Controlling temperature using control logix

I am trying to hold down a tight temperature range customer wants +/-8F on a 800F setting. Currently holding about +/- 15F. I control block set up so that I have a two timers one for heaters on and other for heaters off the .pre of each timer is a varible tag. I change this value depending upon the percentage of the set temp verses the actual measured temp. I have had the temp probes calibrated and there within +/- 2f. But has anybody got any idea's what I can do to tighten up my control loop????

Tonk said:

... I have a two timers one for heaters on and other for heaters off the .pre of each timer is a varible tag. I change this value depending upon the percentage of the set temp verses the actual measured temp.

Click to expand...

What you are doing is simple proportional control. It cannot result in accurate control, you will either have oscillations, or a steady state error. Our distinguished colleague Ron Beaufort has posted a good explanation of why this is, so I won't rehash it here, you can read it here and more here.

What puzzles me is that you are using a controllogix yet you are not using the CLX PID. Why? You should be able to achieve excellent temperature control with it.

Even if PID control is uded the Control Variable output from the PID function, as its name implies, is a variable.

I assume from the original post that the heaters can only be turned on or off; i.e. the heater's power cannot be varied between 0 and 100%. Therefore the Control Variable output needs to be converted to a pulse signal with varying mark:space ratio depending on the value of the CV .

Correct PID tuning aside, generally, the shorter the cycle time of the pulses, the less likely you are to experience temperature oscillations.

The hardware, e.g. a contactor that turns the heaters on and off, may limit how fast the heaters can be cycled (contactor life needs to be considered) and therefore how small temperature oscillations can be achieved.

ControlLogix already provides an excellent instruction for digital heat controls... SRTP - Split Range Time Proportion instruction.

Take the output of the PIDE, send it to the SRTP, and the SRTP will give proportional heat or heat/cool outputs.