Drag-Free Control Design with Cubic Test Masses

Drag-free control is an important technology required for scientific experiments in space that need free-fall conditions. This chapter describes a control design of a drag-free system that uses test masses with cubic shape (rather than spherical or cylindrical). Three interconnected control problems are considered: drag-free control of the test masses, suspension control of the test masses, and attitude control of the spacecraft. The case of two test masses is treated here rather generally, such that an application to more than two test masses or a reduction to a single test mass is straightforward. Both the derivation of the control structure and the performance optimization procedure of the feedback loops are described. It is shown that the proposed control design yields a very simple architecture of the onboard software for drag-free control and furthermore that it leads to an extreme operational flexibility for the experimentalist with respect to redefinition of control modes and performance optimization, on ground and in-flight.