Hi,

if we try to sort out the many noise possibilities:

1. mechanical the wanted steps need some acceleration and deceleration of inertias, these are secured against unwanted rotation by the magnetic spring-like action.

This spring like action - together with inertia is a mechanical oscillator that needs damping for good performance or micro-stepping (low excitation) as suggested above.

Damping may be done nonlinear by pulsing with first and last pulse changed in position or shape for best performance.

Also mechanical: poor mechanical make of motor, magnetic vibration of stacked iron and of coils, both excited by magnetic forces.

2. electromagnetic: most steppers are operated with a pulsed constant current supply - but frequencies are in-between 20 to 200 KHz, so not really within our hearing range, but inter-modulation of two of these may generate unwanted and disturbing (to our ear) difference frequencies.

electromagnetic: the wanted pulses are like steps in a stair, what do you feel if you yourself are hopping down or up with some additional Kg on your back or where else?

So smooth the steps: micro-stepping or noise canceling by distributing the acceleration of one pulse into two of half the amplitude and time lag to cancel the excited vibration of the first half-pulse by the second half-pulse.

3. Feeding back motor noise (back-emf) into the electronics with crosstalk to influence the other motors. Make the shielding of your motor cabling according to recommendations of manufacturer.

4. Coupling of outside em noise into your system: look at proper shielding and grounding. Make one and one only earth connection. Avoid any earth loops. Think about how to avoid capacitively coupled noise: this current shall be carried to the ground by the shield. Think about inductively coupled currents and voltages: avoid any conductor loops, use twisted pairs.

5. Consider your outside world: any high voltage or high current lines nearby? Also nearly never operated pulsed lines may play havoc. (Laser firing we had).

6. What about high power systems in the MHz to GHz range nearby? Some very peculiar responses can be seen with the frequency of rotation of the big radar antennas on ships. Not so easily identified if it is the testing of some ECM system or the next AWACS or any similar system, installed in the metallic birds above us. (I am near a military airport).

RHABE