Advanced Control Systems (ACS) - Dr. Imtiaz Hussain
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Transcript Advanced Control Systems (ACS) - Dr. Imtiaz Hussain
Advanced Control Systems (ACS)
Lecture-8
S-plane Design
Dr. Imtiaz Hussain
Assistant Professor
email: [email protected]
URL :http://imtiazhussainkalwar.weebly.com/
1
Lecture Outline
2
Lead Compensation
β’ Generally Lead compensators are represented by
following transfer function
πΊπ π =
ππ +1
πΎπ πΌ
πΌππ +1
,
(0 < πΌ < 1)
β’ or
πΊπ π = πΎπ
1
π +
π
1
π +
πΌπ
,
(0 < πΌ < 1)
3
πΊπ π =
π +1
3
π +10
(πΌ = 0.1)
,
Pole-Zero Map
Bode Diagram
1
Magnitude (dB)
0
0.5
0
-0.5
-1
-10
-5
-10
-15
-20
60
Phase (deg)
Imaginary Axis
Lead Compensation
-8
-6
-4
Real Axis
-2
0
30
0
-2
10
-1
10
0
10
1
10
2
3
10
10
Frequency (rad/sec)
4
Example-1
β’ Consider the position control system shown in following
figure.
β’ It is desired to design an Electronic lead compensator Gc(s)
so that the dominant closed poles have the damping ratio
0.5 and undamped natural frequency 3 rad/sec.
5
Step-1 (Example-1)
β’ Determine the characteristics of given system using root loci.
C ( s)
10
ο½ 2
R( s) s ο« s ο« 10
β’ The damping ratio of the closed-loop
poles is 0.158.
β’ The undamped natural frequency of
the closed-loop poles is 3.1623
rad/sec.
β’ Because the damping ratio is small,
this system will have a large
overshoot in the step response and
is not desirable.
6
Step-2 (Example-1)
β’ From the performance specifications, determine the
desired location for the dominant closed-loop poles.
β’ Desired performance Specifications are:
ο§ It is desired to have damping ratio 0.5 and undamped natural
frequency 3 rad/sec.
ο·n2
C ( s)
9
ο½ 2
ο½ 2
2
R( s) s ο« 2οΊο·n s ο« ο·n s ο« 3s ο« 9
s ο½ ο1.5 ο± j 2.5981
7
Step-3 (Exampl-1)
β’ To calculate the angle of deficiency apply Angle Condition at desired
closed loop pole.
Desired Closed Loop Pole
s ο½ ο1.5 ο± j 2.5981
ο± d ο½ 180ο° ο 120ο° ο 100.8ο°
-2
ο± d ο½ ο40.89ο°
-1
100.8o
-2
120o
-1
8
Step-5 (Exampl-1)
Solution-1
β’ Solution-1
β If we choose the zero of the
lead compensator at s = -1 so
that it will cancel the plant pole
at s =-1, then the compensator
pole must be located at s =-3.
40.89ο°
9
Step-5 (Example-1)
Solution-1
β’ The pole and zero of compensator are determined as
πΊπ π = πΎπ
1
π
1
π +πΌπ
π +
=
β’ The Value of πΌ
determined as
π +1
πΎπ
π +3
can be
40.89ο°
yields
1
=1
π=1
π
yields
1
=3
πΌ = 0.333
πΌπ
10
Step-6 (Example-1)
Solution-1
β’ The Value of Kc can be
determined using magnitude
condition.
(π + 1) 10
πΎπ
π + 3 π (π + 1)
10
πΎπ
π (π + 3)
=1
π =β1.5+π2.5981
=1
40.89ο°
π =β1.5+π2.5981
π (π + 3)
πΎπ =
10
= 0.9
π =β1.5+π2.5981
π +1
πΊπ π = 0.9
π +3
11
Final Design Check
Root Locus
Solution-1
Root Locus
5
5
0.158 3.16
3
Imaginary Axis
0.5
0
0
0.5
3
0.158 3.16
-5
-4
-3
-2
-1
Real Axis
10
πΊ(π ) =
π (π + 1)
0
1
-5
-4
-3
-2
-1
0
1
Real Axis
9
πΊπ π πΊ(π ) =
π (π + 3)
12
Step-5 (Exampl-1)
Solution-2
β’ Solution-2
-2
40.89ο°
-1
90o
49.2o
-3
-2
-1
13
Solution-2
Step-5 (Exampl-1)
β’ Solution-2
-2
40.89ο°
-1
90o
49.2o
-3
-2
-1
π + 1.5
πΊπ π = 1.03
π + 3.6
14
Exampl-2
β’ Design a lead compensator for following system.
4
s ( s ο« 2)
β’ The damping ratio of closed loop poles is 0.5 and natural
undamped frequency 2 rad/sec. It is desired to modify
the closed loop poles so that natural undamped
frequency becomes 4 rad/sec without changing the
damping ratio.
15
Example-3
β’ Consider the model of space vehicle control system
depicted in following figure.
β’ Design lead compensator such that the damping ratio
and natural undamped frequency of dominant closed
loop poles are 0.5 and 2 rad/sec.
16
Lag Compensation
β’ Lag compensation is used to improve the steady state error
of the system.
β’ Generally Lag compensators are represented by following
transfer function
πΊπ π =
ππ +1
πΎπ π½
π½ππ +1
β’ Or
πΊπ π = πΎπ
1
π
1
π +
π½π
π +
,
,
(Ξ² > 1)
(Ξ² > 1)
β’ Where πΎ πis gain of lag compensator.
17
Lag Compensation
πΊπ π =
π +10
3
π +1
(π½ = 10)
,
Bode Diagram
Pole-Zero Map
30
Magnitude (dB)
1
0.5
20
15
10
5
0
Phase (deg)
0
-0.5
-1
-10
25
-8
-6
-4
Real Axis
-2
0
-30
-60
-2
10
0
10
Frequency (rad/sec)
2
10
18
Lag Compensation
β’ Consider a lag compensator Gc(s), where
1
πΊπ π = πΎπ
π +π
1
π +π½π
,
(Ξ² > 1)
β’ If we place the zero and pole of the lag compensator very close to
each other, then at s=s1 (where s1 is one of the dominant closed
loop poles) then the magnitudes
1
π 1 +
π
and
1
π 1 +
π½π
are almost
equal, or
1
π +
π β
πΎ
πΊπ (π 1 ) = πΎπ
π
1
π +
π½π
19
Lag Compensation
β’ To make the angle contribution of the lag portion of the
compensator small, we require
1
π +
π
β5° < πππππ
1
π +
π½π
< 0°
β’ This implies that if gain πΎπ of the lag compensator is set equal to 1,
the alteration in the transient-response characteristics will be very
small, despite the fact that the overall gain of the open-loop
transfer function is increased by a factor of π½, where π½>1.
20
Lag Compensation
β’ If the open loop transfer function of the uncompensated system is
G(s), then the static velocity error constant Kv of the uncompensated
system is
πΎπ£ = lim π πΊ(π )
π β0
β’ Then for the compensated system with the open-loop transfer
function Gc(s)G(s) the static velocity error constant πΎπ£ becomes
πΎπ£ = lim π πΊπ π πΊ(π ) = πΎπ£ limπΊπ π
π β0
π β0
1
π +
π =πΎ πΎπ½
πΎπ£ = πΎπ£ limπΎπ
π£ π
1
π β0
π +
π½π
21
Design Procedure
β’ The procedure for designing lag compensators by the rootlocus method may be stated as follows.
β’ We will assume that the uncompensated system meets the
transient-response specifications by simple gain adjustment.
β’ If this is not the case then we need to design a lag-lead
compensator which we will discuss in next few classes.
22
Example-4
β’ Consider the system shown in following figure.
β’ The damping ratio of the dominant closed-loop poles is
0.491. The undamped natural frequency of the dominant
closed-loop poles is 0.673 rad/sec. The static velocity error
constant is 0.53 secβ1.
β’ It is desired to increase the static velocity error constant Kv to
about 5 secβ1 without appreciably changing the location of
the dominant closed-loop poles.
23
Example-4 (Step-1)
β’ The dominant closed-loop poles of given system are
s = -0.3307 ± j0.5864
24
Example-4 (Step-3)
β’ The static velocity error constant of the plant (πΎπ£ ) is
1.06
πΎπ£ = limπ πΊ(π ) = lim π
π β0
π β0
π π +1 π +2
= 0.53π β1
β’ The desired static velocity error constant ( πΎπ£ ) of the
compensated system is 5π β1 .
πΎπ£ = lim π πΊπ π πΊ(π ) = πΎπ£ limπΊπ π
π β0
π β0
1
π +
π =πΎ πΎπ½
πΎπ£ = πΎπ£ limπΎπ
π£ π
1
π β0
π +
π½π
25
Example-4 (Step-3)
1
π =πΎ πΎπ½
πΎπ£ = πΎπ£ limπΎπ
π£ π
1
π β0
π +
π½π
π +
πΎπ£ = πΎπ£ πΎπ π½
5 = 0.53π½
π½ = 10
26
Example-4 (Step-4)
β’ Place the pole and zero of the lag compensator
πΊπ π =πΎπ
β’ Since π½ = 10, therefore
πΊπ π =πΎπ
1
π +
π
1
π +
π½π
1
π
0.1
π +
π
π +
27
Example-4 (Step-4)
Solution-1
β’ Place the zero and pole of the lag compensator at s=β0.05
and s=β0.005, respectively.
β’ The transfer function of the lag compensator becomes
πΊπ π
π +0.05
=πΎπ
π +0.005
β’ Open loop transfer function is given as
πΊπ π
π +0.05
1.06
πΊ(π )=πΎπ
π +0.005 π (π +1)(π +2)
πΊπ π
πΎ(π +0.05)
πΊ(π )=
π (π +0.005)(π +1)(π +2)
π€βπππ πΎ = 1.06πΎπ
28
Example-4 (Step-5)
Solution-1
β’ Root locus of uncompensated and compensated systems.
β’ New Closed Loop poles
are
π = β0.31 ± π0.55
29
Example-4 (Step-6)
Solution-1
β’ The open-loop gain K is determined from the magnitude
condition.
πΎ(π + 0.05)
π (π + 0.005)(π + 1)(π + 2)
=1
π =β0.31+π0.55
πΎ = 1.0235
β’ Then the compensator gain πΎπ is determined as
πΎ = 1.06πΎπ
πΎ
πΎπ =
= 0.9656
1.06
30
Example-1 (Step-6)
Solution-1
β’ Then the compensator transfer function is given as
π + 0.05
πΊπ π = 0.9656
π + 0.005
31
Example-4 (Final Design Check)
Solution-1
β’ The compensated system has following open loop transfer
function.
πΊπ π
1.0235(π +0.05)
πΊ(π )=
π (π +0.005)(π +1)(π +2)
β’ Static velocity error constant is calculated as
πΎπ£ = lim π πΊπ π πΊ(π )
π β0
1.0235 π + 0.05
πΎπ£ = lim π [
]
π β0 π π + 0.005 π + 1 π + 2
1.0235 0.05
πΎπ£ =
= 5.12π β1
0.005 1 2
32
Example-4 (Step-4)
Solution-2
β’ Place the zero and pole of the lag compensator at s=β0.01
and s=β0.001, respectively.
β’ The transfer function of the lag compensator becomes
πΊπ π
π +0.01
=πΎπ
π +0.001
β’ Open loop transfer function is given as
πΊπ π
π +0.01
1.06
πΊ(π )=πΎπ
π +0.001 π (π +1)(π +2)
πΊπ π
πΎ(π +0.01)
πΊ(π )=
π (π +0.001)(π +1)(π +2)
π€βπππ πΎ = 1.06πΎπ
33
Example-4 (Step-5)
Solution-2
β’ Root locus of uncompensated and compensated systems.
Root Locus
6
β’ New Closed Loop poles
are
Compensated System
4
Imaginary Axis
π = β0.33 ± π0.55
Actual System
2
0
-2
-4
-6
-8
-6
-4
-2
Real Axis
0
2
4
34
Example-5
β’ Design a lag compensator for following unity
feedback system such that the static velocity error
constant is 50 sec-1 without appreciably changing the
closed loop poles, which are at π = β2 ± π 6.
35
Lag-Lead Compensation
β’ Lag-Lead compensators are represented by following transfer
function
πΊπ π = πΎπ
1
π1
πΎ
π +
π1
π +
1
π2
1
π +
π½π2
π +
, (Ξ³ > 1 πππ Ξ² > 1)
β’ Where Kc belongs to lead portion of the compensator.
Lag-Lead Compensation
πΊπ π =
π +1
πΎπ
π +2
π +0.4
π +0.1
Example-6 (Case-1)
β’ Consider the control system shown in following figure
β’ The damping ratio is 0.125, the undamped natural frequency is 2
rad/sec, and the static velocity error constant is 8 secβ1.
β’ It is desired to make the damping ratio of the dominant closed-loop
poles equal to 0.5 and to increase the undamped natural frequency
to 5 rad/sec and the static velocity error constant to 80 secβ1.
β’ Design an appropriate compensator to meet all the performance
specifications.
Example-6 (Case-1)
β’ From the performance specifications, the dominant closed-loop
poles must be at
π = β2.50 ± π4.33
β’ Since
4
ο
ο½ ο235ο°
s ( s ο« 0.5) s ο½ ο2.50ο« j 4.33
β’ Therefore the phase-lead portion of the lagβlead compensator
must contribute 55° so that the root locus passes through the
desired location of the dominant closed-loop poles.
Example-6 (Case-1)
β’ The phase-lead portion of the lagβlead compensator becomes
πΎπ
1
π +
π1
πΎ
π +
π1
π +0.5
= πΎπ
π +5.02
β’ Thus π1 = 2 and πΎ = 10.04.
β’ Next we determine the value of Kc from the magnitude
condition:
(π + 0.5)
4
πΎπ
π + 5.02 π (π + 0.5)
π (π + 5.02)
πΎπ =
4
=1
π =β2.5+π4.33
= 5.26
π =β2.5+π4.33
Example-6 (Case-1)
β’ The phase-lag portion of the compensator can be designed as
follows.
β’ First the value of π½ is determined to satisfy the
requirement on the static velocity error constant
πΎπ£ = lim π πΊπ π πΊ(π )
π β0
80 = lim π
π β0
1
25.04 π +
π2
π π + 5.02
1
π +
π½π2
80 = 4.988π½
π½ = 16.04
Example-6 (Case-1)
β’ Finally, we choose the value of π2 such that the following
two conditions are satisfied:
Example-6 (Case-1)
β’ Now the transfer function of the designed lagβlead
compensator is given by
πΊπ π = 6.26
π +0.5
π +5.02
π +0.2
π +0.0127
PID
β’ PID Stands for
β P ο Proportional
β I ο Integral
β D ο Derivative
44
Four Modes of Controllers
β’ Each mode of control has specific advantages and
limitations.
β’ On-Off (Bang Bang) Control
β’ Proportional (P)
β’ Proportional plus Integral (PI)
β’ Proportional plus Derivative (PD)
β’ Proportional plus Integral plus Derivative (PID)
45
On-Off Control
β’ This is the simplest form of control.
Set point
Error
Output
46
Proportional Control (P)
β’ In proportional mode, there is a continuous linear relation
between value of the controlled variable and position of the
final control element.
π(π‘)
π(π‘)
π(π‘)
-
ππ(π‘) = πΎπ π(π‘)
πΎπ
πππππ‘
π(π‘)
πππππππ‘πππππ
πΆπππ‘πππ
πΉπππππππ
β’ Output of proportional controller is
ππ(π‘) = πΎπ π(π‘)
β’ The transfer function can be written as
πΆπ(π )
= πΎπ
πΈ(π )
47
Proportional Controllers (P)
β’ As the gain is increased the system responds faster to
changes in set-point but becomes progressively
underdamped and eventually unstable.
48
Proportional Plus Integral Controllers (PI)
β’ Integral control describes a controller in which the output
rate of change is dependent on the magnitude of the
input.
β’ Specifically, a smaller amplitude input causes a slower
rate of change of the output.
49
Proportional Plus Integral Control (PI)
πΎπ β«
π(π‘)
π(π‘)
π(π‘)
πΎπ
πΎπ
π(π‘) ππ‘
πΎπ π(π‘)+
+
πππ π‘
πππππ‘
π(π‘)
πΉπππππππ
πππ π‘ = πΎπ π π‘ + πΎπ
π π‘ ππ‘
50
Proportional Plus Integral Control (PI)
πππ π‘ = πΎπ π π‘ + πΎπ
π π‘ ππ‘
β’ The transfer function can be written as
πΆππ(π )
1
= πΎπ + πΎπ
πΈ(π )
π
51
Proportional Plus derivative Control (PD)
π
πΎπ
ππ‘
π(π‘)
π(π‘)
π(π‘)
πΎπ
ππ(π‘)
πΎπ
ππ‘
πΎπ π(π‘)+
+
πππ π‘
πππππ‘
π(π‘)
πΉπππππππ
πππ
ππ(π‘)
π‘ = πΎπ π π‘ + πΎπ ππ‘
52
Proportional Plus derivative Control (PD)
πππ
ππ(π‘)
π‘ = πΎπ π π‘ + πΎπ ππ‘
β’ The transfer function can be written as
πΆππ(π )
= πΎπ + πΎπ π
πΈ(π )
53
Proportional Plus derivative Control (PD)
β’ The stability and overshoot problems that arise when a
proportional controller is used at high gain can be mitigated by
adding a term proportional to the time-derivative of the error signal.
The value of the damping can be adjusted to achieve a critically
damped response.
54
Proportional Plus derivative Control (PD)
β’ The higher the error signal rate of change, the sooner the final
control element is positioned to the desired value.
β’ The added derivative action reduces initial overshoot of the
measured variable, and therefore aids in stabilizing the process
sooner.
β’ This control mode is called proportional plus derivative (PD) control
because the derivative section responds to the rate of change of the
error signal
55
Proportional Plus Integral Plus Derivative Control (PID)
π
πΎπ
ππ‘
π(π‘)
π(π‘)
π(π‘)
πΎπ
πΎπ
ππ(π‘)
ππ‘
πΎπ π(π‘) +
-
+
ππππ π‘
πππππ‘
π(π‘)
+
πΎπ β«
πΎπ
π(π‘) ππ‘
πΉπππππππ
ππππ π‘ = πΎπ π π‘ + πΎπ
ππ(π‘)
π(π‘) ππ‘ + πΎπ
ππ‘
56
Proportional Plus Integral Plus Derivative Control (PID)
ππππ π‘ = πΎπ π π‘ + πΎπ
ππ(π‘)
π(π‘) ππ‘ + πΎπ
ππ‘
πΆπππ(π )
1
= πΎπ + πΎπ +πΎπ π
πΈ(π )
π
57
Proportional Plus Integral Plus Derivative Control (PID)
β’ Although PD control deals neatly with the overshoot and ringing
problems associated with proportional control it does not cure the
problem with the steady-state error. Fortunately it is possible to
eliminate this while using relatively low gain by adding an integral
term to the control function which becomes
58
The Characteristics of P, I, and D controllers
CL RESPONSE
RISE TIME
OVERSHOOT SETTLING TIME
S-S ERROR
Kp
Decrease
Increase
Small Change
Decrease
Ki
Decrease
Increase
Increase
Eliminate
Kd
Small
Change
Decrease
Decrease
Small
Change
59
Empirical Tuning (Zeigler-Nicholβs First Method)
β’ In the first method, we
obtain
experimentally
the response of the
plant to a unit-step
input.
β’ If the plant involves
neither integrator(s) nor
dominant
complexconjugate poles, then
such
a
unit-step
response curve may look
S-shaped
60
Zeigler-Nicholβs First Method
β’ This method applies if the response to a step input exhibits an
S-shaped curve.
β’ Such step-response curves may be generated experimentally
or from a dynamic simulation of the plant.
Table-1
61
Zeigler-Nicholβs Second Method
β’ In the second method, we first set ππ = β and ππ = 0.
β’ Using the proportional control action only (as shown in
figure), increase Kp from 0 to a critical value Kcr at which
the output first exhibits sustained oscillations.
β’ If the output does not exhibit sustained oscillations for
whatever value Kp may take, then this method does not
apply.
62
Zeigler-Nicholβs Second Method
β’ Thus, the critical gain Kcr
and the corresponding
period Pcr are determined.
Table-2
63
Example-7
C (s)
K ο sL
ο½
e
R ( s ) Ts ο« 1
1
L
t
64
Example-7
Step Response
10
8
Amplitude
C( s )
10 ο 2 s
ο½
e
R( s ) 3 s ο« 1
6
4
2
0
0
5
10
15
Time (sec)
65
Example-8
β’ Consider the control system shown in following figure.
β’ Apply a ZieglerβNichols tuning rule for the determination
of the values of parameters πΎπ , ππ and ππ .
66
Example-8
β’ Transfer function of the plant is
1
πΊ π =
π (π + 1)(π + 5)
β’ Since plant has an integrator therefore Ziegler-Nicholβs
first method is not applicable.
β’ According to second method proportional gain is varied
till sustained oscillations are produced.
β’ That value of Kc is referred as Kcr.
67
Example-8
β’ Here, since the transfer function of the plant is known we can
find πΎππ using
β Root Locus
β Routh-Herwitz Stability Criterion
β’ By setting ππ = β and ππ = 0 closed loop transfer function is
obtained as follows.
πΎπ
πΎπ
πΆ(π )
=
π
(π ) π π + 1 π + 5 + πΎπ
68
Example-8
β’ The value of πΎπ that makes the system marginally unstable so
that sustained oscillation occurs can be obtained as
π 3 + 6π 2 + 5π + πΎπ = 0
β’ The Routh array is obtained as
β’ Examining the coefficients of first
column of the Routh array we find
that sustained oscillations will
occur if πΎπ = 30.
β’ Thus the critical gain πΎππ is
πΎππ = 30
69
Example-8
β’ With gain πΎπ set equal to 30, the characteristic equation
becomes
π 3 + 6π 2 + 5π + 30 = 0
β’ To find the frequency of sustained oscillations, we substitute
π = ππ into the characteristic equation.
(ππ)3 +6(ππ)2 +5ππ + 30 = 0
β’ Further simplification leads to
6(5 β π2 ) + ππ(5 β π2 ) = 0
6(5 β π2 ) = 0
π = 5 πππ/π ππ
70
Example-8
π = 5 πππ/π ππ
β’ Hence the period of sustained oscillations πππ is
2π
πππ =
π
πππ =
2π
5
= 2.8099 π ππ
β’ Referring to Table-2
πΎπ = 0.6πΎππ = 18
ππ = 0.5πππ = 1.405
ππ = 0.125πππ = 0.35124
71
Example-8
πΎπ = 18
ππ = 1.405
ππ = 0.35124
β’ Transfer function of PID controller is thus obtained as
1
πΊπ (π ) = πΎπ (1 +
+ππ π )
ππ π
1
πΊπ (π ) = 18(1 +
+ 0.35124π )
1.405π
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Example-8
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END OF LECTURE-8
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