Compute (skew)$\mu$ lower bounds for mixed perturbations.
This routine computes (skew)$\mu$ lower bounds for an interconnection between a nominal LTI system $M(s)$ and a blockdiagonal operator $\Delta(s)=diag\left(\Delta_1(s),\dots,\Delta_N(s)\right)$, where $\Delta_i(s)=\delta_iI_{n_i}$ ($\delta_i$ real or complex), or $\Delta_i(s)$ is a stable and proper realrational unstructured transfer function. A power algorithm is run at each point of a frequency grid. The considered stability region can be bounded either by the imaginary axis or by a truncated sector.
This routine can also be used to solve a worstcase $\mathcal{H}_\infty$ performance problem, which is nothing but a specific skew$\mu$ problem. A fictitious stable and proper realrational unstructured transfer matrix $\Delta_c(s)$ is first added between the output $y$ and the input $e$, and its size is then minimized under the constraint that there exists a value of $\Delta(s)\in\mathcal{B}_{\bf\Delta}$ such that the interconnection between $M(s)$ and $diag(\Delta(s),\Delta_c(s))$ is unstable. An example is given at the bottom of this page. In this case, the output argument lbnd
is a lower bound on the worstcase $\mathcal{H}_\infty$ norm of the transfer matrix from $e$ to $y$.
[lbnd,wc,pert,tab]=mulb_mixed(sys,blk{,options})
The first two input arguments are mandatory:
sys 
LTI object describing the nominal system $M(s)$. It can also be a 2D or 3D numeric array corresponding to the frequency response of $M(s)$ on a grid. 
blk 
Matrix defining the structure of the blockdiagonal operator $\Delta(s)=diag\left(\Delta_1(s),...,\Delta_N(s)\right)$. Its first 2 columns must be defined as follows for all $i=1,...,N$:
A third column can be used to specify skew uncertainties:
If none, 
The third input argument options
is an optional structured variable with fields:
freq 
Frequency interval $\Omega$ in rad/s on which the lower bound is to be computed. The default value is options.freq=[0 10*max(abs(eig(sys)))] . This parameter is ignored if sys is a numeric array. 
grid 
Frequency grid in rad/s ($1\times m$ array). It can also be a negative integer $m$, in which case a $m$point grid is automatically generated from $\Omega$. The default value is options.grid=50 . This parameter is ignored if sys is a numeric array. 
sector 
Vector $[\alpha\ \xi]$ or $[\alpha\ \xi\ \omega_c]$ characterizing the considered truncated sector (see display_sector for a complete description). The default value is options.sector=[0 0] , which means that the left half plane is considered. This parameter is ignored if sys is a numeric array. 
target 
The algorithm is interrupted if a lower bound is found which is greater than options.target . The default value is options.target=Inf . 
epsilon 
A (skew)$\mu$ lower bound $\tilde{\mu}$ is acceptable if a perturbation $\tilde{\Delta}$ of norm 1/$\tilde{\mu}$ and a frequency $\omega_c$ are found, which satisfy $\mbox{det}(IM(j\omega_c)\tilde{\Delta})<\epsilon$, where $M(j\omega_c)$ is the frequency response of $M(s)$ at $j\omega_c$ (or at the corresponding point on the boundary of the truncated sector defined by options.sector ). The default value is options.epsilon=1e8 . 
trace 
Trace of execution. The default value is options.trace=1 . 
warn 
Warnings display. The default value is options.warn=1 . 
power 
Tuning parameters of the power algorithm (1x4 array). At each frequency:
The default value is 
refine 
If nonzero, the initial frequency grid is refined near peak values of the (skew)$\mu$ lower bound. The default value is options.refine=1 . This parameter is ignored if sys is a numeric array. 
lbnd 
Best (skew)$\mu$ lower bound on the considered frequency interval $\Omega$. 
wc 
Frequency $\omega_c$ in rad/s for which lbnd has been computed. 
pert 
Unless lbnd=0 , every perturbation $\tilde{\Delta}(s)$ whose frequency response $\tilde{\Delta}(\omega_c)$ at $j\omega_c$ (or at the corresponding point on the boundary of the truncated sector defined by options.sector ) is equal to pert satisfies $\mbox{det}(IM(\omega_c)\tilde{\Delta}(\omega_c))\approx 0$, where $M(\omega_c)$ is the frequency response of $M(s)$. 
tab 
Structured variable with fields:

1. Computation of a skew$\mu$ lower bound
System with 10 states and 3 uncertainties. The second one is kept inside the unit ball:
sys=rss(10,5,5)/100;
blk=[2 0 1;1 0 0;2 2 1];
options.sector=[0.1 0];
[lbnd,wc,pert]=mulb_mixed(sys,blk,options);lbnd
display_sector(options.sector,lft(sys,pert));
One of the eigenvalues of the interconnection lies just outside the considered sector.
abs(det(eye(5)calc_freq_resp(sys,wc,options.sector)*pert))
The value of $\mbox{det}(IM(\omega_c)\tilde{\Delta}(\omega_c))$ is almost equal to 0.
2. Computation of a lower bound on the worstcase $\mathcal{H}_\infty$ norm
System with 10 states and 2 uncertainties, which are both kept inside the unit ball. The third block of $\Delta(s)$ corresponds to the performance channel:
blk=[2 0 0;1 0 0;2 2 1];
[lbnd,wc,tab]=mulb_mixed(sys,blk,options);lbnd
mulb
mulb_1real
mulb_nreal
hinflb_real
gen_grid
display_sector
[1]  G. Ferreres and V. Fromion, "Robustness analysis using the $\nu$ tool", in Proceedings of the 35th IEEE Conference on Decision and Control, Kobe, Japan, December 1996, pp. 45664571. 