Accelerometers

(→Using the data)
(→Using the data)
Line 63: Line 63:
   angle_in_radians = '''atan2'''(''x'', -''y'')
   angle_in_radians = '''atan2'''(''x'', -''y'')
-
Similar, ''Y'' and ''Z'' can be used to calculate the pitch.
+
Similar, ''Y'' and ''Z'' can be used to calculate the pitch:
   angle_in_radians = '''atan2'''(''y'', -''z'')
   angle_in_radians = '''atan2'''(''y'', -''z'')

Revision as of 13:03, 18 January 2010

Fremantle provides an accelerometer API. Currently[1] there are two interfaces available:

  1. D-Bus
  2. 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

  1. ↑ #4724 Lack of official documentation on how to use the accelerometer
  2. ↑ Tom Pycke, Accelerometer to pitch and roll

Retrieved from "http://wiki.maemo.org/Accelerometers"