Hi all,

I have to control a temperature (3 channels, J type, 30 ~ 60oC, Solid State Relays). I can use a PID control (PLC’s PID Function or a simpler one) but the existing PLC has only relay type outputs (not transistors nor analog outputs).

Any idea how to build a “hand made” PWM in combination with the PID MV output? I need a large cycle time (some seconds) because the relay contacts.

Thanks in advance

Nikos

P.S. I can’t use external temperature controllers, I have to integrate the temperatures in a HMI – recipe system.

 

Set up a repeating timer with a count of 100 (if you have a 100 millisecond time base) - or 1000 (if you have a 10 millisecond time base).

Set an output relay when the timer rolls over. Compare the current count of the timer with the PID output signal, and reset the output if the count value is greater than the PID output. You'll need to adjust for scaling - the above will work over a 10 second period with a resolution of 1 % for the 100 millisecond case, and 0.1 % for the 10 millisecond case.