cartridge heater control

Question

I am interested in designing with cartridge heaters to implement a temperature control system, with a microcontroller sampling temperature sensors and controlling heating elements. I will probably aim for a cartridge heater since they are cheap.

I was wondering if there are any novel circuits or parts for controlling the temperature of a cartridge heater from a microcontroller. The microcontroller is capable of generating a DAC output, if that helps.

My plan was to have the uC control the temperature by providing a PWM square wave into a solid state relay, controlling the duty ratio of the power. I am unsure if cartridge heaters will naturally average the power into an average temperature, or if they get upset from transients.

Also, I'd like to put in as much safety checking as possible. I'd like an open-loop device that can remove power (and keep the power off until a system reset) if something catastrophic happens causing the temperature to run away.

Answer 1

This is kind of a broad question. Usually time proportioning systems are used with cartridge heaters, with time bases of tens of seconds and the system acts as a low pass filter. You can go a bit faster with an SSR.

A temperature control is conceptually simple- measure the temperature (PV) from a sensor, signal condition and convert to digital if required, feed the PV and the setpoint (SV) into a controller algorithm and get the output and run the output actuator, plus display the PV/SV or whatever else you want to do. The details can be complex.

For overtemperature safety you should endeavor to use something of different technology and as simple as possible. For example, a resettable trip thermostat or a simple mechanical thermostat. Consider that the SSR may (will probably) fail shorted at some point in the future. If injury or significant material damages are possible, great care needs to be taken.

You can buy modular temperature controllers and many industrial control loops use such modules, either discrete, or as part of PLCs or other control equipment.

Answer 2

Any control system should be defined by its control input-output parameters and desired accuracy. In order to do this you need to define what are the variables that go into the "Transfer Function" inputs and outputs, so an engineer, with expertise in Control Systems can decide how to best control it.

• Power , Control and Sensor Input; resolution, range and error tolerance
• Outputs Response desired, with variables that affect rate (mass, volume, power)

For example if you wanted 1'C accuracy and had 0.5'C resolution but the medium temperature does not change more than 0.5'C per minute then you do not need PWM, just on off control. IF you wanted to avoid 1'C overshoot from latency in thermal sensing then you would want some predictor correction or PID loop to control the power. But if the loop response is slow then simple on off control is all you need like in a house thermostat.

How power is controlled and how much power is decided by your specifications above. There may be more specs required as you get into details of desired response like Thermal resistance of power loss in the container measured in ['C/W] or [W/'C] to raise a certain temperature open loop.