Control system MCQ Questions and Answers with explanation -2021

Ques 21. The initial response when the output is not equal to input is called

  1. Transient response
  2. Error response
  3. Dynamic response
  4. Either of the above
Answer.1. Transient response

Explanation:

The transient response of a system is that part of the time response which tends to zero as tends to infinity. It is the response of the system when the input changes from one state to the other. The nature of the transient response is dependent only on the system poles and not on the type of input. The transient response of a system is also called the dynamic response. Transient response The transient response of the system is basically because of stored energy.

In a stable system, the initial response until the system reaches a steady state is called the transient response. After the transient response, the system approaches its steady-state response, which is its approximation for the commanded or desired response.

Transient response of control system means changing so, this occurs mainly after two conditions and these two conditions are

  1. Just after switching ‘on’ the system that means at the time of application of an input signal to the system.
  2. Just after any abnormal conditions. Abnormal conditions may include a sudden change in the load, short-circuiting etc.

In control system engineering, a transient response or natural response is the response of a system to a change from equilibrium or a steady state. Due to inertia, mass, inductance, friction, and other related factors that can’t be avoided the response cannot follow the sudden change in the input therefore transient are observed.

 

Ques.22. A control system working under unknown random actions is called

  1. Computer control system
  2. Digital data system
  3. Stochastic control system
  4. Adaptive control system
Answer.3. Stochastic control system

Explanation:

Stochastic control is a subfield of control theory that deals with the existence of uncertainty either in observation or in the noise that drives the evolution of the system. Methods of stochastic control based on the parametrization of distributions allow the design of practical software tools. A control system is a system which is having feedback and can be both positive and negative and working under unknown random actions is called a stochastic control system.

 

Ques 23. An automatic toaster is a ______ loop control system.

  1. Open-loop
  2. Closed Loop
  3. Semi-closed Loop
  4. None of the above
Answer.1. Open Loop

Explanation:

Automatic toaster system:

In this system, the quality of toast depends upon the time for which the toast is heated.  In an automatic toaster, bread is one of the inputs while the timer which it has to be operated is another input. Depending on the time setting, bread is simply heated in the system. The quality of the toast is the output and once the timer is initiated the action is completed without changes resulting from errors in the toasting quality. The toast quality is judged by the user and has no effect on the input, therefore, the output has no effects on the input. Hence the automatic toaster is an open-loop system.

Toster control system

 

Ques 24. Any externally introduced signal affecting the controlled output is called a

  1. Feedback
  2. Signal
  3. Gain control
  4. Stimulus
Answer.4. Stimulus

Explanation:

In engineering, control is usually restricted to only apply to those systems whose major function is to dynamically or actively command, direct or regulate. Control systems are made up of two main defining divisions that identify with the system, input, and output.

Input:- The input is the stimulus or excitation applied to a control system from an external energy source, usually in order to produce a specified response from the control system.

Output:- The output is the actual response obtained from a control system. It may or may not be equal to the specified response implied by the input. From these inputs and output, the Control System is formed.

Feedback:- Feedback is that property of a closed-loop system that permits the output (or some other controlled variable) to be compared with the input to the system (or input to some other internally situated component or subsystem) so that the appropriate controł action may be formed as some function of the output and input.

Error:- The difference between the input stimulus and the output response. Specifically, it is the difference between the input and the feedback.

 

Ques 25. A closed-loop system is distinguished from open loop system by which of the following?

  1. Servomechanism
  2. Feedback
  3. Output pattern
  4. Input pattern
Answer.2. Feedback

Explanation:

Closed-Loop Control

  • A closed-loop control system utilizes a measure of the actual output to compare the actual output with the desired output response.
  • A Closed-loop Control System is also called the feedback control system which uses the concept of an open-loop system as its forward path but has one or more feedback loops.
  • The measure of the output is called the feedback signal. The elements of a general closed-loop feedback control system are shown in Figure.

Closed loop system

  • A closed-loop control system compares the measurement of the output with the desired output (reference or command input).
  • The difference between the two quantities (the error signal) is then used to drive the output closer to the reference input through the controller and actuator.
  • Often the difference between the output of the process under control and the reference input is amplified and used to control the process so that the difference is continually reduced.

Feedback:- Feedback is that property of a closed-loop system that permits the output (or some other controlled variable) to be compared with the input to the system (or input to some other internally situated component or subsystem) so that the appropriate controł action may be formed as some function of the output and input.

Note:- Essentially, the advantages of closed-loop systems are the disadvantages of open-loop systems, and the disadvantages of closed-loop systems are the advantages of open-loop systems. The introduction of feedback enables the engineer to control the desired output and improve accuracy, but it requires attention to the issue of stability of response. Feedback is used for the purpose of reducing the error between the reference input and the system output. However, the significance of the effects of feedback in control systems is more complex.

 

Ques 26. ______is a part of the human temperature control system.

  1. Digestive system
  2. Perspiration system
  3. Ear
  4. Leg movement
Answer.2. Perspiration system

Explanation:

A part of the human temperature control system is the perspiration system.

When the temperature of the air exterior to the skin becomes too high the sweat glands secrete heavily, inducing cooling of the skin by evaporation. Secretions are reduced when the desired cooling effect is achieved, or when the air temperature falls sufficiently.

The input to this system may be “normal” or comfortable skin temperature, a “set point,” or the air temperature, a physical variable. The output is the actual skin temperature.

 

Ques 27.  _____is a closed-loop system

  1. Direct current generator
  2. Car starter
  3. Auto-pilot for an aircraft
  4. Electric switch
Answer.3. Auto-pilot for an aircraft

Explanation:

Autopilots use gyroscopes to detect changes in the flight path. Any attitude change is sensed by a rate gyroscope which processes and produces an output signal proportional to the rate of change.

A controller compares the actual altitude with the required altitude with the help of a feedback system. If there is a difference between the altitude then the controller computes a corrective signal and sends it to the servo which moves the appropriate flight control and feeds back its position to the controller. As the original attitude is gradually restored, the gyroscope output and error signal are gradually removed, and the controller the pitch command. Thus this system is called a closed-loop system which needs feedback to work.

 

Ques 28. _______ increases the steady-state accuracy.

  1. Integrator
  2. Differentiator
  3. Phase lead compensator
  4. Phase lag compensator
Answer.1. Integrator

Explanation:

Lead compensation and lag compensation are the most commonly used methods of compensation in the conventional frequency domain design of control systems.

Lag compensator

Lag compensator

The transfer function of a lag compensator is given by

$G\left( s \right) = \frac{{\tau s + 1}}{{\beta \tau s + 1}}$

Where

$\beta = \frac{{{Z_c}}}{{{P_c}}} > 1$

  • Both pole and zero lie in LHS of the real plane but PC < ZC i.e. zero is farther away from the origin.
  • Lag Compensation adds a pole at origin (or for low frequencies).
  • It helps to reduce the steady-state error of the system.
  • An unstable system is a system that has at least one pole at the right side of the s-plane.
  • Even if we add a lag compensator to an unstable system, it will remain unstable.

Effects of phase-lag compensator:

  1. Since the lag compensator allows high gain at low frequencies, it is basically a low pass filter. Therefore, it improves the steady-state performance.
  2. In lag compensation, the attenuation characteristics are used for the compensation, whereas the phase lag characteristics are of no use in compensation.
  3. Since the attenuation due to a lag compensator shifts the gain crossover frequency to a lower frequency point, the bandwidth of the system is reduced. Hence a slower response occurs. Therefore, rise time and settling time are usually longer and hence the transient response lasts for a longer time.
  4. The system is very sensitive to parameter variation.
  5. Since a lag compensator approximately acts as a proportional plus integral controller, it thus tends to make a system less stable.

The integrator produces an output proportional to the time integral of error, any finite error will cause the integrator output to keep on increasing and forcing the error to zero. Once the error has reached zero, the integrator output stops increasing but holds the value necessary to remove the error. Phase lag compensation is an integrator, as it reduces the steady-state error and increases steady-state accuracy.

 

Ques 29. As a result of the introduction of negative feedback which of the following will not decrease

  1. Overall Gain
  2. Distortion
  3. Instability
  4. Bandwidth
Answer.4. Bandwidth

Explanation:

If the feedback fraction is opposite in value or anti-phase to the input signal, then the feedback is said to be Negative Feedback or degenerative feedback. If the output of the system changes for any reason, then negative feedback affects the input in such a way as to counteract the change.

The feedback amplification factor is given by:

${A_f} = \frac{A}{{1 + A\beta }}\$

where,

A is open-loop gain

Aβ is the loop gain.

Advantages of Negative feedback

  1. Negative feedback increases the input impedance of the voltage amplifier.\The output impedance of the voltage amplifier can be further lowered by negative feedback.
  2. The transfer gain of the amplifier with feedback can be stabilized against the variations of hybrid parameters of the transistors, or the parameters of the other active devices used in the amplifier.
  3. Negative feedback increases the frequency response and the bandwidth of the amplifier. Bandwidth always increases due to negative feedback as the speed of response is directly proportional to the bandwidth.
  4. Negative feedback increases the linear range of operation of the amplifier.
  5. Negative feedback causes a reduction in noise.
  6. Phase distortion is reduced.

Note:-

Negative feedback reduces the actual input signal applied to the basic amplifier which is appropriate for the active device used because the device has to run for a limited time, this reduced distance on the linear region, making its excursion on the load line faster, means the overall speed is now increased. That means more bandwidth.

 

Ques 30. Regenerative feedback implies feedback with

  1. Oscillations
  2. Step input
  3. Negative sign
  4. Positive sign
Answer.4. Positive sign

Explanation:

  • Feedback can be of two types: negative feedback and positive feedback.
  • When the feedback signal diminishes the magnitude of the input signal, the feedback is called negative, inverse, or degenerative feedback.
  • Negative feedback reduces the overall gain of the amplifier. If the feedback signal enhances the magnitude of the input signal, the feedback is referred to as positive, direct, or regenerative feedback.
  • The overall gain of the amplifier is increased by positive feedback.

Positive feedback causes instability in the gain of the amplifier and distorts the input signal. It is used in narrowband amplifiers and radio receivers because it increases the signal strength.

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