Active Vibration Control in Microgravity Environment

Abstract

The low gravity environment of the Space Station is suitable for experiments or manufacturing processes which require near zero-g. Such experiments are packaged to fit into rack-mounted modules approximately 106.7 cm (42 in.) wide × 190.5 cm (75 in.) high × 76.2 cm (30 in.) deep. The mean acceleration level of the Space Station is expected to be on the order of 10−6 g (9.81 × 10−6 m/s2 ). This steady state acceleration is a superposition of aerodynamic drag, centripetal forces, and the gravitational attraction of the earth and of the moon. Excitations such as crew activity or rotating unbalance of nearby equipment can cause momentary disturbances to the vibration-sensitive payload which degrade the microgravity environment and compromise the validity of the experiment or process. Isolation of the vibration-sensitive payload from structure-borne excitation is achieved by allowing the payload to float freely within an enclosed space. Displacement-sensitive transducers indicate relative drift between the payload and the surrounding structure. Small air jets fixed to the structure direct air flow to impinge on the payload. This thrust force keeps the payload centered within the enclosed space. The mass flow rate of the air jets is controlled such that the resultant acceleration of the payload is less than a criterion of 10−5 g. It is expected that any power or fluid lines that connect the experiment to the Space Station structure can be designed such that their transmitted vibration levels are within the criterion. An experiment has been fabricated to test the validity of the active control process and to verify the flow and control parameters identified in a theoretical model. Zero-g is approximated in the horizontal plane using a low-friction air-bearing table. An analog control system has been designed to activate calibrated air jets when displacement of the test mass is sensed. The experiment demonstrates that the air jet control system introduces an effective damping factor to control oscillatory response. The amount of damping as well as the flow parameters, such as pressure drop across the valve and flow rate of air, are verified by the analytical model.