motorised pots

[Tony Allgood]

JH said:
If you find a good (analogue) control circuit, please tell me.
An opamp that compares the set voltage with the momentary
potentiometer voltage, and thus drives the motor forward and backward,
would be all that's needed. But designing the right time constant, and
probably a little tolerance window where the motor doesn't move,
might be some amount of work.

I designed the motorised sliders on the Soundcraft desks... not easy.
Especially if the load on the motor changes with position or batch. We used
a DAC in the feedback loop which controlled the loop gain. This way the desk
could be programmed with all the different loop gains needed for each
individual pot. Bear in mind as the pots wear out they get sticky and this
has to lead to updated values of loop gain. This was done automatically
everytime the desk was powered up. However, if your motorised pots are good
ones and you are not to bothered with speed you could get away with a single
op-amp servo loop. Switch off the power to motor when the value is reached
to save energy.
For circuit ideas try looking at the various hi-fi's that have motorised
volume pots.

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at enterprise.net

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