There are number of techniques used for process control system that are useful in the analysis and design of modern process control system. Feedback control system is a type used in this type of control system.

In this feedback control system output is measured and compared with input reference signal. The deviation (error) between the two is used by the controller which will send the output so that error of system becomes zero.

In this example, feedback control system can be explained. In which the output Ca is continuously measured using level sensor and feedback to be compared with input (desired, set (or) reference) level Cd. The error e= (Cd-Ca) is used to adjust the control value by means of an actuator.

Advantage of feedback control system

It is very simple technique that compensates for all disturbances

The controller adjusts the controlled variable (affected by the disturbance) with the set point (reference).

The feedback control loop does not care for the disturbance entering into the process. It tries only to minimum the value of error by compensating for all disturbances.

Disadvantage of feedback control system

It does not take corrective action until after the disturbance has upset the process and generates an error signal.

The feedback controllers continue changing its output until measurement and set point are in agreement.

It solves the problems by trial and error method and thus causes oscillatory response of a feedback loop.