Experimental Physics and
Industrial Control System

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" <[email protected]> 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 [[email protected]]
Sent: Monday, August 25, 2014 11:03 AM
To: Dalesio, Leo; 'Benjamin Franksen'; [email protected]; '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:[email protected]]
Sent: Monday, August 25, 2014 9:51 AM
To: Mark Rivers; 'Benjamin Franksen'; [email protected]; '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: [email protected] [[email protected]] on behalf of Mark Rivers [[email protected]]
Sent: Monday, August 25, 2014 10:39 AM
To: 'Benjamin Franksen'; [email protected]; '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: [email protected] [mailto:[email protected]] On Behalf Of Benjamin Franksen
Sent: Monday, August 25, 2014 5:14 AM
To: [email protected]; '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


________________________________

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.

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Geschäftsführung: Prof. Dr. Anke Rita Kaysser-Pyzalla, Thomas Frederking

Sitz Berlin, AG Charlottenburg, 89 HRB 5583

Postadresse:
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--
Bruce Hill
Member Technical Staff
SLAC National Accelerator Lab
2575 Sand Hill Road M/S 10
Menlo Park, CA  94025

Replies:

Re: ePID deadband on output fong

References:

Re: ePID deadband on output Benjamin Franksen

RE: ePID deadband on output Mark Rivers

RE: ePID deadband on output Dalesio, Leo

RE: ePID deadband on output Mark Rivers

RE: ePID deadband on output Dalesio, Leo

Re: ePID deadband on output Pearson, Matthew R.

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