N900 accelerometer
m (Categories) |
(minor tweak) |
||
| Line 6: | Line 6: | ||
It features: | It features: | ||
* Low power consumption | * Low power consumption | ||
| - | * Dynamically selectable 2/8G | + | * Dynamically selectable 2/8G full-scale. |
* Programmable multiple interrupt generator | * Programmable multiple interrupt generator | ||
** Click and double click recognition | ** Click and double click recognition | ||
| Line 12: | Line 12: | ||
The click and double click functionality are not implemented in the current software. | The click and double click functionality are not implemented in the current software. | ||
| + | |||
| + | The accelerometer is uncalibrated - the sensitivity may vary by around 10% plus or minus per-axis. Ideally this would be able to be calibrated by the user. | ||
| + | It is possible to recover the bias and sensitivity by rotating the phone several times, and then finding the best mapping of the resulting ellipsoid to a sphere centered around zero. | ||
The absolute resolution is poor - 18mg (at 2g sensitivity) typical. | The absolute resolution is poor - 18mg (at 2g sensitivity) typical. | ||
| - | This is fine for detecting the orientation of the phone, but makes most uses that would attempt to integrate the | + | This is fine for detecting the orientation of the phone, but makes most uses that would attempt to integrate the acceleration for detection of position useless. |
| + | |||
| + | At the fastest sampling speed, the noise is enough to dither the output, so the resolution is somewhat higher. | ||
For example - with a 9mg error - there is an accelleration error of 9 centimeters per meter per second. | For example - with a 9mg error - there is an accelleration error of 9 centimeters per meter per second. | ||
Revision as of 15:09, 5 May 2010
The accelerometer in the phone is a LIS302DL.
This is supported by the dbus/sysfs infrastructure Accelerometers.
It features:
- Low power consumption
- Dynamically selectable 2/8G full-scale.
- Programmable multiple interrupt generator
- Click and double click recognition
- Zero G detection
The click and double click functionality are not implemented in the current software.
The accelerometer is uncalibrated - the sensitivity may vary by around 10% plus or minus per-axis. Ideally this would be able to be calibrated by the user. It is possible to recover the bias and sensitivity by rotating the phone several times, and then finding the best mapping of the resulting ellipsoid to a sphere centered around zero.
The absolute resolution is poor - 18mg (at 2g sensitivity) typical. This is fine for detecting the orientation of the phone, but makes most uses that would attempt to integrate the acceleration for detection of position useless.
At the fastest sampling speed, the noise is enough to dither the output, so the resolution is somewhat higher.
For example - with a 9mg error - there is an accelleration error of 9 centimeters per meter per second.
This means that after 10 seconds, the velocity may be anywhere within a 1.8m/s (around 4MPH) band, and the position inaccurate by 4.5m. After 100s, the position inaccuracy is up to 450m. (this assumes the orientation of the phone is known exactly)