# LMIs in Control/pages/Schur Stabilization

LMI for Schur Stabilization

Similar to the stability of continuous-time systems, one can analyze the stability of discrete-time systems. A discrete-time system is said to be stable if all roots of its characteristic equation lie in the open unit disk. This provides a condition for the stability of discrete-time linear systems and a linear time-invariant system with this property is called a Schur stable system.

## The System

We consider the following system:

{\begin{aligned}x(k+1)=Ax(k)+Bu(k)\end{aligned}}

where the matrices $A\in \mathbb {R} ^{n\times n}$ , $B\in \mathbb {R} ^{n\times r}$ , $x\in \mathbb {R} ^{n}$ , and $u\in \mathbb {R} ^{r}$  are the state matrix, input matrix, state vector, and the input vector, respectively.

Moreover, $k$  represents time in the discrete-time system and $k+1$  is the next time step.

The state feedback control law is defined as follows:

{\begin{aligned}u(k)=Kx(k)\end{aligned}}

where $K\in \mathbb {R} ^{n\times r}$  is the controller gain. Thus, the closed-loop system is given by:

{\begin{aligned}x(k+1)=(A+BK)x(k)\end{aligned}}

## The Data

The matrices $A$  and $B$  are given.

We define the scalar as $\gamma$  with the range of $0<\gamma \leq 1$ .

## The Optimization Problem

The optimization problem is to find a matrix {\begin{aligned}K\in \mathbb {R} ^{r\times n}\end{aligned}}  such that:

{\begin{aligned}||A+BK||_{2}<\gamma \end{aligned}}

According to the definition of the spectral norms of matrices, this condition becomes equivalent to:

{\begin{aligned}(A+BK)^{T}(A+BK)<\gamma ^{2}I\end{aligned}}

Using the Lemma 1.2 in LMI in Control Systems Analysis, Design and Applications (page 14), the aforementioned inequality can be converted into:

{\begin{aligned}{\begin{bmatrix}-\gamma I&(A+BK)\\(A+BK)^{T}&-\gamma I\end{bmatrix}}<0\\\end{aligned}}

## The LMI: LMI for Schur stabilization

The LMI for Schur stabilization can be written as minimization of the scalar, $\gamma$ , in the following constraints:

{\begin{aligned}&{\text{min}}\quad \gamma \\&{\text{s.t.}}\quad {\begin{bmatrix}-\gamma I&(A+BK)\\(A+BK)^{T}&-\gamma I\end{bmatrix}}<0\\\end{aligned}}

## Conclusion:

After solving the LMI problem, we obtain the controller gain $K$  and the minimized parameter $\gamma$ . This problem is a special case of Intensive Disk Region Design (page 230 in ). This problem may not have a solution even when the system is stabilizable. In other words, once there exists a solution, the solution is robust in the sense that when there are parameter perturbations, the closed-loop system's eigenvalues are not easy to go outside of a circle region within the unit circle .

## Implementation

A link to Matlab codes for this problem in the Github repository: