DYNAMIC ADHESION MEASUREMENT FOR THE VERIFICATION OF THE
GRABBING POSITIONING AND RELEASE MECHANISM FOR THE LISA
PATHFINDER TEST MASS RELEASE
M. Benedetti(1), D. Bortoluzzi(2), M. De Cecco(2), L. Baglivo(3), F. Tondini(2), M. Lapolla(4)
(1)
Department of Materials Engineering and Industrial Technologies, University of Trento, via Mesiano 77, 38050
Trento, Italy, e-mail: matteo.benedetti@ing.unitn.it
(2)
Department of Mechanical and Structural Engineering, University of Trento, via Mesiano 77, 38050 Trento, Italy, e-
mail: daniele.bortoluzzi@ing.unitn.it
(3)
Department of Mechanical Engineering, University of Padova, via Venezia 1, 35131 Padova, Italy, e-mail:
luca.baglivo@unipd.it
(4)
Thales Alenia Space, Italy, s.s. Padana Superiore 290, 20090 Vimodrone (MI), Italy, e-mail:
marco.lapolla@thalesaleniaspace.com
ABSTRACT between contacting bodies is promoted neither by the
gravity field, nor by surface contaminants caused by
A critical phase for both the joint ESA-NASA scientific exposure to the atmosphere, nor by seismic and air-
mission LISA and its technology demonstrator LISA propagated acoustic noise.
Pathfinder has been identified in the release to free A meaningful example of these issues is given by the
floating conditions of the 2kg gold-coated Test Mass ESA-NASA joint scientific space mission LISA (Laser
(TM) on behalf of the Grabbing Positioning and Release Interferometer Space Antenna) and its precursor LISA
Mechanism (GPRM), developed by Thales Alenia Space Pathfinder. Aim of LISA is the first in-flight revealing
and RUAG. The main criticality of such a phase consists of gravitational waves, which will be detected by means
in the limited allowable velocity of the test mass after of laser interferometers arms formed among three
detachment from the GPRM. In this paper we deal with orbiting satellites. The gravitational waves sensing
the on-ground testing activity of such a phase carried out elements, constituting the end-mirrors of the
at the Dept. of Mechanical and Structural Engineering of interferometer arms, will be 2kg Au/Pt cubic masses
the University of Trento, where we have developed a located within the satellites. During the launch, the test
facility for the measurement of the impulse exerted by masses (TM) need to be firmly secured to their housings
the rupture of the adhesive bonds between representative in order to avoid shaking and thus damage. During the
surfaces of the TM and the release-dedicated subsystem experiment, the test masses need to be released in free
of the GPRM. flight. The release of the LISA Pathfinder test masses is
performed by the Grabbing Positioning and Release
1. INTRODUCTION Mechanism (GPRM), that is being developed by Thales
Alenia Space and RUAG.
Adhesion phenomena, even in a space environment, Some boundary conditions make the operation of the
have been extensively investigated in the literature release-dedicated mechanism of the GPRM critical.
([1][2]), but their role on the release of objects to free- First, the TM and any facing surface must be gold
floating conditions in space applications has not been coated, in order to limit stray electric fields that would
thoroughly studied yet. The in-orbit precise release of a convert into force noise on the TM itself. Second, a
body implies the separation of its contacting surfaces limited (µN order) force and torque authority on the
from the engaging surfaces that belong to some kind of floating TM is available supplied by a set of surrounding
caging device. As this separation involves the rupture of electrodes that constitute a capacitive actuation system.
the unavoidable adhesive forces, an impulse is It has been experimented [1] that gold-coated surfaces
developed on the object at any contact point. In absence easily develop a mN-order adhesion force even at low
of constraining non-contact forces that balance the net preloads, that is at least three order of magnitude larger
impulse, the separation of the mating surfaces must rely than the force authority. As a consequence, an impulsive
on the body inertia and result in a transfer of momentum detachment of the release finger from the TM on behalf
to the released object. The critical aspect of this issue is of the GPRM resulted the only viable solution and has
twofold. First, due to the poor repeatability of adhesion been analyzed in [3]. The limits in the TM control force
phenomena, it is not possible to rely on the mutual and the available gaps with the electrodes convert in a
balancing of the developed impulses, even with a requirement on its residual velocity after release that
symmetrical configuration. Second, in the space must be less than 5 µm/s (10-5 Ns linear momentum).
environment, the rupture of the interaction forces Due to the criticality of the release and capture phase of
_____________________________________________________
Proc. ‘12th Euro. Space Mechanisms & Tribology Symp. (ESMATS)’, Liverpool, UK,
19–21 September 2007 (ESA SP-653, August 2007)
, the TM for the entire mission, a ground based finger is retracted, but also along the orthogonal
verification has been requested. directions.
Dynamic adhesion measurements aimed at determining Neglecting, for the moment, the stiff constraint along the
the momentum transfer followed by the separation vertical direction, the simple pendulum model for the
between adhering bodies had not been performed yet. inertial isolation system has been chosen to investigate
An experiment has been designed and its conceptual the possible performance of the transferred momentum
model has been proposed by the authors in [4], measurement experiment. As long as the pendulum
indicating that a basic requirement is to prevent length is compatible with the typical height of a
adhesion rupture induced by force/torque components laboratory ceiling (i.e. meter scale), the preferred
acting along constrained degrees of freedom of the practical implementation is the simple pendulum
experimental device; the measuring apparatus and its characterized by easily determinable dynamic properties
performances in terms of measurement resolution have (quality factor and resonant frequency) and still
been illustrated in [5], the preliminary results of providing good isolation from gravity and micro-seismic
momentum transfer measurement between aluminum noise. The basic concept of the measuring apparatus,
mock-ups have been presented in [6]. The present paper illustrated in Fig. 1, is to suspend both the test mass and
presents for the first time measurements of momentum the release finger from two pendulums. A position
transfer between representative surfaces of the LISA sensor detects the weakly damped oscillation of the test
Pathfinder TM and the GPRM. mass mock-up due to the momentum transferred upon
pulling the contacting finger mock-up away.
2. ON-GROUND TESTING OF THE RELEASE
PHASE
2.1 Dynamic adhesion measurement technique
The on-ground testing approach of the GPRM/TM
release phase is based on the measurement of the
impulse developed by the rupture of adhesive bonds
between replicas of the TM and release finger mating
surfaces in representative conditions of the in-flight
ones. The GPRM release performance is considered
compliant with the requirement if the impulse results
lower than 10-5 Ns.
Precise measurement of impulse performed by other Figure 1. The concept of the release phase ground
authors [7, 8] resulted critical, mainly for two reasons. testing setup: two pendulums with nominally equal
Firstly, the force impulse needs to be entirely converted lengths represent the TM and the finger respectively. A
into momentum; therefore, any other force acting in the position sensor detects the swing motion of the TM due
same direction on the body subjected to the impulse to the momentum transferred upon pulling the finger
must be minimized. Secondly, the momentum must be away from the contact
identified by the measurement of the resulting motion of
the body that is affected by noise sources and by the
unavoidable constraining forces. The conversion of
impulse into momentum may be guaranteed by a
suspension system that minimizes the risk of any
impulsive constraining force in the direction of the
impulse to be measured. Suspension systems based on
simple pendulum, linear rail and torsion pendulum [8, 9]
have been adopted to provide a weakly constrained axis.
The presence of a single weakly constrained degree of
freedom does not limit the measurement, as long as the
impulsive force is a non-contact force and it is
reasonably aligned with the “soft” axis. On the contrary, Figure 2. Schematic representation of the constraining
the impulsive force due to adhesion rupture is affected stiffness of the two initially contacting bodies in the
by the complete three-axial stress status at the contact ground-based experimental configuration. The suffixes 1
patch that depends on how both contacting bodies are and 2 refer to the finger and TM mock-up respectively
constrained to ground. This means that the body
subjected to the adhesive impulse needs to be weakly In the configuration shown in Fig. 1, however, the stress
constrained not only in the direction along which the status on the contact patch may be in principle far
different in the ground experiment from the in-flight
