CONTROL SYSTEM INTERVIEW QUESTIONS AND ANSWERS
Q.1. Which controller is never used and why?
Answer: PI controller. No doubt, PI controller decreases the steady state error of system but it increases the order of system and hence it tends to de-stabilize the system.
Q.2. What do you mean by phase variables?
Answer: The phase variables are those particular state variables which are obtained from one of the system variable and its derivatives.
Q.3. What do you mean by feed forward compensation?
Answer: Disturbance at input introduces error in the system performance. In several systems where the disturbance input can be predicted, its effect can be eliminated by a feed forward compensation.
Q.4. What is Metadyne?
Answer: It is a two stage cross field amplifier. In Metadyne, the cross magnetizing flux of armature reaction is used foe generation of emf in second stage. It is also known as rotating amplifier. Metadyne is used as constant current source.
Q.5. What is Ampidyne?
Answer: It is rotatory or dynamic or mechanical amplifier used when amplification could not be achieved by linear electronic amplifiers. It is a two pole machine. It is also called cross field amplifier.
Q.6. Which information does transient part and steady state part of the response reveals?
Answer: Transient part of the response gives an idea about the nature of response (oscillatory or overdamped) and also gives an indication about its speed. It depends upon the poles and zeros of the system.
The steady part of the response reveals the accuracy of a control system. It depends upon the input signal applied.
Q.7. Why Nyquist plot is symmetric with respect to real axis?
Answer: As Nyquist plot is suitable for linear time invariant systems therefore it is symmetric with respect to real axis.
Q.8. What do you mean by compensation of a control system?
Answer: If a system is to be redesigned so as to meet the required specifications, it is necessary to alter the system by adding an external device to it. Such a redesign or alteration of system using an additional suitable device (compensator) is called compensation of a control system.
Q.9. How gain margin and phase margin can be improved?
Answer: The easiest way to improve is to reduce the gain. However this increases the steady state error and makes the system sluggish. Better methods are available. These methods are adding compensating networks or compensators.
Q.10. 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.11. 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.
Q.12. 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.13. 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.14. 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.15. 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.16. 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.17. 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.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. 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.
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