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BSNL TTA 

Question Paper 2007

Control Systems Specialization

1.An open loop control system has its

• control action independent of the output or desired quantity
• controlling action, depending upon human judgment
• internal system changes automatically taken care of
• both (a) and (b)
• all (a),(b) and (c)

2.A servo system must have

• feedback system
• power amplifier to amplify error
• capacity to control position or its derivative
• all of these
• none of these

3.The major disadvantage of a feedback system may be

• Inaccuracy
• inefficiency
• Unreliability
• instability
• Insensitivity

4.Properties of a transfer function

• It is ratio of two polynomials is S and assumes zero initial conditions
• It depends on system elements and not input and output of the system
• Coefficients of the powers of S in denominator and numerator are all real constant. The order of denominator is usually greater than or equal to the order of numerator
• All of these
• It is a function which transfer one physical system into another physical system.

5.The classical analogous of a simple lever is

• Capacitor bridge
• transformer
• mutual inductor
• either of these

6. Two blocks G1(s) and G2(s) can be cascaded to get resultant transfer function as

• G1(s) + G2(s)
• G1(s) / G2(s)
• G1(s) G2(s)
• 1+G1(s) G2(s)
• 1-G1(s)G2(s)
• two blocks cannot be cascaded

7.The principles of homogeneity and super position can be applied to

• linear time invariant system
• non-linear time invariant system
• digital control system
• both (a) and (b)

8.Pick up the nonlinear system

• automatic voltage regulator
• d.c. servomotor with high field excitation
• temperature control of a furnaces using thermistor
• speed control using SCR
• all of these

9.Signal flow graph (SFG) is a

• polar graph
• semi log graph
• log log graph
• a special type of graph for analyzing modem control system
• a topological representation of a set of differential equations

10.Disadvantages of magnetic amplifier

• time lag, less flexible, non-sinusoidal waveform
• low power consumption and isolation of the active circuit
• saturation of the core
• all of these

11.Pick up false statement regarding magnetic amplifiers

• The gate coil of an ideal magnetic amplifier has either zero or infinite inductance
• Resistance of control and gate winding is very small
• Magnetic amplifier gas dropping load characteristics
• Magnetic amplifiers are not used to control the speed of d.c. shunt motor
• Magnetic amplifiers can be used in automatic control of electric drivers of higher rating.

12.High power amplification is achieved by using

• push pull amplifier
• amplidyne
• magnetic amplifier
• DC amplifier
• D.C. generator

13.Pick up false statement regarding servomotors

• The d.c. servomotors are lighter than equivalent a.c. servomotors
• The d.c. servomotors develops higher starting and reversing torque than equivalent a.c. servomotor.
• A drag cup a.c. servomotor has one windings on stator and other on rotor
• Output power of servomotors varies from 1/20 W to 100 W

14.To reduce steady state error

• decrease natural frequency
• decrease damping
• increase damped frequency
• increase time constant
• increase gain constant of the system

15.A good factor for Mp should be

• less than 1
• lying between 1.1 and 1.5
• more than 2.2
• zero
• infinity

16.Pick up false statement. Routh-Hurwitz criterion

• is used for determining stability of a system
• is an algebraic procedure
• gives the exact location of roots of the characteristic equation
• does not indicate relative degree of stability or instability

17.Which of the following is the time domain method of determining stability of a control system

• Bode plot
• Nyquist plot
• Nicholos chart
• Routh-Hurwitz array

18.The technique which gives transient response quickly as well as stability information is

• Nyquist plot
• Routh-Hurwitz criteria
• Bode plot
• Root locus plot
• Nichols plot

19.The bandwidth can be increased by use of

• phase lag network
• phase lead network
• both (a) and (b) in cascade
• both (a) and (b) in parallel
• none of these

20.Nyquist plot is drawn on

• semi log graph paper
• log log graph paper
• polar graph paper
• centimeter graph paper

21.If the gain margin is positive and the phase margin is negative the system is

• stable
• unstable
• indeterminist

22.The Bode plot is applicable to

• all phase network
• minimum phase network
• maximum phase network
• lag lead network
• none of these

23.The valid relation between setting time ts and rise time tr is

• tr>ts
• ts>tr
• ts=tr
• none of these

24.As a root moves further away from imaginary axis the stability

• increases
• decreases
• not affected
• none of these

25.Flat frequency response means that the magnitude ratio of output to input over the bandwidth is

• variable
• zero
• constant
• none of above

26.How many octaves are between 200 Hz and 800 Hz

• Two octave
• One octave
• Four octave
• None of above

27.Human system can be considered as

• open loop system
• close loop system with single feedback
• close loop system with multivariable feedback
• none of these

28.In a feedback system the transient response

• Decays at constant rate
• gets magnified
• decays slowly
• decays more quickly

29.Transfer function of a system is used to calculate

• the steady state gain
• the main constant
• the order of system
• the output for any given input
• all of the above

30.Transfer function of a system is defined as the ratio of output to input in

• Laplace transform
• Z-transform
• Fourier transform
• Simple algebraic form

31.Introduction of feedback decreases the effect of

• disturbances
• noise signals
• error signals
• all the above

32.The system response of a system can be best tested with

• unit impulse input signal
• ramp input signal
• sinusoidal input signal
• exponentially decaying input signal

33.Which of the following is a closed loop system

• electric switch
• car starter
• de generator
• auto-pilot for an aircraft

34.Which of the following is used as an error detector

• potentiometer
• field controlled ac motor
• amplidyne
• armature controlled ac motor

35.The break away point of root loci are

• open loop poles
• closed loop poles
• open loop zeros
• closed loop zeros

36.Noise in a control system can be kept low by

• reducing the bandwidth
• attenuating such frequencies at which external signals get coupled into the system
• both (a) and (b)
• none of these

37.Main cause of absolute instability in the control system is

• parameters of controlling system
• parameters of controlled system
• parameters of feedback system
• error detector where the two signals are compared

38.Basically a controller is

• a amplifier
• a clipper
• a comparator
• a summer

39.A system with gain margin close to unity or a phase margin close to zero is

• highly stable
• highly oscillatory
• relatively stable
• none of these

40.Which of following elements is not used in an automatic control system

• sensor
• error detector
• oscillator
• final control element

41.AC systems are usually preferred to the DC systems in control applications because

• AC systems are cheaper
• AC systems are more stable
• AC systems have better performance characteristics and smaller in size
• all of these

42.A system has the transfer function (1-s)/(1+s); It is known as

• low pass system
• high pass system
• all pass system
• none of the above

43.In control systems, excessive bandwidth should be avoided because

• noise is proportional to bandwidth
• it leads to low relative stability
• it leads to slow speed of response
• none of these

44.In most systems, an increase in gain leads to

• larger damping ratio
• smaller damping ratio
• constant damping ratio
• none of these

45.A step function is applied to the input of a system and output is of the form y = t, the system is

• stable
• unstable
• not necessarily stable
• conditionally stable

46.Which of the following can be magnified by magnetic amplifier

• voltage
• current
• power
• none of above

47.The inductance is not used in lag network because of

• big size
• time delay and hysteresis losses
• high reactance
• (none of these

48.Saturation in a stable control system can cause

• conditional stability
• over damping
• low level oscillations
• high level oscillations

49.Excessive noise in control systems can cause

• reduction in bandwidth
• reduction in gain
• saturation in amplifying stages
• oscillations

50.The type-0 system has

• net pole at the origin
• no pole at the origin
• simple at one origin
• two poles at the origin

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