Control Design of the Test Mass Release Mode for the LISA Pathfinder Mission

The LISA Technology Package (LTP) onboard the LISA Pathfinder mission features two high precision Gravitational Reference Sensors (GRS). Each GRS encompasses a free floating cubic Test Mass (TM) surrounded by a capacitive housing. The electrodes, hosted by the housing, form a set of variable capacitors with the TM and are used both for TM sensing and suspension. Moreover, each GRS contains a Caging Mechanism which sustains the TM during launch. Before the science phase of the mission begins, each TM is set free and the control system enters the TM Release Mode. This paper presents the design of the TM Release Mode: it covers equipment modelling, actuation strategy selection, disturbances estimation, controller law definition, performance results, and stability and robustness analysis.