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Control Systems
Stability

Practice questions from Stability.

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Q#1 Stability GATE EC 2025 (Set 1) MCQ +2 marks -0.66 marks

Consider the polynomial

. Let  be defined as follows.

 is the number of roots of  with negative real parts.

 is the number of roots of  that are purely imaginary.

 is the number of roots of  with positive real parts.

Which one of the following options is correct?

, and

, and

, and

, and

Explanation:

 

Routh Hurwitz criteria,

No. of sign change in  column of RH table .

Auxiliary equation:

 

    Imaginary Poles

 and

  

 

Hence, option (a) is correct.

Q#2 Stability GATE EC 2024 (Set 1) MCQ +1 mark -0.33 marks

In the feedback control system shown in the figure below .

R(s), Y(s) and E(s) are the Laplace transforms of r(t), y(t), and e(t), respectively. If the input r(t) is a unit step function, then ________.

 does not exist, e(t) is oscillatory

Explanation:

 

 

 

Here,

i.e inner product  outer product

System is marginally stable.

 oscillatory system

 doesn't exist

Q#3 Stability GATE EC 2024 (Set 1) MCQ +2 marks -0.66 marks

Consider a unity negative feedback control system with forward path gain  as shown.

The impulse response of the closed-loop system decays faster than  if

Explanation:

 

Impulse response

 

To decay faster than , poles must lie to the left of

Replace  

If poles lie to the left of imaginary axis after transformation then poles will lie to the left of s=-1

 

   

 

For stability inner producer  outer product

Option (a) satisfies