PC-based PID Temp control

Jul 29, 2003 1 Replies

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I've searched the web extensively and have learned a lot about PID control in general. However, I haven't been able to find specific details about a couple of key aspects of my particular app - controlling water temp with an electric heating element. How to characterize the "plant?" I plan on giving it step commands to go from one temp to the next for periods of time. I envision taking some open-loop measurments of heater voltage vs. thermistor voltage. How should this data be used to form a model of the system? What about lag time? Do I need to include some kind of filter? I'm thinking about using an A/D and D/A for sensor input/controller output to/from parallel port. The algorithm should be pretty simple but what's the best way to implement (from a high level point of view)? Thanks to anyone who can help. Regards, JJ

Thanks for the help Walter. One more question. I think I understand dead time as it relates to motors. It's the point where the motor hasn't started to move yet although the controller is outputting a voltage to it right (or not)? So you're saying that a temperature controller doesn't have an analagous situation, just a delay between the controller command and response of the system (lag time)?

For the measurement, is dead time a period where the sensor doesn't register and is that only a problem for thermistors? Do thermocouples avoid this?

Thanks again, JJ

P.S. I tried to send you email but it bounced. If you don't mind sending me that code, my email is snipped-for-privacy@primewave.L-3com.com

PC-based PID Temp control

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