Accelerometer

What is an accelerometer?
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/second (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 g's.

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 Apple's iPhone and 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.

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

Applications

Zoll's AED Plus uses CPR-D•padz which contain an accelerometer to measure the depth of CPR chest compressions.

The Wii Remote for the Wii game console contains three accelerometers to sense three dimensions of movement and tilt to complement its pointer functionality.

Within the last several years, Nike, Polar and other companies have produced and marketed sports watches for runners that include footpods, containing accelerometers to help determine the speed and distance for the runner wearing the unit.

Apple Inc also uses an LIS302DL accelerometer in the iPhone and iPod Touch allowing the device to know when it is tilted on its side. Third-party developers have expanded its use with fanciful applications such as electronic bobbleheads.

The Nokia 5500 sport features a 3D accelerometer that can be accessed from software. It is used for step recognition (counting) in a sport application, and for tap gesture recognition in the user interface. Tap gestures can be used for controlling the music player and the sport application, for example to change to next song by tapping through clothing when the device is in a pocket. The Nokia N95 and Nokia N70 have accelerometers embedded inside them. The N70's is only used as a tilt sensor for tagging the orientation to photos taken with the built-in camera, while the N95's is also accessible by software thanks to a recent update, with applications using the accelerometer beginning to appear from Nokia and third parties. Some other devices provide the tilt sensing feature with a cheaper component, which is not a real accelerometer.

The Sony Ericsson W580 and Sony Ericsson W910 also have an accelerometer built inside the phone that enables Track Switching on music player known by users as the Shaker Feature but the W910 along with K850 can use the motion sensor feature in gaming, Picture UI AutoRotation and many other applications that require the feature and can be accessible via J2ME application. The first phone from the company to feature an accelerometer was the Sony Ericsson W710.

A small number of modern notebook computers feature accelerometers to automatically align the screen depending on the direction the device is held. This feature is only relevant in Tablet PCs and some smartphones. Some notebooks use an accelerometer to detect drops and prepare the hard disk drive for impact, to avoid data loss.

Camcorders use accelerometers for image stabilization. Still cameras use accelerometers for anti-blur capturing. The camera holds off snapping the CCD "shutter" when the camera is moving. When the camera is still (if only for a millisecond, as could be the case for vibration), the CCD is "snapped". An example application which has used such technology is the Glogger VS2, a phone application which runs on Symbian OS based phone with accelerometer such as Nokia N95. Some digital cameras, such as Canon's PowerShot and Ixus range contain accelerometers to determine the orientation of the photo being taken and also for rotating the current picture when viewing.

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.
Source: Wikipedia

 
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