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I am not a mechanical engineer (I am ECE). At work, we have a stepper motor which is used to tighten or loosen a 'tuning system.' When this motor turns one way it 'squeezes' this thing we call a cavity (think of it as a metal cylinder). When it goes the other way it lessens the squeeze or loosens up the pressure on the cavity. This cavity is under helium pressure which changes (slowly) over time.

We are observing a phenomenon that when the stepper motor moves that sometimes very faint vibrations can be observed. If we move the stepper a few more steps one way or the other, the vibrations go away. The vibrations appear to be coming and going at random. We only care about vibrations that occur once the motor stops moving.

Assuming that the motor itself is not causing the vibrations (i.e. the motor is not shaking or oscillating), what could be causing the oscillations? My thought is that we are hitting sweet spots where the squeeze placed on the cavity compared to the pressure that the cavity is under is somehow causing the oscillations. Is this crazy?

Again, I am not a mechy or civil engineer, but I do remember in school the one example of the bridge that when the wind would blow over it, it would shake and break. My thought is that somehow we are lining up with some kind of mechanical resonance point where the pressure from inside the cavity and the pressure from the squeeze that the motor is applying is causing this resonance. Does this sound plausible? What tests could I do to better understand this?

Thanks! :)

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Without more info about the step motor drive controller, it is hard to say. But here are a few things to look into.

Most importantly, you must measure the oscillation frequency, and then compare it to 1) the clock rate of the microprocessor that controls the motor, 2) the sampling rate of the positional feedback sense system (if so equipped), and 3) the nominal step rate of the pulses being sent to the motor windings. If the oscillation frequency is either a match or an integer submultiple of any of these things, then there is an instability in the motor control system which is causing it to dither at endpoint conditions.

Then, measure the amplitude of the oscillation and compare it to the step resolution of the step motor and/or the shaft position encoder (if so equipped). if the oscillation amplitude is equal to the spacing between successive quadrature positions of the stepper armature or a multiple thereof, or of successive quadrature output states of the position encoder, then you definitely have an endpoint state positional control system instability i.e., an interaction between your test apparatus and the motor control firmware.

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