This paper considers the design of reduced-order state observers for fractional-order time-delay systems with Lipschitz nonlinearities and unknown inputs. By using the Razumikhin stability theorem and a recent result on the Caputo fractional derivative of a quadratic function, a sufficient condition for the asymptotic stability of the observer error dynamic system is presented. The stability condition is obtained in terms of linear matrix inequalities, which can be effectively solved by using existing convex algorithms. Numerical examples and simulation results are given to illustrate the effectiveness of the proposed design approach. KEYWORDS fractional-order systems, Lipschitz nonlinearities, reduced-order observers, time-delay systems
INTRODUCTIONDuring the past decades, fractional-order systems have attracted considerable attention. Various results on fractional-order systems have been reported. 1-6 It was found that a variety of physical and biological systems can be well characterized by fractional-order differential equations, such as the fractional-order Schrodinger equation in quantum mechanics and fractional-order oscillator in damping vibration. 7-12 Fractional calculus offers more advantages than integer-order calculus. For example, there are many dynamical systems described by high integer-order systems that can be expressed more compactly into fractional operators. 13 This fact can be useful not only in the field of control systems but also in areas involving digital signal processing. Moreover, fractional-order systems have been found by directly generalizing integer-order derivatives into the corresponding fractional-order ones, in which way chaotic dynamics has been explored, such as fractional-order Chen's system and fractional-order cellular neural networks. 14 Furthermore, fractional-order controllers have so far been implemented to enhance the robustness and the performance of the closed-loop control systems.As it is well known, in many practical applications, the states of the considered systems are not easily obtained due to technical or economic reasons. In this case, the estimation of actual states and output feedback control law are very necessary. Therefore, the problem of designing state observers for dynamical systems with or without time delays has attracted considerable attention in the literature. [15][16][17][18][19][20][21][22][23][24][25] Observers theory for systems with integer order has been extensively investigated (see, for example, other works 17,18,19,25,26 ). For fractional-order systems, some interesting results on the observer design for fractional-order systems were also reported in other works. 16,20,21,23,24 For instance, a nonfragile fractional-order nonlinear observer design for a class of fractional-order nonlinear systems was derived in the work of Boroujeni and Momeni 16 ; reduced-order fractional descriptor observers for fractional-order descriptor continuous-time linear systems were reported in the work of Kaczorek 20 ; reduced-order perfect nonlinear observe...