PRELIMINARY DYNAMICAL MODEL OF THE LISA/LISA-PATHFINDER RELEASE MECHANISM

LISA Pathfinder (LPF) is a space mission launched in 2015 aimed at testing key technologies for LISA, the first gravitational wave observatory in space. The LPF payload includes two gravitational reference sensors (GRSs), each one hosting a floating test mass (TM). The grabbing, positioning and release mechanism (GPRM) injects the TM into the scientific free-fall state, satisfying strict position and velocity requirements at release. In LPF, the TMs acquired non-compliant velocity at the release, compatible with spurious impacts with the GPRM. The impacts could be caused by oscillations of the mechanism due to the impulsive retraction of a piezo-driven release tip. Ground-based measurements of the GPRM dynamic response show vibrations triggered by the quick tip retraction together with a static deformation at steady state, so far unnoticed. In this work, we build a lumped parameters model, validate it by matching the experimental data, and use it to understand the mechanism dynamics. Moreover, we reshape an existing GPRM finite element model to the analysis of vibrations, in order to provide a physics-based tool to explore different designs and conditions. We present here the preliminary results of these activities.