Re: ePID deadband on output

[Bruce Hill <bhill@slac.stanford.edu>]

There's no need to use the same time constant for both, and
you're right that they shouldn't be coupled.  The feedback loop
should be independent.   However, as Matt pointed out, using
a simple dead band doesn't allow the PID to settle on the target.
- Bruce

On 08/25/2014 03:38 PM, fong@triumf.ca wrote:
> But the "I" in the PID is already an IIR filter.
>
> Putting additional filter inside a feedback loop is tricky.  Too short a
> time constant make the filter ineffective, too long a time constant makes
> the loop unstable.
>
> The motor itself could be another integrator. Having 2 integrators and
> addition phase lag from an IIR filter in a feedback loop is not a good
> idea.
>
> Dead band with hysteresis is much simpler.
>
> Ken
>
> > We have a similar PID controlling a stepper motor.
> >
> > Since we don't have a amplifier for the stepper motor I actually
> > think we probably could allow the motor deadband to handle this,
> > but the "owner" of the stepper motor is more comfortable with
> > us shutting it off while we're close to the target position.
> >
> > Instead of using time within the deadband to turn ours on and
> > off, we use an IIR filter similar to the smoothing algorithm in
> > the AI record with an OFF threshold which can be quite small
> > so we don't shut off till we're centered, and an ON threshold
> > which turns the PID back on when it drifts too far.
> >
> > Time still enters into this calculation via the time constant of
> > the IIR filter factor, but you can ensure your PID gets closer
> > to the target position.
> >
> > Regards,
> > - Bruce
> >
> > On 08/25/2014 10:24 AM, Pearson, Matthew R. wrote:
> > > Hi Folks,
> > >
> > > I initially tried setting MDEL on the ai record that provides the
> > > pressure readback (which is the input to the ePID). I found that the
> > > ePID only controlled the output (the stepper motor) so that the pressure
> > > sat at the low or high limit of the MDEL, depending on which side it was
> > > approached from. So I needed some time factor to take the settling time
> > > into account (to reach the center of the deadband).
> > >
> > > The motor record has deadbands for the monitors (MDEL and ADEL), but not
> > > on the input (except for the NTM logic to deal with backlash
> > > compensation).
> > >
> > > On Aug 25, 2014, at 11:14 AM, "Dalesio, Leo" <dalesio@bnl.gov> wrote:
> > >
> > > > Time can be captured in the integral term of the ePID. If that term
> > > > continues to push the change up, it will exceed the position.
> > > > Is that the argument for putting it in the ePID? We don't integrate the
> > > > error if it is inside the deadband? Is this the only term that does not
> > > > care inside a given deadband?
> > > The output would be completely off (FBON=Off) if we have been within the
> > > deadband for N seconds. It would only turn on again (and reset the
> > > timer) if it drifts outside the deadband.
> > >
> > > The idea is that the controlled variable may sit within the deadband for
> > > minutes or even hours without any further control needed (which in my
> > > case means switching a stepper motor amplifier on and off).
> > >
> > > Cheers,
> > > Matt
> > >
> > >
> > >
> > > > This looks like a good discussion for either the code-a-thon or a side
> > > > meeting at the EPICS meeting.
> > > > ________________________________________
> > > > From: Mark Rivers [rivers@cars.uchicago.edu]
> > > > Sent: Monday, August 25, 2014 11:03 AM
> > > > To: Dalesio, Leo; 'Benjamin Franksen'; tech-talk@aps.anl.gov;
> > > > 'Core-Talk'
> > > > Subject: RE: ePID deadband on output
> > > >
> > > > That's an interesting point.
> > > >
> > > > The motor record already has a deadband.  Matt, did you try just using
> > > > the deadband on the motor record?
> > > >
> > > > I don't think it has the concept of the time inside the deadband
> > > > though.
> > > >
> > > > Mark
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: Dalesio, Leo [mailto:dalesio@bnl.gov]
> > > > Sent: Monday, August 25, 2014 9:51 AM
> > > > To: Mark Rivers; 'Benjamin Franksen'; tech-talk@aps.anl.gov;
> > > > 'Core-Talk'
> > > > Subject: RE: ePID deadband on output
> > > >
> > > > Does the deadband belong on the ePID or the motor record?
> > > > It seems like more of an output issue - not an algorithm issue.
> > > > Bob
> > > > ________________________________________
> > > > From: tech-talk-bounces@aps.anl.gov [tech-talk-bounces@aps.anl.gov] on
> > > > behalf of Mark Rivers [rivers@cars.uchicago.edu]
> > > > Sent: Monday, August 25, 2014 10:39 AM
> > > > To: 'Benjamin Franksen'; tech-talk@aps.anl.gov; 'Core-Talk'
> > > > Subject: RE: ePID deadband on output
> > > >
> > > > Sorry for the delay in replying, this seems good to me as well.
> > > >
> > > > Mark
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: tech-talk-bounces@aps.anl.gov
> > > > [mailto:tech-talk-bounces@aps.anl.gov] On Behalf Of Benjamin Franksen
> > > > Sent: Monday, August 25, 2014 5:14 AM
> > > > To: tech-talk@aps.anl.gov; 'Core-Talk'
> > > > Subject: Re: ePID deadband on output
> > > >
> > > > [cross-posting to core-talk]
> > > >
> > > > On Thursday 14 August 2014 18:13:15 Pearson, Matthew R. wrote:
> > > > > I'm using the ePID record to control a needle valve filling a cryostat
> > > > > with helium. The needle valve is controlled by a stepper motor. The
> > > > > ePID is giving good results, but I found the need for a deadband on
> > > > > the PID output. This is to avoid continually driving the motor to and
> > > > > fro by just a few steps. I implemented it via set of database
> > > > > records, and included a timer to tell me how long the helium pressure
> > > > > has been within a deadband, and then I disable the PID output (by
> > > > > setting FBON).
> > > > >
> > > > > However, I'm wondering if's a common enough use-case to be implemented
> > > > > in the ePID record. It would mean adding some additional fields,
> > > > > something like:
> > > > >
> > > > > DBS - the deadband size
> > > > > DBTS - settling time in the deadband (to avoid sitting on the edge of
> > > > > the deadband) DBT - time in seconds since we entered the deadband
> > > > > window
> > > > > DBIN - binary flag to indicate we are in the deadband
> > > > >
> > > > > If ABS(CVAL-VAL)<=DBS, and DBT>=DBTS seconds, then disable FBON.
> > > > >
> > > > > If ABS(CVAL-VAL)>DBS then reset DBTS and enable FBON.
> > > > >
> > > > > And to maintain current behavior, keep DBS=0 (the default).
> > > > >
> > > > > The database version is working fine, but it's quite a few records and
> > > > > I suspect it would be much simpler in the soft device support.
> > > > Sounds like a pretty good idea to me.
> > > >
> > > > Cheers
> > > > Ben
> > > > --
> > > > "Make it so they have to reboot after every typo." ― Scott Adams
> > > >
> > > >
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> > --
> > Bruce Hill
> > Member Technical Staff
> > SLAC National Accelerator Lab
> > 2575 Sand Hill Road M/S 10
> > Menlo Park, CA  94025
> "The beaver told the rabbit as they stared at Hoover Dam. 'No, I didn't
> build it myself. But it's based on an idea of mine.'"
> Charles Townes
--
Bruce Hill
Member Technical Staff
SLAC National Accelerator Lab
2575 Sand Hill Road M/S 10
Menlo Park, CA  94025