2.mathematical foundation 2.1 the transfer function concept from the mathematical standpoint,...
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2.Mathematical Foundation
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2.1 The transfer function concept
From the mathematical standpoint, algebraic and differential or difference equations can be used to describe the dynamic behavior of a system .In systems theory, the block diagram is often used to portray system of all types .For linear systems, transfer functions and signal flow graphs are valuable tools for analysis as well as for design
If the input-output relationship of the linear system of Fig.1-2-1 is known, the characteristics of the system itself are also known.
The transfer function of a system is the ratio of the transformed output to the transformed input.
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systeminput output
a
TF(s)input output
b
)(
)(
)(
)()(
sr
sc
sinput
soutputsTF
Finger 1-2-1 input-output relationships (a) general (b) transfer function
(2-1)
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Summarizing over the properties of a function we state:
1.A transfer function is defined only for a linear system, and strictly, only for time-invariant system.
2.A transfer function between an input variable and output variable of a system is defined as the ratio of the Lap lace transform of the output to the input.
3.All initial conditions of the system are assumed to zero.
4.A transfer function is independent of input excitation.
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2.2 The block diagram. Figure 2-3-1 shows the block diagram of a linear feedback control
system. The following terminology often used in control systems is defined with preference to the block diagram.
R(s), r (t)=reference input.
C(s), c (t)=output signal (controlled variable).
B(s), b (t)=feedback signal.
E(s), e (t)=R(s)-C(s)=error signal.
G(s)=C(s)/c(s)=open-loop transfer function or forward-path transfer function.
M(s)=C(s)/R(s)=closed-loop transfer function
H(s)=feedback-path transfer function.
G(s)H(s)=loop transfer function.
G(s)
H(s)
Fig2-2-1
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The closed –loop transfer function can be expressed as a function of G(s) and H(s). From Fig.2-2-1we write:
C(s)=G(s)c(s) (2-2)
B(s)=H(s)C(s) (2-3)
The actuating signal is written
C(s)=R(s)-B(s) (2-4)
Substituting Eq(2-4)into Eq(2-2)yields
C(s)=G(s)R(s)-G(s)B(s) (2-5)
Substituting Eq(2-3)into Eq(2-5)gives
C(s)=G(s)R(s)-G(s)H(s)C(s) (2-6)
Solving C(s) from the last equation ,the closed-loop transfer function of the system is given by M(s)=C(s)/R(s)=G(s)/(1+G(s)H(s)) (2-7)
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2.3 Signal flow graphs
Fundamental of signal flow graphs
A simple signal flow graph can be used to represent an algebraic relation
It is the relationship between node i to node with the transmission function A, (it is also represented by a branch).
jiji XAX (2-8)
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2.3.1 Definitions
Let us see the signal flow graphs
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Definition 1: A path is a Continuous, Unidirectional
Succession of branches along which no node is passed
more than once. For example, to to to ,
and back to and to to are paths.
1X 2X3X
4X32 , XX
2X 1X 2X
4X
Definition 2: An Input Node Or Source is a node with only
outgoing branches. For example, is an input node. 1X
Definition 3: An Output Node Or Sink is a node with only
incoming branches. For example, is an output node.
4X
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Definition 4: A Forward Path is a path from the input node to
the output node. For example, to to to and
to to are forward paths.
Definition 5: A Feedback Path or feedback loop is a path
which originates and terminates on the same node. For
example, to , and back to is a feedback path.
Definition 6: A Self-Loop is a feedback loop consisting of a
single branch. For example, is a self-loop.
1X 2X3X 4X
1X 2X4X
2X3X 2X
33A
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Definition 7: The Gain of a branch is the transmission
function of that branch when the transmission function is
a multiplicative operator. For example, is the gain of
the self-loop if is a constant or transfer function.
Definition 8: The Path Gain is the product of the branch
gains encountered in traversing a
path. For example, the path gain of the forward path from,
to to to is
Definition 9: The Loop Gain is the product of the branch
gains of the loop. For example, the loop gain of the
feedback loop from to and back is
33A
33A
1X2X 3X 4X
.2332 AA2X 2X3X
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2.4 Construction of signal flow graphs
A signal flow graph is a graphical representation of a set of algebraic relationship, and it is a directed graph. The arrow represents the relationship between variables. In general, a variable can be represented by a node.
Example: A typical feedback system. (In this case, a
dummy node and a branch are added because the output node C has all outgoing branch).
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Example: Consider the following resistor network. There are five variables, .,,,, 21321 iiVVV
We can write 4 linear equations:
243
32
22
2
2332
21
11
1
11
11
iRv
vR
vR
i
iRiRv
vR
vR
i
i
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Let as input node, the output node can be found as follows:
1v3v
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2.5 General input-output gain transferfunction Let denote the ration between the input and the
output. For the signal flow diagram representation, it becomes
Definition 10: Non-touching two loops, paths, or
loop and path are said to be non-touching if they have no nodes in common.
R
CT
l
n
X
XT
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iP
jkP
j
jkk
k P1)1(1
j
jj j
jj PPP 3211
= the ith forward path gain
= jth possible product of k non-touching loop gains
Definition 11: Signal Flow Graph Determinant (or characteristic function); is defined as follows:
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1-(sum of all loop gains) + (sum of all gain-products of 2 non-touching loops) -(sum of all gain-products of 3 non-touching loops) +…
i
iiiP
R
CT
The general formula for any signal flow graph is
= evaluated with all loops touching Pi eliminated.
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Example:
032
1
PP
GP
,1,1,011
kjP
GHP
jk
There is only one forward path; hence
There is only one (feedback) loop. Hence
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GHP 11 11
1011
GH
GP
R
CT
1
11
then
and
final
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Example:
The signal flow graph of the resistance network,
determine the voltage gain
There is one forward path 1
3
v
vT
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Hence the forward path gain is
21
431 RR
RRP
There are three feedback loops:
Hence the loop gains are
,1
311 R
RP ,
2
321 R
RP
2
431 R
RP
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12P
21
433111 RR
RRPP
12312111 )(1 PPPP
21
43
2
4
2
3
1
31RR
RR
R
R
R
R
R
R
21
4332413121
RR
RRRRRRRRRR
There are no three loops that do not touch. Therefore
There are two non-touching loops, loops one and three. Hence
Gain-Product of the only two non-touching
loops =
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Since all loops touch the forward path, Finally, 11
4332413121
4311
1
3
RRRRRRRRRR
RRP
v
vT
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Problems
1.A control system has the block diagram of Fig1.
(a)Find the system transfer function c/r
(b) Redraw the block diagram to show the control force u as the output and find the transfer function u/r
(c) Redraw the diagram with the actuating signal εas the output and find the transfer function ε/r.
ccG pG
H
-
+r
Fig 1
u
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