# LMIs in Control/pages/DARE

LMIs in Control/pages/DARE

## The System

Consider a Discrete-Time LTI system

{\displaystyle {\begin{aligned}x_{k+1}=A_{d}x_{k}+B_{d}u_{k}\end{aligned}}}
{\displaystyle {\begin{aligned}y_{k}=C_{d}x_{k}\end{aligned}}}

Consider Ad {\displaystyle {\begin{aligned}\mathbb {R} \end{aligned}}}nxn ; Bd {\displaystyle {\begin{aligned}\mathbb {R} \end{aligned}}}nxm

## The Data

The Matrices Ad , Bd , Cd , Q, R are given

Q and R should necessarily be Hermitian Matrices.

A square matrix is Hermitian if it is equal to its complex conjugate transpose.

## The Optimization Problem

Our aim is to find

P - Unique solution to the discrete-time algebraic Riccati equation, returned as a matrix.

K - State-feedback gain, returned as a matrix.

The algorithm used to evaluate the State-feedback gain is given by

{\displaystyle {\begin{aligned}K=(R+B_{d}^{T}PB_{d})^{-1}B_{d}^{T}PA_{d}\end{aligned}}}

L - Closed-loop eigenvalues, returned as a matrix.

## The LMI: Discrete-Time Algebraic Riccati Inequality (DARE)

An algebraic Riccati equation is a type of nonlinear equation that arises in the context of infinite-horizon optimal control problems in continuous time or discrete time

The Discrete-Time Algebraic Riccati Inequality is given by

{\displaystyle {\begin{aligned}A_{d}^{T}PA_{d}-A_{d}^{T}PB_{d}(R+B_{d}^{T}PB_{d})^{-1}B_{d}^{T}PA_{d}+Q-P\geq 0\end{aligned}}}

P , Q {\displaystyle {\begin{aligned}\mathbb {S} \end{aligned}}}n and R {\displaystyle {\begin{aligned}\mathbb {S} \end{aligned}}}m where P > 0, Q ≥ 0, R > 0

P is the unknown n by n symmetric matrix and A, B, Q, R are known real coefficient matrices.

The above equation can be rewritten using the Schur Complement Lemma as :

${\displaystyle {\begin{bmatrix}A_{d}^{T}PA_{d}-P+Q&A_{d}^{T}PB_{d}\\B_{d}^{T}PA_{d}&R+B_{d}^{T}PB_{d}\end{bmatrix}}}${\displaystyle {\begin{aligned}\geq 0\end{aligned}}}

## Conclusion:

Algebraic Riccati Inequalities play a key role in LQR/LQG control, H2- and H∞ control and Kalman filtering. We try to find the unique stabilizing solution, if it exists. A solution is stabilizing, if controller of the system makes the closed loop system stable.

Equivalently, this Discrete-Time algebraic Riccati Inequality is satisfied under the following necessary and sufficient condition:

${\displaystyle {\begin{bmatrix}Q&0&A_{d}^{T}P&P\\0&R&B_{d}^{T}P&0\\PA_{d}&PB_{d}&P&0\\P&0&0&P\end{bmatrix}}}${\displaystyle {\begin{aligned}\geq 0\end{aligned}}}

## Implementation

( X in the output corresponds to P in the LMI )

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

## Related LMIs

LMI for Continuous-Time Algebraic Riccati Inequality

LMI for Schur Stabilization