Nonlinear sliding mode control

Variable structure control (VSC) strategies have been extensively studied and developed over several decades as powerful tools for controlling a broad range of systems, including nonlinear systems. These strategies are primarily aimed at rejecting uncertainties and disturbances while achieving desired control objectives. A key concept within this framework is that of sliding mode. Sliding modes are specific evolutions of the controlled system state that occur within a particular subspace of the state space, known as the sliding manifold. The proposed control laws feature discontinuous control terms and are known as Sliding Mode Control (SMC) strategies. Their main feature is the capability of assigning the desired dynamics to the controlled system in sliding mode, while making it insensitive to some classes of uncertainties. In its original formulation, SMC has however some limitations. Since the 1990s, advancements in SMC theory have addressed many of these limitations, leading to more refined and effective approaches. This chapter focuses on advanced sliding mode control techniques for uncertain nonlinear systems, highlighting recent developments aimed at broadening the application range and improving the robustness of SMC.