Linear Acceleration Sensor in Smartphones and Tablets

The linear acceleration measures the acceleration effect of the device movement, excluding the effect of Earth's gravity on the device. It is typically derived from the accelerometer, where other sensors (e.g. the magnetometer and the gyroscope) help to remove linear acceleration from the data. Linear acceleration units are shown in m/s2 like the accelerometer.

A simple method to determine the linear acceleration using only a three-axes accelerometer is to approximate the gravity sensor readings using a low-pass filter on the accelerometer data, and then subtracting the gravity measurements from the accelerometer full measurement. Better linear acceleratin measurements are obtained when the gravity measurements are enhanced with sensor-fusion following our discussion of the modern gravity sensor. You can use the experiment below to verify the importance of sensor-fusion in measuring linear acceleration.

When used in conjunction with a hand-held device, the device is most of the time stationary, and cannot sustain continuous accelaration in one direction (unless the user travels within a vehicle that is subjected to wild accelerations). Therefore, the linear acceleration components in the accelerometer measurement are transient in nature, with a tendencies to return to 0.

##### Use the Sensor Kinetics app to learn about the Linear Acceleration sensor

Sensor Kinetics includes a unique 3d viewer to demonstrate the linear acceleration concept. It shows a ball suspended in the center of a box with virtual springs. Any linear movement in any direction will cause the ball to move in the movement direction proportionally to the linear acceleration. Once acceleration is stopped (e.g. during constant movement) notice that the ball returns to the center of the box. The following image illustrates the iPhone 3d viewer for linear acceleration sensor:

A similiar viewer is available for the Android version of Sensor Kinetics. The following screen capture was made on a 7" tablet:

Sensor Kinetics displays realtime charts for the three components of the linear acceleration along the x,y,z axes of the device. The linear acceleration charts can be viewed in either portrait or landscape mode.

You can compare the linear acceleration sensor and the accelerometer as follows:

Hold the device face up and make "up and down" movements. The accelerometer's raw readings will show Earth's gravity plus variation on the Z axis due to the movement. The linear acceleration sensor reading will show acceleration only when the device is moved. Earth's gravity effect is zeroed out.

A better demonstration can be made with the Chart Viewer in the Sensor Kinetics app. Run the Chart Viewer by tapping the title of the linear acceleration sensor. Start the chart and slowly rotate (tilt) your device to all directions without making lateral movements. You can see only small changes recorded in the chart. Repeat the same with the accelerometer sensor and you will see that similar rotations record significant readings in the accelerometer chart due to Earth's gravity.

##### Use the RotoView Photo Viewer app to compare detection of a shake with Linear Acceleration sensor or with an Accelerometer

In our RotoView technology, we detect device shaking along the Z axis to activate and deactivate scrolling. RotoView attempts to use the best available sensor for shake detection, so if your device has a Linear Acceleration sensor, it will be automatically selected. However, you can experiment with an accelerometer-only shake detection using the following Setting selection in the RotoView Photo Viewer app:

You will be able to see for yourself that the linear acceleration sensor is somewhat more accurate in detecting device shakes.