PID cascaded temperature control

andreahmed · Jul 24, 2017

I'm doing PID temperature controller using PLC.

I'm doing a cascaded P, and PI controller of both temperature set points.

My question is how would I map the output of the first controller which is PI which should be converted as a temperature set point of the the second controller ?

Dirt · Jul 24, 2017

What PLC/DCS are you using?

If you cannot set the outpt scale in the P/PI-controller, then I guess the output is 0-100? You have to match this to to process value of the second controller.

If output is 0-100 and temperature of second controller is say 0-850degress, then you have to multiply the output with 8.5.

andreahmed · Jul 24, 2017

I'm using beckhoff twincat3

I have two pid loops and one is PI that has a set point temperature. I want to control the second loop. but I don't know how to map the output of the first loop which is Y to the set point input of the second loop. Math ?

Dirt · Jul 24, 2017

andreahmed said:
I want to control the second loop. but I don't know how to map the output of the first loop which is Y to the set point input of the second loop. Math ?

Yes. And you also need to consider desired max and min for setpoint to second controller (the PI). This depends on the how the process is designed. You need some process specification.

mbartoli · Jul 24, 2017

andreahmed said:
I have two pid loops and one is PI that has a set point temperature. I want to control the second loop. but I don't know how to map the output of the first loop which is Y to the set point input of the second loop. Math ?

Dirt said:
Yes. And you also need to consider desired max and min for setpoint to second controller (the PI). This depends on the how the process is designed. You need some process specification.

As DIRT indicates, you will need to determine the allowable max and min setpoints for your final process, and use math to assign the max and min outputs of the outer loop to the setpoints of your inner. In other words, you don't want the first loop (outer) to send a 0 setpoint to the second loop (inner) if your desired process control is 50 - 80 degrees.

shooter · Jul 25, 2017

I use the minimum of both controllers as output to the valve that way it is always on the safe side.
I am controlling the superheat of the coolant and the temperature of the process.

mellis · Jul 25, 2017

It's just a scaling problem and it can be handled in several ways depending on what your PID allows. Some simple solutions:

Scale 0-100% to the range in engineering units of the secondary loop.

Set the output range for the primary loop to be the range in engineering units of the secondary loop instead of 0-100%.

PID cascaded temperature control

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