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Acceleration - The change in velocity as a function of time. Acceleration usually refers to an increasing velocity magnitude while deceleration refers to a decreasing velocity magnitude.

Accuracy - The difference between the expected value of a parameter and its actual value.

Amplifier - Electronics which convert low level command signals to high power voltages and currents to operate a motor.

Angular accuracy - The measure of shaft positioning accuracy on a servo motor.

Bandwidth - The range of frequencies over which a system can faithfully respond to commands.

Back EMF - This is the voltage generated when a permanent magnet motor is rotated. This voltage is proportional to motor speed and is present regardless of whether the motor winding(s) are energized or un-energized. It is numerically equal to the torque sensitivity multiplied by a constant. It is given in volts/rad/sec.

Brushless motor - A class of motors which operates using electronic commutation of phase currents rather than electromechanical (brushes) commutation. Commutation is a function of rotor position.

Closed-loop - A system in which the output is compared to the command with the result being used to force the output to follow the command.

Coercive Force - The field intensity required to demagnetize a magnet. The units of measure are either amp-turns per inch or oersteds.

Cogging - A term used to describe non-uniform angular torque. Cogging appears as a jerkiness especially at low speeds.

Commutation - A term which refers to the action of steering currents or voltages to the proper motor phases so as to produce optimum motor torque. In brush type motors, commutation is done electromechanically via the brushes and commutator. In brushless motors, commutation is done by the switching electronics using rotor position information obtained by Hall sensors or resolver.

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

Controller - A term describing a functional block containing an amplifier, power supplies, and possibly position-control electronics for operating a brushless motor.

Current - A measure of the flow of electrons in an electrical circuit. A positive flow of current is actually the opposite direction of the flow of electrons (the direction of flow was defined before the electron was discovered). The unit of measure is the ampere.

Current at Peak Torque (Motor Winding Constants) - This is the current required to obtain peak torque from the motor. It is given in amperes.

Damping Factor - The ratio of the stall torque to the no load speed. The value is governed by the total amount of resistance in the armature circuit which must include any driving power amplifier's output resistance as well.

DC Resistance (Motor Winding Constant) - This is the DC resistance (in ohms) measured at 25^oC between the motor terminals. It is the sum of the winding and brush resistance. This resistance is usually measured at 1/3 to 1/5 of peak currents.

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.

Duty cycle - A measure of a motor's "on" time or operating time relative to "off" time or rest time. Typically expressed as a percentage of "on" time to total cycle time.

Dynamic braking - A 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.

Electrical Time Constant - The ratio of armature inductance to its resistance is the electrical time constant of a brush type motor.

Electromotive Force - The electrical force that induces current to flow in an electrical circuit. Unit of measure is the volt.

Encoder - A feedback device which converts mechanical motion into electronic signals. Usually an encoder is a rotary device which outputs digital pulses which correspond to incremental angular motion. Example: A 1000 line encoder produces 1000 pulses every mechanical revolution. The encoder consists of a glass or metal wheel with alternating clear and opaque stripes which are detected by optical sensors to produce the digital outputs.

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

Field Intensity (Magneto Motive Force) - The magnetic force that induces flux to flow in a magnetic circuit.

Flux - A measure of the flow of the magnetic lines in a magnetic circuit. The units of measure are the Weber, Maxwell and line.

Flux Density - The amount of flux in a given area. This is significant in that some ferrous (ferrous means to have the characteristics of iron) materials "saturate" at certain flux densities and this is to be avoided in most cases. The units of measure are the Gauss, Tesla and lines per square inch.

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 direction.

Hall sensors - A feedback device which is used in a brushless 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.

Inductance (Motor Winding Constant) - This is the inductance of the motor armature as measured at the motor terminals. It is given in millihenries.

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

Inrush current - The current surge generated when a DC motor is initially connected to a power source. This surge is due to the lack of armature inductance at zero speed and is limited by the motor resistance.

Maximum Theoretical Acceleration - The acceleration developed by the motor alone, from standstill, at the moment when maximum voltage is applied is the maximum theoretical acceleration in radians per second.

Moment of Inertia - The moment of inertia of the armature is measured about the motor's axis of rotation. The ratio of the motor moment of inertia to the damping factor with a zero-impedance power source gives the mechanical time constant of the motor. In direct drive systems, load inertia and damping factor have to be added to the motor inertia and damping factor to determine the mechanical time constant.

Motor Constant - This is the ability of a motor to convert electric power input to torque. This is a figure of merit that can be used to compare motors in their ability to produce torque per unit of power input. It is the ratio of torque to the square-root of the power input.

NEMA - National Electrical Manufacturer's Association. Acronym for an organization which sets standards for motors and other industrial equipment.

No Load Speed - This is the maximum speed of the motor at no load when the voltage that is required to produce peak torque is applied.

Open-loop - A system in which there is no feedback. Motor motion is expected to faithfully follow the input command.

Overshoot - The amount that the parameter being controlled exceeds the desired value.

Peak Torque - This is the maximum useful torque that can be obtained at maximum recommended current input.

Permeability - The relative ease of establishing a magnetic field in a material. A high permeability material is extremely easy to establish magnetic fields in. The units are henrys per meter.

Polarity - The tendency of bodies having opposite magnetic poles to arrange themselves so that their two extremities point to the two magnetic poles of the earth.

Power - The rate at which work is done. Mechanically, this is the torque multiplied by the speed. Electrically, this is the voltage multiplied by the current. The units are horsepower or watt.

Power at Peak Torque - This is the input power required to produce peak torque at stall and at 25^oC winding temperature.

Power Rate - The ratio of peak torque squared to inertia which is useful in applications where the acceleration of a load through a gear train is the prime consideration. An initial motor selection is made which has a power rate of at least 4 times the product of the load inertia and the load acceleration required. A gear ratio is then chosen which will match the motor and load inertia.

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 in contrast to linear control and offers the advantages of greatly improved efficiency.

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.

Reluctance - The magnetic resistance to the flow of flux lines.

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

Residual Induction - The remnant flux density that remains in a permanent magnet after it has been saturated by a magnetic field and then the field has been removed. The units of measure are the Gauss, Tesla and lines per square inch.

Resistance - The characteristic of a material to impede the flow of current in an electrical circuit. The unit is the Ohm.

Resolution - The smallest increment into which a parameter can be broken down.

Resolver - An electromagnetic feedback device which converts angular shaft position into analog signals.

Resonance - Oscillatory behavior caused by mechanical limitations.

Ringing - The oscillation of a system when a change of state occurs.

Ripple Frequency - The number of ripple cycles in one revolution of the armature is the ripple frequency. A higher frequency component caused by the brush phasing also is present, but the fundamental frequency is determined by the number of commutator bars.

Ripple Torque - A small change in torque with armature position is caused by the switching action of the commutator. The armature rotates through a small angle before its field is returned to its original position through commutation. This variation is known as ripple torque and is usually expressed in percent of torque level.

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 period of time.

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

Samarium cobalt - A type of rare-earth permanent magnet material.

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

Stall torque - The torque available from a motor at stall or 0 rpm.

Tachometer - An electromagnetic feedback transducer which produces an analog voltage signal proportional to rotational velocity. Tachometers can be either brush or brushless.

Torque - A measure of angular force which produces rotational motion. This force is defined by linear force multiplied by a radius e.g. lb.-in or oz-in.

Torque constant - A number representing the relationship between motor input current and motor output torque. Typically expressed as lb.-in/A or oz-in/A.

Torque Sensitivity (Motor Winding Constants) - This is the torque output of the motor per ampere of motor input current. It is given in ounce-inches per ampere.

Total Breakaway Torque - The friction contributed by the motor to the system determines the total breakaway torque. It is the sum of the brush-commutator friction plus the magnetic retarding torques such as hysteresis drag and slot effect drag.

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

Voltage at Peak Torque (Motor Winding Constants) - This is the voltage required to produce peak torque when the motor is at standstill and the winding temperature is 25^oC. It is given in volts.

Voltage constant (or Back EMF constant) - A number representing the relationship between Back EMF voltage and angular velocity. Typically expressed as V/Krpm.