Accelerometers
Fremantle provides an accelerometer API. Currently[1] there are two interfaces available:
- D-Bus
- sysfs
See also the related thread at 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 (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[2] 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.
angle_in_radians = atan2(y, -z)
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;
}
References
- ↑ #4724 Lack of official documentation on how to use the accelerometer
- ↑ Tom Pycke, Accelerometer to pitch and roll