Problems with Autotuning for a servo control

What I want: A Autotuning for a servo control

The Process: The control signal is the current of the motor. The motor has an encoder which is used for the field oriented control and additionaly for position control.

The control loop: there is one PID controler for the position control. The Encoder delivers the real position and the error is calculated and input to the PID. The PID outputs the requested current for the motor.

The used Autotuning method: I make a step response experiment. 10% of the max. current is given to the motor and I sample the encodercounts as distance. This process has a PI1-behaviour: The distance increases quadratic until the max speed is reached and the the slope is constant.

With the values slope of the curve, intersection of the tangent with the time axis and the control input to the process its possible to get PID Parameters with the method of CHIEN-HRONES und RESWICK. This procedure delivers also Parameters for integrating processes, like this one.

The Problem: The Parameters for the PID are extremly weak, so no good control is reached. If I tune the PID manually, then I get a proportional part, which is 100-times stronger. Similary happens to the I and D part.

Question: Whats the reason? I am sure I calculated the parameters correct. Who knows the reason and what kind of procedure delivers better results?

Best regards


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First, consider generating a model using ARMAX system identification and designing your controller from that.

First-and-a-half, verify your tuning in some simulation environment like Matlab or Scilab, to make sure your algorithm really does work the way you think it should.

Second, try asking on sci.engr.control -- you may get some help there.

Third, plan on doing some legwork on your own -- autotuning knowledge tends to be closely held, because any controller manufacturer that manages to tune and tweak it to work has a competitive edge.

Tim Wescott
Wescott Design Services
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Tim Wescott

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