______ is usually employed in the open-loop control system.

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ISRO LPSC Technical Assistant Mechanical 4 March 2018 Official Paper
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  1. Servo motor
  2. Stepper motor
  3. Induction motor
  4. Brush-less DC motor

Answer (Detailed Solution Below)

Option 2 : Stepper motor
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Explanation:

Open-loop control system:

The open-loop control system can be described by a block diagram as shown in the figure below.

F1 U.B Deepak 26.03.2020 D2

These are the systems in which the control action is independent of output.

Ex: Traffic signals, washing machine and bread toaster, systems having no sensor, etc.

Advantages:

  • Simple construction and design
  • Cost is less
  • Maintenance is easy
  • No problem with instability
  • Convenient to measure when the output is difficult to measure

Disadvantages:

  • These are less accurate, and their accuracy depends on the calibration.
  • Inaccurate results are obtained with parameter variations within the system.
  • Recalibration of the controller is required from time to time to maintain accuracy. 

Stepper Motor: 

  • A stepper motor is a pulse-driven motor that changes the angular position of the rotor in steps.
  • Due to this nature of a stepper motor, it is widely used in low cost, open-loop position control systems.
  • A stepper motor can also be controlled in this full servo closed-loop mode with the addition of an encoder.

Additional Information

Servomotors 

  • Servomotors are special electromechanical devices that produce precise degrees of rotation. 
  • Servomotors are also called control motors as they are involved in controlling a mechanical system. The servomotors are used in a closed-loop servo system. 
  • A feed drive consists of a feed servomotor and an electronic controller. 
  • The feed motor has certain special characteristics, like constant torque and positioning which are done by servo-motors.

Synchronous Motors: In the synchronous motor the stator poles are wound with coils and the rotor is a permanent magnet and is supplied with current to create fixed polarity poles.

Induction Motor: In the case of an induction motor, the stator is similar to the synchronous motor with windings but the rotors’ construction is different. 

Brushless DC motor (BLDC motor):

  • A BLDC motor is an electronically commuted DC motor which does not have brushes.
  • A typical brushless motor has permanent magnets that rotate around a fixed armature.
  • Brushless DC (BLDC) motors are well-suited for servo applications due to their high efficiency and reliability, low rotor torque (which improves dynamic response), and linear speed-torque relationship.

Closed-loop control system:

The open-loop control system can be described by a block diagram as shown in the figure below.

F1 U.B Deepak 26.03.2020 D3

These are the systems in which the control action depends on the output. These systems have a tendency to oscillate.

Ex: Temperature controllers, speed control of the motor, systems having sensors, etc.

Advantages:

  • More accurate
  • Non-linear distortions are less
  • The output is less sensitive to parameter changes within the system
  • Bandwidth increases

Disadvantages:

  • Design is complicated
  • More expensive
  • May become unstable, if there are malfunctions in the feedback.

The field controlled system is an open-loop control system.

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