On Wed, 09 Mar 2005 08:33:05 +0000, Peter wrote:
....
However, superimposed onto the DC control signal (-10V to +10V or so)
I saw a sinewave ripple, around 500Hz IIRC. The amplitude was only
about 100mV P-P.

The question is whether this is intentional, whether e.g. the servo
uses the presence of this ripple as a confirmation that the computer
unit is still (partially) functioning. It would be quite a clever
trick, but it does mean that it isn't possible to fit a good filter
onto the servo input.

It could very well be intentional - I once worked with a servo system
where they intentionally superimposed a triangle wave on the control V -
they called it "dither". It kept the hydraulic actuator from sticking in
place, so it could move smoothly.

Hope This Helps!
Rich