An accelerometer is a device that measures proper acceleration (“g-force”). Proper acceleration is not the same as coordinate acceleration (rate of change of velocity). For example, an accelerometer at rest on the surface of the Earth will measure an acceleration g= 9.81 m/s2 straight upwards. By contrast, accelerometers in free fall orbiting and accelerating due to the gravity of Earth will measure zero.
Accelerometers have multiple applications in industry and science. Highly sensitive accelerometers are components of inertial navigation systems for aircraft and missiles. Accelerometers are used to detect and monitor vibration in rotating machinery. Accelerometers are used in tablet computers and digital cameras, so screen images are always displayed upright. Accelerometers are used in drones for flight stabilization. Pairs of accelerometers extended over a region of space can be used to detect differences (gradients) in the proper accelerations of frames of references associated with those points.
These devices are called gravity gradiometers, as they measure gradients in the gravitational field. In theory, such pairs of accelerometers may also detect gravitational waves. Single- and multi-axis models of the accelerometer are available to detect the magnitude and direction of the proper acceleration (or g-force) as a vector quantity. They can be used to sense orientation (because the direction of weight changes), coordinate acceleration (so long as it produces g-force or a shift in g-force), vibration, shock, and falling in a resistive medium (a case where the proper acceleration changes, since it starts at zero, then increases).
Micromachined accelerometers are increasingly present in portable electronic devices and video game controllers to detect the device’s position or provide game input.