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15 mars 2011 2 15 /03 /mars /2011 03:24

An accelerometer is a device that measures the proper acceleration of the device. This is not necessarily the same as the coordinate acceleration (change of velocity of the device in space), but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame of reference of the accelerometer device SONY VAIO VGN-FW21E Battery .

For an example of where these types of acceleration differ, an accelerometer will measure a value when sitting on the ground, because masses there have weights, even though they do not change velocity  SONY VAIO VGN-FW21J Battery .

However, an accelerometer in gravitational free falltoward the center of the Earth will measure a value of zero because, even though its speed is increasing, it is in an inertial frame of reference, in which it isweightless  SONY VAIO VGN-FW21L Battery .

An accelerometer thus measures weight per unit of (test) mass, a quantity also known as specific force, or g-force. Another way of stating this is that by measuring weight, an accelerometer measures the acceleration of the free-fall reference frame (inertial reference frame) relative to itself  SONY VAIO VGN-FW21M Battery .

Most accelerometers do not display the value they measure, but supply it to other devices. Real accelerometers also have practical limitations in how quickly they respond to changes in acceleration, and cannot respond to changes above a certain frequency of change SONY VAIO VGN-FW21Z Battery .

Single- and multi-axis models of accelerometer are available to detect magnitude and direction of the proper acceleration (or g-force), as a vector quantity, and can be used to sense orientation (because direction of weight changes), coordinate acceleration (so long as it produces g-force or a change in g-force)  SONY VAIO VGN-FW25T/B Battery ,

vibration, shock, and falling (a case where the proper acceleration changes, since it tends toward zero). Micromachined accelerometers are increasingly present in portable electronic devices and video game controllers, to detect the position of the device or provide for game input SONY VAIO VGN-FW26T/B Battery .

Pairs of accelerometers extended over a region of space can be used to detect differences (gradients) in the proper accelerations of frames of references associated with those points. These devices are called gradiometers, as they measure gradients in the gravitational field Dell Latitude E6400 battery,10400mAh,11.1V .

Such pairs of accelerometers in theory may also be able to detect gravity waves.

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 Dell Latitude E6400 battery,10400mAh,11.1V .

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 measured in terms of g-force Dell Latitude E6500 battery,10400mAh,11.1V .

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 the local inertial frame (the frame of a freely falling object near the surface)  SONY VAIO VGN-FW27/B Battery .

To obtain the acceleration due to motion with respect to the Earth, this "gravity offset" must be subtracted and corrections 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  SONY VAIO VGN-FW27/W Battery ,

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 SONY VAIO VGN-FW31ZJ Battery .

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  SONY VAIO VGN-FW27T/H Battery .

For similar reasons, an accelerometer will read zero during any type offree fall. 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 vacuum. Another example is free-fall at a sufficiently high altitude that atmospheric effects can be neglected  SONY VAIO VGN-FW29/B Battery .

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  SONY VAIO VGN-FW290JRB Battery .

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 g-force (g)  SONY VAIO VGN-FW290JTB Battery .

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  SONY VAIO VGN-FW290JTH Battery .

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 SONY VAIO VGN-FW33G/E1 Battery .

In commercial devices, piezoelectric, piezoresistive and capacitive components are commonly used to convert the mechanical motion into an electrical signal. Piezoelectric accelerometers rely on piezoceramics (e.g. lead zirconate titanate) or single crystals (e.g. quartz, tourmaline)  SONY VAIO VGN-FW33G/W Battery .

They are unmatched in terms of their upper frequency range, low packaged weight and high temperature range. Piezoresistive accelerometers are preferred in high shock applications. Capacitive accelerometers typically use a silicon micro-machined sensing element  SONY VAIO VGN-FW33GW Battery .

Their performance is superior in the low frequency range and they can be operated in servo mode to achieve high stability and linearity.

Modern accelerometers are often small micro electro-mechanical systems (MEMS)  (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk ,

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  Dell Studio XPS 1640 Battery .

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  Dell Vostro 1710 Battery .

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  Dell KM958 battery .

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  Dell Studio 1555 battery .

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 Sony VGP-BPS13 battery .

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  Sony VGP-BPS13/B battery .

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 Sony VGP-BPS13/S battery .

Engineering

Accelerometers can be used to measure vehicle acceleration. They allow for performance evaluation of both the engine/drive train and the braking systems.

Accelerometers can be used to measure vibration on cars, machines, buildings, process control systems and safety installations Sony VGP-BPS13A/B battery .

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  Sony VGP-BPS13B/B battery .

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   Sony VGP-BPL9 battery .

High frequency recordings of bi-axial or tri-axial acceleration (>10 Hz) 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    Sony VGP-BPS13B/B battery ,

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  Sony VGP-BPL15 battery .

This device can be connected to an amplifier to amplify the signal.

Industry - Machinery Health Monitoring

Accelerometers are also used for machinery health monitoring of rotating equipment such as pumps,fans, rollers,compressors,and cooling towers,  Dell Inspiron E1505 battery .

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 Dell Latitude E6400 battery .

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  HP Pavilion dv6000 Battery .

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   HP Pavilion dv8000 battery :

  • 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  HP PAVILION DV6000 Battery,  8800mAh, 10.8V
  • Vehicle collisions
  • Impact loads - falling debris
  • Concussion loads - internal and external explosions
  • Collapse of structural elements
  • Wind loads and wind gusts  HP PAVILION DV8000 Battery,  8800mAh, 10.8V
  • Air blast pressure
  • Loss of support because of ground failure
  • Earthquakes and aftershocks   SONY VAIO VGN-FZ Battery

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     HP Pavilion DV6000 battery - 8800mAh .

[edit]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 HP Pavilion DV8000 battery - 7800mAh .

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  HP Pavilion DV6000 battery - 8800mAh .

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  HP Pavilion DV8000 battery - 6600mAh .

Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial reference platform, and many other variations  SONY VAIO VGN-FZ Battery .

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  SONY VAIO VGN-FZ18 Battery .

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  SONY VAIO VGN-FZ21E Battery .

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  SONY VAIO VGN-FW31J Battery - 5200mAh/7800mAh/10400mAh, 11.10V .

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

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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  SONY VAIO VGN-FW21E Battery .

Vulcanology

Modern electronic accelerometers are used in remote sensing devices intended for the monitoring of active volcanos to detect the motion of magma SONY VAIO VGN-NR11S/S Battery .

Consumer electronics

Accelerometers are increasingly being incorporated into personal electronic devices.

Motion input

Some smartphones, digital audio players and personal digital assistants 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 SONY VAIO VGN-NR11M/S Battery .

Smartphones can download an Automatic Collision Notification (ACN) app such as My-911, similar to the Onstar AACN service, Ford Link's 911 Assist, Toyota's Safety Connect, Lexus Link, orBMW Assist. The phone's accelerometer detects crash-strength G-forces and automatically calls for assistance unless manually cancelled  SONY VAIO VGN-NR11Z/S Battery .

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 SONY VAIO VGN-NR11Z/T Battery .

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 and Burnout Paradise.

The Nokia 5500 sport features a 3D accelerometer that can be accessed from software Sony VAIO VGN-FZ21E Battery .

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  Sony VAIO VGN-FW21E Battery .

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  Sony VAIO VGN-NR11S/S Battery .

Sleep phase alarm clocks use accelerometric sensors to detect movement of a sleeper, so that it can wake the person when he/she is not in REM phase, therefore awakes more easily  Sony VAIO VGN-NR11Z/S Battery .

Orientation sensing

A number of 21st century devices use accelerometers to align the screen depending on the direction the device is held, i.e. switching between portrait and landscape modes. Such devices include many tablet PCs and some smartphones and digital cameras Sony VAIO VGN-NR11M/S Battery .

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 Sony VAIO VGN-NR11Z/T Battery .

The BlackBerry Storm phone was also an early user of this orientation sensing feature.

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 available to other applicationsSONY VAIO VGN-FZ180E Battery .

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  SONY VAIO VGN-FZ220E Battery .

Image stabilization

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  SONY VAIO VGN-FZ340E Battery .

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  SONY VAIO VGN-FZ430E Battery .

Some digital cameras, contain accelerometers to determine the orientation of the photo being taken and also for rotating the current picture when viewing  SONY VAIO VGN-FZ460E Battery .

Device integrity

Many laptops feature an accelerometer which is used to detect drops. If a drop is detected, the heads of the hard disk are parked to avoid data loss and possible head or disk damage by the ensuingshock  SONY VAIO VGN-FZ480E Battery .

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 allvibrations including noise, that cause oscillatory accelerations  SONY VAIO VGN-FZ4000 Battery .

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 SONY VAIO VGN-FZ31E Battery .

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" SONY VAIO VGN-FZ31B Battery .

The g-force (with g from gravitational) associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move  SONY VAIO VGN-FZ31J Battery .

The accelerations that are not produced by gravity are termed proper accelerations, and it is only these that are measured in g-force units. They cause stresses and strains on objects, which are felt as weight (any g-force can thus be simply described, and measured, as a "weight per unit mass") SONY VAIO VGN-FZ31M Battery .

Because of these strains (weight forces), large proper accelerations (large g-forces), may be destructive.

The standard gravitational acceleration at the Earth's surface produces g-force only indirectly  SONY VAIO VGN-FZ31Z Battery .

The 1 g force on an object sitting on the Earth's surface is caused by mechanical force exerted in the upward direction by the ground, keeping the object from going into free-fall. An object on the Earth's surface is accelerating relative to the free-fall condition, which is the path an object would follow falling freely toward the Earth's center SONY VAIO VGN-FZ38M Battery .

It is thus experiencing proper acceleration, even without a change in velocity (which is dv/dt, the familiar "coordinate acceleration" of Newton's laws)  SONY VGP-BPS8 Battery .

Objects allowed to free-fall under the influence of gravity feel no g-force, as demonstrated by the "zero-g" conditions inside a freely-falling elevator falling toward the Earth's center (in vacuum), or (to good approximation) conditions inside a spacecraft in Earth orbit  SONY VGP-BPS13/S Battery .

These are examples of coordinate acceleration (a change in velocity) without proper acceleration. Since the g-force felt is always a measure of proper acceleration (which, in these cases, is zero, even though the objects are freely changing velocity due to gravity) all of these conditions of free-fall produce no g-force  SONY VGP-BPS13A/B Battery .

The experience of no g-force (zero-g), however it is produced, is synonymous with weightlessness.

In the absence of gravitational fields, or in directions at right angles to them, proper and coordinate accelerations are the same, and any coordinate acceleration must be produced by a corresponding g-force acceleration  SONY VGP-BPS13B/B Battery .

An example here is a rocket in free space, in which simple changes in velocity are produced by the engines, and produce g-forces on the rocket and passengers. The same happens in a dragster (see illustration) when it is changing velocity in a direction at right angles to the acceleration of gravity  SONY VGP-BPS13A/S Battery :

such changes must be produced by accelerations that are appropriately measured in g-force units in the horizontal direction, since they produce g-force effects in that direction.

The unit of measure of g-force in the International System of Units (SI) is m/s2  SONY VGP-BPS13AS Battery .

However, for easy comparison with the stationary situation on Earth, and to emphasize the distinction of this acceleration relative to free-fall from simple acceleration (rate of change of velocity), often the unit g is used - the acceleration due to gravity at the Earth's surface; it can be written g, g, or G   Dell Inspiron 1320n Battery .

More accurately, it is the standard gravity (symbol: gn), defined as 9.80665 metres per second squared, or equivalently 9.80665 newtons of force per kilogram of mass. Sometimes the plural Gs is used. The unit g is not one of the SI units, which uses "g" for gram; also, "G" should not be confused with the standard symbol for the gravitational constant  Dell Inspiron 1464 Battery .

Measurement of g-force is typically achieved using an accelerometer (see discussion below in Measuring g-force using an accelerometer). In certain cases, g-forces may be measured using suitably calibrated scales. Specific force is another name that has been used for g-force  Dell Inspiron 1564 Battery .

The term g-force is technically incorrect as it is a measure of acceleration, not force. While acceleration is a vector quantity, g-forces are often expressed as a scalar, with positive g-forces working towards the bottom of a vehicle and negative forces towards the top. However, g-force can also be expressed as a vector acceleration  Dell Inspiron 1764 Battery .

G-forces, when multiplied by a mass upon which they act, are associated with a certain type of mechanical force in the correct sense of the term force, and this force produces compressive stress and tensile stress. If for example a g-force is vertically upward and applied by the ground or the floor of an elevator to a standing person   Dell Studio 1450 Battery ,

most of the body experiences compressive stress which at any height, if multiplied by the area, is the related mechanical force, which is the product of the g-force and the supported mass (the mass above the level of support, including arms hanging down from above that level)  Dell Studio 1457 Battery .

At the same time, the arms themselves experience a tensile stress, which at any height, if multiplied by the area, is again the related mechanical force, which is the product of the g-force and the mass hanging below the point of mechanical support. The mechanical resistive force spreads from points of contact with the floor or supporting structure  Dell Latitude D610 Battery ,

and gradually decreases toward zero at the unsupported ends (the top in the case of support from below, such as a seat or the floor, the bottom for a hanging part of the body or object). With compressive force counted as negative tensile force, the rate of change of the tensile force in the direction of the g-force  Toshiba NB100 Battery ,

per unit mass (the change between parts of the object such that the slice of the object between them has unit mass), is equal to the g-force plus the non-gravitational external forces on the slice, if any (counted positive in the direction opposite to the g-force) Toshiba Satellite M65 battery .

For a given g-force the stresses are the same, regardless of whether this g-force is caused by gravity, by acceleration, or a combination. Hence, for people it feels exactly the same, and both for people and objects the question whether they can withstand the g-force is the same  Toshiba Satellite M60 battery .

For example, upward acceleration (e.g. increase of speed when going up or decrease of speed when going down) on Earth feels the same as being stationary on a celestial body with a higher surface gravity.

Examples of important situations involving g-forces include  Dell Latitude D830 Battery :

  • The g-force acting on a stationary object resting on the Earth's surface is 1 g (upwards) and results from the resisting reaction of the Earth's surface bearing upwards equal to an acceleration of 1 g, and is equal and opposite to gravity. The number 1 is approximate, depending on location  Dell Latitude D620 Battery .
  • The g-force acting on an object in any weightless environment such as free-fall in a vacuum is 0 g.
  • The g-force acting on an object under acceleration can be much greater than 1 g, for example, the dragster pictured right can exert a horizontal g-force of 5.3 when accelerating  Dell Inspiron Mini 10 Battery .
  • The g-force acting on an object under acceleration may be downwards, for example when cresting a sharp hill on a roller coaster  Sony VGN-FW11S Battery .
  • If there are no other external forces than gravity, the g-force in a rocket is the thrust per unit mass. Its magnitude is equal to the thrust-to-weight ratio times g, and to the consumption of delta-v per unit time.
  • In the case of a shock, e.g. a collision, the g-force can be very large during a short time  Sony VGN-FW11M Battery .

A classic example of negative g-force is in a fully inverted roller coaster which is accelerating (changing velocity) toward the ground. In this case, the roller coaster riders are accelerated toward the ground faster than gravity would accelerate them, and are thus pinned upside down in their seats  Sony VGN-FW139E/H battery .

In this case, the mechanical force exerted by the seat causes the g-force by altering the path of the passenger downward in a way that differs from gravitational acceleration. The difference in downward motion, now faster than gravity would provide, is caused by the push of the seat, and it results in a g-force toward the ground  Dell Latitude E5400 Battery .

All "coordinate accelerations" (or lack of them), are described by Newton's laws of motion as follows:

The Second Law of Motion, the law of acceleration states that: F =?ma., meaning that a force F acting on a body is equal to the massm of the body times its acceleration a  Dell Latitude E4200 Battery .

The Third Law of Motion, the law of reciprocal actions states that: all forces occur in pairs, and these two forces are equal in magnitude and opposite in direction. Newton's third law of motion means that not only does gravity behave as a force acting downwards on  Dell Inspiron 300M Battery ,

say, a rock held in your hand but also that the rock exerts a force on the Earth, equal in magnitude and opposite in direction.

In an airplane, the pilot’s seat can be thought of as the hand holding the rock, the pilot as the rock. When flying straight and level at 1 g, the pilot is acted upon by the force of gravity  Dell Vostro A840 Battery .

His weight (a downward force) is 725 newtons (163 lbf). In accordance with Newton’s third law, the plane and the seat underneath the pilot provides an equal and opposite force pushing upwards with a force of 725 N (163 lbf)   Dell Studio 1737 battery .

This mechanical force provides the 1.0 g-force upward proper acceleration on the pilot, even though this velocity in the upward direction does not change (this is similar to the situation of a person standing on the ground, where the ground provides this force and this g-force)  Dell Inspiron E1505 battery .

If the pilot were suddenly to pull back on the stick and make his plane accelerate upwards at 9.8 m/s2, the total g?force on his body is 2 g, half of which comes from the seat pushing the pilot to resist gravity, and half from the seat pushing the pilot to cause his upward acceleration—a change in velocity which also is a proper acceleration because it also differs from a free fall trajectory Dell RM791 battery .

Considered in the frame of reference of the plane his body is now generating a force of 1,450 N (330 lbf) downwards into his seat and the seat is simultaneously pushing upwards with an equal force of 1,450 N (330 lbf)  Dell XPS M1530 battery .

Unopposed acceleration due to mechanical forces, and consequentially g-force, is experienced whenever anyone rides in a vehicle because it always causes a proper acceleration, and (in the absence of gravity) also always a coordinate acceleration (where velocity changes)  Dell XPS M2010 battery .

Whenever the vehicle changes either direction or speed, the occupants feel lateral (side to side) or longitudinal (forward and backwards) forces produced by the mechanical push of their seats.

The expression "1 g = 9.80665 m/s2" means that for every second that elapses, velocity changes 9.80665 meters per second (?35.30394 km/h)  Acer Aspire One battery .

This rate of change in velocity can also be denoted as 9.80665 (meter per second) per second, or 9.80665 m/s2. For example: An acceleration of 1 g equates to a rate of change in velocity of approximately 35 kilometres per hour (22 mph) for each second that elapses   Toshiba Satellite P10 Battery .

Therefore, if an automobile is capable of braking at 1 g and is traveling at 35 kilometres per hour (22 mph) it can brake to a standstill in one second and the driver will experience a deceleration of 1 g. The automobile traveling at three times this speed, 105 km/h (65 mph), can brake to a standstill in three seconds  SONY VGN-FZ210CE Battery .

In the case of an increase in speed from 0 to v with constant acceleration within a distance of s this acceleration is v2/(2s).

Preparing an object for g-tolerance (not getting damaged when subjected to a high g-force) is called g-hardening. This may e.g. apply to instruments in aprojectile shot by a gun Dell Precision M70 Battery .

Human tolerances depend on the magnitude of the g-force, the length of time it is applied, the direction it acts, the location of application, and the posture of the body.

The human body is flexible and deformable, particularly the softer tissues   Toshiba Satellite L305 Battery .

A hard slap on the face may briefly impose hundreds of g locally but not produce any real damage; a constant 16 g for a minute, however, may be deadly. When vibration is experienced, relatively low peak g levels can be severely damaging if they are at theresonance frequency of organs and connective tissues  Toshiba Satellite T4900 Battery .

To some degree, g-tolerance can be trainable, and there is also considerable variation in innate ability between individuals. In addition, some illnesses, particularlycardiovascular problems, reduce g-tolerance  Toshiba PA3399U-2BRS battery .

Vertical axis g-force

Aircraft, in particular, exert g-force along the axis aligned with the spine. This causes significant variation in blood pressure along the length of the subject's body, which limits the maximum g-forces that can be tolerated  Toshiba Satellite A200 Battery .

Positive, or "upward" g, drives blood downward to the feet of a seated or standing person (more naturally, the feet and body may be seen as being driven by the upward force of the floor and seat, upward around the blood)  Toshiba Satellite 1200 Battery .

Resistance to positive g varies. A typical person can handle about 5 g (49 m/s²) before losing consciousness ("G-LOC"), but through the combination of special g-suits and efforts to strain muscles—both of which act to force blood back into the brain—modern pilots can typically handle a sustained 9 g (88 m/s²) (see High-G training)  Toshiba Satellite M300 Battery .

Resistance to "negative" or "downward" g, which drives blood to the head, is much lower. This limit is typically in the ?2 to ?3 g (?20 m/s² to ?30 m/s²) range. This condition is sometimes referred to as red out where vision is literally reddened due to expansion of the capillaries in the eye   SONY VGP-BPS13A/Q Battery .

Negative g is generally unpleasant and can cause damage. Blood vessels in the eyes or brain may swell or burst under the increased blood pressure.

In aircraft particularly, vertical g-forces are often positive (force blood towards the feet and away from the head) SONY VGP-BPS13A/Q Battery ;

this causes problems with the eyes and brain in particular. As positive vertical g-force is progressively increased (such as in a centrifuge) the following symptoms may be experienced:

  • Grey-out, where the vision loses hue, easily reversible on levelling out  SONY VGP-BPS13B/Q Battery .
  • Tunnel vision, where peripheral vision is progressively lost.
  • Blackout, a loss of vision while consciousness is maintained, caused by a lack of blood to the head.
  • G-LOC a loss of consciousness ("LOC" stands for "Loss Of Consciousness").
  • Death, if g-forces are not quickly reduced, death can occur  SONY VGP-BPS13B/Q Battery .

Horizontal axis g-force

The human body is better at surviving g-forces that are perpendicular to the spine. In general when the acceleration is forwards (subject essentially lying on their back, colloquially known as "eyeballs in" )  SONY VGP-BPS13/Q Battery

a much higher tolerance is shown than when the acceleration is backwards (lying on their front, "eyeballs out") since blood vessels in the retina appear more sensitive in the latter direction.

Early experiments showed that untrained humans were able to tolerate 17 g eyeballs-in (compared to 12 g eyeballs-out) for several minutes without loss of consciousness or apparent long-term harm  SONY VGP-BPS13/Q Battery .

The record for peak experimental horizontal g-force tolerance is held by acceleration pioneer John Stapp, in a series of rocket sled deceleration experiments culminating in a late 1954 test in which he was stopped in a little over a second from a land speed of Mach 0.9  SONY VGP-BPS21A/B Battery .

He survived a peak "eyeballs-out" force of 46.2 times the force of gravity, and more than 25 g for 1.1 sec, proving that the human body is capable of this. Stapp lived another 45 years to age 89, but suffered lifelong damage to his vision from this last test  SONY VGP-BPS21/S Battery .

Short g-force durations and jerk

Toleration of g-force also depends on its duration. Shock is a short-term transient exitiation and is often measured as an acceleration. Jerk is the rate of change in acceleration. In SI units, jerk or horizontal g force g is expressed as m/s3. In non-SI units, jerk can be expressed simply as gees per second (g/s)  SONY VGP-BPS21B Battery .

Very short durations g-forces of 100 g have been survivable in racing car crashes.

An accelerometer, in its simplest form, is a damped mass on the end of a spring, with some way of measuring how far the mass has moved on the spring in a particular direction, called an 'axis'  SONY VGP-BPS21A Battery .

Accelerometers are often calibrated to measure g-force along one or more axes. If a stationary, single-axis accelerometer is oriented so that its measuring axis is horizontal, its output will be 0 g, and it will continue to be 0 g if mounted in an automobile traveling at a constant velocity on a level road   SONY VGP-BPS21 Battery .

When the driver presses on the brake or gas pedal, the accelerometer will register positive or negative acceleration.

If the accelerometer is rotated by 90° so that it is vertical, it will read +1 g upwards even though stationary. In that situation, the accelerometer is subject to two forces  Sony VGP-BPS21A/B Battery :

the gravitational force and the ground reaction force of the surface it is resting on. Only the latter force can be measured by the accelerometer, due to mechanical interaction between the accelerometer and the ground. The reading is the acceleration the instrument would have if it were exclusively subject to that force  Sony VGP-BPS21/S Battery .

A three-axis accelerometer will output zero?g on all three axes if it is dropped or otherwise put into a ballistic trajectory (also known as an inertial trajectory), so that it experiences "free fall," as do astronauts in orbit (astronauts experience small tidal accelerations called microgravity, which are neglected for the sake of discussion here)   Sony VGP-BPS21A Battery .

Some amusement park rides can provide several seconds at near-zero g. Riding NASA’s “Vomit Comet” provides near-zero g for about 25 seconds at a time.

A single-axis accelerometer mounted in an airplane with its measurement axis oriented vertically reads +1 g when the plane is parked   Sony VGP-BPS21 Battery .

This is the g-force exerted by the ground. When flying at a stable altitude (or at a constant rate of climb or descent), the accelerometer will continue to indicate 1 g, as the g-force is provided by the aerodynamic lift, which now acts in place of the ground to keep the plane from free-falling   Sony Vaio PCG-5G2L Battery .

Under such conditions, the upward force acting upon the pilot’s body (which keeps him from falling) is the normal value of about 9.8 newtons per kilogram (N/kg), and it is provided by his seat, which in turn is supported by the lift of the wings  Sony Vaio PCG-5G3L Battery .

If the pilot pulls back on the stick until the accelerometer indicates 2 g, the g-force acting upwards on him through the seat doubles to 19.6 N/kg.

A gyroscope is a device for measuring or maintaining orientation, based on the principles of conservation of angular momentum  Sony Vaio PCG-5J1L Battery .

A mechanical gyroscope is essentially a spinning wheel or disk whose axle is free to take any orientation. This orientation changes much less in response to a given external torque than it would without the large angular momentum associated with the gyroscope's high rate of spin   Sony Vaio PCG-5K2L Battery .

Since external torque is minimized by mounting the device in gimbals, its orientation remains nearly fixed, regardless of any motion of the platform on which it is mounted.

Gyroscopes based on other operating principles also exist  Sony Vaio PCG-5J2L Battery ,

such as the electronic, microchip-packaged MEMS gyroscope devices found in consumer electronic devices, solid state ring lasers, fibre optic gyroscopes and the extremely sensitive quantum gyroscope  Sony Vaio PCG-5K1L Battery .

Applications of gyroscopes include navigation (INS) when magnetic compasses do not work (as in the Hubble telescope) or are not precise enough (as in ICBMs) or for the stabilization of flying vehicles like radio-controlled helicopters or UAVs. Due to their high precision, gyroscopes are also used to maintain direction in tunnel mining  Sony Vaio PCG-6S2L Battery .

Within mechanical systems or devices, a conventional gyroscope is a mechanism comprising a rotor journalled to spin about one axis, the journals of the rotor being mounted in an inner gimbal or ring, the inner gimbal is journalled for oscillation in an outer gimbal which is journalled in another gimbal for a total of three gimbals  Sony Vaio PCG-6S3L Battery .

The outer gimbal or ring which is the gyroscope frame is mounted so as to pivot about an axis in its own plane determined by the support. This outer gimbal possesses one degree of rotational freedom and its axis possesses none   Sony Vaio PCG-6V1L Battery .

The next inner gimbal is mounted in the gyroscope frame (outer gimbal) so as to pivot about an axis in its own plane that is always perpendicular to the pivotal axis of the gyroscope frame (outer gimbal). This inner gimbal has two degrees of rotational freedom   Sony Vaio PCG-6W1L Battery .

Similarly, next innermost gimbal is attached to the inner gimbal which has three degrees of rotational freedom and its axis possesses two.

The axle of the spinning wheel defines the spin axis    Sony Vaio PCG-6W2L Battery .

The rotor is journaled to spin about an axis which is always perpendicular to the axis of the innermost gimbal. So, the rotor possesses four degrees of rotational freedom and its axis possesses three. The wheel responds to a force applied about the input axis by a reaction force about the output axis   Sony Vaio PCG-6W3L Battery .

The behaviour of a gyroscope can be most easily appreciated by consideration of the front wheel of a bicycle. If the wheel is leaned away from the vertical so that the top of the wheel moves to the left, the forward rim of the wheel also turns to the left. In other words, rotation on one axis of the turning wheel produces rotation of the third axis   Sony Vaio PCG-7111L Battery .

A gyroscope flywheel will roll or resist about the output axis depending upon whether the output gimbals are of a free- or fixed- configuration. Examples of some free-output-gimbal devices would be the attitude reference gyroscopes used to sense or measure the pitch, roll and yaw attitude angles in a spacecraft or aircraft   Sony Vaio PCG-7112L Battery .

The centre of gravity of the rotor can be in a fixed position. The rotor simultaneously spins about one axis and is capable of oscillating about the two other axes, and thus, except for its inherent resistance due to rotor spin, it is free to turn in any direction about the fixed point    Sony Vaio PCG-7113L Battery .

Some gyroscopes have mechanical equivalents substituted for one or more of the elements, e.g., the spinning rotor may be suspended in a fluid, instead of being pivotally mounted in gimbals     Sony Vaio PCG-7133L Battery .

Acontrol moment gyroscope (CMG) is an example of a fixed-output-gimbal device that is used on spacecraft to hold or maintain a desired attitude angle or pointing direction using the gyroscopic resistance force  Sony Vaio PCG-7Z2L Battery .

In some special cases, the outer gimbal (or its equivalent) may be omitted so that the rotor has only two degrees of freedom. In other cases, the centre of gravity of the rotor may be offset from the axis of oscillation and thus the centre of gravity of the rotor and the centre of suspension of the rotor may not coincide  Sony Vaio PCG-8Y1L Battery .

History

The earliest known gyroscope-like instrument was made by German Johann Bohne

nberger, who first wrote about it in 1817. At first he called it the "Machine". Bohnenberger's machine was based on a rotating massive sphere  Sony Vaio PCG-8Y2L Battery .

In 1832, American Walter R. Johnson developed a similar device that was based on a rotating disk.The French mathematician Pierre-Simon Laplace, working at the École Polytechnique in Paris, recommended the machine for use as a teaching aid, and thus it came to the attention of Léon Foucault   Sony Vaio PCG-8Z1L Battery .

In 1852, Foucault used it in an experiment involving the rotation of the Earth. It was Foucault who gave the device its modern name, in an experiment to see (Greek skopeein, to see) the Earth's rotation (Greek gyros, circle or rotation), which was visible in the 8 to 10 minutes before friction slowed the spinning rotor   Sony Vaio PCG-8Z2L Battery .

In the 1860s, the advent of electric motors made it possible for a gyroscope to spin indefinitely; this led to the first prototype gyrocompasses. The first functional marine gyrocompass was patented in 1907 by German inventor Hermann Anschütz-Kaempfe   SONY VAIO VGN-FW31J Battery .

The American Elmer Sperry followed with his own design later that year, and other nations soon realized the military importance of the invention—in an age in which naval prowess was the most significant measure of military power—and created their own gyroscope industries   SONY VAIO VGN-FW31E Battery .

The Sperry Gyroscope Company quickly expanded to provide aircraft and naval stabilizers as well, and other gyroscope developers followed suit.

In 1917, the Chandler Company of Indianapolis, Indiana, created the "Chandler gyroscope", a toy gyroscope with a pull string and pedestal   SONY VAIO VGN-FW31B Battery .

Chandler continued to produce the toy until the company was purchased by TEDCO inc. in 1982. The chandler toy is still produced by TEDCO today  SONY VAIO VGN-FW31M Battery .

In the first several decades of the 20th century, other inventors attempted (unsuccessfully) to use gyroscopes as the basis for early black box navigational systems by creating a stable platform from which accurate acceleration measurements could be performed (in order to bypass the need for star sightings to calculate position)  SONY VAIO VGN-FW31Z Battery .

Similar principles were later employed in the development of inertial guidance systems for ballistic missiles.

During World War Two, the gyroscope became the prime component for aircraft and anti-aircraft gun sights  SONY VAIO VGN-FW38M Battery .

Properties

A gyroscope exhibits a number of behaviours including precession and nutation. Gyroscopes can be used to construct gyrocompasses which complement or replace magnetic compasses  SONY VAIO VGN-AW41JF Battery

(in ships, aircraft and spacecraft, vehicles in general), to assist in stability (Hubble Space Telescope, bicycles, motorcycles, and ships) or be used as part of an inertial guidance system. Gyroscopic effects are used in tops, boomerangs, yo-yos, and Powerballs  SONY VAIO VGN-AW41JF/H Battery .

Many other rotating devices, such as flywheels, behave gyroscopically although the gyroscopic effect is not being used.

The fundamental equation describing the behavior of the gyroscope is   SONY VAIO VGN-AW41MF Battery :

\boldsymbol\tau={{d \mathbf{L}}\over {dt}}={{d(I\boldsymbol\omega)} \over {dt}}=I\boldsymbol\alpha

where the vectors ? and L are, respectively, the torque on the gyroscope and its angular momentum, the scalar I is its moment of inertia, the vector ? is its angular velocity, and the vector ? is its angular acceleration  SONY VAIO VGN-AW41MF/H Battery .

It follows from this that a torque ? applied perpendicular to the axis of rotation, and therefore perpendicular to L, results in a rotation about an axis perpendicular to both ? and L. This motion is called precession. The angular velocity of precession ?P is given by the cross product   SONY VAIO VGN-FW30B Battery :

Precession can be demonstrated by placing a spinning gyroscope with its axis horizontal and supported loosely (frictionless toward precession) at one end. Instead of falling, as might be expected, the gyroscope appears to defy gravity by remaining with its axis horizontal   SONY VAIO VGN-FW50B Battery ,

when the other end of the axis is left unsupported and the free end of the axis slowly describes a circle in a horizontal plane, the resulting precession turning. This effect is explained by the above equations. The torque on the gyroscope is supplied by a couple of forces   SONY VAIO VGN-FW51B/W Battery :

gravity acting downwards on the device's centre of mass, and an equal force acting upwards to support one end of the device. The rotation resulting from this torque is not downwards, as might be intuitively expected, causing the device to fall, but perpendicular to both the gravitational torque   SONY VAIO VGN-FW51MF Battery

(horizontal and perpendicular to the axis of rotation) and the axis of rotation (horizontal and outwards from the point of support), i.e. about a vertical axis, causing the device to rotate slowly about the supporting point  SONY VAIO VGN-FW51MF/H Battery .

Under a constant torque of magnitude ?, the gyroscope's speed of precession ?P is inversely proportional to L, the magnitude of its angular momentum:

\tau = \mathit{\Omega}_{\mathrm{P}} L \sin\theta,\!

where ? is the angle between the vectors ?P and L   SONY VAIO VGN-FW51ZF Battery .

Thus if the gyroscope's spin slows down (for example, due to friction), its angular momentum decreases and so the rate of precession increases. This continues until the device is unable to rotate fast enough to support its own weight, when it stops precessing and falls off its support, mostly because friction against precession cause another precession that goes to cause the fall SONY VAIO VGN-FW51ZF/H Battery .

By convention, these three vectors, torque, spin, and precession, are all oriented with respect to each other according to the right-hand rule.

To easily ascertain the direction of gyro effect, simply remember that a rolling wheel tends, when it leans to the side, to turn in the direction of the lean  SONY VAIO VGN-FW52JB Battery .

Gyrostat

A gyrostat is a variant of the gyroscope. It consists of a massive flywheel concealed in a solid casing. Its behaviour on a table, or with various modes of suspension or support, serves to illustrate the curious reversal of the ordinary laws of static equilibrium due to the gyrostatic behaviour of the interior invisible flywheel when rotated rapidly  SONY VAIO VGN-FW54FB Battery .

The first gyrostat was designed by Lord Kelvin to illustrate the more complicated state of motion of a spinning body when free to wander about on a horizontal plane, like a top spun on the pavement, or a hoop or bicycle on the road  SONY VAIO VGN-FW70DB Battery .

MEMS

A MEMS gyroscope takes the idea of the Foucault pendulum and uses a vibrating element, known as a MEMS (Micro Electro-Mechanical System). The MEMS-based gyro was initially made practical and producible by Systron Donner Inertial (SDI). Today, SDI is a large manufacturer of MEMS gyroscopes  SONY VAIO VGN-FW71DB/W Battery .

FOG

A fiber optic gyroscope (FOG) is a gyroscope that uses the interference of light to detect mechanical rotation. The sensor is a coil of as much as 5 km of optical fiber. The development of low loss single mode optical fiber in the early 1970s for the telecommunications industry enabled the development of Sagnac effect fiber optic gyros  SONY VAIO VGN-FW72JGB Battery .

VSG or CVG

A vibrating structure gyroscope (VSG), also called a coriolis vibratory gyroscope (CVG), uses a resonator made of different metallic alloys. It takes a position between the low accuracy, low costMEMS gyroscope and the higher accuracy and higher cost fiber optic gyroscope (FOG)  SONY VAIO VGN-FW73JGB Battery .

Accuracy parameters are increased by using low intrinsic damping materials, resonator vacuumization, and digital electronics to reduce temperature dependent drift and instability of control signals.

High-Q Wine-Glass Resonators for precise sensors like HRG or CRG are based on Bryan's "wave inertia effect"  SONY VAIO VGN-FW74FB Battery .

They are made from high-purity quartz glass or from single-crystalline sapphire.

DTG

A dynamically tuned gyroscope (DTG) is a rotor suspended by a universal joint with flexure pivots. The flexure spring stiffness is independent of spin rate       SONY VAIO VGN-FW81HS Battery .

However, the dynamic inertia (from the gyroscopic reaction effect) from the gimbal provides negative spring stiffness proportional to the square of the spin speed (Howe and Savet, 1964; Lawrence, 1998). Therefore, at a particular speed, called the tuning speed, the two moments cancel each other, freeing the rotor from torque, a necessary condition for an ideal gyroscope SONY VAIO VGN-FW81NS Battery .

London moment

A London moment gyroscope relies on the quantum-mechanical phenomenon whereby a spinning superconductor generates a magnetic field whose axis lines up exactly with the spin axis of the gyroscopic rotor  SONY VAIO VGN-FW81S Battery .

A magnetometer determines the orientation of the generated field, which is interpolated to determine the axis of rotation. Gyroscopes of this type can be extremely accurate and stable, for example those used in the Gravity Probe B experiment measured changes in gyroscope spin axis orientation to better than 0.5 milliarcseconds (1.4×10?7 degrees) over a one-year period SONY VAIO VGN-FW82DS Battery .

This is equivalent to an angular separation the width of a human hair viewed from 32 kilometers (20 miles) away.

The GP-B gyro consists of a nearly-perfect spherical rotating mass made of fused quartz which provides a dielectric support for a thin layer of niobium superconducting material  SONY VAIO VGN-FW82JS Battery .

To eliminate frictionfound in conventional mechanical bearings, the rotor assembly is suspended by six electromagnets that form a magnetic bearing. After the initial spin-up by a jet of helium brings the rotor to 4,000RPM, the polished gyroscope housing is evacuated to a ultra-high vacuum to further reduce drag on the rotor  SONY VAIO VGN-FW82XS Battery .

Provided the suspension electronics remain powered, the extreme rotational symmetry, lack of friction, and low drag will allow the angular momentum of the rotor to keep it spinning for about 15,000 years.

A sensitive DC SQUID magnetometer able to discriminate changes as small as one quantum, or about 2 ×10?15 Wb, is used to monitor the gyroscope  SONY VAIO VGN-FW83DS Battery .

A precesses, or tilt, in the orientation of the rotor causes the London moment magnetic field to shift relative to the housing. The moving field passes through a superconducting pickup loop fixed to the housing, inducing a small electric current. The current produces a voltage across a shunt resistance, which is resolved to spherical coordinates by a microprocessor SONY VAIO VGN-FW83JS Battery .

The system is designed to minimize Lorentz torque on the rotor.

In addition to being used in compasses, aircraft, computer pointing devices, etc., gyroscopes have been introduced into consumer electronics   SONY VAIO VGN-FW83XS Battery .

Since the gyroscope allows the calculation of orientation and rotation, designers have incorporated them into modern technology. The integration of the gyroscope has allowed for more accurate recognition of movement within a 3D space than the previous lone accelerometer within a number of smartphones   SONY VAIO VGN-FW90NS Battery .

Scott Steinberg, known for his critiques on newly released technology, says that the new addition of the gyroscope in the iPhone 4 may "completely redefine the way we interact with downloadable apps".

Nintendo has integrated a gyroscope into the Wii console's Wii Remote controller by an additional piece of hardware called "Wii MotionPlus"  SONY VAIO VGN-FW90S Battery .

The term geophone derives from the Greek word "geo" meaning "earth" and "phone" meaning "sound".

A geophone is a device which converts ground movement (displacement) into voltage, which may be recorded at a recording station        SONY VAIO VGN-FW91NS Battery .

The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the earth.

Geophones have historically been passive analog devices and typically comprise a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal   SONY VAIO VGN-FW91S Battery .

Recent designs have been based on Microelectromechanical systems technology which generates an electrical response to ground motion through an active feedback circuit to maintain the position of a small piece of silicon  SONY VAIO VGN-FW91S Battery .

The response of a coil/magnet geophone is proportional to ground velocity, while microelectromechanical systems devices usually respond proportional toacceleration. Microelectromechanical systems have a much higher noise level (50 dB velocity higher) than geophones and can only be used in strong motion or active seismic applications SONY VAIO VGN-FW92DS Battery .

The frequency response of a geophone is that of a harmonic oscillator, fully determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). Since the corner frequency is proportional to the inverse root of the moving mass, geophones with low corner frequencies (< 1Hz) become unpractical  SONY VAIO VGN-FW92JS Battery .

It is possible to lower the corner frequency electronically, at the price of higher noise and cost.

Although waves passing through the earth have a three-dimensional nature, geophones are normally constrained to respond to single dimension - usually the vertical   SONY VAIO VGN-FW93DS Battery .

However, some applications require the full wave to be used and three-component or 3-C geophones are used. In analog devices, three moving coil elements are mounted in an orthogonal arrangement within a single case.

The majority of geophones are used in reflection seismology to record the energy waves reflected by the subsurface geology SONY VAIO VGN-FW93JS Battery .

In this case the primary interest is in the vertical motion of the Earth's surface. However, not all the waves are upwards travelling. A strong, horizontally transmitted wave known as ground-roll also generates vertical motion that can obliterate the weaker vertical signals  SONY VAIO VGN-FW139E/H Battery .

By using large areal arrays tuned to the wavelength of the ground-roll the dominant noise signals can be attenuated and the weaker data signals reinforced.

Analog geophones are very sensitive devices which can respond to very distant tremors   SONY VAIO VGN-FW145E/W Battery .

These small signals can be drowned by larger signals from local sources. It is possible though to recover the small signals caused by large but distant events by correlating signals from several geophones deployed in an array. Signals which are registered only at one or few geophones can be attributed to small, local events and thus discarded  SONY VAIO VGN-FW190EBH Battery .

It can be assumed that small signals that register uniformly at all geophones in an array can be attributed to a distant and therefore significant event.

The sensitivity of passive geophones is typically 30 Volts/(meter/second), so they are in general not a replacement for broadband seismometers  SONY VAIO VGN-FW190EDH Battery .

Conversely, some applications of geophones are interested only in very local events. A notable example is in the application of Remote Ground Sensors (RGS) incorporated in Unattended Ground Sensor (UGS) Systems. In such an application there is an area of interest which when penetrated a system operator is to be informed, perhaps by an alert which could be accompanied by supporting photographic data SONY VAIO VGN-FW21E Battery SONY VAIO VGN-FW21J Battery SONY VAIO VGN-FW21L Battery SONY VAIO VGN-FW21M Battery SONY VAIO VGN-FW21Z Battery .

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