20.10.2015

So how exactly do they work?

Mjolnir

The Mjolnir, also known as the worlds toughest anemometer works like a traditional cup anemometer, but is modified specifically for use with smartphones. The physical design is optimised to be as thin as possible to fit snugly in the pocket.

The overall size and aerodynamics are designed so the device doesn’t turn too fast that it cannot be detected by the phone but is not too big that it slows down the device. The top section runs at a point directly at the centre of this aerodynamic pressure and mass. This, combined with a smooth plastic POM with Teflon allows the top part to rotate freely.

When the wind blows, the cups capture this and the top spins accordingly. Inside the Mjolnir are very powerful magnets that disturb the device’s magnetic fields while it is rotating. The built-in digital compass in a nearby smartphone detects these changes in the magnetic field from all three dimensions. Sine waves enter the compass with a frequency dependent on how fast the device is rotating. We then analyse this pattern of sine waves given to us at roughly 50 times a second. On each of the three signals that correspond to the three dimensions, we run a Fourier analysis. The output we get is the actual rotation frequency, which we then translate into wind speed. When users move the phone around, this affects the patterns we receive, which poses a challenge. To solve it, we filtered this data out to make sure it wasn’t part of the estimates of the rotation.

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Sleipnir

With the Sleipnir, Also known as the smartest anemometer, we wanted to try something different by also measuring wind direction. Instead of using the magnetic field technology, we decided to use the audio jack, which has an updated frequency that’s about 1000x faster than the compasses of some phones. This was also more standardised since every phone has an audio jack. Since we wanted to measure wind direction, using the audio jack prevented us from disturbing the internal compass.

From the jack, signals in the form of two sine waves give power to send an infrared beam from the outer surface to the middle. When the top section rotates, the uniformly spaced teeth block the travelling infrared beam. This creates a pattern, which then transfers back into the phone. We take these chunks of small noises and figure out how long a time there is between the beeps. The faster it rotates, the smaller the time between the beeps.

Now to measure wind direction we look at the physique of the Sleipnir, which includes an asymmetric rotor design, or this hole in the blades. Because of the hole, some moments in the rotation are slower or faster. These moments are shown in the pattern given by the blocked infrared lights, which we also analyse once the signals are sent back to the phone.

A big challenge to measure direction was that the device had to be very light so it could accelerate or decelerate properly. The wings are very, very thin: only 0.4mm while the ridges have structural agility. The device also needs to be perfectly balanced because it rotates very fast, up to 100 revolutions per second. Any slight imbalance causes it to vibrate and affect measurements. Another tricky part was the plastic material. It needed to absorb the infrared light waves, but, because only this dark grey colour was able to do that, coming up with a red wind meter was a challenge. To combat it, we made this very special compound of polycarbonate plastic so that the red one could absorb these light waves.

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