Peltier control with PID

[Blokk]

Hello,

I have a Keithley 2400 source-meter, and a Keithley 2010 multimeter. I measure temperature of a water loop with a RTD, and I measure its 4W resisitivity with the K2010.

I can cool and also heat this water loop via a Peltier heat-pump. The applicable range is -1Amp < current < +1 Amp. So I use the Keithley source-meter to drive the Peltier in this range. Negative current cools the water, positive current heats.

I have the PID tool-kit with LabView 2011.

My only question is that, if I give  +-1 as range input to the PID.vi, is it a proper way to implement this task?

Thanks!

[GerdW · Accepted solution]

Hi blokk,

maybe you should set the output range to [-1...+1]?

[Blokk]

thanks, that was the only question, I did not name it properly, so the output range what I need to set to +-1 Ampere. So this PID.vi actually written in a way, it "knows" positive output will increase the process value, and negative will decrease? (this is what I did not know)

[GerdW]

Hi blokk,

"it "knows" positive output will increase the process value, and negative will decrease?"

No, it doesn't know that. But you can change the kp (proportional gain) from positive to negative values to change the PID behaviour...

[Blokk]

ok. so this is not implemented in the Pid toolkit. So when the PID vi using negative output, I have to set the Kc to negative. Should I alter the pid.vi inside? Because it should change the sign of Kc as early as it switches range polarity continuously. Strange why this is not implemented in the toolkit...

[GerdW]

Hi blokk,

"ok. so this is not implemented in the Pid toolkit. So when the PID vi using negative output, I have to set the Kc to negative. ... Because it should change the sign of Kc as early as it switches range polarity continuously."

You don't need to change anything, when the PID control is running. You have to set the Kc once to the correct value, that's it! Kc is describing the PID behaviour in terms of "higher output results in heating up and lower output means cooling mode"...

"Should I alter the pid.vi inside?"

Never ever change vi.lib VIs! If you really want to change them (that isn't needed here!), make a copy of the VI and change the copy!

[Blokk]

okey, thanks for lighting up this for me 🙂 I think I overcomplicated the actual problem in my mind a "bit".