At what power level ? A current low noise microstepping design of mine employs a pair of linear audio fullbridges. Since the rotational speed as well as the torque is close to zero, a single chip stereo amp that can be operated DC coupled was choosen as optimal solution.
Rene
About five watts. We can't go linear because of battery operation. Every milliwatt counts. We'll just roll our own PWM, then there will be peace and quiet, plus one less part to buy for the purchasing guys ;-)
Nice idea. I naively started out thinking these motors were just like a DC motor except they stepped. Little realising that any number of techniques and 'approaches' can make a world of difference to their performance. Once I'd moved from full to half to micro step, there was no turning back :)
The extension of my current modulation scheme to micro-stepping would be to include a map of torque versus position per amp for your particular motor design, to approximately unravel the nonlinearities of the magnetic structure. Stepper motors are not very good sine wave generators. They have all sorts of cogging (salient poles) and harmonic factors in their operation.
One method to improve the situation is to minimize the noise coming from the motor but that requires somewhat luxurious software handles for tweaking. Ideally the motor should just release a very faint hiss, not the usual "eeeep". However, once they asked me to introduce some "eeeep" again because they wanted acoustic feedback indicating that it's turning. Shook my head but did it. The client's wishes must be my wishes ...
That sounds like a problem with your software. ;-)
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