Control systems Interview questions and Answers gate questions




1: What Is Meant By System ?

Answer: A system is defined as any physical object comprises of various components or elements which are interconnected with each other in a sequence to perform a specific function.

Example: Automobile, Automatic washing machine

2: What Is Meant By Control System ?

Answer: Control system is a system that can control its output(s) to a particular value or perform a sequence of events or perform an event if the specified conditions are satisfied based on the input(s) given.

3: List a few examples for a control system. ?

Answer: Control of a variable: In an automobile, the speed can be controlled by adjusting the accelerator to the desired level.

Control a sequence of events: In an automatic washing machine, a number of options are available like ‘normal’, ‘heavy’, ‘whites’ etc. The machine undergoes a sequence of washing cycle appropriate to the washing option chosen.

Control whether an event occurs or not: The automatic washing machine has a safety lock on the door so that the machine does not operate if the door is open. Here, the condition is that the door is to be closed for the machine to operate.

4: What are the major types of control systems ?

Answer: The two major types of control systems are open loop and closed loop systems.

Read here about the Comparison between open loop and closed loop control systems.




5: What is an open loop system ?

Answer: The control system in which the output is generated based on the reference input only is called as an open loop system. The actual value of the output is not measured and it does not modify the input to the process. Also, it has no influence on control action.

Example: Traffic control system where the traffic signals are operated based on the pre-programmed time slot given for each signal.

6: What are the basic elements of an open loop system ?

Answer: The basic elements of an open loop control system are, controller, final control element and process.

7: What is a closed loop system ?

Answer: The control system in which the actual value output is measured and is used to modify the input to the process and thereby maintain the output at a required value is called as a closed loop system.

Example: Traffic control system where the time slots of each traffic signal are decided based on the density of traffic.



8: What are the basic elements of a closed loop system ?

Answer: The basic elements of a closed loop system are, comparator, controller, a final control element, process, and measurement element.

9: What is a feedback in control systems ?

Answer: Feedback is a means by which the signal related to the output of the system is feedback to modify the input signal to a process.

When the feedback signal subtracts the input value, it is called negative feedback. It is the negative feedback that is required to control a system.

Positive feedback occurs when the feedback signal adds to the input.

10: Compare the effect of positive and negative feedback on control systems.
Answer: Positive feedback increases error signal and makes the system unstable. Sometimes, it is used in minor loops of control systems to amplify signals or parameters.
The negative feedback rejects disturbance signals and it is less sensitive to parameter variations. It reduces the system gain that results in better stability.

 

11: Define the mathematical model of a system.

Answer: The mathematical model consists of a set of differential equations that describes the input-output relations of various physical components of a system. The output of the system can be obtained by solving the differential equations for various input conditions.

12: Define Transfer Function.

Answer: Transfer Function of a control system is defined as the ratio of the Laplace transform of Output to the Laplace transform of the Input with zero initial conditions.



13. What is time-invariant System?

The time required to change from one state to another state is known as transient time, the value of current and voltage during this period is called transient response and the system in which the input and output characteristics of the system does not change with time is called the Time-Invariant System.

14. What are the advantages and disadvantages of open loop control System?

Advantages of the open-loop control system

Open loop systems are simple.

These are economical.

Less maintenance is required and is not difficult.

Disadvantages of the open-loop control system

Open loop systems are inaccurate.

These systems are not reliable.

These are slow.

Optimization is not possible.

15) What are the advantages and disadvantages of closed-loop control System?

Advantages of closed-loop systems

The closed loop systems are more reliable.

Closed loop systems are faster.

Many variables can be handled simultaneously.

Optimization is possible.

Disadvantages of closed-loop systems

Closed loop systems are expensive.

Maintenance is difficult.

Installation is difficult for these systems.

16. What are the necessary components of the feedback control system?

The processing system (open loop system), feedback path element, an error detector, and controller are the necessary components of the feedback control system.



17. What is the feedback in the control system?

When the input is fed to the system and the output received is sampled, and the proportional signal is then fed back to the input for automatic correction of the error for further processing to get the desired output is called as feedback in control system.

18. What is Gain Margin?

Gain margin is the gain which varies before the system becomes stable because if we continuously increase the gain up to a certain threshold, the system will become marginally stable, and if the gain varies further then it leads to instability. Mathematically, it is the reciprocal of the magnitude of the G(jω)H(jω) at phase cross-over frequency.

19. What is Signal Flow Graph?

The graphical representation of the system’s relationship between the variables of a set of linear equations is called SFG (Signal Flow Graph). Signal flow graphs do not require any reduction technique or process.

more details..

20. What are the essential characteristics of Signal Flow Graphs?

The essential characteristics of the signal flow graph are:

It represents a network in which nodes are used for the representation of system variable which is connected by direct branches.

SFG is a diagram which represents a set of equations. It consists of nodes and branches such that each branch of SFG having an arrow which represents the flow of the signal.

It is only applicable to the linear system.



21. What is the basic rule for block diagram reduction technique?

The basic rule for block diagram reduction is that if we make any changes in the diagram, then that changes do not create any changes in the input-output relationship of the system.

22. What is an order of a system?

Order of the system is the highest derivative of the order of its equation. Similarly, it is the highest power of ‘s’ in the denominator of the transfer function.

23. What is the resonant peak?

The maximum value of the closed-loop transfer function is called the Resonant Peak. A large value of resonant peak means that it has large overshoot value in the transient response.

24. What is the cut-off rate?

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

 



25. What is phase cross-over frequency?

When the phase of the open loop transfer function reaches 180? at a particular frequency then it is called as Phase crossover frequency.

26. What is Phase Margin?

When we have to bring the system to the edge of instability, the additional phase lag required at the gain crossover frequency is called the Phase Margin.

27. What Is Pole Of The System?

The value at which the function F(s) becomes infinite is called the Pole of the function F(s), where F(s) is a function of complex variables.

28. What Is Zero Of The System?

The value at which the function F(s) becomes zero is called the Zero of the function F(s), where F(s) is a function of complex variables.

29. What Is The Use For Cable Entry In Control Room?

When there is an emergency, i.e., fire/explosion takes place in the plant, and we have to restrict it from entering to the control room then MCT (Multiple Cable Transient) blocks are used, and the process control rooms are built for the non-hazardous area.

30. What is the effect of positive feedback on the Stability of the systems?

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



31. What is Servomechanism?

When a specific type of motor known as servomotor is combined with a rotary encoder or a potentiometer, to form a Servomechanism. In this setup, a potentiometer provides an analog signal to indicate the position and an encoder provide position and speed feedback.

32. Where is Servomechanism used?

Servomechanism is used in the control system so that the mechanical position of a device can be varied with the help of output.

The Servomechanism is widely used in a Governor value position control mechanism that is used in power plants to take the speed of turbine and process it using the transducer, and the final value is taken as a mechanical movement of the value. However, nowadays Governor value position control is done with Electronic controls that use power Thyristors. This mechanism is also used in robotic hand movement.

33. How many types of instrument cables are there?

The following types of instrument cables are there:

IS cables

NIS cables.

IS – Intrinsic safety and NIS – Non Intrinsic safety.

Depending upon the condition of hazards the type of cable is decided.

34. What are the temperature elements?

The temperature elements are-

Thermocouple.

Resistance temperature detectors (RTDs).



35. What is Cable Tray, its Type, and its Support?

The media or way through which we lay the field cables in plants is called as cable tray. These are made of aluminum, steel or fiber reinforced plastic (FRP) and are available in six types-

Ladder type Cable Tray(made of Rungs type construction)

Solid Bottom Cable Tray

Trough Cable Tray

Channel Cable Tray

Wire Mesh Cable Tray

Single Rail Cable Tray

The main points which we have to consider before laying the cables are the site conditions and the adequate space where we have to lay the cable.

36. How to decide cable tray size?

Based on the occupancy in the cable tray and a number of cables required we have to choose the size of the cable tray. These are available in all sizes like 80,150, 300, 450, 600 and 900.

37. What is the cut-off Rate?

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

38. Enlists the applications of Sampled Data Systems?

Sampled data system has the following applications-

Quantized data is used for controlling in High-speed tinplate rolling mills.

Digitally controlled or pulse controlled electric drives.

For machine tool operations which are numerically controlled.

It is used in large systems using telemetry links based on pulse modulation (PM) translational data.

39. What are DCS and PLC?

DCS and PLC are the control systems which handles fields I/Os. DCS is Distributed control system, and PLC is the Programmable logic controller.



40. What are stable systems?

Stable systems are the system in which all the roots of the characteristic equations lie on the right half of the ‘S’ plane.

41. What are marginally stable systems?

Marginally stable systems are the system in which all the roots of the characteristic equations lie on the imaginary axis of the ‘S’ plane

42. What are unstable systems?

Unstable systems are the system in which all the roots of the characteristic equations lie on the left half of the ‘S’ plane.

43. What is Routh Hurwitz Stability Criterion?

Routh Hurwitz criterion states that a system is stable if and only if all the roots of the first column have the same sign and if all the signs are not same then number of time the 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.

more details..

44. What is an Automatic Controller?

Automatic Controllers are the device which compares the actual value of plant output with the desired value. These systems produce the control system that reduces the deviation to ?0? or to a small value and determines the deviation.



45. What is the Control Action?

Control action is the manner in which the automatic controller produces the control signal.

46. Why Negative Feedback Is Preferred In The Control System?

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 ( deviation from the desired result).

Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.

47. What Is Pole Of The System?

Pole of a function F(s) is the value at which the function F(s) becomes infinite, where F(s) is a function of the complex variable s.

48. What Is Time Response Of The Control System?

Time response of the control system is defined as the output of the closed loop system as a function of time. Time response of the system can be obtained by solving the differential equations governing the system or time response of the system can also be obtained by transfer function of the system.

49 . What Is Mathematical Model Of A Control System?

Control system is a collection of physical elements connected together to serve an objective. The output and input relations of various physical system are governed by differential equations. Mathematical model of a control system constitutes set of differential equations. The response of the output of the system can be studied by solving the differential equations for various input conditions.



50. What Is The Basic Rule For Block Diagram Reduction Technique?

The rules of the Block Diagram reduction Techniques are designed in such a manner that any modifications made in the diagram will not alter the input and output relation of the system.



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