Older version of Android devices implement accelerometer and magnetometer to monitor the motion of the device. Accelerometer measures the linear acceleration of the device relative to the gravitational acceleration along one dimension, while magnetometer measures direction and strength of magnetic field along one dimension (like a compass). However, the use of magnetometer in motion sensor has become obsolete because earth magnetic field is weaker than common disturbances around such as steel furniture, so it is not effective in reading the lateral orientation of the device. Thus, Android devices now impelement gyroscope as a part of their motion sensors. Gyroscope measures the angular velocity of the device along one rotational axis.
The combination of accelerometer and gyroscope creates a powerful motion sensor. In a single Android motion sensor device there are 3 axis-perpendicular mounted accelerometers and 3 rotational-axis mounted gyroscopes. This motion sensor is applicable in health management apps such as movement tracking. Movement such as walking, running, and jumping can easily be recognized by the sensor based on the value of the acceleration (read by the accelerometer) and the angular velocity (read by the gyroscope) of the device.
Accelerometers use the standard sensor coordination system that applies the following conditions when a device is laying flat on a table in its natural orientation:
- If it is pushed to the right, the x-axis acceleration value is positive.
- If it is pushed away, the y-axis acceleration value is positive.
- If it is pushed towards the sky with an acceleration of A m/s2, the z-axis acceleration value is equal to A + 9.81 (A minus the force of gravity which is -9.81 m/s2).
- If it is not pushed at all, the z-axis acceleration value is equal to 9.81 m/s2.
Gyroscopes use the same coordinate system as accelerometers, whereas it applies on all axis that the rotation is positive if the device is rotated counter-clockwise.
In conclusion, current technology of motion sensors implements the use of accelerometer and gyroscope in monitoring the motion. The use of gyroscope is more effective than magnetometer because gyroscope can work on any enviroment without getting disturbed. Based on the reading of the value from the accelerometer and gyroscope, the device can pinpoint the exact motion of it.