Limitations of earthquake detection with the E-puck's accelerometer

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Par Rodrigo De Pablo Peña et Fabian Baerenbold

Introduction

Theoretical part

– Functioning

– Measure limitations

– Earthquakes

Experimental part

– Choose a test-frequency and design a filter

– Try to make the E-puck find a source of vibrations

Accelerometer – How it works

Accelerometer consists of 3 beams in series where the middle one is attached to a movable mass. The 3 beams form 2 capacitors in series.

An applied force changes position of the mass and therefore the value of the capacitors.

The capacitors values are linked to an output voltage between 0 and 3300[mV], which on the E-puck is a binary code between 0 and 4096

The range of the E-puck's accelerometer is set to +/- 2[g], g=gravity

Measure limitations and noise

• Resolution:

• Noise RMS in data sheet : 4.7 [mV]

• Noise RMS calculated :

• Minimum detectable amplitude:

• Maximum detectable amplitude : 2[g], (up to 2.75[g], but less accurate above 2[g])

• Sampling frequency: Up to 11[kHz]

][0.4][33004096

0.5mVmV

][001.04096]/[600

][3300g

gmV

mV

][02.0]/[600

][0.43 g

gmV

mV

Earthquakes

• E-puck can recognize frequencies up to 11/2[kHz]

• Frequencies of an earthquake between 0.1 and 20[Hz]

=> E-puck is able to measure this

• E-puck detects earthquakes having amplitudes bigger than 0.02[g] (=0.2[m2/s]) if it is right by the epicenter

• 0.2[m2/s] corresponds to an earthquake where “dishes in a cupboard rattle”!

=> E-puck is not very useful in this case

Experimental part

Signal of the razor

Frequency Detection - FIR Filter• Razor frequencies: 12-14[Hz]

• FIR equiripple bandpass filter

• Sampling frequency : 200[Hz] (to minimize the filter order)

Band: 10 -16 [Hz], Filter order: 143

Unfiltred vs. Filtred Signal

Filter with attenuation of low frequencies of 20dB

Filter with attenuation of low frequencies of 40dB

Shaking by hand (2-3[Hz]) --- Razor (13[Hz]) --- Cell phone (25[Hz])

Unfiltred vs. Filtred Signal

Is it possible to find the source of the « earthquake » ?

“Bacteria” AlgorithmMobile experiment I

•Goal: E-puck finds the source of vibrations

•Assumption: There is a continuous drop of amplitude with distance to the source.

•How: E-puck moves with constant speed. As long as the amplitude measured by the E-puck increases, it continues to move forward. If not, it changes direction.

•Amplitude computation: Take the sum of the filtered signal elements squared and divide by an appropriate constant to avoid overflow.

•Random term: A random term is added to the constant speed to make the E-puck able to move randomly in any direction.

“Bacteria” AlgorithmMobile experiment I

But:But: Although the E-puck sometimes finds the source, it often gets stuck far away Although the E-puck sometimes finds the source, it often gets stuck far away from it…. from it….

Why ?Why ?=>>> =>>> IdeaIdea: Scan the whole surface with the E-puck to have an idea of the : Scan the whole surface with the E-puck to have an idea of the vibration pattern of it. Then repeat the experiment.vibration pattern of it. Then repeat the experiment.

Material for experiments

Relative amplitude responseStatic experiment

Relative amplitude responseMobile experiment II

Relative amplitude responseMobile experiment II

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Relative amplitude responseMobile experiment

References

• http://www.naun.org/journals/geology/20-052.pdf

• http://en.wikipedia.org/wiki/Japan_Meteorological_Agency_seismic_intensity_scale

• http://pdf1.alldatasheet.com/datasheet-pdf/view/103487/MOTOROLA/MMA7260Q.html