Sliding Mode Control Using MATLAB

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In the formulation of any control problem, there will typically be discrepancies between the actual plant and the mathematical model developed for controller design. This mismatch may be due to unmodelled dynamics, variation in system parameters or the approximation of complex plant behavior by a straightforward model.

The engineer must ensure that the resulting controller has the ability to produce the required performance levels in practice
despite such plant /model mismatches. This has led to intense interest in the development of robust control methods that seek to solve this problem. One particular approach to robust controller design is the sliding mode control methodology.

One of the most intriguing aspects of sliding mode is the discontinuous nature of the control action, whose primary function of each of the feedback channels is to switch between two distinctively different system structures (or components) such that a new type of system motion, called the sliding mode, exists in a manifold. This peculiar system characteristic is claimed to result in superb system performance, which includes insensitivity to parameter
variations, and complete rejection of disturbances.

Sliding mode control is a particular type of variable structure control. In sliding mode control, the control system is designed to drive and then constrain the system state to lie within a neighborhood of the switching function. There are two main advantages to this approach. Firstly, the dynamic behavior of the system may be tailored by the particular choice of switching function. Secondly, the closed-loop response becomes totally insensitive to a particular class of uncertainty. The latter invariance property clearly makes the methodology an appropriate candidate for robust control. In addition, the ability to specify performance directly makes sliding mode control attractive from a design perspective.

The sliding mode design approach consists of two components. The first involves the design of a switching function so that the sliding motion satisfies design specifications. The second is concerned with the selection of a control law that will make the switching function attractive to the system state. Note that this control law is not necessarily discontinuous.