All space experiments which involve freely falling or softly suspended bodies require
them to be locked during launch, and properly unlocked once in orbit to start the
experiment. First of all we find it important to avoid any danger of the payload hitting
the spacecraft walls. This is done simply by having each suspended mass and the PGB
laboratory constrained to only slight movements by means of mechanical stops. Gaps of a
few millimetres in size make the very soft mechanical suspension dominate during the
experimental phase but at the same time constrain the body to within a small range of
movements in case anything unpredictable should happen. As for the launch phase, when the
system is subject to strong accelerations, we envisage having a static mechanical locking
for each body, typically made of 3 lockers 
apart on each suspension side. As for the forces acting on the springs
themselves during launch, we recall that their mass is very small; it is also possible to
use mechanical stops in order to avoid large displacements. Estimates show that there is
no danger for the elasticity regime to be exceeded during launch, even though some time
for relaxation should probably be allowed at the beginning of the mission. Once the
spacecraft has been injected in its orbit and given the required attitude and spin rate
the static mechanical lockers can be released and never used again. A symmetrical locking
consisting of 4 inch-worms placed at 
from one another as shown in Fig. 19 is
provided, each inch-worm being equipped with a force sensor sensitive to 
. It gives a measure of the
centrifugal force in that direction, and therefore provides the driving signal to the
inch-worms for reducing the distance offset from the rotation axis. Once this has been
reduced to 
, which means a
centrifugal force of 
for the suspended test masses, active centering with inch-worms can be stopped; the
electrostatic dampers will then stabilize whirling and precessional motions around the
equilibrium position of supercritical rotation as shown in Section
3 . While the static lockers are meant not to be reused, the inch-worms can. Together
with the mechanical stops they make the system in principle safe from unexpected
occurrences.
Fig. 19. Top view of a set of four inch-worms actuators for locking and unlocking the suspended masses. Each mass needs two such sets placed at its two axial ends (see also Fig. 2 ). Between the inch-worms are the electrostatic plates used for active damping. The rod, hence the suspended masses, is locked during launch and until the spacecraft has reached the final spin angular velocity
. Then the inch-worms equipped with pressure sensors sensitive to
are used for initial centering until the centrifugal forces detected by the pressure sensors become smaller than the forces that can be generated by the electrostatic plates. At this point the inch-worm will be retracted and the electrostatic system will complete the centering and will keep it stable.
Copyright © 1998 Elsevier Science B.V., Amsterdam. All Rights Reserved.
(Anna Nobili- nobili@dm.unipi.it)
