Accelerometers

Fremantle provides an accelerometer API. Currently there are two interfaces available:

1. D-Bus
2. sysfs

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

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 (provided on one line, separated by white space). Values are in mG (milli G). 1000 = 1 G

Position	X	Y	Z
Lying on table (back down)	0	0	-1000
Lying on table (face down)	0	0	1000
Sitting on table (bottom edge down)	0	-1000	0
Sitting on table (right edge down)	-1000	0	0
Sitting on table (left edge down)	1000	0	0
Bottom right corner down (approx.)	-500	-500	0

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

Using the data

The X and Y values can be used to calculate the roll (that is, clockwise rotation) using the atan2 function (note the inverted sign of y):

 angle_in_radians = atan2(x, -y)

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

Python

Using the sysfs interface:

  def get_rotation():
    f = open("/sys/class/i2c-adapter/i2c-3/3-001d/coord", 'r' )
    coords = [int(w) for w in f.readline().split()]
    f.close()
    return coords

Smoothed C interface

This is a C function which returns smooth values from the accelerometer. Designed for applications running at >=25fps (otherwise it lags)

(GPL code extracted from libliqbase: liqaccel.c)

static int ocnt=0;
static int oax=0;
static int oay=0;
static int oaz=0;
	
static const char *accel_filename = "/sys/class/i2c-adapter/i2c-3/3-001d/coord";

int liqaccel_read(int *ax,int *ay,int *az)
{
	FILE *fd;
	int rs;
	fd = fopen(accel_filename, "r");
	if(fd==NULL){ liqapp_log("liqaccel, cannot open for reading"); return -1;}	
	rs=fscanf((FILE*) fd,"%i %i %i",ax,ay,az);	
	fclose(fd);	
	if(rs != 3){ liqapp_log("liqaccel, cannot read information"); return -2;}
	int bx=*ax;
	int by=*ay;
	int bz=*az;
	if(ocnt>0)
	{
		*ax=oax+(bx-oax)*0.1;
		*ay=oay+(by-oay)*0.1;
		*az=oaz+(bz-oaz)*0.1;
	}
	oax=*ax;
	oay=*ay;
	oaz=*az;
	ocnt++;
	return 0;
}