Definitions

bi-axial

Accelerometer

[ak-sel-uh-rom-i-ter]

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 and video game controllers.

Physical principles

An accelerometer measures the acceleration and gravity it experiences. Both are typically expressed in SI units meters/second2 (m/s2) 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 counter-intuitively, 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 be 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). Mechanically the accelerometer behaves as a mass-damper-spring system; the damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not result in a lower sensitivity.

Under the influence of gravity or acceleration the proof mass deflects from its neutral position. This deflection is measured in an analog or digital manner. Most commonly the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple and reliable; it also does not require additional process steps making it inexpensive. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process are needed. For very high sensitivities quantum tunneling is also used; this requires specific fabrication steps making it more expensive. Optical measurement has been demonstrated on laboratory scale.

Another, far less common, 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.

Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.

Micromechanical accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of g's. The designer must make a compromise between sensitivity and the maximal acceleration that can be measured.

Applications

Science and Engineering

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.

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.

Notebooks equipped with accelerometers can to contribute to the Quake-Catcher Network. QCN is a BOINC project aimed at scientific research of earthquakes.

Accelerometers are also increasingly used in the Biological Sciences. High frequency recordings of bi-axial or tri-axial acceleration (>10 Hz) allows the discrimination of behavioural 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 or measures such as Overall Dynamic Body Acceleration. 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.

Medical applications

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

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.

In Belgium, accelerometer-based step counters are promoted by the government to encourage people to walk a few thousand steps each day.

Herman Digital Trainer uses accelerometers to measure strike force in physical training.

Transport

Accelerometers are used to detect apogee in both professional and in amateur rocketry.

Accelerometers are used alongside gyroscopes in inertial guidance systems.

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. Another common automotive use is in electronic stability control systems, which use a lateral accelerometer to measure cornering forces. The widespread use of accelerometers in the automotive industry has pushed their cost down dramatically.

Tilting trains use accelerometers and gyroscopes to calculate the required tilt.

Electronic devices

Many laptops feature an accelerometer, such as Apple's Sudden Motion Sensor, which is used to detect drops. If a drop is detected, the heads of the hard disk are parked to avoid data loss by the ensuing shock.

Accelerometers are increasingly being incorporated into personal electronic devices such as media players and gaming devices. Some smartphones and personal digital assistants contain accelerometers for user interface control. The Wii Remote for the Wii game console contains a three-axis accelerometer from Analog Devices to sense movement which complements its pointer functionality. This provides more realistic game control.

A number of modern notebook computers feature accelerometers to automatically align the screen depending on the direction the device is held, i.e. switching between portrait and landscape modes. This feature is only relevant in Tablet PCs and some smartphones.

For example, Apple uses an LIS302DL accelerometer in the iPhone, iPod Touch and the 4th generation iPod Nano 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 N73 have accelerometers embedded inside them. The N73'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 true accelerometer.

The Sony Ericsson W580, Sony Ericsson W910, Sony Ericsson K850i and Sony Ericsson C902 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.

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.

Also, Apple's latest addition to the iPod family, the iPod Touch has an accelerometer to detect your motions during any of the numerous games featured on the iPod touch to move your character, car, or person.

Types of accelerometers

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