Is it OK to repeatedly reset the set point on a PID controller?
Is it OK to repeatedly reset the set point on a PID controller?
(OP)
Hey everyone, I have a 1/16 DIN temperature controller with a single setpoint with remote communications. I have written a simple time based program for ramping and holding (soak). The program basically just resets the setpoint periodically in order to achieve the desired temperature based on a rate or time. There is no PID control algorithm with the program. The "system" relies on the PID in the controller to maintain the temperature. The device for which I am controlling the temperature is a somewhat fast response system; in the order of 10s of seconds to to change from 25C to 100C. Knowing that a PID controller is constantly calculating and setting the control output, my question is: Will resetting the setpoint every 10 seconds throw off the PID?
Thanks in advance!
Thanks in advance!





RE: Is it OK to repeatedly reset the set point on a PID controller?
Why not give it a try and monitor actual temperature?
There are numerous PID based ramp/soak controllers available on the market.
-AK2DM
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"It's the questions that drive us"
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RE: Is it OK to repeatedly reset the set point on a PID controller?
A commercial PID controller ought to be able to handle ANY setpoint withing its specified allowable range. The only concern one might have is a technical one: if the response time is on the order of "10s of seconds" and you change the setpoint every 10 seconds, then you may not have the temperature ramp that you think you should be getting. That is, you might wind up with a bunch of smaller responses with one giant step at the end.
You should do some testing to make sure.
TTFN
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RE: Is it OK to repeatedly reset the set point on a PID controller?
Finding out whether this is the case or not with a particular model is very problematic, because the one guy who might know lives half way around the world and your contact is a sales office in the other half of the world. That manufacturer has no clue whether today's equivalent model has the same limitation.
RE: Is it OK to repeatedly reset the set point on a PID controller?
Pamela K. Quillin, P.E.
Quillin Engineering, LLC
RE: Is it OK to repeatedly reset the set point on a PID controller?
analogkid2digitalman: I am aware that there are numerous PID based ramp/soak controllers available. The goal is to have everything monitored and programmed from a common front end using LabVIEW. This will also allow us to subsitute the PID controllers in the future without having to retrain the research people.
IRstuff: Very good point. I have done some testing "on the bench" but nothing is as good as testing in the actual system. Thanks for the vote of confidence on modern PID controller being able to handle a setpoint change within its range. This directly addressed my concern.
danw2: Great point too. My particular controller has both a volatile and non-volatile write instruction. I am writing to the volatiel one for that reason. As an added note - writing to the non-volatile causes and annoying reset of the display/controller which wasn't desirable anyways.
lacajun: Yes, retuning the PID was exactly my concern.
Thanks again!
RE: Is it OK to repeatedly reset the set point on a PID controller?
PID B — Unlike the PID A equation, the controller gives only an integral response to a setpoint change, with no effect on the output due to the gain or rate action, and it gives full response to PV changes. Otherwise controller action is as described for the PID A equation.
>This will also allow us to subsitute the PID controllers in the future.
I assume you are writing values via some digital protocol, ASCII or Modbus (as opposed to generating a voltage signal to a 2nd input on the controller). You should be aware that there's absolutely no standard for mapping digital comm values or the format used.
Your current model might take signed integer, the next one floating point, or signed integer*20, whatever. Not that those issues can't be addressed, but changing brands is never plug and play when it comes to digital comm.
So whatever you pick and stick with should have a suitable lifespan because changing brands will require changing the program you use to write the values.
RE: Is it OK to repeatedly reset the set point on a PID controller?
RE: Is it OK to repeatedly reset the set point on a PID controller?
Fe
RE: Is it OK to repeatedly reset the set point on a PID controller?
Thanks!
RE: Is it OK to repeatedly reset the set point on a PID controller?
Neglect in engineering is the reason for many faults and accidents. Have fun with your elementary PID controller.
Fe
RE: Is it OK to repeatedly reset the set point on a PID controller?
RE: Is it OK to repeatedly reset the set point on a PID controller?
If in the user interface only the setpoint is changed then any controller can be programmed into place.
I know a lot of the times what ends up happening is that the designer just tests physically it to ensure operation. This is alright I guess, but not really ideal and not engineering. That's all.
Have a good one.
Fe
RE: Is it OK to repeatedly reset the set point on a PID controller?
Thanks.
RE: Is it OK to repeatedly reset the set point on a PID controller?
Try to figure out (derive) the dynamic model for your temperature control system. And work from there.
Good luck
Fe
RE: Is it OK to repeatedly reset the set point on a PID controller?
Any given PID set is good for @ +/- 200˚ from its tuning point. Only if you exceed this should you consider using another PID set. Some controllers have a feature called Gain Scheduling. You'd tune it at several setpoints and set a boundary at which it would change PID sets.
You could use another feature of some controllers called 'adaptive tune' which recalculates the PID settings on the fly when the deviation is at a set threshold, but this is disruptive to many processes.
Thanks again for all your input.