Optimized Thruster Control Algorithm for Drag-free Spacecraft

Abstract

Several space missions for fundamental physics research require a disturbance-free environment for their demanding science experiments. The concepts of these missions incorporate a drag-free technology for the spacecraft in order to counteract any external forces and torques. With a disturbance force, originated by the solar radiation pressure and other effects in the range of several Micronewton, micro-propulsion systems are necessary for the spacecraft AOCS. Today, electrical and coldgas engines are available or in development to provide a continuous and controlled thrust in the Micronewton range. Though, the different technologies for the thrust production have different advantages and disadvantages. An ideal thruster control algorithm should be able to deal with these different attributes in an optimal manner. The content of this paper is the analysis of the currently used methods to control the spacecraft thrusters of a drag-free mission like MICROSCOPE, LISA Pathfinder or LISA and an introduction of a more advanced method. It shows the basic equations and problems of a Thruster Actuation System (TAS) for a simultaneous control of six degrees of freedom. The focus is on the modification of the standard linear programming algorithm SIMPLEX to meet robustness requirements for the application on a satellite onboard system. This new method takes care of the thrust range limitations (Control Authority) and the dynamics (change of thrust level) of the engines. As a true optimization algorithm, it also provides suitable control strategies for different types of thrusters. In comparison to the standard optimization method, the required memory and computational effort could also be decreased by a special modification, called “Solution Range Placement”. The algorithm that is developed is finally applied in a closed-loop, nonlinear simulation environment for the LISA mission. In this context, the science mode performance is demonstrated in which two proof masses on board of the LISA spacecraft are isolated from external disturbances along a sensitive axis down to a level of 3×10-15 m/s²/sqrt(Hz). At the end of the paper, a small overview is given, how numerical instabilities in the original optimization algorithm can be avoided.