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I'm using two stepper motors (datasheet below) to pan and tilt a platform. The bottom motor spins fine without the breadboard glued to the top platform. However, it behaves unpredictably and skips steps when the breadboard is glued in place (shown below). My question is why does this happen? Is it related to rotor inertia?

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I assume the bottom stepper shaft is affixed directly to the clear plastic platform that supports the top motor and the breadboard, is this correct?

If so, there are two possible causes of your problem. first, the rotary inertia of the assembly consisting of the top motor and the PC board on its lever arm may be too much for the bottom stepper to rotate at the speed with which you are sending it pulses. try slowing down the pulse rate.

second, the combination of board + support arm + top motor may have a torsional resonance close to the step signal frequency. trying to rotate the assembly with a string of discrete pulses may then excite the resonance and kick the torque impulses back to the bottom motor, causing it to jump between cog positions and lose steps. try hitting the bottom motor with one, two, three, ... pulses in a string and watch the assembly closely to see if it has been set into vibration by the end of the pulse string.

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