43 + Interview Questions in Stability In Frequency Domain in Control Systems Page 1 InterviewSolution

1.	The polar plot of the transfer function G(s) = 10(s+1)/s+10 will be in the :(a) First quadrant(b) Second quadrant(c) Third quadrant(d) Fourth quadrantI have been asked this question during a job interview.My question is from Assessment of Relative Stability Using Nyquist Criterion topic in division Stability in Frequency Domain of Control Systems
Answer» Right OPTION is (d) Fourth quadrant The best I can EXPLAIN: Polar PLOT of the given transfer FUNCTION lies in the fourth quadrant.

Discussion

2.	The phase angle of the system, G(s) =s+5/s^2+4s+9, varies between :(a) 0° and 90°(b) 0° and -90°(c) 0° and -180°(d) -90° and -180°I had been asked this question during an interview.Query is from Assessment of Relative Stability Using Nyquist Criterion topic in section Stability in Frequency Domain of Control Systems
Answer» CORRECT option is (a) 0° and 90° Easiest EXPLANATION: As it is the type 0 SYSTEM so the PHASE angle can be 0° and 90°.

Discussion

3.	Which of the following is the non-linearity caused by servomotor?(a) Static friction(b) Backlash(c) Saturation(d) None of the mentionedThe question was asked during an interview.This interesting question is from Mathematical Preliminaries topic in portion Stability in Frequency Domain of Control Systems
Answer» The correct option is (c) Saturation For explanation: Linearity in the servo-motor is due to the saturation present and which is the important CONSIDERATION for any type of the PRACTICAL DEVICE.

Discussion

4.	Closed loop frequency response is very useful as :(a) It approximately predicts the time response of the systems(b) Time response are converted into time domain specifications(c) After design time domain specification is converted into frequency domain(d) All of the mentionedThe question was posed to me in homework.I need to ask this question from Closed-Loop Frequency Response in portion Stability in Frequency Domain of Control Systems
Answer» The correct choice is (d) All of the mentioned Easy explanation: Closed loop frequency response is very useful as it enables to USE SECOND ORDER CORRELATIONS between frequency and transient response.

Discussion

5.	When the period of the observation is large, the type of the error will be:(a) Transient error(b) Steady state error(c) Half-power error(d) Position error constantI got this question in an interview.The origin of the question is Time Domain Analysis in section Stability in Frequency Domain of Control Systems
Answer» RIGHT CHOICE is (b) Steady state error For explanation I would say: The error will be the steady state error if the period of observation is LARGE as the time if large then the final value THEOREM can be directly APPLIED.

Discussion

6.	A system has a single pole at origin. Its impulse response will be:(a) Constant(b) Ramp(c) Decaying exponential(d) OscillatoryI got this question in semester exam.The query is from Time Domain Analysis topic in division Stability in Frequency Domain of Control Systems
Answer» Correct answer is (a) Constant For explanation I WOULD say: For a single pole at origin the SYSTEM is of TYPE 1 and impulse response of the system with single pole at the origin will be constant.

Discussion

7.	The system with the open loop transfer function 1/s(1+s) is:(a) Type 2 and order 1(b) Type 1 and order 1(c) Type 0 and order 0(d) Type 1 and order 2I have been asked this question by my college professor while I was bunking the class.This intriguing question originated from Time Domain Analysis topic in chapter Stability in Frequency Domain of Control Systems
Answer» The correct answer is (d) Type 1 and order 2 For explanation: Type is defined as the number of POLES at ORIGIN and order is defined as the total number of poles and this is CALCULATED with the help of the TRANSFER function from the above transfer function the type is 1 and order is 2.

Discussion

8.	If a Nyquist plot of G (jω) H (jω) for a closed loop system passes through (-2, j0) point in GH plane, what would be the value of gain margin of the system in dB?(a) 0 dB(b) 2.0201 dB(c) 4 dB(d) 6.0205 dBThis question was posed to me by my school teacher while I was bunking the class.This intriguing question comes from Nyquist Stability Criterion topic in division Stability in Frequency Domain of Control Systems
Answer» Right option is (d) 6.0205 dB The best I can EXPLAIN: GAIN Margin is calculated by taking inverse of the gain where the Nyquist PLOT CUTS the real AXIS.

Discussion

9.	The important figures of merit to the system in frequency domain are :(a) Maximum value of resonant peak(b) Value of resonance frequency(c) Maximum value of resonant peak and Value of resonance frequency(d) Minimum value of resonance frequencyI got this question in a national level competition.The above asked question is from Closed-Loop Frequency Response in portion Stability in Frequency Domain of Control Systems
Answer» RIGHT option is (c) Maximum value of RESONANT peak and Value of resonance frequency The EXPLANATION is: The important figures of MERIT to the system in frequency DOMAIN are maximum value of resonance peak and frequency at which it occurs.

Discussion

10.	As the polar plot moves toward the point (-1, 0) then the system becomes :(a) Stable(b) Marginally stable(c) Conditionally stable(d) UnstableThis question was addressed to me in final exam.My enquiry is from Assessment of Relative Stability Using Nyquist Criterion topic in section Stability in Frequency Domain of Control Systems
Answer» Right choice is (d) UNSTABLE The explanation: As the polar plot MOVES toward the point (-1, 0) then the SYSTEM becomes unstable.

Discussion

11.	Laplace transform is used for the systems with:(a) Initial conditions infinite(b) Initial conditions zero(c) Initial conditions finite(d) Initial conditions equal to the initial inputThe question was asked during a job interview.I'd like to ask this question from Mathematical Preliminaries topic in division Stability in Frequency Domain of Control Systems
Answer» Correct ANSWER is (b) Initial conditions zero To explain I would say: By definition of the TRANSFER FUNCTION by Laplace transform the initial conditions of the SYSTEMS are considered to be zero.

Discussion

12.	The parameter which is constant along the constant N circle :(a) Frequency(b) Phase angle(c) Magnitude(d) Open loop gainI got this question in homework.My query is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems
Answer» RIGHT choice is (B) Phase angle The BEST explanation: Phase angle remains constant in constant N-circle.

Discussion

13.	Cut-off rate is the ability of the system to distinguish the signal from _____________(a) Bandwidth(b) Noise(c) Resonance peak(d) MagnitudeI had been asked this question in my homework.My doubt stems from Closed-Loop Frequency Response in section Stability in Frequency Domain of Control Systems
Answer» CORRECT option is (b) Noise Explanation: Cut-off rate is the ABILITY of the SYSTEM to DISTINGUISH the signal from noise.

Discussion

14.	A differentiator is usually not a part of a control system because it(a) Reduces damping(b) Reduces the gain margin(c) Increases input noise(d) Increases errorThis question was addressed to me in my homework.Origin of the question is Mathematical Preliminaries in division Stability in Frequency Domain of Control Systems
Answer» Correct option is (c) INCREASES input noise For explanation: A differentiator is generally not the part of the control system as it increases the input noise DUE to reduced gain and INCREASED BANDWIDTH.

Discussion

15.	______________technique gives quick transient and stability response(a) Root locus(b) Bode(c) Nyquist(d) NicholsThis question was addressed to me by my school teacher while I was bunking the class.My doubt stems from Mathematical Preliminaries in division Stability in Frequency Domain of Control Systems
Answer» Correct CHOICE is (a) Root locus For explanation: Root locus is the right technique for the QUICK TRANSIENT and stability response and gives the final response and QUICKLY.

Discussion

16.	The concepts used to measure relative stability are:(a) Phase margin(b) Gain margin(c) Phase and Gain margin(d) StableThis question was addressed to me in an interview for internship.I want to ask this question from Assessment of Relative Stability Using Nyquist Criterion topic in division Stability in Frequency Domain of Control Systems
Answer» RIGHT choice is (c) PHASE and GAIN margin The best I can explain: The concepts used to MEASURE relative stability are phase margin and gain margin.

Discussion

17.	In q(s) plane Nyquist plot is symmetrical about the :(a) Imaginary axis(b) Real axis(c) Origin(d) None of the mentionedI have been asked this question by my college professor while I was bunking the class.My question comes from Closed-Loop Frequency Response in section Stability in Frequency Domain of Control Systems
Answer» The correct answer is (B) REAL AXIS For explanation I would say: By definition, In Q(s) plane Nyquist plot is symmetrical about the real axis.

Discussion

18.	A third order system is approximated to an equivalent second order system. The rise time of this approximated lower order system will be:(a) Same as the original system for any input(b) Smaller than the original system for any input(c) Larger than the original system for any input(d) Larger or smaller depending on the inputI had been asked this question during an online exam.The question is from Time Domain Analysis topic in portion Stability in Frequency Domain of Control Systems
Answer» Right CHOICE is (B) SMALLER than the ORIGINAL system for any input To explain: As order of the system increases the system approaches more towards the ideal characteristics and if the third order system is approximated to an equivalent second order system then the rise time of this will be smaller than the original system for any input.

Discussion

19.	With negative feedback in a closed loop control system, the system sensitivity to parameter variation:(a) Increases(b) Decreases(c) Becomes zero(d) Becomes infiniteThis question was addressed to me in homework.I would like to ask this question from Time Domain Analysis topic in portion Stability in Frequency Domain of Control Systems
Answer» Correct ANSWER is (b) Decreases Best explanation: Sensitivity is defined as the change in the output with respect to the change in the input and due to NEGATIVE feedback REDUCES by a factor of 1/ (1+GH).

Discussion

20.	Which of the following is an electromechanical device?(a) Induction relay(b) Thermocouple(c) LVDT(d) All of the mentionedI have been asked this question in homework.The question is from Mathematical Preliminaries topic in chapter Stability in Frequency Domain of Control Systems
Answer» The CORRECT choice is (c) LVDT The best I can explain: LVDT is the electromechanical DEVICE which converts the displacement of the CORE to the ELECTRICAL energy by the action of the transformer.

Discussion

21.	The phase margin (in degrees) of a system having the loop transfer function G(s)H(s)=2√3/s(s+1) is:(a) 45°(b) -30°(c) 60°(d) 30°This question was posed to me during an interview.My doubt is from Assessment of Relative Stability Using Nyquist Criterion topic in section Stability in Frequency Domain of Control Systems
Answer» The CORRECT choice is (d) 30° For explanation I would say: PHASE margin is calculated at gain CROSS over FREQUENCY where magnitude of the TRANSFER function is 1.

Discussion

22.	Consider a feedback system with gain margin of about 30. At what point does Nyquist plot crosses negative real axis?(a) -3(b) -0.3(c) -30(d) -0.03I got this question in my homework.Query is from Nyquist Stability Criterion in division Stability in Frequency Domain of Control Systems
Answer» RIGHT choice is (b) -0.3 To explain: Gain Margin is always INVERSE of the point which CUTS the Nyquist on the real AXIS.

Discussion

23.	A polar plot intersects the unit circle at a point making -45° to the negative real axis then the phase margin of the system is :(a) -45°(b) 45°(c) 180°-45°(d) 180°+45°This question was addressed to me in an online interview.My doubt stems from Closed-Loop Frequency Response topic in section Stability in Frequency Domain of Control Systems
Answer» The correct CHOICE is (b) 45° The EXPLANATION: As the angle is with the negative REAL axis HENCE the Phase MARGIN will be 45°.

Discussion

24.	According to Nyquist stability criterion, where should be the position of all zeros of q(s) corresponding to s-plane?(a) On left half(b) At the center(c) On right half(d) RandomI have been asked this question during an interview for a job.I would like to ask this question from Nyquist Stability Criterion topic in division Stability in Frequency Domain of Control Systems
Answer» Correct answer is (a) On LEFT HALF Easiest explanation: ACCORDING to Nyquist stability criterion zeroes must LIE on the left half on the s plane.

Discussion

25.	The identical first order system have been cascaded non-interactively. The unit step response of the systems will be:(a) Overdamped(b) Underdamped(c) Undamped(d) Critically dampedI had been asked this question in unit test.I'd like to ask this question from Time Domain Analysis in section Stability in Frequency Domain of Control Systems
Answer» Right answer is (d) CRITICALLY DAMPED To explain I WOULD SAY: Since both the SYSTEMS that is the first order systems are cascaded non-interactively, the overall unit step response will be critically damped.

Discussion

26.	Which principle specifies the relationship between enclosure of poles & zeros by s-plane contour and the encirclement of origin by q(s) plane contour?(a) Argument(b) Agreement(c) Assessment(d) AssortmentI had been asked this question in an online quiz.I'd like to ask this question from Nyquist Stability Criterion topic in division Stability in Frequency Domain of Control Systems
Answer» Correct CHOICE is (a) Argument The explanation is: Argument PRINCIPLE specifies the relationship between enclosure of poles & zeros by s-PLANE CONTOUR and the encirclement of origin by q(s) plane contour.

Discussion

27.	For Nyquist contour, the size of radius is _______(a) 25(b) 0(c) 1(d) ∞This question was posed to me by my college professor while I was bunking the class.Origin of the question is Nyquist Stability Criterion in division Stability in Frequency Domain of Control Systems
Answer» Right OPTION is (d) ∞ The EXPLANATION: For Nyquist contour, the SIZE of RADIUS is ∞.

Discussion

28.	Due to an addition of pole at origin, the polar plot gets shifted by ___ at ω = 0 ?(a) -45°(b) -60°(c) -90°(d) -180°The question was posed to me in semester exam.This intriguing question comes from Nyquist Stability Criterion in portion Stability in Frequency Domain of Control Systems
Answer» CORRECT choice is (c) -90° To explain: ADDITION of pole causes INSTABILITY to the SYSTEM.

Discussion

29.	The system with the open loop transfer function G(s) H(s) =1/s(s^^2+s+1) has the gain margin of :(a) -6 dB(b) 0 dB(c) 3.5 dB(d) 6 dBI have been asked this question during a job interview.This question is from Assessment of Relative Stability Using Nyquist Criterion topic in portion Stability in Frequency Domain of Control Systems
Answer» Correct option is (b) 0 dB To explain: Gain margin is calculated at PHASE CROSS over FREQUENCY where the phase is 180°.

Discussion

30.	When the unit step response of a unity feedback control system having forward path transfer function G (s) =80/s(s+18)?(a) Overdamped(b) Critically damped(c) Under damped(d) Un Damped oscillatoryThe question was posed to me in quiz.Query is from Time Domain Analysis in division Stability in Frequency Domain of Control Systems
Answer» Right option is (a) Overdamped The best explanation: The open loop transfer function is first CONVERTED into the CLOSED loop as unity FEEDBACK is used and then VALUE of damping factor is calculated.

Discussion

31.	____________can be extended to systems which are time-varying?(a) Bode-Nyquist stability methods(b) Transfer functions(c) Root locus design(d) State model representativesThe question was posed to me in an interview for internship.Query is from Mathematical Preliminaries topic in section Stability in Frequency Domain of Control Systems
Answer» Correct option is (d) State model representatives The BEST explanation: State model representation is the best advantage for the systems over transfer FUNCTION APPROACH as it can be APPLIED for the non-linear systems.

Discussion

32.	Which of the following devices is used for conversion of co-ordinates?(a) Microsyn(b) Selsyn(c) Synchro-resolver(d) Synchro-transformerThe question was asked in homework.My doubt is from Mathematical Preliminaries in chapter Stability in Frequency Domain of Control Systems
Answer» Right CHOICE is (c) Synchro-resolver Easy EXPLANATION: Synchro-Resolver is ROTARY transformer used for MEASURING degree of ROTATION and used for the conversion of the co-ordinates.

Discussion

33.	Given a unity feedback system with G (s) =K/ s (s+4). What is the value of K for a damping ratio of 0.5?(a) 1(b) 16(c) 4(d) 2I got this question in an internship interview.I'd like to ask this question from Time Domain Analysis in section Stability in Frequency Domain of Control Systems
Answer» RIGHT CHOICE is (b) 16 The best explanation: Comparing the EQUATION with the standard characteristic equation gives the value of damping factor, natural frequency and value of GAIN K.

Discussion

34.	Consider the system represented by the equation given below. What would be the total phase value at ω = 0?200/[s3 (s + 3) (s + 6) (s + 10)].(a) -90°(b) -180°(c) -270°(d) -360°The question was asked in class test.The above asked question is from Nyquist Stability Criterion in chapter Stability in Frequency Domain of Control Systems
Answer» Right option is (c) -270° To ELABORATE: The PHASE can be calculated by the basic FORMULA for calculating phase ANGLE.

Discussion

35.	Gain margin is:(a) It is a factor by which the system gain can be increased to drive it to the verge of instability(b) It is calculated at gain cross over frequency(c) It is calculated at phase cross over frequency(d) Both a and cI got this question in an interview.I want to ask this question from Assessment of Relative Stability Using Nyquist Criterion topic in chapter Stability in Frequency Domain of Control Systems
Answer» RIGHT choice is (d) Both a and c Explanation: Gain MARGIN is a factor by which the system gain can be INCREASED to drive it to the verge of instability and is calculated at PHASE CROSS over frequency.

Discussion

36.	Bandwidth is :(a) Range of frequencies for which the system gain is more than-3dB(b) This is considered good range for transmission of signals(c) For better communication bandwidth of signal must be less(d) All of the mentionedThe question was posed to me in a national level competition.The query is from Closed-Loop Frequency Response in division Stability in Frequency Domain of Control Systems
Answer» Correct option is (d) All of the mentioned Easiest EXPLANATION: Bandwidth of a signal must be less but CHANNEL must be more foe BETTER TRANSMISSION or communication.

Discussion

37.	In polar plots, if a pole is added at the origin, what would be the value of the magnitude at Ω = 0?(a) Zero(b) Infinity(c) Unity(d) UnpredictableThe question was posed to me during an online interview.I'd like to ask this question from Nyquist Stability Criterion topic in chapter Stability in Frequency Domain of Control Systems
Answer» RIGHT ANSWER is (b) Infinity Easiest explanation: ADDITION of pole CAUSES instability to the system.

Discussion

38.	Maximum peak overshoot in time domain corresponds to :(a) Resonance peak(b) Resonant frequency(c) Bandwidth(d) Cut-off rateI had been asked this question at a job interview.Enquiry is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems
Answer» Correct answer is (a) Resonance PEAK To explain: Resonance peak in frequency DOMAIN CORRESPOND to MAXIMUM peak overshoot in time domain.

Discussion

39.	Frequency of oscillation in time domain correspond to :(a) Resonance peak(b) Resonant frequency(c) Bandwidth(d) Cut-off rateThe question was posed to me in a national level competition.Query is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems
Answer» RIGHT answer is (b) Resonant frequency For EXPLANATION: Resonant frequency in frequency domain correspond to frequency of OSCILLATION in TIME domain.

Discussion

40.	An underdamped second order system with negative damping will have the roots :(a) On the negative real axis as roots(b) On the left hand side of complex plane as complex roots(c) On the right hand side of complex plane as complex conjugates(d) On the positive real axis as real rootsThe question was posed to me by my school teacher while I was bunking the class.I'd like to ask this question from Time Domain Analysis topic in division Stability in Frequency Domain of Control Systems
Answer» CORRECT answer is (c) On the right HAND side of complex plane as complex CONJUGATES The best explanation: An underdamped second order SYSTEM is the system which has damping factor less than unity and with negative damping will have the roots on the right hand side of complex plane as complex conjugates.

Discussion

41.	Phase and gain margin are applicable to open and closed loop systems both.(a) True(b) FalseThis question was posed to me in an online quiz.I want to ask this question from Assessment of Relative Stability Using Nyquist Criterion in portion Stability in Frequency Domain of Control Systems
Answer» Right answer is (B) False The best I can EXPLAIN: PHASE and gain margin are applicable only to open loop systems.

Discussion

42.	If the system is represented by G(s) H(s) = k (s+7) / s (s +3) (s + 2), what would be its magnitude at ω = ∞?(a) 0(b) ∞(c) 7/10(d) 21The question was asked at a job interview.This key question is from Nyquist Stability Criterion in section Stability in Frequency Domain of Control Systems
Answer» Correct CHOICE is (a) 0 The best EXPLANATION: On calculating the magnitude of the SYSTEM and PUTTING the VALUE of frequency one gets the magnitude as 0.

Discussion

43.	Polar plots moving toward the imaginary axis makes the system:(a) Stable(b) Marginally stable(c) Conditionally stable(d) UnstableThis question was addressed to me in an internship interview.I want to ask this question from Assessment of Relative Stability Using Nyquist Criterion in chapter Stability in Frequency Domain of Control Systems
Answer» Right ANSWER is (d) UNSTABLE To elaborate: POLAR plots moving TOWARD the imaginary axis MAKES the system unstable.

Discussion

Explore topic-wise InterviewSolutions in .

A system has a single pole at origin. Its impulse response will be:(a) Constant(b) Ramp(c) Decaying exponential(d) OscillatoryI got this question in semester exam.The query is from Time Domain Analysis topic in division Stability in Frequency Domain of Control Systems

The parameter which is constant along the constant N circle :(a) Frequency(b) Phase angle(c) Magnitude(d) Open loop gainI got this question in homework.My query is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems

Cut-off rate is the ability of the system to distinguish the signal from _____________(a) Bandwidth(b) Noise(c) Resonance peak(d) MagnitudeI had been asked this question in my homework.My doubt stems from Closed-Loop Frequency Response in section Stability in Frequency Domain of Control Systems

Which of the following is an electromechanical device?(a) Induction relay(b) Thermocouple(c) LVDT(d) All of the mentionedI have been asked this question in homework.The question is from Mathematical Preliminaries topic in chapter Stability in Frequency Domain of Control Systems

Consider a feedback system with gain margin of about 30. At what point does Nyquist plot crosses negative real axis?(a) -3(b) -0.3(c) -30(d) -0.03I got this question in my homework.Query is from Nyquist Stability Criterion in division Stability in Frequency Domain of Control Systems

For Nyquist contour, the size of radius is _______(a) 25(b) 0(c) 1(d) ∞This question was posed to me by my college professor while I was bunking the class.Origin of the question is Nyquist Stability Criterion in division Stability in Frequency Domain of Control Systems

Due to an addition of pole at origin, the polar plot gets shifted by ___ at ω = 0 ?(a) -45°(b) -60°(c) -90°(d) -180°The question was posed to me in semester exam.This intriguing question comes from Nyquist Stability Criterion in portion Stability in Frequency Domain of Control Systems

Which of the following devices is used for conversion of co-ordinates?(a) Microsyn(b) Selsyn(c) Synchro-resolver(d) Synchro-transformerThe question was asked in homework.My doubt is from Mathematical Preliminaries in chapter Stability in Frequency Domain of Control Systems

Given a unity feedback system with G (s) =K/ s (s+4). What is the value of K for a damping ratio of 0.5?(a) 1(b) 16(c) 4(d) 2I got this question in an internship interview.I'd like to ask this question from Time Domain Analysis in section Stability in Frequency Domain of Control Systems

Maximum peak overshoot in time domain corresponds to :(a) Resonance peak(b) Resonant frequency(c) Bandwidth(d) Cut-off rateI had been asked this question at a job interview.Enquiry is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems

Frequency of oscillation in time domain correspond to :(a) Resonance peak(b) Resonant frequency(c) Bandwidth(d) Cut-off rateThe question was posed to me in a national level competition.Query is from Closed-Loop Frequency Response topic in division Stability in Frequency Domain of Control Systems

Phase and gain margin are applicable to open and closed loop systems both.(a) True(b) FalseThis question was posed to me in an online quiz.I want to ask this question from Assessment of Relative Stability Using Nyquist Criterion in portion Stability in Frequency Domain of Control Systems

If the system is represented by G(s) H(s) = k (s+7) / s (s +3) (s + 2), what would be its magnitude at ω = ∞?(a) 0(b) ∞(c) 7/10(d) 21The question was asked at a job interview.This key question is from Nyquist Stability Criterion in section Stability in Frequency Domain of Control Systems