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36138Re: [SeattleRobotics] Re: Robotic control algorithms

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  • Peter Balch
    May 9, 2008
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      Dave

      > In the real world, the D term is low pass filtered for frequencies
      > above the loop response. The effect is to cap the D term gain for
      > frequencies above the loop response.

      I hadn't realised that. I guess it's obvious when someone points it out.

      I knew that D was sensitive to noise but I'd always been wary of filtering
      it because filters introduce delay and delay causes oscillation.

      > > To start with, there is a delay in the system of 1/20 sec at 20 Hz.
      > I took a guess from your statement: "Here is a plot of a PD velocity
      > controller, showing the controller variables and output for
      > 6 seconds at 20 Hz, 120 samples, for my SR04 robot."

      (That was dpa's statement. And graphs and robot.)

      > I jumped to the conclusion that the 20 Hz was the servo pulse update
      > rate. I hope my parachute opened.

      Ah. I was searching for something subtle in the graphs.

      > > If you remove the D term, check the overshoot and ringing.
      > > I bet it will go up.

      I just realised something. You were suggesting in your previous post that "a
      conventional D term cannot compensate for this [20mS] delay". Therefore the
      D term isn't there for that purpose. It's a velocity controller so D is not
      there because of momentum. It must be there simply for "balancing the I and
      D terms". So if you remove both I and D then you wouldn't expect extra
      overshoot and ringing.

      And why can't D compensate for a fixed delay?

      Peter
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