Stefan..

An idea.  You have implemented a bang-bang driver with a dead zone.
Switched hard on in either direction,  or stopped close to the middle.

Change the circuit so you linearly amplify the output from the phase
detector, and make youself a bidirectional bridge driver, using say an
op-amp and NPN-PNP pair - as in audio amplifiers.  Or it may even be
possible to use an audio amplifier chip if the gain can be brought
down low enough..

Now, the motor will automatically slow as the phase error is reduced,
and you 'shouldn't' have to include any dead zone.
You may have to compensate the loop to prevent overshoot or high
frequency instabilities if there is too much mechanical lag in the
system or inertia, but that is something that can be done pure with CR
networks.

Andy
www.g4jnt.com


2011/12/23 Stefan Schäfer <[email protected]>:
> Jim,
>
> Another thought:
>
> Am 23.12.2011 12:17, schrieb James Moritz:
>>
>> [...] I guess the inertia of the motor and gearbox will produce some
>> hysteresis, moving the variometer to a very slightly over-adjusted position
>> after the motor drive is removed, which will also help to prevent the system
>> continuously searching around the correct tuning point.[...]
>
>
> This effect can be compensated by choosing the dead band treshold value.
> Then the motor gets stopped a bit before the resonance point and will
> exactly land on it. As good as having an infinitesimal small dead band ;-)
> Anyway we are talking about phase angles which are very very small, much
> below that what could become critical for a PA.
>
> 73, Stefan/DK7FC
>