Accelerometers can be used in many
applications. For those who are studying
vulcanology, accelerometers can be used
for determining which direction the
magma is moving using the Z axis. The
sensor being utilized measures in three
dimensions, using an ST Microelectronics
2-16 g LIS3DSH accelerometer.
In my first design, I used a Freescale
Semiconductor 2-8 g three-axis
accelerometer eight-bit A/D. However,
the ST model (which is 16 bits) is
256 times more sensitive, and has a
FIFO register which allows you to see
activity just prior to an earthquake.
ST also makes a 100 g to 400 g sensor
which plugs in, replacing the lower g unit
if you are interested in impact gs or a
shipping monitor.
Idesigned this project so that it can be programmed utilizing the same SD card as the previous eismograph article used. Figure 1 shows the low g accelerometer which can be used as a seismograph,
continuous monitor, or a tripping monitor. The data
collected on the SD card can be downloaded into a
computer and displayed using either Excel or the Dataq
web browser (which is free software from Dataq). All three
dimensions are displayed at the same time, giving the
date, time, and parameters.
There are two main types of scales for measuring
earthquakes: the Richter and the Mercalli. The Richter
scale is logarithmic and was developed in 1935. Each
number represents 10 to that power. An earthquake of
three is about what most people can feel: 103 = 1000.
The San Francisco, CA quake in 1906 was estimated to be
an eight: 108 = 100,000,000. This is 100,000 times more
powerful than a three.
The Mercalli scale is an arbitrary measurement based
on what people feel. It was revised in 1902, and is a
12-point scale. The San Francisco quake equates to an
X or XI on the Mercalli.
Gravity is the force pulling things down, and
accelerates objects 9.8m/s2. For something to leap in the
air during an earthquake requires a force wave traveling
above 9.8m/s2 (1 g) which is equivalent to an 8.1 on the
Richter, or an 11 on the Mercalli scale.
The sensor used in this article can measure down to
.00006 per digit, or 0.00006m/s2 (Figure 2). This would
be a 2-3 on the Mercalli scale or about a 2 on the Richter
scale. The accelerometer is more akin to the Mercalli scale
than the Richter scale. What the accelerometer does over
the seismograph is to give you a three-dimensional view of
the g forces involved. This is an engineer’s dream tool for
studying structural engineering.
In the seismograph mode, once an earthquake above
.001 g is detected on the X or Y axis, it records all three
axes onto the SD chip for one minute using the 2 g
sensitive mode. As far as using it as a seismograph, it will
not be as sensitive as the poor man’s version (Figure 3)
but will work for slightly larger earthquakes, showing the
direction of the waves and their g force. The earthquake is
determined using the zero crossing mode which makes it
very sensitive.
For a continuous monitor, all three axes are watched
on any of the settings of 2 g, 4 g, or 8 g. This is also set
using an Excel spreadsheet. It will continue monitoring
until the program button is pressed.
The continuous monitor has many applications, e.g.,
racing cars, model airplanes, rocketry, etc. The SD card
size will determine how long it will record; it records
50 samples per second. How about a physics project of
building a container that contains an egg and dropping it
from a height of 20 feet without breaking it? This device
November 2014 33
■ FIGURE 1.
■ FIGURE 2.
■ FIGURE 3.
