Midband Resonance in Stepper Motors

All stepper motors have some mid band resonance, sometimes you can accelerate through it unaffected, but you can't operate in this zone as running in this band will stall the motor no matter what the load is.

Mid-band resonance is caused by oscillations in the system that build up over time so great that there is no torque left an the motor is not able to take commanded step and losses synchronization, so these oscillations need to be damped like a shock absorber in motor cars.

It can be done mechanically by adding a damper on motor shaft or the load it is driving or electronically by sensing the load change and compensating for it by modifying the driving current waveform. Electronic damping is obviously best method as mechanical damping will take away useful power or torque from the system that otherwise be used for useful work.
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