Flatness, backstepping and sliding mode controllers for nonlinear systems

Sliding mode control (SMC) is a powerful and robust control method. SMC methods have been widely studied in the last three decades from theoretical concepts to industrial applications [1]-[3]. Higher-order sliding mode controllers have recently been addressed to improve the system responses [1]. However, when designing a control for a plant it is sometimes more beneficial to use combined techniques, using SMC in conjunction with other methods such as backstepping, passivity, flatness and even other traditional control design methods including H  ∞ , proportional-integral-derivative (PID) and self-tuning. Note that PID control design techniques may also be used for designing the sliding surface. A drawback of the SMC methods may be unwanted chattering resulting from discontinuous control. There are many methods which can be employed to reduce chattering, for example, using a continuous approximation of the discontinuous control, and a combination of continuous and discontinuous sliding mode controllers. Chattering may also be reduced using the higher-order SMC [4] and dynamic sliding mode control [4, 5].