ITPEC FE Morning April 2022 Question 6

It detects disturbance and takes a corrective action to prevent its influence — the official key's wording for feedback control.

Feedback control measures the output, compares it with the target value, and corrects the deviation. It reacts to the effect a disturbance has on the output, so it copes with disturbances it cannot foresee.

Note on the key: ITPEC's official answer is (a), and that is what the exam scores. In the usual textbook terms, (d), correcting after detecting the influence of a disturbance, is the closer description of feedback, while acting before the influence appears, as (a) reads, is how feedforward control is usually described. The key takes (a)'s "detects disturbance" as detecting its effect on the output. On the exam, choose the official answer.

Answer (a)

Why not others:

• •(b) — describes an unstable system; negative feedback is used precisely to suppress disturbances, not amplify them
• •(c) — predicting a disturbance and correcting in advance is feedforward control
• •(d) — the textbook description of feedback, but not the official answer to this question

Key rule: Feedback control corrects by measuring the output against the target; feedforward control acts on a predicted disturbance before it reaches the output.

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