Experimental-Analytical Qualification of a Piezoelectric Mechanism for a Critical Space Application

Modern launch-lock and release mechanisms are mechatronics systems that are often responsible for critical tasks in space applications. In the European Space Agency mission LISA-Pathfinder, a piezoelectrical mechanism performs the dynamic release of an object (in very demanding conditions). A 1.96-kg cubic test mass has to be held during launch and then released into free fall with a velocity below 5 μm/s. Two identical and opposed mechanisms, moved by piezostack actuators, are designed for a nominally symmetrical release in order to minimize this residual velocity. However, deviations from this symmetric release can produce a momentum transfer to the mass that can result in an excessive velocity. Two main critical effects are highlighted. First, metallic adhesion at the contact interface between the mechanisms and the test mass has a limited repeatability and behaves differently on the two nominally identical sides, producing a net force on the test mass. Second, a net force also arises if the retractions of the two opposing mechanisms are not symmetrical during the interaction with the test mass. Models of the electromechanical dynamics of the mechanisms and their physical interaction with the test mass have been formulated, validated with experimental data, and used to run a large set of Monte Carlo simulations. The effect of uncertainty in the piezoelectrical, mechanical, and electrical parameters is included, allowing us to obtain a statistical distribution of the release velocity and to predict the in-flight performance of the mechanism. This paper is also intended as a reference for future scientific and commercial drag-free missions.