# LMIs in Control/pages/Discrete Time Minimum Gain Lemma
## The Concept

The output of the system y(t) is fed back through a sensor measurement F to a comparison with the reference value r(t). The controller C then takes the error e (difference) between the reference and the output to change the inputs u to the system under control P. This is shown in the figure. This kind of controller is a closed-loop controller or feedback controller.
This is called a single-input-single-output (SISO) control system; MIMO (i.e., Multi-Input-Multi-Output) systems, with more than one input/output, are common. In such cases variables are represented through vectors instead of simple scalar values. For some distributed parameter systems the vectors may be infinite-dimensional (typically functions).
If we assume the controller C, the plant P, and the sensor F are linear and time-invariant (i.e., elements of their transfer function C(s), P(s), and F(s) do not depend on time), the systems above can be analysed using the Laplace transform on the variables. This gives the following relations:
    $Y(s)=P(s)U(s)$
    $U(s)=C(s)E(s)$
    $E(s)=R(s)-F(s)Y(s).$
Solving for Y(s) in terms of R(s) gives
    $Y(s)=\left({\frac {P(s)C(s)}{1+P(s)C(s)F(s)}}\right)R(s)=H(s)R(s).$
The expression $H(s)={\frac {P(s)C(s)}{1+F(s)P(s)C(s)}}$ is referred to as the closed-loop transfer function of the system. The numerator is the forward (open-loop) gain from r to y, and the denominator is one plus the gain in going around the feedback loop, the so-called loop gain. If $|P(s)C(s)|\gg 1$, i.e., it has a large norm with each value of s, and if $|F(s)|\approx 1$, then Y(s) is approximately equal to R(s) and the output closely tracks the reference input. This page gives an LMI to reduce the gain so that the output closely tracks the reference input.

## The System

Consider a discrete-time LTI system, ${\mathcal {G}}:{\mathcal {l}}_{2e}\rightarrow {\mathcal {l}}_{2e}$, with minimal state-space relization $({\mathcal {A}}_{d},{\mathcal {B}}_{d},{\mathcal {C}}_{d},{\mathcal {D}}_{d})$, where ${\mathcal {A}}_{d}\in {\mathcal {R}}^{n\times n},{\mathcal {B}}_{d}\in {\mathcal {R}}^{n\times m},{\mathcal {C}}_{d}\in {\mathcal {R}}^{p\times n},$ and ${\mathcal {D}}_{d}\in {\mathcal {R}}^{p\times m}$.
    $x(k+1)={\mathcal {A}}_{d}x(k)+{\mathcal {B}}_{d}u(k)$
    $y(k)={\mathcal {C}}_{d}x(k)+{\mathcal {D}}_{d}u(k),k=0,1...$

## The Data

The matrices ${\mathcal {A}}_{d},{\mathcal {B}}_{d},{\mathcal {C}}_{d}$ and ${\mathcal {D}}_{d}$

## LMI : Discrete-Time Minimum Gain Lemma

The system ${\mathcal {G}}$ has minimium gain γ under any of the following equivalent sufficient conditions.
    1. There exists $P\in {\mathcal {S}}^{n},$ and γ $\in {\mathcal {R}}_{\geq 0}$ where $P\geq 0$ such that
    ${\begin{bmatrix}A_{d}^{T}PA_{d}-P-C_{D}^{T}C_{D}&A_{d}^{T}PB_{d}-C_{d}^{T}D_{d}\\(A_{d}^{T}PB_{d}-C_{d}^{T}D_{d})^{T}&B_{d}^{T}PB_{d}+\gamma ^{2}I-(D_{d}^{T}+D_{d})\end{bmatrix}}\leq 0.$
    2. There exists $P\in {\mathcal {S}}^{n},$ and $\gamma \in {\mathcal {R}}_{\geq 0}$ where $P\geq 0$ such that
    ${\begin{bmatrix}A_{d}^{T}PA_{d}-P-C_{D}^{T}C_{D}&A_{d}^{T}PB_{d}-C_{d}^{T}D_{d}&0\\(A_{d}^{T}PB_{d}-C_{d}^{T}D_{d})^{T}&B_{d}^{T}PB_{d}-(D_{d}^{T}+D_{d})&\gamma I\\0&\gamma I&I\end{bmatrix}}\leq 0.$
$proof$ : Applying the Schur complement lemma to the γ$^2$ term in equation 1 gives equation 2.

## Conclusion:

If there exist a positive definite $P$ for the system ${\mathcal {G}}$, then the minimum gain of the system is γ can be obtaied from above defined LMIs.

## Implementation

Code for implementation of this LMI using MATLAB. https://github.com/VJanand25/LMI

## Related LMIs

KYP Lemma
State Space Stability KYP Lemma without Feedthrough

## References

1. J. C. Willems, “Dissipative dynamical systems - part I: General theory,” Archive Rational Mechanics and Analysis, vol. 45, no. 5, pp. 321–351, 1972.
2. D. J. Hill and P. J. Moylan, “The stability of nonlinear dissipative systems,” IEEE Transac- tions on Automatic Control, vol. 21, no. 5, pp. 708–711, 1976.
3. LMI Properties and Applications in Systems, Stability, and Control Theory, by Ryan James Caverly1 and James Richard Forbes2
4. Brogliato B., Maschke B., Lozano R., Egeland O. (2007) Kalman-Yakubovich-Popov Lemma. In: Dissipative Systems Analysis and Control. Communications and Control Engineering. Springer, London