Control system MCQ Questions and Answers with explanation -2021

Ques 91. A signal other than the reference input that tends to affect the value of controlled variable is known as

  1. Disturbance
  2. Command
  3. Control element
  4. Reference input
Answer.1. Disturbance

Explanation:

Disturbamce:- Disturbance is the unwanted signal (e.g, gust) in a control system that tends to affect the controlled variable. The disturbance may be introduced into the system at many places.

Noise:- Noise is the unwanted signal (eg., engine vibration) that tends to affect the measured variable. A filter may be utilized to get rid of noise in the measurement device.

Reference (command) input is a signal supplied to the control system which represents the desired value (or variation) of the controlled output.

Control signal is the output of the controller that will be used to bring the output of the system as close to the desired value as possible.

control system distrubamce

 

Ques 92. The transfer function is applicable to which of the following?

  1. Linear and time-invariant systems
  2. Linear and time-variant systems
  3. Linear systems
  4. Non-linear systems
Answer.1. Linear and time- Invariant system

Explanation:

The transfer function of a linear, time-invariant system is defined as the ratio of the Laplace transform of the output to the Laplace transform of the input with all initial conditions being zero.

The transfer function is a frequency-domain concept that is used to calculate the output of the linear system to any input.

$TF = \frac{{C\left( s \right)}}{{R\left( s \right)}}$

A transfer function may only be defined for a linear, time-invariant (constant parameter) single-input-single-output system. Transfer function is an input-output description of the behaviour of a system where the information about the initial conditions is lost. Thus, the transfer function description does not include any information concerning the internal structure of the system and its behaviour.

Properties of Transfer function

  1. The transfer function of a system is the Laplace transform of its impulse response.
  2. The transfer function concept is applicable only to linear, time-invariant systems.
  3. The transfer function does not take care of the initial conditions of the system. This is the greatest disadvantage. T
  4. The transfer function is applicable to single-input-single-output systems, though for multivariable systems, the transfer matrix can be obtained by using the principle of superposition.
  5. The degree of the denominator polynomial of the transfer function is the order of the system. The denominator polynomial gives the poles and the numerator polynomial gives the zeros.

 

Ques 93. From which of the following transfer function can be obtained?

  1. Analogous table
  2. Output-input ratio
  3. Signal flow graph
  4. Standard block system
Answer.3. Signal flow graph

Explanation:

Signal Flow Graph

The signal flow graph for a system can be reduced to obtain the transfer function of the system using the following rules.

Rule 1: Incoming signal to a node through a branch is given by the product of a signal at the previous node and the gain of the branch.

Rule 2: Cascaded branches can be combined to give a single branch whose transmittance is equal to the product of individual branch transmittance.

Rule 3: Parallel branches may be represented by a single branch whose transmittance is the sum of individual branch transmittances.

Rule 4: A mixed node can be eliminated by multiplying the transmittance of the outgoing branch (from the mixed node) to the transmittance of all incoming branches to the mixed node.

Rule 5: the equations to find the ratio of output to input. This ratio gives the gain of the resultant branch. A loop may be eliminated by writing equations at the input and output node and rearranging.

The signal flow graph of a system can be reduced either by using the rules of a signal flow graph algebra (i.e.) by writing equations at every node and then rearranging these equations to get the ratio of output and input (Transfer function).

The signal flow graph reduction by the above method will be time-consuming and tedious S.J. Mason has developed a simple procedure to determine the transfer function of the system represented as a signal flow graph. He has developed a formula called by his name Mason’s gain formula which can be directly used to find the transfer function of the system.

 

Ques 94. ______ is the reference input minus the primary feedback.

  1. Manipulated variable
  2. Actuating signal
  3. Primary feedback
  4. Zero sequence
Answer.2. Actuating Signal

Explanation:

Actuating signal is obtained from a comparison measuring device and is the reference input minus the primary feedback. This signal, usually at a low energy level, is the input to the control elements that produce the manipulated variable.

Manipulated variable  is the quantity obtained from the control elements that is applied to the controlled system. The manipulated variable is generally at a higher energy level than the actuating signal and may also be modified in form.

 

Ques 95. The term backlash is associated with

  1. Servomotor
  2. Induction relay
  3. Gear train
  4. Any of the above
Answer.3. Gear train

Explanation:

The term backlash is associated with Gear train. Backlash or gear play arises in the mechanical system involving gear trains.

  1. If an input sinusoid is used, then the output of a backlash will be periodic, but the output will remain constant whilst the input changes from being increasing to decreasing.
  2. The gear meshes are manufactured for minimal backlash, but the resulting increase the friction between gear teeth thus reduces the efficiency of the system.
  3. The main difficulty is the phase shift which is introduced by the backlash that can cause stability problems in feedback control loops.
  4. An interesting feature of backlash is its multi-valued behaviour, since for each input there are two possible values for the output of the nonlinearity.
  5. The output that is appropriate depends upon the previous motion of the input signal, and hence the output depends upon the history of the input signal movements.
  6. If backlash occurs the analysis will be non-linear because the backlash can cause undesired low-level oscillation in the feedback control system.
  7. Backlash can be useful sometimes as it increases the damping.

 

Ques 96. By which of the following the system response can be tested better

  1. Ramp input signal
  2. Sinusoidal input signal
  3. Unit impulse input signal
  4. Exponentially decaying signal
Answer.3. Unit impulse input  signal

Explanation:

The system response can be tested better with Unit impulse input signal.

A unit step or unit impulse is a baseline input or test input. A unit impulse response reveals a plant time constant, time delay, and gain in a transparent way. These parameters can be determined by the graphical display. For this reason, the step response test is widely used by the process engineer as the first step to access the plant information.

 

Ques 97.  In a system zero initial condition means that

  1. The system is at rest and no energy is stored in any of its components
  2. The system is working with zero stored energy
  3. The system is working with zero reference signal
  4. Laplace transform is not possible

Answer.1. The system is at rest and no energy is stored in any of its components

Explanation:

If the initial condition is assumed to be zero means that system is initially relaxed at time t =0 and no energy is stored in any of the components.

Zero initial condition for a system:

  • Any parameter or physical quantity of a system like temperature, voltage, energy, etc is most of the time not constant. It varies with the time.
  • These can be represented as differential equations or state equations in s – domain.
  • If the system is linear and shift-invariant, then the principle of superposition can be used and the output can be determined as a linear combination of the output due to the initial state (initial conditions) and the output due to the input sequence.
  • When all of the initial conditions of a system are equal to zero, the system is designated to be relaxed (at rest) and no energy is stored in any of its components.
  • We use LTI systems in practice. That means the system should be LINEAR. Zero initial conditions ensure Linearity. That is why Initial Conditions are assumed to be Zero in transfer function model.

 

Ques 98. Hydraulic torque transmission system is analog of

  1. Amplidyne set
  2. Resistance-capacitance parallel circuit
  3. Motor-generator set
  4. Any of the mentioned
Answer.2. Motor-generator set

Explanation:

Hydraulic torque transmission system is analog of Motor-generator set.

A servomotor of the hydraulic type has been variously known as hydraulic transmission, speed gear, oil gear, and positive displacement transmission and combine this device are termed as the simply hydraulic motor.

The electric analogy of a hydraulic motor is similar to an electric motor.There is no essential difference between a hydraulic pump and motor, like as there no essential difference between a dc motor and a dc generator

The hydraulic motor  is classified as an electric motor if the input is hydraulic flow or pressure and the output is mechanical power

The hydraulic pump is classified as an electric generator if the input is mechanical torque and output is hydraulic pressure or flow.

 

Ques 99. The frequency and time domain are related to which of the following?

  1. Fourier Integral
  2. Laplace Transform and Fourier Integral
  3. Fourier Integral
  4. None of the above
Answer.2. Laplace Transform and Fourier Integral

Explanation:

The frequency domain and time domain are related through Laplace transform but not through Gauss elimination.

The analysis of a system with respect to time is known as time-domain analysis and with respect to frequency is frequency domain analysis. we usually change our systems from time to frequency by using (Fourier, Laplace ) to make it easy to understand the response of the system because the time domain is more complex for higher orders.

 

Ques 100. An increase in gain, in most systems, leads to

  1. Smaller damping ratio
  2. Larger damping ratio
  3. Constant damping ratio
  4. Zero damping ratio
Answer.1. Smaller damping ratio

Explanation:

  • The damping ratio is inversely proportional to the gain, therefore, the larger proportion of the gain results in a faster response and a smaller steady-state error.
  • With the increase in gain damping ratio decreases and the damped frequency increases.
  • The overshoot increase with an increase in the gain while delay time and rise time of the system is reduced hence the system response faster.

Effect of controller parameters on system performance

Item Increase proportional gain Increase Derivative time Increase Rest time
Damping factor Decreases Increases Decreases
Overshoot Increases Decreases Increases
Decay ratio Increases Decreases Increases
Steady state error Decreases No change Eliminated
Stability Decreases Increases Decreases
Closed loop gain Decreases No change No change


FOR TRANSMISSION AND DISTRIBUTION SYSTEM MCQ CLICK HERE

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FOR SYNCHRONOUS GENERATOR OR ALTERNATOR MCQ CLICK HERE

FOR SYNCHRONOUS MOTOR MCQ CLICK HERE

FOR DC MOTOR MCQ CLICK HERE

FOR DC GENERATOR MCQ CLICK HERE

FOR POWER SYSTEM MCQ CLICK HERE

FOR TRANSFORMER MCQ CLICK HERE

FOR BASIC ELECTRICAL MCQ CLICK HERE

 

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