PID - Heater and PWM output - Doesn't stabilize the temperature after autotune.

[Fran]

Hi,

I implemented default PID. After 3 cycles of autotune the PID can't control the temperature.

I followed help:

https://www.unitronicsplc.com/Download/SoftwareHelp/UniLogic_Knowledgebase/index.htm#PID/PID.htm

 

And Video:

 

 

Attached:

Autotune and PID control result

Function that I am using.

Setup:

Controlling the SSR. Output of PWM is connected to A2 of the SSR
 

 

Heater_PID.ulle

[Joe Tauser]

Autotune is not the end-all do-all in the process world.  All it does it get you close, it's not a guaranteed that your controller will be Awesome when it completes.

If you haven't any experience with tuning a PID loop I would do some Googling and learn a bit about what the parameters do.  Looking at your graphs above I think your Proportional Band is still too small.  I don't even use the Derivative parameter for most loops as it's not needed and complicates things.  Autotune calculates one anyway - the first thing I do is set this to zero.

Post the loop parameters you have.  How tightly are you trying to control?

Joe T.

[Arne]

I guess there is "to parts" of a stable closed loop control system. Half of it is the controller and the other half is the process itself, what is contains of time constants and dead time.

If it is a some kind of heating fan with a temperature sensor, it might give a proses that easy get unstable.

https://blog.opticontrols.com/archives/499

Maybe the problem is more related to the process than to the controller?    

A more detailed information about the process itself might make things a bit clearer.