Experimental Physics and
Industrial Control System

"Pal Trivan" <[email protected]> · Wed, 27 Feb 2013 14:58:36 +0100

Hello all,

A humble opinion/word: in the literature, for the 'standard' form (cf 'parallel') it is 'ok' as is in the EPID. Note, however, the caution in the in-line comment of the code that Ki is actually 1/Ki w.r.t (same) literature.

The tuning of the parameters, as given in the example of the epid record manual, works well with tools such as mathworks 'pidtune', when all the above are taken into account.

Hope this helps a bit.

Sincerely,

Trivan
PSI
GFA/Controls

________________________________

Von: [email protected] im Auftrag von Mark Rivers
Gesendet: Mi 2/27/2013 2:33
An: Bruce Hill
Cc: [email protected]
Betreff: RE: Question about epid record

Hi Bruce,

As I recall I copied that logic from the old PID record from EPICS base.  I agree that KP should probably not be in the I and D terms, but I think it's too late to change it, since users have tuned their EPID systems using this formula.

Mark

________________________________
From: Bruce Hill [[email protected]]
Sent: Wednesday, February 27, 2013 12:36 AM
To: Mark Rivers
Cc: [email protected]
Subject: Question about epid record

Hi Mark,
I'm in the process of replacing a genSub based PID controller with
the epid record that you and Bob developed, and I'm puzzled by a difference
in how the two compute the I and D terms.

In the classical PID algorithm, the value of M(n), the manipulated variable is
M(n) = KP*(E(n) + KI*SUMi(E(i)*dT(i)) + KD*(E(n) -E(n-1))/dT(n)

In fact, the above is from line 33 of devEpidSoft.c, and agrees with
other descriptions I've found of the PID calculation.

However, in the do_pid() function, and in epidRecord.html, the
calculation includes the KP gain in the integral and derivative terms.
M(n) = KP*(E(n) + KP*KI*SUMi(E(i)*dT(i)) + KP*KD*(E(n) -E(n-1))/dT(n)

I can scale my KI and KD gain factors by KP to get the equivalent result,
but I don't see why the effective integral gain should be affected when
the proportional gain is tweaked.  Ditto for KD.

Any comments or clarification would be appreciated.

Thanks,
- Bruce

--
Bruce Hill
Member Technical Staff
SLAC National Accelerator Laboratory

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System