Principles of Motion Sensing

InvenSense MotionTracking device illustrating  linear acceleration and tilt angle and angular rate of rotation about one or more axes

Various sensors capable of detecting motion in free space have been commercially available for several decades and have been used in automobiles, aircraft and ships. Initial size, power consumption and price, however, prevented their mass adoption in consumer electronics until the past few years.

While there are other motion sensor technologies available, the following four fundamental motion sensors are the most relevant for tracking motion for consumer electronics:

Accelerometers (G-sensors)

Accelerometers measure linear acceleration and tilt angle. Single and multi-axis accelerometers detect the combined magnitude and direction of linear and gravitational acceleration. They can be used to provide limited motion sensing functionality. For example, a device with an accelerometer can detect movement from a vertical to horizontal state in a fixed location. As a result, accelerometers are primarily used for sensing device orientation with respect to gravity, and delivering simple functions, such as changing the screen on a mobile device from portrait to landscape mode.

Gyroscopes (Gyros)

Gyroscopes measure the angular rate of rotation about one or more axes. Gyroscopes can measure complex motions accurately in free space, hence, making it a required motion sensor for tracking the position and rotation of a moving object. Unlike accelerometers and compasses, gyroscopes are not dependent on any external forces such as gravity or magnetic fields, and can therefore function fairly autonomously.

Magnetic Sensors (E-Compasses)

Compasses are used to detect heading based on the Earth’s magnetic field. Consumer electronics applications for eCompasses include correctly orienting a down-loaded map on a mobile screen or providing basic heading information for navigation applications. As the earth’s magnetic field is relatively weak compared to magnetic interference from electronic equipment and building materials, the compass sensor output can easily be impacted by varying environmental conditions particularly indoor. As such, eCompasses require frequent calibration in order to maintain their heading accuracy.

Pressure Sensors (Barometers)

Pressure sensors measure relative and absolute altitude through the analysis of changes in the atmospheric pressure. Pressure sensors are being used in consumer devices for sports and fitness, and for location-based applications where map information can be adjusted as a consumer moves to different floors in a building.

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