The figure shows the block diagram of close loop system or process control system. The process control system consists of process or plant, feedback elements, error detector, automatic controller, actuator or control element.
Block diagram – process control system
Plant or Process :
Plant or process is an important element of process control system in which variable of process is to be controlled. The Process means some manufacturing sequence. It has one variable or multivariable output.
Feedback element or Sensor :
The feedback element or sensor is the device which converts the output variable into another suitable variable which can acceptable by error detector.
Error detector :
The error detector compares between actual signal and reference input i.e. set point .The error detector is subtract summing points whose output is an error signal to controller for comparison and for the corrective action.
E(t)= r(t) b(t)
Automatic controller :
The controller detects the actuating error signal, which is usually at a very low power level, and amplifies it to a sufficiently high level i.e. means automatic controller comprises an error detector and amplifier.
Actuator or control element :
The actuator is nothing but pneumatic or valve, a hydraulic motor or an electric motor, which produces an input to the plant according to the control signal getting from controller.