Question about epid record

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

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