BeagleBone - 3 Axis Accelerometer Interface and Plotting in 3D

Overview

The accelerated pace of adoption and applications of accelerometers are creating excitement in the engineering and design worlds. Easy to interface via the I2C bus, and very sensitive, an accelerometer measures tilt and motion. They are commonly used in portable devices and game controllers to detect orientation.

In this tutorial we will demonstrate how to interface the ADXL345 accelerometer to the BeagleBone Black and display, in 3D, the sensor results. We will use Python, GnuPlot and Debian in this tutorial. Let's take a look at these steps together.

Special thanks to contributing author: David St-Pierre

What You Will Need

• BeagleBone Black
• BeagleBone expandable case Orange or Black (recommended)
• Mini Breadboard
• Jumper Wire Pack for Breadboard
• AC adapter 5 Volts, 2 Amp (suggested)
• Debian Linux 
• ADXL345 Accelerometer from Analog Devices available at MPJA

Introduction

In this project, we will interface an ADXL345 3-Axis Accelerometer to the BeagleBone Black. A python program will communicate over the digital I2C Bus and collect X-Y-Z data from the sensor. The data will be stored in a file. The data file will then be plotted in 2D showing the X,Y,Z motion and activity during the recorded period. A 3D plot will then be generated representing the 3D travel of the sensor.

Instructions

Connections to the accelerometer can be made with a solderless breadboard and a jumper wire kit.

STEP 1 - Circuit Connections

Turn OFF the BeagleBone Black power and make the following connections.
Interfacing to the sensor is simple. Only 4 connections are required.

Device Pin	BeagleBone	Signal
GND	P9-1	GND
VCC	P9-3	+3.3V
SDA	P9-20	I2C2-SDA
SCL	P9-19	I2C2-SCL

STEP 2 - Required Software

Download Gnuplot

sudo apt-get update && upgrade
sudo apt-get install gnuplot
sudo apt-get install gnuplot-x11

You may also need to download VNC (if you have not previously done so)

sudo apt-get install x11vnc

Download ADXL345 pythonLibrary File (modified for BeagleBone Black)

STEP 3 - Program Listing

This is a python program, that interfaces with the ADXL345 library, which has been adapted for the BeagleBone Black.

Download getAccel.py File (and save it without the txt extension)

#!/bin/python
from adxl345 import ADXL345
from time import sleep

adxl345 = ADXL345()

myfilename = "accel.dat"
with open(myfilename,'w') as f:
 print"ADXL345 on address 0x%x:" % (adxl345.address)
 f.write(str("#X,Y,Z ADXL345 accelerometer readings\n"))
 try:
  for i in range(1000):
   axes = adxl345.getAxes(True)
   xVal = axes['x']
   yVal = axes['y']
   zVal = axes['z']  

   print "x = %.3fG" % ( xVal )
   print "y = %.3fG" % ( yVal )
   print "z = %.3fG" % ( zVal )

   f.write(str(xVal) + ",")
   f.write(str(yVal) + ",")
   f.write(str(zVal) + '\n')
   sleep(.1) 
 except KeyboardInterrupt:
  exit()

STEP 4 - 2D Plotting

This is a gnuPlot program that takes the collected sensor data and plots in 2D the X,Y and Z values measured by the accelerometer.

Download graphData.gp File (and save it without the txt extension)

#!/usr/bin/gnuplot -persist
#Author:David St.Pierre
#Date:July 31, 2014
reset
set yrange[-2:2]
set datafile separator ","
set mytics 10
set title "ADXL345 2D Plot"
set xlabel "Time (0.1 secs)"
set ylabel "Gs" 
set grid
plot 'accel.dat' using 0:1 with lines lt 1 title "X axis", 'accel.dat' using 0:2 with lines lt 2 title"Y axis", 'accel.dat' using 0:3 with lines lt 3 title"Z axis"

set terminal png
set output "myAccelData2d.png"
replot

pause -1 "Heres your graph\n\nPress enter..."
#    EOF

STEP 5 - 3D Plotting

This is a gnuPlot program that takes the collected sensor data and plots in 3D the X,Y and Z values measured by the accelerometer.

Download graphData3d.gp File (and save it without the txt extension)

#!/usr/bin/gnuplot -persist
#
#    
#     G N U P L O T
#     Version 4.6 patchlevel 0    last modified 2012-03-04 
#     Build System: Linux armv7l
#    
#     Copyright (C) 1986-1993, 1998, 2004, 2007-2012
#     Thomas Williams, Colin Kelley and many others
#    
#     gnuplot home:     http://www.gnuplot.info
#     faq, bugs, etc:   type "help FAQ"
#     immediate help:   type "help"  (plot window: hit 'h')
# set terminal wxt 0
# set output
reset
unset clip points
set clip one
unset clip two
set bar 1.000000 front
set border 31 front linetype -1 linewidth 1.000
set timefmt z "%d/%m/%y,%H:%M"
set zdata 
set timefmt y "%d/%m/%y,%H:%M"
set ydata 
set timefmt x "%d/%m/%y,%H:%M"
set xdata 
set timefmt cb "%d/%m/%y,%H:%M"
set timefmt y2 "%d/%m/%y,%H:%M"
set y2data 
set timefmt x2 "%d/%m/%y,%H:%M"
set x2data 
set boxwidth
set style fill  empty border
set style rectangle back fc lt -3 fillstyle   solid 1.00 border lt -1
set style circle radius graph 0.02, first 0, 0 
set style ellipse size graph 0.05, 0.03, first 0 angle 0 units xy
set dummy x,y
set format x "% g"
set format y "% g"
set format x2 "% g"
set format y2 "% g"
set format z "% g"
set format cb "% g"
set format r "% g"
set angles radians
set grid nopolar
set grid xtics nomxtics ytics nomytics noztics nomztics \
 nox2tics nomx2tics noy2tics nomy2tics nocbtics nomcbtics
set grid layerdefault   linetype 0 linewidth 1.000,  linetype 0 linewidth 1.000
set raxis
set key title ""
set key inside right top vertical Right noreverse enhanced autotitles nobox
set key noinvert samplen 4 spacing 1 width 0 height 0 
set key maxcolumns 0 maxrows 0
set key noopaque
unset label
unset arrow
set style increment default
unset style line
unset style arrow
set style histogram clustered gap 2 title  offset character 0, 0, 0
unset logscale
set offsets 0, 0, 0, 0
set pointsize 1
set pointintervalbox 1
set encoding default
unset polar
unset parametric
unset decimalsign
set view 60, 30, 1, 1
set samples 100, 100
set isosamples 10, 10
set surface
unset contour
set clabel '%8.3g'
set mapping cartesian
set datafile separator ","
unset hidden3d
set cntrparam order 4
set cntrparam linear
set cntrparam levels auto 5
set cntrparam points 5
set size ratio 0 1,1
set origin 0,0
set style data points
set style function lines
set xzeroaxis linetype -2 linewidth 1.000
set yzeroaxis linetype -2 linewidth 1.000
set zzeroaxis linetype -2 linewidth 1.000
set x2zeroaxis linetype -2 linewidth 1.000
set y2zeroaxis linetype -2 linewidth 1.000
set ticslevel 0.5
set mxtics default
set mytics default
set mztics default
set mx2tics default
set my2tics default
set mcbtics default
set xtics border in scale 1,0.5 mirror norotate  offset character 0, 0, 0 autojustify
set xtics autofreq  norangelimit
set ytics border in scale 1,0.5 mirror norotate  offset character 0, 0, 0 autojustify
set ytics autofreq  norangelimit
set ztics border in scale 1,0.5 nomirror norotate  offset character 0, 0, 0 autojustify
set ztics autofreq  norangelimit
set nox2tics
set noy2tics
set cbtics border in scale 1,0.5 mirror norotate  offset character 0, 0, 0 autojustify
set cbtics autofreq  norangelimit
set rtics axis in scale 1,0.5 nomirror norotate  offset character 0, 0, 0 autojustify
set rtics autofreq  norangelimit
set title "" 
set title  offset character 0, 0, 0 font "" norotate
set timestamp bottom 
set timestamp "" 
set timestamp  offset character 0, 0, 0 font "" norotate
set rrange [ * : * ] noreverse nowriteback
set trange [ * : * ] noreverse nowriteback
set urange [ * : * ] noreverse nowriteback
set vrange [ * : * ] noreverse nowriteback
set xlabel "" 
set xlabel  offset character 0, 0, 0 font "" textcolor lt -1 norotate
set x2label "" 
set x2label  offset character 0, 0, 0 font "" textcolor lt -1 norotate
set xrange [ -2 : 2 ] noreverse nowriteback
set x2range [ * : * ] noreverse nowriteback
set ylabel "" 
set ylabel  offset character 0, 0, 0 font "" textcolor lt -1 rotate by -270
set y2label "" 
set y2label  offset character 0, 0, 0 font "" textcolor lt -1 rotate by -270
set yrange [ -2 : 2 ] noreverse nowriteback
set y2range [ * : * ] noreverse nowriteback
set zlabel "" 
set zlabel  offset character 0, 0, 0 font "" textcolor lt -1 norotate
set zrange [ -2 : 2 ] noreverse nowriteback
set cblabel "" 
set cblabel  offset character 0, 0, 0 font "" textcolor lt -1 rotate by -270
set cbrange [ * : * ] noreverse nowriteback
set zero 1e-08
set lmargin  -1
set bmargin  -1
set rmargin  -1
set tmargin  -1
set locale "en_US.UTF-8"
set pm3d explicit at s
set pm3d scansautomatic
set pm3d interpolate 1,1 flush begin noftriangles nohidden3d corners2color mean
set palette positive nops_allcF maxcolors 0 gamma 1.5 color model RGB 
set palette rgbformulae 7, 5, 15
set colorbox default
set colorbox vertical origin screen 0.9, 0.2, 0 size screen 0.05, 0.6, 0 front bdefault
set style boxplot candles range  1.50 outliers pt 7 separation 1 labels auto unsorted
set loadpath 
set fontpath 
set psdir
set fit noerrorvariables
GNUTERM = "wxt"
splot 'accel.dat' u 1:2:3 with lines lt 1 title"3Axis in 3d"
pause -1 "Heres your graph\n\nPress enter..."
#    EOF

STEP 6 - Auto Measure and Plotting

Finally, we put everything together in a single file.

Create this file using:

nano autoPlot.sh

and enter the following:

#!/bin/bash
sudo python getAccel.py
./graphData.gp
./graphData3d.gp

Cntl+o, (enter), Cntl+x to write and exit the editor.

This bash file calls the getAccel.py program that acquires the sensor data, until Cntl+C is pressed. Then call the gnuPlot programs to plot the accelerometer data in 2D and next in 3D.

Before we can run this file, we need to make the scripts executable.

chmod a+x graphData.gp
chmod a+x graphData3d.gp
chmod a+x autoPlot.sh

STEP 7 - Running the Program

Start this program and move the sensor in the air (creating acceleration data).
This command will start collecting sensor data until you press Cntl+c, then plot the results.

In order to see the graphical output (graphs) you must access the BeagleBone via a VNC session, or directly connected to a monitor and keyboard.

./autoPlot.sh

Sample 2D Plot - Not Moving

Z-Axis is showing Earth gravity (acceleration 9.8m/s^2)

Sample 3D Plot - Not Moving

Single (tiny red) dot in Space (sensor not moving)

Sample 2D Plot - Spinning on itself

Sensor rotating on Axis (note shift in rotation/waves)

Sample 3D Plot - Spinning on itself

Free Spinning in space (hand-held)

Random Motion -2D

Random Motion -3D