1. Feedback Control Systems (FCS)
Lecture-8-9
Block Diagram Representation of Control Systems
Dr. Imtiaz Hussain
email: imtiaz.hussain@faculty.muet.edu.pk
URL :http://imtiazhussainkalwar.weebly.com/
2. Introduction
• A Block Diagram is a shorthand pictorial representation of
the cause-and-effect relationship of a system.
• The interior of the rectangle representing the block usually
contains a description of or the name of the element, gain,
or the symbol for the mathematical operation to be
performed on the input to yield the output.
• The arrows represent the direction of information or signal
flow.
x
d
dt
y
3. Introduction
• The operations of addition and subtraction have a special
representation.
• The block becomes a small circle, called a summing point, with
the appropriate plus or minus sign associated with the arrows
entering the circle.
• The output is the algebraic sum of the inputs.
• Any number of inputs may enter a summing point.
• Some books put a cross in the circle.
4. Introduction
• In order to have the same signal or variable be an input
to more than one block or summing point, a takeoff (or
pickoff) point is used.
• This permits the signal to proceed unaltered along
several different paths to several destinations.
5. Example-1
• Consider the following equations in which 𝑥1 , 𝑥2 , 𝑥3 , are
variables, and 𝑎1 , 𝑎2 are general coefficients or mathematical
operators.
x3 a1 x1 a2 x2 5
9. Characteristic Equation
• The control ratio is the closed loop transfer function of the system.
C( s )
G( s )
R( s ) 1 G( s ) H ( s )
• The denominator of closed loop transfer function determines the
characteristic equation of the system.
• Which is usually determined as:
1 G( s )H ( s ) 0
10. Example-3
1. Open loop transfer function
B( s )
G( s ) H ( s )
E( s )
2. Feed Forward Transfer function
C( s )
G( s )
R( s ) 1 G( s ) H ( s )
3. control ratio
4. feedback ratio
5. error ratio
C (s)
G (s)
E (s)
G(s )
B( s )
G( s ) H ( s )
R( s ) 1 G ( s ) H ( s )
E( s )
1
R( s ) 1 G( s ) H ( s )
6. closed loop transfer function
H (s )
C( s )
G( s )
R( s ) 1 G( s ) H ( s )
7. characteristic equation 1 G( s )H ( s ) 0
8. Open loop poles and zeros if 9. closed loop poles and zeros if K=10.
15. Example-5
• For the system represented by the following block diagram
determine:
1.
2.
3.
4.
5.
6.
7.
8.
Open loop transfer function
Feed Forward Transfer function
control ratio
feedback ratio
error ratio
closed loop transfer function
characteristic equation
closed loop poles and zeros if K=10.
16. Example-5
– First we will reduce the given block diagram to canonical form
K
s 1
18. Example-5 (see example-3)
1. Open loop transfer function
B( s )
G( s ) H ( s )
E( s )
2. Feed Forward Transfer function
C( s )
G( s )
E( s )
C( s )
G( s )
3. control ratio
R( s ) 1 G( s ) H ( s )
4. feedback ratio
5. error ratio
G(s )
B( s )
G( s ) H ( s )
R( s ) 1 G ( s ) H ( s )
E( s )
1
R( s ) 1 G( s ) H ( s )
6. closed loop transfer function
C( s )
G( s )
R( s ) 1 G( s ) H ( s )
7. characteristic equation 1 G( s )H ( s ) 0
8. closed loop poles and zeros if K=10.
H (s )
19. Example-6
• For the system represented by the following block diagram
determine:
1.
2.
3.
4.
5.
6.
7.
8.
Open loop transfer function
Feed Forward Transfer function
control ratio
feedback ratio
error ratio
closed loop transfer function
characteristic equation
closed loop poles and zeros if K=100.
32. G4
R(s)
G1
G A G
G 4 G2G3
B
Y (s )
3
2
H2
H1G2
G4 G2G3
3. Moving pickoff point B behind block
II
R(s)
G1
B
G4 G2G3
H2
H1G2
1 /(G4 G2G3 )
C
Y (s )
33. 4. Eliminate loop III
R(s)
G1
G4 4 G2G3
G G2G3
H
1 H 2 (G4 2 G2G3 )
C
C
Y (s )
G2 H1
G4 G2G3
R(s)
G1 (G4 G2G3 )
1 G1G 2 H1 H 2 (G4 G2G3 )
G1 (G4 G2G3 )
Y (s)
R( s ) 1 G1G 2 H1 H 2 (G4 G2G3 ) G1 (G4 G2G3 )
Y (s )
34. Example 9
Find the transfer function of the following block diagrams
H4
R(s)
Y (s )
G1
G3
G2
H3
H2
H1
G4
35. Solution:
1. Moving pickoff point A behind block
G4
I
H4
R(s)
Y (s )
G1
G3
G2
H3
H2
H3
G4
H2
G4
H1
1
G4
1
G4
A
G4
B
36. 2. Eliminate loop I and Simplify
R(s)
G2G3G4
1 G3G4 H 4
G1
II
Y (s )
B
H3
G4
H2
G4
III
H1
II
feedback
G2G3G4
1 G3G4 H 4 G2G3 H 3
III
Not feedback
H 2 G4 H 1
G4
37. 3. Eliminate loop II & IIII
R(s)
G1G2G3G4
1 G3G4 H 4 G2G3 H 3
Y (s )
H 2 G4 H 1
G4
G1G2G3G4
Y (s)
R( s ) 1 G2G3 H 3 G3G4 H 4 G1G2G3 H 2 G1G2G3G4 H1