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An accelerometer is a tool that measures proper acceleration.Extract of page 33
/ref> Proper acceleration is the (the rate of change of ) of a body in its own instantaneous rest frame;Extract of page 61
/ref> this is different from coordinate acceleration, which is acceleration in a fixed
coordinate system In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the Position (geometry), position of the Point (geometry), points or other geometric elements on a manifold such as Euclidean space ...

coordinate system
. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards (by definition) of g ≈ 9.81 m/s2. By contrast, accelerometers in (falling toward the center of the Earth at a rate of about 9.81 m/s2) will measure zero. Accelerometers have many uses in industry and science. Highly sensitive accelerometers are used in inertial navigation systems for aircraft and missiles. Vibration in rotating machines is monitored by accelerometers. They are used in tablet computers and digital cameras so that images on screens are always displayed upright. In unmanned aerial vehicles, accelerometers help to stabilise flight. 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 . Gravity gradiometry is useful because absolute gravity is a weak effect and depends on the local density of the Earth, which is quite variable. Single- and multi-axis accelerometers can detect both the magnitude and the direction of the proper acceleration, as a vector quantity, and can be used to sense orientation (because the direction of weight changes), coordinate acceleration, vibration, shock, and falling in a resistive medium (a case in which the proper acceleration changes, increasing from zero). Micromachined
microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
(MEMS) accelerometers are increasingly present in portable electronic devices and video-game controllers, to detect changes in the positions of these devices.


Physical principles

An accelerometer measures proper acceleration, which is the acceleration it experiences relative to freefall and is the acceleration felt by people and objects. Put another way, at any point in spacetime the equivalence principle guarantees the existence of a local inertial frame, and an accelerometer measures the acceleration relative to that frame. Such accelerations are popularly denoted ; i.e., in comparison to standard gravity. An accelerometer at rest relative to the Earth's surface will indicate approximately 1 g ''upwards'' because the Earth's surface exerts a normal force upwards relative to the local inertial frame (the frame of a freely falling object near the surface). To obtain the acceleration due to motion with respect to the Earth, this "gravity offset" must be subtracted and corrections made for effects caused by the Earth's rotation relative to the inertial frame. The reason for the appearance of a gravitational offset is Einstein's equivalence principle, which states that the effects of gravity on an object are indistinguishable from acceleration. When held fixed in a gravitational field by, for example, applying a ground reaction force or an equivalent upward thrust, the reference frame for an accelerometer (its own casing) accelerates upwards with respect to a free-falling reference frame. The effects of this acceleration are indistinguishable from any other acceleration experienced by the instrument so that an accelerometer cannot detect the difference between sitting in a rocket on the launch pad, and being in the same rocket in deep space while it uses its engines to accelerate at 1 g. For similar reasons, an accelerometer will read ''zero'' during any type of . This includes use in a coasting spaceship in deep space far from any mass, a spaceship orbiting the Earth, an airplane in a parabolic "zero-g" arc, or any free-fall in a vacuum. Another example is free-fall at a sufficiently high altitude that atmospheric effects can be neglected. 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. At terminal velocity, the accelerometer will indicate 1 g acceleration upwards. For the same reason a skydiver, upon reaching terminal velocity, does not feel as though he or she were in "free-fall", but rather experiences a feeling similar to being supported (at 1 g) on a "bed" of uprushing air. Acceleration is quantified in the SI unit metres per second per second (m/s2), in the cgs unit gal (Gal), or popularly in terms of standard gravity (''g''). 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. This can be obtained either by calibrating the device at rest, or from a known model of gravity at the approximate current position.


Structure

Conceptually, an accelerometer is a damped mass, a proof mass, on a spring. When the accelerometer experiences an acceleration, the mass is moved to the point that the spring can push (accelerate) the mass at the same speed as the casing. The measurement of the spring's compression measures the acceleration. The system is damped so that oscillations (wiggles) of the mass and spring do not affect the needed measurements. Because of the damping, accelerometers always respond in different ways to different frequencies of acceleration. This is called the "frequency response." Many animals have sensory organs to detect acceleration, especially gravity. 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. The hairs form the springs, with the neurons as sensors. The damping is usually by a fluid. Many vertebrates, including humans, have these structures in their inner ears. Most invertebrates have similar organs, but not as part of their hearing organs. These are called statocysts. Mechanical accelerometers are often designed so that an electronic circuit senses a small amount of motion, then pushes on the proof mass with some type of linear motor to keep the proof mass from moving far. The motor might be an or in very small accelerometers,
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word f ...
. Since the circuit's electronic behavior can be carefully designed, and the proof mass does not move far, these designs can be very stable (i.e. they do not oscillate), very linear with a controlled frequency response. (This is called servo mode design.) In mechanical accelerometers, measurement is often electrical, , piezoresistive or capacitive. Piezoelectric accelerometers use piezoceramic sensors (e.g. lead zirconate titanate) or single crystals (e.g.
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra ...

quartz
,
tourmaline Tourmaline ( ) is a crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in ...

tourmaline
). They are unmatched in high frequency measurements, low packaged weight, and resistance to high temperatures. Piezoresistive accelerometers resist shock (very high accelerations) better. Capacitive accelerometers typically use a silicon micro-machined sensing element. They measure low frequencies well. Modern mechanical accelerometers are often small ''micro-electro-mechanical systems'' ( MEMS), and are often very simple MEMS devices, consisting of little more than a with a proof mass (also known as ''seismic mass''). 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 external accelerations, 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, reliable, and inexpensive. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process steps are needed during the fabrication sequence. For very high sensitivities is also used; this requires a dedicated process making it very expensive. Optical measurement has been demonstrated in laboratory devices. Another MEMS-based accelerometer is a thermal (or ) accelerometer. It contains a small heater in a very small dome. This heats the air or other fluid inside the dome. The thermal bubble acts as the proof mass. An accompanying temperature sensor (like a thermistor; or thermopile) in the dome measures the temperature in one location of the dome. This measures the location of the heated bubble within the dome. When the dome is accelerated, the colder, higher density fluid pushes the heated bubble. The measured temperature changes. The temperature measurement is interpreted as acceleration. The fluid provides the damping. Gravity acting on the fluid provides the spring. Since the proof mass is very lightweight gas, and not held by a beam or lever, thermal accelerometers can survive high shocks. Another variation uses a wire to both heat the gas and detect the change in temperature. The change of temperature changes the resistance of the wire. A two dimensional accelerometer can be economically constructed with one dome, one bubble and two measurement devices. 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 another ''out-of-plane'' device, three axes can be measured. Such a combination may have 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 compromise between sensitivity and the maximum acceleration that can be measured.


Applications


Engineering

Accelerometers can be used to measure vehicle acceleration. Accelerometers can be used to measure
vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...

vibration
on cars, machines, buildings,
process control An industrial process control in Continuous production, continuous production processes is a discipline that uses industrial control systems to achieve a production level of consistency, economy and safety which could not be achieved purely by h ...
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 Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Units of measurement Gr ...
, are called
gravimeter Gravimetry is the measurement of the strength of a gravitational field In physics, a gravitational field is a scientific model, model used to explain the influences that a massive body extends into the space around itself, producing a force ...
s. Notebook computers equipped with accelerometers can contribute to the '' Quake-Catcher Network'' (QCN), a BOINC project aimed at scientific research of earthquakes.


Biology

Accelerometers are also increasingly used in the biological sciences. High frequency recordings of bi-axial or tri-axial acceleration 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 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. For example, accelerometers have been used to study flight energy expenditure of
Harris's Hawk The Harris's hawk (''Parabuteo unicinctus''), formerly known as the bay-winged hawk, dusky hawk, and sometimes a wolf hawk, and known in Latin America as peuco, is a medium-large bird of prey that breeds from the southwestern United States south ...

Harris's Hawk
(''Parabuteo unicinctus''). Researchers are also using smartphone accelerometers to collect and extract mechano-biological descriptors of resistance exercise. Increasingly, researchers are deploying accelerometers with additional technology, such as cameras or microphones, to better understand animal behaviour in the wild (for example, hunting behaviour of
Canada lynx The Canada lynx (''Lynx canadensis''), or Canadian lynx, is a medium-sized North American lynx that ranges across Alaska, Canada, and northern areas of the contiguous United States. It is characterized by its long, dense fur, triangular ears w ...

Canada lynx
).


Industry

Accelerometers are also used for machinery health monitoring to report the vibration and its changes in time of shafts at the bearings of rotating equipment such as turbines,
pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the metho ...

pump
s, fans, rollers,
compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...
s, or bearing fault which, if not attended to promptly, can lead to costly repairs. Accelerometer vibration data allows the user to monitor machines and detect these faults before the rotating equipment fails completely.


Building and structural monitoring

Accelerometers are used to measure the motion and vibration of a structure that is exposed to dynamic loads. Dynamic loads originate from a variety of sources including: * Human activities – walking, running, dancing or skipping * Working machines – inside a building or in the surrounding area * Construction work – driving piles, demolition, drilling and excavating * Moving loads on bridges * Vehicle collisions * Impact loads – falling debris * Concussion loads – internal and external explosions * Collapse of structural elements * Wind loads and wind gusts * Air blast pressure * Loss of support because of ground failure * Earthquakes and aftershocks Under structural applications, measuring and recording how a structure dynamically responds to these inputs is critical for assessing the safety and viability of a structure. This type of monitoring is called Health Monitoring, which usually involves other types of instruments, such as displacement sensors -Potentiometers, LVDTs, etc.- deformation sensors -Strain Gauges, Extensometers-, load sensors -Load Cells, Piezo-Electric Sensors- among others.


Medical applications

Zoll's Plus uses CPR-D•padz which contain an accelerometer to measure the depth of CPR chest compressions. Within the last several years, several 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. It has been suggested to build
football Football is a family of team sports that involve, to varying degrees, Kick (football), kicking a Football (ball), ball to score a Goal (sport), goal. Unqualified, Football (word), the word ''football'' normally means the form of football tha ...

football
helmets with accelerometers in order to measure the impact of head collisions. Accelerometers have been used to calculate gait parameters, such as stance and swing phase. This kind of sensor can be used to measure or monitor people.


Navigation

An inertial navigation system is a
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navi ...

navigation
aid that uses a computer and motion sensors (accelerometers) to continuously calculate via
dead reckoning In navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general c ...
the position, orientation, and (direction and speed of movement) of a moving object without the need for external references. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial reference platform, and many other variations. 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.


Transport

Accelerometers are used to detect
apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary (astronomy), primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two ...

apogee
in both professional and in amateur rocketry. Accelerometers are also being used in Intelligent Compaction rollers. Accelerometers are used alongside
gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...

gyroscope
s in inertial navigation systems. One of the most common uses for MEMS accelerometers is in
airbag An airbag is a vehicle occupant-restraint system using a bag designed to inflate extremely quickly, then quickly deflate during a collision. It consists of the airbag cushion, a flexible fabric bag, an inflation module, and an impact sensor. Th ...

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 Electronic stability control (ESC), also referred to as electronic stability program (ESP) or dynamic stability control (DSC), is a computerized technology that improves a car handling, vehicle's stability by detecting and reducing loss of Tract ...
systems, which use a lateral accelerometer to measure cornering forces. The widespread use of accelerometers in the automotive industry has dramatically. Another automotive application is the monitoring of
noise, vibration, and harshness Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks. While noise and vibration can be readily measured, ...
(NVH), conditions that cause discomfort for drivers and passengers and may also be indicators of mechanical faults.
Tilting train A tilting train is a train that has a mechanism enabling increased speed on regular rail tracks. As a train (or other vehicle) rounds a curve at speed, objects inside the train experience centrifugal force. This can cause packages to slide abo ...
s use accelerometers and gyroscopes to calculate the required tilt.


Volcanology

Modern electronic accelerometers are used in remote sensing devices intended for the monitoring of active
volcano A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Ear ...

volcano
es to detect the motion of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

magma
.


Consumer electronics

Accelerometers are increasingly being incorporated into personal electronic devices to detect the orientation of the device, for example, a display screen. A ''free-fall sensor'' (FFS) is an accelerometer used to detect if a system has been dropped and is falling. It can then apply safety measures such as parking the head of a
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...

hard disk
to prevent a head crash and resulting data loss upon impact. This device is included in the many common computer and consumer electronic products that are produced by a variety of manufacturers. It is also used in some
data logger A data logger (also datalogger or data recorder) is an electronic device that records data over time or about location either with a built-in instrument or sensor A sensor is a device that produces an output signal for the purpose of se ...

data logger
s to monitor handling operations for
shipping container A shipping container is a container with strength suitable to withstand shipment, storage, and handling. Shipping containers range from large reusable steel boxes used for intermodal shipments to the ubiquitous corrugated box design, corrugated b ...

shipping container
s. The length of time in free fall is used to calculate the height of drop and to estimate the shock to the package.


Motion input

Some
smartphone A smartphone is a Mobile device, portable computer device that combines Mobile phone, mobile telephone and Mobile computing, computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities ...

smartphone
s, digital audio players and
personal digital assistant A personal digital assistant (PDA), also known as a handheld PC, is a variety mobile device which functions as a personal information manager. PDAs have been mostly displaced by the widespread adoption of highly capable smartphones, in partic ...
s contain accelerometers for user interface control; often the accelerometer is used to present landscape or portrait views of the device's screen, based on the way the device is being held.
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wh ...
has included an accelerometer in every generation of ,
iPad The iPad is a brand of iOS and iPadOS-based tablet computers that are developed by Apple Inc., Apple Inc. The iPad was conceived before the related iPhone but the iPhone was developed and released first. Speculation about the development, ...

iPad
, and
iPod touch The iPod Touch (stylized as iPod touch) is a discontinued line of iOS-based mobile device, mobile devices designed and marketed by Apple Inc. with a Touchscreen, touchscreen-controlled user interface. As with other iPod models, the iPod Touch ...

iPod touch
, as well as in every
iPod nano The iPod Nano (stylised and marketed as iPod nano) is a discontinued portable media player A portable media player (PMP) (also including the related digital audio player (DAP)) is a portable consumer electronics device capable of storin ...
since the 4th generation. Along with orientation view adjustment, accelerometers in mobile devices can also be used as
pedometer A pedometer, or step-counter, is a device, usually portable and electronic or electromechanical In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including br ...

pedometer
s, in conjunction with specialized
applications Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a c ...
. Automatic Collision Notification (ACN) systems also use accelerometers in a system to call for help in event of a vehicle crash. Prominent ACN systems include
OnStar OnStar Corporation is a subsidiary of General Motors that provides subscription-based telecommunication, communications, in-vehicle security, emergency services, turn-by-turn navigation, and remote diagnostics systems throughout the United States, ...

OnStar
AACN service, Ford Link's 911 Assist, Toyota's Safety Connect, Lexus Link, or BMW Assist. Many accelerometer-equipped smartphones also have ACN software available for download. ACN systems are activated by detecting crash-strength accelerations. Accelerometers are used in vehicle
Electronic stability control Electronic stability control (ESC), also referred to as electronic stability program (ESP) or dynamic stability control (DSC), is a computerized technology that improves a car handling, vehicle's stability by detecting and reducing loss of Tract ...
systems to measure the vehicle's actual movement. 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. This can help prevent the vehicle from spinning or rolling over. Some
pedometer A pedometer, or step-counter, is a device, usually portable and electronic or electromechanical In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including br ...

pedometer
s use an accelerometer to more accurately measure the number of steps taken and distance traveled than a mechanical sensor can provide. Nintendo's
Wii The Wii ( ) is a home video game console A home video game console is a video game console that is designed to be connected to a display device, such as a television, and an external power source as to play video games. Home consoles ...

Wii
video game console uses a controller called a
Wii Remote The Wii Remote, also known Colloquialism, colloquially as the Wiimote, is the primary game controller for Nintendo's Wii home video game console. An essential capability of the Wii Remote is its Motion controller, motion sensing capability, which ...

Wii Remote
that contains a three-axis accelerometer and was designed primarily for motion input. Users also have the option of buying an additional motion-sensitive attachment, the
Nunchuk is a traditional Okinawan martial arts weapon consisting of two sticks (traditionally made of wood), connected to each other at their ends by a short metal chain or a rope. It is approximately 30 cm (sticks) and 1 inch (rope). A person wh ...

Nunchuk
, so that motion input could be recorded from both of the user's hands independently. Is also used on the
Nintendo 3DS The is a handheld game console produced by Nintendo. It was announced in March 2010 and unveiled at E3 2010 as the successor to the Nintendo DS. The system features backward compatibility with Nintendo DS video games. As an eighth-generation ...
system. The Sony
PlayStation 3 The PlayStation 3 (PS3) is a home video game console developed by Sony Interactive Entertainment, Sony Computer Entertainment. The successor to the PlayStation 2, it is part of the PlayStation brand of consoles. It was first released on Novemb ...
uses the
DualShock 3 The DualShock (originally Dual Shock; trademarked as DUALSHOCK or DUAL SHOCK; with the PlayStation 5 The PlayStation 5 (PS5) is a home video game console developed by Sony Interactive Entertainment. Announced as the successor to the PlayStat ...

DualShock 3
remote which uses a three axis accelerometer that can be used to make steering more realistic in racing games, such as '' MotorStorm'' and '' Burnout Paradise''. 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. Other uses for accelerometer in
Nokia Nokia Corporation (natively Nokia Oyj, referred to as Nokia) is a Finnish multinational corporation, multinational telecommunications industry, telecommunications, technology company, information technology, and consumer electronics corporatio ...

Nokia
phones include
Pedometer A pedometer, or step-counter, is a device, usually portable and electronic or electromechanical In engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including br ...

Pedometer
functionality in Nokia Sports Tracker. Some other devices provide the tilt sensing feature with a cheaper component, which is not a true accelerometer. Sleep phase
alarm clock An alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they ar ...

alarm clock
s use accelerometric sensors to detect movement of a sleeper, so that it can wake the person when he/she is not in REM phase, in order to awaken the person more easily.


Sound recording

A microphone or eardrum is a membrane that responds to oscillations in air pressure. These oscillations cause acceleration, so accelerometers can be used to record sound. A 2012 study found that voices can be detected by smartphone accelerometers in 93% of typical daily situations. Conversely, carefully designed sounds can cause accelerometers to report false data. One study tested 20 models of (MEMS) smartphone accelerometers and found that a majority were susceptible to this attack.


Orientation sensing

A number of 21st-century devices use accelerometers to align the screen depending on the direction the device is held (e.g., switching between portrait and landscape modes). Such devices include many
tablet PC A tablet computer, commonly shortened to tablet, is a mobile device, typically with a mobile operating system and touchscreen display processing circuitry, and a rechargeable battery in a single, thin and flat package. Tablets, being comput ...
s and some
smartphone A smartphone is a Mobile device, portable computer device that combines Mobile phone, mobile telephone and Mobile computing, computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities ...

smartphone
s and
digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...

digital camera
s. The Amida
Simputer The Simputer was a self-contained, open hardware Open-source hardware (OSH) consists of physical artifact (software development), artifacts of technology designed and offered by the open-design movement. Both free and open-source software (FO ...

Simputer
, a handheld Linux device launched in 2004, was the first commercial handheld to have a built-in accelerometer. It incorporated many gesture-based interactions using this accelerometer, including page-turning, zoom-in and zoom-out of images, change of portrait to landscape mode, and many simple gesture-based games. 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.


Image stabilization

Camcorders use accelerometers for
image stabilization Image stabilization (IS) is a family of techniques that reduce blurring associated with the motion of a camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced mode ...
, either by moving optical elements to adjust the light path to the sensor to cancel out unintended motions or digitally shifting the image to smooth out detected motion. Some stills cameras use accelerometers for anti-blur capturing. The camera holds off capturing the image when the camera is moving. When the camera is still (if only for a millisecond, as could be the case for vibration), the image is captured. An example of the application of this technology is the Glogger VS2, a phone application which runs on
Symbian Symbian is a discontinued mobile operating system (OS) and computing platform designed for smartphones. It was originally developed as a proprietary software OS for personal digital assistants in 1998 by the Symbian Ltd. consortium. Symbian OS ...
based phones with accelerometers such as the
Nokia N96 The Nokia N96 is a discontinued high-end smartphone A smartphone is a Mobile device, portable computer device that combines Mobile phone, mobile telephone and Mobile computing, computing functions into one unit. They are distinguished from ...
. Some digital cameras contain accelerometers to determine the orientation of the photo being taken and also for rotating the current picture when viewing.


Device integrity

Many laptops feature an accelerometer which is used to detect drops. If a drop is detected, the heads of the
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...

hard disk
are parked to avoid data loss and possible head or disk damage by the ensuing shock.


Gravimetry

A gravimeter or gravitometer, is an instrument used in
gravimetry Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Units of measurement Gr ...
for measuring the local . A gravimeter is a type of accelerometer, except that accelerometers are susceptible to all
vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...

vibration
s including
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...

noise
, that cause oscillatory accelerations. This is counteracted in the gravimeter by integral vibration isolation and
signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission med ...

signal processing
. Though the essential principle of design is the same as in accelerometers, gravimeters are typically designed to be much more sensitive than accelerometers in order to measure very tiny changes within the
Earth's gravity The gravity of Earth, denoted by , is the net force, net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a Eucli ...
, of 1 ''g''. In contrast, other accelerometers are often designed to measure 1000 ''g'' or more, and many perform multi-axial measurements. The constraints on
temporal resolution Temporal resolution (TR) refers to the discrete resolution of a measurement with respect to time. Physics Often there is a trade-off between the temporal resolution of a measurement and its spatial resolution, due to Heisenberg's uncertaint ...
are usually less for gravimeters, so that resolution can be increased by processing the output with a longer "time constant".


Types of accelerometer

* Bulk micromachined capacitive * Bulk micromachined piezoelectric resistive * Capacitive spring mass system base * DC response * Electromechanical servo (Servo Force Balance) * High gravity * High temperature * Laser accelerometer * Low frequency * Magnetic induction * Modally tuned impact hammers * Null-balance * Optical * Pendulous integrating gyroscopic accelerometer (PIGA) * Piezoelectric accelerometer * Quantum (Rubidium atom cloud, laser cooled) * Resonance * Seat pad accelerometers * Shear mode accelerometer *
Strain gauge A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports ...

Strain gauge
*
Surface acoustic wave A surface acoustic wave (SAW) is an acoustic wave Acoustic waves are a type of energy propagation through a medium by means of adiabatic loading and unloading. Important quantities for describing acoustic waves are acoustic pressure, particle ve ...
(SAW) * Surface micromachined capacitive ( MEMS) * Thermal (submicrometre
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOS ...
process) * Triaxial * * potentiometric type * LVDT type accelerometer


Exploits and privacy concerns

Accelerometer data, which can be accessed by third-party apps without user permission in many mobile devices, 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). If done without a user's knowledge or consent, this is referred to as an inference attack. Additionally, millions of
smartphone A smartphone is a Mobile device, portable computer device that combines Mobile phone, mobile telephone and Mobile computing, computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities ...

smartphone
s could be vulnerable to software cracking via accelerometers.


See also

* Accelerograph *
Degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variable Dependent and independent variables are variables in mathematical modeling, statistical model A statistical model is a mathematical model that emb ...
* *
Geophone A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The Standard deviation, deviation of this measured voltage from the base line is called the seismic response and is analyz ...

Geophone
*
Gyroscope A gyroscope (from Ancient Greek γῦρος ''gŷros'', "round" and σκοπέω ''skopéō'', "to look") is a device used for measuring or maintaining Orientation (geometry), orientation and angular velocity. It is a spinning wheel or disc in ...

Gyroscope
*
Inclinometer An inclinometer or clinometer is an measuring instrument, instrument used for measuring angles of slope, elevation, or depression (geology), depression of an object with respect to gravity's direction. It is also known as a ''tilt indicator'', ' ...

Inclinometer
*
Inertial measurement unit An inertial measurement unit (IMU) is an electronic device that measures and reports a body's specific force Specific force is defined as the non-gravitational force per unit mass. :\mbox = \frac Specific force (also called g-force and mass- ...
*
Inertial navigation system An inertial navigation system (INS) is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (direc ...
*
Magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, on ...
*
Seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, Types of volcanic eruptions, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to f ...

Seismometer
* Vibration calibrator


References

{{Reflist, 30em Acceleration