Hi everybody,

we are using an HP-Vee-PID-Algorithm to control the heating of our instrument. Due to its task we need a very constant temperature. But after shutting them down for like 2h they overheat up to 10 degrees C which almost ruins our data? Is there the possibility to enter something like an upper limit into the program? I would be happy over every help!!

Thanks Jan

 

If you mean that the loop continues to be calculated for two hours while the actual control to the device is turned off, it sounds like you are seeing what is called "Integral Wind-Up". The integral term becomes large when the device is off setpoint for a long time, result in a big burst of energy to try to compensate when control
over the heater is available again. Most PID loops have a setting to limit how big the integral term can get (sometimes called "anti-reset windup"), or you could reset the loop terms when you power up the instrument.

 

I agree. as another alt. you can monitor your s/d switch and feed it back into the controller-device (if possible of course). this can be an external limit switch that will limit output of controller-device to whatever you like. though this method is not usually available on most controllers due to the boolean & CIN switches required. but it all depends on what controller-device you're using.

 

Just another possibility of what may have happened. If you switch off the temperature control system and switch it on again – the controller PID parameters are tuned the way it gives you overshoot at start-up equal to 10 degrees of overheat. In this case you need to retune the controller to make it work without overshoot.

Igor Boiko

 

I'd guess that you are describing "reset windup", where the integrated error saturates the PID output.

You should either look for a feature to deal with this (it is called anti-reset windup) in your PID algorithm, or put the loop into manual after shutdown to avoid saturating the I term.

Cheers,
-Ron