Question about a P and I control

bigrthanur · Aug 28, 2007

I am having a brain block about using control with only gain and reset If i was to set up my controller to work optimal. I would think i would want the controller to look at the error as much as possible. But i am trying to think back to basic process and forgot?

ANyway my question is i set my reset high(to look at the error more often) Wouldnt my controller lets say heat control in a water tank, Work better or more smoothly?

ANy info would help

Thanks

Gerry · Aug 28, 2007

Check the units.
Reset is normally expressed as minutes per repeat.
So entering larger numbers makes the controller check the error less often.

bigrthanur · Aug 28, 2007

yes that is correct but my question is if i check the controller more often will it smooth out more easily or will the temp fluxuate more?

Peter Nachtwey · Aug 28, 2007

Minutes per repeat? Yuk.

That is old terminology. Integrator gains are normally experessed as time constants.

bigrthanur, making the integrator time constant shorter doesn't make the controller 'look' at the error more often. This is done by making the controller update the PID loop more often. The sample time is divided by the time constant and mulitplied by the gain to find the true integrator gain. This means the integrator gain gets smaller as the same time is smaller. This way less gets added to the integrator when it is update more often keeping the over response to the error pretty much the same as long as the sample time is 10 times shorter than the minimum of the plant time constant and closed loop time constant.

bigrthanur · Aug 28, 2007

Old termanology, or i think you are misunderstanding what i am asking. (or i am lost somewhere and not getting the basics?)

"I" standing for Integral or Reset i thought to mean how often the product controller will look at the error.
Error i understand it as the difference between the set point and the actual value

Mickey · Aug 28, 2007

bigrthanur,
Heres some good reading material for you.

http://www.ronbeaufort.com/sample_lessons.htm

Then after you have read the above go here:

ControlGuru is a blog about Automatic Process Control

And if you like real old school stuff go here and scroll down to "Fundamentals of Three-Mode Controllers"

http://www.fisherregulators.com/technicalmonographs/various/

Peter Nachtwey · Aug 28, 2007

bigrthanur"I" standing for Integral or Reset i thought to mean how often the product controller will look at the error.[/QUOTE said:
No, how often the PID 'looks' at the error is dependent on how often the PID routine is called. The PID is called when a timer times out or from an periodic interrupt. You must select the sample period to be fast enough for your application. The integrator time constant is one of the tuning parameters.

Now y'all know why I don't like the term resets or repeats. It is misleading. What is repeating or reseting? Think in terms of time constants. A PID with a time constant of 1 second will take 5 time constants or 5 seconds to wind up to within 1% of the final value. That is simple and gives one a gut feel for how to adjust the integrator time constant.

bigrthanur · Aug 29, 2007

Thanks to all

That helped alot!!!!!!!!!!!!!!!!!

Question about a P and I control

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