Tuning a PID with both fast and slow response.

Forum 2005-2010 (read only) Software Interfacing
1

GOAL: I have a kiln that I need to maintain at 1200 degrees "F" for 10 minutes.

I'm using the PID beta .6 library and I would like to get some thoughts on PID tuning.

Here is the scenario:

MAX6675 Break Out Board with High Temp thermocouple.
25 Amp Solid state relay
15 Amp 2000F Small Tabletop Kiln. (yes, 2000 degrees F)

Temp range: 70F - 2000F

So, here is the issue.

The basic code is done and the parts all try to work together nicely.

I can create a setpoint and ask the PID to reach it. OUTPUT is driving a PWM pin that is in turn driving the SSR feeding power to the KILN heaters. The setup is nicely doing it's best to ramp up the heaters and reach the temperature selected... but then the process overshoot is like 100 degrees or more even though cut off occurred very close to the right temp indicated by the thermocouple.

Questions:
Slower ramp is needed? Ramp up in stages?

It takes the kiln 5 minutes to begin making headway in losing that excess temperature because the kiln is very very efficient.

So I want to minimize overshoot.

ideas?

2

Slower ramp is needed? Ramp up in stages?

You shouldn't need to do either.

I believe the term is "critically damped". You should be able to adjust the three parameters to get exactly what you want.

Unfortunately, I have no idea how to tune a loop to be critically damped.

3

Yeah... and my mind is a bit overwhelmed with the research information available...

4

Skip the research. Look for "pid tuning critically damped".

This looks promising...
http://newton.ex.ac.uk/teaching/CDHW/Feedback/Setup-PID.html

The "Tuning" section here looks promising...
http://www.embedded.com/2000/0010/0010feat3.htm

5

I have used the PID libary to control my pellet boiler. It works very nice. I can recommend http://www.controlguru.com/ if you want to have a "down to earth" introduction to PID.

Jens