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.

**Air Gap: **

A low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.

**Anisotropic Magnet:**

A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one vector direction.

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.

This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.

**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, H _{c}:**

The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, (B), to zero after the magnet has previously been brought to saturation.

**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.

**Curie Temperature, T _{c}**

_{:}

The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetization

**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.

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^{o}C 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.

**Demagnetization Curve:**

The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.

**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.

Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs, which should be accounted for or minimized.

**Electrical Time Constant:**

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

**Electromagnet:**

A magnet, consisting of a solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.

**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.

**Energy Product**:

Indicates the energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve. Calculated as B_{d} x H_{d}, and measured in Mega Gauss Oersteds, MGOe.

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

**Ferromagnetic Material: **

A material whose permeability is very much larger than 1 (from 60 to several thousand times 1), and which exhibits hysteresis phenomena.

**Field Intensity** **(Magneto Motive Force):**

The magnetic force that induces flux to flow in a magnetic circuit.

**Flux **

The condition existing in a medium subjected to a magnetizing force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The cgs unit of flux is the Maxwell.

**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.

**Fluxmeter:**

An instrument that measures the change of flux linkage with a search coil.

**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.

**Fringing Fields:**

Leakage flux particularly associated with edge effects in a magnetic circuit.

Lines of magnetic flux per square centimeter, cgs unit of flux density, equivalent to lines per square inch in the English system, and Webers per square meter or Tesla in the SI system.

**Gaussmeter:**

An instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following: the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

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.

**Hysteresis Loop:**

A closed curve obtained for a material by plotting corresponding values of magnetic induction, B, (on the abscissa) against magnetizing force, H, (on the ordinate).

The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units.

**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.

**Intrinsic Coercive Force,** **Hci:**

Measured in Oersteds in the cgs system, this is a measure of the materials inherent ability to resist demagnetization. It is the demagnetization force corresponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

**Intrinsic Induction, Bi:**

The contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relationship is expressed as: Bi = B-H.

**Irreversible Loss:**

Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by re-magnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.

**Isotropic Magnet:**

A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetized in any direction without loss of magnetic characteristics.

A piece of soft iron that is placed on or between the poles of a magnet, decreasing the reluctance of the air gap and thereby reducing the flux leakage from the magnet.

**Knee of the Demagnetization Curve:**

The point at which the B-H curve ceases to be linear. All magnet materials, even if their second quadrant curves are straight line at room temperature, develop a knee at some temperature. Alnico 5 exhibits a knee at room temperature. If the operating point of a magnet falls below the knee, small changes in H produce large changes in B, and the magnet will not be able to recover its original flux output without re-magnetization.

That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gaps, and is therefore unable to be used.

**Length of air-gap, L**_{g}**: **

The length of the path of the central flux line in the air-gap.

**Load Line: **

A line drawn from the origin of the Demagnetization Curve with a slope of -B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see Permeance Coefficient.

An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.

**Magnetic Flux:**

The total magnetic induction over a given area. When the magnetic induction, B, is uniformly distributed over an area A, Magnetic Flux = BA.

**Magnetizing Force, H: **

The magnetomotive force per unit length at any point in a magnetic circuit. Measured in Oersteds in the cgs system.

**Magnetomotive Force, F:**

Analogous to voltage in electrical circuits, this is the magnetic potential difference between any two points.

**Maximum Energy Product, BHmax:**

The point on the Demagnetization Curve where the product of B and H is a maximum and the required volume of magnet material required to project a given energy into its surroundings is a minimum. Measured in Mega Gauss Oersteds, MGOe.

**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.

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

**North Pole: **

That pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often best to clarify by using "north seeking pole" instead of "north pole" in specifications.

**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.

A cgs unit of measure used to describe magnetizing force. The English system equivalent is Ampere Turns per Inch, and the SI systems is Ampere Turns per Meter.

**Open-loop:**

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

**Orientation Direction: **

The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties. Also known as the "axis", "easy axis", or "angle of inclination".

**Overshoot:**

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

A material having a permeability slightly greater than 1.

**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.

**Permeance Coefficient, Pc:**

Ratio of the magnetic induction, Bd, to its self demagnetizing force, Hd. Pc = Bd / Hd. This is also known as the "load line", "slope of the operating line", or operating point of the magnet, and is useful in estimating the flux output of the magnet in various conditions. As a first order approximation, Bd / Hd = Lm/Lg, where Lm is the length of the magnet, and Lg is the length of an air gap that the magnet is subjected to. Pc is therefore a function of the *geometry* of the magnetic circuit.

**Pole Pieces:**

Ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.

**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^{o}C 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.

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

**Relative Permeability:**

The ratio of permeability of a medium to that of a vacuum. In the cgs system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the cgs system.

**Reluctance, R:**

Analogous to resistance in an electrical circuit, reluctance is related to the magnetomotive force, F, and the magnetic flux by the equation R = F/(Magnetic Flux), paralleling Ohm's Law where F is the magnetomotive force (in cgs units).

**Remanence, Bd: **

The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remanence will be less than the residual induction, Br.

**Repeatability:**

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

**Residual Induction, Br**_{:}** **

This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.

**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.

**Return Path: **

Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux.

**Reversible Temperature Coefficient:**

A measure of the reversible changes in flux caused by temperature variations.

**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.

A type of rare-earth permanent magnet material.

**Saturation:**

The condition under which all elementary magnetic moments have become oriented in one direction. A ferromagnetic material is saturated when an increase in the applied magnetizing force produces no increase in induction. Saturation flux densities for steels are in the range of 16,000 to 20,000 Gauss.

**Search Coil:**

A coil conductor, usually of known area and number of turns that is used with a fluxmeter to measure the change of flux linkage with the coil.

**Settling time:**

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

**Stabilization:**

Exposure of a magnet to demagnetizing influences expected to be encountered in use in order to prevent irreversible losses during actual operation. Demagnetizing influences can be caused by high or low temperatures, or by external magnetic fields.

**Stall torque:**

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

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

**Temperature Coefficient:**

A factor, which describes the change in a magnetic property with change in temperature. Expressed as percent change per unit of temperature.

**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.

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^{o}C. 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.

The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.