Best way to control a thermoelectric device with PID

[ajsteinm]

In my application, I am trying to control a Peltier, or thermoelectric device.  Would you help me understand the logic in the program I inherited?

 

The program has two PID loops, one that outputs heat, and another that outputs cool.  When heating, cooling is inhibited.

 

Any suggestions on better ways to do this?  Maybe a single PID with two outputs?

 

Just in case, a peltier device has two sides, a main and a waste.  When current is flows though one way, the main side gets colder and the waste side heats.  When the polarity is reversed, the main side heats and the waste side cools.

[Joe Tauser]

The program is written that way because the Unitronics PID block does not do heat/cool (two outputs).  I ran into the same problem trying to control a motorized variac with a CW and a CCW input signal.

 

What problems are you having with the program?

 

Joe T.