119 Gravity-Offloading System for Large-Displacement  Ground Testing of Spacecraft Mechanisms 

Olyvia Han*, David Kienholz*, Paul Janzen* and Scott Kidney* 

Abstract     Gravity offloading of deployable spacecraft mechanisms during ground testing is a long-standing problem.  Deployable structures which are usually too weak to  support their own weight under gravity require a  means of gravity offloading as they unfurl. Conventional solutions to this problem have been helium-filled balloons or mechanical pulley/counterweight system s. These approaches, however, suffer from the  deleterious effects of added inertia or friction forces. The changing form factor of  the deployable structure  itself and the need to track the trajectory of the center of gravity also pose a challenge to these  conventional technologies. This paper presents a novel testing apparatus for high-fidelity zero-gravity simulation for special application to deployable s pace structures such as solar arrays, magnetometer  booms, and robotic arms in class 100,000 clean room environments.   Introduction   The WAGM (Walking Anti-Gravity Machine) is an active  approach to gravity offloading of space structures  that deploy in three degrees of freedom. At its heart is the proven AGM (Anti-Grav ity-Machine) technology  [1][2] – a single degree-of-freedom gravity offloading de vice that supports a wide range of payload weight  over several centimeters or more of vertical travel with arbitrarily low stiffness, zero st atic deflection,  minimal added mass, freedom from spurious modes, and perhaps most importantly, zero friction. The deployable payload in the present WAGM application is  single-point suspended from the AGM via a load  cable. The AGM is itself mounted onto a pair of commercial off-the-shelf sealed linear belt drive tracking stages. Horizontal drag forces on the payload are minimized by an active system that actuates the linear belt drive stages such that the AGM is centered ov er the payload and the load cable is kept close to  vertical as the payload moves under its own power. The horizontal and vertical systems work together to  deliver gravity offloading for arbitrary trajectories of  the payload with a total range of motion of about 2  meters (6 feet) in all three spatial directions, with  capability to scale up to greater distances in each  direction. In particular, one horizontal direction can have an arbitrarily long range of motion, a feature that  would be useful for testing of unfoldi ng solar arrays or extending booms. 

Comparison to Conventional Gravity Offloaders   A common approach to gravity offloading deployable structures ignores the most difficult DOF (vertical) and is restricted to applications involving only translation in one or two horizontal degrees of freedom. In this approach, overhead air bearings or ball bearing dollies moving in the horizontal dire ction(s) carry the  payload weight as it deploys [3]. Often, the payload it self must pull the dollies or air bearings along. This  approach is limiting in that it not only artifically constrains the vertical degree of freedom but also imposes artificial lateral drag forces on the payload.  Buoyancy systems using floats in a water tank can deliv er a large range of horizontal motion. However, in  addition to logistical and cleanliness problems, they suffer from a high level of viscous damping as well as excessive vertical stiffness. Overhead helium ballo ons solve some of these problems but add a minimum  of 16% of the payload mass, an import ant limitation in dynamic testing. 

                                                  * CSA Engineering, Mountain View, CA 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  NASA/CP-2010-216272