Accelerometers

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Fremantle provides an accelerometer API. Currently<ref>[https://bugs.maemo.org/show_bug.cgi?id=4724 #4724] ''Lack of official documentation on how to use the accelerometer''</ref> there are two interfaces available:
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#REDIRECT [[N900 accelerometer]]
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# D-Bus
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# sysfs
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See also the [http://talk.maemo.org/showthread.php?p=288990 related thread] at talk.maemo.org.
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== D-Bus ==
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Thomas Thurman ([http://twitter.com/marnanel 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/
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== sysfs ==
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Another way is to use the sysfs file information:
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/sys/class/i2c-adapter/i2c-3/3-001d/coord
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When reading that file you get 3 values X, Y and Z (provided on one line, separated by white space). Values are in mG (milli G). 1000 = 1 G
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{| class="wikitable"
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|-
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! Position
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! X
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! Y
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! Z
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|-
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| Lying on table (back down)
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| 0
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| 0
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| -1000
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|-
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| Lying on table (face down)
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| 0
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| 0
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| 1000
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|-
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| Sitting on table (bottom edge down)
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| 0
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| -1000
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| 0
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|-
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| Sitting on table (right edge down)
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| -1000
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| 0
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| 0
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|-
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| Sitting on table (left edge down)
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| 1000
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| 0
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| 0
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|-
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| Bottom right corner down (approx.)
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| -500
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| -500
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| 0
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|}
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These are theoretical values. In real life your mileage will vary.
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== Using the data ==
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The ''X'' and ''Y'' values can be used to calculate<ref>Tom Pycke, ''[http://tom.pycke.be/mav/69/accelerometer-to-attitude Accelerometer to pitch and roll]''</ref> the roll (that is, clockwise rotation) using the ''atan2'' function (note the inverted sign of ''y''):
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  angle_in_radians = '''atan2'''(''x'', -''y'')
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Similar, ''Y'' and ''Z'' can be used to calculate the pitch.
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  angle_in_radians = '''atan2'''(''y'', -''z'')
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== Python ==
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Using the sysfs interface:
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<pre>
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  def get_rotation():
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    f = open("/sys/class/i2c-adapter/i2c-3/3-001d/coord", 'r' )
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    coords = [int(w) for w in f.readline().split()]
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    f.close()
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    return coords
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</pre>
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== Smoothed C interface ==
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This is a C function which returns smooth values from the accelerometer.
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Designed for applications running at >=25fps  (otherwise it lags)
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(GPL code extracted from libliqbase: liqaccel.c)
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<pre>
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static int ocnt=0;
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static int oax=0;
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static int oay=0;
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static int oaz=0;
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static const char *accel_filename = "/sys/class/i2c-adapter/i2c-3/3-001d/coord";
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int liqaccel_read(int *ax,int *ay,int *az)
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{
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FILE *fd;
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int rs;
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fd = fopen(accel_filename, "r");
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if(fd==NULL){ liqapp_log("liqaccel, cannot open for reading"); return -1;}
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rs=fscanf((FILE*) fd,"%i %i %i",ax,ay,az);
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fclose(fd);
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if(rs != 3){ liqapp_log("liqaccel, cannot read information"); return -2;}
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int bx=*ax;
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int by=*ay;
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int bz=*az;
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if(ocnt>0)
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{
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*ax=oax+(bx-oax)*0.1;
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*ay=oay+(by-oay)*0.1;
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*az=oaz+(bz-oaz)*0.1;
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}
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oax=*ax;
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oay=*ay;
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oaz=*az;
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ocnt++;
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return 0;
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}
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</pre>
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== References ==
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<small>
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<references/>
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</small>
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[[Category:Development]][[Category:Fremantle]]
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Latest revision as of 14:12, 16 June 2010

  1. REDIRECT N900 accelerometer
Retrieved from "http://wiki.maemo.org/Accelerometers"