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Glossary of terms and concepts

Cogging

Cogging is a resistive torque or force caused by the interaction of a magnetic field with a ferrous (magnetic, iron-containing) material, even when there is no current present. Cogging causes jerky, uneven motion in servo systems.

Because our Linear Shaft Motor contains no ferrous material, it does not experience cogging effects.

Current

Continuous current

Continuous current is current that can be supplied from the driver indefinitely.

RMS Current

RMS stands for "Root Mean Square." It is the effective average current. It is most commonly used when referring to AC current.

Peak current

The peak current refers to the maximum amount of current the driver outputs.

In non-microstepping drivers

When using a driver that only does full stepping, the rated current is the same as the peak current.

In microstepping drivers

When using a driver that is capable of taking microsteps (at a rate of a half-step, fourth-step or any other fraction of a step), the definition of peak current becomes 1.4 times the rated current. Microstepping drivers are made differently in order to maximize their ability to drive the stepper motor. Therefore, step motors can handle up to their rated current multiplied by 1.4. This will not damage the motor because the power output is approximately the same.

Encoders

There are many types of encoders. The basic function of an incremental encoder is to output signals that help the control electronics determine the speed and direction of travel of the motor. The control electronics then calculate the relative position of the motor. The basic function of an absolute encoder is to output signals that help the control electronics determine the exact position of the motor. The control electronics then calculate the relative speed and direction of travel of the motor.

Microstepping

Microstepping increases the number of steps required to move between each motor pole by controlling the phase-current ratio. Microstepping allows a motor to run more smoothly and with less noise, though it does not improve step accuracy. When microstepping, you should always stop on either a multiple of the microstep or the full step position every time. This will allow the motor to stop at a magnetic pole, which is the rotor's natural position, giving you the best accuracy.

Step accuracy is inherent in a motor's mechanical design and is controlled by the torque stiffness. Microstepping increases the number of steps required to move between each motor pole but does not increase the step accuracy. Microstepping a motor without good step accuracy will not provide smooth motion.

Motor power duty cycle for a motor

Duty cycle for a linear motor is different than for other types of systems. While it is defined as (time on) / (time on + time off) per cycle, in servo systems the motor can be on even when not in motion. Thus, for a servo motor, the duty cycle is based upon the time the motor is actually working (when current is applied) and NOT the percentage of time the motor is moving. It is possible for motor power duty to be 100 percent while the motor is not moving, or for the motor's motion duty to be nearly 100 percent with very low motor power duty.

Stepper Motor

A stepper motor uses input pulses to take proportional steps. These motors can be used for positioning and/or speed control in various applications. To change phases, steppers require power and sequence circuits.

System resonance

Although resonance frequency cannot be completed eliminated, it can be changed by altering the rotor or system inertia or by altering the torque stiffness.

Torque

Holding torque vs pull out Torque

Holding torque is the maximum torque generated to prevent the motor from moving. Pull out torque is the maximum dynamic torque that can be generated at a given speed to start the motor moving. Pull out torque varies at different speeds with different drivers and power input.

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