Model and Test Validation of Gravity Offload System

It is necessary to test the behavior of deployable space structures under realistic conditions, without the influence of gravity. Gravity offload systems are used to simulate the necessary environment. There are several types of gravity offload systems, including freefall systems, air bearings, buoyancy systems, and suspension cable systems. The ideal gravity offload device is a zero stiffness spring that would still support the test article. In reality, an active ZSRM (zero spring rate mechanism) system provides a very soft equivalent spring constant over a limited throw. The system presented in this paper is a passive system which utilizes a counterweight and trolley system and provides a constant gravity compensation force for a test article while following the free motion of the article both vertically and in one horizontal direction for up to 1 m. Experiments are performed to verify proper system performance. Then videogrammetry tests characterize the behavior of the test article and gravity offload system. A mathematical model of the system is developed to predict the movement of the gravity offload system’s cart. The model results are compared with the videogrammetry data to verify that the mathematical model accurately predicts the motion of the system.