FEEDBACK CONTROL

Feedback control

Feedback control is a control action of altering the manipulated variable in response to the deviation between the controlled variable and its set point in such a way to eliminate the deviation.

Block Diagram of Feedback Control

The feedback control is a Closed Loop Control as it measures the controlled variable and feeds it back to the controller through a loop. The feedback loop helps to verify how well the process responded to the past manipulations. If the past adjustments failed to produce the desired output, then the feedback control make further manipulations accordingly. This process will continue until the controlled variable and its set point matches. It makes the feedback control a self-correcting control scheme. 

Feedback control is also called Reactive Control because it takes corrective action only after an error (the deviation between the controlled variable and set point) is generated. An error arises because of disturbance inputs, process parameter variations, and imperfect modeling.

EXAMPLE OF FEEDBACK CONTROL

Temperature Control Using Feedback Control

Consider a process outlet temperature control problem in a furnace. The furnace is used to heat the process fluid stream and its outlet temperature is to be controlled. In feedback control, the outlet temperature is measured and it is compared against the reference set point. The deviation between the desired and measured temperatures is found. The flow rate of the fuel gas is adjusted in accordance with the magnitude of deviation by the controller through the control valve. The outlet stream temperature is kept at the desired value by controlling fuel gas flow rate. (Read how Feedforward control and Cascade control  solves this temperature control problem more efficiently)

LIMITATIONS OF FEEDBACK CONTROL

1. Feedback control takes corrective action only after the disturbances has affected the process and generated an error.

2. The feedback control corrects the output deviations by trial and error method which results in the oscillatory response of the feedback loop.

3. Feedback control continues to change its output until the error is zero. There exists a finite delay between the detection of disturbance and the effect of control action on the controlled variable.

4. Feedback control cannot achieve perfect control because its effectiveness is limited by the process response to continuous manipulation of the process variable.

5. The inclusion of feedback control to a process affects stability, sensitivity and overall gain.

Also read other control schemes

Inferential Control 

Self-Adaptive Control

Programmed or Gain Scheduled Adaptive Control

Adaptive Control - An Introduction

Override Control

Feedforward Control

Cascade Control

Ratio Control

Feedforward control vs Feedback control