A depiction of an accelerometer
designed at Sandia National Laboratories.
An
accelerometer measures the
proper acceleration it experiences
relative to freefall.Single- and multi-axis models are available to
detect magnitude and direction of the acceleration as a
vector quantity, and can be used to sense
orientation, vibration and shock.
Micromachined
accelerometers are increasingly present in portable electronic
devices and video game controllers, to detect the orientation of
the device or provide for game input.
Physical principles
An accelerometer measures the
proper
acceleration it experiences relative to freefall. Put another
way, at any point in spacetime the
equivalence principle guarantees the
existence of a locally
inertial
frame, and an accelerometer measures the acceleration relative
to that frame.
As a consequence an accelerometer at rest relative to the Earth's
surface will indicate approximately 1 g
upwards, because
any point on the earth's surface is accelerating upwards relative
to a locally inertial frame. To obtain the acceleration due to
motion with respect to the earth, this "gravity offset" should be
subtracted. No such subtraction is necessary along horizontal
directions.
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 of the reference frame. 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 effect of this
reference frame acceleration is indistinguishable from any other
acceleration experienced by the instrument.
An accelerometer will read
zero during
free fall.This includes use in a spaceship
orbiting earth, 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 1 g acceleration
upwards.
Acceleration is quantifiedin the
SI unit
metres per second per second
(m/s
2),in the
cgs unit
gal (Gal),or popularly in terms of
g-force (
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 behaves as a damped mass on a
spring. When the accelerometer experiences an acceleration, the
mass is displaced to the point that the spring is able to
accelerate the mass at the same rate as the casing. The
displacement is then measured to give the acceleration.
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). 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
quantum tunneling 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 maximum acceleration that can be
measured.
Applications
In engineering
Accelerometers can be used to measure vehicle acceleration. 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.
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 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. This device can be
connected to an amplifier to amplify the signal.
Machinery health monitoring
Accelerometers are also used for machinery health monitoring of
rotating equipment such as pumps, fans, rollers, compressors, and
cooling towers,. 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 or bearing fault 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, machine tool applications, pharmaceutical
production, power generation and power plants, pulp and paper, food
and beverage production, water and wastewater, hydropower,
petrochemical and steel manufacturing.
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
- Earthquake.
Measuring and recording how a structure responds to these inputs is
critical for assessing the safety and viability of a structure.
This type of monitoring is called Dynamic Monitoring.
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.
Navigation
An
Inertial Navigation System (INS) is a
navigation aid that uses a
computer and motion sensors (accelerometers) to
continuously calculate via
dead
reckoning the position, orientation, and
velocity (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 in
both professional and in amateur rocketry.
Accelerometers are also being used in Intelligent Compaction
rollers. 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. Another automotive application is the
monitoring of
noise,
vibration and harshness (NVH), conditions that cause discomfort
for drivers and passengers and may also be indicators of mechanical
faults.
Tilting trains use accelerometers and
gyroscopes to calculate the required tilt.
Consumer electronics
Accelerometers are increasingly being incorporated into personal
electronic devices. Some
smartphones,
Digital audio players and
personal digital
assistants contain accelerometers for user interface control.
Prominent examples includethe
Apple iPhone,
Samsung Omnia,
Samsung Omnia HD,
Samsung innov8,
Nokia
N96,
Nokia 5800,
Sony Ericsson W910i,
Palm Pre,
Blackberry
Storm,
HTC Touch Diamond,
HTC Dream, Microsoft
Zune HD,
Apple iPod touch
and
Apple iPod Nano 5G.
Nintendo's
Wii video game console uses a
controller called a
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, so
that motion input could be recorded from both of the user's hands
independently.
The Sony
PlayStation 3 uses the
DualShock 3 remote which uses a six-axis
accelerometer that can be used to make steering more realistic in
racing games, such as
Motorstorm.
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.
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 .
For example, Apple uses an
LIS302DL accelerometer in the
iPhone,
iPod Touch and the
4th&5th 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 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. Other uses for accelerometer in Nokia phones include
Pedometer functionality in
Nokia Sports Tracker. 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 W760,
Sony Ericsson W910,
Sony Ericsson W902,
Sony Ericsson K850i,
Sony Ericsson C905 and
Sony Ericsson C510 also have an
accelerometer built inside the phone that enables Track Switching
on music player known by users as the Shaker Feature but the W760,
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 W910 and the
Sony Ericsson K850.
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 N96. 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.
As of January 2009, almost all new mobile phones and digital
cameras contain at least a tilt sensor (sometimes an accelerometer)
for the purpose of auto image rotation, motion-sensitive
mini-games, and to correct shake when taking photographs.
Gravimetry
A
gravimeter or
gravitometer, is an instrument used in
gravimetry for measuring the local
gravitational field. A gravimeter is a
type of accelerometer, except that accelerometers are susceptible
to all
vibrations including
noise, that cause oscillatory accelerations. This is
counteracted in the gravimeter by integral vibration isolation and
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, 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 are usually
less for gravimeters, so that resolution can be increased by
processing the output with a longer "time constant".
Types of accelerometer
By construction
By interface type
See also
External links
References
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Industrial Monitoring Instrumentation
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