Multiple Choice Questions and Answers on Control Systems

1)   Which terminology deals with the excitation or stimulus applied to the system from an external source for the generation of an output?

a. Input signal
b. Output signal
c. Error signal
d. Feedback signal

ANSWER: (a)  Input signal

2)   Which among the following is not an advantage of an open loop system?

a. Simplicity in construction & design
b. Easy maintenance
c. Rare problems of stability
d. Requirement of system recalibration from time to time

ANSWER: (d) Requirement of system recalibration from time to time

3)   Which notation represents the feedback path in closed loop system representation?

a. b(t)
b. c(t)
c. e(t)
d. r(t)

ANSWER: (a) b(t)

4)   Which among the following represents an illustration of closed loop system?

a. Automatic washing machine
b. Automatic electric iron
c. Bread toaster
d. Electric hand drier

ANSWER: (b) Automatic electric iron

5)   How is an output represented in the control systems?

a. r(t)
b. c(t)
c. x(t)
d. y(t)

ANSWER: (b) c(t)

6)   The output is said to be zero state response because ______conditions are made equal to zero.

a. Initial
b. Final
c. Steady state
d. Impulse response

ANSWER: (a) Initial

7)   Basically, poles of transfer function are the laplace transform variable values which causes the transfer function to become ___________

a. Zero
b. Unity
c. Infinite
d. Average value

ANSWER:(c)  Infinite

8)   By equating the denominator of transfer function to zero, which among the following will be obtained?

a. Poles
b. Zeros
c. Both a and b
d. None of the above

ANSWER: (a) Poles

9)   The output signal is fed back at the input side from the _________point

a. Summing
b. Differential
c. Take-off
d. All of the above

ANSWER: (c) Take-off

10)   In a parallel combination, the direction of flow of signals through blocks in parallel must resemble to the main _______

a. Forward
b. Feedback
c. Opposite
d. Diagonal

ANSWER: (a) Forward

11)   While shifting a take-off point after the summing point, which among the following should be added?

a. Summing point in series with take-off point
b. Summing point in parallel with take-off point
c. Block of reciprocal transfer function
d. Block of inverse transfer function

ANSWER: (a) Summing point in series with take-off point

12)   Consider the assertions related to block diagram. Which among them represents the precise condition?

A. Block diagram is used for analysis & design of control system.
B. Block diagram also provides the information regarding the physical construction of the system.

a. A is true, B is false
b. A is false, B is true
c. Both A & B are true
d. Both A & B are false

ANSWER: (a) A is true, B is false

13)   In a signal flow graph, nodes are represented by small _____

a. Circles
b. Squares
c. Arrows
d. Pointers

ANSWER: (a) Circles

14)   According to signal flow graph, which among the following represents the relationship between nodes by drawing a line between them?

a. Branch
b. Self-loop
c. Semi-node
d. Mesh

ANSWER: (a) Branch

15)   Which type of node comprises incoming as well as outgoing branches?

a. Source node
b. Sink node
c. Chain node
d. Main node

ANSWER: (c) Chain node

16)   Where are the dummy nodes added in the branch with unity gain?

a. At input & output nodes
b. Between chain nodes
c. Both a and b
d. None of the above

ANSWER: (a) At input & output nodes

17)   According to the property of impulse test signal, what is the value of an impulse at t = 0?

a. Zero
b. Unity
c. Infinite
d. Unpredictable

ANSWER: (c) Infinite

18)   What is the value of parabolic input in Laplace domain?

a. 1
b. A/s
c. A/s2
d. A/s3

ANSWER: (d) A/s3

19)   Which among the following is/are an/the illustration/s of a sinusoidal input?

a. Setting the temperature of an air conditioner
b. Input given to an elevator
c. Checking the quality of speakers of music system
d. All of the above

ANSWER: (c) Checking the quality of speakers of music system

20)   If a system is subjected to step input, which type of static error coefficient performs the function of controlling steady state error?

a. Position
b. Velocity
c. Acceleration
d. Retardation

ANSWER: (a) Position

21)   On the basis of an output response, into how many parts can the s-plane be divided?

a. 2
b. 3
c. 4
d. 6

ANSWER: (b) 3

22)   If the complex conjugate poles are located at RHP, what would be the nature of corresponding impulse response?

a. Exponential
b. Damping oscillations
c. Increasing amplifier
d. Constant amplitude oscillations

ANSWER: (c) Increasing amplifier

23)   Which among the following are solely responsible in determining the speed of response of control system?

a. Poles
b. Zeros
c. Speed of input
d. All of the above

ANSWER: (a) Poles

24)   If a pole is located at s = -5 in left-hand plane (LHP), how will it be represented in Laplace domain?

a. 1/ s + 5
b. 1/ s – 5
c. s/ s + 5
d. s/ s – 5

ANSWER: (a) 1/ s + 5

25)   In second order system, which among the following remains independent of gain (k)?

a. Open loop poles
b. Closed loop poles
c. Both a and b
d. None of the above

ANSWER: (a) Open loop poles

26)   Root locus specifies the movement of closed loop poles especially when the gain of system ________

a. Remains constant
b. Exhibit variations
c. Gives zero feedback
d. Gives infinite poles

ANSWER:(b) Exhibit variations

27)   Which condition is used to verify the existence of a particular point on the root locus?

a. Amplitude
b. Frequency
c. Magnitude
d. Angle

ANSWER: (d) Angle

28)   While specifying the angle and magnitude conditions, angles are added whereas magnitudes get ________ 

a. Subtracted
b. Multiplied
c. Divided
d. All of the above

ANSWER: (b) Multiplied

29)   The magnitude & phase relationship between _________input and the steady state output is called as frequency domain.

a. Step
b. Ramp
c. Sinusoidal
d. Parabolic

ANSWER: (c) Sinusoidal

30)   If a linear system is subjected to an input r(t) = Asin(ωt), what output will be generated?

a. c(t) = B sin (ωt + Φ)
b. c(t) = B cos (ωt + Φ)
c. c(t) = B tan (ωt + Φ)
d. c(t) = B cot (ωt + Φ)

ANSWER: (a) c(t) = B sin (ωt + Φ)

31)   Which unit is adopted for magnitude measurement in Bode plots?

a. Degree
b. Decimal
c. Decibel
d. Deviation

ANSWER: (b) Decibel

32)   In an octave frequency band, the ratio of f2 / f1 is equivalent to ________

a. 2
b. 4
c. 8
d. 10

ANSWER: (a) 2

33)   In polar plots, what does each and every point represent w.r.t magnitude and angle?

a. Scalar
b. Vector
c. Phasor
d. Differentiator

ANSWER: (c) Phasor

34)   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. Unpredictable

ANSWER: (b) Infinity

35)   Conventional control theory is applicable to ______ systems

c. Time varying
d. Non-linear


36)   State space analysis is applicable even if the initial conditions are _____

a. Zero
b. Non-zero
c. Equal
d. Not equal

ANSWER: (b) Non-zero

37)   If an error signal e(t) of an ON-OFF controller is found to be greater than zero, what would be its output?

a. 10%
b. 50%
c. 80%
d. 100%

ANSWER: (d) 100%

38)   Which time is responsible for introducing an error in the temperature regulation of applications associated with ON-OFF controllers?

a. Rise time
b. Dead time
c. Switching time
d. Decay time

ANSWER: (b) Dead time

39)   Which controller has the potential to eliminate/overcome the drawback of offset in proportional controllers?

a. P-I
b. P-D
c. Both a and b
d. None of the above


40)   In P-I controller, what does an integral of a function compute?

a. Density of curve
b. Area under the curve
c. Volume over the curve
d. Circumference of curve

ANSWER: (b) Area under the curve

41)   Which among the following controls the speed of D.C. motor?

a. Galvanometer
b. Gauss meter
c. Potentiometer
d. Tachometer

ANSWER: (d) Tachometer

42)   Into which energy signal does the position sensor convert the measured position of servomotor in servomechanisms?

a. Mechanical
b. Electrical
c. Thermal
d. Light

ANSWER: (b) Electrical

43)   What is the value of steady state error in closed loop control systems?

a. Zero
b. Unity
c. Infinity
d. Unpredictable

ANSWER: (a)  Zero

44)   A good control system should be sensitive to __________

a. Internal disturbances
b. Environmental parameters
c. Parametric variations
d. Input signals (except noise)

ANSWER: (d) Input signals (except noise)

45)   For the transfer function given below, where does the zero of the system lie?

G(s) = 5s -1 / s2 + 5s + 4

a. s = -1 & s = -1/4
b. s = -4 & s = -1
c. s = 1/5
d. s = -1/5

ANSWER:  (c) s = 1/5

46)   If a signal is passed through an integrator, it _____the amplitude of noise signal.

a. Enhances
b. Reduces
c. Stabilizes
d. Factorizes

ANSWER: (b) Reduces

47)   Laplace transform of an impulse response is regarded as _____function of the system

a. Analytic
b. Parabolic
c. Transfer
d. Hypothetical

ANSWER: (c) Transfer

48)   The fundamental function of a tachometer is the conversion of angular _______ into voltage 

a. Velocity
b. Displacement
c. Acceleration
d. Current

ANSWER: (a) Velocity

49)   If finite number of blocks are connected in series or cascade configuration, then how are the blocks combined algebraically?

a. By addition
b. By multiplication
c. By differentiation
d. By integration

ANSWER: (b) By multiplication

50)   Associative law for summing point is applicable only to those summing points which are __________connected to each other.

a. Directly
b. Indirectly
c. Orthogonally
d. Diagonally

ANSWER: (a) Directly

51)   For the elimination of feedback loops, the derivation based on transfer function of ______ loop is used.

a. Open
b. Closed
c. Both a and b
d. None of the above

ANSWER: (b) Closed

52)   In block diagram representation, what do the lines connecting the blocks, known as?

a. Branches
b. Nodes
c. Datums
d. Sources

ANSWER:(a)  Branches

53)   In a signal flow graph method, how is an overall transfer function of a system obtained?

a. Poisson’s equation
b. Block diagram reduction rules
c. Mason’s equation
d. Lagrange’s equation

ANSWER: (c) Mason’s equation

54)   While solving signal flow graph using Mason’s gain equation, what does the second letter in two subscript notation of ‘L’ stand for?

a. Serial number of loop
b. Parallel number of loop
c. Number of touching loops
d. Number of non-touching loops

ANSWER: (d) Number of non-touching loops

55)   For which systems are the signal flow graphs applicable?

a. Causal
b. Invertible
c. Linear time invariant system
d. Dynamic

ANSWER: (c) Linear time invariant system

56)   Two loops are said to be non-touching only if no common ______exists between them.

a. Loop
b. Feedback path
c. Branch
d. Node

ANSWER: (d) Node

57)   In time domain system, which response has its existence even after an extinction of transient response?

a. Step response
b. Impulse response
c. Steady state response
d. All of the above

ANSWER: (c) Steady state response

58)   Which among the following is represented by a parabolic input signal?

a. Position
b. Force
c. Velocity
d. Acceleration

ANSWER: (d) Acceleration

59)   Type 0 systems are unsuitable ___________

a. For ramp inputs
b. If the input is parabolic in nature
c. Both a and b
d. None of the above

ANSWER:  (c) Both a and b

60)   If a type 0 system is subjected to step input, what is its effect on steady state error?

a. It increases continuously
b. It remains constant
c. It decreases monotonically
d. It gets subjected to another input

ANSWER: (b) It remains constant

61)   Consider that the pole is located at origin and its laplace representation is 1/s. What would be the nature of pole response?

a. Rising exponential
b. Decaying exponential
c. Sinusoidal
d. Constant value

ANSWER: (d) Constant value

62)   In accordance to relative stability, the settling time exhibits inversely proportional nature to ________parts of roots

a. Real positive
b. Real negative
c. Imaginary positive
d. Imaginary negative

ANSWER: (b) Real negative

63)   In Routh array, if zero is found in the first column, then by which term it needs to be replaced?

a. δ
b. η
c. σ
d. ε

ANSWER: (d) ε

64)   In a second order system, if the damping ratio is greater than equal to ‘1’, then what would be the nature of roots?

a. Imaginary
b. Real and equal
c. Real but not equal
d. Complex conjugate

ANSWER: (c) Real but not equal

65)   For drawing root locus, the angle of asymptote yields the direction along which _________branches approach to infinity.

a. p + z
b. p – z
c. p / z
d. p x z

ANSWER: (b) p – z

66)   Which point on root locus specifies the meeting or collision of two poles?

a. Centroid
b. Break away point
c. Stability point
d. Anti-break point

ANSWER: (b) Break away point

67)   What should be the nature of root locus about the real axis?

a. Assymetric
b. Symmetric
c. Exponential
d. Decaying

ANSWER: (b) Symmetric

68)   If the system is specified by open loop transfer function G(s)H(s) = k / s(s+3) (s + 2), how many root loci proceed to end at infinity?

a. 2
b. 3
c. 5
d. 6

ANSWER: (b) 3

69)   Which plots in frequency domain represent the two separate plots of magnitude and phase against frequency in logarithmic value?

a. Polar plots
b. Bode plots
c. Nyquist plots
d. All of the above

ANSWER: (b) Bode plots

70)   How is the sinusoidal transfer function obtained from the system transfer function in frequency domain?

a. Replacement of ‘jω’ by ‘s’
b. Replacement of ‘s’ by ‘ω’
c. Replacement of ‘s’ by ‘jω’
d. Replacement of ‘ω’ by ‘s’

ANSWER: (c) Replacement of ‘s’ by ‘jω’

71)   According to the principle of log-scales, if the ratio between two points is same, then the two points get ________equally.

a. United
b. Separated
c. Multiplexed
d. Mixed

ANSWER: (b) Separated

72)   If a pole is located at origin, how does it get represented on the magnitude plot?

a. -10 log (ω) dB
b. -20 log (ω) dB
c. -40 log (ω) dB
d. -60 log (ω) dB

ANSWER: (b) -20 log (ω) dB

73)   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°

ANSWER: (c) -90°

74)   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°

ANSWER:  (c) -270°

75)   Which among the following are the interconnected units of state diagram representation? 

a. Scalars
b. Adders
c. Integrators
d. All of the above

ANSWER: (d) All of the above

76)   Which among the following plays a crucial role in determining the state of dynamic system?

a. State variables
b. State vector
c. State space
d. State scalar

ANSWER: (a) State variables

77)   In P-D controller, the derivative action plays a significant role in increasing _______ of response.

a. Time
b. Distance
c. Speed
d. Volume

ANSWER:(c) Speed

78)   In addition to storage instructions, PLC controls __________

a. Logic sequence timing
b. Counting
c. Arithmetic operations
d. All of the above

ANSWER: (d) All of the above

79)   Which is the correct sequence of operational steps necessary for proper operation of an elevator (lift) control mechanism?

1. Up switch
2. Stop switch
3. Down switch
4. Start switch

a. 1-2-3-4
b. 2-1-4-3
c. 4-2-1-3
d. 3-1-2-4

ANSWER: (c) 4-2-1-3

80)   How many digital inputs are present in PLCs?

a. 4
b. 8
c. 16
d. 32

ANSWER: (c) 16

81)   Which system exhibits the initiation of corrective action only after the output gets affected?

a. Feed forward
b. Feedback
c. Both a and b
d. None of the above

ANSWER: (b) Feedback

82)   What should be the nature of bandwidth for a good control system?

a. Large
b. Small
c. Medium
d. All of the above

ANSWER: (a) Large

83)   If an impulse response of a system is e-5t, what would be its transfer function?

a. 1/ s – 5
b. 1/ s + 5
c. (s+1) / (s+5)
d. (s2 – 5s)/ (s-5)

ANSWER: (b) 1/ s + 5

84)   Which among the following are the elements of rotational motion?

a. Mass, Spring, Friction
b. Inertia, Damper, Spring
c. Work, Energy, Power
d. Force, Pressure, Viscocity

ANSWER: (b) Inertia, Damper, Spring

85)   Match the following notations with their meanings:

A. G(s) ———- 1) Laplace of error signal
B. H(s) ———- 2) Laplace of output signal
C. C(s) ———- 3) Forward transfer function
D. E(s) ———- 4) Feedback transfer function

a. A- 2, B- 3, C- 1, D- 4
b. A- 3, B- 4, C- 2, D- 1
c. A- 2, B- 3, C- 4, D- 1
d. A- 1, B- 2, C- 3, D- 4

ANSWER: (b) A- 3, B- 4, C- 2, D- 1

86)   At summing point, more than one signal can be added or _________

a. Subtracted
b. Multiplied
c. Both a and b
d. None of the above

ANSWER: (a) Subtracted

87)   The value of variables at each node is _________the algebraic sum of all signals arriving at that node.

a. Less than
b. Equal to
c. Greater than
d. None of the above

ANSWER: (b) Equal to

88)   In signal flow graph, the product of all ______gains while going through a forward path is known as ‘Path gain’.

a. Branch
b. Path
c. Node
d. Loop

ANSWER: (a) Branch

89)   If a type 1 system is subjected to parabolic input, what will be the value of steady state error?

a. 0
b. 100
c. Constant k
d. Infinite

ANSWER: (d) Infinite

90)   On which factor does the steady state error of the system depend?

a. Order
b. Type
c. Size
d. Prototype

ANSWER: (b) Type

91)   Consider the equation S3 + 3s2 + 5s + 2 = 0. How many roots are located in left half of s-plane?

a. Zero
b. Two
c. Three
d. Four

ANSWER: (c) Three

92)   If the system is represented by characteristic equation s6 + s4 + s3 + s2 + s + 3 = 0, then the system is_________ 

a. Stable
b. Unstable
c. Marginally stable
d. Unpredictable

ANSWER: (b) Unstable

93)   If poles are added to the system, where will the system tend to shift the root locus?

a. To the left of an imaginary axis
b. To the right of an imaginary axis
c. At the center
d. No shifting takes place

ANSWER: (b) To the right of an imaginary axis

94)   For a unity feedback system with G(s) = 10 / s2, what would be the value of centroid?

a. 0
b. 2
c. 5
d. 10

ANSWER: (a) 0

95)   If ‘ξ’ approaches to zero, the peak resonance would ________

a. Also be zero
b. Be unity
c. Tend to infinity
d. Become equal to peak overshoot

ANSWER: (c) Tend to infinity

96)   At which condition of ‘ξ’, resonant peak does not exist and its maximum value is considered to be unity along with zero resonant frequency?

a. 0 < ξ < 0.707
b. ξ > 0.707
c. ξ = 0
d. ξ = 1

ANSWER: (b) ξ > 0.707

97)   If the damping of the system becomes equal to zero, which condition of the resonant frequency is likely to occur?

a. ωr = ωd
b. ωr > ωn
c. ωr < ωn
d. ωr = ωn

ANSWER: (d) ωr = ωn

98)   If the resonant peak is estimated to be ‘5’, which among the following would be the correct value of damping?

a. ξ = 0.3
b. ξ = 1
c. ξ = 3.2
d. ξ = 5.55

ANSWER: (a) ξ = 0.3

99)   If a system is said to have a damping ξ = 0.5532 with the natural frequency ωn = 2 rad/sec, what will be the value of resonant frequency (ωr)?

a. 1.2456 rad/s
b. 1.7352 rad/s
c. 2.3421 rad/s
d. 3.66 rad/s

ANSWER: (a) 1.2456 rad/s

100)   In frequency response, the resonance frequency is basically a measure of _______ of response.

a. Speed
b. Distance
c. Angle
d. Curvature

ANSWER: (a) Speed

101)   The frequency at which the phase of the system acquires ____ is known as ‘Phase crossover frequency’.

a. 90°
b. -90°
c. 180°
d. -180°

ANSWER: (d)  -180°

102)   At which frequency does the magnitude of the system becomes zero dB?

a. Resonant frequency
b. Cut-off frequency
c. Gain crossover frequency
d. Phase crossover frequency

ANSWER: (c) Gain crossover frequency

103)   If the phase angle at gain crossover frequency is estimated to be -105°, what will be the value of phase margin of the system?

a. 23°
b. 45°
c. 60°
d. 75°

ANSWER: (d) 75°

104)   The system is said to be marginally stable, if gain margin is ______

a. 0
b. 1
c. +∞
d. None of the above

ANSWER: (c) +∞

105)   If the constant ‘k’ is positive, then what would be its contribution on the phase plot?

a. 0°
b. 45°
c. 90°
d. 180°

ANSWER: (a) 0°

106)   If the unity feedback system is given by the open loop transfer function G(s) = ks2 / [(1 + 0.3s) (1+ 0.05s)], what would be the initial slope of magnitude plot?

a. 20 dB/decade
b. 40 dB/decade
c. 60 dB/decade
d. Unpredictable

ANSWER: (b) 40 dB/decade

107)   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. 21

ANSWER: (a) 0

108)   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. Random

ANSWER: (a) On left half

109)   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. -0.03

ANSWER:(b)  -0.3

110)   For Nyquist contour, the size of radius is _______

a. 25
b. 0
c. 1
d. ∞

ANSWER:(d)  ∞

111)   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 dB

ANSWER: (d) 6.0205 dB

112)   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. Assortment

ANSWER: (a) Argument

113)   Which among the following constitute the state model of a system in addition to state equations?

a. Input equations
b. Output equations
c. State trajectory
d. State vector

ANSWER: (b) Output equations

114)   State model representation is possible using _________

a. Physical variables
b. Phase variables
c. Canonical state variables
d. All of the above

ANSWER: (d) All of the above

115)   Which mechanism in control engineering implies an ability to measure the state by taking measurements at output?

a. Controllability
b. Observability
c. Differentiability
d. Adaptability

ANSWER: (b) Observability

116)   According to the property of state transition method, e0 is equal to _____

a. I
b. A
c. e-At
d. -eAt


117)   Which among the following is a disadvantage of modern control theory?

a. Implementation of optimal design
b. Transfer function can also be defined for different initial conditions
c. Analysis of all systems take place
d. Necessity of computational work

ANSWER: (d) Necessity of computational work

118)   Which among the following is a unique model of a system?

a. Transfer function
b. State variable
c. Both a and b
d. None of the above

ANSWER: (a) Transfer function

119)   Which architectural unit/block of PLC decides the sequence of different operations to be executed by means of instructions written in memory?

a. Memory
b. Programming software
c. I/O interface
d. CPU


120)   Which among the following units of PLC is adopted to convey the control plan to CPU?

a. Memory
b. Power supply unit
c. I/O interface
d. Programming software

ANSWER: (d) Programming software