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Mina Gerges
WRIT 340, Martha Townsend
Illumin Article, Portfolio draft
1 May 2012
Accelerate into the 21st Century: The Growing Popularity of Accelerometers in Diverse Applications
Accelerometers are devices that measure the acceleration force of the object they are attached to. They
convert a physical force to an electrical signal which can be processed by computers to indicate the
acceleration, velocity, and/or position of the object. Since their initial development in 1880, advances in
accelerometers have made them smaller, lighter, and tolerant of higher temperatures and acceleration
levels. Although their intended use was for measurement in engineering applications such as airplane
testing, they have grown to satisfy a need in videogames, smart phones and even in the health industry.
Since accelerometers can easily be integrated in diverse applications, their usage is likely to grow in the
world of social networking and artificial intelligent robots.
Introduction
Rolling on your side, twitching your legs, or even flinging your arms above your head during your sleep
can be detected and recorded. This is the case for iPhone users who have purchased one of the latest
trending applications, “Sleep Cycle Alarm Clock”. When you sleep, you go through a range of phases
from deep sleep to light sleep. During light sleep, you usually toss and turn, but during deep sleep, you
rarely move any part of your body. The phase you are in when you wake up greatly determines how
rested you feel. With the help of an embedded accelerometer, this iPhone application monitors and
analyzes sleep patterns and wakes you up during the lightest sleep phase for the best possible rested
feeling. The ability to monitor such extremely sensitive motions, and thus to create such an application,
is only possible thanks to the technology of accelerometers.
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Accelerometers are devices that measure the acceleration force of the object they are attached to. They
have been very useful in making smart phone applications more exciting and stimulating for users.
Additionally, accelerometers are used not only in iPhones, but also in the medical and aerospace
industry. The significance and growing use throughout many industries of accelerometers is illustrated
here through their background, functionality, current and future applications.
Technical History
The development of the technology behind the accelerometer began as early as 1880, when Pierre and
Jacques Curie experimented with the properties of a crystal [1]. After many experiments, they began to
understand how the crystal structure reacts with different forces. This led to their popular theory,
known as the piezoelectric effect, which is now utilized to make most modern accelerometers. The
piezoelectric effect is the generation of an electric charge when a crystal is subjected to a force or
pressure. The first accelerometer was developed and built in the 1920s by McCollum and Peters [2]. It
was almost two pounds and used simple electrical components such as resistors instead of crystals to
measure acceleration. Within the next decade, this instrument spread to eight overseas countries,
including Germany, and found a niche for measuring acceleration in applications such as airplane
catapults, passenger elevators, aircraft shock absorbers, and vibrations of steam turbines. Since crystals
weigh less and tolerate a higher temperature range than resistors, the use of crystals was still of great
interest. However, the use of crystals in accelerometers did not successfully transition from theory to
reality until the 1940s [1]. This only became possible due to the advancement of technology that
permitted engineers to amplify the signals from the crystals. In the 1950s, the technology of
amplification and insulation materials drastically improved the piezoelectric effect and enabled the
commercialization of accelerometers with crystals. In 1964, there was a need to measure vibrations for
the Apollo mission. This required more research and development to build accelerometers that could
tolerate the requirements of the Apollo voyage including incredibly high temperatures [2]. As the need
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for accelerometers increased, the research and development of advanced products also increased.
Accelerometers were produced to answer customer requirements including smaller accelerometers,
higher accuracy, and/or higher ranges. In 1988, Endevco, an accelerometer company, released an
accelerometer than could handle temperatures up to 1400o F and acceleration up to 100,000g, a force
100,000 times the force of gravity [2]. Within the past decade, the use of accelerometers grew from
strictly engineering applications to entertainment applications such as smart phones and video games.
Under the Hood
Materials Used for Accelerometers
Accelerometers can utilize many different materials in measuring acceleration. The three most common
materials are crystals, resistors, and capacitors, as shown in Fig. 1.
Figure 1: Three Most Commonly Used Materials in Accelerometers. Modified Source: http://4.bp.blogspot.com/Owu4F4pAroo/T0SjHxOwnaI/AAAAAAAABGE/eFljUP7dRf8/s1600/Quartz-Crystal-Cluster1.jpg
The most popular accelerometers use crystal structures that produce a voltage when they experience
acceleration forces [3]. This is the most popular kind because these accelerometers weigh less and can
tolerate a much higher temperature range. A less popular method includes the use of resistors, rather
than a change of voltage, to indicate acceleration. Another method uses capacitors, devices that
measure the storage of voltage. If the capacitor moves by an acceleration force, the energy stored in
the capacitor will change. The capacitance change indicates the level of acceleration. All three methods
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measure acceleration, but only crystal can produce a voltage, whereas the resistor and capacitor can
only measure the voltage produced.
How Accelerometers Work
Piezoelectric accelerometers behave as a system with a mass and a spring (Fig.2). As an accelerometer
experiences an acceleration force, the mass (a material with mass such as plastic or ceramic) inside the
accelerometers pushes on the crystal [3]. The spring is attached to the mass and keeps it in line with the
crystal. Then the crystals produce a certain amount of energy proportional to the amount of
acceleration that was generated. A physical force (the accelerometer moving) is thus converted into an
electrical signal (voltage). The electrical signal is sent to a data processor, a computer for example, that
filters the data. There is a low frequency filter, which only accepts low frequencies such as a slowly
swinging pendulum, and a high frequency filter, which accepts high frequencies such as the vibration
from a mechanical toothbrush or a jackhammer. The purpose of using a filter is to get rid of unwanted
data (or noise) that distorts the desired data. After the data is filtered, it has to be processed. If the
acceleration of an object is known, and the time it took to accelerate is also known, mathematical
integration can be used to find the object’s velocity and position.
Figure 2: The Inside of an Accelerometer. Modified Source: http://www.data-acquisition.us/images/10_117.jpg
Current Applications
Accelerometers are versatile devices that can be used for many different applications from
entertainment to mechanical testing and even to helping the disabled. Some popular current
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applications include use of accelerometers in smart phones, videogames, airplane testing, structural
monitoring, and muscle stimulators.
Smart Phones
Accelerometers are embedded inside the iPhone to provide a more interactive experience for the user.
The most basic functionality of the accelerometers in the iPhone is to detect which way the screen is
being held. Depending on what is on the screen, the user might prefer a landscape or portrait view. As
the user turns the iPhone vertically or sideways, the accelerometer senses this tilt and quickly sends a
signal to change the visual graphics (Fig.3) [5].
Figure 3. iPhone Screen Tilting. Source: http://news.cnet.com/i/bto/20090528/apple-iphone-accelerometer-small.jpg
Additionally, the accelerometer can be utilized in many other impressive ways. Application developers
have taken advantage of accelerometers to make their applications more interactive and fun for the
user. One interesting use is to create music by simply moving the iPhone around. When the iPhone is
lying flat on its back, the accelerometer yields a zero value for forces in all three directions (x,y, and z).
The user can change the x and y coordinates of the iPhone by moving it to create musical notes. As the
phone is moved, it produces a force, and each force value is programmed with a certain pitch [6]. Tilt
and TouchOSC are applications that utilize accelerometers to control music production on the iPhone
which can also be downloaded directly to the user’s computer. These iPhone applications can be
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downloaded from the App store not only to produce interesting music but to also experience the power
of accelerometers and their fast response to user movement.
Nike has developed an application to go along with its Nike+ Fuelband that monitors data that might be
interesting to a runner (Fig.4). The embedded accelerometer in the Fuelband can track the user’s every
move and outputs the number of steps taken, calories burned, and time spent [7]. The user can then use
the iPhone application to upload their statistics and the application automatically creates a timeline of
activity rate throughout the day. The innovation behind this product enables the customer to measure
performance and set goals while using exciting technology and a stylish wristband.
Figure 4. Nike+ Fuelband & iPhone application. Source: http://www.contagiousmagazine.com/magazine/upload/motherone.jpg
Accelerometers are also used to enhance the capability of the built-in features of the iPhone. With the
5-megapixel camera, the iPhone has made it easy to take pictures on the go. Night Camera and Moon
Lighter are iPhone applications that use the accelerometer to trigger the shutter when it detects the
iPhone is not accelerating [6]. This helps take sharper photos at any light condition.
Accelerometers only measure acceleration forces, and on their own, do not necessarily add any benefit
to the iPhone. It is the imaginative ability of the developers to harness the basic power of
accelerometers in order to create innovative features that makes smart phones so engrossing and
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enjoyable for users. Nevertheless, none of these applications would be possible without the use of
accelerometers.
Videogames
The use of accelerometers has even expanded to gaming consoles for more user interaction. The times
of simply pressing buttons on a game controller are almost gone. With some games, the user has to
actually move his or her arms, legs, or even the entire body in order to play the game. For example, the
Wii joystick utilizes the accelerometer’s capabilities to sense the tilt and movement of the user [5]. Thus
the user can use his hand movements to interact with the game, making the game much more realistic
and interesting. Nintendo heavily advertises this feature to encourage users to be active while playing
videogames. A user can control a sword’s every movement with his or her Wii controller simply by
moving the controller as if it were the sword. The movements are recorded by the accelerometer and
processed by the Wii’s computer; the results are then portrayed on the screen. This breakthrough in the
gaming industry is proving to be an excellent way to engage gamers, especially older generations. More
parents play the Wii with their kids because the hand motions are much more natural and intuitive than
pressing a series of button combinations on the controller.
Airplane Testing
On May 25th, 2002, structural failure of the tail section of a Boeing 747 caused 225 deaths. The ability to
thoroughly test airplanes for any possible failure is of critical importance to the airline industry and its
flyers for reasons like this. One type of airplane failure is known as flutter. This occurs when the
frequency of the airplane during flight matches its natural frequency when it is not moving (the way an
airplane vibrates due to its structure). When flutter occurs, the airplane experiences increasing levels of
vibration until it reduces speed, flies at a lower altitude, or loses a wing or tail. Reducing speed or flying
at a lower altitude is very difficult to do because the pilot does not have enough time to react. It can
take as little as 5 seconds from the start of flutter to the destruction of a tail of wing. Prior to the 1950s,
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there was no system to test for flutter, engineers were not concerned with flutter because airplanes at
that time did not try to fly particularly fast or high (this is where flutter usually occurs). Now, the testing
of this powerful failure is possible due to accelerometers. In this test, more than two hundred
accelerometers are mounted onto the airplane to monitor the airplane’s response (Fig. 5).
Airplane surface
Shaker
Accelerometer
Figure 5. Accelerometer on the Surface of a Plane (left), a Ground Vibration Test (right). Modified Source:
http://www.lmsintl.com/_resources/2009100209394851832.jpg
To test for natural frequencies, a shaker (seen in Fig.5 in the blue box) vibrates the airplane at a high
frequency, similar to a mechanical toothbrush. The accelerometers are mapped so that they cover all
outside surfaces. The signals from the accelerometers are directed by wires to computers that convert
the data into a frequency. The engineer is then able to tell the natural frequencies of the airplane. At
this point, only one half of the data is known. Flutter occurs when the natural frequency matches the
frequency caused by flight conditions, thus frequencies also have to be measured during test flights.
With the help of the accelerometers, a safe flight envelope is established where flutter will not occur.
Various flights are flown at different speeds and different altitudes to establish this envelope. The
accelerometers used for flight testing are incredibly small (0.25 cubic inch.) and lightweight [4].
Accelerometers are thus reliable devices that allow engineers to build safe airplanes. Without these
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small devices, airplanes might not have been able to push the boundary of the flight envelope. Airplanes
now can fly higher and faster, and thus more efficiently, while never crossing their safety limit.
Structural Monitoring
Another interesting application of accelerometers is to monitor the health of machines such as fans,
turbines, and pumps. The accelerometer can keep track of the forces on the bearings of machines in
order to determine the strength of the mechanical components, instead of using an estimated time
frame to determine when to replace machines. This is a more accurate process that relies on the actual
condition of the machine, rather than a predetermined timeframe that might result in the unnecessary
replacement of a functioning machine. This process saves money and improves safety by detecting
failures before they occur. In addition, accelerometers are used in bridges to monitor the structural
health of the beams. When earthquakes occur, the structural integrity of the beam is precisely known
because the acceleration levels inside the beams are known. This allows the prediction of any bridge
failures and can allow the engineers to retrofit the beams if needed.
Muscle Stimulator
Accelerometers are also used in the health industry as a neuromuscular stimulator [8]. If a person only
has nerves working in one leg, for example, the use of an accelerometer can help this person walk or
even run. The accelerometer is worn on the healthy leg to sense whether the leg is moving up or down.
Depending on the direction measured, the appropriate electronic signal is sent to stimulate the other
foot to flex. For example, as the healthy leg moves down, a mild shock is sent to the nerves of the other
leg to move it up. This application is the result of the technological abilities of the accelerometers as well
as the desire of engineers to expand and explore the many unique uses of accelerometers. The
capabilities of accelerometers extend to biological applications and can help disadvantaged or injured
people regain their lost abilities.
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Future Applications
Accelerometers are multipurpose transducers that have found functionality in numerous applications
and fields. However, some areas of implementation have been neglected. For example, the integration
of accelerometer data and desktop applications has not been fully explored. Perhaps the motion paths
from an accelerometer could be imported to Photoshop to create designs [6]. There is also a huge
opportunity for integrating accelerometer data with social networking. Perhaps a mobile device’s
motions could be used to call a particular person in the phonebook, or programmed to open the
Facebook application. Lastly, televisions and mobile devices are transitioning to 3D technology. With the
help of the accelerometer, a user could potentially visualize objects in 3D on the iPhone screen,
depending on the phone’s orientation to the object. Thus, as the user turns and rotates the iPhone, the
physical object might appear differently [6].
Accelerometers might potentially expand in their applications to include artificially intelligent robots in
the near future. With the ability to detect motion paths and process them, accelerometers could help
robots know how fast to move or even how much force they have to exert to pick up a cup of water.
Try It Yourself
Instructables.com has a full tutorial on how to convert a console game pad into a USB keyboard mouse
with a built in accelerometer [9]. This is an easy and quick way to familiarize yourself with
accelerometers. This project calls for a game pad, a teensy, a tri-accelerometer, a USB cable, and a
ribbon cable. The step by step instruction guide can be found here:
http://www.instructables.com/id/USB-Game-Pad-With-Tilt-accelerometer-Mouse/
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About the Author
Mina Gerges is a senior studying Aerospace Engineering at the University of Southern California. He
currently works for Boeing and will be pursuing a M.S. in stress analysis.
References
[1] G. Kulwanooski, (2004, February 1). The Principles of Piezoelectric Accelerometers [Online]. Available:
http://www.sensorsmag.com/sensors/acceleration-vibration/the-principles-piezoelectricaccelerometers-1022
[2] P. Walter. The History of the Accelerometer. Available:
http://www.sandv.com/downloads/0701walt.pdf
[3] Dimension Engineering, (2012, January 21). A Beginner's guide to accelerometers [Online]. Available:
http://www.dimensionengineering.com/accelerometers.htm
[4] Aerospace & Defense Division. "Aerospace Flight Test" PCB Piezotronics Catalog. Available:
http://www.pcb.com/Linked_Documents/Aerospace/AD_FlightTest_LowRes.pdf
[5] How Stuff Works. What does the iPhone accelerometer do?[Online].
Available: http://electronics.howstuffworks.com/iphone-accelerometer.htm
[6] F. Visnjic, (2009, January 13). 10 Creative Ways to Use the Accelerometer [iPhone][Online].
Available: http://www.creativeapplications.net/iphone/10-creative-ways-to-use-the-accelerometeriphone
[7] L. McGregor, (2012, January 21). Nike unveils the next generation of accelerometer with Nike+
Fuelband[Online]. Available: http://recombu.com/news/nike-unveils-the-next-generation-ofaccelerometer-with-nike-fuelband_M16532.html
[8] H. Weinberg. Accelerometers- Fantasy & Reality. Available:
http://www.analog.com/library/analogDialogue/archives/43-05/accelerometer.html
[9] Adafruit . USB Game Pad with Tilt-accelerometer Mouse. Available:
http://www.instructables.com/id/USB-Game-Pad-With-Tilt-accelerometer-Mouse/