MEMS gravimeter takes underground mapping to the masses

April 01, 2016 // By Julien Happich
Using a proprietary asymmetric cantilever design, researchers from the University of Glasgow have designed a portable MEMS-based gravimeter boasting the smallest resonant frequency ever reported for a MEMS device together with unprecedented stability.

With its resonant frequency of 2.3Hz, an acceleration sensitivity of 40μGal Hz-1/2 and a stability guaranteed for days or even weeks thanks to very accurate servo control loops that maintain the system's temperature to within 1mK, the device can be used to monitor minute variations in gravity over long time spans, making it applicable to many fields of geology and underground exploration.

In a paper titled ‘Measurement of the Earth tides with a MEMS gravimeter’ published in Nature, the researchers detail the unique geometry of their 15x15mm MEMS die featuring an anti-spring flexure pair at the bottom (constraining the motion of the proof mass) and a curved cantilever at the top.

The proof mass motion is measured using an optical shadow sensor (a photodiode placed on one side measures the proof mass' shadow variations as the MEMS is illuminated from the other side by a LED – see figure 1). This sensor setup achieves a high sensitivity, equating to an acceleration noise floor of under 10μGal at the sampling frequency of 0.03Hz, while allowing a large dynamic range of up to 50mm, explain the researchers. Key to the MEMS' long term stability is the precise temperature control mechanism, achieved through the use of four small platinum resistors (one on the outer frame of the MEMS and three placed equidistantly around the copper shield of the full device).

Fig. 1: Both the MEMS device and the shadow sensor sit on an aluminium plate and are encased in a copper thermal shield. At the top left is a photograph and scanning electron microscope (SEM) image (copyright for both images R.P.M., 2015) of the MEMS device. At the bottom left is a photograph of the MEMS device mounted on the optical shadow sensor with glue holding the heater and thermometer in place (copyright G.D.H., 2015).