Synergetic approach in attitude control of very flexible satellites by means of thrusters and PZT devices

The interaction between angular motion and flexible vibrations can heavily affect the stability of the spacecraft. Many control strategies have been developed for solving this issue. Some of them are focused on the attitude dynamics while a different approach consists in facing the problem from the structural point of view, trying to actively damp the vibrations induced by the attitude control, using smart material (like piezoelectric) devices. In this research, an approach unifying the two aspects is proposed. The satellite is modeled as a flexible multibody system, in which the two sets of actuators (attitude and structural devices) are commanded by means of a common sliding mode control algorithm. In such a way, the two systems are not considered as competitors (trying each one to cancel the disturbing effects caused by the other one), but they are cooperating for the common goal of acquiring a desired attitude in a given time without residual oscillations. This synergetic approach is first developed in a high-fidelity numerical environment, then it is tested by means of a free-floating platform equipped with flexible appendages, designed and built as a multilayer composite material with a net of embedded PZT patches (sensors and actuators). The overall navigation and control loop is based on the information coming from the Inertial Measurement Unit and from the PZT sensors, which are filtered and sent to the Synergetic controller, with the goal of reaching a desired attitude. The output of the Synergetic controller consists in both the thrusters firing sequence and the PZT actuators voltage difference required to reach the goal. The experimental results are compared with the ones obtained by more classic approaches (attitude and structural control computed independently), commenting both advantages and drawbacks of the different approaches.