On Tue, 15 Sep 1998, Stefan Sczekalla-Waldschmidt wrote:

> But I4ve 2 more problems:
>
> The Moror is a series-wound-type ( mdrklin ) so how to get backemf
> without a constant field.

By making a constant field ;) The stator should be polarized all the time
for best results, but that means a 2nd PWM switch.

imho, the best idea is a reflective opto coupler. Alternately, if there is
a ferromagnetic gearwheel in the gearing, use a SMT Hall sensor. This is
real easy to use, you'd need a small magnet glued to it and a pole piece
to close the field behind it. You also need a little 'L'-bent piece of
soft steel sheet in front to act as concentrator (read one tooth as
opposed to 2 or more). The signal is conditioned by a 358, one section is
a differential voltage amp, the 2nd a self-biased comparator. The airgap
needs to be <= 1/2 tooth pitch for good results.

hope this helps,

Peter