الفهرس 
Introduction and historical reviewOnly 14 pages are availabe for public view
 |  | from | 188 |  |  |
| | | from | 188 | | |
Abstract
Considerable attention has been focused on the robust control of uncertain dynamical nonlinear systems that are subject to external disturbances and parameter variations. Adaptive sliding mode control, based on the theory of variable structure systems, has attracted a lot of research on control systems for the last two decades and widely recognized as a potential approach to this problem. In sliding mode control, the control action forces the system trajectories to intersect a manifold of the state space, which is called the sliding surface, designated by the designers. The system trajectories are then constrained to the sliding surface for all subsequent time via the use of high speed switching controls. The salient advantage of variable structure control with sliding mode is robustness against structured and unstructured uncertainty changes in system parameters or disturbances. In path tracking systems, however, the system invariance properties are observed only during the sliding phase. In the reaching phase, tracking may be hindered by disturbances or parameter variations. The straightforward way to reduce tracking error and reaching time is to increase the control discontinuity gain. Nevertheless, this may cause severe chattering which is undesirable in some dynamic systems. Chattering associated with sliding mode control is, however, the main drawback because it can excite undesirable high frequency dynamics. Several methods to reduce chattering have been reported. One approach uses a boundary layer around the switching. Another method replaces a maxmin type control by a unit vector function with a small positive constant. Continuous sliding mode control can exponentially drive the system state to the chattering free sliding mode but tends to produce conservative designs. However, these approaches provide no guarantee for the exact convergence to the sliding mode and there is a tradeoff between chattering and robustness. Reducing chattering can be achieved without sacrificing robust performance by combining the attractive features of sliding mode control and fuzzy control. Fuzzy logic has been proven to be a powerful tool for ill defined or parametervariant control systems. A combination of fuzzy logic tuning schemes and sliding mode control is described for accelerating the reaching phase and reducing the influence of unmodelled uncertainties, thus improving system robustness. A fuzzy logic controller may, however, suffer from a heavy computational burden when implemented. Fuzzy tuning of the control action or the sliding surface parameters with explicit expressions for tuning can avoid this problem. By encapsulating heuristic engineering rules a fuzzy logic controller can cope well with severe uncertainties.