abbey road d enfer said:
Unfortunately I don't have a copy to hand. I don't think this was published on the web. If I find a copy in the attic I'll let you know !
Long time since I was doing this now - 90s.
I also looked for it on the website but nowhere to be found.
I think the circuit was developed as an example for manufacturers etc.
I don't know if Curtis Wright can supply it or an equivalent now ?
From memory the circuit is relatively simple - 'Parallel'PID implemented around a single amplifier stage. The 'gain expansion' less complex in implementation than might be imagined. basically 2 x 3V3 Zener Diodes back to back making the 'resting voltage ' approx 3V3 one way or the other.
I think it gives a smoother response overall. Considering that a 'long fader move' is, in automation terms, a series of short fader moves then it means that the string tends to be under tension for the whole move rather than losing tension along the way then starting off with a 'jolt' as the driving voltage rapidly increases from zero. Bear in mind that there is a minimum voltage at which the motor will significantly operate - eg you don't get anything really at, say, 2V. So the driving voltage would increase rapidly from zero giving the PID 'more to do' to control it as the motor starts to move the fader.
Of course this depends on sample rate and mechanical friction and other factors.
As for changing direction. When the fader has stopped the string will be tensioned but it's sort of random which way ! Depends on state of PID drive when fader stops - whether it corrected any overshoot etc. So it might give a smoother start or the string may need to transition through no tension then tension the opposite way.
In use the fader response is good. BUT I have to add the caveat that this was test bench stuff in the factory and not on an in use desk after mechanical wear etc.
I'm not sure if you mean tactile feel but for manual operation / takeover the motor is turned off via touch detection so the feel is then down to the fader itself together with the mechanical resistance of the string / pulley / motor - independent of the electronics.
Yes - mechanical wear will affect the system and might get to exceed the PID capabilities.
Time to clean the faders
Might have misunderstood what I wrote re SSL faders.
Yes - better damping reduces / eliminates 'bouncing'.
IIRC the P&G circuit exhibited better characteristics in this regard and the 'stops' were less abrupt than the SSL circuit. IIRC I monitored this by both measurements off the wiper and current waveforms into the motor.
Also the 'gain expansion / tension' function was different which affects track wear.
Basically the P&G circuit was 'softer'.
Admittedly this implies a less time accurate response but I'd say that in reality the original manual fader moves are relatively slow wrt the system response.
SSL was not digital solution.
The background to this was that SSL sent some faders etc back to the factory for investigation as they had reports of excessive fader wear.
This was back when recording the Oscilloscope images was a matter of an adaptor on a Polaroid camera so I don't have any images to post :-\
As for the micro based system. Well the algorithm wasn't good enough for real use with short moves etc.
Faster micro might be advantageous in terms of sampling rate etc (I can't remember what was used) but it's not something I'm looking to resolve now !
If I were I might be looking to something like a dsPIC controller which might be better suited to the computation requirements.
From memory I used Intel 80C196 at the time.
I'll mention that I'm referring to P&G faders with the bigger Swiss made vertically mounted motor rather than the smaller horizontally mounted motor shown in the OP post so details may be different but the general ideas apply.
Hope this helps.
