Experimental Physics and
Industrial Control System

Ben Franksen via Tech-talk <tech-talk at aps.anl.gov> · Sat, 13 Feb 2021 10:50:44 +0100

I have a question regarding the epid record type, which is part of
https://github.com/epics-modules/std.

According to the docs, the formula to calculate the output is

  OVAL = P + I + D
  P = KP * E(n)
  I = KP * KI * SUMi ( E(i) * dT(n) )
  D = KP * KD * ( (E(n) - (E(n-1) ) / dT(n) )

I checked that the code (in the soft device support) really does that. I
am ignoring the extra functionality that limits the output to be between
DRVL and DRVH as this is irrelevant here.

Suppose we have been at the target position for some time, i.e.
VAL=OVAL, so that E(n)=0 for all n. Then all terms become 0 and the
output is OVAL=0 as well. But shouldn't it be OVAL=VAL instead?

Either I am mising something here or this is a bug and line 171 of
stdApp/src/devEpidSoft.c should not be

            oval = p + i + d;

but rather

            oval += p + i + d;

and the formula above start with

  OVAL += P + I + D

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