# LMIs in Control/pages/Discrete Time Modified Minimum Gain Lemma

**The Concept**[edit | edit source]

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:

Solving for *Y*(*s*) in terms of *R*(*s*) gives

The expression 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 , i.e., it has a large norm with each value of *s*, and if , 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 ouput closely tracks the reference input.

**The System**[edit | edit source]

Consider a discrete-time LTI system, , with minimal state-space relization **, where and .
**

**The Data**[edit | edit source]

The matrices and

**LMI : Discrete-Time Modified Minimum Gain Lemma**[edit | edit source]

The system has minimium gain γ under any of the following equivalent sufficient conditions.

- 1. There exists and γ where such that

. The term in Discrete Time Minimum Gain Lemma makes the matrix inequality more negative definite. Therefore,

- 2. There exists and where such that

: Applying the Schur complement lemma to the γ^{2} term in equation 1 gives equation 2.

**Conclusion:**[edit | edit source]

If there exist a positive definite for the system , then the minimum gain of the system γ can be obtaied from above defined LMIs.

**Implementation**[edit | edit source]

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

**Related LMIs**[edit | edit source]

KYP Lemma

State Space Stability

KYP Lemma without Feedthrough

Discrete Time Minimum Gain Lemma

**References**[edit | edit source]

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