Design and Simulation of a MEMS Gravimeter for Lunar Missions

Lunar gravity filed determines the optimization design of lunar orbiters and selections for landing sites. Direct gravity measurement by deploying high-resolution gravimeter is the only option for obtaining precise local gravity data rather than the satellite gravity survey. However, there are both light-weight and shock-resistance demands for payloads of lunar landers. Therefore, this paper proposes a Micro-Electromechanical-Systems (MEMS) based gravimeter for lunar gravity survey, taking the advantages of both light and robust. The basis of this MEMS lunar gravimeter is: a silicon-based spring-mass system performs as the gravity-sensitive element. VVhen gravity varies with locations or time, the spring length will change and so the displacement of mass. Using the optical displacement measurement technology, the gravitational acceleration can be worked out. The gravity-sensitive element requires both an extremely low fundamental frequency for a higher sensitivity and lower noise floor and large spurious resonant frequencies for low cross-sensitivities. Shock-resistance structures are required for robustness of the gravimeter. According to the above design rationales, the gravity-sensitive element is designed and the finite element analysis software COMSOL has been used to simulate the vibration modes of the spring-mass system with the beam width varying from 20 mu m to 30 mu m. The gravimeter design with a beam width of 24 mu m has a fundamental frequency of 5Hz. The mechanical thermal noise can be worked out as 0.3 mu Gal/Hz(1/2) under the room temperature and atmosphere with a quality factor of 200. The equivalent noise of the optical displacement transducer is 10 mu Gal/Hz(1/2). Hence, the overall noise floor of the MEMS lunar gravimeter is 10 mu Gal/Hz(1/2), which can meet the requirement of precise measurement of lunar gravity field.