1. Desired Bode plot shape
High low freq gain for steady state tracking
Low high freq gain for noise attenuation
Sufficient PM near wgc for stability
Want high gain
Use PI or lag control
wgc w
High freq
0dB
Low freq
Mid freq
Use low pass filters
Use lead or PD control
Want low gain w
Want sufficient
Phase margin
-90
-180
PM+Mp=100

2. Bode-based Control design
C(s)
Gp(s)
Use series controller
Specs: => desired Bode shape of C(s)Gp(s)
Start with Bode plot of Gp(s)
Use poles/zero of C(s) to change Bode shape

3. Overall Loop shaping strategy
Determine mid freq requirements
Speed/bandwidth  wgc
Overshoot/resonance  PMd
Use PD or lead to achieve wgc
Use overall gain K to enforce wgc
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

4. 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 wgcd to be the freq at which PM is sufficient, and KP = 1/|G(jwgcd)|
For fixing speed: from td, tr, tp, or ts requirement, find out wn, let wgcd = (0.65~0.8)*wn and KP = 1/|G(jwgcd)|

5. Lead Design From specs => PMd and wgcd From plant, draw Bode plot
Find PMhave = angle(G(jwgcd))
DPM = PMd - PMhave + a few degrees
Choose a=plead/zlead so that fmax = DPM and it happens at wgcd

6. Alternative use of lead
Use Lead and Proportional together
To fix overshoot and ess
No speed requirement
Select K so that KG(s) meet ess req.
Find wgc and PM from KG(s), find PMd from specs
Let phi_max = PMd – PM +5~7, find alpha
Place phi_max a little higher than wgc

7. Lag and lead-lag Design Steps
From plant, draw Bode plot
From specs => PMd and wgcd
If there is speed or BW req,  wgcd,
In this case, if PM not enough, design PD or lead
Otherwise, choose wgcd to have PM>PMd
Find K to enforce wgcd:
Find Kp,v,a-have with K and C above
Find Kp,v,a-des from ess specs
Let zlag/plag = Kp,v,a-des/Kp,v,a-have
Let zlag= wgcd/10~20, depending on PM room
Compute plag

8. PD control design

9. PD control design Variation
Restricted to using KP = 1
Meet Mp requirement
Find wgc and PM
Find PMd
Let f = PMd – PM + (a few degrees)
Compute TD = tan(f)/wgcd
KP = 1; KD=KPTD

10. Basic PI Design Steps From plant, draw Bode plot
From specs => PMd and wgcd
If there is speed or BW req,  wgcd,
In this case, if PM not enough, design PD or lead
Otherwise, choose wgcd to have PM>PMd
Find K to enforce wgcd:
Let KP = K
And KI = Kwgcd/10~20, depending on extra PM room to spare
Need to increase type to make a nonzero ess to be zero.
But no requirement on ess after type increase.

11. PI Design with ess specs
From plant, draw Bode plot
From specs => Kv,a-des, PMd and wgcd
For required ess, Kv,a-des =1/ess
With C(s)=1/s, compute Kv,a-have
If there is speed or BW req,  wgcd,
In this case, if PM not enough, design PD or lead
Otherwise, choose wgcd to have PM>PMd
Find K to enforce wgcd:
Let KP = K, KIdes= Kv,a-des/Kv,a-have
If KIdes <= Kwgcd/5~20, done, let KI = KIdes
Else, increase wgcd and go back to previous step
Need to increase type by 1 to make a nonzero ess to be zero,
and after type increase, there is further requirement on ess.

12. 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 wgc and PM
Find PMd
Let f = PMd – PM + (a few degrees)
Compute TD = tan(f)/wgcd
KP = KI*TD

13. 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

14. Two d.o.f. Control design C(s) Gp(s) C1(s) Gp(s) C2(s)+ r + e
C1(s)
Gp(s) y + _ _
C2(s)
C1(s) = C(s) – C2(s)

15. 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

16. 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

17. 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

18. y _
C2(s)
C2 = -2s
Implement as: y _ -2

19. 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

20. s=tf('s');
Gp=10/s/(s+1);
figure; margin(Gp); grid;
ts = 1; %specification
t=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 target
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;
Gwgc = evalfr(Gp, j*wgcd);
PM=angle(Gwgc)/pi* ;

21. PMd = 70 - Mp + 10; %add 10 deg because we need PI later
DPM = PMd - PM; %phase margin deficiency
z_PD = wgcd /tan(DPM*pi/180); %PD control
K = 1/abs(evalfr((s+z_PD)*Gp,j*wgcd));
C=K*(s+z_PD);
figure; margin(C*Gp); grid; %Bode with PD
figure; 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 PD
figure; margin(C*Gp); grid;

22. Plant Bode plot

23. Original step response

24. Bode plot after PD

25. Step response with initial PD

26. Step response after PD
Ts is two large, need to increase wgc by 1.5X

28. wgc=0.7*wn*1.6;
z_PI = wgc/20;

29. Wgc increased from 4.47 to 7.47
After PD

30. Ts is now about right
After PD

31. After PID

32. After PID
Ts is OK
Mp is OK

33. PD
+0.1s y PI _ _
-0.1