Inria Associate team
2016 – 2019
Overview
Estimation (of the state variables, parameters or inputs) and control in finite time is the principal direction of research of NonA team applying algebraic tools and the homogeneity approach. Sliding mode algorithms are ones of the most popular solutions providing finitetime convergence. The main advantages of sliding mode control and estimation solutions include convergence in finite time, compensation of matched disturbances and robustness with respect to measurement noises. The basic disadvantage is a chattering phenomenon that occurs due to discontinuity of the righthand side in such systems. During the last decades a big effort has been applied to overcome the chattering, and new generations of slidingmode algorithms have been developed avoiding discontinuity as much as possible and having even fixedtime convergence. The last fifth generation of sliding mode was founded just recently and waits for its theoretical development and application for design of control and estimators.
The goal of the project is development of control and estimation algorithms converging in fixed or in finite time by applying the last generation sliding mode techniques and the homogeneity theory.
People involved in the project
NonA team, Inria Lille – Nord Europe:
The NonA team (Inria) is developing an estimation theory, built around differential algebra and operational calculation on the one hand, and high gain algorithms (such as sliding mode, etc.) on the other hand. Both approaches allow derivatives of noisy signals to be estimated in finite or fixed time, which opens a lot of prospects in control and signal processing. It has resulted in relatively simple, rapid algorithms. Unlike traditional methods, the majority of which pertain to asymptotic statistics, the NonA estimators are ”nonasymptotic”. These estimators found their applications in electrical engineering, robotics, aerospace and bioscience.
Denis Efimov (project leader), CR Inria, is a recognised expert in automatic control theory. His main research interests include nonlinear oscillation analysis, observation and control, nonlinear system stability. Andrey Polyakov, CR Inria, is working with different fields of mathematical control theory. Rosane Ushirobira, CR Inria, is working in algebraic methods for signal and parameter estimation, based on differential algebra and Weyl algebra. Wilfrid Perruquetti, Full Professor at École Centrale de Lille specializes in estimation and control theory, nonlinear systems with applications to robotics. Francisco Lopez, PhD student 20152018, financially supported by Inria.
Sliding mode control laboratory:
The Universidad Nacional Autonoma de Mexico (UNAM) team comes from the “Sliding Mode Control” laboratory. A detailed survey on advantages of the variable structure systems can be briefly summarized as follows:
 Insensitivity/uniformity (more than robustness) with respect to socalled matched uncertainties/disturbances acting in the same channels as the control
 Finitetime, exact and uniform convergence.
 The possibility to use the equivalent output injection in order to reveal additional information
UNAM team :
 Leonid Fridman, Full Professor
 Jaime Alberto Moreno Pérez, Full Professor
 Rafael Iriarte, Professor
 Andrea Aparicio, PhD student 2014−2017
Scientific progress
 First year
 Linear system stabilization by nsign feedback. The problem of a linear system stabilization using a control dependent on the signs of the state variables is revisited [4]. It is shown that depending on the properties of the linear part of the system, the stabilization by nsign feedback is possible (the previous results show that for the chain of integrators of order 3 and higher it is impossible). The conditions of stability are expressed in the form of linear matrix inequalities, and the domain of stability is evaluated. Next, the control law is augmented by a linear feedback, then an inputtostate stability property of the closedloop system is established. The problem of finitetime convergence in the obtained system is discussed.
 On converse design of homogeneous Lyapunov functions. It is a wellknown fact that (weighted) homogeneous and asymptotically stable dynamical systems possess homogeneous Lyapunov functions [Zubov (1964); Rosier (1992)]. However, the existing methods [Zubov (1964); Rosier (1992)] do not provide an explicit expression for the design of such Lyapunov functions, since they assume that a nonhomogeneous Lyapunov function is obtained by the converse Lyapunov theorem, and next its nonlinear transformation is performed. In [1], several analytic expressions of homogeneous Lyapunov functions are proposed. For this purpose, modifications of converse Lyapunov approaches are carried out to directly obtain the desired homogeneous Lyapunov functions from converse arguments.
 Oscillatory global output synchronization of nonidentical nonlinear systems. A global output synchronization problem for nonidentical nonlinear systems having relative degree two or higher is studied in [3]. The synchronization is based on a partial projection of the dynamics of individual subsystems into Brockett oscillators. The approach uses a higher order sliding mode observer to estimate the states and perturbations of the synchronized nonlinear systems.
 Second year
 Numeric algorithms for building homogeneous Lyapunov functions. Using previously obtained extensions of the converse Lyapunov function constructions for homogeneous systems, numeric algorithm has been developed in [1], which quantitatively derives a homogeneous Lyapunov function for a system starting from numeric discretization of its solutions initiated on a sphere, and next by a suitable approximation. The proposed algorithm can be used for verification stability of homogeneous systems.
 Homogeneity framework for discretetime systems. Homogeneity demonstrated its utility for continuoustime systems granting the dynamics by a superior rate of convergence or robustness, but for discretetime systems the existing notions were not so useful. In [5] we have proposed a new concept which allows to establish (local) stability and convergence rate from degree for a wide class of discretetime systems.
 Fixedtime converging estimators under delayed measurements. Estimation in linear and nonlinear systems under delayed measurements is an important topic of research nowadays due to omnipresence of lags and samplings in networked systems. Usually, appearance of a delay in measurements degrades performance (rate of convergence, stability or robustness) of an observer, and quality of estimation in such a case can be guaranteed for a known bounded delay only. In our collaborative work [6] an observer is designed, which use a new structure proposed by our colleagues from UNAM, which ensures a uniform convergence for any delay value.
Next year’s work program
New estimation algorithms have been proposed recently in UNAM’s team, which ensure finitetime or fixedtime convergence and based on solution in real time of an adjoint system. These algorithms can be applied for estimation of the state or the parameters, or both. An interesting problem consists in robustness analysis of these algorithms with respect to external disturbances, measurement noises and time delays in measurement channel. This subject has been already partially investigated before (see [4]), but we are planning to continue these investigations in the next year.
Developing the new concept of homogeneity introduced in [3] for discretetime systems, this year we are going to analyse local homogeneous approximation for these class of systems, and also check the robustness abilities for discretetime systems and their global fixedtime convergence. Study of these properties has to be useful in a future for design of estimation and control algorithms in discrete time.
During the previous two years of cooperation, we proposed analytical and numeric tools for construction of homogeneous Lyapunov functions for homogeneous systems using the converse arguments [1]. A restriction we imposed for all these analysis and design was the positiveness of homogeneity degree for a system under consideration (in this case the systems are also locally Lipschitz, which simplifies a lot application of many methods). It is planned to further develop these results to the systems with negative degree (nonLipschitz case), since these dynamics demonstrate finitetime convergence and have a special importance in applications. Slidingmode control and estimation systems are all have negative homogeneity degree.
The Lyapunov function method is one of the main tools for analysis stability of nonlinear systems. Since there is no common routine for such a function construction for an application, there are many methods oriented on special classes of systems or using special forms of Lyapunov functions. One of such an approach is implicit Lyapunov function (ILF) method, which is welldeveloped in NonA team, and it is applicable for the chains of integrators, where all other terms are treated as perturbations. Placing a lot of items in disturbances introduces conservatism in the method and in the obtained estimates on the time of convergence. Another solution is adaptation of ILF approach to linear –parametervarying (LPV) systems, which we are going to try next year.
Record of activities
Visits of Mexican team to France:
First year:

 Leonid Fridman, 10/07/2016 − 22/07/2016, “Stability analysis of a slidingmode control algorithm of second order with time delays”
 Jaime Alberto Moreno Pérez, 27/06/2016 − 08/07/2016, “Recursive design of Lyapunov functions for finitetime stable systems”
 Tonámetl Sánchez Ramírez, 24/10/2016 − 18/11/2016, “Homogeneity for discrete time systems”
 Juan Gustavo Rueda Escobedo, 24/10/2016 − 18/11/2016, “Finitetime and fixed time identification of parameters”
 Second year:

 Jaime Alberto Moreno Pérez, 27/06/2017 − 08/07/2017, “Numeric design of Lyapunov functions for homogeneous systems”
 Leonid Fridman, 10/07/2017 − 22/07/2017, “Stability analysis of a slidingmode control algorithm of high order with time delays”
 Willy Alejandro Apaza Perez, 24/10/2017 − 18/11/2017, “Control and estimation in finite time of mechanical systems”
 Alan Tapia, 24/10/2017 − 18/11/2017, “Development of implicit Lyapunov function method for polytypical systems”
Visits of French team to Mexico (also partially supported by ANR projects):
First year:

 Rosane Ushirobira, 03/12/2016 − 10/12/2016, “Numerical design of homogeneous Lyapunov functions for homogeneous systems with positive degree”
 Denis Efimov, 03/12/2016 − 10/12/2016, “Stability analysis of a slidingmode control algorithm of second order with time delays”
Production
 Efimov D., Ushirobira R., Moreno J.A., Perruquetti W. On converse design of homogeneous Lyapunov functions. Proc. 56th IEEE Conference on Decision and Control (CDC), Melbourne, Australia, 2017.
 Rios H.,Efimov D., Moreno J.A., Perruquetti W., RuedaEscobedo J.G. TimeVarying Parameter Identification Algorithms: Finite and FixedTime Convergence. IEEE TAC, issue 99, 2017.
 Ahmed H., Ushirobira R., Efimov D., Fridman L. Oscillatory global output synchronization of nonidentical nonlinear systems. Proc. IFAC World Congress, Toulouse, 2017.
 Aparicio Martínez A., Efimov D., Fridman L. Stabilization of a triple integrator by a 3 sign feedback. Proc. 55th IEEE Conference on Decision and Control (CDC), Las Vegas, 2016.
 Sanchez Ramirez T., Efimov D., Polyakov A., Moreno J.A., Perruquetti W. A homogeneity property of a class of discretetime systems. Proc. 56th IEEE Conference on Decision and Control (CDC), Melbourne, 2017.
 RuedaEscobedo JG., Ushirobira R., Efimov D., Moreno J., A Gramianbased observer with uniform convergence rate for delayed measurements. Proc. European Control Conference, Limassol, Cyprus, 2018. (submitted)