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What is Feedback control?

Control that measures the controlled variable with a sensor, compares it with the setpoint and changes the manipulated variable to reduce the difference (the error). Because the signal goes around a loop, it is also called closed-loop control. It corrects deviations in the result whatever their cause, so it is robust to disturbances and plant changes, but it corrects only after an error appears, so there is a delay, and too high a gain leads to oscillation or instability. PID control is the typical example, widely used to control temperature, speed, voltage, position and more.

Japanese name: フィードバック制御

Feedback control appears in 3 practice questions across 1 topic, including Control Engineering Basics.

Field: Control Engineering Basics

Try it: questions about Feedback control

Which of the following correctly describes feedback control (closed-loop control)?

  1. AIt runs only on a predetermined manipulated variable or procedure without measuring the controlled variable, so a deviation caused by a disturbance is not corrected automatically
  2. BIt measures the controlled variable, compares it with the setpoint and changes the manipulated variable to reduce the difference (the error), so it can reduce the effect of disturbances and of changes in the plant
  3. CIt measures a disturbance and applies a manipulated variable that cancels it before it reaches the controlled variable, so it always drives the error to zero, even for unmeasured disturbances and model errors
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Answer: B. It measures the controlled variable, compares it with the setpoint and changes the manipulated variable to reduce the difference (the error), so it can reduce the effect of disturbances and of changes in the plant

Feedback control compares the controlled variable measured by a sensor with the setpoint and decides the manipulated variable according to the error. Because the signal returns from the output to the input side and goes once around a loop, it is also called closed-loop control. Its strength is robustness to disturbances and to changes in the plant, because it corrects the deviation of the result whatever the cause. On the other hand, it corrects only after an error has appeared, so there is a delay, and too much gain makes the loop oscillate or become unstable.

Which of the following is the correct reason for combining feedforward control with feedback control?

  1. AFeedforward can respond to measurable disturbances and setpoint changes before an error appears, but it cannot correct unmeasured disturbances or model errors, so feedback takes care of the rest
  2. BAdding feedforward changes the gain and phase of the loop transfer function and increases the phase margin, so the loop stays stable even with a larger feedback gain
  3. CWith feedback alone a steady-state error always remains even with integral action, so feedforward is essential to bring the steady-state error to a constant setpoint or disturbance to zero
Show answer and explanation

Answer: A. Feedforward can respond to measurable disturbances and setpoint changes before an error appears, but it cannot correct unmeasured disturbances or model errors, so feedback takes care of the rest

Feedforward control measures disturbances or setpoint changes and, before they affect the controlled variable, applies a manipulated variable calculated from a model. It does not wait for an error to appear, so it responds quickly, but it assumes an accurate model and measurable disturbances. A common combination is therefore to let feedforward handle the main disturbances and setpoint tracking and to let feedback correct the remaining deviation. The feedforward path lies outside the loop, so it does not change the stability of the loop (the phase margin and so on).

Feedback control in the explanations

Questions that mention Feedback control (3)

  1. Which of the following correctly describes feedback control (closed-loop control)?
  2. Which of the following is the correct reason for combining feedforward control with feedback control?
  3. Which of the following correctly describes sequence control?

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Comparisons that include this term

Related terms

Feedforward control Phase margin Block diagram Bode plot First-order system Gain margin Integral windup Laplace transform Overshoot PID control

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