Re: PID control

Assuming that a control system is digital (a good assumption these
days) with a finite scan rate (usually 4 times/sec or slower) there is
an inherent time delay of 1/2 the scan time.

A first order process will have sustained oscillations if the gain is
high enough. If the single lag time constant is not much different from
the scan time, the gain does not have to be very large.

A second order system with two equal lags and the small scan time will
often have sustained oscillations at a moderate gain.

There are certainly loops that will not oscillate at even a high gain.
Then the ZN closed loop is not useful. The ZN open loop method may be
used if the "pseudo dead time" (the small lag before the point of
inflection in the response curve) can be measured. If that cannot be
measured, ZN (and other tuning methods) cannot be of much help. A high
gain may be used, with the limitation on the gain being the effect of
any noise or sepoint adjustments on the valve (not wanting the valve to
be always moving back and forth).

John Shaw
www.jashaw.com/pid


Tim Wescott wrote:
> Curtis wrote:
> > I don't have near the training you guys have, but I thought that
> > anything over a first order process showed some "inherent deadtime",
> > and therefore would oscillate. I thought that only the first order
> > process with no deadtime would not oscillate...
> >
> > curtis
> >
> Even a first-order system has some delay. In a linear system the delay
> shows up as a certain phase shift vs. frequency relation. A 2nd-order
> non-minimum phase system approaches the requisite 180 degrees of phase
> shift asymptotically at the same time that it's gain approaches zero.
>
> --
>
> Tim Wescott
> Wescott Design Services
> http://www.wescottdesign.com

.

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