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Re: PID loop frequency

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On 10/05/2018 05:04 PM, Chris Jones wrote:
> On 06/10/2018 00:07, 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
> 
> Maybe do reciprocal counting - measure the time between transitions of 
> the encoder, and from this time difference generate a velocity estimate 
> (and run the PID) on every edge from the encoder, with variable timesteps.
> 

I'll see if I can implement and test this easily; may be worth a shot 
but I don't expect much of it.

Measuring the time between the encoder edges doesn't seem a good 
solution as the time between encoder edges varies due to manufacturing 
errors in the encoder, see [1].

[1] https://www.embeddedrelated.com/showarticle/444.php

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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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