Control Systems Important interview questions Part 4

By Kajal Vats|Updated : May 24th, 2020

Q1. What is an all pass system?

Ans. An all pass system is one which has unity magnitude, due to symmetry of poles and zeros with respect to imaginary axis, and provides phase lag to the system.

Q2. In Routh stability criterion what happens if one of the element of first column becomes zero and there is a sign change in first column?

Ans. If there is a sign change it represents a root on RHS of s-plane, system is unstable.

Q3. In Routh stability criterion when all the elements of any row become zero and there is a sign change in first column then comment about stability of system.

Ans. If there is a sign change it represents symmetric roots w.r.t. origin and one root is on LHS and other will be on RHS of s-plane and the system is unstable.

Q4. In what condition the Bode Gain Kequals to DC gain of OLTF ?

Ans. When there is no pole or zero at origin then Bode gain becomes equal to DC gain.

Q5. What do you mean by phase cross over frequency? Give its significance.

Ans. Frequency at which Nyquist plot or the polar plot crosses the negative real axis or at which Bode plot crosses 180o phase line is known as phase cross over frequency. At phase cross over frequency we calculate Gain Margin.

Q6. What do you mean by gain cross over frequency? Give significance.

Ans. Frequency at which magnitude Bode plot crosses 0 dB axis is known as gain cross over frequency. At gain cross over frequency we calculate phase margin.

Q7. Give stability condition in terms of gain cross over frequency and phase cross over frequency.

Ans. For a system to be stable phase cross over frequency should be greater than gain cross over frequency.

Q8. Differentiate between conditionally stable system and marginally stable system in terms of Gain Margin.

Ans. For conditionally stable system 0 < GM < ∞ and for marginally stable system GM = 0 dB as for conditionally stable system gain K is less than one and for marginally stable system gain is equals to 1.

Q9. Give methods for determining controllability of a system.

Ans. Controllability Matrix and Gilbert test are two methods for determining controllability of a system. In Controllability Matrix , if determinant of matrix is non-zero, the system is controllable.

Q10. Give methods for determining observability of a control system.

Ans. Observability Matrix and Gilbert text are two methods for determining observability of a system . If determinant of Observability Matrix is non-zero then system is said to be observable.

Q11. What is state transition matrix? Give significance of it.

Ans. A state transition matrix is given by

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If is a powerful method for determining the behavior of dynamic systems and represent the output in terms of the input variables.

Q12. Given DC gain of a closed loop system is one then what should be the steady state error for unit step input?

Ans. Since ,

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 So where  T(0) = 1

 then ess = 0.

Q13. What do you mean by term control in control system ?

Ans. Control means to keep something under limits i.e. the system performs as per intended design. In Electronics control word is used to keep output of a system within specified limits.

Q14. Give differences between open and closed loop systems?

Ans. Open loop systems are simple to construct and cheap. They are generally stable but at the same time less accurate than closed loop systems. The closed loop system is faster as compared to open loop system.

Q15. What do you understand by the term relative stability of a system?

Ans. The relative stability of a system can be ascertained using techniques such as Nyquist plot in which the degree stability can be estabilished. The absolute stability techniques gives the result in yes-no manner.

Q16. Define transfer function of a system.

Ans. Transfer function is the ratio of the Laplace transform of the output of a system to the Laplace transform of the input, considering its initial conditions to be zero.

Q17. What is lead-lag compensator?

Ans. It consists of a lead compensator cascaded with lag compensator and is given as

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Q18. Why is it necessary to stabilize a system?

Ans. The stability of a control system is often extremely important and is also a safety issue. Unstable systems in sensitive places like power-plants, nuclear reactors, automated vehicles  can generally yield catastrophic results.

Q19. What are the different types of stability criterion ?

Ans. A linear system can be characterized as Absolutely stable : With respect to a parameter of the system if a system is stable for all values of this parameter then it is absolutely stable.

Conditionally stable : With respect to a parameter, if the system is stable for only certain bounded ranges of values of this parameter then it is conditionally stable.

Q20. What do you understand by transient response and steady state response?

Ans. Transient state : A system is said to be in transient state when the input condition changes abruptly and its response varies with time or their is sudden change in loading.

       Steady state : If the output of the system does not change with respect to time.

Q21. Why is stability of a system checked by bounded input signal?

Ans. A bounded signal is a signal such that there exists a value such that the absolute value of the signal is never greater than some value, which means that at no point can the signal tend to infinity. Thus, these signals are used to test stability.

Q22. What do you understand by marginal stability?

Ans.   In marginal stability the response of the system is constant i.e. oscillations with constant amplitude. A linear time-invariant system is marginally stable if it is neither asymptotically stable nor unstable.

Q23. Is it possible to destabilize the system by using feedback?

Ans. Generally Positive feedback can destabilize the system but negative feedback can also destabilize the system if used with very high forward path gain.

Q24. Is it possible that a stable system has positive PM and negative GM?

Ans. For stable system PM and GM should positive and for system to be unstable PM and GM would be negative.

Q25. What is the use of Nicole’s chart?

Ans. Nicole’s chart is the dB version of constant M and N circles. Nicole’s chart is used for gain adjustment based on desirable values of resonant peak.

Q26. Can stability be given by using M and N circle?

Ans. Not directly because constant M-N circles are used to determine closed loop resonance peak and resonance frequency.

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