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.
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