Robust Adaptive Variable Structure Control of a Flexible Spacecraft Containing Input Nonlinearity/Dead-Zone

Fine attitude control and stabilization are key technologies required for modern flexible spacecraft, whose missions, such as stereoscopic mapping, require high pointing accuracy and stabilization. However, in the realistic environment, knowledge about system parameters such as inertia matrix and modal frequencies are usually not known, and various disturbances also occur. Therefore, disturbance rejection control strategies that are also robust to parametric uncertainty are of great interest in spacecraft applications. Variable structure control (VSC) is an efficient control technique that is applicable to a wide class of nonlinear systems subject to modeling uncertainty and external disturbances. In those works, VSC is limited to systems with full-state feedback. In practical application, however, fullstate measurement might be neither possible nor feasible. The problem of designing output feedback controllers via VSC to stabilize multi-variable plants has been investigated by Wang and Yallapragada. In addition, the inputs of a system are usually restricted by its physical structure and energy consumption, which also gives the system inputs such nonlinear characteristics as saturation, dead-zone, etc. The existence of input nonlinearity is a source of degradation or even worse, instability in system performance. Hsu et al. have proposed VSC schemes for uncertain dynamics systems with dead-zone nonlinearity for solving regulation problems. These control schemes can possess insensitivity to matching uncertainties and disturbances. However, information on the upper boundaries of uncertainty is required, which is generally not available or too expensive to assess. Adaptive approaches may offer a simple and effective tool to solve this problem. The contribution of this paper is the design of a nonlinear controller to achieve attitude maneuvering of a three-axis stabilized flexible spacecraft under parametric uncertainty, external disturbances and control input nonlinearity/deadzone. The proposed controller ensures the global reaching condition of the sliding mode of an uncertain system in the presence of input nonlinearity/dead-zone without the limitation of knowing the boundaries of uncertainty and the perturbations in advance. Numerical simulations show that precise attitude control and vibration suppression can be accomplished using the derived controller for both asymptotically and exponentially stable design cases.