Q: What is a Temperature Controller and how does it work?
A : A temperature controller is an device that is used to control temperature. It does this by first measuring the temperature (process variable), it then compars it to the desired value (set value). The difference between these values is known as the error (Deviation). Temperature controllers use this error to decide how much heating or cooling is required to bring the process temperature back to the desired value. Once this calculation is complete the controller will produce an output signal that effects the change required. This output signal is known as the (manipulated value) and is normally connected to a heater, control valve, fan or some othe "final control element" which actually injects or removes heat from the process.
Temperature controllers form one of the four parts of a temperature controlled system. To help visualize this we will consider an oven. The four parts would be :
1 The oven
2. The heater
3. The thermometer (or thermocouple)
4. The controller
The role of the temperature controller is to measure the temperature on the thermocouple, compare it to the set point and to calculate the amount of time the heater should remain switched on to maintain a constant temperature.
Many factors change the amount of time that the heater needs to on, to maintain the process temperature. For example the size of the heater, the size of the oven, the amount of insulation surrounding the oven and the ambient temperature will all change the rate at which the oven will heat up or cool down. Other factors such as the circulation of air within the oven, the humidity of the air. The mass of product being place inside the oven and many more that are described in minute detail at http://newton.ex.ac.uk/teaching/CDHW/Feedback/OvSimForm-gen.html
At the end of the day a temperature controller replaces the function of a person who's job description would read something like:-
Watch that thermometer
Keep the temperature stable at 80Deg C
If you need more heat, switch on the heater.
The important point, is that the temperature controller has one input, one output and one set point.