Process Control - Manual Pid Setting

[Yorg]

I have a steam powered mash system with a PID Controller.
These controllers have an autotune function - they will oscillate around a set point and then decide on the appropriate PID parameters.
Unfortunately, in an insulated mash tun, the overshoot takes so long to come back down that the autotune function gives up before it does.
Anyone know how to manually approach determining the PID parameters for such a system?

Cheers.

 

[newguy]

If you can actually change the individual constants, start with Tim Westcott's PID Without a PhD. His article is an easy read and it should help you figure out what to do.

 

[Adamt]

You could use the Cohen-Coon method... not sure how accurate it will be but anyway:

Input a step change in set point (temperature) and plot response.

You need to work out the dead time (DT) and time constant (TAU) in the way shown below...



Tuning parameters are:

T_i = DT*(23+6*DT/TAU)/(13+2*DT/TAU)

T_d = DT*4/(11+2DT/TAU)

Hope this helps.. this is based on theory I've learnt at uni, never used it in practice.

 

[Thirsty Boy]

maybe ... $8 for an uninsulated storage box to use as a faux mash tun. Use the auto tune in that to get a ball park set of parameters (I know it works, thats what my PID controlled mash tun is) then you can change to your "real" mash tun and tweak the D (damping) constant and/or the "souf" (or thats what its called on my PID) up till the bloody thing stops overshooting.

It'll take a bit of mucking about but should get you there eventuially.

Or just ditch the insulation... you have heat. Who needs insulation?

TB

 

[Tony]

Basic rule of thumb is increase proportional band till system starts to occilate then set Integral time at 4X the time it takes to do one complete cycle.

Like i said...... basic rules but will get you close.

Dont bother using any derivitive........ only needed in very complex systems...... home brew systems not included.

Just remember...... increase the P to slow the response down, decrease it to speed it up.

The I works to reduce error over time so when you get a good P your happy with increase the I to take the error out of the control

Good PID control should over shoot a bit then under shoot a bit then the I slowly chips away at the error to acheive setpoint

hope that helps a bit

cheers

 

[Yorg]

OK thanks guys, I'll give that all some thought.