Absolute Position Feedback

With the shaft in any position at motor start, the data from an absolute position feedback mechanism always relays the position of the shaft. A non-absolute feedback mechanism must start from a known rotational position, such as the index pulse.

Absolute Programming

A positioning coordinate reference where all positions are specified relative to some reference, or “home” position. This is different from incremental programming, where distances are specified relative to the current position.


A measure of the difference between expected position and actual position. Position sensor accuracy is usually specified as an angle representing the maximum deviation from expected position.


Electronics which convert low-level command signals to high power voltages and currents to operate a servo motor. Electronics that convert low level command signals into high power voltages and currents to operate an electrical device.

Amplifier, PWM

A device which draws power from an independent source to produce a magnified pulse width modulated power signal form of a controlling input signal.


A device or circuit in which the output varies as a continuous function of the input.

Asynchronous Machine

Any machine in which the output speed is not dependent of the excitation frequency.

B. EMF Harmonic Distortion

Root sum squares of the rms values of non-fundamentals as a percentage of fundamental rms values.


The frequency ranges in which the magnitude of the system gain expressed in dB is greater than -3 dB.

BCD (Binary Coded Decimal)

A binary numbering system in which the decimal digits 0 to 9 are represented by a 4-bit binary number. Each digit in a decimal number assigned a four bit binary code.
Example: 156 = 0001 0101 0110.

Base two numbering system. Each number being expressed in the powers of two, by 0 or 1.

Bit (Binary Digit)

A unit of information equal to 1 binary decision or having only a value 0 or 1.

Bode Plot

A plot of the magnitude of system gain in dB and the phase of system gain in degrees versus the sinusoidal input signal frequency in logarithmic scale.


Bits per second

Biss Interface

(bidirectional/serial/synchronous) real time protocol interface .It enables a digital, serial and secure communication between the position sensor and the controller. The BiSS protocol is designed in B mode and C mode (continuous mode). It is used in industrial applications which require transfer rates, safety, flexibility and a minimized implementation effort. The BiSS interface has roots in SSI protocol.

C-face mounting

A standard NEMA mounting design, where the mounting holes in the face are threaded to receive the mating mount.

Cantilevered Load

Any load not symmetrically mounted on a stage. Such loads exert torque moments upon the ways, and the resulting deformations can degrade accuracy.


A device which, when connected in an alternating-current circuit, causes the current to lead the voltage in time phase. The peak of the current wave is reached ahead of the peak of the voltage wave. This is the result of the successive storage and discharged of electric energy used in single-phase motors to start, or in three-phase motors for power factor correction.


The designation shows that a product such as a motor or control meets European Standards for safety and environmental protection. A CE mark is required for products used in most European Countries.

Closed loop

A broadly applied term, relating to any system in which the output is measured and compared to the input. The output is then adjusted to reach the desired condition. In motion control, the term typically describes a system utilizing a velocity and/or position transducer to generate correction signals in relation to desired parameters.

Closed Loop Positioning

The use of information from external feedback devices, such as encoders, in controlling a motor’s movement to reach a desired position.

Command Position

The desired angular or linear position of an actuator. The command position is typically a signal that represents a realization of a motion control strategy.


  1. A term which refers to the action of steering currents or voltages to the proper motor phases so as to produce optimum motor torque. In brush type motors, commutation is done electromechanically via the brushes and commutator. In brushless motors, commutation is done by the switching electronics using rotor position information obtained by Hall sensors or other position sensors.
  2. Commutation of step motors is normally done open loop. Feedback from the motor is not required to hold rotor position precisely.


A device where the feedback signal is subtracted from the command signal. The difference output of the comparator is called the error signal.


The adjustment of gain and frequency parameters in a closed-loop system to achieve the desired dynamic response and also to insure a stable (non-oscillating) condition.


The amount of displacement per unit of applied force.

Compliant Coupling

The limited motion of one coupled shaft without causing motion of the other coupled shaft that does not permit permanent displacement of one shaft with respect of the other.


A material such as copper or aluminum which offers low resistance or opposition to the flow of electric current.


The measure of electrical flow, measured in Amps

Daisy Chain

A term used to describe the linking of several RS232C devices in sequence such that a single data stream flows through one device and on to the next. Daisy-chained devices usually are distinguished by device addresses which serve to indicate the desired destination for data in the stream.

DC (Direct Current)

A current that flows only in one direction in an electric circuit. It may be continuous or discontinuous and it may be constant or varying.


The rate velocity decreases as a function of time.

Decibel (db)

A logarithmic measurement of gain. If G is a systems gain (ratio of output to input) then 20 log G = gain in decibels (db)

Dielectric Material

A substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. If the flow of current between opposite electric charge poles is kept to a minimum while the electrostatic lines of flux are not impeded or interrupted.


An electrical input or output signal which uses two lines of opposite polarity referenced to the legal signal ground.


A device or circuit in which the output varies in discrete steps, i.e., pulses or on-off operation.

Distributed Processing

A technique to gain increased performance and modularity in control systems utilizing multiple computers or processors.


An electronic device that controls torque, speed and/or position of an AC or brushless motor. Typically, a feedback device (position sensor) is mounted on the motor shaft for closed-loop control of  velocity and position.


Electrically Erasable Programmable Read Only Memory. Re-writable non-volatile memory chip. The contents of the EEPROM is not lost after power is turned off.

Effective Torque

The root mean square value of the instantaneous torque value over a given cycle.


The ratio of power output to power input.

Electrical Degree

A unit of measurement of time as applied to alternating current. One complete cycle equals 360 electrical degrees. Once cycle in a rotating field moves from one pole to the next pole of the same polarity. There are 360 electrical degrees in this time period. Therefore, in a two pole machine there are 360 degrees in one revolution, and the electrical and mechanical degrees are equal. In a machine with more than two poles, the number of electrical degrees per revolution is obtained by multiplying the number of pairs of poles by 360.

EMI (Electro-Magnetic Interference)

EMI is noise which, when coupled into sensitive electronic circuits, may cause problems.

Encoder – position sensor

A feedback device which converts mechanical motion into electronic signals. The most commonly used, rotary encoders, output digital pulses corresponding to incremental angular motion or absolute position via serial interface. (Optical , Magnetic , Inductive , Capacitive)

Encoder Resolution

A measure of the smallest positional change which can be detected by the encoder.

Encoder, Absolute

A digital position sensor  in which the output is representative of the absolute position of the input shaft within one (or more) revolutions. Output is usually a parallel digital word.

Encoder, Incremental

A position encoding device in which the output represents incremental changes in position.

Encoder, Linear

A digital position transducer which directly measures linear position.

Encoder, Marker

A one-per-revolution signal provided by some incremental encoders to specify a reference point within that revolution. Also known as Zero Reference signal or index pulse.

Feed Forward

A technique used to pre-compensate, control a loop for known errors due to motor, drive, or lead characteristics. Provides improved response.


A signal which is transferred from the output back to the input for use in a closed loop system.


A type of permanent magnet consisting of ceramic compounds made up of oxides of iron, barium and strontium.

Filter (control systems)

A transfer function used to modify the frequency or time response of a control system.

Flatness OT Travel

The extent to which the straight line movement of a linear actuator is parallel to the base of the unit.


Flutter is an error of the basic cycle of an encoder per one revolution.


The magnetic field which is established around an energized conductor or permanent magnet. The field is represented by flux lines creating a flux pattern between opposite poles. The density of the flux lines is a measure of the strength of the magnetic field.

Following Error

The positional error curing motion resulting from use of a position control loop with proportional gain only.


The action of one body on another which tends to change the state of motion of that body. Typically described in terms of magnitude, direction, and point of application.

Form factor

The ratio of RMS current to average current. This number is a measure of the current ripple in a SCR or other switch-mode type of drive. Since motor heating is a function of RMS current while motor torque is a function of average current, a form factor greater than 1.00 means some fraction of motor current is producing heat but not torque.


The ratio of system output signal to system input signal. The control loop parameter that determines system performance characteristics.


The portion of a gear motor, which contains the actual gearing which, converts the basic motor speed to the rated output speed.

Gear motor

A gearhead and motor combination to reduce the speed of the motor to obtain the desired speed or torque.


Graphical user interface; a program that lets users select icons representing commands and prompts for parameter entries through dialog boxes

Hall sensor

A feedback device which is used in a brushless servo system to provide information for the amplifier to electronically commutate the motor. The device uses a magnetized wheel and hall effect sensors to generate the commutation signals.


Human-machine interface

Holding torque

Sometimes called static torque, holding torque specifies the maximum external torque that can be applied to a stopped, energized motor without causing the rotor to rotate. Generally used as a figure of merit when comparing motors.


A reference position in a motion control system. Often designated as the zero position.

Home Limit Switch

Any device which locates the position of the zero reference.

Home Position

A reference position for all absolute positioning movements. Usually defined by a home limit switch and/or encoder marker. Normally set at power up and retained for as long as the control system is operational.


An index of the amount of work a machine or motor can perform. One horsepower is equal to 746 watts. Since power is equal to torque multiplied by speed, horsepower is a measure of a motor’s torque and speed capability; e.g., a 1 HP motor will produce 36 lb-in. at 1,750 rpm.

HP = Torque (lb-in.) x Speed (RPM)/63,025
HP = Torque (lb-ft.) x Speed (RPM)/5,252
HP = Volts x Amps x Efficiency/746

Host Computer

An auxiliary computer system which is connected to a controller or controllers. The host computer in distributed control systems is frequently involved with controlling many remote and distributed motion control devices. It may also be used for off-line tasks such as program preparation, storage, supervisory control and evaluation.


The counter-force stored in an elastic material or mechanism after the outside forces acting on it have been changed. Commonly referred to as the difference between the position when approached from one direction and the same position when approached from the opposite direction. An example is the mechanical wind-up in the lead screw assembly.

I/O (Input/Output)

The reception and transmission of information between control devices. In modern control systems, I/O has two distant forms: switches, relays, etc. which are either an on or off state, or analog signals that are continuous in nature such as speed, temperature, flow, etc.


Losses due to current flowing in a conductor caused by resistance (equals the current squared times the resistance).

Incremental Motion

A motion control term that is ued to describe a device that produces one step of motion for each step command (usually a pulse) received.

Incremental Move

A move referenced from the current set position.


The characteristic of an electric circuit by which varying current in it produces a varying magnetic field which causes voltages in the same circuit or in a nearby circuit.

Inductance (L) (mH – millihenries line-to-line)

The electrical equivalent to mechanical inertia; that is, the property of a circuit, which has a tendency to resist current flow when no current is flowing, and when current is flowing has a tendency to maintain that current flow.

Inductance (mutual)

Mutual inductance is the property that exists between two current carrying conductors or coils when magnetic lines of force from one link with those of the other.

Inductance (self)

The self-inductance of a coil is the constant by which the time rate of change of the current in the coil must be multiplied to give the self-induced counter EMF.


The property of an object to resist change in velocity unless acted upon by an outside force. Higher inertia objects require larger torques to accelerate and decelerate. Inertia is dependent upon the mass and shape of the object.

Inertia Match

An inertial match between motor and load is obtained by selecting the coupling ratio such that the load moment of inertia reflected to the motor shaft is equal to the motor moment of inertia.

Inertial match

For most efficient operation, the system coupling ratio should be selected so that the reflected inertia of the load is equal to the rotor inertia of the motor.

Inrush Current

The current surge generated when a piece of equipment such as a servo amplifier is connected to an AC line. This surge is typically due to the impulse charging of a large capacitor located in the equipment.


Undesirable motion of an actuator that is different from the command motion. Instability can take the form or irregular speed or hunting of the final rest position.

Instantaneous Start-Stop Rate

The maximum switching rate at which an unloaded step motor can run without losing synchronism, or missing steps from a standstill, or stop without taking more steps than pulses.

Insulation Class

The rating assigned to the maximum temperature capability of the insulating components in a motor or other piece of equipment.

Information Solutions Practice; Internet Service Provider; Interoperable System Project


The steel portion of the rotor and stator cores make up a series of thin laminations (sheets) which are stacked and fastened together by cleats, rivets or welds. Laminations are used instead of a solid piece in order to reduce eddy-current losses.

LSB – Least Significant Bit

The bit in a binary number that is the least important, or having the least weight.

Limit Switch

A sensor, typically hall effect, optical, eddy current, or mechanical, which is used to sense the end of travel of a linear motion assembly. In addition to preventing over travel, it is frequently used to establish a precision reference. A sensor that signals the control to do something when a preset location is reached.


Motion control systems may have sensors called limits that alert the control electronics that the physical end of travel is being approached and that motion should stop.

Line Shaft

A shaft rotated by the primary motor drive. The line shaft transmits power from the motor to a load or series of loads. In the multiple load case, the loads are synchronized to one another because they are connected to the common shaft.

Linear Coordinated Move

A coordinated move where the path between endpoints is a line.

Linear Position Accuracy

The error between desired move and actual position achieved by a linear positioning components or stage system. The linear accuracy of components: motor accuracy, leadscrew accuracy, stage accuracy (Pitch and Yaw), and thermal expansion.


For a speed control system it is the maximum deviation between actual and set speed expressed as a percentage of set speed.


A device or mass to be moved or manipulated.


The burden imposed on a motor by the driven machine. It is often stated as the torque required to overcome the resistance of the machine it drives. Sometimes “load” is synonymous with “required power.”

Load Angle – Dynamic

The angle between the loaded and unloaded position (theoretical zero) of the rotor at a given instant under otherwise identical conditions at a specified command pulse rate, mode of winding excitation and phase current.

Load Angle – Static

The angle between the magnetic axis of the stator and rotor magnetic fields with the motor driving a specified load.

Logic Ground

An electrical potential to which all control signals in a particular system are referenced.

Loop Gain, Open

the product of the forward path and feedback path gains.

Loop, Feedback Control

A control method that compares the input from a measurement device, such as an encoder or tachometer, to a desired parameter, such as a position or velocity and causes action to correct any detected error. Several types of loops can be used in combination (i.e. velocity and position together) for high performance requirements.

Loop, PIC (Proportional, Integral, and Derivative Loop)

Specialized very high performance control loop which gives superior response.

Loop, Position

A feedback control loop in which the controlled parameter is motor position.

Loop, Velocity

A feedback control loop in which the controlled parameters is mechanical veloctiy.


A motor converts electrical energy into a mechanical energy and in so doing, encounters losses. These losses are all the energy that is put into a motor and not transformed to usable power but are converted into heat causing the temperature of the windings and other motor parts to rise.


The quantity of matter that an object contains.

Master Slave Motion Control

A type of coordinated motion control where the master axis position is used to generate one or more slave axis position commands.

Matched Load

A load inertia equal to the driving element (motor rotor) inertia.


Megabps, one million bps

Mechanical time constant ™ (Seconds)

In a simple first order system, the time required for the motor’s speed to attain 63.2% of its final value for a fixed voltage level. Can be calculated from: where: J is inertia in lb-in./s2 R is resistance in ohms KT is torque constant in lb-in./amp. 8.87 is a conversion factor tM is calculated in seconds


Million instructions per second; a measure of computational speed


Mind-machine interface

Moment Load

Pitch, Roll, and Yaw.

Msb – Most Significant Bit

The bit in a binary number that is the most important or that has the most weight.

Motion Profile

A method of describing a move operation in terms of time, position, and velocity. Typically velocity is characterized as a function of time or distance which results in a triangular or trapezoidal profile.


A device that takes electrical energy and converts it into mechanical energy to turn a shaft.

Motor Constant

The ratio of the motor torque to motor input power.

Motor, AC

A device that converts electrical alternating current into mechanical energy. requires no commutation devices such as brushes. Normally operated off commercial AC power. Can be single or multiple phase.

Motor, AC Asynchronous or Induction

An AC motor in which speed is proportional to the frequency of the applied AC. Requires no magnets or field coil. Usually used for non-precise constant speed applications.

Motor, AC Synchronous

Another term for brushless DC motor.

Motor, DC

A device that converts electrical direct current into mechanical energy. It requires a commutating device, either brushes or electronic. Usually requires a source of DC power.

Motor, DC Brushless

A type of direct current motor that utilizes electronic commutation rather than brushes to transfer current.

Motor, DC Permanent Magnet

A motor utilizing permanent magnets to produce a magnetic field. Has linear torque speed characteristics.

Motor, DC Wound Field

A direct current utilizing a coil to produce a magnetic field. Usually used in high power applications where constant horsepower operation is desired.

Motor, Stepping

A specialized AC motor that allows discrete positioning without feedback. Normally used for non-critical, low power applications, since positional information is easily lost if acceleration or velocity limits are exceeded. Load variations can also cause loss of position. If encoders are used, these limitations can be overcome.

Multi-axis Control System

Is a system designed to control more than one actuator. This type of controller allows the actuators to work independently or as a coordinated group to perform more complex tasks.

Multi-turn Encoder

A multi-turn encoder stores the number of rotations made by the shaft in non-volatile memory. See also Single-turn encoder.

No Load Speed

Motor speed with no external load and full voltage applied.

Non-Volatile Memory

Information that is stored on a device that does not require continuous power to be maintained. Typically associated with an EEPROM (Electrically Erasable Programmable Memory) chip for memory storage.


A system in which there is no feedback. Motor motion is expected to faithfully follow the input command. Stepping motor systems are an example of open-loop control.

Open-Loop Systems

A system where the command signal results in actuator movement but because the movement is not sensed, there is no way to correct the error. Open loop means no feedback.

Optical Encoder

A position or angular feedback device typically producing two pulse trains, phase shifted 90 degrees from each other providing positioning and directional information.

Peak Rating

The maximum value obtained over a complete cycle, usually intermittent within the system limits.

PID (Proportional-Integral-Derivative)

An Acronym that describes the compensation structure that can be used in a closed-loop system.

PLC (Programmable Logic Controller)

A programmable device which utilizes user defined logic to control a bank of inputs and outputs which are interfaced to external devices.

PMDC Motor (Permanent Magnet Direct Current)

A motor comprising a wound iron-core armature and a permanent magnet stator. These brushed motors operate on DC power.

Point to Point Move

A multi-axis moves from one point to another where each axis is controlled independently. (NO coordination between axis is required.)


A frequency at which the transfer function of a system goes to infinity.

Pole Pair, Electromechanical

The number of cycles of magnetic flux distribution in the air gap of a rotary electromechanical device.


Refers to the number of magnetic poles arranged on the rotor of the brushless motor. Unlike an AC motor, the number of poles has no direct relationship to the base speed of the motor.

Position Error

The difference between the present actuator (feedback) value and the desired position command for a position loop.

Position Feedback

Present actuator position is measured by a position sensor.


  1. The rate at which work is done. In motion control, power is equal to torque multiplied by speed.
  2. The rate of doing work or expending energy. It may be written as: Power (watts) = force x distance/time. Expressed in electrical terms it is voltage x current = power (watts)

Power factor

Ratio of true power (kW) to apparent power (kVA).


The compression of a bearing used to remove play with the anticipation of a load.

Primary Winding

The winding of a motor, transformer or other electrical device, which is connected to the power source.

Pulse rate

The frequency of the step pulses applied to a step motor driver. The pulse rate, multiplied by the resolution of the motor/driver combination (in steps per revolution), yields the rotational speed in revolutions per second.

Pulse Width Modulation (PWM)

  1. A PWM controller (amplifier) switches DC supply voltage on and off at fixed frequencies. The length of the on/off interval or voltage waveform is variable.
  2. Pulse width modulation (PWM), describes a switch-mode (as opposed to linear) control technique used in amplifiers and drivers to control motor voltage and current. PWM offers greatly improved efficiency compared to linear techniques.


Refers to signal characteristics of interfaces to positioning devices such as encoders or resolver’s. Specifically, that property of position transducers that allows them to detect direction of motion using the phase relationship of two signal channels.

RAM (Ram Access Memory)

A memory chip that can be read from and written to. Used for temporary information storage. Data is lost after power loss.

Rated Torque

The torque producing capacity of a motor at a given speed. This is the maximum continuous torque the motor can deliver to a load and is usually specified with a torque/speed curve.

RDC: Resolver-to-Digital Converter

An electronic component that converts the analogue signals from a resolver into a digital word representing angular position. R to D (Resolver to Digital) is sometimes used.


The degree to which a parameter such as position or velocity can be duplicated.

Repeatability (Bi-Directional)

The difference in the final position reached by moving away and then returning to a common point from both plus and minus directions. The error or non-repeatability is usually the sum of the backlash, hysteresis, and one unit of the system resolution.

Repeatability (Uni-Directional)

The ability of a system to repeat to a desired location, approaching that location from both plus and minus directions.


The degree of obstacle presented by a material to the flow of electric current is known as resistance and is measure to Ohms.


The smallest increment into which a parameter can be broken down. For example, a 1000 line encoder has a resolution of 1/1000 of a revolution.


An electromagnetic feedback device which converts angular shaft position into analog signals. These signals can be processed in various ways, such as with an RDC (resolver-to-digital converter) to produce digital position information. There are two basic types of resolvers; transmitter and receiver. A transmitter-type is designed for rotor primary excitation and stator secondary outputs. Position is determined by the ratio of the sine output amplitude to cosine output amplitude. A receiver-type is designed for stator primary excitations and rotor secondary output. Position is determined by the phase shift between the rotor output signal and one of the primary excitation signals.


Oscillatory behavior caused by mechanical limitations.

Restart torque

The maximum friction load, at a particular inertial load, that can be applied to the shaft of a synchronous motor without causing it to lose synchronism when accelerating to a constant speed from standstill.


Radio Frequency Interference.


Oscillation of a system following a sudden change in state.

RMS (Root Mean Square)

The square root of the average of the squares of the values of a periodic quantity taken throughout one complete period.

RMS Current – Root Mean Square Current

In an intermittent duty cycle application, the RMS current is equal to the value of steady state current which would produce the equivalent motor heating over a period of time.

RMS Torque – Root Mean Square Torque.

In an intermittent duty cycle application, the RMS torque is equal to the value of steady state torque which would produce the equivalent motor heating over a period of time.


A re-programmable multifunctional manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks.

Robot Control

A computer-based motion control device to control the servo-axis motion of a robot.

ROM (Read Only Memory)

A memory chip that can be read but not altered.


The moving part of the motor, consisting of the shaft and magnets. These magnets are analogous to the field winding of a brush-type DC motor.


Revolutions Per Minute

RPM (Revolutions Per Minute)

The number of times per minute the shaft of the motor (machine) rotates. This is a function of design and the power supply.


A popular protocol for transmitting digital data over two lines in a bit-serial format. RS232C specifies signal levels, data formats, maximum transmission distances…etc.


A high speed, differential method of serial communication that allows for multiple network “drop”, or “nodes”, to share the transmit and receive lines. rS485 is commonly used in automation due to its extended range (4000 ft) and its outstanding noise immunity.


supervisory control and data acquisition [system]

Secondary Winding

Winding which is not connected to the power source, but which carries current induced in it through its magnetic linkage with the primary winding.

Serial Port

A digital data communications port configured with a minimum number of signal lines. This is achieved by passing binary information signals as a timed series of “1”s and “0”s on a signal line.


A system consisting of several devices which continuously monitor actual information (position, velocity), compares those values to desired outcome and makes necessary corrections to minimize that difference.

Servo Amplifiers/Servo Drive

An electronic device which produces the winding current for a servo motor. The amplifier converts a low level control signal into a high voltage and current levels top produce torque in the motor.

Servo Motor

A DC motor in which position is achieved by precise linear or duty cycle control of motor current and accurate monitoring of an external feedback device. Converts electrical energy (winding current)into mechanical energy (torque).

Servo System

An automatic feedback control system for mechanical motion in which the controlled or output quantity is position, velocity, or acceleration. Servo systems are closed loop systems.

Settling time

The time required for a parameter to stop oscillating or ringing and reach its final value.

Shock loading

A load that produces extremely high peak torques for very short durations. This type of load is associated with conveyorized grinding, crushing and separation processes.

Single Point Ground

The common connection point for signal grounds in a control wiring environment.

Single-turn Encoder

A single-turn encoder cannot distinguish between one turn and the next without the aid of external circuitry. See also Multi-turn encoder.


In motion control, the portion of a move made at a constant non-zero velocity.

Slew Speed

The maximum velocity at which an encoder will be required to perform.


Describes the linear or rotational velocity of a motor or other object in motion.

Speed Regulation

For a speed control systems, speed regulation is the variation in actual speed expressed as a percentage of set speed.

SPS (Steps-Per-Second)

A measure of velocity used with stepping motors.

Stall Torque

The amount of torque developed with voltage applied and shaft locked, or not rotating. Also known as locked-rotor torque.

Starting Current

Amount of current drawn at the instant a motor is energized. Same as locked rotor current.

Starting Torque

The torque of twisting force delivered by a motor at the instant it is energized. Starting torque is often higher than rated running or full load torque.


The non-rotating part of a  structure. In Netzer Electric encoder, the stator usually contains the mounting surface,housing and cover.

Step angle

The angular distance the shaft rotates upon receipt of a single step command.

Stepper Motor

Motor that receives electric pulses to command a move in discrete angular increments (steps). Through appropriate drive circuitry, controlling the rate and quantity of pulses will control the motor’s velocity and position.


The ability to resist movement induced by an applied torque. Stiffness is often specified as a torque displacement curve, indicating the amount a motor shaft will rotate upon application of a known external force when stopped.

Straightness of Travel

The deviation from ideal straight-line motion of a linear motion assembly. This can result from rotation around any of three perpendicular axes or translation in two linear axes.


Synchronous Serial Interface (SSI) is a widely used serial interface standard for industrial applications between a master (e.g. controller) and a slave (e.g. sensor). SSI is based on RS-422[1] standards and has a high protocol efficiency in addition to its implementation over various hardware platforms.


A device attached to a moving shaft that generates a voltage signal directly proportional to rotational speed. Tachometers are typically attached to the output shaft of DC or AC variable-speed motor requiring close speed regulation. The tachometer feeds its signal to a control which adjusts its output to the DC or AC motor accordingly.

Thermistor (Thermally Sensitive Resistor)

A semiconductor used to measure temperature that can be attached to an alarm or meter to detect motor overheating.

Thermocouple (Thermal Detection Device)

A temperature-detecting device made of two dissimilar metals, which generates a voltage as a function of temperature. Thermocouples can be attached to a meter or alarm to detect overheating of motor windings or bearings.


A temperature sensitive pilot duty device mounted on the interior of the motor to protect it from overheating.


Measurement of linear force. The total force required to move the load, including gravity, friction, and acceleration.


A measure of angular force which produces rotational motion. This force is defined by a linear force multiplied by a radius; e.g. lb-in. Torque is an important parameter of any motion control system. Formula: Torque (lb-ft.) = 5,250 x HP/RPM


Any device that translates a physical parameter into an electrical parameter. Tachometers and encoders are examples of transducers.

Transfer Function

The ratio of the Laplace transforms of a system output signal and a system input signal.

User interface

Variable Frequency drives

An electronic device used to control the speed of a standard AC induction motor. The device controls the speed by varying the frequency of the winding current used to drive the motor.

Vector Control

A method of obtaining servo type performance from an AC motor by controlling two components of motor current.


The change in position as a function of time. Velocity has both a magnitude and sign.


Difference in electrical potential between two points.

Voltage constant (KE) (V/kRPM peak, line-to-line)

May also be termed back-EMF constant. When a motor is operated, it generates a voltage proportional to speed, but opposing the applied voltage. The shape of the voltage waveform depends upon the specific motor design. For example, in a brushless motor, the wave shape may be trapezoidal or sinusoidal in nature. All Pacific Scientific brushless motor designs have a sinusoidal voltage constant. For a sine waveform, the voltage constant can be measured from line-to-neutral or line-to-line and expressed as a peak value or “RMS” value.

Voltage constant (or Back EMF Constant)

A number representing the relationship between Back EMF voltage and angular velocity. Typically expressed as V/KRPM.

Voltage Drop

Loss encountered across a circuit impedance from power source to applicable point (motor) caused by the resistance in conductor. Voltage drop across a resistor takes the form of heat released into the air at the point of resistance.


One horsepower equals 746 watts.

Watts Loss

The amount of lost power due to inefficiencies of a device.


Force of gravity acting on a body. Determined by multiplying the mass of the object by the acceleration due to gravity.