PlacementPrepA closed-loop control system differs from an open-loop system by having:
Solution
Feedback compares the output with the input to correct errors.
Answer: (B) Feedback
Technical · Electrical and Electronics (EEE)
Control Systems MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
The transfer function of a system is defined with:
Solution
It is the ratio of Laplace transforms of output to input with zero initial conditions.
Answer: (C) Zero initial conditions
For a unity negative feedback system with forward gain G(s), the closed-loop transfer function is:
Solution
C/R = G/(1 + GH) with H = 1.
Answer: (B) G(s)/(1 + G(s))
A system is stable if all poles of its closed-loop transfer function lie:
Solution
Left-half-plane poles give decaying responses.
Answer: (C) In the left half of the s-plane
The Routh-Hurwitz criterion is used to determine:
Solution
It counts right-half-plane roots without solving the characteristic equation.
Answer: (C) Stability
The damping ratio of an underdamped second-order system lies between:
Solution
ζ = 1 is critical damping and ζ > 1 is overdamped.
Answer: (D) 0 and 1
For a second-order system, ωn = 10 rad/s and ζ = 0.5. The damped natural frequency is about:
Solution
ωd = ωn√(1 − ζ²) = 10 × √0.75 ≈ 8.66 rad/s.
Answer: (A) 8.66 rad/s
The type of a system is given by the number of:
Solution
Type 1 has one integrator, type 2 has two, and so on.
Answer: (A) Poles at the origin in the open-loop transfer function
A type-1 system has zero steady-state error for a:
Solution
It has finite error for a ramp and infinite error for a parabola.
Answer: (A) Step input
The Bode plot shows:
Solution
Gain in dB and phase are plotted against log frequency.
Answer: (C) Magnitude and phase against frequency
The gain margin of a stable system is measured at the:
Solution
It is how much gain can increase before the system becomes unstable.
Answer: (D) Phase crossover frequency
Root locus shows how the closed-loop poles move as:
Solution
It is plotted as K goes from 0 to infinity.
Answer: (A) The loop gain K varies
The integral (I) term in a PID controller mainly:
Solution
Integral action keeps correcting until the error is zero.
Answer: (C) Removes steady-state error
The derivative (D) term in a PID controller mainly:
Solution
It anticipates error changes and reduces overshoot.
Answer: (B) Improves damping by reacting to the rate of change of error
The time taken by a step response to rise from 10% to 90% of its final value is the:
Solution
Rise time is commonly measured from 10% to 90%.
Answer: (C) Rise time
A pole at the origin in G(s) contributes a phase of:
Solution
1/s has phase −90° at all frequencies.
Answer: (C) −90°
Signal flow graphs are reduced using:
Solution
Mason's formula gives the overall transfer function from forward paths and loops.
Answer: (A) Mason's gain formula
Increasing the gain of a type-0 system generally:
Solution
Higher gain lowers error but can cause oscillation.
Answer: (D) Reduces steady-state error but may reduce stability
The characteristic equation of a closed-loop system with open-loop G(s)H(s) is:
Solution
Its roots are the closed-loop poles.
Answer: (D) 1 + G(s)H(s) = 0
The Nyquist criterion is based on:
Solution
It counts encirclements of −1 by the open-loop frequency response.
Answer: (C) The principle of the argument
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