Interview Questions On Control System

Q. What is a control system?

A. A control system consists of subsystems and processes (or plants) assembled for the purpose of obtaining a desired output with desired performance, given a specified input.

Q. When is a control system said to be robust?

A. control system is said to be robust when
1. It has low sensitivities
2. It is stable over a wide range of parameter variations; and
3. The performance stays within prescribed (but practical) limit bounds in presence of changes in the parameters of the controlled system and disturbance input.

Q. What is the main difference between open loop and closed loop systems?

A. Open-loop systems, do not correct for disturbances and are simply commanded by the input.
The closed-loop system compensates for disturbances by measuring the output response, feeding that measurement back through a feedback path, and comparing that response to the input at the summing junction. If there is any difference between the two responses, the system drives the plant, via the actuating signal, to make a correction. If there is no difference, the system does not drive the plant, since the plant's response is already the desired response.

Q. What are the advantages of closed loop systems over open loop systems?

A. Closed-loop systems have greater accuracy than open-loop systems. They are less sensitive to noise, disturbances, and changes in the environment. Transient response and steady-state error can be controlled more conveniently and with greater flexibility in closed-loop systems.

Q. Why Negative Feedback is preferred in the Control System?

A. The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error. Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations.

Q. What is the effect of positive feedback on stability of the system?

A. Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters.

Q. What are the components of feedback control system?

A. The components of feedback control system are plant, feedback path elements, error detector and controller.

Q. When a system is said to be stable?

A. A system is stable if every bounded input yields a bounded output.

Q. How to determine if a closed loop system is stable from transfer function?

A. If the poles of the closed loop system lie on the left half of the plane i.e have a negative real part then the system is stable.

Q. What is Routh-Hurwitz criterion?

A. Routh-Hurwitz criterion declares that the number of roots of the polynomial that are in the right half-plane is equal to the number of sign changes in the first column. The Routh-Hurwitz criterion lets us find how many poles are in each of the sections of the s-plane without giving us the coordinates of the poles. Just knowing that there are poles in the right half-plane is enough to determine that a system is unstable.

Q. What is a steady state error?

A. Steady state error is the difference between the input and output for a prescribed test input as t  ∞

Q. What is gain margin and phase margin?

A. The gain margin is the change in open-loop gain, expressed in decibels (dB), required at 180° of phase shift to make the closed-loop system unstable.

The phase margin is the change in open-loop phase shift required at unity gain to make the closed-loop system unstable.

Q. What is the advantage of P+I controller over P (Purely proportional) controller?

A.  If the controller is based on proportional logic, then in the new steady state, a non-zero error must exist to get a non-zero value of control signal. The operator must then reset the set point to bring the output to the desired value. We need a controller that automatically brings the output to the set point which is done by PI, which gives a steady control signal with system error equal to zero.

Q. What is frequency response?

A frequency response is the steady state response of a system when the input to the system is a sinusoidal signal.

Q. What is Bode plot? What are its advantages?

A. Bode plot is the frequency response plot of the transfer function of a system. A Bode plot consists of two graphs. One is the plot of magnitude of sinusoidal transfer function versus log w. The other is a plot of the phase angle of a sinusoidal function versus log w.
Advantages:
1 Multiplication of magnitude can be in to addition.
2 A simple method for sketching an approximate log curve is available.
3 It is based on asymptotic approximation. Such approximation is sufficient if rough information on the frequency response characteristic is needed.
4 The phase angle curves can be easily drawn if a template for the phase angle curve of 1+ jw is available.

Q. What is a compensator?

A device inserted into the system for the purpose of satisfying the specifications is called as a compensator.

Q. What are the three types of compensators?

A. 1. Lag compensator: Improves the steady state behavior of a system, while nearly preserving its transient response.
2. Lead compensator: Speeds up the transient response, increases the margin of stability of a system and increases the system error constant to a limited extent.
3. Lag-Lead compensator: The lag lead compensator is required when both the transient and steady state response of a system has to be improved

Q. What is a servomotor?

A. The motors used in automatic control systems or in servomechanism are called servomotor. They are used to convert electrical signal into angular motion.