# LMIs in Control/pages/H2SO

LMIs in Control/pages/H2SO We treat the problem of designing a full-order state observer for the system mentioned below. The aim of it is to mitigate the effect of disturbance $w(t)$ to the estimate error is prohibited to a desired level.

## The System

Consider the continuous-time generalized plant P with state-space realization

{\begin{aligned}{\dot {x}}(t)&=Ax(t)+B_{1}u(t)+B_{2}w(t)&x(0)=x_{0}\\y(t)&=C_{1}x(t)+D_{1}u(t)+D_{2}w(t)\\&z(t)=C_{2}x(t)\end{aligned}}

where it is assumed that $(A,C_{2})$  is detectable. An observer of the form

## The Data

• $x$ {\begin{aligned}\mathbb {R} \end{aligned}} n, $y$ {\begin{aligned}\mathbb {R} \end{aligned}} l , $z$ {\begin{aligned}\mathbb {R} \end{aligned}} m are respectively the state vector, the measured

output vector, and the output vector of interests

• $w$ {\begin{aligned}\mathbb {R} \end{aligned}} p and $u$ {\begin{aligned}\mathbb {R} \end{aligned}} r are the disturbance vector and the control vector,

respectively

• A, B1, B2, C1, C2, D1, and D2 are the system coefficient matrices of

appropriate dimensions

## The Optimization Problem

For the system we introduce a full state observer in the following form:
${\dot {\hat {x}}}=(A+LC_{1}){\hat {x}}-Ly+(B_{1}+LD_{1})u$  ${\hat {x}},L$  are the observation vector and the observer gain.
The transfer function for this case is
$G_{zw}(s)=C_{2}(sI-A-LC_{1})^{-1}(B_{2}+LD_{2})$
and thus the problem of $H_{2}$  state observer design is to find L such that
||$G_{zs}(s)||_{2}<\gamma$
The error :$e=x-{\hat {x}}$

## The LMI: LMI for H2 State Observer Design

The H2 state observer problem has a solution if and only if there exists a matrix $W$ , a symmetric matrix $Q$  and a symmetric matrix $X$  such that

${\begin{bmatrix}XA+WC_{1}+(XA+WC_{1})^{T}&XB_{2}+WD_{2}\\(XB_{2}+WD_{2})^{T}&-I\end{bmatrix}}$ {\begin{aligned}<0\end{aligned}}

${\begin{bmatrix}-Q&C_{2}\\C_{2}^{T}&-X\end{bmatrix}}$ {\begin{aligned}<0\end{aligned}}

{\begin{aligned}trace(Q)<\gamma ^{2}\end{aligned}}

and from the solution of the above LMIs we can obtain the observer matrix as

$L=X^{-1}W$

## Conclusion

Thus by formulation, we have converted the problem of H2 state observer design into an LMI feasibility problem by optimizing the above LMIs. In application we are often concerned with the problem of finding the minimal attenuation level $\gamma$

On implementation and optimization of the above LMI using YALMIP and MOSEK (or) SeDuMi we get 3 matrices as output, $X,WandQ$  and also $\rho$  which is used to calculate $\gamma$  which is the H2 norm of the system.

There exists another set of LMIs which holds true for the same optimization problem as above.

{\begin{aligned}A^{T}Y+C_{1}^{T}V^{T}+YA+VC_{1}+C_{2}^{T}C_{2}<0\\\end{aligned}}

${\begin{bmatrix}-Z&YB_{2}+VD_{2}\\(YB_{2}+VD_{2})^{T}&-Y\end{bmatrix}}$ {\begin{aligned}<0\end{aligned}}

{\begin{aligned}trace(Z)<\gamma ^{2}\end{aligned}}

When a minimal $\rho$  is obtained, the minimal attenuation level is $\gamma ={\sqrt {\rho }}$

## Implementation

A link to the Matlab code for a simple implementation of this problem in the Github repository:

## Related LMIs

H$\infty$  State Observer Design
Discrete time H2 State Observer Design