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Accelerometer

Device that measures proper acceleration

An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change of velocity) of the object relative to an observer who is in free fall (that is, relative to an inertial frame of reference). Proper acceleration is different from coordinate acceleration, which is acceleration with respect to a given coordinate system, which may or may not be accelerating. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity straight upwards of about g ≈ 9.81 m/s2. By contrast, an accelerometer that is in free fall will measure zero acceleration. Highly sensitive accelerometers are used in inertial navigation systems for aircraft and missiles. In unmanned aerial vehicles, accelerometers help to stabilize flight. Micromachined micro-electromechanical systems (MEMS) accelerometers are used in handheld electronic devices such as smartphones, cameras and video-game controllers to detect movement and orientation of these devices. Vibration in industrial machinery is monitored by accelerometers. Seismometers are sensitive accelerometers for monitoring ground movement such as earthquakes. When two or more accelerometers are coordinated with one another, they can measure differences in proper acceleration, particularly gravity, over their separation in space—that is, the gradient of the gravitational field. Gravity gradiometry is useful because absolute gravity is a weak effect and depends on the local density of the Earth, which is quite variable. A single-axis accelerometer measures acceleration along a specified axis. A multi-axis accelerometer detects both the magnitude and the direction of the proper acceleration, as a vector quantity, and is usually implemented as several single-axis accelerometers oriented along different axes.