This paper studies the stability of a class of nonlinear fractional-order (FO) systems under input control saturation. Based on the Gronwall-Bellman lemma and the sector-bounded condition, sufficient conditions are provided to stabilize such systems by means of a state-feedback controller. The performance of the proposed controller is tested with 2 FO chaotic systems, namely, the FO brushless direct-current motor and the Chen systems. The results illustrate the good performance of the new controller under saturation effects.
KEYWORDSchaotic system, fractional-order system, Gronwall-Bellman lemma, input saturation, sectorbounded condition
INTRODUCTIONFractional-order (FO) systems have been studied in the past 2 decades for describing long memory and hereditary attributes of complex phenomena in different fields, such as energy fuels, 1 imaging science, 2 biomedicine 3 and accident analysis. 4 Therefore, the stability analysis of FO systems attracted considerable attention 5 and several results were reported in the literature addressing this topic. [6][7][8][9][10][11] Due to the simplicity in its design, most of the systems to be controlled are modeled in the perspective of linear dynamics. However, many physical phenomena involve nonlinear relationships between the variables, and there is a model mismatch between the real and the nominal models. Consequently, the stability problem of FO nonlinear dynamics represents an interesting and challenging topic. 12 In general, 2 techniques are addressed in the literature to study the stability of FO systems. The first is the FO extension of Lyapunov direct technique. [13][14][15][16][17][18] However, the use of this approach is often difficult because deriving Lyapunov functions is more complex for the FO case than for integer-order models. The second technique is based on solving or estimating the solution of models without considering control input saturation. [19][20][21] These methods allow some conclusions about stability, asymptotic stability, and Mittag-Leffler stability of FO systems but have the limitations referred previously.Both actuator and control input saturations are important issues in real-world control systems because actuators cannot provide unlimited efforts. Controllers that ignore the actuator saturation may cause undesirable responses and even closed-loop instability. 22 Therefore, it is important to compensate or at least to smoothen the saturation effects and to consider these options in the control design of FO systems. The stability analysis of FO systems under saturation has already received some attention. 14,15,[23][24][25] In related works, 14,15,23,24 simple models involving merely linear dynamics were used to study the stability of such systems, whereas the domain of attraction of the resultant closed-loop system subject to saturation was estimated by different techniques. In the work of Shahri et al, 25 a robust FO dynamic output-feedback sliding-mode controller was designed for a class of uncertain nonlinear FO systems under forced satu...