An accelerometer is a sensor for testing the acceleration along a given axis. When a physical body accelerates at a certain direction, it becomes subject to a force equal to:
F = ma
in accordance with Newton's Second Law. In this formula, m is the mass, a is the acceleration. Therefore, accelerometers are built on the principle of measuring the force exerted on a test body of a known mass along a given axis.
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The following drawing schematically shows the structure of an accelerometer.
In Newton's day, accelerometers where built using a test mass (shown in red) held at rest with springs and having a scale showing the acceleration along the sensitivity axis. Note that the unit g is equal to the acceleration subject to all bodies at the surface of the earth due to gravity, and is equal to about 9.8 meter/second². The same gravity is the acceleration that translates our body mass to a weight we can measure when we stand on a scale.
In the early 1950s, accelerometers were used in inertial
navigation systems, and their structure has been modernized to include an easy
electronic interface, and to replace the springs with magnetic forces. In the
early 1990s, a new generation of MEMS devices integrated the accelerometer into
a single silicon structure. With modern MEMS technology, the sensors are easily included in miniature electronic boards, like in RotoView.
The accelerometer can detect movement based on double integration of the measured acceleration and addition of the initial position and speed. However, since the Earth exerts a gravity acceleration on all bodies, we can also use the accelerometer to measure tilt.
When the sensitivity axis points directly to the center of the Earth, it measures 1g (assuming no additional hand acceleration in this direction). When the accelerometer sensitivity axis lies parallel to the surface of the Earth, it measures 0 acceleration.
The actual tilt angle may be inferred with the following formula:
Tilt Angle = ArcSin (measured acceleration / 1g).
When using an accelerometer to measure the tilt of an hand-held device, the movements of the hand create additional accelerations components which distort the exact calculation of the tilts. Therefore, RotoView ballistics algorithms are used to allow easy and intuitive view navigation, as you can experiment with this development system.