Thermal control system design

John,

The controller I had in mind was an Omron unit and I believe it had an current output instead of the voltage output I had stated. The temperature I am looking for is 30-40 degrees C. Do you think a thermistor would be better for that range?

Thanks, Bob

Many PID controllers provide a pulsing relay power output, that if the pulse time can be made significantly shorter than the dominant thermal time constant of the process, approximates a continuously variable power output. There also solid start relay outputs that control integer half cycles of the line waveform of on time for a low noise version of this sort of burst control.

Other controllers provide an analog output, usually 4 to 20 milliamps that represents the 0 to 100% power output. The simplest way to connect that to a small heater is with a solid state relay that converts the variable current into a phase controlled output (the sine wave has various early parts of each half cycle of the line waveform turned off, with the rest of each half cycle turned on. This is still a form of pulse width modulation, but is much higher frequency than the relay output.

The ultimate in low noise, continuously variable heat control would be a linear voltage regulator controlled by the 4-20 ma output, but that would be used only for very low power heaters with extreme precision requirements.

I have never seen a PID controller with a millivolt output.

A big problem for your controller will be the tiny voltage represented by a less than .2 degree error (and the controller has to be able to measure the temperature to a finer precision than you can expect it to control). If the temperature is moderate, you may do better with a thermistor even though the absolute calibration will not be as good, because of the higher sensitivity to very small temperature changes.

That is an ideal temperature range for many thermistor sensors. You will probably need a transmitter (signal conditioner) to convert the variable resistance of the thermistor to the standard 4-20 ma controller input. If you pick a transmitter that allows a narrow range of resistance to be expanded to the full input signal range, you will increase the resolution, though the stability and calibration issues remain. some examples that you can mine for key words for searches:

Maybe I'm just not familiar with it, but why would you want to use a PID controller for a thermal process? A thermal system is much more complicated than a simple spring/mass/damper system.

I would assume a PD controller with an experimentally obtained offset would be adequate.

Dave

(A) because you can buy off-the-shelf PID controllers. (B) because everyone knows how to tune a PID (well, sorta). (C) because PID is often good enough. (D) because if PD is good, PID is better and no harder (see A).

Surely Omron has one with relay output? That's a very very common output for temperature control.

I have used these units to control both laser current and temperature. And the peltier control units accept thermistor sensors, directly and control clean DC to run the peltier. I suspect one of these units could be used as is, if with slight modification, for the complete heater control.

very

the

Robert, have a look at the stuff from wavelength electronics, it may give you some ideas - there is not a lot in some of the controllers. I used one of their small PID units for controlling a Peltier, had to add extra heatsinking. It provided direct drive of the load and had analog in & out signals as well.

rob