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Glossary of Motion Control Terms
Refers to a motion control system employing position feedback devices (absolute encoders) to maintain a given mechanical location.
Position referenced to a fixed zero position.
A positioning coordinate referenced wherein all positions are specified relative to some reference, or zero position. This is different from incremental programming, where distances are specified relative to the current position.
A linear positioning error caused by a combination of an angular error in the ways, and an offset between the precision determining element (lead screw, feedback device, etc.) and the actual point of interest.
AC Adjustable-Speed Drive
All equipment required to adjust the speed or torque of AC electric motor(s) by controlling both frequency and voltage applied to the motor(s).
AC Servo Drive:
A servo drive used to control either or both synchronous or induction AC motors.
A device which creates mechanical motion by converting various forms of energy to mechanical energy.
A technique to allow the control to automatically compensate for changes in system parameters such as load variations.
Electronics which convert low level command signals to high power voltages and currents to operate a servomotor.
A general term referring to a motor drive that generates sinusoidal shaped motor currents in a brushless motor wound as to generate sinusoidal back EMF.
The change in velocity as a function of time. Acceleration usually refers to increasing velocity and deceleration describes decreasing velocity.
A measure of the difference between expected position and actual position of a motor or mechanical system. Motor accuracy is usually specified as an angle representing the maximum deviation from expected position.
The temperature of the cooling medium, usually air, immediately surrounding the motor or another device.
American Standard Code for Information Interchange. This code assigns a number series of electrical signals to each numeral and letter of the alphabet. In this manner, information can be transmitted between machines as a series of binary numbers.
A measure of system response. It is the frequency range that a control system can follow.
A lead screw which has its threads formed as a ball bearing race; the carriage contains a circulating supply of balls for increased efficiency.
The number of binary bits transmitted per second on a serial communication link (such as RS-232).
Binary Coded Decimal is an encoding technique used to describe the numbers 0 through 9 with four digital (on or off) signal lines. Popular in machine tool equipment, BCD interfaces are now giving way to interfaces requiring fewer wires such as RS-232C.
Abbreviation of Binary Digit, the smallest unit of memory equal to 1 or 0.
The voltage produced across a winding of a motor due to the winding turns being cut by a magnetic field while the motor is operating. This voltage is directly proportional to rotor velocity and is opposite in polarity to the applied voltage. Sometimes referred to as counter EMF.
A simplified schematic representing components and signal flow through a system.
A graph of system gain and phase versus input frequency which graphically illustrates the steady state characteristics of the system.
Frequency(ies) at which the gain changes slope on a Bode plot (break frequencies correspond to the poles and zeroes of the system).
Conducting material which passes current from the DC motor terminals to the rotating commutator.
Brushless DC Servo
A general term referring to a motor drive that generates trapezoidal shaped motor currents in a motor wound as to generate trapezoidal Back EMF.
Brushless Servo Drive
A servo drive used to control a permanent magnet synchronous AC motor. (may also be referred to as AC Servo Drive.)
A group of parallel connections carrying preassigned digital signals. Buses usually consist of address and data information and miscellaneous control signals for the interconnection of microprocessors, memories, and other computing elements.
A group of 8 bits treated as a whole, with 256 possible combinations of ones and zeros, each combination representing a unique piece of information.
A broadly applied term relating to any system where the output is measured and compared to the input. The output is then adjusted to reach the desired condition. In motion control, the term describes a system wherein a velocity or position (or both) transducer is used to generate correction signals by comparison to desired parameters.
Coefficient of Friction:
This is defined as the ratio of the force required to move a given load to the magnitude of that load. Typical values for the ball and crossed roller slides are 0.001 to 0.005.
A term used to describe non-uniform angular velocity. Cogging appears as a jerkiness especially at low speeds.
The desired angular or linear position of an actuator.
The switching sequence of drive voltage into motor phase windings necessary to assure continuous motor rotation. A brushed motor relies upon brush/bar contact to mechanically switch the windings. A brushless motor requires a device that senses rotor rotational position, feeds that information to a drive that determines the next switching sequence.
A mechanical cylinder consisting of alternating segments of conductive and insulating material. This cylinder used in DC motors passes currents from the brushes into the rotor windings and performs motor commutation as the motor rotates.
The corrective or control action in a feedback loop system which is used to improve system performance characteristics such as accuracy and response time.
A control action which depends on the command only and not the error to improve system response time.
A control action which is proportional to the integral or accumulative time error value product of the feedback loop error signal. It is usually used to reduce static error.
A control action which causes the lag at low frequencies and tends to increase the delay between the input and output of a system while decreasing static error.
A control action which causes the phase to lead at high frequencies and tends to decrease the delay between the input and output of a system.
Compensation, Lead Lag:
A control action which combines the characteristics of lead and lag compensations.
Compensation, Proportional: .
A control action which is directly proportional to the error signal of a feedback loop. It is used to improve system accuracy and response time.
The amount of displacement per unit of applied force.
Multi-axis motion where the position of each axis is dependent on the other axis such that the path and velocity of a move can be accurately controlled (requires coordination between
A system is critically damped when the response to a step change in desired velocity or position is achieved in the minimum possible time with little or no overshoot.
The frequency at which the gain intercepts the 0 dB point on a Bode plot (used in reference to the open-loop gain plot).
A term used to describe the linking of several RS-232C 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.
An indication of the rate of decay of a signal to its steady state value. Related to settling time.
Ratio of actual damping to critical damping. Less than one is an underdamped system and greater than one is an overdamped system.
DC Adjustable-Speed Drive:
All equipment required to adjust the speed or torque of DC motor(s) by controlling the voltages applied to the armature and/or field of the motors.
An electronic control unit for running DC motors. The DC drive converts AC line current to a variable DC current to control a DC motor. The DC drive has a signal input that controls the torque and speed of the rotor.
A range of input signals for which there is no system response.
A logarithmic measurement of gain. If G is a systems gain (ratio of output to input), then 20 log G = gain in decibels (dB).
The minimal torque present in an unenergized motor. The detent torque of a step motor is typically about 1% of its static energized torque.
Usually referring to any type of motor drive in which the input is an analog signal.
Usually referring to any type of motor drive in which the tuning or compensation is done digitally. Input may be an analog or digital signal.
A motor drive in which the output is directly proportional to either a voltage or current input. Normally both inputs and outputs are analog signals.
A motor drive utilizing Pulse-Width Modulation techniques to control power to the motor. Typically a high efficiency drive that can be used for high response applications.
A motor drive which utilizes internal feedback loops for accurate control of motor current and/or velocity.
Electronics which convert step and direction inputs to high power currents and voltages to drive a stepping motor. (The stepping motor drive is analogous to the servomotor amplifier).
A passive technique for stopping a permanent magnet brush or brushless motor. The motor windings are shorted together through a resistor which results in motor braking with an exponential decrease in speed.
Direct Drive Servo
A high-torque, low-speed servo motor with a high resolution encoder or resolver intended for direct connection to the load without going through a gearbox.
For a repetitive cycle, the ratio of on time to total cycle time. Duty Cycle = On Time/(On Time + Off Time) x 100%
The ratio of power output to power input.
Electrical Time Constant
The ratio of armature inductance to armature resistance. The time it takes for a step current input to the coil to reach 63% of its value by overcoming the resistance and the inductance of the coil.
A device that translates mechanical motion into electronic signals used for monitoring position or velocity. Typical encoders are designed with a printed disc and a light source. As the disc turns with the actuator shaft, the light source shines through the printed pattern onto a sensor. The light transmission is interrupted by the patterns of the disc. These interruptions are sensed and converted to electrical pulses. By counting these pulses, actuator shaft position is determined.
A digital position transducer 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.
A position encoding device in which the output represents incremental changes in position.
A digital position transducer which directly measures linear position.
A once-per-revolution signal provided by some incremental encoders to specify a reference point within that revolution.
A measure of the smallest positional change which can be detected by the encoder.
A motor classification that indicates a motor is capable of withstanding internal explosions without bursting or allowing ignition to reach beyond the confines of the motor frame.
A signal which is transferred from the output back to the input for use in a closed loop system.
Filter (Control Systems):
A transfer function used to modify the frequency or time response of a control system.
Flatness of Travel:
Deviation from ideal straight line travel in a vertical plane, also referred to as vertical runout.
The positional error during motion resulting from use of a position control loop with proportional gain only.
The ratio of the RMS value of a harmonic signal to its average value in one half-wave.
A resistance to motion. Friction can be constant with varying speed (Coulomb friction) or proportional to speed (viscous friction).
Full Load Current:
The armature current of a motor operated at its full load torque and speed with rated voltage applied.
Full Load Speed:
The speed of a motor operated with rated voltage and full load torque.
The ratio of system output signal to system input signal. The control loop parameter that determines system performance characteristics
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.
Sometimes called static torque, it specifies the maximum external force or torque that can be applied to a stopped, energized motor without causing the rotor to rotate continuously.
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.
One horsepower is equal to 746 watts. Since Power = Torque x Speed, horsepower is a measure of a motor's torque and speed capability (e.g. a 1 HP motor will produce 35 in-lb. at 1,800 RPM).
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
The oscillation of the system response about a theoretical steady-state value.
Hybrid Stepping Motor:
A motor designed to move in discrete increments or steps. The motor has a permanent magnet rotor and wound stator. These motors are brushless and phase currents are commutated as a function of time to produce motion.
A brushless servo motor based on a conventional hybrid stepper. It may use either a resolver or encoder for commutation feedback.
For a system with an analog input and digital output, the output value is dependent on both the input value and output state such that there is an input range over which the output can be high or low.
A digital data communications standard popular in instrumentation electronics. This parallel interface is also known as GPIB, or General Purpose Interface Bus.
A motion control term that describes a device that produces one step of motion for each step command (usually a pulse) received.
A coordinate system where positions or distances are specified relative to the current position.
Electronics which convert high level motion commands from a host computer, programmable controller, or operator panel into step and direction pulse streams for use by the stepping motor driver.See PMC
The tendency of a motor coil to store energy in a magnetic field. High speed stepping motor performance is inversely proportional to motor inductance.
A measure of an objects resistance to a change in velocity. The larger an objects inertia, the larger the torque that is required to accelerate or decelerate it. Inertia is a function of an objects mass and its shape.
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.
The reception and transmission of information between control devices. In modern control systems, I / O has two distinct forms: switches, relays, etc. which are in either an on or off state, or analog signals that are continuous in nature such as speed, temperature, flow, etc.
A mathematical equation implemented by a computer to decrease the delay between the input and output of a system.
A device for translating rotary motion into linear motion, consisting of an externally threaded screw and an internally threaded carriage (nut). Typically leadscrews are inefficient when compared to Ball Screws or Belt Drives.
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.
Linear Coordinated Move:
A coordinated move where the path between endpoints is a line.
For a speed control system it is the maximum deviation between actual and set speed expressed as a percentage of set speed.
An electrical potential to which all control signals in a particular system are referenced.
Loop, Feedback Control:
A control method that compares the input from a measuring device, such as an encoder or tachometer, to a desired parameter, such as 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 Gain, Open:
The product of the forward path and feedback path gains.
Proportional, Integral, and Derivative Loop: Specialized very high performance control loop which gives superior response.
A feedback control loop in which the controlled parameter is motor position.
A feedback control loop in which the controlled parameter is mechanical velocity.
A type of coordinated motion control where the master axis position is used to generate one or more slave axis position commands.
Mechanical Time Constant
The time for an energized DC motor to reach 2/3rds of its set velocity. Based on a fixed voltage applied to the windings.
Designates the condition resulting from energizing a motor at a multiple of its natural frequency (usually the third orders condition). Torque loss and oscillation can occur in underdamped open-loop systems.
An electronic control technique that proportions the current in a step motors windings to provide additional intermediate positions between poles. Produces smooth rotation over a wide speed range and high positional resolution.
A phenomenon in which a stepping motor can fall out of synchronism due to loss of torque at mid-range speeds. The loss of torque is due to interaction between the motor's electrical characteristics and the driver electronics. Some drivers have circuitry to eliminate or reduce this phenomenon.
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 speed66 applications.
Motor, AC Synchronous:
Another term for brushless DC motor.
The ratio of the motor torque to motor input power.
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.
A specialized AC motor that allows discrete positioning without feedback. Normally used for non-critical, lowpower 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.
Usually refers to any type of automated equipment or process used for contouring or positioning.
No Load Speed:
Motor speed with no external load.
A term used to describe a signal output that is performed with a transistor. An open collector output acts like a switch closure with one end of the switch at ground potential and the other end of the switch accessible.
A system where the command signal results in actuator movement but, because the movement is not sensed, there is no way to correct for error. Open lop means no feedback.
A device that allows the operator to communicate with a machine. This device typically has a keyboard or thumbwheel to enter instructions into the machine. It also has a display device that allows the machine to display messages.
A linear or angular position feedback device, typically providing incremental two channel information in quadrature format (sine or square waves with a 90-degree phase shift between each channel). Such two channel information allows simple counter circuits to function as absolute position indicators.
A method of sending a signal from one piece of equipment to another without the usual requirement of common ground potentials. The signal is transmitted optically with a light source (usually a Light Emitting Diode) and a light sensor (usually a photosensitive transistor). These optical components provide electrical isolation.
The degree of perpendicularity, or squareness, between the two axes in an X-Y or X-Z table. This parameter is usually measured in arc-seconds or microradians.
An effect that varies periodically between two values.
The amount that the parameter being controlled exceeds the desired value for a step input.
Refers to a data communication format wherein many signal lines are used to communicate more than one piece of data at the same time.
May also refer to a motor mounting style on an actuator which brings the motor back parallel with the actuator body versus inline with the actuator body.
Phase-Locked Servo System:
A hybrid control system in which the output of an optical tachometer is compared to a reference square wave signal to generate a system error signal proportional to both shaft velocity and position errors.
The angle at which the steady state input signal to a system leads the output signal.
The difference between 180° and the phase angle of a system at its crossover frequency.
An angular deviation possible in positioning systems, in which the tables leading edge rises or falls as the table translates along its direction of travel. This represents rotation around a horizontal axis, perpendicular to the direction of travel.
An acronym that describes the compensation structure that can be used in a closed-loop system.
Programmable logic controller; a machine controller that activates relays and other I/ O units from a stored program. Additional modules support motion control and other functions.
Programmable motion controller, primarily designed for single- or multi-axis motion control with I/O as an auxiliary function.
A frequency at which the transfer function of a system goes to infinity.
A multi-axis move from one point to another where each axis is controlled independently. (No coordination between axes is required).
The difference between the present actuator (feedback) value and the desired position command for a position loop.
Present actuator position as measured by a position transducer.
The rate at which work is done. In motion control, Power = Torque x Speed.
A term used to describe the control of machine or manufacturing processes, especially in continuous production environments.
The frequency of the step pulses applied to a motor driver. The pulse rate multiplied by the resolution of the motor/ drive combination (in steps per revolution) yields the rotational speed in revolutions per second.
Pulse Width Modulation. A method of controlling the average current in a motors phase windings by varying the on-time (duty cycle) of transistor switches.
Refers to signal characteristics of interfaces to positioning devices such as encoders or resolvers. Specifically, that property of position transducers that allows them to detect direction of motion using the phase relationship of two signal channels. (A type of incremental encoder output in which the two square wave outputs are offset by 90 degrees).
The acceleration and deceleration of a motor. May also refer to the change in frequency of the applied step pulse train.
The torque producing capacity of a motor at a given sped. This is the maximum torque the motor can deliver to a load and is usually specified with a torque/speed curve.
Usually refers to a circuit in a drive amplifier that accepts and drains energy produced by a rotating motor either during deceleration or free-wheel shutdown.
Changing the predefined move profile that is being executed, to a different predefined move profile following receipt of an input or interrupt.
The degree to which the positioning accuracy for a given move performanced repetitively can be duplicated.
The smallest positioning increment that can be achieved. Frequently defined as the number of steps required for a motors shaft to rotate one complete revolution.
A feedback device with a construction similar to a motors construction (stator and rotor). Provides velocity and position information to a drives microprocessor or DSP to electronically commutate the motor.
Designates the condition resulting from energizing a motor at a frequency at or close to the motors natural frequency. Lower resolution, open-loop systems will exhibit large oscillations from minimal input.
Radio Frequency Interference
Oscillation of a system following a sudden change in state.
A reprogrammable multifunctional manipulator designed to move material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks.
A computer-based motion control device to control the servo-axis motion of a robot.
An angular deviation from ideal straight line motion, in which the positioning table rotates around its axis of travel as it translates along that axis.
Root Mean Square Current (RMS Current):
In an intermittent duty cycle application, the RMS current is equal to the value of steady state current which would produce the equivalent resistive heating over a long period of time.
Root Mean Square Torque (RMS Torque):
For an intermittent duty cycle application, the RMS torque is equal to the steady state torque which would produce the same amount of motor heating over long periods of time.
The rotating part of a magnetic structure. In a motor, the rotor is connected to the motor shaft.
A data communications standard that encodes a string of information on a single line in a time sequential format. The standard specifies the proper voltage and time requirements so that different manufacturers devices are compatible.
A digital data communication port configured with a minimum number of signal lines. This is achieved by passing binary information signals as a time series of "1"s and "0"s on a single 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 Amplifier/Servo Drive:
An electronic device which produces the winding current for a servomotor. The amplifier converts a low level control signal into high voltage and current levels to produce torque in the motor.
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.
The time required for a step response of a system parameter to stop oscillating or ringing and reach its final value.
SlewIn motion control, the portion of a move made at a constant non-zero velocity.
The maximum velocity at which an encoder will be required to perform.
Static Torque (Stall Torque)
The maximum torque available at zero speed.
The non-rotating part of a magnetic structure. In a motor, the stator usually contains the mounting surface, bearings, and nonrotating windings or permanent magnets.
The angle the shaft rotates upon receipt of a single step command.
The ability to resist movement induced by an applied torque. 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:
Deviation from straight line motion in a horizontal plane. Also referred to as horizontal runout. This error is usually traceable to an underlying angular error of the ways.
A motor rotating at a speed correctly corresponding to the applied step pulse frequency is said to be in synchronism. Load torques in excess of the motors capacity (rated torque) will cause a loss of synchronism. The condition is not damaging to a step motor.
An electromagnetic feedback transducer which produces an analog voltage signal proportional to rotational velocity. Tachometers can be either brush or brushless.
This stands for Total Indicator Reading, which reflects the total absolute deviation from a mean value (versus a + value which indicates the deviation from a nominal value).
The rotary equivalent to force. Equal to the product of the force perpendicular to the radius of motion and distance from the center of rotation to the point where the force is applied.
A number representing the relationship between motor input current and motor output torque. Typically expressed in units of torque / amp.
The cyclical variation of generated torque at a frequency given by the product of motor angular velocity and number of commutator segments or magnetic poles.
Defined as a motors holding torque divided by the inertia of its rotor. The higher the ratio, the higher a motors maximum acceleration capability will be.
Any device that translates a physical parameter into an electrical parameter. Tachometers and encoders are examples of transducers.
A mathematical means of expressing the output to input relationship of a system. Expressed as a function of frequency.
A motion profile in which the velocity vs. time profile resembles a trapezoid. Characterized by constant acceleration, constant velocity, and constant deceleration.
Inputs on a controller that initiate or œtrigger the next step in a program.
Transistor-Transistor Logic. Describes a common digital logic device family that is used in most modern digital electronics. TTL signals have two distinct states that are described with a voltage “ a logical œzero or œlow is represented by a voltage of less than 0.8 volts and a logical œone or œhigh is represented by a voltage from 2.5 to 5 volts.
The change in position as a function of time. Velocity has both a magnitude and a direction.
Disturbances in the programmed velocity profile due to changes in magnetic flux and commutation switching.
Voltage Constant: (or Back EMF Constant)
A number representing the relationship between Back EMF voltage and angular velocity. Typically expressed as V / kRPM.
An angular deviation from ideal straight line motion, in which the positioning table rotates around the Z (vertical) Axis as it translates along its travel axis.
A frequency at which the transfer function of a system goes to zero.
Parker Hannifin Corporation, Electromechanical and Drives Division,
9225 Forsyth Park Drive, Charlotte, NC 28273
704-588-3246 or 800-358-9068
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