A temperature controller is an instrument used to control temperature. It does this by comparing the process temperature ( process variable ) with 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 a 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
4. The controller
The role of the temperature controller is to measure the temperature on the thermometer, 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 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 are some of the more obvious. But other factors such as the circulation of air within the oven, the humidity of the air.
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 a basic temperature controller has one input, one output and one set point.