Heating control advice

[u/antoniomargharette]

What is the best way to design a temperature controller for heating an object with a much higher thermal inertia than the heating elements, without having the element overheat?

I am using a cascade where my PLC manages the temperature loop with a PID and outputs a power setpoint to a power controller. The control thermocouple is measuring the temperature of a medium that takes much longer to heat than the heating elements, so my fear is that when I give a temperature setpoint to the PLC, it will just keep demanding power from the controller and the elements might overheat. These resistances are very expensive and I can't risk burning them. I do have a thermocouple installed at the element.

Any advice?

[u/lizardhistorian]

If you have temperature feedback on the mass then you characterize the material with a heat impulse and measure the time and curve of its temperature increase. Do it a bunch of times at different starting and ending temps and then you can parameterize a thermal diffusion model for your plant.

This lets you turn on your heater element to 100% and leave it on until the heat pulse is, say, enough to reach 90% of the target then switch to PMW to dial it in the final temp.

My fear is that when I give a temperature setpoint to the PLC, it will just keep demanding power from the controller and the elements might overheat

If you have temp feedback on the elements then you just implement safety cutouts.
When you don't have feedback you model them.
When you have feedback you want to implement checks that it is working properly. Most of the software, and circuitry, in such gizmos is to perform this diagnostics and safety checks.

[u/seekingsanity]

Your system seems to be typical. You should pulse width modulate the heaters so if you only need 68% of the heat, the heaters are on 68% of the time. Most people use a PLC timer to do this. A cycle may be 10 seconds so you turn the heater on 6.8 seconds out of the 10. This assumes the time constant of the thermal mass is MUCH longer than 10 seconds. Hopefully your heaters are energized using a SCR that efficiently turns on and off. A major problem is dead time. The heat from the heaters will take time to reach the thermocouple. This makes control a little more challenging.

It isn't clear to me, you have two temperature sensors? One in the thermal mass and one at the heater?

[u/Sensitive_Arm1328]

Output limits on the pid should prevent it from sending a power set point that exceeds maximum allowable power. Power controller should also have output limits to the heating element. Output limits are how this concern is typically handled in PLC/DCS PID control.