Control systems normally employ three means of manipulation of variables.
- Pneumatic control systems: Used in explosive areas, local control, and in simple systems.
- Electronic control systems: Most versatile and dependable, ideally suited for remote control arrangements.
- Hydraulic systems: used in high power and torque applications. The basic block diagram of a control system is shown below:
In an Electronic temperature controller this is modified as below:
The PID controller utilises operational amplifiers to produce the three terms, Proportional, Integral and Derivative. Any other algorithm can also be generated in the controller, to make a good control action suited to the process.
The thermocouple produces DC millivolts proportional to the temperature in the chamber, which is amplified and then given to the PID block. The Controller output is fed to a TRIAC unit to vary the power fed to the heater to increase or decrease the temperature as set by the controller.