TESTING THE INJECTION OF THE LISA-PATHFINDER TEST MASS INTO GEODESIC CONDITIONS

摘要

LISA Pathfinder is a technology demonstration missionrnaimed at testing the free-fall purity level of a referencernmass (or test mass, TM). Such an object can be usedrnas a geodesic tracker for the detection of gravitationalrnwaves and is the core of the Gravitational Reference Sensorrn(GRS). The low acceleration noise level pursued forrnthe tracker sets tight design constraints on the GRS. Anyrnnon-gravitational force has to be reduced to the level ofrn10 fN in the measurement bandwidth (1-30 mHz).rnThe TM is firmly constrained by the Caging and VentingrnMechanism (CVM) during the spacecraft launch andrnis then handed over to the Grabbing Positioning and ReleasernMechanism (GPRM). This mechanism is designedrnto handle the TM during in-orbit operations and injectrnit into the geodesic trajectory (or free-fall condition) byrnmeans of a quick retraction of two release tips on two opposedrnsides of the TM. After these operations, the TMrnresidual velocity must be below 5 u0016m/s, otherwise thernGRS capacitive position control is not able to capture andrncentre the TM in its electrode housing. Due to this criticality,rnthe requirement on the maximum release velocityrnis verified on-ground both by means of an experimentalrnapparatus (the Transferred Momentum Measurement Facility,rnTMMF) and by means of simulations. The TMMFrnis specifically designed to assess the contribution of adhesionrnbetween TM and release tips to the total TM momentum.rnOn top of this contribution, any deviation fromrna symmetrical action of the two opposed release mechanismsrndetermines an extra transferred momentum. Greatrncare must be taken to correlate the experimental resultsrnwith the in-flight conditions, where a different actuatorrnis used to perform the TM injection. The extrapolationrnof the experimental results to in-flight conditions and thernMontecarlo simulation of the GPRM-TM combined dynamicsrnshow the presence of a 2.4 margin factor with respectrnto the 5 u0016m/s requirement.rnu0003carlo.zanoni@unitn.itrnThe on-ground measured and simulated performance ofrnthe injection phase, its challenges and critical points arernhere presented and discussed. For the first time, in thisrnpaper, the simulations take advantage of the large amountrnof data available from the testing campaigns.