> From: Shawn Ellis <sellis@MERC.RX.UGA.EDU>
> >I can't see offhand how the system described could be causing this error,
> >especially if you've used an external clock source.  I wonder if you have
> >a problem caused by the measurement technique?  If you're measuring the
> >period of a single cycle, you may be seeing the effect of a bit of
> >hysteresis in the input conditioning.  Well, it's a thought.
> >
> Well, I've used the frequency counter on a fluke oscilliscope but the
> ultimate sign of error is the time between activations of an optical sensor
> I have looking at a hole in a wheel attached to the motor is 5 microseconds
> too fast.
> Also bear in mind that the 62.5 Hz square wave I described before runs this
> same setup at the correct frequency, but with more acceleration/deceleration
> variences than I'd like.
>

I'm in a very analogue mood today, so I would suggest feeding your
existing square wave into an analogue band pass centred on that freq.
I don't think the motor would notice a little 3rd harmonic distortion.
If the motor still runs fast you definitely have a problem in your measuring
equipment.

Certain analogue filters will also give sine _and_ cosine output, which
could be put to good use generating a nice rotating magnetic field:
two coils angled at 90 deg to each other.  Stick a permanent magnet
inside this and voila - a synchronous motor.

Regards,
SJH
Canberra, Australia