h_inf - H-infinity (central) controller
h_inf computes H-infinity optimal controller for the continuous-time plant P.
The partition of P into four sub-plants is given through the 2-vector r which is the size of the 22 part of P.
P is given in state-space e.g. P=syslin('c',A,B,C,D) with A,B,C,D = constant matrices or P=syslin('c',H) with H a transfer matrix.
returns ro in [romin,romax] and the central controller Sk in the same representation as P.
(All calculations are made in state-space, i.e conversion to state-space is done by the function, if necessary).
Invoked with three LHS parameters,
returns ro and the Parameterization of all stabilizing controllers:
a stabilizing controller K is obtained by K=lft(Sk,r,PHI) where PHI is a linear system with dimensions r' and satisfy:
H_norm(PHI) < gamma. rk (=r) is the size of the Sk22 block and ro = 1/gama^2 after nmax iterations.
Algorithm is adapted from Safonov-Limebeer. Note that P is assumed to be a continuous-time plant.