Glossary

Common Motion Control Words and Equations:

Acceleration
The change in velocity as a function of time. Acceleration usually refers to increasing velocity and deceleration to decreasing velocity.

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

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

Back EMF
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 unenergized.

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

Cogging
A term used to describe non-uniform angular velocity. Cogging appears as a jerk in motion, especially at low speeds.

Commutation
A term that 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 a resolver.

Detent Torque
The maximum torque that can be applied to an unenergized motor without causing continuous rotating motion. This value is negligible in slotless motors.

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

Efficiency
The ratio of power output to the power input.  

Feedback
This is a device that is located on the motor that provides motor position and velocity information to the controller or drive. Common examples of these devices are resolvers, incremental encoders, and absolute encoders.

Inertia
A measure of an object's resistance to a change in velocity. The larger an object's inertia, the larger the torque required to accelerate or decelerate it. Inertia is a function of the shape and size of the motor's rotor.  

Inertial Match
The ratio of the reflected inertia of the load is equal to the rotor inertia of the motor. The larger the ratio between the load and the motor's inertia the slower response out of the system. Good ratios with high response times lie between 1 and 5 with 10 being an average system. 

Kit Motors
A partial motor consisting of just the rotor and stator. The feedback device, mounting and bearings are handled and supplied by the purchaser. This option is great for OEMs that require reduced costs, weight and have the resources to design this style of motor into a project. 

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

PID
Proportional-Integral-Derivative. An acronym that describes the compensation structure used in a closed-loop system. Compensation can be done using P, PI, or PID.

Profile Move
A velocity or position move that is made up of time segments consisting of acceleration, constant speed, deceleration and dwell. Some examples of a profile move are triangular, trapezoidal, S-Curve, and step response. Below is an example of a trapezoidal move. 

*Photo from Motion Control Tips. HERE

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.

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

Resolution
The smallest increment into which a parameter can be broken down. For example, a 2,048 pulse per revolution (PPR) can be broken down into 1/2,048 of a revolution. With incremental encoders and the PAC drive, you have the option of using the quadrature signal (rising and falling edge of channel A) to get 4 times the resolution. The resolution would then be 8,192 counts per revolution (CPR) or 1/8,192 of a revolution.

Resonance
Oscillatory behavior occurring in a mechanical system.

RMS Current (Arms)
Root Mean Square Current. In an intermediate 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. Our values used in datasheets for motors and drives are expressed in Arms.

              Irms [Arms] = Io-peak * 0.707

Rotor
The rotor is the portion of the servo motor that rotates and transmits torque.
For example, in a brushless servo motor permanent magnets are attached to the rotor. 

Servo Motor
is an electromechanical device that produces torque and velocity based on the supplied current and voltage regulated by a feedback device. A servo motor works as part of a closed-loop system providing torque and velocity as commanded from a servo controller utilizing a feedback device to close the loop. A servo motor can be linear or rotary.

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

Slotless Stator Technology
The stator of a slotless brushless servo motor has no iron teeth to hold the windings. The removal of the iron teeth yields significant motor performance advantages including smooth no cogging motion, high torque and power density, higher efficiency.

Slotted Stator Technology
The stator of a slotted brushless servo motor has iron teeth to hold the windings. The iron teeth take up space resulting in less space to add windings. The teeth also create attraction between the rotor magnets and the stator that produces cogging torque.

Stall Torque
The torque available from a motor at stall or 0 RPM.

Stator
The stator is the stationary part within a servo motor that does not move.
For example, in a brushless servo motor, the stator houses copper windings and iron laminations that create a magnetic field.  

Stiffness 
The ability to resist movement induced by an applied torque.

Torque Constant (Kt)
A number representing the relationship between motor input current and motor output torque. Typically expressed by lb-in/Arms or Nm/Arms.

        Kt [Nm/Arms] = Ke [V/Rad/s] * SQRT(3)

Torque-to-Inertia Ratio
Defined as the motor's holding torque (stall) divided by the inertia of its rotor. The higher the ratio, the higher a motor's maximum acceleration capability will be.

Voltage Constant (Ke) 
A number representing the relationship between back EMF voltage and angular velocity. Typically expressed in V/Rad/s or V/kRPM.