Motor and Motion Control Glossary of Terms

A
Absolute Positioning: Refers to a motion control system employing position feedback devices (absolute encoders) to maintain a given mechanical location.
Absolute Programming: A positioning coordinate reference wherein all positions are specified relative to some reference, or "home" position. This is different from incremental programming, where distances are specified relative to current position.
Acceleration: The change in velocity as a function of time. Acceleration usually refers to increasing velocity and deceleration describes decreasing velocity.
Accuracy: The difference between the expected value of a parameter and its actual value.
Actuator: A device which creates mechanical motion by converting various forms of energy to mechanical energy.
Adaptive Control: A technique to allow the control to automatically compensate for changes in system parameters such as load variations.
Ambient Temperature: The temperature of the cooling medium, usually air, immediately surrounding the motor or another device.
Amplifier: Electronics which convert low level command signals to high power voltages and currents to operate a servomotor.
ASCII (American Standard Code for Information Interchange): This code assigns a number to each numeral and letter of the alphabet. In this manner, information can be transmitted between machines as a series of binary numbers.

B
Back-EMF: Electromotive force generated when a conductor passes through a magnetic field. In a motor it is generated any time the armature is moving in the field whether the motor is under power or not. The term "back" or "counter" EMF is referring to the polarity of the voltage and the direction of the current flow as being opposed to the supply voltage and current to the motor under power.
Back_EMF constant: [mV/rpm] It is the constant corresponding to the relationship between the induced voltage in the rotor and the speed of rotation. In brushless motors the back-EMF constant is the constant corresponding to the relationship between the induced voltage in the motor phases and the rotational speed.
Backlash: This is the typically undesirable quality of "play" or "slop" in a mechanical system. Gearheads, depending on the level of the precision of the parts and the type of gearing system involved can have varying degrees of backlash internally.
Baud Rate: The number of binary bits transmitted per second on a serial communication link (such as RS-232).
Bit (Binary Digit): A unit of information equal to 1 binary decision or having a value of 0 or 1.
Block Diagram A simplified representation of a system, with each component represented by a block, and each block positioned in order of signal flow through the system.
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.
Brushes: A motor may have or more pairs of brushes which served as the positive and negative voltage terminals to the motor. Current is delivered through the brushes to armature (the commutator and from there the motor windings). Typical brush materials are highly conductive alloys containing precious metals or sintered materials like copper or silver graphite.
Bus: A group of parallel connections carrying pre-assigned digital signals. Busses usually consist of address and data information and miscellaneous control signals for the interconnection of microprocessors, memories, and other computing elements.

C
CAM Profile: A technique used to perform nonlinear motion electronically similar to that achieved with mechanical cams.
Characteristic Equation: Closed Loop: A broadly applied term relating to any system where the output is compared to the input. The output is then adjusted to reach the desired condition. In motion control the term is used to describe a system wherein a velocity or position (or both) transducer is used to generate correction signals by comparison to desired parameters.
Cogging: This term refers to the typically undesirable property of conventional DC motors to assume a preferred rotor position due to the change in reluctance in the slot wound laminated iron rotor. This means that accurate positioning is simpler and more accurate with a coreless motor and operation is smooth even at low speed.
Commutation: A term that refers to the action of steering currents or voltage 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 typically obtained by hall sensors, a tachsyn, a resolve, or an encoder.
Commutator: Mounted on the shaft of the armature, this element consists of segments or bars that are directly connected to the different phases of the motor windings. Brushes contact the bars and when the armature rotates, current is switched from one phase of the winding to the next.
Compensation: The corrective or control action in a feedback loop system which is used to improve system performance characteristics such as accuracy and response time.
Compensation, Feedforward: A control action which depends on the command only and not the error to improve system response time.
Compensation, Integral: 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.
Compensation, Lag: 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.
Compensation, Lead: 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.
Compliance: The amount of displacement per unit of applied force.
Computer Numerical Control (CNC): A computer-based motion control device programmable in a numerical word address format. A computer numerical control product typically includes a CPU section, operator interface devices, input/output signal and data devices, software and related peripheral apparatus.
Coupling Ratio: The ratio of motor velocity to load velocity for a load coupled to motor through a gear or similar mechanical device.
Critical Damping: 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.

Current: [A] The flow of electrons through a conducting material. By convention, current is considered to flow from positive to negative potential. The electrons, however, actually flow in the opposite direction. The unit of measurement is the Ampere and 1 Amp is defined as the constant current produced between two straight infinitely long parallel conductors with negligible cross section diameter and spaced one meter apart in a vacuum.

Current Constant: [A/oz-in] The constant corresponding to the relationship between motor current and motor output torque.


D
Damping: An indication of the rate of decay of a signal to its steady state of value. Related to settling time.
Damping Ration Ratio of actual damping. Ratio values of less than one can indicate an under-damped system and greater than on can indicate an over-damped system.
DC (Direct Current): [V] Is the type of current where all electrons are flowing in the same direction continuously. If the flow of electrons reverses periodically, the current is called AC (Alternating Current).
Dead Band: A range of input signals for which there is no system response.
Decibel (dB): A logarithmic measurement of gain. If G is a system gain (ratio of output to input) then 20 log G = gain in decibels (dB).
Demagnetization (Demag) Current: The current level at which the motor magnets will be demagnetized. This is an irreversible effect which will alter the motor characteristics and degrade performance.
Detent Torque: The maximum torque that can be applied to an un-energized stepper motor without causing continuous rotating motion.
Dielectric Test: A high voltage breakdown test of insulation's ability to withstand an AC voltage. Test criterion limits the leaking current to a specified magnitude and frequency, applied between the specified test points.
Differential: An electrical input or output signal which uses two lines of opposite polarity referenced to the local signal ground.
DNC, Direct Numerical Control: Technique of transferring part program data to a numerical control system via direct electrical connection in place of paper tapes.
Drive, Analog: Usually referring to any type of motor drive in which the input is an analog signal.
Drive, Digital: 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.
Drive, Linear: 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. This is a relatively inefficient drive type.
Drive, PWM: 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.
Drive, SCR: A DC motor drive which utilizes internal silicon controlled rectifiers as the power control elements. Usually used for low bandwidths, higher power applications.
Drive, Servo: A motor drive which utilizes internal feedback loops for accurate control of motor current and/or velocity.
Drive, Stepper: Electronics which convert step and direction inputs to high power currents and voltages to drive a stepping motor. The stepping motor driver is analogous to the servo motor amplifier.

Duty Cycle: For a repetitive cycle, the ratio of on time to total cycle time.

Dynamic Braking: 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.
E
Eddy Current Losses: Efficiency: [%] It is the ratio between the absorbed electrical power and the resulting mechanical output power of the motor. Maximum efficiency does not always necessarily correspond to the optimum duty points of the motor in any specific application.
Electronic Gearing: A technique used to electrically simulate mechanical gearing. Causes one closed loop axis to be slaved to another open or closed loop axis with a variable ratio.
EMI (Electro-Magnetic Interference): EMI is noise which, when coupled into sensitive electronic circuits, may cause problems.
Encoder: A type of feedback device which converts mechanical motion into electrical signals to indicate actuator position. 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 pattern on the disc. These interruptions are sensed and converted to electric pulses. By counting the pulses, actuator shaft position is determined.
Encoder, Absolute: 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.
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, Index: A once-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.
Encoder, Resolution: A measure of the smallest positional change which can be detected by the encoder.
Explosion-proof: 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.

F
Fall Time: The time for the amplitude of system response to decay to 37% of its steady-state value after the removal of a steady-state step input signal.
Feedback: A signal which is transferred from the output back to the input for use in a closed loop system.
Feed Forward: A technique used to pre-compensate control a loop for known errors due to motor, drive, or lead characteristics. Provides improved response.
Following Error: The positional error during motion resulting from use of a position control loop with proportional gain only.
Form Factor: The ratio of RMS current to average current. This number is a measure of the current ripple in a PWM or other switch mode type of controller. Since motor heating is a function of RMS current while motor torque is a function of average current, a form facter greater than 1.00 means some fraction of motor current is producing heat but not torque.
Four Quadrant: Refers to a motion system that can operate in all four quadrants, i.e. velocity in either direction and torque in either direction. This means that the motor can accelerate, run, and decelerte in either direction.
Friction Torque: [oz-in] The sum of torque losses independent of motor speed. These losses include those caused by static mechanical friction of the ball bearings and magnetic hysteresis of the stator.

G
Gain: The ratio of system output signal to system input signal. It is a control loop parameter that determines system performance characteristics.


H
Holding Torque: 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.
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.
Hunting: The oscillation of the system response about a theoretical steady-state value.
Hysteresis (Electronic): 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.
Hysteresis Losses: A loss in power due to the cyclic reversal of the magnetic state of a rotating steel armature. It is proportional to the amount of magnetic flux and the speed of the armature.     
I
Idle Current Reduction: A stepping motor driver feature that reduces the phase current to the motor when no motor motion (idle) is commanded for a specified period of time. This reduces motor heating and allows high machine throughput to be obtained form a given motor.
Incremental Motion: A motion control term that is used to describe a device that produces one step of motion for each step command (usually a pulse) received.
Inductance: The characteristic of a coil of wire or of an electrical circuit to oppose any change in state. An analogy would be inertia in a mechanical system. The unit of measurement is the henry and is equal to the production of 1 Volt EMF produced when current varies in a circuit at a rate of one amp per second.
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.     
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L
Lamination: A stamped form from electrical steel that is stacked to form the basis of a conventional DC motor armature.
Lead Screw: A device for translating rotary motion into linear motion, consisting of an externally threaded screw and an internally threaded carriage (nut).
Least Significant Bit: The bit is a binary number that is the least important, or having the least weight.
Limits: 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.
Linearity: For a speed control system it is the maximum deviation between actual and set speed expressed as a percentage of set speed.
Logic Ground: 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 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 Gain, Open: The product of the forward path and feedback path gains.

M
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.
Microstepping: An electronic control technique that proportions the current in a step motor's windings to provide additional intermediate positions between poles. Produces smooth rotation over a wide speed range and high positional resolution.
Most Significant Bit: The bit in a binary number that is the most important or that has the most weight.
Mechanical Time Constant: [ms] The time required by the motor to reach a speed of 63% of its final no-load speed from standstill.

N
NC, Numerical Control: Usually refers to any type of automated equipment or process used for contouring or positioning.
Negative Feedback: The type of feedbacks used in a closed loop system where the output value is inverted and combined with the input to be used to stabilize or improve system characteristics.
Nominal Voltage: [V DC] The voltage applied to the armature at which the nominal motor specifications are measured or calculated.
No-load speed: [rpm] The maximum speed the motor attains with no additional torque load at a given voltage. This value varies according to the voltage applied to the motor.
No-load current: [A] The current consumption of the motor at nominal voltage and under no-load conditions. This value varies proportionally to speed and is influenced by temperature.

O
Open Collector: 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.
Open-Loop System: 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 loop means no feedback.
Optically Isolate: A system or circuit that transmits signals with no direct electrical connection. Used to protectively isolate electrically noisy machine signals from low level control logic.
Oscillation: Periodic variation between two values.
Output Power: [W] The mechanical power that the motor generates based on a given input power. Mechanical power can be calculated in a few different ways. For motors, one common way is the multiplication of the output speed and torque and conversion factor.

P
PID: Proportional-Integral-Derivative. An acronym that describes the compensation structure that can be used in a closed-loop system.
PLC: Programmable Logic Controller. Also known as a programmable controller, these devices are used for machine control and sequencing.
PMDC Motor: A motor consisting of a permanent magnet stator and a wound iron-core rotor. These are brush type motors and are operated by application of DC current.
Pull-In Torque: The maximum torque at which an energized stepping motor or synchronous motor will start and run in synchronism.
Pull-Out Torque: The maximum torque that can be applied to a stepping motor or synchronous motor running at constant speed without causing a loss of synchronism.
PWM: Pulse width modulation. An acronym which describes a switch-mode control technique used in amplifiers and drivers to control motor voltage and current. This control technique is used in contrast to linear control and offers the advantages of greatly improved efficiency.
Pulse Rate: The frequency of the step pulses applied to a stepper motor driver. The pulse rate divided by the resolution of the motor/drive combination (in steps per revolution) yields the rotational speed in revolutions per second.     
Q
Quadrature: 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.

R
Ramping: The acceleration and deceleration of a motor. May also refer to the change in frequency of the applied step pulse signal.
Regeneration: The action during motor braking, in which the motor acts as a generator and takes kinetic energy from the load, converts it to electrical energy, and returns it to the amplifier.
Resistance: [Ohm] It is the measure of opposition to current flow through a given medium. Substances with high resistances are called insulators and those with low resistances are called conductors. Those in between are known as semiconductors. The unit is the Ohm. 1 Ohm is defined as the resistance between two points on a conductor when an electric potential difference of one volt applied between those points produces a current of one Amp and when that conductor is not the source of any electro motive force.
Resonance: The effect of a periodic driving force that causes large amplitude increases at a particular frequency. (Resonance frequency.)
RFI: Radio frequency interference.
Ringing: Oscillation of a system following sudden change in state.
Rise Time: The time required for a signal to rise from 10% of its final value to 90% of its final value.
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 resistive heating over a long period of time.
RMS Torque: Root mean square 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.
Rotor inductance: [µH] Inductance limits the rate at which current can build up in the windings of a DC motor. It causes current to lag substantially behind voltage thereby limiting torque output. Typically, the lower the inductance, the better.
Rotor inertia: [oz-in-sec2] This is the moment of inertia for the rotor and is influenced by rotor mass and geometry.     
S
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 step response of a system parameter to stop oscillating or ringing and reach its final value.
Shunt Resistor: A device located in a servo amplifier for controlling regenerative energy generated when braking a motor. This device dissipates or "dumps" the kinetic energy as heat.
Single Point Ground: The common connection point for signal grounds in a control wiring environment.
Slew: 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.
Slope of n-M curve: [rpm/oz-in] Describes how speed varies with torque in a DC motor. The smaller this value is the more powerful the motor.
Speed constant: [rpm/V] The speed variation per Volt applied to the motor phases at constant load.
Stall torque: [oz-in]The torque developed by the motor at zero speed and nominal voltage.
Stator: The non-rotating part of a magnetic structure. In a motor the stator usually contains the mounting surface, bearings, and non-rotating windings or permanent magnets.
Stiffness: Ratio of an applied force or torque to change in position for a mechanical system.

T
Tachometer: An electromagnetic feedback transducer which produces an analog voltage signal proportional to rotational velocity. Tachometers can be either brush or brushless.
Terminal inductance, phase to phase: [µH] The inductance measured between two phases at 1 kHz.
Terminal resistance, phase to phase: [Ohm} The resistance measured between two motor p hases. The coil temperature directly affects the value.
Thermal resistance Rth 1 / Rth 2: [K/W] Rth 1 corresponds to the value between the coil and housing. Rth 2 corresponds to the value between the housing and the ambient air. Rth 2 can be reduced by enabling exchange of heat between the motor and the ambient air (for example using a heat sink or forced air cooling.

U



V
Viscous damping factor: [oz-in/rpm] The multiplier defining the torque losses proportional to speed. These torque losses are in part due to the viscous friction of the ball bearings, as well as to the Foucault (Eddy) currents in the stator generated by the rotating magnetic field.
Voltage: [V] The force that causes a current to flow in an electrical circuit. The unit is the Volt. 1 Volt is defined as the difference of electric potential between two points on a conductor that is carrying a constant current of one ampere when the power dissipated between those points is one watt.     
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