Control Systems Interview Questions

Control Systems Interview Questions

  • Anuj Awasthi
  • 17th Apr, 2021

Control Systems Interview Questions

1) What is a control system?

A control system is a set of mechanical or electronic devices that regulates other devices or systems by way of control loops. Typically, control systems are computerized.

Control systems are a central part of the industry and of automation. The types of control loops that regulate these processes include industrial control systems (ICS) such as supervisory control and data acquisition (SCADA) and distributed control systems (DCS).

2) List different types of control system?

Different types of Control Systems are

  • Linear Control Systems.
  • Non-linear Control Systems.
  • Analog or Continuous System.
  • Digital or Discrete System.
  • Single Input Single Output Systems.
  • Multiple Input Multiple Output Systems.
  • Lumped Parameter System.
  • Distributed Parameter System.

3) What is time-invariant System?

A time-invariant (TIV) system has a time-dependent system function that is not a direct function of time. Such systems are regarded as a class of systems in the field of system analysis. The time-dependent system function is a function of the time-dependent input function.

4) What is a Controller?

A controller is the most important component of the control system. It is responsible for the performance of the control system. It is a device or an algorithm that works to maintain the value of the controlled variable at set point.

5) What are Disturbance signals?

Disturbance signals represent unwanted inputs that affect the control systems output and result in an increase of the system error. It is the job of the control-system engineer to properly design the control system to partially eliminate the effects of disturbances on the output and system error.

6) What is SisO and MIMO?

SISO: A single-input and single-output (SISO) system is a simple single variable control system with one input and one output. In radio, it is the use of only one antenna both in the transmitter and receiver.

MIMO: Systems with more than one input and/or more than one output is known as Multi-Input Multi-Output systems (MIMO).

7) What is Open loop system?

An Open-loop system also known as a non-feedback system, is a type of continuous control system in which the output has no influence or effect on the control action of the input signal.

An example of an Open-loop system is the clothes dryer that does not monitor or measure the condition of the output signal, which is the dryness of the clothes. Then the accuracy of the drying process or success of drying the clothes will depend on the experience of the user (operator).

8) What are linear and non-linear systems?

Linear system: Linear systems are the systems which possess the property of homogeneity and superposition. The term superposition means that an input r1(t) gives an output c1(t) and r2(t) will give the output c2(t). If we apply both the input r1(t) and r2(t) together, then the output will be the sum of c1(t) and c2(t).

r1 (t) + r2 (t) = c1 (t) + c2 (t)

Non-Linear System: Non-linear systems are the systems that do not possess the property of superposition and homogeneity, and the output of these systems is not directly proportional to its input. In these types of systems, the stability of the system depends upon the input and initial state of the system.

9) What is closed loop System?

A Closed-loop Control System, also known as a feedback control system is a control system which uses the concept of an open-loop system as its forward path but has one or more feedback loops (hence its name) or paths between its output and its input.

10) What is Cut-off Rate?

The slope of the log-magnitude curve near the cut-off frequency is called the cut-off rate. The cut-off rate indicates the ability of the system to distinguish between the signal and the noise.

11) What is Gain Margin?

The gain margin is defined as the reciprocal of the magnitude |G(jω)| at the frequency at which the phase angle is −180°. Defining the phase crossover frequency ω1 to be the frequency at which the phase angle of the open-loop transfer function equals −180° gives the gain margin Kg:

Gain margin

12) What is a Block diagram?

Block diagrams consist of a single block or a combination of blocks. These are used to represent the control systems in pictorial form. The basic elements of a block diagram are a block, the summing point, and the take-off point.

13) What are metadyne?

A metadyne is a direct current electrical machine with two pairs of brushes. It can be used as an amplifier or rotary transformer. A metadyne can convert a constant-voltage input into a constant-current, variable-voltage output.

14) What is Resonant Frequency?

Resonant frequency is the frequency at which the magnitude of the frequency response has peak value for the first time. It is denoted by ωr. At ω=ωr, the first derivate of the magnitude of T(jω) is zero.

15) What is Routh Hurwitz Stability Criterion?

In Control System, the Routh–Hurwitz stability criterion is a mathematical test that is a necessary and sufficient condition for the stability of a linear time-invariant (LTI) control system.

Routh Hurwitz criterion states that any system can be stable if and only if all the roots of the first column have the same sign and if it does not has the same sign or there is a sign change then the number of sign changes in the first column is equal to the number of roots of the characteristic equation in the right half of the s-plane i.e. equals to the number of roots with positive real parts.

16) What is Servo Motors?

Servo Motors are also known as Control motors. They are used in feedback control systems as output actuators and does not use for continuous energy conversion. The principle of the Servomotor is similar to that of the other electromagnetic motor, but the construction and the operation are different. Their power rating varies from a fraction of a watt to a few hundred watts.

17) What is stroboscope?

A stroboscope also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It is used to set the ignition timing of internal combustion engines is called a timing light.

18) What Is Resonant Peak?

It is the peak (maximum) value of the magnitude of T(jω). It is denoted by Mr

Resonant Peak

Resonant peak in frequency response corresponds to the peak overshoot in the time domain transient response for certain values of damping ratio δ. So, the resonant peak and peak overshoot are correlated with each other.

19) What is signal flow graph?

SFG (Signal flow graph) is a graphical representation of the relationship between the variables of a set of linear algebraic equations.A single flow graph consists of a network in which nodes are connected by directed branches.

20) What is Mason's Gain formula?

Mason's gain formula is a method for finding the transfer function of a linear signal-flow graph. The formula was derived by Samuel Jefferson Mason, whom it is also named after.


21) What is meant by analogous System?

When the two differential equations are of same order or form such type of systems is called analogous systems.

22) What is Automatic Controller?

An instrument that continuously measures the value of a variable quantity or condition and then automatically acts on the controlled equipment to correct any deviation from a desired preset value is known as an Automatic controller or regulator.

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