A force-balance MEMS gravimeter with area-changed capacitors for transducing and actuating

The measurements of gravity have been widely applied to earthquake monitoring, resource exploration and identification of natural voids. Traditional gravimeters, restricted by the exorbitant price and bulk, are hardly to be utilized in newly developed applications such as AUV-borne gravimetry and the multi-pixels gravimetry. Fortunately, this is changing with the development of portable MEMS gravimeters.

Here, we have already developed a relative MEMS gravimeter with a quasi-zero stiffness spring-mass system and an arrayed area-changed capacitor for displacement transducing and actuating. This quasi-zero stiffness spring-mass system transfers the acceleration variation to the displacement of the proof-mass with high sensitivity. The displacement is then detected by the arrayed area-changed capacitor. The spring-mass system was installed inside a vacuum chamber for reducing the thermal noise and environmental disturbances. However, the response time of the gravimeter was enlarged when operating in open-loop mode dues to the decreased air damping. Therefore, a force-balance system is utilized in a MEMS gravimeter to improve the response time and the measurement range. This paper proposes an area-changed capacitors for transducing the displacement and balancing the inertial force on the proof-mass. As no additional structures are required on the MEMS chip, the design of the force-balance system is beneficial to simplifying the fabricating process and avoiding additional noise sources.

We calibrated our MEMS gravimeter statically in our cave lab. After linear correction of the drift, the output of the MEMS agrees well with the theoretical earth tide with a coefficient of association of 0.92. The self-noise is evaluated to be 1 μGal/√Hz, which is one of the most sensitive MEMS-based gravimeter reported. In addition, the response time of the MEMS gravimeter was calibrated to be 0.2 s in close-loop mode, which is 2000 times faster than that in open-loop mode. This will promote improved accessibility of mobile gravity measurements at an affordable cost.