Analytical study of a new gravity compensator to allow for the accurate weightless motion in the vertical direction

The purpose of this paper is to show a new gravity compensator which allows for the passive vertical weightless motion. The conventional zero gravity simulators could not simulate the vertical balance and weightless motion passively at the same time. For example, the simulator using the counter weight of the same mass can only simulate half the acceleration of the weightless motion because an external force acts on both the object and the counter weight. On the other hand, the simulator using the low stiffness spring is difficult to balance. Firstly in this paper for such a trade-off, a new concept which is named degree of zero gravity (DOZ for short) is introduced. By using DOZ we can estimate how similarly the system simulates the acceleration in zero gravity. Secondly, a new gravity compensator which uses a heavy counter weight and a pantograph mechanism is introduced and evaluated by means of DOZ. When an external force acts on the object of the gravity compensator, most of the energy acts on the relatively lightweight object by using the heavy counter weight. It follows the passive vertical weightless motion of the object. Finally, numerical results show the effectiveness for the passive vertical weightless motion. The main results obtained are 0.86 times the acceleration in zero gravity when an external force is applied and the disturbance acceleration of 2.3 × 10−4g or less when an external force is not applied.