# Control system MCQ Questions and Answers with explanation -2021

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 an effect on the control output
4.  None of the above
Answer.1. Output is independent of control system

Explanation:

### Open-loop control system

• In any open-loop control system, the output is not compared with the reference input.
• Thus, to each reference input, there corresponds a fixed operating condition, as a result, the accuracy of the system depends on the calibration.
• In open-loop system control action is independent of the output.
• In the presence of disturbances, an open-loop control system will not perform the desired task because when the output changes due to disturbances, it is not followed by changes in input to correct the output.
• In open-loop control systems, the changes in output are corrected by changing the input manually.
• Open-loop control systems can be used in practice only if the relationship between the input and the output is known and if there are neither internal nor external disturbances.
• In other words, the output is neither measured nor fed back for comparison with the reference point. Hence the output is independent of the input.

For example, let us consider a washing machine in which soaking, washing, and rinsing in the washer operates on a time basis. The machine does not measure the output signal, that is, the cleanliness of clothes. Such open-loop control systems can be used in practice if the relationship between input and output is known and there are no external or internal disturbances.

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

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

Explanation:

### Open-loop control system

• In any open-loop control system, the output is not compared with the reference input.
• Thus, to each reference input, there corresponds a fixed operating condition, as a result, the accuracy of the system depends on the calibration.
• In open loop system control action is independent of the output.
• In the presence of disturbances, an open-loop control system will not perform the desired task because when the output changes due to disturbances, it is not followed by changes in input to correct the output.

In an open-loop, if the systems undergo rapid changes in either the external variables or in their own behavior. In such situations, open-loop control will not be satisfactory, since it would often require recalibration and adjustments of the calibration setting.

The main disadvantages are the following:

1. Disturbances, internal or external, cause drifts in the desired output.
2. Changes in calibration cause errors in the system.
3. Re-calibration of the system may be necessary from time to time in order to maintain the required quality of 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

Explanation:

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

Explanation:

### Closed-Loop Control

• A closed-loop control system utilizes a measure of the actual output to compare the actual output with the desired output response.
• The measure of the output is called the feedback signal. The elements of a general closed-loop feedback control system are shown in Figure.

• A closed-loop control system compares the measurement of the output with the desired output (reference or command input).
• 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.
• Often the difference between the output of the process under control and the reference input is amplified and used to control the process so that the difference is continually reduced.

The transfer function of the closed-loop transfer function, with the positive value of feedback gain is given by

$\frac{{G\left( s \right)}}{{1 – G\left( s \right)H\left( s \right)}}$

• If the feedback signal enhances the magnitude of the input signal, the feedback is referred to as positive, direct, or regenerative feedback.
• The overall gain of the amplifier is increased by positive feedback.
• Regenerative feedback implies feedback with a Positive sign.

Positive feedback causes instability in the gain of the amplifier and distorts the input signal. It is used in narrowband amplifiers and radio receivers because it increases the signal strength.

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

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

Explanation:

Field controlled D.C. servomotor is an open loop control system.

• 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. The 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.

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 accurate than closed loop system
Answer.4. Less accurate than closed loop system

Explanation:

The systems in which output has no effect on the control action are called open-loop control systems. I

## 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 Less accurate

Open-loop control systems are those control systems in which the output has no effect on the control action are called open-loop control systems. In other words, in an open-loop control system, the output is neither measured nor fedback for comparison with the input. One practical example is a washing machine. Soaking, washing, and rinsing in the washer operate on a time basis. The machine does not measure the output signal, that is, the cleanliness of the clothes.

In an open-loop control system, the output is not compared with the refer once input. Thus, to each reference input, there corresponds a fixed operating condition, and as a result, the accuracy of the system depends on calibration. In the presence of disturbances, an open-loop control system will not perform the desired task. Open-loop control can be used, in practice, only if the relationship between the input and output is known and if there are neither internal nor external disturbances. Clearly, such systems are not feedback control systems. Note that any control system that operates on a time basis is an open-loop control system. For example, traffic control by means of signals operated on a time basis is another example of open-loop control.

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

Explanation:

### Open-loop control system

• In any open-loop control system, the output is not compared with the reference input.
• Thus, to each reference input, there corresponds a fixed operating condition, as a result, the accuracy of the system depends on the calibration.
• In open loop system control action is independent of the output.
• In the presence of disturbances, an open-loop control system will not perform the desired task because when the output changes due to disturbances, it is not followed by changes in input to correct the output.
• In open-loop control systems, the changes in output are corrected by changing the input manually.
• Open-loop control systems can be used in practice only if the relationship between the input and the output is known and if there are neither internal nor external disturbances.
• In other words, the output is neither measured nor fed back for comparison with reference point.

For example, let us consider a washing machine in which soaking, washing and rinsing in the washer operates on the time basis. The machine does not measure the output signal, that is, cleanliness of clothes. Such open-loop control systems can be used in practice, if the relationship between input and output is known and there are no external or internal disturbances.

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

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

Explanation:

### Open-loop control system

• In any open-loop control system, the output is not compared with the reference input.
• Thus, to each reference input, there corresponds a fixed operating condition, as a result, the accuracy of the system depends on the calibration.
• In open loop system control action is independent of the output.
• In the presence of disturbances, an open-loop control system will not perform the desired task because when the output changes due to disturbances, it is not followed by changes in input to correct the output.
• Open loop control system has no feedback is used and gain of the system is very high due to this damping is very less and hence system has tendency to oscillate.
• In open-loop control systems, the changes in output are corrected by changing the input manually.
• Open-loop control systems can be used in practice only if the relationship between the input and the output is known and if there are neither internal nor external disturbances.
• In other words, the output is neither measured nor fed back for comparison with reference point.

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

Explanation:

A good control system has all the following features except slow Response.

A well-designed control system must have less error, good accuracy, good speed of response, good relative stability, good damping so that undue overshoots etc. can be avoided. Usually, first priority is given to gain adjustment. Practically, it has been found that adjusting the gain alone, it is very difficult to obtain a satisfactory

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