Upload
arturo-langridge
View
239
Download
2
Embed Size (px)
Citation preview
1
سیستمهای کنترل خطی
1389پاییز
بسم ا... الرحمن الرحيم
دکتر حسين بلندي- دکتر سید مجید اسما عیل زاده
2
Recap.
• Steady State Error• Stability• Routh Horowitz Method
3
Root Locus
The root locus is a plot of the roots of the characteristic equation of the closed-loop system as a function of the gain of the open-loop transfer function.
This graphical approach yields a clear indication of the effect of gain adjustment.
INTRODUCTION:
4
• Performance (stability and response) of a closed-loop system (feedback system) depends on its pole locations.
• In feedback control system design, it is usually necessary to adjust one or more parameters to achieve desired pole locations for the closed-loop system.
• Root locus is a plot of closed-loop system pole locations when the gain K is varied.
• A graphical technique exists for plotting the root locus which does not involve calculating the roots of a closed-loop polynomial.
• Root locus is a powerful graphical tool for analysis and design of feedback control systems.
Root Locus
5
i.e; A specific technique which shows how changes in one of a system’s parameter
(usually the controller gain, K)
will modify the location of the closed-loop poles
in the s-domain.
6
Graphical Evaluation )Hostetter):
A rational function
1 2
1 2
( ) m
n
s z s z s zF s K
s p s p s p
When evaluated at a specific value of the variable , is:0s s
0 1 0 2 00
0 1 0 2 0
( ) m
n
s z s z s zF s K
s p s p s p
7
On a pole-zero plot, suppose a directed line segment is drawn from the position of a pole, say to the value at which the function is to be evaluated.
0s1p
The segment has length and makes the angle With the real axis.
0 1s p
As shown below, at a root s1
10 ps
8
Thus:
0 1 0 2
0 1 0 2
0 1 0 2
0
0 1 0 2
...( )
...
j s z j s z
j s p j s p
s z e s z eF s K
s p e s p e
00
0
( )
product of the length of the directed
line segments from the zero to sF s K
product of the length of the directed
line segments from the poles to s
00
sum of the angles of the directed line segments from zeros to s( )
sum of the pole angles 180 if k is negativeF s
9
Consider the system:
Calculation of Magnitude and Angles (Ogata)
1
1 2 3 4
2 3
( ) ( )
where and are complex-conjugate poles.
s zG s H s K
s p s p s p s p
p p
1 1 2 3 4
1
1 2 3 4
( ) ( )
( ) ( )
G s H s
KBG s H s
A A A A
10
Definition
01 sHsKG
• The closed-loop poles of the negative feedback control:
are the roots of the characteristic equation:
01 sHsKG
The root locus is the locus of the closed-loop poles when a specific parameter (usually gain, K)
is varied from 0 to infinity.
11
Root Locus Method Foundations
• The values of s in the s-plane that make the loop gain KG(s)H(s) equal to -1 are the closed-loop poles
(i.e. )
• KG(s)H(s) = -1 can be split into two equations by equating the magnitudes and angles of both sides of the equation.
101 sHsKGsHsKG
12
Root Locus for Feedback Systems
magnitude conditions angle conditions
13
Angle and Magnitude Conditions
Independent of K
,,,l 210
,,,l 210
12180
1
12180
1
1
0
lsHsG
KsHsG
lsHsKG
sHsKG
sHsKG
o
14
APPLICATION OF THE MAGNITUDE AND ANGLE CONDITIONS:
Once the open-loop transfer function G(s)H(s) has been determined and put into the proper form, the poles and zeros of this function are plotted in the S plane.
As an example, consider:
with the damping ratio < 1, the complex-conjugate poles are:
15
The plot of the poles and zeros is shown in bellow:
16
17
magnitude conditions:
angle conditions:
All angles are considered positive, measured in the counter clock wise sense.
18
PLOTTING ROOTS OF A CHARACTERISTIC EQUATION:
Example1 : position-control system
19
Example1 (Continue) : position-control system
20
Example1 (Continue) : position-control system
21
Example1 (Continue) : position-control system
22
Example1 (Continue) : position-control system
These curves are defined as the root-locus plot of
23
Example1 (Continue) : position-control system
Corresponding to the selected roots there is a required value of K that can be determined from the plot . When the roots have been selected, the time response can be obtained.
The value of K is normally considered to be positive. However, it is possible for K to be negative.
For an increase in the gain K of the system, analysis of the root locus reveals the following characteristics:
24
Example1 (Continue) : position-control system
25
simple second-order system:
A zero is added to the simple second-order system:
QUALITATIVE ANALYSIS OF THE ROOT LOCUS:
Compare
26
If a pole, instead of a zero, is added to the simple second-order system,the resulting transfer function is:
root locus of the closed-loop control system
27
root-locus configurations for negative feedback control systems
?
28
Example2:
Determine the locus of all the closed-loop poles of C(s)/R(s), for
29
Root Locus
Real Axis
Imag
inar
y A
xis
-6 -5 -4 -3 -2 -1 0 1-10
-8
-6
-4
-2
0
2
4
6
8
10
System: sysGain: 0.35Pole: -1.51 + 1.72e-008iDamping: 1Overshoot (%): 0Frequency (rad/sec): 1.51
System: sysGain: 0Pole: -5Damping: 1Overshoot (%): 0Frequency (rad/sec): 5
System: sysGain: InfPole: -4Damping: 1Overshoot (%): 0Frequency (rad/sec): 4
System: sysGain: 44Pole: -1.93 + 6.42iDamping: 0.288Overshoot (%): 38.8Frequency (rad/sec): 6.71
>> sys = zpk( -4, [-1 -2 -5],1) Zero/pole/gain: (s+4)----------------------(s+1) (s+2) (s+5)
>> rlocus(sys)
Rlocus in MATLAB:
30
GEOMETRICAL PROPERTIES (CONSTRUCTION RULES):
To facilitate the application of the root-locus method, the following rules are established for K > 0.
These rules are based upon the interpretation of the angle condition and an analysis of the characteristic equation.
These rules can be extended for the case where K < 0.
These rules provide checkpoints to ensure that the solution is correct.
which provides a qualitative idea of achievable closed-loop system performance.
31
Rule 1: Number of Branches of the Locus
32
33
• RL starts from n poles (k=0)• RL ends at m zeroes (k=infinity)• # of branches= max( n, m)
34
35
36
37
Rule 2: Real-Axis Locus
38
39
• The locus includes all points along the real axis to the left of an odd number of poles plus zeros of GH
40
41
42
44
Rule 3: Asymptotes of Locus as s Approaches Infinity
45
Rule 3(continue):
46
Rule 4: Real-Axis Intercept of the Asymptotes
47
Rule 5: Breakaway Point on the Real Axis
breakaway pointbreakin point
48
The breakaway and break-in points can easily be calculated for an open-loop pole-zero combination for which
the derivatives of W(s)=-K
As an example, if
then
Multiplying the factors together gives
49
by taking the derivative of this function and setting it equal to zero, the points can be determined
or
50
Rule 6: Complex Pole (or Zero): Angle of Departure
51
In a similar manner the approach angle to a complex zero can be determined
52
Rule 7: Imaginary-Axis Crossing Point
53
Rule 7(continue):
54
Learning by doing Example
Sketch the root locus of the following system:
55
#1Assuming n poles and m zeros for G(s)H(s):
• The n branches of the root locus start at the n poles.
• m of these n branches end on the m zeros• The n-m other branches terminate at infinity
along asymptotes.
First step: Draw the n poles and m zeros of G(s)H(s) using x and o respectively
56
Draw the n poles and m zeros of G(s)H(s) using x and o respectively.
• 3 poles: p1 = 0; p2 = -1; p3 = -2
• No zeros
21
1
ssssHsG
57
Draw the n poles and m zeros of G(s)H(s) using x and o respectively.
• 3 poles: p1 = 0; p2 = -1; p3 = -2
• No zeros
21
1
ssssHsG
58
#2
• The loci on the real axis are to the left of an ODD number of REAL poles and REAL zeros of G(s)H(s)
Second step: Determine the loci on the real axis. Choose a arbitrary test point. If the TOTAL number of both real poles and zeros is to the RIGHT of this point is ODD, then this point is on the root locus
59
Determine the loci on the real axis:
• Choose a arbitrary test point.
• If the TOTAL number of both real poles and zeros is to the RIGHT of this point is ODD, then this point is on the root locus
60
Determine the loci on the real axis:
• Choose a arbitrary test point.
• If the TOTAL number of both real poles and zeros is to the RIGHT of this point is ODD, then this point is on the root locus
61
#3Assuming n poles and m zeros for G(s)H(s):
• The root loci for very large values of s must be asymptotic to straight lines originate on the real axis at point:
radiating out from this point at angles:
Third step: Determine the n - m asymptotes of the root loci. Locate s = α on the real axis. Compute and draw angles. Draw the asymptotes using dash lines.
mn
lo
l
12180
mn
zps m
in
i
62
Determine the n - m asymptotes:• Locate s = α on the real axis:
• Compute and draw angles:
• Draw the asymptotes using dash lines.
13
210
03321
ppp
s
mn
ll
12180
0
0
1
00
0
18003
112180
6003
102180
,,,l 210
63
Determine the n - m asymptotes:• Locate s = α on the real axis:
• Compute and draw angles:
• Draw the asymptotes using dash lines.
13
210
03321
ppp
s
mn
ll
12180
0
0
1
00
0
18003
112180
6003
102180
,,,l 210
64
Breakpoint Definition
• The breakpoints are the points in the s-domain where multiples roots of the characteristic equation of the feedback control occur.
• These points correspond to intersection points on the root locus.
65
#4Given the characteristic equation is KG(s)H(s) = -1
• The breakpoints are the closed-loop poles that satisfy:
Fourth step: Find the breakpoints. Express K such as:
Set dK/ds = 0 and solve for the poles.
0ds
dK
.sHsG
K1
66
Find the breakpoints.
• Express K such as:
• Set dK/ds = 0 and solve for the poles.
4226057741
0263
21
2
.s,.s
ss
sssK
sss)s(H)s(G
K
23
211
23
67
Find the breakpoints.
• Express K such as:
• Set dK/ds = 0 and solve for the poles.
4226057741
0263
21
2
.s,.s
ss
sssK
sss)s(H)s(G
K
23
211
23
68
#5Assuming n poles and m zeros for G(s)H(s):
• The n branches of the root locus start at the n poles.
• m of these n branches end on the m zeros• The n-m other branches terminate at infinity
along asymptotes.
Last step: Draw the n-m branches that terminate at infinity along asymptotes
69
Applying Last Step
Draw the n-m branches that terminate at infinity along asymptotes
70
Points on both root locus & imaginary axis?
• Points on imaginary axis satisfy:
• Points on root locus satisfy:
• Substitute s=jω into the characteristic equation and solve for ω.
js jω?
- jω
01 sHsKG
20 or
71
EXAMPLE : ROOT LOCUS
72
EXAMPLE (continue): ROOT LOCUS
73
EXAMPLE (continue): ROOT LOCUS
74
EXAMPLE (continue): ROOT LOCUS
75
EXAMPLE (continue): ROOT LOCUS
76
EXAMPLE (continue): ROOT LOCUS
77
EXAMPLE (continue): ROOT LOCUS
78
EXAMPLE (continue): ROOT LOCUS
79
Example of an application of the root-locus method
The systematic application of the for the construction of a root locus is shown in the following non-trivial example for the open-loop transfer function :
40122
1)()(
20
ssss
sksHsG
The degree of the numerator polynomial is m=1. This means that the transfer function has one zero (s=-1). The degree of the denominator polynomial is n=4 and we have the four poles (s=0, s=-2 s=-6+2j, s=-6-2j). First the poles (x) and the zeros (o) of the open loop are drawn on the S plane as
80
40122
1)()(
20
ssss
sksHsG
Values of are in red 0k
81
We have (n-m=3) branches that go to infinity and the asymptotes of these three branches are lines which intercept the real axis according to rule. the crossing is at :
33.4
3
16620
a
the slopes of the asymptotes are:
3,,3
3
12180
pk
The asymptotes are shown in Figure as blue lines:
82
83
Using Rule 2 it can be checked which points on the real axis are points on the root locus. The points -1<s<0 and s<-2 belong to the root locus, because to the right of them the number of poles and zeros is odd.
According to rule 4 breakaway and break-in points can only occur pairwise on the real axis to the left of -2.
68.31
Bs
47.51
Bs
866.076.04,3
jsB
The real roots and are the
positions of the breakaway and the break-in point
68.31
Bs 47.51
Bs
0ds
dK
84
The angle of departure of the root locus from the complex pole at can be determined from
jsp 263
8.661808.2463 p
)12(1802.1586.1614.153903 kp
85
40122
1)()(
20
ssss
sksHsG
0k
Breakaway Point Example
Consider the following loop transfer function.
Real axis loci exist for thefull negative axis.
Asymptotes: angles
jw
sXX
–4X
–2
–2j
2j
+1
60°
asymptotes
23)()(
ss
ksHsG
5,,3
3
12ppp
pk
5p
23
0)033(
a
Breakaway Point Example
Determine the breakaway points from
then
jw
sXX
–4X
–2
–2j
2j
+1
3,1
0)3)(1(342
--=
=++=++
s
ssss
0)96(
)9123(
96)3(
223
2
232
=++
++-=
úû
ùêë
é
++=ú
û
ùêë
é
+
sss
ssK
sss
K
ds
d
ss
K
ds
d
Root Locus Plot Example
Loop Transfer function:
Roots:s = 0, s = –4, s = –2 4j
Real axis segments:between 0 and –4 .
Asymptotes:
angles = 2
4)0224( -=----=as
47
,4
5,
43,
404)12( =
-+k ppppp
asymptotes
jw
sX
–4X
–2
–2j
2j
+1
45°
4j
–4jX
X
2044)()(
2
ssss
ksHsG
Root Locus Plot Example
Breakaway points:
Three points that breakaway at 90° .
solving,
jw
sX
–4X
–2
–2j
2j
+1
45°
4j
–4jX
X
2
0
80368
8072244
80368
2234
23
234
ssss
sssk
ssss
k
ds
d
jsb 45.22,2
Root Locus Plot Example
The imaginary axis crossings:
Characteristic equation
Routh table
s4 1 36 K
s3 8 80 0
s2 26 K 0
s 80-8K/26 0 0
s0 K 0 0
Condition for critical stability80-8K/26 > 0 or K<260
The auxiliary equation26 s 2 + 260 = 0solving
080368 234 =++++ Kssss
jjs 16.310 ±=±=
The final plot is shown on the right.
What is the value of the gain K corresponding to the breakaway point at
Root Locus Plot Example
sX
–4X
–2
–2j
2j
4j
–4jX
X
3.16j
jw
?45.22 jsb
From the general magnitude condition the gain corresponding to the point s1 on the loci is
For the point s1 = –2 + 2.45j
K = |–2 + 2.45j| ·|–2 + 2.45j + 4| ·|–2 + 2.45j + 2 + 4j| ·|–2 + 2.45j + 2 – 4j| / 1.0
= 3.163 · 3.163 · 6.45 · 1.55 = 100.0
Root Locus Plot ExampleGain Calculations
ÕÕ==
++=m
ii
n
ii zspsK
11
11
Is there a gain corresponding to a damping ratio of 0.707 or more for all system modes?z = 0.707 = cos( q )q = 45°
Root Locus Plot Example
sX
–4X
–2
–2j
2j
4j
–4jX
X
3.16j
jw
45°
Examine the responses for the various gains and relate them to the location of the closed-loop roots.
K = 64, roots are –2, –2, –2±3.46j K = 100, roots are –2±2.45j, –2±2.45j K = 260, roots are ±3.16j, –4±3.16j
sX
–4X
–2
–2j
2j
4j
–4jX
X
3.16j
jw
K=64 K=100
K=260
Root Locus Plot ExampleTime Responses
Root Locus Plot ExampleTime Responses
Time (sec.)
Am
plit
ud
eStep Response, K = 64
0 1 2 3 4 5
0
0.2
0.4
0.6
0.8
1
s = –2, –2
s = –2±3.46j
whole response
Root Locus Plot ExampleTime Responses
Time (sec.)
Am
plit
ud
eStep Response, K = 100
0 1 2 3 4 5
0
0.2
0.4
0.6
0.8
1
whole response
s = –2±2.45j (repeated)
Root Locus Plot ExampleTime Responses
Time (sec.)
Am
plit
ud
eStep Response K = 260
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
whole response
s = ±3.16j
s = –4±3.16j
Root Locus with Two Parameters
The root locus method focuses on the roots of the characteristic equation.
There can be several different loop transfer functions that have the same closed-loop characteristic equation.
To construct the root locus for a characteristic equation which has two parameters, we construct fictitious loop transfer functions and apply the normal methods.
Root Locus with Two Parameters
Consider the following characteristic equation.s3 + s2 + b s + a = 0 .
Write this in the general form of 1 + GH(s) = 0 with b as a multiplying gain.
This will allow the plotting of the root locus with respect to the gain b for any given value of a.
The roots of the characteristic equation of GH´(s) define the starting points for the root loci. Consider the loci of these roots.
01 23=
+++
absss
GH´(s)
Root Locus with Two Parameters The characteristic
equation of GH´(s) is s3 + s2 + a = 0 , which may be written as
Now construct the root locus of GH´´(s) in terms of the gain a .
0)1(
1 2=
++
ss
a
GH´´(s)
jw
sX
–2 –1X X
a1a1a2
a2
a1 = 0.3s = –1.2, 0.1±0.5j
a2 = 1.8s = –1.66, 0.33±1.0j
Root Locus with Two Parameters Now construct the
root locus for
where the open-loop poles correspond to the previous root locus for varying a .
Asymptotes: ± 90°
a = 0.3 sa = (-1.2 +0.1 +0.1)/2
= -0.5 a = 1.8
sa = (-1.66 +0.33 +0.33)/2 = -0.5
23=++ a
bsss
GH´(s)
Root Locus with Two Parameters Imaginary axis
crossings:s3 + s2 + b s + a = 0
s3 1 b 0s2 1 a 0s -b a 0 0s0 a 0 0 a = , b s2 + = 0a
ajs -=
jw
s
–2 –1
a1a1a2
a2
O
103
104
105
106
SUMMARY OF ROOT-LOCUS CONSTRUCTION RULESFOR NEGATIVE FEEDBACK
107