Editorial

Abstract

This December 2000 issue of the ASME Journal of Dynamic Systems, Measurement, and Control is focused on the topic of Variable Structure Systems (VSS), as described in the Editorial. I would like to thank Professors Eduardo Misawa and Vadium Utkin for organizing this special issue, which I believe will be of high interest to our readership. A second special issue on Identification of Mechanical Systems is already under preparation for publication in December 2001. (See the Call for Papers that appears in this issue.) A. Galip Ulsoy Technical Editor A fundamental issue in feedback control is the robust stability and performance of control systems in the presence of plant uncertainties and disturbances. The majority of current research in control theory addresses different aspects of this problem. The theory of variable structure systems (VSS) has made several important contributions in this area since its early developments pioneered by S. Emelyanov in the 1950s. The interest of the international control community to research in this area has been increased significantly within the last two decades as it follows from intensity of publications and presentations in leading journals and conferences on control. This special issue on VSS intends to provide a collection of several recent important developments in this area.The idea of this special issue was born at one of the VSS workshops, in which the need to have a single point of reference to recent developments was identified. Professor Galip Ulsoy, editor of the Journal of Dynamic Systems, Measurement, and Control (JDSMC), and the associate editors, enthusiastically supported the idea of a special issue. Several excellent investigators around the globe in the area of VSS submitted a large number of papers for the issue and provided excellent technical contributions. The result is this edition. An attempt is made to provide coverage of both recent theories and applications. We hope that the readers enjoy this issue as much as the organizers did.The idea of modifying the controller structure in real time is a natural concept in the implementation of control systems, starting from the conventional relay systems in the 1930s. The use of microprocessors and digital signal processors make this idea even more attractive, since switching control structure means logical decisions and variations in the control algorithm in software. From this point-of-view, most practical control systems have some sort of variable structure.The formal theory of VSS is based on geometrical arguments and on formal stability theories to perform the structure switching. An important class of VSS, which is almost synonymous to VSS is sliding mode control. In this approach, the trajectories in the state-space are constrained to be in a sub-space through the use of a discontinuous control law, resulting in invariance with respect to uncertainties and disturbances. Wide use of power converters as actuators with “on-off” as the only admissible operation mode leads to natural implementation of discontinuous control. Many contributions in this special issue fall into this category.Several articles in this special issue address important theoretical issues in VSS. Topics that are covered include discrete-time systems, infinite dimensional systems, optimization, simulation of system of differential-algebraic equations, estimation, techniques tailored to specific mechanical systems, and alternative forms of VSS and corresponding design techniques. Several of these techniques were successfully applied in practice and the results show their effectiveness.Contributions include not only theory but also applications. Several articles stress applications in areas such as mobile robots and mechanical manipulators, automotive, aircraft, missile, and chemical processes.The variety of both theoretical and application-oriented contributions show that the area of VSS is mature and will continue to grow. A temptation at this point