Hardware

In this study, a hardware-in-the-loop implementation of attitude control algorithms based on high-order sliding mode theory is performed and then both compared in terms of performance and functionality. The spacecraft simulator consists of a free-floating platform hinged on a spherical air-bearing support. In order to compensate for the unreliable performance of the rate gyros, a super-twisting sliding mode observer is incorporated to estimate the angular velocities fed back to the controller unit. The proposed scheme makes full use of novel sliding mode methods to alleviate the chattering effects without increasing the control effort; both controllers and observer are tuned to deal with the saturation of reaction wheels with respect to momentum and its rate. The ab initio simulations compared well with the simulator responses, implying that involved instruments including actuators and sensors have been properly emulated. This also proved that the external disturbances were modeled in a reliable manner. The robustness and effectiveness of the proposed scheme have been validated experimentally under extreme circumstances of disturbances and levels of uncertainties