JNTU
JNTU B.Tech III CONTROL SYSTEMS ENGINEERING Previous Question Paper (November/December - 2018 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
Distinguish between open loop and closed loop control systems with the help of neat diagrams.
Answer
An open loop control system is a system in which the control action is independent of the output. It does not have feedback, making it simple and inexpensive but sensitive to external disturbances. The configuration is: Input → Controller → Process → Output.
A closed loop control system is a system in which the control action is dependent on the output. It utilizes feedback to compare the actual output with the desired input, generating an error signal to adjust the process. This provides higher accuracy and stability, though it is more complex and costly. The configuration is: Input → Error Detector → Controller → Process → Output, with a feedback path from the Output back to the Error Detector.
Explain with block diagrams the various effects of feed-back signal on the control system.
Answer
Feedback in a control system is the process where a portion of the output is returned to the input to compare it with the reference signal. The primary effects of feedback are:
- Reduction in Sensitivity: Feedback reduces the sensitivity of the system gain to variations in internal component parameters, making the system more stable and robust.
- Improvement in Disturbance Rejection: Feedback allows the system to detect and counteract the effects of external disturbances, ensuring the output remains closer to the desired reference.
- Bandwidth Improvement: Feedback increases the operating bandwidth of the system, allowing it to respond faster to input changes, though this often comes at the cost of reduced overall gain.
The basic block diagram consists of a forward path gain G(s) and a feedback path H(s). The closed-loop transfer function is given by:
Where the positive sign denotes negative feedback (stability) and the negative sign denotes positive feedback (instability/oscillation).
Define steady state error of a control system and explain steady state error constants.
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Briefly discuss the effects of proportional derivative, proportional integral systems.
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Give the limitations of Routh’s stability criterion.
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Two first order systems have pole at -1 and -5 respectively. Comment on their relative stability.
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Explain how polar plots are useful in finding the stability of a system.
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Explain the terms gain margin and phase margin.
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Define state transition matrix.
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Briefly explain the concepts of Controllability and Observability.
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Part B
State different rules which govern the block diagram reduction technique to obtain transfer function of a system.
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Reduce the given block diagram shown in figure 1 and hence obtain the transfer function C(s)/R(s).
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Describe the analogy between electrical and mechanical systems.
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For a signal flow graph in figure 2, draw the block diagram, there by obtain transfer function.
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A unity feedback control system has an open loop transfer function G(s) = 10 / (s(s+1)). Find the time response of the system and the time constant and percentage overshoot, for a unit step input.
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What are standard test signals? Explain their significance.
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For an under damped second order system, define various time domain specifications.
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The forward path transfer function of a unity feed-back control system is given by G(s) = 2 / (s(s+3)). Obtain the expression for unit step response of the system.
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The open-loop transfer function of a unity feedback control system is given by G(s) = K / (s(s+2)(s+4)(s2+6s+25)). By applying the Routh criterion, discuss the stability of the closed-loop system as a function of K. Determine the values of K, which will cause sustained oscillations in the closed-loop system. What are the corresponding oscillation frequencies?
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Sketch the root locus diagram for a feedback system with the characteristic equation G(s) = K / (s(s+2)(s2+2s+2)). Show clearly the steps involved.
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Explain how exact Bode plot is obtained from asymptotic plot, with a figure.
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The open loop transfer function of a unity feed-back system is G(s) = 50K / (s(s+10)(s+5)(s+1)). (i) Determine the gain margin and phase margin. (ii) Determine the value of steady state error coefficient for a gain of 10 dB and the value which will make the closed loop system marginally stable. Use Bode diagram.
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Sketch the Nyquist plot and find the stability of the following system G(s)H(s) = 100 / ((s+2)(s+4)(s+8)).
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Obtain the transfer function of a control system whose state model is defined by matrices A, B, and C as provided in the state equations.
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Obtain the state model of a field controlled d.c. shunt motor.
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Construct the canonical state model for the system characterized by the following differential equation y'' + 3y' + 2y = u' + u.
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