Q.1. Define control.

Answer: Control means to keep something under limits. In electronics control word is used to keep the output of a system within specified limits. A control system manages commands, directs or regulates the behavior of other devices or systems.

Q.2. What is servomechanism?

Answer: In control engineering a servomechanism, sometimes shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism. It usually includes a built-in encoder or other position feedback mechanism to ensure the output is achieving the desired effect.

Q.3. what is open loop and closed loop control system?

Answer: Open loop control system- In open loop system any change in the output has no effect on the input i.e. feedback does not exists. It is inaccurate and unreliable. It is simple to construct and cheap. Closed loop control system- In closed loop system any change in the output, affects the input which is possible by use of feedback. It is highly accurate and reliable. It is complicated to design and hence costly.

Q.4. How can you define the stability of a control system?

Answer: The stability of a system relates to its response to inputs or disturbances. A system which remains in a constant state unless affected by an external action and which returns to a constant state when the external action is removed can be considered to be stable. Systems stability can be defined in terms of its response to external impulse inputs. A system is stable if every bounded input produces a bounded output. A system is stable if its impulse response approaches zero as time approaches infinity.

Q.5. What do you mean by BIBO stability?

Answer: A system is said to be BIBO stable if a bounded input results in bounded output.

Q.6. Which system is always stable?

Answer: A second order system is always stable.

Q.7. Define transfer function.

Answer: The transfer function of a control system is defined as the ratio of the Laplace transform of the output variable to Laplace transform of the input variable. Transfer function is always calculated with zero initial conditions because zero initial conditions ensure the linearity.

Q.8. What is the difference between absolute and relative stability of a system?

Answer: Absolute stability only tells about that the system is stable or not whereas relative stability not tells about the stability but also tells the margin of stability.

Q.9. Why transfer function is not suitable in dynamic systems?

Answer: Since transfer function is always calculated with zero initial conditions because zero initial conditions ensure the linearity and does not take initial conditions of a system into account and dynamic systems are not linear time invariant system and also possesses initial conditions.

Q.10. What are time domain methods of determining stability of a system?

Answer: Routh stability criterion and root locus are time domain methods of determining stability of a system.

Q.11. What are frequency domain methods of determining stability of a system?

Answer: Bode plot, Nyquist plot, Nichlos chart and constant M-N circle are frequency domain methods of determining stability of a system.

Q.12. What is Routh’s stability criterion?

Answer: This states that, the necessary and sufficient condition for system to be stable is “All the terms in the first column of Routh’s array must have same sign. There should not be any sign change in the first column of Routh’s array.” If there are any sign changes existing then, system are unstable and the number of sign changes equals the number of roots lying in the right half of the s-plane.

Q.13. What is Tachometer?

Answer: Tachometer is an electromechanical unit which generates an electrical output proportional to the speed of the shaft. In automatic control system tachometer performs two main functions, stabilization of system and computation of closed loops in a control system.

Q.14. What is regenerative feedback?

Answer:  Feedback in which the portion of the output signal that is returned to the input has a component that is in phase with the input signal. The regenerative feedback is sometimes used for increasing the loop gain of the feedback systems.

Q.15. What is ‘type’ of system?

Answer: Type denotes number of integrations in the system which involves Sn  term in denominator where n is called type of system. For example type-1 system would have ‘S’ in the denominator, type-2 system would have ‘S2’ in denominator.

Q.16. What is the order of the system?

Answer: The highest power of the complex variable ‘S’ in the denominator of the transfer function determines the order of the system.

Q.17. What is viscous friction force?

Answer: The force of friction between moving surfaces separated by viscous fluid or force between solid body and fluid motion.

Q.18. What are minimum phase systems?

Answer: The system having transfer function having neither poles nor zeros in the right half of the s-plane is called as minimum phase system or in other words system with all poles and zeros in left half of the s-plane are called as minimum phase systems.

Q.19. What are non minimum phase system?

Answer: The system having transfer function containing either a pole or a zero or both in the right half of the s-plane is called as non minimum phase system.

Q.20. What are all pass systems?

Answer: The function in which zeros lie on right half side of s-plane while poles in left half side of s-plane and the locations of poles and zeros are symmetric about imaginary axis is called all pass function or all pass system.

Q.21. What are the factors on which transient response and steady state response of a system depends?

Answer: Transient response only depends on the system component and their layout not on input. Steady state response depends on both input as well as system components and their layout.

Q.22. What is steady state error?

Answer: Steady state error is difference between steady state output and desired output. Steady state error is caused by imperfection in the system components, deterioration and ageing of components.

Q.23. What do you mean by asymptotic stability?

Answer: A system without any input and having some initial condition is said to be stable if response decays to zero regardless of magnitude of initial conditions.

Q.24. What is the effect of adding a zero to closed loop transfer function of second order system?

• Peak overshoot increases
• Rise time decreases
• Peak time decreases

Q.25. What is the effect of adding a zero to open loop transfer function of second order system?

Answer: If zero is added very near to origin then damping of the system worsens. If zero is added far from origin then damping of the system improves.

Q.26. What is zero order hold?

Answer: The simplest hold circuit used in reconstruction of signal is zero order hold. In this scheme the reconstructed signal acquires the same value as the last received sample for the entire sampling period. The simple zero order hold when used in conjunction with a high sampling rate provides a satisfactory performance.

Q.27. What do you mean by peak overshoot?Is it desirable or not?

Answer: Difference between the maximum value attained by a response and final value of response, is known as peak overshoot. It should be as low as possible because it increases noise in the system.

Q.28. How can you differentiate controller and compensator?

Answer: Both controller and compensator has the function of improving stability and response of a system. But controller is incorporated in the system during manufacturing while compensator is incorporated when system fails to work according to specifications.

Q.29. What will be the effect of adding a pole to the origin on the response of system?

Answer: Addition of a pole always tend to make system more slow and unstable so it should be always avoided to add a pole closer to origin.

Q.30. What is root locus technique?

Answer: This is a graphical method, in which movement of poles in the s-plane is sketched when a particular parameter of a system is varied from zero to infinity. Note that the parameter is usually the gain but any other parameter may be varied. But for root locus method, gain is assumed to be a parameter which is to be varied from zero to infinity.

Q.31. Why root locus plot is symmetrical about the real axis?

Answer: Because roots are either real or occur in complex conjugate pairs, so root locus is symmetrical about real axis.

Q.32. What is the effect of adding poles on the shape of root locus?

Answer: The complex part of root locus has tendency to shift towards right side. The range of K for which system is stable decreases which makes system relatively less stable. The addition is somewhat equivalent to applying integral control to the system.

Q.33. What is the effect of adding zeros on the shape of root locus?

Answer: The complex part of root locus has tendency to shift towards left side. The range of K for which system is stable increases which makes system relatively more stable. The addition is somewhat equivalent to applying derivative control to the system.

Q.34. What is break away point?

Answer: Breakaway point is a point on the root locus where multiple roots of the characteristic equation occurs for a particular value of K. As breakaway point indicates values of multiple root, it is always on the root locus.

Q.35. Which segment of real axis are part of root locus?

Answer: Segments of the real axis having an odd number of real axis open loop poles plus zeros to their right are parts of the root locus.

Q.36. What is PD controller?

Answer: A controller in the forward path, which changes the controller output corresponding to proportional plus derivative of error signal is called PD controller.

Q.37. What are the effects of PD controller on the system?

Answer: PD controller has following effects on system:

• It increases damping ratio.
• Natural frequency for system remains unchanged.
• ‘TYPE’ of the system remains unchanged.
• It reduces peak overshoot.
• It reduces settling time.
• Steady state error remains unchanged.
• It improves gain margin and phase margin of the system.

Q.38. What is PI controller?

Answer: A controller in the forward path, which changes the controller output corresponding to proportional plus integral of error signal is called PI controller.

Q.39. What are the effects of PI controller on the system?

Answer: PI controller has following effects on system:

• It increases order of the system.
• It increases ‘TYPE’ of the system.
• It improves damping and reduces maximum overshoot.
• It improves gain margin and phase margin of the system.
• Steady state error reduces tremendously for same type of inputs.

Q.40. What is PID controller?

Answer: A controller in the forward path, which changes the controller output corresponding to proportional plus integral plus derivative of error signal is called PID controller. As PD improves transient and PI improves steady state response, combination of two may be used to improve overall time response of the system.

Q.41. What is integral error compensation?Where is it used?

Answer: In an integral error compensation scheme, the output response depends in some manner upon the integral of the actuating signal. A significant contribution of integral error compensation to the system steady state performance is , however obvious as the additional integration in the forward path changes the system from type-1 to type-2 and the error of velocity input is eliminated or considerably reduced as the practical integration may not be perfect.

Q.42. What is the frequency response of a system?

Answer: The steady state response of a system to a purely sinusoidal input is defined as frequency response of a system. In such method frequency of the input signal is to be varied over a certain range and the resulting response of system is to be studied. Such response is called frequency response. In other words, the magnitude and phase relationship between the sinusoidal input and the steady state output of a system is called as frequency response.

Q.43. What are Bode plots?

Answer: In general Bode plots consists of two plots which are-

• Magnitude expressed in logarithmic values against logarithmic values of frequency called Magnitude Plot.
• Phase angle in degrees against logarithmic values of frequency called Phase Angle Plot.

Q.44. Bode Plot is suitable for which type of system?

Answer: It is suitable only for minimum phase transfer function systems.

Q.45. What is semilog paper?

Answer: In such paper the X-axis is divided into logarithmic scale which is non linear one, while Y-axis is divided into linear scale and hence it is called semilog paper.

Q.46. What is Gain Margin and Phase Margin?

Answer: Bode Plots provide relative stability in terms of Gain Margin and Phase Margin.

Gain Margin- Greater the gain margin greater will be the stability of the system. It refers to the amount of gain, which can be increased or decreased without making the system unstable.

Phase Margin- Greater the phase margin greater will be the stability of the system. It refers to the phase which can be increased or decreased without making the system unstable.

Q.47. What do you mean by Gain cross over frequency? Give significance.

Answer: Frequency at which magnitude bode plot crosses 0 dB axis is known as gain cross over frequency. At Gain cross over frequency we can calculate   Phase margin.

Q.48. What do you mean by Phase cross over frequency? Give significance.

Answer: Frequency at which Nyquist plot crosses the negative real axis or at which bode plot crosses 180 degree phase line is known as phase cross over frequency. At phase cross over frequency we can calculate gain margin.

Q.49. How can you differentiate stable, unstable and marginally stable systems on the basis of gain cross over frequency and phase cross over frequency?

Answer: For stable systems gain cross over frequency should be less than phase cross over frequency.

For unstable systems gain cross over frequency should be greater than phase cross over frequency.

For marginally stable systems gain cross over frequency should be equal to phase cross over frequency.

Q.50. How can you differentiate stable, unstable and marginally stable systems on the basis of gain margin and phase margin?

Answer: For stable systems, gain margin and phase margin both are positive.

For unstable systems, gain margin and phase margin both are negative.

For marginally stable systems, gain margin and phase margin both are zero.