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GATE 2017 EC – Question 30

Control Systems · Compensators and PID Controller · 1 mark · Multiple choice

Which of the following can be the pole-zero configuration of a phase-lag controller (lag compensator)?

Four pole-zero plots on the s-plane (poles marked ×, zeros marked ○). In (A) a zero is to the left of a pole, and both are on the negative real axis. In (B) a pole is to the left of a zero. In (C) the order along the axis is ×, ○, ○, ×. In (D) the order is ○, ×, ×, ○.
  1. Plot (A)
  2. Plot (B)
  3. Plot (C)
  4. Plot (D)

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Show answer and explanation

Correct answer: (A) Plot (A)

Explanation

A lag compensator has the form $\frac{s + z}{s + p}$ with the pole closer to the origin than the zero ($p < z$). On the negative real axis that means the pole is to the right of the zero, nearer the origin. That is the pattern in plot (A). In plot (B) the zero is nearer the origin, which is a lead compensator.