Ground Testing and In-Flight Performance of a Space Mechanism
Creators: name: Zambotti, Andrea
Publisher: name: Universit� degli studi di Trento
Year: 2019

description: LISA Pathfinder is a mission designed for testing the key technologies of the future LISA mission, whose goal is the detection of gravitational waves through the measurement of the relative motion of dedicated proof masses.
In LISA Pathfinder, a critical task is the release of two Test Masses (TMs); each TM has to be injected into free fall by a dedicated Grabbing Positioning and Release Mechanism (GPRM). Despite the symmetrical design of the GPRM, during the release, as an effect of asymmetric impulses exchanged by the TM and the release tips of the GPRM, the TM can acquire a residual momentum. The release is successful if the residual momentum of the TM can be compensated by the force authority of the capacity control, which allows to centre the TM in its housing; as a consequence, a residual momentum of the TM higher than a maximum requirement can be critical for the mission.
In the nominal release configuration, which assumes a monodimensional dynamics of the mechanism along the axis of the release tips, the residual momentum can be produced by the asymmetry of pushing forces (due to relative time delays between the two tips) or by two unbalanced adhesive pulls on the two sides. In particular, the low repeatability of the adhesive pulls suggests their characterization through a dedicated on-ground experimental campaign.
The characterization of the adhesive pulls exchanged by the TM and the GPRM has been the focus of the on-ground experimental campaigns performed by the University of Trento since the early 2000s. The Transferred Momentum Measurement Facility (TMMF) has been developed: a mock-up of the TM release, which allows a high measurability of the adhesive pulls and guarantees the representativeness of the experiment, has been tested in order to estimate the properties of the adhesive force at the contact between the two bodies. The estimated parameters, applied to a model of the in-flight release, allowed to predict that the effect of the asymmetric adhesive pulls applied by the GPRM to the TM should not be critical for the residual momentum.

In this thesis we report the completion of the research on the effect of adhesion in the TM release of LISA Pathfinder, by means of additional on-ground experimental campaigns, and by comparing the predictions with the actual behaviour of the GPRM in the releases performed during the early stages of the LISA Pathfinder mission (2016).
Prior to the launch of the mission, the on-ground TMMF facility has been modified in 2015 in order to host a copy of the GPRM, thus increasing the representativeness of the experiment w.r.t. the nominal release. The on-ground test campaign, consisting in several release tests, allowed to obtain a new (conservative) estimation of the effect of adhesion in the TM release of LISA Pathfinder. 
The estimation of the adhesive effect, which yielded first a conservative prediction, has been then improved by investigating in detail the release dynamics of the TMMF. Thanks to a vibration mode-based model of the TMMF, the effect of the adhesive pull on the measured dynamics has been estimated with its uncertainty, thus yielding a more precise prediction for the in-flight case.
The launch of the LISA Pathfinder mission occurred on December 3, 2015, and prior to the beginning of the scientific operations the two TMs have been injected into free fall. Due to the criticalities observed in the releases, an additional in-flight release test campaign has been planned during the end-of-life activities (June 2017), by alternating several times grab and release of each TM. The in-flight release campaign yielded a statistical distribution of the residual momentum of the TM at the release, which we analysed in detail in order to characterize the actual in-flight GPRM performance. In particular, we focused on the deviation of the predictions (based on the assumption of nominal release) w.r.t. the in-flight observations, by looking for the motivations of the residual momentum measured in the in-flight case.