control signal. The control signal to the motor is based on electrical pulse of variable width called pulse with modulation or PWM. The angle of the motor shaft is determined by the duration of a pulse that is applied to the control wire. The servo motor expects to see a pulse every 20 milliseconds (ms). The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position. Different motion control parameters are established by different types of PWM pulse train such as: minimum pulses; maximum pulses and retentive pulses.

When the servo motor is commanded to move it will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is the torque rating of the servo. The servo will not hold their position forever; the position pulse must be repeated to instruct the servo to stay in position.

When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo motor. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.

Another parameter that varies from servo to servo is the turn rate. This is the time it takes from the servo to change from one position to another. The worst case turning time is when the servo is holding at the minimum rotation and it is commanded to go to maximum rotation. This can take several seconds on very high torque servos.

When compared with an open loop stepper system, a servo motor paired with its position feedback device and servo amplifier offer: better transient response times; reduced steady state errors; reduced sensitivity to load parameters; and better performance and accuracy. Additionally servo systems have inherently better characteristic to handle system disturbances such as a torque disturbance.

When selecting a servomotor, it is important to consider the following:

  • What shaft speed does the application required?  Manufacturers usually identify their shaft speeds as the no-load speed at the rated terminal voltage.
  • What terminal voltage is required?
  • What continuous current does the application require?  Continuous current is the maximum rated current that can be supplied to the motor without it overheating.
  • What continuous torques is required during constant running conditions?
  • The continuous output power or the mechanical power your application requires.
  •  What are the physical space limitations in terms of shape, diameter and housing length?

There are many types of servomotors. Some of the most common types and their applications include:

  • DC permanent magnet and shunt wound motors which provide constant speed with varying load, so they are a good fit for machine tools, fans and blowers.
  • DC series wound motors provide high starting torque, so they are good for constant loads such as in heavy industrial applications.
  • Compound wound motors provide a heavy starting torque and are typically used where adjustable speed is not required, such as in elevators and hoists.
  • AC brushless servo motors are usually considered the highest performance servo systems because of their response and high acceleration. AC servo amplifiers typically provide a sinusoidal output to the AC brushless motor.

Stepper Drives

To understand difference between stepper motor motion control versus servo based motion control one need to understand open loop control versus close loop feedback control. Open loop control means that there is no performance feedback once an actuator is told (by a control system) to start and complete a task. An automobile’s automatic window button is an example of an open loop control systems. Once the window open button is pressed the window opens all the way unless you hit the button again to stop the window.

A stepper motor motion control system is based on open loop control versus the servo’s closed loop control. A stepper motor gets its name because the motor divides a single shaft rotation into a number of steps or small fixed increments. The size of the increment is measured in degrees. Typical increments are 0.9 or 1.8 degrees, with 400 or 200 increments representing one full motor shaft rotation. The stepper motor’s position is then commanded to move and hold at one of these steps without any feedback sensor. Thus a stepper system is considered an open loop motion control.

A stepper motor system consists of three basic elements and is sometimes combined with some type of user interface such as an operator terminal:

  • An indexer or controller which is a microprocessor capable of generating step pulses and direction signals for the driver.
  • A driver or amplifier that converts the indexer command signals into the power necessary to energize the motor windings. There are numerous types of drivers, with different voltage and current ratings and construction technology. Not all drivers are suitable to run all motors, so when designing a stepper motion control system the driver selection process is critical.
  • A stepper motor which is an electromagnetic device that converts digital pulses into mechanical shaft rotation creating the motion.

The stepper motor drive provides the motor with pulses that determine the number degrees or steps the motor is to increment and the direction of rotation. The speed of the motor is determined by the time delay between each incremental movement. When the desired position is reached and command pulses cease, the stepper motor shaft stops and there is no need for clutches or brakes. The stepper motor is generally left energized at a stop position. Once stopped, the stepper motor resists dynamic movement up to the value of the holding torque.

In general, a stepper motor has high torque at low speeds or a low pulse rate to easily accelerate a load and low torque at high speeds. The stepper motors movement at low speeds tends to be choppy (unless the drive has microstepping capability which means that each motor’s incremental steps are divided into fractional step increments).    At higher speeds, the stepper motor is not as choppy, but the motor does not have as much torque.

Advantage of stepper motors are: low cost; no motion feedback is required for operation (open loop) thus not loop tuning is required; excellent holding torque – eliminating the need to brakes and clutches; excellent torque at low speeds; rugged construction; typically low maintenance; and excellent for precise position control applications. Disadvantages are: rough performance at low speed (unless the motor has micro positioning capabilities); consumes current regardless of load condition (correct motor sizing is important to avoid overheating); stepper motor are relatively noisy in operation; torques decrease as speed increase (applications requiring higher torques at high speed will require an oversized motor); and stepper motors and stall or lose position without a feedback control loop.

Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment.

Stepper Motors

There are three basic types of stepper motors: the permanent magnet motor, the variable-reluctance motor, and the hybrid motor, which is a combination of the previous two.

The permanent-magnet (PM) stepper motor operates on the reaction between a permanent-magnet rotor and an electromagnetic field. The stepper motor’s rotor, called a canstack rotor consists of multiple magnets each with north and south poles called teeth. The stator is also consist or multiple magnetic windings poles. The teeth on the rotor surface and the stator pole faces are offset so that there will be only a limited number of rotor teeth aligning themselves as stator poles are sequential independently energized. The rotor aligns itself with the stator coil that is energized. By pulsing the stator coils in a desired sequence, it is possible to control the speed and direction of the motor.

The number of teeth on the rotor and stator determine the step angle or degree of rotation that will occur each time the polarity of the winding is reversed; the greater the number of teeth, the smaller the step angle. For example, if the rotor has 50 teeth and the stator has 8 poles with 5 teeth each (total of 40 teeth), the stepper motor is able to move 200 distinct steps to make one complete revolution. This means that shaft of the motor will turn 1.8° per step.

When a PM stepper motor has a steady DC signal applied to one stator winding, the rotor will overcome the residual torque and line up with that stator field. The holding torque is defined as the amount of torque required to move the rotor one full step with the stator coil energized. An important characteristic of the PM stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. When no power is applied to the windings, a small magnetic force is developed between the permanent magnet and the stator. This magnetic force is called a residual, or detent torque. The detent torque can be noticed by turning a stepper motor by hand and is generally about one-tenth of the holding torque.

The main feature of the permanent magnet motor is that a permanent magnet is used for the rotor, which means that no brushes are required. The drawback of this type of motor is that it has relatively low torque and is usually used for low-speed applications.

The variable-reluctance (VR) stepper motor differs from the PM stepper in that it has no permanent-magnet rotor and no residual torque to hold the rotor at one position when turned off. The stator of a VR stepper motor has a magnetic core constructed with a stack of steel laminations each with coil winding that look like magnetized teeth. The rotor is made of unmagnetized soft steel with teeth and slots.

When the stator coils are sequential energized the rotor teeth align with the energized stator poles (teeth) creating an increment of rotation. This type of motor operates on the principle of minimizing the reluctance along the path of the applied magnetic field. By alternating the teeth windings that are energized in the stator, the stator field changes, and the rotor is moved to a new position.

As a simple example of how a VR stepper motor works suppose the motor has six stator teeth and the rotor has four teeth. The stator coils are energized in groups referred to as phases or two stator teeth. Each phase has a north and south pole (teeth) opposite of each other (total three phases 30° apart). If the first phase is energized the closest rotor tooth will rotate to the first phase pole. As long as this phase is energized, the rotor will be held stationary. When the first phase is switched off and the second phase energized, the rotor will turn 30° until two poles of the rotor are aligned under the north and south poles established by second phase. Turning off the second phase and energizing the third phase will again cause the rotor to move another 30°. After the third phase is switch off and the first phase again energized the rotor has moved one complete rotation or one cycle. By repeating this pattern, the motor will rotate in a clockwise direction. The direction of the motor is changed by reversing the pattern of turning ON and OFF each phase.

The amount of torque produced by a VR stepper motor is still small, so it is generally used for small positioning tables and other small positioning loads. Since this type of motor does not have permanent magnets, it cannot use the same type of stepper controller as other types of stepper motors.

The disadvantage of this design for a stepper motor is that the steps are generally quite large usually above 15°. Multistack stepper motors are offered that can produce smaller step sizes because the motor is divided along its axial length into magnetically isolated sections, or stacks.

A hybrid stepper motor is the most widely used stepper motor. As its name implies the hybrid stepper motor combines the principles of, and has of the advantages of both permanent magnet (PM)and variable-reluctance (VR) stepper motors. The hybrid stepper has a permanent magnet rotor with teeth just like a PM stepper motor. The stator is just like the VR stepper motor (it has small magnetized teeth and poles) and are the stator winding poles (teeth) and activated in phases just like the VR stepper.

Combining the qualities of the VR and the PM, the hybrid motor has some of the desirable features of each. Hybrid steppers have high detent torque and excellent holding and dynamic torque, and they can operate at high stepping speeds. When phases are energized sequential, the rotor typically rotates in increments of 1.8 degrees. This motor can also be driven two phases at a time to yield more torque, or alternately one then two then one phase, to produce half steps or 0.9 degree increments.

The rotation angle or degree of increment of a stepper can be increased using a technique called microstepping. Microstepping increases the position resolution and smoothness of conventional hybrid step motors. This is done with electronic control in the drive circuits. The drive subdivides each full step electronically into a large number of smaller steps. For example, a microstepping drive that subdivides each full step of a 200-step/rev motor into 125 microsteps produces 25,000 steps/rev (200 X 125 = 25,000).

Microstepping offers three primary benefits: (1) It allows a stepping motor to stop and hold its position between the full or half-step positions; (2) It largely eliminates the noise at intermediate speeds and also the jerky character of low speed operation; (3) it offers a reduction of resonance problems.