Small And Inexpensive MEMS Gravimeter

A gravimeter, as the name suggests, measures gravity. These specialized accelerometers can find underground resources and measure volcanic activity. Unfortunately, traditional instruments are relatively large and expensive (nearly 20 pounds and $100,000). Of course, MEMS accelerometers are old hat, but none of them have been stable enough to be called gravimeters. Until now.

In a recent edition of Nature (pdf), researchers at the University of Glasgow have built a MEMS device that has the stability to work as a gravimeter. To demonstrate this, they used it to measure the tides over six days.

The device functions as a relative gravimeter. Essentially a tiny weight hangs from a tiny spring, and the device measures the pull of gravity on the spring. The design of the Glasgow device has a low resonate frequency (2.3 Hz).

Small and inexpensive devices could monitor volcanoes or fly on drones to find tunnels or buried oil and gas (a job currently done by low altitude aircraft). We’ve covered MEMS accelerometers before, although not at this stability level.  We’ve even seen an explanation from the Engineer Guy.

15 thoughts on “Small And Inexpensive MEMS Gravimeter

  1. For my team’s hackaday prize entry (lasercut optics bench), one of the proposed experiments is measuring the pull of gravity. We figure it’s possible to sense the pull of the moon with a simple optical setup.

    (And our .io page is totally blank ATM because we’ve just got started, and I’m composing the 1st build log now.)

    I remember (I think it was) an Amateur Scientist article that had a weight hanging from a spring with a magnet on one side. A coil would pull the magnet up or down, and a beam of light bounced off of a mirror on the weight leading to a pair of phototransistors in opposition.

    The DC current on the coil moved the magnet to keep the light centered between the two sensors, and the system recorded the voltage needed to do that.

    Over time, a signal from the attraction of the moon was clearly evident.

    1. As a followup, assume a 1KG weight:

      (Gravitational Constant) * 7.34767309 * 10^22 kilograms * 1 kilogram / ((370300000 * 1 meter)^2)= 3.576×10^-5 kg m/s^2 (kilogram meters per second squared)

      =^= 36 micro newtons

      Assume the weight hangs from a rubber band, with a spring constant of 88 N/M

      Displacement due to moon is 409 nM, or about 1 wavelength of blue light.

      Easily detected with a laser interferometer, which is easy to set up. I’ve actually done this (set up an interferometer) on my kitchen table.

      Twice the mass makes twice the displacement, a ballpoint pen (spring constant: 221 N/M) about 1/3 the displacement.

      The weight is also pulled up when the moon is on the opposite side of the planet – the moon does *not* pull the weight down when it’s directly underneath our feet.

      Can you explain why?

      1. Moon – Earth system is like a carousel. There is a gravity force and centrifugal force. If you are nearer the Moon, the Moon gravity is bigger and you are lighter. If you are on the opposite site, the centrifugal force is bigger and you are lighter too.

        Sorry for my bad English.

    1. You mount the gravimeter on its own stabilized platform, and subtract off the motion caused by the aircraft, measured independently from LiDAR and/or GPS. It’s been a solved problem form more than a decade.

      It would take some doing to cram that in something less than Robinson R22, but I’d guess it would be possible these days.

        1. True. I suggested a R22 because the previous commenter wants to drone-mount it, and the R22 is the least expensive rotorcraft I know of (except maybe a Bell 47). It’s maybe a third the operating cost of the more usual Bell 206, for example (we used to pay $6/minute all-in for a 206+jockey, which probably says more about how long it’s been since I was flying…). But you’re right, a 172 is a better platform and less than half the operating cost of even the R22. I have no experience with Pipers, but it’s probably similar.

  2. The Nature abstract and the summary here neglect to mention that the accelerometer, to function to the precision needed, must be in high vacuum and temperature controlled to within 1 milliKelvin. Do-able, but it’s not quite as simple as just a thin slab of silicon.

    1. High Vacuum is not to hard to do on die , it won’t be penny/chip cheap but is common enough for high Q MEMS stuff. Regulating temp to a mK is bog standard for precision equipment like voltage and timing references. Not the lowest power, but something doable for a few bucks and a few watts.

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