How similar is Insight’s seismometer to the one in your mobile phone?

This was made for All About Mars.

The seismometer aboard InSight is extraordinarily sensitive. The working principle of the very first seismometers is a simple pendulum such that vibrations or oscillations cause the mass to oscillate in different directions, etching said oscillations onto a rolling piece of paper which can be analysed later on. This works based on the principle of inertia. The pendulum is attached to a frame, and should be firmly fixed to the ground, so the mass vibrates in the frame of reference of the frame itself, and can thus accurately plot seismic oscillations for analysis. This design is however very old and has a multitude of issues, including lack of sensitivity and is bulky.

We need ever increasing sensitivity in seismometers so that we can effectively use them to analyse P and S waves, so seismometers have evolved adequately through time. In order to increase sensitivity, the device could then be mounted upside down, so that the centre of mass of the pendulum is above the pivot point.

However, this seismometer is still only able to detect vibrations to plot them on paper.

A mobile phone contains an accelerometer, which essentially has the same general function as a seismometer — to detect movement, however a mobile phone accelerometer has a high range and is not very sensitive. Accelerometers are simply used to detect movement in different axes, which can be useful for navigation and such, however seismometers need to supply data for geophysicists to analyse and understand and so an accelerometer cannot be used in place of a seismometer. We shall start by discussing the working principle of phone accelerometers and compare and contrast seismometers with these devices to explain their similarities and differences.

Accelerometers need to be suitably sensitive to detect movement effectively, but they also need to be incredibly small. In order to achieve this we use MEMs, or microelectromechanical devices. These are tiny devices that incorporate both mechanical and electronic components but exist at the 10^-6 order of magnitude.

An accelerometer works using one such device. Let us first consider a normal spring-mass system. Movement of the system will cause the mass to move and displacement of the mass is a function of acceleration (or vice versa) so x = f(a) (as F = ma, F = kx thus ma=kx thus x = (k/m)*a). We can therefore work out acceleration of the system if we know the displacement. Now consider a capacitor. The capacitance is equal to the electrostatic constant, multiplied by the area and divided by the plate distance, so capacitance is inversely proportional to distance and we can thus work out distance from capacitance. If we made a system that combines these two, that would be an excellent method of detecting acceleration. This is fabricated on a tiny scale using microelectromechanical devices, a combination of analogs of parallel-plate capacitors and springs .

This is what the final accelerometer product looks like. We have a fixed support which acts as the frame, electrodes which act as the the capacitors and a proof mass, which is what oscillates when moved. Using the same system we devised above, we can use a basic circuit to find capacitance, and thus find a fairly exact measurement of the displacement and acceleration.

Furthermore, we can orient these sensors in all three x, y, z planes to measure acceleration in all of these directions

This is a basic description of how phone accelerometers work using MEMS, or microelectromechanical devices.

The main difference between accelerometers and seismometers is down to their sensitivity. Accelerometers are much less sensitive but have a significantly greater range than seismometers, and therefore are much better at detecting strong movements. They can detect 2g or above of ground acceleration, seismometers would clip after tapping them.

The seismometer used in InSight is a cutting edge device with unparalleled sensitivity, many many orders of magnitude more than other seismometers, let alone an accelerometer, it can detect movements at the order of a 10th of a hydrogen atom. The working principle however is remarkably similar to the microelectromechanical device used in a mobile phone.

The InSight seismometer works using a pendulum. As discussed previously, seismometers and accelerometers both require one part to be moving freely, and one part to be fixed in place. When earthquakes occur and the ground shakes. Vibrations are transmitted to this freely moving part and causes it to move with respect to the fixed part. We require these movements to happen with a minimal friction, so that they can be detected effectively.

The InSight pivot is shown here. It has a set of copper lamellae, which are incredibly thin — around 50 microns thick (half the thickness of paper). The lamellae flex frictionlessly allowing the pendulum to be displaced.

The next important mechanical component of the InSight seismometer is the spring, which links the mobile and fixed components of the VBB sensor and allows the pendulum system to be in equilibrium with gravity. It exerts a constant restoring force, bringing the mass to its initial position after a vibration, thus preventing it from irreversibly moving out of equilibrium.

Perhaps the place where the InSight Seismometer and a standard mobile phone accelerometer have the most in common is the way that these tiny movements are sensed, using capacitors. The seismometer uses a Differential Capacity Sensor (DCS). The sensor measures the differences in capacitance between the mobile mass electrode and the fixed electrode. This allows tiny motion to be quantified very precisely.

One final incredibly interesting component of the VBB pendulum is the TCDM, the Thermal Compensation Device Mechanism, is a device that compensates for temperature variations. The mechanism adjusts the pendulum’s centre of gravity in response to variations in environmental temperature. This is a big difference between the standard phone accelerometer and the seismometer. This is because incredibly sensitive seismometers need to take into account temperature, while accelerometers do not need to. The TCDM consists of two thermal compensation devices positioned at each end of the shaft, with two metals that have a different COE (coefficient of expansion). This means that upon a change in temperature, one has a significant expansion, while the other has very little.

When the temperature rises, the TCDM elongates, stopping the effect of increasing temperature. This means that the TCDM is actually a passive device, which means it doesn’t actually require energy to work.

In conclusion, the InSight seismometer and a mobile phone accelerometer system have the main difference of sensitivity. The InSight system has various components optimising its performance in minimising noise such as thermal noise, and being incredibly sensitive. Accelerometers need to be small and detect motion effectively. Both sensors share one major thing; capacitance. Capacitors are used in all of these.

Bibliography

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‌www.siliconsensing.com. (n.d.). Silicon Sensing | MEMS Accelerometers. [online] Available at: https://www.siliconsensing.com/technology/mems-accelerometers/.

Seismology Research Centre. (2014). Strong Motion Accelerographs. [online] Available at: https://www.src.com.au/strong-motion-accelerographs/#:~:text=Accelerometers%20are%20much%20 less%20sensitive [Accessed 18 Mar. 2022].

‌Tech, V. (2015). How does a seismograph work? — Virginia Tech. YouTube. Available at: https://www.youtube.com/watch?v=XWwHqOec-KA [Accessed 25 Nov. 2020].

‌Seis-insight.eu. (2016). How a seismometer works — SEIS / Mars InSight. [online] Available at: https://www.seis-insight.eu/en/public-2/planetary-seismology/how-a-seismometer-works.

‌www.seis-insight.eu. (n.d.). VBB sensors — SEIS / Mars InSight. [online] Available at: https://www.seis-insight.eu/en/public-2/seis-instrument/vbb-sensors.

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