(Aktu Btech) Control System Important Unit-1 Control System Concepts

Using Aktu’s Quantum Notes to Master Control Systems. To ace your B.Tech studies, have access to critical insights and frequently asked questions. Your road to success begins here! Unit-1 Control System Concepts

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Q1. Explain open loop and closed loop control system with the help of suitable examples.

Ans. A. Open loop control system: 

• 1. A control system without feedback or non-feedback control system is another name for an open loop control system.
• 2. The control action in open loop systems is unrelated to the desired output.
• 3. This system does not compare the output to the reference input.
• 4. The controller and controlled process are the elements of open loop systems.

• 5. Examples: 
  • i. Immersion rod 
  • ii. A field control DC motor 
  • iii. Door lock system 
  • iv. Traffic control system. 

B. Closed loop control system: 

• 1. Closed loop control system is also known as feedback control system. 
• 2. In closed loop control system the control action is dependent on the desired output. 

• 3. A closed loop system is any system with one or more feedback pathways. In closed loop systems, an error signal is generated when the output is compared to the reference input.
• 4. The controller receives the error signal and uses it to lower the error to produce the desired output.
• 5. Examples: 
  • i. Air conditioners 
  • ii. Autopilot aeroplane 
  • iii. Electric iron. 

Q2. Determine the transfer function of the circuit given below: 

Ans. Step 1:

Q3. What are the basic elements of control system?

Ans. The various elements of closed loop system are shown in Fig.

• 1. Command: The command is an input that is produced externally and is unrelated to the feedback control mechanism.
• 2. Reference input element: This produces the standard signals proportional to the command.
• 3. Error detector: The measured signal is delivered to the error detector, who then compares it to the reference input. The error signal is created by the difference between two signals. 
• 4. Control element: This regulates the output according to the signal obtained from error detector. 
• 5. Controlled system: This represents what we are controlling by the feedback loop. 
• 6. Feedback element: This element feedback the output of the error detector for comparison with the reference input.

Q4. Explain construction and working of AC servomotor.

Ans. A. Construction: 

• 1. Fig. shows the schematic diagram of a 2 𝛟 AC servomotor 
• 2. The stator has two distributed windings which are displaced from each other by 90 electrical degrees. 
• 3. One winding is called the reference or fixed phase and other winding is called control phase.

B. Working principle: 

• 1. Reference phase is supplied from a constant voltage source 

• 2. The control phase, the other winding, is given a variable voltage at the same frequency as the reference phase, but its phase is 90° off (electrically).
• 3. A servo amplifier is typically used to supply the control phase.
• 4. The phase difference between the control voltage and the reference phase voltages regulates the rotor’s speed and torque.
• 5. By switching the phase difference between the control phase voltage and the reference phase voltage from leading to lagging, the direction of the rotor’s spin can be changed.

Q5. What is tachometer ? Also explain different types of tachometer. 

Ans. A. Tachometer: 

• 1. A tachometer converts mechanical energy into electrical energy using electromechanical means.
• 2. The tool functions as a generator, with the output voltage inversely proportional to the angular velocity.

B. Types of tachometer:

a.  DC tachometer: 

• 1. The iron-core rotor is present in the most popular kind of DC tachometer.
• Permanent magnets are used to create the magnetic field, so no external power source is required.
• 3. The output voltage is applied across a pair of brushes that ride on the commutator segments, which are connected to the windings on the rotor (armature).
• 4 The DC tachometers display a rotating shaft’s speed visually. These tachometers are directly connected to a rpm-calibrated voltmeter.

b. AC tachometer:  

• 1. It comprises of two stator field coils or windings that are positioned at right angles to one another or spaced 90° electrically apart. Reference winding and quadrature winding are these windings.
• 2. In the space between the magnetic structure and the air, a rotating element, or rotor, is inserted. In order to reduce losses and have low inertia, the rotor is often made of a thin aluminium cup.
• 3. The primary winding receives an applied sinusoidal voltage of the specified value. Mechanically, a secondary winding is positioned at a 90-degree angle to the primary winding.
• 4. The sinusoidal output voltage’s magnitude is related to the rotor speed when the rotor shaft is rotated.

Q6. What are the characteristics of servomotors ? Compare the AC and DC servomotors.

Ans. A. Characteristics of AC servomotor:   

• 1. The torque-speed characteristics of AC servomotor (two phase) as shown in Fig. 
• 2. The motor receives positive damping from the negative slope, which indicates a high rotor resistance and improves stability.
• 3. The curve is linear for almost various control voltages.  

B. Characteristics of DC servomotor : 

• 1. The torque-speed characteristics of DC servomotor as shown in Fig.  
• 2. It is evident from the features that the slope is negative. As a result, a negative slope gives the servo drive system viscous damping.

B. Comparison :

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