Which PID gains should I use to control an oven's temperature?

[nikosfs]

Hello,

I whould like some advice on which PID gains I must use to control an oven's temperature that works with electrical resistants.
The oven's power is 14 KW.
I use LabVIEW 8.2 , FieldPoint cFP-TC-120 & cFP-DO-400 , a thermocouple and a relay to control the oven.
I have tested the following values: Proportional gain (Kc) = 80 , Integral Time (Ti,min) = 0,015 , Derivative Time (Td,min) = 0
                                                       Proportional gain (Kc) = 1 ,   Integral Time (Ti,min) = 0,010 , Derivative Time (Td,min) = 0
but the overshoot was big.
During the first test, the setpoint was 40 degrees Celsius but the oven's temperature rised to 85 °C.
During the second test, the setpoint was 30 degrees Celsius but the oven's temperature rised to 65 °C.
The reason of the above overshoots is that the oven had been working until its temperature reached the setpoint.
What is more, even if there was ­­little overshoot, it would cause problems as the oven cools very slowly.
After all, which PID gains should I use ?
I attach an image that shows partially the additional code that I have written in LabVIEW.

Thank you in advance.

[michaeld]

Nikolis,

Unfortunately, it is very difficult for anyone to be able to recommend values for your PID gains. It is really going to be dependent on your system, and the only way to really find values that will give you the response you would like out of your system, is to manually tune it using trial and error. Here is a great article that describes a method for tuning the PID gains for a servo motor. It is not exactly like your system, but the method works really well and should really help you in developing a strategic plan for getting the response you want out of your system.

A Simple Method for Servo Motor Tuning

Also, here is another great article that will help to explain how the values you set for P, I, and D affect your system.

PID Theory Explained