Accelerometers

Fremantle offers the possibility to play with accelerometers. There is a plan to offer a proper interface for accelerometers in Maemo, but if you want to try out before there are a couple of ways to do it.

See also the related thread in talk.maemo.org.

Contents

D-Bus

Thomas Thurman (marnanel) has put together a simple demo of an application using accelerometers using the D-Bus interface. You can find sources and .deb up at http://people.collabora.co.uk/~tthurman/sandcastle/

sysfs

Another way is to use the sysfs file information. 

/sys/class/i2c-adapter/i2c-3/3-001d/coord

When reading that file you get 3 values X, Y and Z. Values are in mG (milli G). 1000 = 1 G

  • On the table face up values are 0, 0, -1000 (face down would read 0, 0, 1000)
  • On the table on botton side (landscape): 0, -1000, 0
  • On the table on right side (portrait): -1000, 0, 0
  • In any other position any mix of those 3 values.

These are theoretical values. In real life your mileage will vary.

Using the data

The X and Y values can be used to calculate[1] the roll (that is, clockwise rotation) using the atan2 function: 

 angle_in_radians = atan2(x, y)

Similar, Y and Z can be used to calculate the pitch.

Python

Simple example {{{ 

 def get_rotation():
   try:
       f = open("/sys/class/i2c-adapter/i2c-3/3-001d/coord", 'r' )
       input=f.readline( )
       f.close()
       coords = input.split(' ')
       x=0
       y=0
       z=0
       try:
           x=int(coords[0])
           x=int(coords[1])
           z=int(coords[2])
       except:
           return (0,0,0)
       return (x,y,z)
   except:
       return (0,0,0)

}}}

The above is probably far from optimal, so please fix if you know how to read the data faster.

References

  1. ↑ Tom Pycke, Accelerometer to pitch and roll

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