Experimental Physics and
Industrial Control System

Ben Franksen via Tech-talk <tech-talk at aps.anl.gov> · Sun, 14 Feb 2021 16:42:49 +0100

Am 14.02.21 um 03:18 schrieb Mark Rivers:
> The soft device support contains this code:
> 
>             if (pepid->fbon) {
>                 if (!pepid->fbop) {
>                     /* Feedback just made transition from off to on.  Set the integral
>                        term to the current value of the controlled variable */
>                     if (pepid->outl.type != CONSTANT) {
>                         if (dbGetLink(&pepid->outl,DBR_DOUBLE,&i,0,0)) {
>                             recGblSetSevr(pepid,LINK_ALARM,INVALID_ALARM);
>                         }
>                     }
> 
> Whenever feedback make the transition from off to on the value of the
> integral term is set to the current value of the controlled
> variable.
Thanks Mark, I completely missed that. This is very nice.

>  So, for example:
> 
> - Your DAC is set to the value 1.64 V that sets the furnace to 400 C
> - You set the setpoint (.VAL) to 400 C.
> - You turn on feedback.  The integral term (I) will be set to the
> current value of the DAC, 1.64.  This means that turning on feedback
> is "bumpless".> - Now you can change the setpoint to 500 C, and it will use PID to get
there.
> - If you set the VAL to 500 before turning on the feedback it still
> behaves reasonably, since the I term is set to the DAC value for the
> current temperature, 400 C.>
> I apologize that this behavior is not documented.  The documentation
> is clearly very out of date, since it refers to MPF and not to ASYN,
> which is what it currently uses for fast-feedback.>
> Pull requests for documentation updates welcome.

I can't promise anything ATM, but I may give it a try.

Cheers
Ben
-- 
I would rather have questions that cannot be answered, than answers that
cannot be questioned.  -- Richard Feynman

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System