Experimental and model-based analysis of the LISA Pathfinder GPRM force sensor interference

The LISA mission, planned for mid-2030, aims at detecting gravitational waves from space by precisely deploying a cubic test mass (TM) into a geodesic trajectory using the Grabbing Positioning and Release Mechanism (GPRM). The GPRM features two end effectors: a plunger for grabbing and centering the TM, and a release tip for releasing the TM into free fall. Once the TM is grabbed and centered by the plunger, it is transferred to the release tip for final deployment via a dedicated handover procedure. Requirements for TM release specify that the residual velocity relative to the spacecraft must be below $15 \mu \mathrm{~m} / \mathrm{s}$, to enable capture using limited electrostatic control forces. Between 2015 and 2017, the LISA Pathfinder mission (LPF) successfully demonstrated core technologies for LISA. However, the TM release performed in LPF revealed unexpected and non-compliant residual velocity components of the TM after release. Subsequent analyses linked these anomalies to unexpected impacts between the GPRM plungers and the TM. On-ground experiments conducted at the University of Trento identified a fictitious force signal generated by the GPRM qualification model (QM) during release tip extension. This signal may interfere with the actual force measurement used in the GPRM control system, disrupting system functionality and leading to an unconstrained TM before release, increasing the risk of impacts with the GPRM plunger. Further testing and FEM analysis indicated that the force interference originates from the deformation of the plunger connected to the forcesensing element. This study presents a multi-body dynamic model that incorporates the origins of force measurement interference. The model is used to simulate the closed-loop system behavior and quantify the effects of plunger deformation, sensor noise, mechanism compliance, highlighting their impact on the TM handover and release process.