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Resonances

The stepper motor is an oscillatory system. Due to its construction, the stepper motor shows distinctive resonances in the lower speed range. When the motor is initiated to carry out one single full step, a dying out oscillation in the final position can be noticed. The vibration depends on the selected current, or in other words: the motor’s spring constant is proportional to the energizing current. The oscillation frequency is in accordance with the motor’s natural frequency. Activated at this natural frequency, the motor will most likely not execute a controlled movement, but will exhibit unregulated vibrations.

This tendency to oscillate is also influenced by the amplitude of the motor current change. In the full step mode, there is the maximum current change, therefore, in the full step mode the resonances in the lower frequency ranges are most pronounced.

In the worst case, these resonances could stimulate the motor to lose synchronism.

Countermeasures:

In order to restrict the effects of the resonance points in the motor frequency bands, the following measures are recommended:

1) Do not to operate the stepper motor in the full step mode.

In this mode, the current changes are most significant. Proceeding from step to step is effectively like a relatively strong stimulus to the oscillating system. Therefore, it is recommended not to operate the stepper motor in the full step mode, but in the half step or, even better, in the micro step mode (1/4, 1/8, ... 1/512). The higher the step resolution, the smoother the motor run and the lower or “softer” the undesired stimulation of the stepper motor.

2) Set the motor current as low as possible.

The oscillation intensity depends on the intensity of the stimutating impulse. It is recommended to supply the motor with the lowest current setting that also provides safe operation.

3) Quickly accelerate the stepper motor through the resonance ranges.

As far as possible, the stepper motor should always be operated away from the resonance ranges. Fast acceleration across the resonance ranges normally makes the resonance effects negligibly small.  

4) Dampen the stepper motor system.

The stepper motor system is already damped by mounting external loads and mass inertias that also shifts the resonance frequencies. The resonance behavior can be strongly influenced by modifying the mounting and suspension of the stepper motor. If even electronic damping measures, e.g. raising the step resolution, are not sufficiently (optical systems, for example), there is the possibility of fitting an external inertial damping disc at the motor shaft.

5) Modify the controller.

How strongly a stepper motor is affected by oscillations also depends on the controlling driver unit. A choppered driver normally stimulates stronger resonance effects than a linear driver would. The characteristic of the controlling current shape influences the oscillation behavior of the motor. 

 

 

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