Limitations of earthquake detection with the E-puck's accelerometer
?
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
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])
“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.
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