# Kalman–Yakubovich–Popov lemma

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The Kalman–Yakubovich–Popov lemma is a result in system analysis and control theory which states: Given a number ${\displaystyle \gamma >0}$, two n-vectors B, C and an n x n Hurwitz matrix A, if the pair ${\displaystyle (A,B)}$ is completely controllable, then a symmetric matrix P and a vector Q satisfying

${\displaystyle A^{T}P+PA=-QQ^{T}}$
${\displaystyle PB-C={\sqrt {\gamma }}Q}$

exist if and only if

${\displaystyle \gamma +2Re[C^{T}(j\omega I-A)^{-1}B]\geq 0}$

Moreover, the set ${\displaystyle \{x:x^{T}Px=0\}}$ is the unobservable subspace for the pair ${\displaystyle (C,A)}$.

The lemma can be seen as a generalization of the Lyapunov equation in stability theory. It establishes a relation between a linear matrix inequality involving the state space constructs A, B, C and a condition in the frequency domain.

The Kalman–Popov–Yakubovich lemma which was first formulated and proved in 1962 by Vladimir Andreevich Yakubovich[1] where it was stated that for the strict frequency inequality. The case of nonstrict frequency inequality was published in 1963 by Rudolf E. Kalman[2]. In that paper the relation to solvability of the Lur’e equations was also established. Both papers considered scalar-input systems. The constraint on the control dimensionality was removed in 1964 by Gantmakher and Yakubovich[3] and independently by Vasile Mihai Popov[4]. Extensive review of the topic can be found in [5].

## Multivariable Kalman–Yakubovich–Popov lemma

Given ${\displaystyle A\in \mathbb {R} ^{n\times n},B\in \mathbb {R} ^{n\times m},M=M^{T}\in \mathbb {R} ^{(n+m)\times (n+m)}}$ with ${\displaystyle \det(j\omega I-A)\neq 0}$ for all ${\displaystyle \omega \in \mathbb {R} }$ and ${\displaystyle (A,B)}$ controllable, the following are equivalent:

1. for all ${\displaystyle \omega \in \mathbb {R} \cup \{\infty \}}$
${\displaystyle \left[{\begin{matrix}(j\omega I-A)^{-1}B\\I\end{matrix}}\right]^{*}M\left[{\begin{matrix}(j\omega I-A)^{-1}B\\I\end{matrix}}\right]\leq 0}$
2. there exists a matrix ${\displaystyle P\in \mathbb {R} ^{n\times n}}$ such that ${\displaystyle P=P^{T}}$ and
${\displaystyle M+\left[{\begin{matrix}A^{T}P+PA&PB\\B^{T}P&0\end{matrix}}\right]\leq 0.}$

The corresponding equivalence for strict inequalities holds even if ${\displaystyle (A,B)}$ is not controllable. [6]

## References

1. ^ Yakubovich, Vladimir Andreevich (1962). "The Solution of Certain Matrix Inequalities in Automatic Control Theory". Dokl. Akad. Nauk SSSR. 143 (6): 1304–1307.
2. ^ Kalman, Rudolf E. (1963). "Lyapunov functions for the problem of Lur'e in automatic control" (PDF). Proceedings of the National Academy of Sciences. 49 (2): 201–205. doi:10.1073/pnas.49.2.201. PMC 299777.
3. ^ Gantmakher, F.R. and Yakubovich, V.A., (1964). Absolute Stability of the Nonlinear Controllable Systems, Proc. II All-Union Conf. Theoretical Applied Mechanics. Moscow: Nauka.CS1 maint: Multiple names: authors list (link)
4. ^ Popov, Vasile M. (1964). "Hyperstability and Optimality of Automatic Systems with Several Control Functions". Rev. Roumaine Sci. Tech. 9 (4): 629–890.
5. ^ Gusev S. V. and Likhtarnikov A. L. (2006). "Kalman-Popov-Yakubovich lemma and the S-procedure: A historical essay". Automation and Remote Control. 67 (11): 1768–1810.
6. ^ "Anders Rantzer" (1996). "On the Kalman–Yakubovich–Popov lemma". Systems & Control Letters. 28 (1): 7–10. doi:10.1016/0167-6911(95)00063-1.