PID control with relays and heaters

[Herche]

I am trying to find the correct PID values for 5 separate heaters. My configuration is very similar to the block diagram show on page 2-6 of the PID Control Toolset User Manual. I have a heater that is monitored by a therocouple that is connected to FP-TC 120. I am monitoring the temperature change on the computer screen using the PID toolkit. The FP-DO 403 will send out a voltage to trip the relay that supplies the voltage to the heater. I can control how long the relay is on by adjusting the duty cycle on the program. My question is, how do I obtain the correct PID values to make my heater reach the desired overshoot and settling time. I have run the open-loop test with a step of 1 and 10. The manual suggests using
the tangent line to find the Td, T, and K, but my tangent line always crosses prior to the step change. So, next, I tried to find the transfer function from the step response and calculate the PID that way. I'm still having trouble finding a consistent method of obtaining the correct PID values. I need to find a way that works for any heater that I configure this way. I'm attaching two excel spreadsheets that display the output I'm getting. If you have any suggestions, please let me know. Thanks, Jason

[duffeyt]

Where exactly are you measuring the temperature? This data is not representative of a first order plus deadtime system leading me to suspect that thermocouple placement may be an issue. As an example this is more like what you should be seeing...

http://aabi.tripod.com/optimumpid/zieglar/optimum-pid-concept.htm

Tom

[Herche]

Thanks for your feedback. I failed to mention the fact that the heater is connected to pressuried air. Basically this heater is used to heat the air that flows through it. The rate of the air is 30L/min at 20psi. I don't know if this makes a big difference. I didn't think this would make a big difference. However, you mentioned placement of the thermocouple and how it may cause this problem. Do you have any suggestions for better thermocouple placement? I agree with your statement that it does not look first order. I have done some work with PID in the past and I could see that it was a little off. If you have any other suggestions, I'd greatly appreciated them. Thanks for the html link too.

Jason

[duffeyt]

Jason, without knowing more about your specific application, it would be hard to make any suggestions other than your thermocouple should obviously be as close as possible to whatever it is you're trying to control the temperature of. Since I see no delay in temperature rise from the time you apply power, this leads me to believe your thermocouple is placed directly on the heater? I also see an oscillation in the data that occurs as soon as you apply power suggesting that it may be very close. If it is, perhaps you should try moving it away from it, maybe a half inch? Tom

[Herche]

Thanks again Tom for all of your suggestions. I think you may be onto something with the thermocouple placement. There are two pipe heaters that are used to heat the air flowing through them. The thermocouple is mounted in another pipe that is connected to the heaters. Perhaps the T/C is still too closed to the heater though. The other 3 heaters are aluminum chambers that have anywhere up to 5 Watlow orange surface heaters attached to them. The T/C placement on these chambers is directly on metal itself, but at least 1/2 inch from any heaters. However, aluminum tape is used to hold the T/C in place. Could this tape, that is also in contact with the heaters, be conducting heat directly from the heater to the T/C? I
'm kind of new at T/C's and their placement, so again, any suggestions would be greatly appreciated. The open-loop responses I get for the surface heaters do not look first order at all and I'm really confused. I have attached some files of recent data I've collected. Heater 2 is the pipe heater, and 3 and 4 are the surface heaters. Thanks again for all your help.

Jason

[duffeyt]

Jason, are your T/Cs grounded or ungrounded and have you followed the proper grounding instructions per the FP-TC-120 manual? You might be picking up AC voltage from the heaters which could be causing a measurement error. I find it hard to believe that things are heating up that quick but if indeed they are, what you might need to do is sample the data at a much higher rate to measure the delay accurately. It appears, though it's hard to tell because of the noise, that your delay may be on the order of four seconds or less and you're sampling every two seconds. Tom