desired bode plot shape
DESCRIPTION
Desired Bode plot shape. High low freq gain for steady state tracking Low high freq gain for noise attenuation Sufficient PM near w gc for stability. Want high gain. Use PI or lag control. w gc. High freq Noise immu. w. Mid freq Speed, BW. 0dB. Low freq ess , type. - PowerPoint PPT PresentationTRANSCRIPT
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Desired Bode plot shape
0
-90
-180
0dB
gc
High low freq gain for steady state trackingLow high freq gain for noise attenuationSufficient PM near gc for stability
Low freqess, type
High freqNoise immu
Want high gain
Want low gain
Mid freqSpeed, BW
Want sufficientPhase margin
Use low pass filters
Use PI or lag control
Use lead or PD control
PM+Mp=70
Mr, Mp
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Overall Loop shaping strategy• Determine mid freq requirements
– Speed/bandwidth gc
– Overshoot/resonance PMd
• Use PD or lead to achieve PMd@ gc
• Use overall gain K to enforce gc
• PI or lag to improve steady state tracking– Use PI if type increase neede – Use lag if ess needs to be reduced
• Use low pass filter to reduce high freq gain
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Proportional controller design• Obtain open loop Bode plot
• Convert design specs into Bode plot req.
• Select KP based on requirements:
– For improving ess: KP = Kp,v,a,des / Kp,v,a,act
– For fixing Mp: select gcd to be the freq at which PM is sufficient, and KP = 1/|G(jgcd)|
– For fixing speed: from td, tr, tp, or ts requirement, find out n, let gcd = (0.65~0.8)*n and KP = 1/|G(jgcd)|
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gcd
gcd
gcd
gcd
gcd
gcd
From specs, find and
( )
a few degrees
tan( ) /
1/ (1 ) ( )
; ( )
( ) ( ) ( ) / 1 ( ) ( )
Perform c.l. step response, tune C
d
D
P D s j
D D P P D
cl
PM
PM angle G j
PM PMd PM
T PM
K T s G s
K T K C s K K s
G s C s G s C s G s
(s) as needed
PD control design
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PD control design Variation
• Restricted to using KP = 1
• Meet Mp requirement
• Find gc and PM
• Find PMd
• Let = PMd – PM + (a few degrees)
• Compute TD = tan()/wgcd
• KP = 1; KD=KPTD
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Lead Design • From specs => PMd and gcd
• From plant, draw Bode plot
• Find PMhave = 180 + angle(G(jgcd)
• PM = PMd - PMhave + a few degrees
• Choose =plead/zlead so that max =PM and it happens at gcd
1
gcdgcdgcd
gcdgcd
max
max
)/()()(
*,/
sin1
sin1
leadlead
leadlead
pjjGzjK
pz
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Alternative use of lead1.Select K so that KG(s) meet ess req.
2.Find wgc and PM, also find PMd
3.Determine phi_max, and alpha
4.Place phi_max a little higher than wgc
maxgc gc
max
1 sin/ , *
1 sin
( )
lead lead
lead lead lead
lead lead lead
z p
p s z s zC s K K
z s p s p
0.25 0.75gc gc/ ,Or: *lead leadz p
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Lag and lead-lag Design Steps• From plant, draw Bode plot
• From specs => PMd and gcd
– If there is speed or BW req, gcd, • In this case, if PM not enough, design PD or lead
– Otherwise, choose gcd to have PM>PMd
• Find K to enforce gcd:
• Find Kp,v,a-have with K and C above
• Find Kp,v,a-des from ess specs
• zlag/plag = Kp,v,a-des/Kp,v,a-have
• Let zlag= gcd/5~20, depending on PM room
• Compute plag
lead
lead
s zC
s p
1C
1
gcd )(
jCGK
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Basic PI Design Steps• From plant, draw Bode plot
• From specs => PMd and gcd
– If there is speed or BW req, gcd, • In this case, if PM not enough, design PD or lead
– Otherwise, choose gcd to have PM>PMd
• Find K to enforce gcd:
• Let KP = K
• And KI = Kgcd/5~20, depending on extra PM room to spare
1
gcd( )K CG j
Need to increase type to make a nonzero ess to be zero. But no requirement on ess after type increase.
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PI Design with ess specs• From plant, draw Bode plot
• From specs => Kv,a-des, PMd and gcd
– For required ess, Kv,a-des =1/ess
– With C(s)=1/s, compute Kv,a-have
– If there is speed or BW req, gcd, • In this case, if PM not enough, design PD or lead
– Otherwise, choose gcd to have PM>PMd
• Find K to enforce gcd:
• Let KP = K, KIdes= Kv,a-des/Kv,a-have
– If KIdes <= Kgcd/5~20, done, let KI = KIdes
– Else, increase gcd and go back to previous step
1
gcd )(
jCGK
Need to increase type by 1 to make a nonzero ess to be zero, and after type increase, there is further requirement on ess.
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PI Design with PD Design Steps• From required ess, Kv,a-des =1/ess
• With C(s)=1/s, compute Kv,a-have
• Let KI = Kv,a-des/Kv,a-have
• Multiply G(s) by KI/s
• Do a PD design for KIG(s)/s, with DC gain=1:– Find gc and PM
– Find PMd
– Let = PMd – PM + (a few degrees)
– Compute TD = tan()/wgcd
• KP = KI*TD
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Alternative PI Design Steps• For required ess, Kv,a-des =1/ess
• With C(s)=1/s, compute Kv,a-have
• Let KI = Kv,a-des/Kv,a-have
• Rewrite char eq: (KP + KI/s)G(s) + 1=0• KP*n/d + KI*n/d/s +1 = 0• KP *n*s + KI*n+d*s =0, KP*n*s/(KI*n+d*s) + 1 =0• So do a KP design for n*s/(KI*n+d*s), with KI above
– Draw Bode plot for n*s/(KI*n+d*s)– Select max PM frequency– Compute KP to make that frequency wgc
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C(s) Gp(s)
Two d.o.f. Control design
C1(s) Gp(s)
C2(s)
C1(s) = C(s) – C2(s)
rd
e y++
+
_ _
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• Loop transfer function is still C(s)Gp(s)
• TF from d to y: Gp(s)/(1+C(s)Gp(s))
• TF from r to y: C1(s)Gp(s)/(1+C1(s)Gp(s) +C2(s)Gp(s))=C1(s)Gp(s)/(1+C(s)Gp(s))
– Design C(s) to achieve desired loop shape– Split C(s) into C1 and C2– Dual loop implementation to individually
control disturbance TF and reference TF
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Dual loop control to increase type• Recall: type with respect to r = #integrators
in TF from e to where r enters– This = #integrator in C1 + #integrator in inner loop– Therefore, choose C2 so that #integrators in inner
loop is more than #integrators in Gp(s)
• Overall loop TF not affected• Input/output TF Poles not affected• Input/output TF Zeros affected• Steady state tracking improved
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• Inner loop TF
• Not easy to see in this general form
• But simply pick C2 to cancel one or two lowest order terms in Gp
• See example
2 2
( ) ( )
1 ( ) ( ) ( ) ( ) ( )p p
p p p
G s n s
C s G s d s C s n s
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C2(s)
y
_
1
( 2)s s
-2
y
_
1
2s1
s
C2 = -2s
1
s
2
1
s
22 2( ) ( ) ( ) ( 2) ( )
set
p pd s C s n s s s C s s
Implement as:
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Example• Design problem from text book
• But our solution is much more meaningful and much easier– Plant TF Gp(s) = 10/s(s+1)– Specs:
• Mp <19%, but >2%• Ts <1 sec• Ess to step, ramp, acc all = 0
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s=tf('s');Gp=10/s/(s+1);figure; margin(Gp); grid;ts = 1; %specificationt=linspace(0,3*ts,301);Gcl=Gp/(1+Gp); figure; step(Gcl,t); grid;%specification for Mp is 2% to 19%%can use the mid point as initial targetMp=10;zeta=0.6; %for Mp=10%sigma=4/ts; %no tolerance band is given, so use 2%wn=sigma/zeta;wgcd=0.7*wn;Gwgc = evalfr(Gp, j*wgcd);PM=angle(Gwgc)/pi*180 + 180;
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PMd = 70 - Mp + 10; %add 10 deg because we need PI laterDPM = PMd - PM; %phase margin deficiencyz_PD = wgcd /tan(DPM*pi/180); %PD controlK = 1/abs(evalfr((s+z_PD)*Gp,j*wgcd));C=K*(s+z_PD);figure; margin(C*Gp); grid; %Bode with PDfigure; step(C*Gp/(1+C*Gp),t); grid;%now add PI, place zero of PI at wgcd/10%Mp target of 10% together with 10 deg extra PMd%can tolerate a little more phase delay from PI.z_PI = wgcd/10;C=C*(s+z_PI)/s; %multiply PI and PDfigure; margin(C*Gp); grid;figure; step(C*Gp/(1+C*Gp),t); grid;
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-100
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0
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Mag
nit
ud
e (d
B)
10-2
10-1
100
101
102
-180
-135
-90
Ph
ase
(deg
)Bode Diagram
Gm = Inf dB (at Inf rad/sec) , Pm = 18 deg (at 3.08 rad/sec)
Frequency (rad/sec)
Plant Bode plot
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1
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1.6
1.8Step Response
Time (sec)
Am
plit
ud
e
Original step response
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-40
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0
20
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80M
agn
itu
de
(dB
)
10-2
10-1
100
101
102
-120
-110
-100
-90
Ph
ase
(deg
)Bode Diagram
Gm = Inf , Pm = 70 deg (at 4.67 rad/sec)
Frequency (rad/sec)
Bode plot after PD
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1
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1.4Step Response
Time (sec)
Am
plit
ud
e
Step response with initial PD
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1
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1.4Step Response
Time (sec)
Am
plit
ud
e
Step response after PD
Ts is two large, need to increase wgc by 1.5X
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-50
0
50
100M
agn
itu
de
(dB
)
10-2
10-1
100
101
102
-180
-135
-90
Ph
ase
(deg
)Bode Diagram
Gm = -Inf dB (at 0 rad/sec) , Pm = 64.4 deg (at 4.69 rad/sec)
Frequency (rad/sec)
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wgc=0.7*wn*1.6;
z_PI = wgc/20;
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-40
-20
0
20
40
60
80M
agn
itu
de
(dB
)
10-2
10-1
100
101
102
-130
-120
-110
-100
-90
Ph
ase
(deg
)Bode Diagram
Gm = Inf , Pm = 70 deg (at 7.47 rad/sec)
Frequency (rad/sec)
Wgc increased from 4.47 to 7.47
After PD
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0 0.5 1 1.5 2 2.5 30
0.2
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1
1.2
1.4Step Response
Time (sec)
Am
plit
ud
e
Ts is now about right
After PD
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-50
0
50
100M
agn
itu
de
(dB
)
10-2
10-1
100
101
102
-180
-135
-90
Ph
ase
(deg
)
Bode DiagramGm = -Inf dB (at 0 rad/sec) , Pm = 67.2 deg (at 7.47 rad/sec)
Frequency (rad/sec)
After PID
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0 0.5 1 1.5 2 2.5 30
0.2
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1
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1.4Step Response
Time (sec)
Am
plit
ud
e
After PID
Ts is OKMp is OK
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-0.1
y
_
10
1s1
s_PI
PD+0.1s
C2(s) = -0.1s
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-0.1
y
_
10
1s1
s_
PI*PD+0.1s
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System: untitled1Time (seconds): 0.4Amplitude: 1.22
Step Response
Time (seconds)
Am
plit
ud
e
The 2 DOF implementation caused 5% additional overshoot.
Increase PMd by 7 to target 15% Mp
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… …Mp=10; zeta=0.6; %for Mp=10%sigma=4/ts; %no tolerance band is given, so use 2%wn=sigma/zeta;wgcd=0.7*wn*1.6;Gwgc = evalfr(Gp, j*wgcd);PM=angle(Gwgc)/pi*180 + 180;PMd = 70 - Mp + 17; %add 10 deg because we need PI laterDPM = PMd - PM; %phase margin deficiencyz_PD = wgcd /tan(DPM*pi/180); %PD controlK = 1/abs(evalfr((s+z_PD)*Gp,j*wgcd));C=K*(s+z_PD);… ….z_PI = wgcd/20;C=C*(s+z_PI)/s; %multiply PI and PDfigure; margin(C*Gp); grid;figure; step(C*Gp/(1+C*Gp),t); grid;figure; step((C+0.1*s)*Gp/(1+C*Gp),t); grid;
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0 0.5 1 1.5 2 2.5 30
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System: untitled1Time (seconds): 0.43Amplitude: 1.18
Step Response
Time (seconds)
Am
plit
ud
e