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GATE 2021 BM – Question 56

Measurements and Control Systems · Control System Transfer Functions · 2 marks · Numerical answer

A unity-feedback system has closed-loop transfer function $C(s)/R(s)=(s+90)/(s^2+10s+90)$. Find steady-state error to a unit ramp, to one decimal place.

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Correct answer: 0.1 to 0.1

Explanation

For unity feedback, E/R=1−T=(s²+9s)/(s²+10s+90). A unit ramp has R=1/s². The final-value theorem gives ess=lim sE=lim (s+9)/(s²+10s+90)=9/90=.1. The closed-loop poles are stable, so the steady-state limit is valid.

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