John Larkin wrote: > What a coincidence... I've been thinking about the same problem. >
I'm mostly interested in very smooth motion at small scales, which is why I want an ironless BLDC. The gizmo's operation will require a lot of curve fitting to pull out the amplitude and phase of a small-amplitude tone burst of about 10k cycles over about 5 degrees of shaft rotation, once per rev. Any cogging or other bad behaviour of the motor will cause nasty spurious peaks in the spectrum, among other problems.
Steppers are never sufficiently well made to avoid periodic errors--I'm at the level where I have to worry about whether the ball bearings are smooth enough, or whether I need to use jewels, which would be fragile and expensive enough to dim my enthusiasm quite a bit. (A galvo is another possibility, but those cost the Earth.) My hope is that because the balls' motion doesn't have the same period as the shaft rotation, I can sort out the bearing junk from the desired signal.
In the real system, I'm expecting to have optical clues as to what the actual motor phase is, but I'm not too worried about that at this point.
I'm currently gearing up to do a sanity test with a nice Maxon brush motor from my junk box, a He-Ne, and an HP 35665A dynamic signal analyzer to do the data acq and so on. (I just got a Prologix GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data in and out.)
Cheers
Phil Hobbs