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/
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
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.
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.
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
Example-1
x3  a1 x1  a2 x2  5
Example-2
• Draw the Block Diagrams of the following equations.
(1)
( 2)
dx1 1
x2  a1
  x1dt
dt
b
x3  a1
d 2 x2
dt 2
dx1
3
 bx1
dt
Canonical Form of A Feedback Control System
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
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.
Reduction techniques
1. Combining blocks in cascade
G2
G1
G1G2
2. Combining blocks in parallel
G1
G2
G1  G2
3. Eliminating a feedback loop
G
G
1  GH
H
G
H 1
G
1 G
Example-4: Reduce the Block Diagram to Canonical Form.
Example-4: Continue.
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.
Example-5
– First we will reduce the given block diagram to canonical form
K
s 1
Example-5
K
s 1
K
G
 s 1
K
1  GH
1
s
s 1
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 )
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.
Reduction techniques
4. Moving a summing point behind a block
G
G
G
5. Moving a summing point ahead a block
G
G
1
G
6. Moving a pickoff point behind a block
G
G
1
G
7. Moving a pickoff point ahead of a block
G
G
G
8. Swap with two neighboring summing points
A
B
B
A
Example-7
• Reduce the following block diagram to canonical form.
H2
_
R
+_
+
+
G1
+
H1
C
G2
G3
Example-7
H2
G1
_
R
+_
+
+
+
C
G1
H1
G2
G3
Example-7
H2
G1
_
R
+_
+
+
C
+
G1G2
H1
G3
Example-7
H2
G1
C
_
R
+_
+
+
G1G2
+
H1
G3
Example-7
H2
G1
_
R
+_
+
G1G2
1  G1G2 H1
C
G3
Example-7
H2
G1
_
R
+_
+
G1G2G3
1  G1G2 H1
C
Example-7
R
+_
G1G2G3
1  G1G2 H1  G2G3 H 2
C
Example 8
Find the transfer function of the following block diagram
G4
R (s )
Y (s)
G1
G2
G3
H2
H1
I
G4
R(s)
B
G1
G2
A
G3
H2
H1
G2
Solution:
1. Moving pickoff point A ahead of block
2. Eliminate loop I & simplify
B
G4  G2G3
G2
Y (s)
G4
R(s)
G1
A G G
2G 3
GG4 
B
2
Y (s)
3
H2
H 1G2
G4  G2G3
3. Moving pickoff point B behind block
II
R(s)
G1
B
G4  G2G3
H2
H 1G2
1/(G4  G2G3 )
C
Y (s)
4. Eliminate loop III
R(s)
G1
GG4 4GG
2G
3 3
2G
1  H 2 (GH4 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  G2 G3 )
Y (s)

R( s ) 1  G1G 2 H 1  H 2 (G4  G2 G3 )  G1 (G4  G2 G3 )
Y (s)
Example 9
Find the transfer function of the following block diagrams
H4
R(s)
Y (s)
G1
G2
G3
H3
H2
H1
G4
Solution:
1. Moving pickoff point A behind block
G4
I
H4
R(s)
Y (s)
G1
G2
G3
H3
H2
H3
G4
H2
G4
H1
1
G4
1
G4
A
G4
B
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 H1
G4
3. Eliminate loop II & IIII
R(s)
G1G2G3G4
1  G3G4 H 4  G2G3 H 3
Y (s)
H 2  G4 H1
G4
G1G2G3G4
Y (s)

R( s ) 1  G2G3 H 3  G3G4 H 4  G1G2G3 H 2  G1G2G3G4 H 1
Example-10: Reduce the Block Diagram.
Example-10: Continue.
Example-11: Simplify the block diagram then obtain the closeloop transfer function C(S)/R(S). (from Ogata: Page-47)
Example-11: Continue.
Superposition of Multiple Inputs
Example-12: Multiple Input System. Determine the output C
due to inputs R and U using the Superposition Method.
Example-12: Continue.
Example-12: Continue.
Example-13: Multiple-Input System. Determine the output C
due to inputs R, U1 and U2 using the Superposition Method.
Example-13: Continue.
Example-13: Continue.
Example-14: Multi-Input Multi-Output System. Determine C1
and C2 due to R1 and R2.
Example-14: Continue.
Example-14: Continue.
When R1 = 0,
When R2 = 0,
Block Diagram of Armature Controlled D.C Motor
Ra
La
c
Va
ia
eb
T
J

La s  Ra I a(s)  K b(s)  Va(s)
Js  c (s)  K m I a(s)
Block Diagram of Armature Controlled D.C Motor
La s  Ra I a(s)  K b(s)  Va(s)
Block Diagram of Armature Controlled D.C Motor
Js  c ( s)  K ma I a( s)
Block Diagram of Armature Controlled D.C Motor
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END OF LECTURES-8-9