Control Engineering Basics
Which of the following correctly describes feedback control (closed-loop control)?
- 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
- 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
- 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
Show answer and explanation
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.
- A. This describes open-loop control, which does not measure the controlled variable. It does not look at the result, so it cannot correct a deviation.
- B. Correct. Comparing the measured result with the setpoint and correcting it is the essence of feedback control.
- C. This describes feedforward control, but it cannot correct unmeasured disturbances or model errors, so "always drives the error to zero" is wrong.