In that case - you could try a non-linear amplifier, with a high gain
until the magnitude of the output voltage is around 4V then reducing.
Makie the opamp feedback R of two elements, then shunt one of these
with a pair of back to back zeners of about 4V rating.  At low Vout,
the two resistors operate in series giving a high gain.   As 4V output
is reached, the diodes start to conduct, removing the contribution
from the R they are shunting, and leaving the remaining series R for
reduced gain.  The speed of the gain change can be modified with
another R in series with the diode arm

Bit hit and miss, perhaps, but it ought to be possible to compensate a
non-linear load  by a non-linear drive network.

Andy
www.g4jnt.com



2011/12/23 Stefan Schäfer <[email protected]>:
> Hi Andy,
>
> Thanks for the hint.
>
> I intended to use a hard drive since the motor does not rotate when supplied
> with < 4 V DC. So it would always stop before the resonance is reached. That
> problem may be reduced by increasing the gain of the input stage but then i
> expect oscillation problems and small currents running through the motor all
> the time...
>
> 73, Stefan
>
> Am 23.12.2011 20:24, schrieb Andy Talbot:
>
>> 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
>>>
>>>
>>
>>
>
>