Control System Objective Question And Answer with Explanation

Most Important Control system Multiple Choice Question (MCQ) With Explanation

Ques1.  In an open loop control system

  1. Output is independent of control input
  2. Output is dependent on control input
  3. Only system parameters have effect on the control output
  4.  None of the above
Answer.1. Output is independent of control system


When the action of the system is independent of the output i.e the output has no influence or effect on the control action of the input signal then the system is called as the open loop control system.An Open-loop system is also called as the non-feedback system. 


Ques 2. For open control system which of the following statements is incorrect?

  1.  Less expensive
  2.  Recalibration is not required for maintaining the required quality of the output
  3. Construction is simple and maintenance easy
  4.  Errors are caused by disturbance
Answer.2. Recalibration is not required for maintaining the required quality of the output


There are several limitations of open-loop systems which include: slow system response, poor disturbance rejection, poor tracking under uncertainties, high sensitivity to system parameter errors (e.g., errors in plant or controller gains), and high sensitivity to changes in calibration errors (hence recalibration is usually necessary).


Ques 3. A control system in which the control action is somehow dependent on the output is known as

  1. Closed loop system
  2. Semiclosed loop system
  3. Open system
  4. None of the above
Answer.1. Closed loop control system


A closed-loop control system compares a measurement of the actual output with the desired output (reference or command input). The measure of the output is called the feedback signal.  The difference between the two quantities (the error signal) is then used to drive the output closer to the reference input through the controller and actuator.


Ques 4. In closed loop control system, with positive value of feedback gain, the overall gain of the system will

  1. Decrease
  2. Increase
  3. Be unaffected
  4. Any of the above
Answer.1. Increase


In an automated process, positive feedback would result in the system becoming unstable because an increase in the output leads to an increase in the input which creates an increase in the output and so the loop starts again.

Negative Feedback is often used in automated Control processes because it works to change the input in such a way that the output is decreased to provide stable operating conditions.


Ques 5. Which of the following is an open loop control system?

  1. Field controlled D.C. servomotor
  2. Ward Leonard control
  3. Metadyne
  4. Stroboscope
Answer.1. Field controlled D.C. servomotor


  • In this motor the control signal obtained by the servo amplifier is applied to the field winding and with the constant current source, the armature current is maintained constant.
  • This type of motor has large reactance to resistance ratio (L/R ratio). Hence the time constant of the motor is high. Therefore it can’t give the rapid response to the quick changing control signals.
  • It is an open loop control system because any change in the output has no effect on the input i.e speed will not be constant for load variations on the motor since There is not any feedback to the controller to indicate the change in speed due to load.

Block diagram of field-control DC motor


Ques 6. Which of the following statements is not necessarily correct for open control system?

  1. Input command is the sole factor responsible for providing the control action
  2. Presence of non-linearities causes malfunctioning
  3. Less expensive
  4. Less stable than closed loop system

Answer.4. Less stable than closed loop system


Advantages of Open Loop Control System

  1. Better reliability and stability
  2. More suitable where accurate positioning of the tools is not required
  3. Easily buildable
  4. Less costly

Disadvantages of Closed Loop Control system

  1. Error cannot be corrected
  2. Control action depend upon input command
  3. Presence of non-linearities can cause malfunction.

Why Open Loop control system is more accurate

  • Open loop systems are set up to achieve desired output for particular input in given conditions and in general, gives stable bounded output for bounded input.
  • Mathematically, all the open loop poles(in Laplace domain) need to be to the left of the s- plane for dampening response. Any pole to the right might cause the system to become unstable.


Ques 7. In open loop system

  1. Control action depends on the size of the system
  2. Control action depends on system variables
  3. Control action depends on the input signal
  4. Control action is independent of the output
Answer.4. Control action is independent of the output


Control action is the quantity responsible for activating the system to produce the output.

An open loop system is one in which the control action is independent of the output.

Examples of open-loop control systems are a toaster, in which the amount of heat is set by a human, and an automatic washing machine, in which the controls for water temperature, spin-cycle time, etc are preset by a human. ln both these cases, the control actions are not a function of the output of the toaster or the Washing machine.

Examples of feedback control are a room temperature thermostat, which senses room temperature and activates a heating or cooling unit when a certain threshold temperature is reached.


Ques 8.  Which system has the tendency to oscillate?

  1. Open loop system
  2. Closed-loop System
  3. Both 1 & 2
  4. None of the above
Answer.2. Closed-loop system


Due to the presence of feedback action, the closed-loop system has the tendency towards oscillation or instability.


Ques 9. A good control system has all the following features except

  1. Good stability
  2. Slow Response
  3. Good Accuracy
  4. Sufficient power handling capacity
Answer.2. Slow Response


For any control system, it is desirable that the system should react quickly to a control input or excitation.


Ques 10. A car is rising at a constant speed of 50 km/h, which of the following is the feedback element for the driver?

  1. Clutch
  2. Eyes
  3. Needle of the speedometer
  4. Steering wheel
Answer.3. Needle of speedometer


A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle.

Here the change in speed will be sense by speedometer thus speedometer is used as a feedback for  the variation of the speed of the car

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Q 4 Answer should be option 2 increase


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