Control Systems Interview Questions & Answers

Control Systems Interview Questions

Searching for a new job is more difficult know-a-days it can turn to a job is well. So, to make it easy we has provided a complete detail about the Control Systems Interview Question and Answers along with the positions. To be precise about Control Systems is a system of devices or set of devices, that manages, commands, directs or regulates the behaviour of other device(s) or system(s) to achieve desire results. In other words, the definition of control system can be rewritten as A control system is a system, which controls other system. If you are familiar with Control Systems Concepts then there are various companies that offer various job positions like Senior Hardware Engineer, DeltaV Automation Eng, Automation Engineer and many other roles too.

Control Systems Interview Questions And Answers

Control Systems Interview Questions
    1. Question 1. What Is Meant By System?

      Answer :

      A system is defined as any physical object comprises of various components which are inter connected with each other by some characteristics.

      Example: Automobile.

    2. Question 2. What Is Meant By Control System?

      Answer :

      It is possible to vary any of the characteristics of components in a system according to the requirements, and then the system is called a controllable system or control system.

      For example consider an automobile, where the speed can be varied by adjusting the accelerator to the desired level.

    3. Question 3. What Are The Major Types Of Control System?

      Answer :

      The major types of control systems are:

      • Open loop control system.
      • Closed loop control system.

    4. Question 4. What Is Time Invariant System?

      Answer :

      If system's output characteristics do not change with time, then it is called as time invariant system.

      A linear time invariant system can be represented by constant co-efficient differential equations.

    5. Question 5. What Is Meant By Analogous Systems?

      Answer :

      The systems for which the differential equations have similar form are known as analogous system.

    6. Question 6. What Are The Advantages And Disadvantages Of Open Loop Control Systems?

      Answer :


      1. Open loop control is much simpler and less expensive.
      2. No sensors are needed to control the variables.


      1. No accurate control.
      2. There is no compensation for any disturbances entering the system since it has fixed input.

    7. Question 7. What Are The Advantages And Disadvantages Of Closed Loop Control System?

      Answer :


      1. More accurate control.
      2. It compensates for disturbances.
      3. Desired response can be obtained by appropriate design.


      1. More complex and expensive.
      2. Reduces the gain of the system.
      3. If the closed loop system is not properly designed, the feedback may lead to an oscillatory system.

    8. Question 8. What Is An Order Of A System?

      Answer :

      The order of the differential equation governing the system is called as the order of a system.
      From the transfer function of the given system, we can obtain the order of the system.

    9. Question 9. What Are Analog Input/output & Digital Input/output?

      Answer :

      • 4-20 mA signals from instrument (transmitters) are analog input to control system.
      • 4-20 mA signals to instrument (I/p & electro pneumatic positioner) are analog output from control system.
      • Volt free (24V) contact (NO/NC) by instruments (all type of switches, ex. Limit switch, press,temp,flow,level switch) are digital input to control system.
      • All powered signals (24V, 48V, 110V….) from system to instruments (SOV) are Digital outputs.

    10. Question 10. Types Of Instrument Cables?

      Answer :

      • IS cables & NIS cables.
      • IS – Intrinsic safety & NIS – Non Intrinsic safety.
      • Which cable to use, depends upon hazards condition.

    11. Question 11. Instrument Jb’s?

      Answer :

      • Instrument JB’s depending upon hazards area classification.
      • JB’s also can be IS or NIS.
      • For IS signal IS JB’s used for NIS signal NIS JB’s used.
      • For analog I/p & o/p signal we can use same JB.
      • But for Digital I/p & o/p we have to use separate JB’s. Because digital outputs are powered signal, by wrong connection there may be chances to damage the card.
      • For DCS I/p & o/p and PLC I/p & o/p we used separate JB’s.

    12. Question 12. What Is Open Loop & Close Loop?

      Answer :

      OPEN LOOP: This is nothing but to sense the process signals from the field and to send it to the control room for operator monitoring only.

      CLOSE LOOP: This is measuring the process signals for operator’s action. Means Tx sense the process and send it to the control room. Where the PID controller takes control action and that given to the final control element as per the process requirement.

    13. Question 13. What Is Dcs & Plc?

      Answer :

      Distributed control system (DCS) and Programmable logic controller. These are the control systems which handles fields I/Os.

      Basic difference between DCS & PLC is:

      1. DCS handles more nos of I/Os rather than PLC.
      2. PLC is faster system than DCS.
      3. DCS can handles handsome quantity of I/Os so that can be used for total plant automation. Whereas PLC has own limitations so it generally used for small but for important (Safety point of view) units, like boiler automations, Make-up compressor automations Etc.
      4. In the above mentioned case the PLCs can be get connected with the DCS with the help of soft link. Generally this is used to make alert to both the operator.
      5. As I heard the PLC used to handle the DI/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI/AO.
      6. DCS & PLC’s speed depends on the scan rate of I/Os.
      7. For both the system Marshalling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.
      8. According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.

    14. Question 14. While Making Datasheets Which Things Are To Be Considered?

      Answer :

      Basically it depends on the instrument item for which you are preparing the datasheet. As an Example. Temperature element. We have collect following information to prepare D/s.

      1. Type of Temperature Sensor (T/C or RTD, in case of T/c which type J,K,R,S etc.)
      2. Head details a: Screw-cap and chain, b: material, c: conduit connections, d: terminal block> single or duplex.
      3. Thermo well Details a: material, b: Construction, c: dimension, e: connection. Etc.   

      In case of valve:

      1. Process Details
      2. Material of construction (Valve body, Trim),
      3. Operating action,
      4. Type of operating characteristics
      5. Stroke time
      6. Accessories attached Etc.

    15. Question 15. What Are Temperature Elements?

      Answer :

      Thermocouples and rtds.

    16. Question 16. What Is Cable Tray? Its Type? Its Size? Its Support?

      Answer :

      Cable tray is nothing but the way or media through which we lay the field cables in plant.

      There are two basic types:

      1. Ladder type (made in Rungs type construction)
      2. Perforated Type(Solid sheath consist of Holes for ventilation). Basically discussing about tray support than we could say it depends on the site conditions. Only care has to be taken considering adequate space for laying cable, considering their bends.Etc.

    17. Question 17. How To Decide Cable Tray Size?

      Answer :

      According to the no. of cable occupancy in the cable tray and available tray size we have to choose it. They are available in all. Types 80,150,300,450,600 & 900.

    18. Question 18. What Is Meant By Instrument Location & Jb Location?

      Answer :

      This consists of Instrument location considering the piping drawing given from piping dept. We identify the locations of the instrument in the equipment layouts and put the bubbles and elevation and JB nos for the location. Same way depending upon the accessibility we decide the location of JB and marking of it into the instrument location plan is JB location. There is no need to make a different drawing for this.

    19. Question 19. While Locating Instrument & Jb Which Things We Have To Consider?

      Answer :

      Transmitter: Tube routing, maintenance area, man approach, Valve : Hand wheel operations, Maintenance area, Etc. Loop power indicators: Man approach Illumination from Electrical if instrument is not going to provide.

    20. Question 20. What Is The Use For Cable Entry In Control Room? (sleeves & Mct)

      Answer :

      In the process plant Control room built considering the non-hazardous area. So in case if fire/Explosion takes place in the plant than that has to be restricted from entering into the control room. So MCT (Multiple cables transient) blocks are used. They are designed to sustain the fire for fixed time duration. That block holds the cables which are entering into the CCR.

    21. Question 21. What Is Control System?

      Answer :

      A control system is an arrangement of physical components connected or related in such a manner as to command, direct or regulate in such a manner as to command, direct or regulate itself or another system.

      Control system can be of two viz. open-loop control system and closed-loop control system.

    22. Question 22. What Is Feedback In Control System? What Are Its Characteristics?

      Answer :

      Feedback is that property of a closed loop control system which allows the output or some other controlled variable of the system, to be compared with the input to the system, so that the appropriate control action may be formed as some function of the output and input.

      A feedback is said to exist in a system if the cause and effect between variables is interrelated.

      The characteristics of feedback are: increased bandwidth, increased accuracy, reduced distortion and non linearity and tendency towards oscillations or instability.

    23. Question 23. What Is System Error?

      Answer :

      The system error is the ideal value of the controlled variable minus the value of the final controlled variable.

    24. Question 24. What Are Control System Components?

      Answer :

      Various components employed in control systems are servo-motors, amplidyne, synchros, stepper motors, tachometers, gyroscopes etc.

    25. Question 25. Which Types Of Servo Motors Generally Lend Themselves To Small Horse Power Requirements?

      Answer :

      Two phase and shaded pole type induction motor are used.

    26. Question 26. Give One Advantage Of The Two Phase Servomotor Over The Shaded Pole Type?

      Answer :

      Better response to very small controls signals.

    27. Question 27. Why Are Drag-cup-construction Used In Rotors Of Certain Fhp Machines For Control Purposes?

      Answer :

      In drag-cup-construction all heavy iron lamination remain stationary and only a light cup rotates, so the inertia is quite small.

    28. Question 28. Why Is Series Compensating Winding Employed In The Amplidynes?

      Answer :

      Compensating winding is connected in series with the power output brush terminals of the amplidynes to neutralize the armature reaction mmf set up by the load current and increase amplification factor.

    29. Question 29. Why Area Commutating Poles Required In Amplidynes?

      Answer :

      Com mutating poles are required in amplidynes to give satisfactory commutation.

    30. Question 30. Why Amplidyne Are Not Extremely Over-compensated?

      Answer :

      In case of extreme over-compensation, undesirable self-excitation of the amplidyne that is spontaneous rise in voltage and load current at constant control current (or with the control circuit open) can occur. Self-excitation can cause considerable over-loading of the amplidyne and totally disrupt operation of system containing the amplidyne.

    31. Question 31. Why Amplidynes Are Provided With Split-poles?

      Answer :

      Amplidynes are provided with split-poles to provide space for inter poles.

    32. Question 32. What Is Stepper Motor?

      Answer :

      A motor in which the rotor turns in discrete movements is called a stepper motor.

    33. Question 33. What Are The Two Types Of Stepper Motors?

      Answer :

      Permanent –magnet type and variable reluctance type motor are stepper motor.

    34. Question 34. For The Same Number Of Poles Which Stepper Motor Produces?

      Answer :

      1. Largest stepping angle.
      2. Smallest stepping angle.
      • Permanent magnet type- largest stepping angle.
      • Variable reluctance type-smallest stepping angle.

    35. Question 35. Where Is Stepper Motor Used?

      Answer :

      Super motors are used in industrial situations which call for precise positioning of an object or precise control of speed without having to resort to closed-loop feedback. ,, it is most important question of control system.

    36. Question 36. How An Ac Tachometer Is Superior To Dc Tachometer?

      Answer :

      AC tachometer does not cause any maintenance problem of commentators and brushes.

    37. Question 37. What Is Transfer Function?

      Answer :

      It is the mathematical expression that relates the system output to the system input and so describes the behavior of a system. It is expressed as ratio of the Laplace transform of the output variable to the lap lace transform of the input variable with the assumption that all the initial conditions are zero.

    38. Question 38. What Is Linear System?

      Answer :

      A linear system is a system that follows the law of superposition and homogeneity.

    39. Question 39. What Is Analogous System?

      Answer :

      Systems which are governed by the same types of equations are called analogous systems. Analogous systems may have entirely different physical appearances. For example, a given electrical circuit consisting of resistances, inductance and capacitance may be analogous to a mechanical system consisting of a suitable combination of dash pots, weights and springs or it may be analogous to an acoustical device consisting of an appropriate arrangement of fine mesh screens, tubes and cavities.

    40. Question 40. What Is Signal Flow Graph?

      Answer :

      A signal flow graph is pictorial representation of the simultaneous equations describing a system. It graphically displays the transmission of signals through the system, as does the block diagram.

    41. Question 41. What Is Meant By Overshoot?

      Answer :

      Over shoot is maximum difference between the transient and steady-state solutions for a unit step function input.

    42. Question 42. Enlist The Applications Of Sampled Data Systems?

      Answer :

      Sampled data system finds applications in

      1. High speed tin plate rolling mills using quantized data for control.
      2. Numerically controlled machine tool operations.
      3. Pulse controlled or digital controlled electric drives.
      4. Large complex systems using telemetry links based on pulse modulation (PM) translation of data.

Control Systems Related Tutorials

Control Systems Related Interview Questions

Popular Interview Questions

All Interview Questions

Control Systems Practice Test

All rights reserved © 2020 Wisdom IT Services India Pvt. Ltd Protection Status

Control Systems Tutorial