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Re: PID loop frequency

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On 10/05/2018 04:58 PM, bitrex wrote:
> On 10/05/2018 10:07 AM, Fitzgerald wrote:
>> I'm working on a digital PID motion controller. The target/goal is to 
>> move at (very) constant velocity. Position feedback is from a linear 
>> encoder with quadrature output. The input to the controller is a 
>> linear ramp (i.e. constant velocity) and the PID changes the motor 
>> output to minimize the error. The system works, but I'm looking to 
>> improve its performance at low velocities.
>>
>> At low velocities there are only a few pulses from the encoder per 
>> servo cycle. In the error signal I can clearly see this quantization. 
>> If I lower the frequency of the servo cycle; the controller 
>> performance increases (i.e. smaller velocity variations).
>>
>> Two questions:
>> - Is there a method to determine optimal servo frequency?
>> - Would it be beneficial to separate the controller in two control 
>> loops; an inner velocity loop and an outer position loop. Perhaps with 
>> both running at different frequencies?
>>
>> Any pointers to good documentation are appreciated.
>>
>> Thanks,
>> Fitzgerald
> 
> 
> I did something similar for just a "dumb" brushed DC motor controller 
> PID a while back, a cascade loop worked pretty well for me. Since your 
> goal is a constant velocity I wonder why you need a position loop? if 
> it's just that the shaft turn at a constant angular velocity of some RPM 
> then what its precise position is at any time doesn't seem like a 
> relevant state variable.
> 
> I believe I used a fast sampling inner loop, PI, to achieve coarse 
> regulation of motor torque sensed from a current shunt in series with 
> the motor. And a slower outer loop, PID, for velocity, sensed indirectly 
> from motor back EMF generated by the PWM drive switching waveform.
> 
> I found a little "D" coefficient in the outer loop goes a long way, at 
> least for my little motor.
> 
> All DC motors and servos are finally current/torque driven for whatever 
> control variable it is you want to regulate, angular position, velocity, 
> acceleration etc. so I think it's a good idea to have torque be the 
> primary state variable of interest in the process.
> 
> I recall a problem I encountered with my "dumb" motor with just a single 
> PID loop that only "watched" velocity is it that a single set of 
> coefficients which achieved good control when it's at speed are too 
> aggressive trying to bring it up out of stall. "Oh shit 0 RPM? 10 amps, 
> right now!!" except the PSU size I needed to use for the project size 
> not capable of that.
> 
> <https://controlguru.com/the-cascade-control-architecture/>
> 
> 

The stage will be used for both point-to-point moves as well as constant 
velocity operation. I needed the position loop anyway; so I started 
using that. I will implement a cascaded loop to see if that improves 
performance.

Thanks for providing the link

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PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-05 16:07 +0200
  Re: PID loop frequency bill.sloman@ieee.org - 2018-10-05 07:53 -0700
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:08 +0200
      Re: PID loop frequency bill.sloman@ieee.org - 2018-10-09 00:11 -0700
  Re: PID loop frequency Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:58 -0400
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:12 +0200
  Re: PID loop frequency bitrex <user@example.net> - 2018-10-05 10:58 -0400
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:18 +0200
  Re: PID loop frequency Chris Jones <lugnut808@spam.yahoo.com> - 2018-10-06 01:04 +1000
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:30 +0200
      Re: PID loop frequency bill.sloman@ieee.org - 2018-10-09 00:21 -0700
      Re: PID loop frequency Chris Jones <lugnut808@spam.yahoo.com> - 2018-10-09 23:03 +1100
        Re: PID loop frequency bill.sloman@ieee.org - 2018-10-09 05:44 -0700
          Re: PID loop frequency Chris Jones <lugnut808@spam.yahoo.com> - 2018-10-10 00:01 +1100
            Re: PID loop frequency bill.sloman@ieee.org - 2018-10-09 07:33 -0700
  Re: PID loop frequency Lasse Langwadt Christensen <langwadt@fonz.dk> - 2018-10-05 08:49 -0700
  Re: PID loop frequency Jon Elson <elson@pico-systems.com> - 2018-10-05 13:59 -0500
  Re: PID loop frequency Hul Tytus <ht@panix.com> - 2018-10-05 19:52 +0000
    Re: PID loop frequency Jon Elson <elson@pico-systems.com> - 2018-10-05 15:46 -0500
      Re: PID loop frequency George Herold <gherold@teachspin.com> - 2018-10-05 18:05 -0700
      Re: PID loop frequency Hul Tytus <ht@panix.com> - 2018-10-06 18:49 +0000
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:38 +0200
  Re: PID loop frequency DJ Delorie <dj@delorie.com> - 2018-10-05 16:19 -0400
    Re: PID loop frequency Fitzgerald <john.smith@mailinator.com> - 2018-10-09 08:48 +0200
  Re: PID loop frequency tabbypurr@gmail.com - 2018-10-05 14:33 -0700
    Re: PID loop frequency Jasen Betts <jasen@xnet.co.nz> - 2018-10-06 02:14 +0000
      Re: PID loop frequency bill.sloman@ieee.org - 2018-10-05 19:49 -0700
        Re: PID loop frequency Klaus Kragelund <klauskvik@hotmail.com> - 2018-10-06 01:19 -0700
          Re: PID loop frequency bill.sloman@ieee.org - 2018-10-06 03:26 -0700
            Re: PID loop frequency George Herold <gherold@teachspin.com> - 2018-10-06 11:56 -0700
            Re: PID loop frequency Klaus Kragelund <klauskvik@hotmail.com> - 2018-10-08 06:27 -0700
              Re: PID loop frequency Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-08 10:22 -0400
                Re: PID loop frequency Klaus Kragelund <klauskvik@hotmail.com> - 2018-10-09 14:46 -0700

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