AskDefine | Define accelerometers

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Noun

accelerometers
  1. Plural of accelerometer

Extensive Definition

An accelerometer is a device for measuring acceleration and gravity induced reaction forces. Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a vector quantity. Accelerometers can be used to sense inclination, vibration, and shock. They are increasingly present in portable electronic devices.

Physical principles

An accelerometer measures the acceleration and gravity it experiences. Both are typically expressed in SI units meters/second2 (m·s-2) or popularly in terms of g-force.
The effects of gravity and acceleration are indistinguishable, following Einstein's equivalence principle. As a consequence, the output of an accelerometer has an offset due to local gravity. This means that, perhaps counterintuitively, an accelerometer at rest on the earth's surface will actually indicate 1 g along the vertical axis. To obtain the acceleration due to motion alone, this offset must be subtracted. Along all horizontal directions, the device yields acceleration directly. Conversely, the device's output will zero during free fall, where the acceleration exactly follows gravity. This includes use in an earth orbiting spaceship, but not a (non-free) fall with air resistance, where drag forces reduce the acceleration until terminal velocity is reached, at which point the device would once again indicate the 1 g vertical offset.
For the practical purpose of finding the acceleration of objects with respect to the earth, such as for use in an inertial navigation system, the correction due to gravity along the vertical axis is usually made automatically, e.g. by calibrating the device at rest.

Structure

Modern accelerometers are often small micro electro-mechanical systems (MEMS), and are indeed the simplest MEMS devices possible, consisting of little more than a cantilever beam with a proof mass (also known as seismic mass) and some type of deflection sensing circuitry. Under the influence of gravity or acceleration the proof mass deflects from its neutral position. The deflection is measured in an analog or digital manner. Another type of MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor.
Single-axis, dual-axis, and triple-axis models exist to measure acceleration as a vector quantity or just one or more of its components. MEMS accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of gs.

Uses

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. Applications for accelerometers that measure gravity, wherein an accelerometer is specifically configured for use in gravimetry, are called gravimeters.
Accelerometers are increasingly being incorporated into personal electronic devices such as media players, gaming devices, or step counters. Smartphones and personal digital assistants (such as HTC Touch Diamond, Apple's iPhone, iPod Touch and the Nokia N95) contain accelerometers for user interface control, e.g., switching between portrait and landscape modes. Apple's laptops since 2005 feature an accelerometer known as Sudden Motion Sensor, which is used to protect against hard disk crashes in the event of shock. In game controllers such as the Wii Remote accelerometers may provide realistic game control. The Sony Ericsson W910i also features a 3-axis accelerometre, which can be used in gaming, switching between landscape and portrait in the interface and its well known 'shake control feature' to change song tracks in its WALKMAN player.
One of the most common uses for MEMS accelerometers is in airbag deployment systems for modern automobiles. In this case the accelerometers are used to detect the rapid negative acceleration of the vehicle to determine when a collision has occurred and the severity of the collision. The widespread use of accelerometers in the automotive industry has pushed their cost down dramatically.
Accelerometers may be used alongside gyroscopes in inertial guidance systems. Those may be constructed with only two accelerometers instead of the usual three when gyroscopes are available.

Types of accelerometers

Accelerometers are used in rocketry to detect apogee.
Herman Digital Trainer uses accelerometers to measure strike force in physical training (The Contender 3 Episode 1 SPARQ testing ESPN).
Accelerometers can be used to calculate vehicle acceleration and deceleration. They allow for performance evaluation of both the engine/drive train and the braking systems. Useful numbers like 0-60mph, 60-0mph and 1/4 mile times can all be found using accelerometers. Tazzo Motorsports and G-Tech have taken this technology and packaged it into a convenient self contained dash mounted unit that can be added to a vehicle without any modifications.
Also used on the PlayStation 3 SIXAXIS controller

See also

accelerometers in Danish: Accelerometer
accelerometers in German: Beschleunigungssensor
accelerometers in Estonian: Kiirendusandur
accelerometers in Spanish: Acelerómetro
accelerometers in French: Accéléromètre
accelerometers in Croatian: Mjerač ubrzanja
accelerometers in Italian: Accelerometro
accelerometers in Dutch: Versnellingsmeter
accelerometers in Japanese: 加速度計
accelerometers in Polish: Przyspieszeniomierz
accelerometers in Portuguese: Acelerômetro
accelerometers in Russian: Акселерометр
accelerometers in Slovak: Akcelerometer
accelerometers in Swedish: Accelerometer
accelerometers in Ukrainian: Акселерометр
accelerometers in Chinese: 加速規
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