A self-levelling nano-g silicon seismometer

Abstract

We demonstrate a microseismometer with a 2ng/rtHz noise floor capable of autonomous operation over a wide range of tilts. This represents the highest performance yet achieved by a silicon-based vibration sensor. The microseismometer builds on previous development of a shortperiod seismometer for NASA's 2016 InSight mission to Mars . The deep-reactive-ion-etched sensor element is unique in that it uses a spring-mass system with a proof mass that moves laterally. This minimizes the damping of the spring mass systems without the need for vacuum encapsulation. The proof-mass position is sensed by a periodic linear capacitive array transducer allowing highly sensitive position detection combined with feedback control at multiple null points. Operation at any of these points enables the sensor to function over a large tilt range without compromising the noise performance. As well as the capacitive sensing elements, the proof mass has planar coils on the surface to electromagnetic actuator when placed in a static magnetic field. The MEMS sensor element is connected to an electronics feedback circuit similar to those used in broad-band seismometers allowing the sensor to act as a velocity output force balance transducer.

A seesaw-lever force-balancing suspension design for space and terrestrial gravity-gradient sensing

“…The electronics for the gradiometer displacement transducer we are going to use are inherited from the NASA's InSight microseismometer which has been developed recently in our group. 22 The estimated electronic noise is negligible, around 0:2 E= ffiffiffiffiffiffi Hz p for our gradiometer design. The total noise floor of our MEMS gravity gradiometer would be 10 E= ffiffiffiffiffiffi Hz p listed in Table IV.…”

“…A N DT of 1 pm= ffiffiffiffiffiffi Hz p has been demonstrated for MEMS sensors, 22 while a resonance of 1.3 Hz is achievable with a proof mass of 1 g (20 Â 20 Â 0.5 mm silicon). A Q of 100 000 can be obtained in vacuum.…”

“…A lateral capacitive array transducer (LCAT) 22 offers a possible solution, with the output dependent on the change in overlap between a series of moving electrodes on the two proof masses and a corresponding series of fixed electrodes on a facing die. A noise floor of 1 pm= ffiffiffiffiffiffi Hz p is achievable with a 12 lm gap between the moving and fixed electrodes, with a lateral separation of around 12 lm.…”

A micromachined angular-acceleration sensor for geophysical applications

“…6 However, MEMS sensors for geophysical applications are emerging in the past two decades. MEMS seismic-grade accelerometers [7][8][9] have been developed to determine the surface seismic waves for either monitoring earthquakes or subsurface imaging oil exploration using the seismic reflection method, 10 including the 0.2 ng/ ffiffiffiffiffiffi Hz p short-period seismometer 11,12 recently developed in our group as a contribution to NASA's InSight Mars mission. There are also several reported MEMS angular accelerometers.…”

“…When there are relative movements close to the null position of the RCAT, changes in capacitance are proportionally to the rotation angle and are then picked up by the proven front-end circuit of the prior work. 11 The bootstrapping technology 18 has been applied to eliminate the effect of stray capacitors by using guard electrodes, whose voltage potentials are driven to be the same as the pickup electrodes, underneath and opposite to the pickup electrodes. Both the stator and rotor dies were implemented simultaneously by microfabrications on the same 4-in.…”