Feedforward Vs Feedback Control

Feedback Control

Feedback Control is considered as an important technique widely used in around all process industries.

Feedback controller

Its main advantages are as follows.

  • As the controlled variable/control objective deviates from the set point, corrective action taken very instant; irrespective of source and type of disturbances.
  • Feedback is reactive in nature.
  • Here, Corrective action taken by the controller occurs after the disturbance observed in the output.
  • Minimal knowledge requires about the processed to be controlled, helpful in control system design.
  • The ubiquitous PID controller is both versatile and robust. If process conditions change, retuning the controller usually produces satisfactory control.

However, feedback control also has certain inherent disadvantages:

  • No corrective action is taken until after a deviation in the controlled variable occurs. Thus, perfect control, where the
    controlled variable does not deviate from the set point during disturbance or set-point changes, is theoretically impossible.
  • Feedback control does not provide predictive control action to compensate for the effects of known or measurable
    disturbances.
  • It may not be satisfactory for processes with large time constants and/or long time delays. If large and frequent
    disturbances occur, the process may operate continuously in a transient state and never attain the desired steady state.
  • In some situations, the controlled variable cannot be measured on-line, and, consequently, feedback control is not
    feasible.

Feedforward Vs Feedback control Images

Feedforward control

feedforward control

Feedback control

Feedback control

Let’s talk about Feedforward Control

Feedback control also has certain inherent disadvantages:

Feedforward Control Loop

  • No corrective action is taken until after a deviation in the controlled variable occurs. Thus, perfect control, where the controlled variable does not deviate from the set point during disturbance or set-point changes, is theoretically impossible.
  • Corrective action taken before the actual disturbance occurs on the output.
  • Feedback control does not provide predictive control action to compensate for the effects of known or measurable disturbances.
  • It may not be satisfactory for processes with large time constants and/or long time delays. If large and frequent disturbances occur, the process may operate continuously in a transient state and never attain the desired steady state.
  • In some situations, the controlled variable cannot be measured on-line, and, consequently, feedback control is not feasible.

ImageSource : ResearchGate; Instrumentation Forum

ArticleSource : sites.chemengr.ucsb.edu

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