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Glossary Terms & Definitions

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Acceleration: A change in velocity as a function of time.  Acceleration is increasing velocity, while deceleration is decreasing velocity.

Acceleration Feedforward: In a velocity loop, acceleration feedforward acts to inject additional torque/force to compensate for delays inherent during high rates of change in velocity.

Accuracy : A measure of the difference between expected or programmed position and actual position of a motor or mechanical system.  Motor accuracy is usually specified as an angle in degrees or arc minutes representing the maximum deviation from expected position.

Actuator: In a servo system, any device (such as a motor) that outputs mechanical motion using electricity as the power source. While a motor can technically be referred to as an actuator mostly used to describe a device that delivers motion in a linear fashion.

Alternating Current: An electrical power format in which the electricity is supplied with a sinusoidal waveform. It can be delivered in both single and three phase. In the US the standard values are 120V and 208V single phase and 208V, 230V, 460V three phase.

Ambient Temperature: The temperature of the cooling medium typically air, immediately surrounding a motor or another device.

Ampere: The basic unit of measure for electric current, also known as amps. Designated by an I in formulas.

Amplifier: In a motion control system this is the component that follows the command from a controller and provides power to the motor. In the strictest sense, amplifiers operate in current mode where the output current is proportional to the input command signal. In the motion control industry, the terms amplifiers and drives are often interchanged.

Analog: Relating to, using signals or information represented by a continuously variable physical quantity such as spatial position or voltage.

Angular Accuracy : The measure of shaft positioning accuracy on a servo or a stepping motor.

Axis: The components that control each degree of freedom in a machine or robot can be considered an axis. An X-Y-Z machine is a three axis machine where the X and Y axes control movement in the horizontal plane and the Z axis controls vertical motion. Each axis can consist of a controller, drive, motor, and transmission components necessary to couple to the load.

Back EMF: The voltage generated when a permanent magnet motor is rotated.  This voltage is directly proportional to motor speed and is generated regardless of whether the motor winding(s) are energized or de-energized.

Backlash: The play caused by loose connections between mechanical components. Backlash becomes a issue when an axis reverses direction. When a motor turns a gearhead it pushes all the gears together in one direction. When the motor reverses direction, the gear teeth separate from one side and contact the other side. The distance of the separation is the backlash usually measured in Arc minutes.

Balance Pot: A potentiometer used to set the output of a drive so it doesn't have an offset. This balances the output so that one direction is not favored over the other.

Bandwidth: Bandwidth is a measurement of the overall responsiveness of a system. The higher the bandwidth the faster and more responsive it is. Bandwidth can be measured by first determining the output of a slow sine wave command, then increasing the frequency of the sine wave until the output has decreased by a multiple 0.707. The resulting frequency is the bandwidth.

Breakaway Torque: The torque required to start a motor in motion.  Breakaway torque almost always exceeds the running toque.

Brushless Motor: Any motor that operates by using electronic commutation of phase currents, instead of electromechanical (brush-type) commutation.  Brushless motors typically have a permanent magnet rotor and a wound stator.

Bus Voltage: The voltage level of the main power supply of a system.

Capacitance: The measurement of how much charge can be stored in a capacitor. Capacitance is measured in Farads (F). Since 1F is a very large amount of capacitance, a more common unit is the microfarad or 1/1000000 of a Farad.

Capacitive Coupling: The transfer of energy from one circuit to another by means of the mutual capacitance between them. In feedback and control systems this is considered to be electrical noise and is a common problem. Shielding of wires and filters are used to limit capacitive coupling.

Capacitor: A device that stores an electrical charge.

Closed Loop: A broadly applied term, relating to any system in which the output is continually monitored and compared to the input.  The output is then adjusted to maintain  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 preset parameters.

Cogging (Cogging Torque): A term used to describe non-uniform angular velocity.  Cogging appears as a jumpy motion, especially at low speeds.


Commutation: A term which refers to the action of directing currents or voltages to the proper motor phases so as to produce optimum motor torque.  Proper commutation means the relationship of the Rotor to the Stator must be known in real time.
1.  In brush type motors, commutation is accomplished electromechanically via the brushes and commutator.
2. In brushless motors, commutation is done by the switching electronics utilizing rotor position information obtained by Hall sensors, a single turn absolute encoder, or a resolver.

Continuous: Constant or unending. Can refer to torque or current ratings of components.

Control Transformers (Synchro): Have 3-phase stators of medium to high impedance which are excited by other synchros, Transmitters or Differentials, establishing electrically a directional field whose heading or angle is detected by the proper null output of the single phase winding on a cylindrical rotor.  Of course cylindricalness and perfect symmetry are essential to minimize or avoid distortions of the stator established directional field, i.e. C.T. calibration error.  C.T.’s with their impedances and cylindrical rotors should not generally be used as Transmitters to other C.T.’s without wider tolerances on system accuracy, null levels, and phase shift variations than that obtainable in Transmitter- C.T. systems.

Controller: A broad term describing a for a device containing an amplifier power supplies, and possibly position-control electronics for operating a servomotor or step motor.

Current: The flow of electric charge

Current Limit Potentiometer: A potentiometer that controls the output current limit of a drive or amplifier.

Current Loop: The current loop regulates the current in most servo drives. The current loop is the innermost loop and the foundation for high performing velocity and position loops. Since motor torque is proportional to motor current the current loop is also referred to as the torque loop.

Current Mode: A drive mode where only the current loop is active. In current mode, the velocity loop and position loop are either disabled, non-existent or closed outside of the drive.

Current at Peak Torque (IPK) (Amperes): The amount of input current required to develop a “peak Torque”. This often exceeds the linear torque/current relationship.

Current, Rated: The maximum allowable continuous current a motor can accept without exceeding motor temperature limits.

DC: Direct Current. A current source that is constant.

DC Shunt: Usually a calibrated low value resistor used to measure currents that are too high for a volt meter. Also a power resistor used to draw excess voltage off of a power supply.

Derivative: The slope or rate of change in a function. The derivative of position is velocity. The derivative of velocity is acceleration.

Derivative Gain: The derivative gain effects the damping on a system. It determines the contribution of restoring force proportional to the rate of change (derivative) of position error. This force is much like the viscous damping in a shock absorber. A tuning function in a servo system.

Detent: The magnets in a motor create points that the motor favors, and points that the motor tries to stay away from, these are called detents. As a disconnected motor is turned, the detent points can be felt by hand. The less detent there is, the smoother the motor will be when running.

Detent Torque: The maximum torque that can be applied to an un-energized step motor without causing continuous rotating motion.

Digital: Expression of discrete numerical values. Digital components communicate and interact using the 1's and 0's of binary code.

Differential (Synchro): Synchro differentials are the Synchro analog of their mechanical namesake. Units consist of 3-phase stators and 3-phase cylindrical rotors.  In servo control systems, the stator primary is excited by the stator voltages of a Transmitter and the rotor leads are wired to those of another stator, most often those of a C.T. and occasionally another Differential in a synchro chain.  Sometimes a Differential is used to provide a pointer-dial readout of two Transmitters between which it is connected.  In this mode it is subject to the end-play and friction factors of Receivers.  Differentials must be double-tested for electrical accuracy.  The stator calibration error is that observed by locking the differential rotor and looking at the rotor output relative to an electrically rotating stator input; and the rotor calibration is the output error in minutes as a function of different rotor positions.

Differential Resolvers: A resolver that has a three-phase cylindrical rotor and a two-phase stator. The construction is the inverse of a transolver and its uses are the same- it can be used either as a CG or a CT with no loss in accuracy.  The advantage of the differential resolver over the transolver lies in the fact that the outputs not picked off of slip rings, thus reducing noise and making four-wire outputs less costly.

Direct Drive: When the motor is directly coupled to a load with no gearing or other mechanical transmission.

Directional Inhibit: A type of inhibit that disables motion in one direction. Typically used near the end-stops on a machine. This can be used to prevent a load from crashing into the end-stop while allowing the drive to apply power to reverse direction. They can also be used to ensure that a drive does not reverse direction on one-way machines.

Drive: In motion control this component follows the command from a controller and provides power to the motor. A drive can operate in current mode, velocity mode or position mode. It can be commanded by many means such as analog signals, step and direction, encoder following and through network commands among others. Also referred to as an amplifier.

Driver: A device that controls or regulates another device. For example, gate driver controls the output of an IGBT power device, or a line driver can be used to boost an encoder signal.

Dual Loop: A control scheme where velocity and torque feedback is on the motor (1st loop) and position feedback is on the load (2nd loop). Dual loop control increases accuracy at the load and, compensates for system backlash and stabilizes the system.

Duty Cycle: For any continuously repetitive operating cycle, the ratio of on time to total cycle time. Duty cycle (%) = [On time/ (On time + Off time)] x 100%

Dynamic Braking: A passive technique for stopping a permanent magnet brush or brushless motor.  To accomplish the braking the motor windings are shorted together through a resistor which results in motor braking with an exponential decrease in speed.

EEPROM: Electrically Erasable Programmable Read Only Memory. Non-Volatile Memory that can be erased and reprogrammed.

Electronic Gearing: Mechanical gears can be replaced with electronic gearing where a motor will follow another motor, but at a fraction or some multiple of the first motor’s speed.

Efficiency: The ratio of power output to power input.

Electrical Resolvers: In the Synchro family, resolver is the name given to units used for the vector addition and also to resolve a vector representing voltage into its orthogonal components. The classic application is to solve the unknowns of a right triangle. As a vector adder, single frequency, sinusoidal voltages are applied, generally, to the 2-phase windings of the stator establishing a resultant field within which a 2-phase servo rotated rotor is turned to produce a null on one winding. The output of the other rotor winding represents the magnitude of the resultant, and the physical rotor angle is the direction. 

Electrical Time Constant (te) (Seconds): The time required for current to reach 63.2% of its final value for a fixed voltage level. Can be calculated from the relationship te+L/R where L is inductance (henries) and R is resistance (ohms).

Encoder: A feedback device which converts mechanical motion into electronic signals to provide both velocity and positional feedback. The most common type are incremental rotary encoders, which produce output digital pulses corresponding to incremental angular motion. For example, a 500-line encoder produces 500 pulses every mechanical revolution. The encoder consists of a glass or metal wheel with alternating transparent and opaque stripes, a light source shining through the transparent stripes is detected by optical sensors to produce the digital outputs.

Encoder Following: A master slave control scheme where the encoder from the master is used to command a second axis (slave). This is a useful feature when it is desired to have one axis follow a second such as in a gantry or conveyor. This feature can be combined with electronic gearing.

Feedback: A signal, either analog or digital which is transferred from the output of a system back to the input for use in a closed loop system. Feedback is the measurement of the parameter that is being controlled. For a positioning system to accurately compensate for an error, the actual position must be known relative to the commanded position. In this case, position feedback would be used to provide the actual position

Feedforward: Feedforward control provides corrective action before the disturbance affects the output.

FET: Field Effect Transistors are used  as the output power devices in PWM drives.


Ferrite: A ceramic material with magnetic properties. Used in suppression cores to reduce electronic noise emissions.

Filter: A hardware component, or some software or firmware that removes unwanted portions of a signal.

Firmware: Firmware is the programming of a device that has been permanently or semi-permanently stored. Firmware is commonly stored in flash or EEPROM memory which both have the ability to keep their information, even when powered off. It is also easy to re-program flash and EEPROM memory when new firmware updates become available. When used in devices that are frequently powered off, firmware allows these devices to turn on again without having to be re-programmed.

Floating Ground: Any electrical ground that is not referenced to earth ground. Floating grounds can make dangerous situations if not handled correctly. For example, when 120VAC is rectified into DC, the ground is considered to be a floating ground. Any attempt to tie the rectified AC ground to earth ground would likely blow a breaker or a fuse. The solution in this case is to first isolate the AC using a transformer before rectification.

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

Four Quadrant: Refers to a motion system which 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 decelerate in either CW or CCW direction.

Friction: A resistance to motion caused by contact with a surface.  Friction can be constant with varying speed (Coulomb friction) or proportional to speed (Viscous friction).

Gain: Gain is a multiplier between an input and an output. It is defined as the output divided by the input. Unity gain occurs when the output equals the input. A gain of 5 causes 1V input to result in 5V output. A gain of 0.5 causes 2V input to result in 1V output. Units are also associated with gain values. In a velocity mode servo drive the gain could be 1000(rpm/Volt). This means 1V input will result in 1000rpm output.

Hall Sensor: A type of feedback device which is used in a brushless servo system to provide rotor position information for the amplifier to electronically commutate the motor. The device uses a magnetized wheel and sensors to generate commutation signals.

Hardware: The physical components in a device like resistors and capacitors and IC chips.

Hertz: A unit of frequency in units of 1/s. 60Hertz is 60/s or 60 times per second. 5kHz is 5,000 times/second.

Hz: The abbreviation for Hertz.

Holding Torque: Often referred to as 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 when comparing motors.

Horsepower: A Unit of measure of Power.  One horsepower is equal to 746 watts. (W=V*I)  Horsepower is a measure of a motor’s torque and speed capability.
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

I/O: The abbreviation for Inputs and Outputs. '24V I/O' means the inputs and outputs operate at 24V. Other versions may use 5VDC or 115 VAC  

IGBT: Insulated Gate Bipolar Transistor. Used as the output power devices in PWM drives. IGBT's are good for high voltage and high current drives.

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 align with those of the other.

Inductive: An electric property that is related to the resistance to change in current.

Inductor: A coiled wire usually wrapped around an iron core. Inductors and the property of inductance are very important to the operation of PWM drives. An inductor resists change in current. This property is useful since it keeps the current from rising too quickly when the PWM is ON and it keeps the current from falling too quickly when the PWM is OFF. The net effect is that it stabilizes the current loop by making the current more manageable and predictable.

Inhibit: A logic input on a servo or stepper drive that disables the output power. The inhibit line is used to turn off the drive's output while the drive is powered. Often employed in e-stop conditions or when the control system detects improper operation.

Input: A contact that senses the voltage state of a line. Inputs can be either digital or analog.

Integrator: A mathematical function that determines the area under a curve.

Integrator Gain: Integrator gain uses the integrator function to determine the output of a gain stage. This is a time based gain where the magnitude of the output is determined by how much error there is and how long the error has occurred.

Interpolate: To calculate values between two intermediate points. Linear interpolation connects the points with straight lines with no smoothness. 3rd order interpolation uses smooth curves between points.

Inertia: The property of an object to resist change in velocity unless acted upon by any outside force.  Higher inertia objects require larger torques to accelerate and decelerate.  Inertia is dependent upon the mass and shape of an object. Inertia should always be considered when sizing motors for motion systems.

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.

INV: INV causes the functionality of the Inhibit lines to be inverted on servo drives.

IO: Another abbreviation for Inputs and Outputs.

IR Compensation Mode: This mode is a 'pseudo-velocity' mode because the actual motor velocity is not measured. The velocity loop is closed by estimating motor velocity by using the motor terminal voltage and the motor current. An increase in the motor current is interpreted as the motor slowing down because of an increase in load. Positive feedback into the velocity loop acts as compensation. IR compensation mode can cause the system to become unstable because of the positive feedback, therefore one must be cautious when adjusting the gains.

Kd: The abbreviation for derivative gain, a servo tuning function.

Ki: The abbreviation for Integral gain,a servo tuning function.

Kp: The abbreviation for proportional gain, a servo tuning function.


Legacy: Legacy products are older products that are no longer manufactured. Our website features some legacy products as our staff knows how to propose the proper replacement product.

Loop Gain: Loop gain refers to the velocity loop on analog servo drives.

Low Pass Filter: A filter that only allows frequencies below a set value to pass. Commonly used to filter the input command and feedback signals to reduce annoying high frequency buzzing and oscillations that can affect system operation.

Microstepping: A step motor drive technique in which each motor step is divied into spaller steps. Common resolutions divide each step by 10, 25,50 and 256 microsteps per step. While these re the most common many resolutions are available.

MOSFET: Metal-Oxide Semiconductor Field-Effect Transistors are used as the output power devices in PWM drives. MOSFET's are good for lower voltage and lower current drives.

MTBF: Mean Time Before Failure is a measurement of the expected reliability of a component or device. Typically measured in number of hours or years, it is how long the average device will continue to operate before it fails. The MTBF is often estimated by adding up the individual failure rates of all the components used in the device. This method is popular but can be grossly inaccurate.

Negative Inhibit: A type of inhibit that disables motion in one direction. Typically used near the end-stops on a machine. This can be used to prevent a load from crashing into the end-stop while allowing the drive to apply power to reverse direction. Can also be used to ensure that a drive does not reverse direction on one-way machines.


Negative Logic: In electronics, logic levels are indicated with voltage levels. In negative logic, the 'true' condition is indicated by the lower voltage and the 'false' condition is indicated by the higher voltage. In 0-5V logic, 0V would be a 'true' and 5V would be a 'false'. In (-5)-0V logic, (-5V) would be a 'true' and 0V would be a 'false'.

Noise: Any unwanted signals that interfere with normal operation.

Notch Filter: A filter that targets a specific frequency or range of frequencies and removes it. Can be used to remove a frequency that cause a harmonic vibration of some part in a system.

NRE: Non Recurring Engineering During the process of designing a custom part or system we will sometimes charge an NRE to help pay for the cost of development. NRE charges only occur once at the time of development.

Open-loop: A system in which there is no feedback. Motor motion is expected to faithfully follow the input command. Most stepping motor systems are open-loop control.

Overload Capacity: The ability of a drive to sustain currents above its continuous current rating. It is defined by NEMA as 150% of the rated full-load current for “standard industrial DC motors” for one minute.

Peak Torque (Tpk) (lb-in): The maximum torque a brushless motor can deliver for short periods of time.  Operating a permanent magnet motors above the maximum torque value can cause demagnetization of the rare-earth magnets as excessive current is required to do so.  This is an irreversible effect that will alter the motor characteristics and degrade performance.  Some motors give the rating as  peak current.  Not to be confused with system peak torque, which is often determined by amplifier peak current limitations, where peak current is typically two times continuous current.

Poles: 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 Loop: The position loop regulates the position of the motor or system. The position loop uses a feedback device such as an encoder to ensure that the actual position is equal to the commanded position. The position loop will correct position errors by applying more torque to bring the system back into the commanded position.

Positive Inhibit: A type of inhibit that disables motion in one direction. Typically used near the end-stops on a machine. This can be used to prevent a load from crashing into the end-stop while allowing the drive to apply power to reverse direction. Can also be used to ensure that a drive does not reverse direction on one-way machines

Positive Logic: The opposite of Negative Logic. In electronics, logic levels are indicated with voltage levels. In positive logic, the 'true' condition is indicated by the higher voltage and the 'false' condition is indicated by the lower voltage. In 0-5V logic, 0V would be a 'false' and 5V would be a 'true'. In (-5)-0V logic, (-5V) would be a 'false' and 0V would be a 'true'.

Pot: Abbreviated name for potentiometer.

Potentiometer: A variable resistor to control the voltage output. Can mechanically be either rotary or linear. Potentiometers use a contact brush along a resistive surface to create variable resistance.

Power: The rate at which work is done. Energy per unit of time, E/t. Common units of measurement are: Watts (W) and Horse Power (HP).  In motion control, power is equal to torque multiplied by speed.
Power (watts) = force x distance/time.  Power = voltage x current

Power Factor: Ratio of true power (kW) to apparent power (kVA).

Power Supply: The source that supplies voltage and current to another device or system.

Proportional: A mathematical function that relates two variables with a fixed multiplier.


Proportional Gain: Proportional gain gives an output that is directly proportional to the error signal.

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

Pulse Width Modulation (PWM): 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 is the most common control technique. By changing the Pulse Width (duty cycle) the output voltage and current can be controlled.

PVT: Position Velocity Time. In PVT mode a controller sends Position, Velocity and Time signals to a servo drive.

Ramping: The acceleration and deceleration of a motor. So named for the sloped side of a trapezoid.

Receivers (Synchro): Receivers have a lie excited salient pole rotor free to turn within a 3-phase stator electronically connected to the corresponding stator leads of the driving Torque Transmitter allowing the receiver to follow the transmitter. Receivers feature free end-play (.002-.005”) and an internal means of damping the rotor’s tendency to oscillate or even spin when subjected to transients.  Despite high torque gradients the equilibrium position of Receiver rotors is a torqueless one and the electrical accuracy is obscured by the friction of bearings and brushes.

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. This occurs when the motor torque is in the opposite direct of the motor velocity. Care must be taken to make certain the voltage being returned does not become excessive and cause an over voltage shutdown of the amplifier. Resistors are often used to limit the voltage.

Ref Gain: Reference Gain. The Reference Gain is the first gain stage after the reference input or command input. This gain adjusts the overall gain of the amplifier.

Reference Gain Potentiometer: On a servo amplifier the reference gain is typically adjusted using the Reference Gain Potentiometer.

Regen: abbreviation for regeneration

Resolution: The smallest increment into which a parameter can be broken down.  For example, a 500 line encoder has a resolution of 1/500 of a revolution.

Resolver: A high resolution feedback device that is an alternative to encoders. Resolvers are suited to harsh environments such as high temperatures and severe vibration since they do not rely on optical sensors or glass disks which can fail in these conditions. Like synchros, resolvers utilize inductive coupling to determine rotor position. Since resolver signals are not discrete, resolution is determined by the interface circuitry in the servo amplifier or servo controller. Typical resolutions range from 1000 to 65000 counts / revolution.

Resonance: Oscillation of a system following a sudden change in state.

RMA: Return Materials Authorization. Servo systems Co. uses RMA numbers to track returns. An RMA number can be obtained from the sales department or from anyone in customer service.

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. The RMS for some common waveforms can be calculated using a multiplier and the amplitude. For sine waves the multiplier is 0.707, square waves 1, and triangle waves 0.577.

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.

Rotor: The moving part of a motor, In a brushless motor it consists of the shaft and magnets.  These magnets are analogues to the field winding of a brush-type DC Motor. In a brush type motor it consists of the shaft and windings around an iron core.

Run Away: Also known as going open loop. In control systems, Run Away occurs when feedback is lost or inverted. When there is no feedback the system will try to add more power. The end result is a system that is outputting full power.

Scaling: A ratio that relates one thing to another. In velocity mode of a servo amplifier the scaling could be 1000rpm/V. Meaning for 1V input on the command the motor velocity would be 1000rpm. A 5V input with the same scaling the motor velocity would be 5000rpm.

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

Servo: The control of motion. Servo control, which is also referred to as "motion control" is used in industrial processes to move a specific load in a constantly correcting controlled fashion.

Servo Amplifier: The servo drive is the link between the system controller and a motor. Also referred to as servo amplifiers, their job is to translate the low energy analog or digital reference signals from the controller into high energy power signals to the motor.

Servo Drive: Synonym of Servo Amplifier.

Servo Systems Co.: Your number one choice for motion control components.

Set Speed: The speed at which the system is commanded or programmed.

Shunt: An alternative path for current flow.

Shunt Regulator: A device that clamps the power supply voltage from exceeding rated values. Shunt regulators are used to solve regeneration problems.

Shunt Resistor: A power resistor used in a shunt regulator. The shunt regulator directs excess energy into the shunt resistor to dissipate excess voltage.


Single Phase: AC power delivered over two wires. Usually either 115 volts or 208 volts. Single phase AC has more voltage ripple and less output power compared with three phase AC power.

Sinking Input (NPN): An input where the direction of current flows into the input. Sinking inputs are compatible with sourcing outputs.

Sinking Output (NPN): An output where the direction of current flows into the output. Sinking outputs are compatible with sourcing inputs. A sinking output works by pulling low a line that is pulled up with a pull up resistor.


Sinusoidal: A wave form in shape of a sine wave. In motor commutation instead of the six distinct states of trapezoidal commutation, sinusoidal commutation uses smooth transitions from state to state. This typically reduces overall torque and speed by approximately 20%. When viewing a complete cycle or cycles the result is a sine-wave pattern.

Software: Computer programs that make hardware work.

Sourcing Input (PNP):  An input where the direction of current flows out of the input. Sourcing inputs are compatible with sinking outputs. A sourcing input works by internally pulling the input line high through a pull-up resistor. The output however is able override this voltage and pull it low.

Sourcing Output (PNP): An output where the direction of current flows out of the output. Sourcing outputs are compatible with sinking inputs.

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

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

Stator: The non-moving part of the motor.  Specifically, it is the iron core with the wire winding in it that is pressed into the frame shell.  The winding pattern determines the voltage constant of the motor.

Step and Direction: A type of command that uses two digital lines to control motion. One line is used to increment position or velocity y inputting a pulse train, the other line is used to indicate direction by either a high or low state of the input. While it was originally developed to control stepper motors some servo amplifiers can accept step and direction inputs.

Stepper: A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The most common units move 1.8 degree per pulse (200 Steps per revolution) . Special drive circuits can divide each step resulting in resolutions of up to 51,200 discrete steps per revolution.

Stiffness: 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.

Supply: The source that supplies voltage and current to the drive. Related Terms: power supply, supply.

Suppression Core: Used for electrical noise suppression. Suppression cores are made out of a ceramic ferrite material. They have inductive properties that target specific frequencies to attenuate (reduce) electrical noise without adversely affecting performance. Related Terms: ferrite, suppression core.

Switcher: Typically refers to PWM switching to control current, as opposed to linear control.

Tachometer: A velocity feedback device that outputs a voltage that is proportional to rotary velocity. Can be a stand alone component or an integral part of a motor.

Tachometer Mode: A velocity mode that uses tachometer feedback to close the feedback loop.

Three Phase: AC power delivered over three wires. Typical voltages are 230 VAC and 460 VAC. Three phase AC power has lower voltage ripple and more power when compared to single phase AC power.

Toroid: A geometrical shape similar to a doughnut. Ferrite suppression cores are often in the shape of a toroid.

Torque: 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.  Formula: Torque (lb-ft.) = 5,250 x HP/RPM.

Torque Loop: The current loop regulates the current in a servo amplifier. The current loop is the innermost loop and the foundation for high performing velocity and position loops. Since motor torque is proportional to motor current the torque loop is also referred to as the current loop.

Torque Constant (KT = lb-ft./A): An expression of the relationship between input current and output torque.  For each ampere of current, a fixed amount of torque is produced.
NOTE: Torque constants ARE NOT linear over the operating range of a motor.  They apply best at ~75% of no load maximum speed or where the peak and continuous torque curves meet.

Torque-to-inertia Ratio: Defined as the motor’s holding torque divided by the inertia of its rotor. The higher the ration, the higher a motor’s maximum acceleration capability will be.

Transconductance: The relationship of voltage to current in an electronic device. In a servo system, transconductance specifically refers to a current mode drive. The value of the transconductance is the gain in units of A/V. A drive with a gain of 5A/V would output 25A with a 5V input command.

Trap: Abbreviation of trapezoidal.

Trapezoidal: A type of commutation for brushless motors. The waveform of each commutation point is shaped like a trapezoid Trapezoidal commutation directs current through the three motor phases in 6 steps per electrical cycle.

Transmitters (Synchro): Transmitters have a single phase salient pole rotor mechanically positioned with respect to a symmetrical 3-phase stator. Low impedance and relatively high power capabilities characterize Torque Transmitters used to drive synchro receivers, or combinations of Differentials and Control Transformers. Control Transmitters use less exciting current and they are most frequently connected to a Control Transformer or, less often, to a Differential and a C.T. in series. Tuning - To adjust the gains in a control loop to optimize performance.

Turnkey: A system or product that has been fully configured, programmed, installed and is ready for operation.


Under Damped: In tuning, the response is under damped when there is overshoot or oscillations in the response. Could also be described as system being jumpy.

Unstable: As it pertains to servo systems a system not able to hold a commanded value because of poor tuning.


V: The unit of electric potential. The volt is the electrical equivalent of fluid pressure or the mechanical force.

VDC: Abbreviation for Volts DC.

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

Voltage Constant (KE) (V/kRPM eak, 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.

Velocity Feedforward: In the position loop, velocity feedforward acts to inject additional velocity to compensate for delays inherent during high rates of change in position. Related Terms: acceleration feedforward, feedforward, velocity feedforward.

Velocity Loop: The velocity loop is the control loop that regulates velocity.

Velocity Mode: A servo drive configured for velocity mode turns the motor at a velocity that is proportional to the command.

Volt: SI unit of electric potential. The volt is the fluid equivalent of pressure or the mechanical equivalent of force.

Voltage: The strength of an electric potential expressed in volts.

Voltage Mode: A servo drive mode where the output voltage is directly proportional to the input command.


Watt: SI unit of power.