Closed Loop Performance and Limitations of the LISA Pathfinder Drag-Free Control System

LISA Pathfinder is a technology satellite project that is currently in full implementation phase, with a launch date scheduled for 2010. One of the key technologies to be tested is the drag-free control system. It consists of two cubic test masses with measurement and actuation provisions along all degrees of freedom. In total, 15 degrees of freedom must be controlled such that the differential free-fall requirement between the two test masses is met. In this paper the control design approach is revisited and the control requirements breakdown is discussed, based on the differential measurement equation. Then emphasis is put on the expected control performance of the drag-free control system with comprehensive numerical results. Performance limitations are addressed together with its driving parameters, and further possibilities for performance improvements are discussed, including test mass DC alignment aspects.