The computation of H 2 and H 2,Ω norms for LTI Time-Delay Systems (TDS) are important challenging problems for which several solutions have been provided in the literature. Several of these approaches, however, cannot be applied to systems of large dimension because of the inherent poor scalability of the methods, e.g., LMIs or Lyapunov-based approaches. When it comes to the computation of frequency-limited norms, the problem tends to be even more difficult. In this chapter, a computationally feasible solution using H 2 model reduction for TDS, based on the ideas provided in [3], is proposed. It is notably demonstrates on several examples that the proposed method is suitable for performing both accurate model reduction and norm estimation for large-scale TDS.
IntroductionForewords: Modeling is an essential step to well understand and interact with a physical dynamical phenomena. It, among other, permits to analyze, simulate, optimize, and control dynamical processes. The values of a model lies on its ability to describe the reality as accurately as possible. In general, dynamical models are described by equations and their complexity is somehow linked to its number of equations and variables. Although complex models have a high degree of likeness with reality, in practice, due to numerical limitations, they are problematic to manipulate. Actually, complex models are difficult to analyze and to control, due to limited computational capabilities, storage constraints, and finite machine precision. Therefore, a good model has to reach a trade-off between its accuracy and complexity.In addition to high state-space complexity, we will be interested in how any delay may affect the system. This kind of models fall then in the class of infinitedimensional systems. As a matter of consequence, classical analysis and control methods are not applicable as it. Even if many dedicated approaches have been derived to handle TDS problems (see [18]), most of them are limited to delay systems with low-order state-space vector and associated methods are not scalable when the number of state variables is increasing. Many examples can be found in the context of network systems, where delays appear naturally as the amount of time necessary to transmit some information between different systems (communication lag). In other systems, they are intrinsic part of the natural phenomena as it can be seen in chemical reactions, traffic jam, and heating systems.This context justifies the search of simpler models in order to avoid numerical issues and to apply the classical methods of analysis and control. This is the philosophy of model approximation methods and they will be the main tool of this chapter. These methods permit to obtain a simpler model which well approximates the original one, even of infinite dimension.Contribution: In this chapter, we will work in the framework of linear timeinvariant dynamical systems and the complexity will be associated with the dimension of the state space of the systems. The aim of this chapter is ...