Temperature control fails. Will a PID solve the problem?

Question

I made a temperature control system without PID (just on/off with an offset) to control the temperature in a tank.
I can only cool the tank with a cooling coil inside the tank (so there is no heating element.) However, I can never reach the target temperature 20C due to the remaining cold water inside the coil (although the pump which transfer the cooling water inside the coil is turned off,) so the temperature drops after some hour or hours below 20C.
If I buy a commercial PID controller can I solve the problem?
If not, what can I do?

Pete W · Answer

This will sound a little counter-intuitive, but adding a very slight amount of constant heating, so that the cooling control loop has something to work against, might be something to explore.

Marko Buršič · Answer

just on/off with an offset

You probably mean an ON/OFF controller with hysteresis? The ON/OFF controller is always stable, meanwhile the PID is not. The PID controller has to be tuned to work properly. It can be very painful to properly tune it, even for an engineer that has a big theoretical knowledge on closed loop control. The ON/OFF control drawback compared to PID, is that ON/OFF control produces higher overshoots/undershoots as it is constantly oscillating around the setpoint value, but for rude regulation it is OK. Also not complicated, easy to setup.
If this kind of control can't provide a stable oscillating control value around setpoint, neither a PID would be able to control.

Temperature control fails. Will a PID solve the problem?

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