Learning Objectives:

• Reduce a block diagram of multiple subsystems to a single block representing the transfer function from input to output.

• Draw the open-loop control system block diagram.

• Draw the closed-loop control system block diagram.

• Compare and contrast open and closed loop control.

• State the principle disadvantage of open-loop control.

• List the advantages of closed loop control.

where G\(_1\)(s) is a controller or compensator, G\(_2\)(s) represents a plant model, and H(s) represents the measurement dynamics. Y(s) represents the controlled variable, R(s) represents the reference input, and L(s) represents a disturbance.

Solved Example: 86-1-01

In an open loop control system: (Based on BSNL JE(TTA) Sep 2016-Shift II)

A. Control input is independent of output

B. Output is independent on control input

C. Only system parameters have effect on the control output

D. None of the above

Correct Answer: A

Solved Example: 86-1-02

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

A. Less expensive

B. Recalibration is not required for maintaining the required quality of the output

C. Construction is simple and maintenance easy

D. Errors are caused by disturbances

Correct Answer: B

Solved Example: 86-1-03

A control system in which the control action is somehow dependent on the output is known as: (RSMSSB JE Elec/Mech Dec 2020)

A. Closed loop system

B. Semi-closed loop system

C. Open system

D. None of the above

Correct Answer: A

Solved Example: 86-1-04

In open loop system, the control action:

A. Depends on the size of the system

B. Depends on system variables

C. Depends on the input signal

D. Independent of the output

Correct Answer: D

Solved Example: 86-1-05

A good control system has all the following features except:

A. Good stability

B. Slow response

C. Good accuracy

D. Sufficient power handling capacity

Correct Answer: B

Solved Example: 86-1-06

A closed loop system is distinguished from open loop system by which of the following? (Vizag MT Elect 2017)

A. Servomechanism

B. Feedback

C. Output pattern

D. Input pattern

A closed loop control system has feedback elements which are absent in open loop control system.

Correct Answer: B

Learning Objectives:

• Determine the overall transfer function of an interconected system, using block diagram reduction techniques.

Rule 1: Blocks in cascade (series):

Rule 2: (Combining blocks in parallel):

Rule 3: (Eliminating a feedback loop):

Solved Example: 86-2-01

Find the equivalent transfer function, T(s) = C(s)/R(s), for the system shown in Figure.

A. $\dfrac{1}{s+5}$

B. $\dfrac{5}{s+5}$

C. $\dfrac{s+5}{5}$

D. (s+5)

The transfer function will be given by: \begin{align*} T(s) = \dfrac{C(s)}{R(s)} &= \dfrac{G(s)}{1 + G(s) H(s)}\\ &= \dfrac{\left(\dfrac{1}{s}\right)}{1 + \left(\dfrac{1}{s}\right) (5)}\\ &= \dfrac{\left(\dfrac{1}{s}\right)}{\left(1 + \dfrac{5}{s}\right)}\\ &= \dfrac{1}{s+5} \end{align*}

Correct Answer: A