
Institute for Systems Theory and Automatic Control
Prof. Dr.-Ing. F. Allgwer
Robust Control
http://www.ist.uni-stuttgart.de/education/courses/robust/
Uni Stuttgart
02.07.2009
jj, ssc
H
∞
Control in Matlab
1 Introduction
Typically the concept of H
∞
controller design is fairly easy to grasp. However, as
controller synthesis is done numerically, a major problem for people new to the subject
is how to write the Matlab code. I w ill here try to give a short overview of some useful
Matlab functions. Hopefu lly this will help you when trying to design your first H
∞
-
controller.
There are many H
∞
related functions available in Matlab and its toolboxes. The im-
portant toolboxes are, in addition to the Contr ol System Toolbox, the mu-Analysis and
Synthesis Toolbox (mu-tools), the Robust Control Toolbox (RCT) and the LMI Control
Toolbox. LMI and mu-tools are both included in RCT v.3.0.1 which comes with Matlab
7, in earlier versions they are separate.
I have also prepared an m-file where I have tried to use as many of the fun ctions discussed
here as possible. The m-file is included in the appendix and can also be downloaded
from the r ob ust control webpage.
A mixed S/KS synthesis problem will be used to illustrate the use of a handful of useful
functions. Let’s take a look at the this problem first.
2 Shaping closed loop transfer functions
The mixed S/KS problem can be illustrated with the block diagram shown in Figure 1.
The closed loop transfer fun ction T = F
l
(P, K) from w to z can be found by visual
inspection as
z
1
z
2
=
W
S
S
W
KS
KS
r. (1)
The generalized plant P (s) (see Figure 2) is
z
1
z
2
e
=
W
S
−W
S
G
0 W
KS
I −G
r
u
. (2)
If we have the state space realizations
G
s
=
A B
C D
, W
S
s
=
A
s
B
s
C
s
D
s
, W
KS
s
=
A
ks
B
ks
C
ks
D
ks
,
it can be s hown that a possible state space r ealization for P (s) is given by
P
s
=
A
s
0 −B
s
C
B
s
−B
s
D
0 A
ks
0 0 B
ks
0 0 A 0 B
C
s
0 −D
s
C D
s
−D
s
D
0 C
ks
0
0 D
ks
0 0 −C
I −D
. (3)
1