Jump to content

Accelerometer: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
mNo edit summary
Wolfkeeper (talk | contribs)
31,832 edits
(3 intermediate revisions by the same user not shown)
Line 1: Line 1:
{{Refimprove|date=October 2007}}
{{Refimprove|date=October 2007}}
[[Image:Accelerometer.png|thumb|A depiction of an accelerometer designed at Sandia National Laboratory.]]
[[Image:Accelerometer.png|thumb|A depiction of an accelerometer designed at Sandia National Laboratory.]]
An '''accelerometer''' is a device for measuring [[acceleration]] and gravity induced reaction forces.
An '''accelerometer''' is a device for measuring non-gravitational [[acceleration]].<ref name=Eshbach>Eshbach's Handbook of Engineering Fundamentals By Ovid W. Eshbach, Byron pg 9</ref>

Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a [[Euclidean vector|vector]] quantity,<ref name="analog.com">{{cite web | url = http://www.analog.com/en/technical-library/faqs/design-center/faqs/CU_faq_MEMs/resources/fca.html | accessdate = 2008-12-23 | title = Accelerometer Design and Applications | publisher = [[Analog Devices]] }}</ref> and can be used to sense orientation, vibration and shocks. Accelerometers are present in numerous portable electronic devices and video game controllers, including the [[iPhone]]<ref>{{cite web | url = http://www.apple.com/iphone/features/accelerometer.html | title = Accelerometer. Made to move. | publisher = [[Apple Inc.]] | accessdate = 2008-12-23 }} </ref>, the ipod nano 4G and Nintendo's [[Wii]] Remote.<ref>{{cite video |title=Gyroscopes and Accelerometers Compared |url=http://invensense.com/support/FLVPlayer_Progressive.swf?skinName=Halo_Skin_3&streamName=../Library/InvenSense5_VP6_512K |format=Flash Video |publisher=InvenSense |accessdate=2008-07-17 |time=00:00:37 }}</ref>
Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a [[Euclidean vector|vector]] quantity,<ref name="analog.com">{{cite web | url = http://www.analog.com/en/technical-library/faqs/design-center/faqs/CU_faq_MEMs/resources/fca.html | accessdate = 2008-12-23 | title = Accelerometer Design and Applications | publisher = [[Analog Devices]] }}</ref> and can be used to sense orientation, vibration and shocks. Accelerometers are present in numerous portable electronic devices and video game controllers, including the [[iPhone]]<ref>{{cite web | url = http://www.apple.com/iphone/features/accelerometer.html | title = Accelerometer. Made to move. | publisher = [[Apple Inc.]] | accessdate = 2008-12-23 }} </ref>, the ipod nano 4G and Nintendo's [[Wii]] Remote.<ref>{{cite video |title=Gyroscopes and Accelerometers Compared |url=http://invensense.com/support/FLVPlayer_Progressive.swf?skinName=Halo_Skin_3&streamName=../Library/InvenSense5_VP6_512K |format=Flash Video |publisher=InvenSense |accessdate=2008-07-17 |time=00:00:37 }}</ref>


==Physical principles==
==Physical principles==
An accelerometer measures the [[acceleration]] and gravity it experiences.
An accelerometer measures the non-gravitational [[acceleration]] it experiences.<ref name=Eshbach/> Both are typically expressed in SI units [[metre per second squared|meters/second<sup>2]] (m/s<sup>2</sup>) or popularly in terms of [[g-force]].
Both are typically expressed in SI units [[metre per second squared|meters/second<sup>2]] (m/s<sup>2</sup>) or popularly in terms of [[g-force]].


The effects of gravity and acceleration are indistinguishable, following [[Equivalence principle|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.
The effects of gravity and acceleration are indistinguishable, following [[Equivalence principle|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.

Revision as of 05:58, 21 January 2009

File:Accelerometer.png
A depiction of an accelerometer designed at Sandia National Laboratory.

An accelerometer is a device for measuring non-gravitational acceleration.[1]

Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a vector quantity,[2] and can be used to sense orientation, vibration and shocks. Accelerometers are present in numerous portable electronic devices and video game controllers, including the iPhone[3], the ipod nano 4G and Nintendo's Wii Remote.[4]

Physical principles

An accelerometer measures the non-gravitational acceleration it experiences.[1] 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 by automatically calibrating the device at rest.[2]

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 steps are needed during the fabrication sequence. For very high sensitivities quantum tunnelling is also used; this requires a dedicated process making it very 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

In engineering

Accelerometers can be used to measure vehicle acceleration and deceleration. They allow for performance evaluation of both the engine/drive train and the braking systems[citation needed]. Useful numbers like 0-60mph, 60-0mph and 1/4 mile times can all be found using accelerometers.

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[5]

Accelerometers are also increasingly used in the Biological Sciences. High frequency recordings of bi-axial[6] or tri-axial acceleration[7] (>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[8] or measures such as Overall Dynamic Body Acceleration[9] 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.

Machinery Health Monitoring

Accelerometers are also used for machinery health monitoring of rotating equipment such as pumps,[10] fans,[11] rollers,[12] compressors,[13] and cooling towers,[14]. Vibration monitoring programs are proven to save money, reduce downtime, and improve safety in plants worldwide by detecting conditions such as shaft misalignment, rotor imbalance, gear failure[15] or bearing fault[16] which can lead to costly repairs. Accelerometer vibration data allows the user to monitor machines and detect these faults before the rotating equipment fails. Vibration monitoring programs are utilized in industries such as automotive manufacturing,[17] machine tool applications,[18] pharmaceutical production,[19] power generation[20] and power plants,[21] pulp and paper, [22] food and beverage production, water and wastewater, hydropower, petrochemical and steel manufacturing.

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.[23][24]

Transport

Accelerometers are used to detect apogee in both professional[25] and in amateur[26] rocketry.

Accelerometers are also being used in Intelligent Compaction rollers. Accelerometers are used alongside gyroscopes in inertial guidance systems.[27]

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.[28]

Tilting trains use accelerometers and gyroscopes to calculate the required tilt.[29]

Electronic devices

Many laptops feature an accelerometer, such as Lenovo's (formerly IBM's) Active Protection System, and 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 relevant in Tablet PCs and some smartphones and digital cameras[citation needed].

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.[30]

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 N82 have accelerometers embedded inside them. It was primarily used as a tilt sensor for tagging the orientation to photos taken with the built-in camera, later thanks to a firmware update it became possible to use it in other applications. Some other devices provide the tilt sensing feature with a cheaper component, which is not a true accelerometer.

The HTC Touch Pro, HTC Touch Diamond, Sony Ericsson G705, Sony Ericsson W595, Sony Ericsson W910, Sony Ericsson W902, Sony Ericsson K850i and Sony Ericsson C905 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, W595, W902 and 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 W705.

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[31], 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.

Types of accelerometers

References

  1. ^ a b Eshbach's Handbook of Engineering Fundamentals By Ovid W. Eshbach, Byron pg 9
  2. ^ a b "Accelerometer Design and Applications". Analog Devices. Retrieved 2008-12-23.
  3. ^ "Accelerometer. Made to move". Apple Inc. Retrieved 2008-12-23.
  4. ^ Gyroscopes and Accelerometers Compared (Flash Video). InvenSense. Event occurs at 00:00:37. Retrieved 2008-07-17.
  5. ^ http://qcn.stanford.edu/
  6. ^ Yoda et al. (2001) Journal of Experimental Biology204(4): 685-690
  7. ^ Shepard et al (2008) Endangered Species Research http://www.int-res.com/articles/esr2008/theme/Tracking/TMVpp1.pdf
  8. ^ Kawabe et al. (2001) Fisheries Science 69 (5):959 - 965
  9. ^ Wilson et al. (2006) Journal of Animal Ecology:75 (5):1081 - 1090
  10. ^ http://www.wilcoxon.com/knowdesk/Know%20the%20health%20of%20your%20pumps.pdf
  11. ^ http://www.wilcoxon.com/knowdesk/Guidance%20for%20mounting%204-20mA%20sensors%20on%20fans.pdf
  12. ^ http://www.wilcoxon.com/knowdesk/Vibration%20monitoring%20of%20slow%20speed%20rollers.pdf
  13. ^ http://www.wilcoxon.com/knowdesk/LF%20VM%20on%20compressor%20gear%20set.pdf
  14. ^ http://www.wilcoxon.com/knowdesk/PT104%20VM%20of%20cooling%20towers%20and%20fans.pdf
  15. ^ http://www.wilcoxon.com/knowdesk/gear.pdf
  16. ^ http://www.wilcoxon.com/knowdesk/bearing.pdf
  17. ^ http://www.wilcoxon.com/knowdesk/auto.pdf
  18. ^ http://www.wilcoxon.com/knowdesk/rep11.pdf
  19. ^ http://www.wilcoxon.com/knowdesk/pharmac.pdf
  20. ^ http://www.wilcoxon.com/knowdesk/rep9.pdf
  21. ^ http://www.wilcoxon.com/knowdesk/pwrplnt.pdf
  22. ^ http://www.wilcoxon.com/knowdesk/pulp_pap.pdf
  23. ^ The Contender 3 Episode 1 SPARQ testing ESPN
  24. ^ http://www.goherman.com/martialarts.aspx
  25. ^ http://westrocketry.com/articles/DualDeploy/DualDeployment.html
  26. ^ http://www.picoalt.com/
  27. ^ "Design of an integrated strapdown guidance and control system for a tactical missile" WILLIAMS, D. E.RICHMAN, J.FRIEDLAND, B. (Singer Co., Kearfott Div., Little Falls, NJ) AIAA-1983-2169 IN: Guidance and Control Conference, Gatlinburg, TN, August 15-17, 1983, Collection of Technical Papers (A83-41659 19-63). New York, American Institute of Aeronautics and Astronautics, 1983, p. 57-66.
  28. ^ http://mafija.fmf.uni-lj.si/seminar/files/2007_2008/MEMS_accelerometers-koncna.pdf
  29. ^ http://www.memagazine.org/backissues/membersonly/june98/features/tilting/tilting.html
  30. ^ http://blog.medallia.com/2007/08/fun_with_the_iphone_accelerome.html
  31. ^ Glogger
  32. ^ a b c IMI - Industrial Monitoring Instrumentation

See also

Retrieved from "https://en.wikipedia.org/w/index.php?title=Accelerometer&oldid=265436024"