Accelerometer
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.
The reason for the appearance of a gravitational offset is Einstein's equivalence principle,[5] which states that the effects of gravity on an object are indistinguishable from acceleration.
However, this does not include a (non-free) fall in which air resistance produces drag forces that reduce the acceleration until constant terminal velocity is reached.
For the practical purpose of finding the acceleration of objects with respect to the Earth, such as for use in an inertial navigation system, a knowledge of local gravity is required.
In these, the proof mass is usually one or more crystals of calcium carbonate otoliths (Latin for "ear stone") or statoconia, acting against a bed of hairs connected to neurons.
Piezoelectric accelerometers use piezoceramic sensors (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline).
For very high sensitivities quantum tunnelling is also used; this requires a dedicated process making it very expensive.
Since the proof mass is very lightweight gas, and not held by a beam or lever, thermal accelerometers can survive high shocks.
Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die.
Accelerometers can be used to measure vibration on cars, machines, buildings, process control systems and safety installations.
They can also be used to measure seismic activity, inclination, machine vibration, dynamic distance and speed with or without the influence of gravity.
High frequency recordings of bi-axial[8] or tri-axial acceleration[9] allows the discrimination of behavioral patterns while animals are out of sight.
Furthermore, recordings of acceleration allow researchers to quantify the rate at which an animal is expending energy in the wild, by either determination of limb-stroke frequency[10] or measures such as overall dynamic body acceleration[11] Such approaches have mostly been adopted by marine scientists due to an inability to study animals in the wild using visual observations, however an increasing number of terrestrial biologists are adopting similar approaches.
Accelerometer vibration data allows the user to monitor machines and detect these faults before the rotating equipment fails completely.
[21][22] It has been suggested to build football helmets with accelerometers in order to measure the impact of head collisions.
An accelerometer alone is unsuitable to determine changes in altitude over distances where the vertical decrease of gravity is significant, such as for aircraft and rockets.
In the presence of a gravitational gradient, the calibration and data reduction process is numerically unstable.
Another common automotive use is in electronic stability control systems, which use a lateral accelerometer to measure cornering forces.
[31] Another automotive application is the monitoring of noise, vibration, and harshness (NVH), conditions that cause discomfort for drivers and passengers and may also be indicators of mechanical faults.
[32] Modern electronic accelerometers are used in remote sensing devices intended for the monitoring of active volcanoes to detect the motion of magma.
This device is included in the many common computer and consumer electronic products that are produced by a variety of manufacturers.
Along with orientation view adjustment, accelerometers in mobile devices can also be used as pedometers, in conjunction with specialized applications.
A computer compares the vehicle's actual movement to the driver's steering and throttle input.
The stability control computer can selectively brake individual wheels and/or reduce engine power to minimize the difference between driver input and the vehicle's actual movement.
Some pedometers use an accelerometer to more accurately measure the number of steps taken and distance traveled than a mechanical sensor can provide.
[36] A 2012 study found that voices can be detected by smartphone accelerometers in 93% of typical daily situations.
One study tested 20 models of (MEMS) smartphone accelerometers and found that a majority were susceptible to this attack.
As of January 2009, almost all new mobile phones and digital cameras contain at least a tilt sensor and sometimes an accelerometer for the purpose of auto image rotation, motion-sensitive mini-games, and correcting shake when taking photographs.
Some digital cameras contain accelerometers to determine the orientation of the photo being taken and also for rotating the current picture when viewing.
Accelerometer data, which can be accessed by third-party apps without user permission in many mobile devices,[41] has been used to infer rich information about users based on the recorded motion patterns (e.g., driving behavior, level of intoxication, age, gender, touchscreen inputs, geographic location).
