This document provides an overview of transfer functions and stability analysis of linear time-invariant (LTI) systems. It discusses how the Laplace transform can be used to represent signals as algebraic functions and calculate transfer functions as the ratio of the Laplace transforms of the output and input. Poles and zeros are introduced as important factors for stability. A system is stable if all its poles reside in the left half of the s-plane and unstable if any pole resides in the right half-plane. Examples are provided to demonstrate calculating transfer functions from differential equations and analyzing stability based on pole locations.

1. Feedback Control Systems (FCS)
Lecture-2
Transfer Function and stability of LTI systems
Dr. Imtiaz Hussain
email: imtiaz.hussain@faculty.muet.edu.pk
URL :http://imtiazhussainkalwar.weebly.com/
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2. Transfer Function
• Transfer Function is the ratio of Laplace transform of the
output to the Laplace transform of the input.
Considering all initial conditions to zero.
u(t)
If
Plant
u(t ) U ( S )
y(t )
y(t)
and
Y (S )
• Where  is the Laplace operator.
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3. Transfer Function
• Then the transfer function G(S) of the plant is given
as
G( S )
U(S)
Y(S )
U(S )
G(S)
Y(S)
3

4. Why Laplace Transform?
• By use of Laplace transform we can convert many
common functions into algebraic function of complex
variable s.
• For example
 sin t
s
Or
e
at
2
2
1
s a
• Where s is a complex variable (complex frequency) and
is given as
s
j
4

5. Laplace Transform of Derivatives
• Not only common function can be converted into
simple algebraic expressions but calculus operations
can also be converted into algebraic expressions.
• For example
dx(t )

sX ( S ) x(0)
dt
2

d x(t )
dt
2
2
s X ( S ) x( 0)
dx( 0)
dt
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6. Laplace Transform of Derivatives
• In general
d x(t )
n

dt
n
s X (S ) s
n
n 1
x( 0)  x
n 1
( 0)
• Where x(0) is the initial condition of the system.
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7. Example: RC Circuit
• u is the input voltage applied at t=0
• y is the capacitor voltage
• If the capacitor is not already charged then
y(0)=0.
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8. Laplace Transform of Integrals
 x(t )dt
1
X (S )
s
• The time domain integral becomes division by
s in frequency domain.
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9. Calculation of the Transfer Function
• Consider the following ODE where y(t) is input of the system and
x(t) is the output.
• or
A
d 2 x(t )
dy(t )
C
dt
dt 2
Ax' ' (t )
dx(t )
B
dt
Cy ' (t ) Bx' (t )
• Taking the Laplace transform on either sides
A[ s 2 X ( s ) sx( 0)
x' ( 0)]
C[ sY ( s )
y( 0)] B[ sX ( s )
x( 0)]
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10. Calculation of the Transfer Function
A[ s 2 X ( s ) sx( 0)
x' ( 0)]
C[ sY ( s )
y( 0)] B[ sX ( s )
x( 0)]
• Considering Initial conditions to zero in order to find the transfer
function of the system
As 2 X ( s )
CsY ( s ) BsX ( s )
• Rearranging the above equation
As 2 X ( s ) BsX ( s )
X ( s )[ As 2
X (s)
Y (s)
Bs ]
Cs
As 2
Bs
CsY ( s )
CsY ( s )
C
As B
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11. Example
1. Find out the transfer function of the RC network shown in figure-1.
Assume that the capacitor is not initially charged.
Figure-1
2. u(t) and y(t) are the input and output respectively of a system defined by
following ODE. Determine the Transfer Function. Assume there is no any
energy stored in the system.
6u' ' (t ) 3u(t )
y(t )dt
3 y' ' ' (t )
y(t )
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12. Transfer Function
• In general
• Where x is the input of the system and y is the output of
the system.
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13. Transfer Function
• When order of the denominator polynomial is greater
than the numerator polynomial the transfer function is
said to be ‘proper’.
• Otherwise ‘improper’
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14. Transfer Function
• Transfer function helps us to check
– The stability of the system
– Time domain and frequency domain characteristics of the
system
– Response of the system for any given input
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15. Stability of Control System
• There are several meanings of stability, in general
there are two kinds of stability definitions in control
system study.
– Absolute Stability
– Relative Stability
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16. Stability of Control System
• Roots of denominator polynomial of a transfer
function are called ‘poles’.
• And the roots of numerator polynomials of a
transfer function are called ‘zeros’.
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17. Stability of Control System
• Poles of the system are represented by ‘x’ and
zeros of the system are represented by ‘o’.
• System order is always equal to number of
poles of the transfer function.
• Following transfer function represents nth
order plant.
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18. Stability of Control System
• Poles is also defined as “it is the frequency at which
system becomes infinite”. Hence the name pole
where field is infinite.
• And zero is the frequency at which system becomes
0.
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19. Stability of Control System
• Poles is also defined as “it is the frequency at which
system becomes infinite”.
• Like a magnetic pole or black hole.
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20. Relation b/w poles and zeros and frequency
response of the system
• The relationship between poles and zeros and the frequency
response of a system comes alive with this 3D pole-zero plot.
Single pole system
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21. Relation b/w poles and zeros and frequency
response of the system
• 3D pole-zero plot
– System has 1 ‘zero’ and 2 ‘poles’.
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22. Relation b/w poles and zeros and frequency
response of the system
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23. Example
• Consider the Transfer function calculated in previous
slides.
G( s )
X (s)
Y (s)
C
As B
the denominator polynomial is
As
B
0
• The only pole of the system is
s
B
A
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24. Examples
• Consider the following transfer functions.
– Determine
•
•
•
•
i)
iii)
Whether the transfer function is proper or improper
Poles of the system
zeros of the system
Order of the system
G( s )
G( s )
s 3
s( s 2)
( s 3) 2
s( s
2
10)
ii)
iv)
G( s )
G( s )
s
( s 1)(s 2)(s 3)
s 2 ( s 1)
s( s 10)
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25. Stability of Control Systems
• The poles and zeros of the system are plotted in s-plane
to check the stability of the system.
j
LHP
Recall s
RHP
j
s-plane
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26. Stability of Control Systems
• If all the poles of the system lie in left half plane the
system is said to be Stable.
• If any of the poles lie in right half plane the system is said
to be unstable.
• If pole(s) lie on imaginary axis the system is said to be
marginally stable.
j
LHP
RHP
s-plane
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27. Stability of Control Systems
• For example
G( s )
C
,
As B
if A 1, B
3 and C
10
• Then the only pole of the system lie at
pole
3
j
LHP
RHP
X
-3
s-plane
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28. Examples
• Consider the following transfer functions.





i)
iii)
Determine whether the transfer function is proper or improper
Calculate the Poles and zeros of the system
Determine the order of the system
Draw the pole-zero map
Determine the Stability of the system
G( s )
G( s )
s 3
s( s 2)
( s 3) 2
s( s
2
10)
ii)
iv)
G( s )
G( s )
s
( s 1)(s 2)(s 3)
s 2 ( s 1)
s( s 10)
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END OF LECTURES-2
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