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PID loop frequency

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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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#518563

Thanks Jon.

Hul

Jon Elson <elson@pico-systems.com> wrote:
> On Fri, 05 Oct 2018 19:52:00 +0000, Hul Tytus wrote:

> > A question regarding terms: what is a "linear encoder with quatrature
> > output" that produces "pulses"?
> There are things called linear encoders which produce a pair of square 
> wave signals that are 90 degrees out of phase with each other.  The phase 
> relationship allows you to determine direction of motion as well as 
> distance.  The signals are identical to rotary quadrature encoders.
> The 90 degree phase relationship is the definition of quadrature.

> The individual signals consist of pulses.

> Jon

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#518796

On 10/05/2018 09:52 PM, Hul Tytus wrote:
> A question regarding terms: what is a "linear encoder with quatrature
> output" that produces "pulses"?
> 
> Hul
> 

The encoder has two 50% duty cycle square wave outputs A and B. 
Depending on the direction of travel the A output either leads or lags 
the B output by 90 degrees. E.g. clockwise is AB = 10 > 11 > 01 > 00 and 
counter-clockwise is AB = 10 > 00 > 01 > 11.

Each edge/pulse corresponds to a fixed distance traveled by the stage. 
Hence, the frequency of the square wave is a measure for its velocity.

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#518457

Fitzgerald <john.smith@mailinator.com> writes:
> I'm working on a digital PID motion controller.

Isn't everyone?  ;-)

> - Is there a method to determine optimal servo frequency?

In my case, the pid loops ran at the PWM 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?

I ended up with three PID loops:

* the innermost one managed the current through each phase of the
  motor.  I.e. this is a torque loop.

* the next one is the RPM loop

* the outermost one is the position loop.


For high RPM, you can drive the RPM loop directly.  For low RPM, you can
calculate the desired position vs time and drive the position loop
instead.  It will lag a bit according to the holding torque, but follow
the desired RPM.

I can usually keep the motors within a few counts of the desired
position, at 4000 counts/rot.

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#518797

On 10/05/2018 10:19 PM, DJ Delorie wrote:

> I can usually keep the motors within a few counts of the desired
> position, at 4000 counts/rot.
> 

At which RPM is that? I have about 50k pulses/s input from the encoder; 
so that's equivalent to 750 RPM at 4000 counts/rot.

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#518469

On Friday, 5 October 2018 15:07:48 UTC+1, 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

Cassette deck motors solved this a while ago. IIRC they used a bit of negative resistance, no PID. You could switched-mode it.


NT

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#518498

On 2018-10-05, tabbypurr@gmail.com <tabbypurr@gmail.com> wrote:
> On Friday, 5 October 2018 15:07:48 UTC+1, 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
>
> Cassette deck motors solved this a while ago. IIRC they used a bit of negative resistance, no PID. You could switched-mode it.

The ones I dissasembled when I was doing learn by destroying had
centrifugal switches on the rotor.  but a recent thread on SX had a
circuit which did the negative resistance thing (for a record player).
if you cancel the resistance of the windings you can command the
back-emf, and thus the speed, directly.



-- 
  Notsodium is mined on the banks of denial.

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#518502

On Saturday, October 6, 2018 at 12:31:10 PM UTC+10, Jasen Betts wrote:
> On 2018-10-05, tabbypurr@gmail.com <tabbypurr@gmail.com> wrote:
> > On Friday, 5 October 2018 15:07:48 UTC+1, 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
> >
> > Cassette deck motors solved this a while ago. IIRC they used a bit of negative resistance, no PID. You could switched-mode it.
> 
> The ones I dissasembled when I was doing learn by destroying had
> centrifugal switches on the rotor.  but a recent thread on SX had a
> circuit which did the negative resistance thing (for a record player).
> if you cancel the resistance of the windings you can command the
> back-emf, and thus the speed, directly.

As NT has pointed out, Philips was doing that thirty years ago. I put it into a circuit I put together back in 1992 and it worked fine. You do have to kill off the negative resistance at high frequencies, but that was obvous enough that I anticipated it at the design stage.

-- 
Bill Sloman, Sydney

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#518518

Talking about digital control, this has been bothering me:

Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us

Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system

That’s how the classical books determine the phase corruption 

But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?

Cheers

Klaus

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#518522

On Saturday, October 6, 2018 at 6:19:11 PM UTC+10, Klaus Kragelund wrote:
> Talking about digital control, this has been bothering me:
> 
> Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us
> 
> Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system
> 
> That’s how the classical books determine the phase corruption 
> 
> But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?

The saving grace in motion control is that moving parts have inertia. You have to keep track of where they are, but they do move pretty smoothly at the millisecond level, and getting them to change velocity takes quite a lot of joules.

-- 
Bill Sloman, Sydney

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#518566

On Saturday, October 6, 2018 at 6:26:38 AM UTC-4, bill....@ieee.org wrote:
> On Saturday, October 6, 2018 at 6:19:11 PM UTC+10, Klaus Kragelund wrote:
> > Talking about digital control, this has been bothering me:
> > 
> > Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us
> > 
> > Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system
> > 
> > That’s how the classical books determine the phase corruption 
> > 
> > But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?
> 
> The saving grace in motion control is that moving parts have inertia. You have to keep track of where they are, but they do move pretty smoothly at the millisecond level, and getting them to change velocity takes quite a lot of joules.
> 
> -- 
> Bill Sloman, Sydney

I think that's right, as long as you're sampling faster 
than the time constant of the system, your model should give
a good guess at the next position, and you adjust on the 
difference.  (I do mostly analog loops.)

George H.

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#518712

On Saturday, October 6, 2018 at 12:26:38 PM UTC+2, bill....@ieee.org wrote:
> On Saturday, October 6, 2018 at 6:19:11 PM UTC+10, Klaus Kragelund wrote:
> > Talking about digital control, this has been bothering me:
> > 
> > Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us
> > 
> > Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system
> > 
> > That’s how the classical books determine the phase corruption 
> > 
> > But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?
> 
> The saving grace in motion control is that moving parts have inertia. You have to keep track of where they are, but they do move pretty smoothly at the millisecond level, and getting them to change velocity takes quite a lot of joules.
> 
> -- 
True, but I was thinking about power supplies, where the crossover frequency is typically above 1kHz, so a 1000us delay would cause a 360 degrees phase lag, which of course is not acceptable ;-)

Cheers

Klaus

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#518716

On 10/08/2018 09:27 AM, Klaus Kragelund wrote:
> On Saturday, October 6, 2018 at 12:26:38 PM UTC+2, bill....@ieee.org wrote:
>> On Saturday, October 6, 2018 at 6:19:11 PM UTC+10, Klaus Kragelund wrote:
>>> Talking about digital control, this has been bothering me:
>>>
>>> Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us
>>>
>>> Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system
>>>
>>> That’s how the classical books determine the phase corruption 
>>>
>>> But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?
>>
>> The saving grace in motion control is that moving parts have inertia. You have to keep track of where they are, but they do move pretty smoothly at the millisecond level, and getting them to change velocity takes quite a lot of joules.
>>
>> -- 
> True, but I was thinking about power supplies, where the crossover frequency is typically above 1kHz, so a 1000us delay would cause a 360 degrees phase lag, which of course is not acceptable ;-)
> 

Switching to absolute encoders with continuous PWM readout (US Digital
and others) makes the problem go away.

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
https://hobbs-eo.com

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#518843

On Monday, October 8, 2018 at 4:22:43 PM UTC+2, Phil Hobbs wrote:
> On 10/08/2018 09:27 AM, Klaus Kragelund wrote:
> > On Saturday, October 6, 2018 at 12:26:38 PM UTC+2, bill....@ieee.org wrote:
> >> On Saturday, October 6, 2018 at 6:19:11 PM UTC+10, Klaus Kragelund wrote:
> >>> Talking about digital control, this has been bothering me:
> >>>
> >>> Say we have a PWM controller running at 100kHz switching frequency, so the period is 10us
> >>>
> >>> Say we then have a digital loop that samples the voltage at 1kHz and when it samples it calculates new PWM duty in 1us, the we will have 36 degrees phase lag introduced by the sampling system
> >>>
> >>> That’s how the classical books determine the phase corruption 
> >>>
> >>> But the loop is blind for a full 1000us before it actually sampled again, so shouldn’t the phase corruption (phase lag introduced by digital loop), be much more than 36 degrees?
> >>
> >> The saving grace in motion control is that moving parts have inertia. You have to keep track of where they are, but they do move pretty smoothly at the millisecond level, and getting them to change velocity takes quite a lot of joules.
> >>
> >> -- 
> > True, but I was thinking about power supplies, where the crossover frequency is typically above 1kHz, so a 1000us delay would cause a 360 degrees phase lag, which of course is not acceptable ;-)
> > 
> 
> Switching to absolute encoders with continuous PWM readout (US Digital
> and others) makes the problem go away.
> 
Encoders are not used in switch mode power supplies :-)

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