Accelerometer

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A depiction of an accelerometer designed at Sandia National Laboratory.

An accelerometer is a device that measures non-gravitational accelerations.. These are accelerations produced by mechanically accelerating the accelerometer via its casing.

Single- and multi-axis models are verticle to detect magnitude and direction of the acceleration as a vector quantity, and can be used to sense orientation, vibration and shocks. Accelerometers are present in numerous portable electronic devices and video game controllers, including the Nokia N95, Nokia 5800, Sony Ericsson W910i, Blackberry Storm, iPhone, the iPod Nano 4G and Nintendo's Wii Remote.

Accelerometers may be used in some cases to measure acceleration produced by gravity forces, but only indirectly. This is done by assessing the device's output in situations where gravity forces are known not to vary in time (for example, with the device is at rest at a given altitude), and then inferring from the overall zero vertical acceleration of the device, that the measured non-gravitational forces must be equal (but opposite) to any gravitational fields which are present (see gravimeter).

Accelerometers are also used in inertial navigation systems where the exact gravity is unknown, and then an estimate of the gravity based on the position/altitude is used. This allows the output to be integrated to convert it into changes in vertical velocity and to calculate distance (with respect to the center of the Earth) via essentially dead reckoning.

Physical principles

An accelerometer measures the non gravitational acceleration it experiences. By definition, non-gravitational acceleration is that which is produced by forces other than gravity or inertial/fictitious forces. Such forces include all simple mechanical forces. These are transmitted to the accelerometer device through mechanical stress on its mounting.

The result of such forces, and the output of the accelerometer, is typically expressed in SI units meters/second² (m/s²) or popularly in terms of g-force. It does not measure the "acceleration" due to gravity, and an accelerometer in free fall in a gravity field, even though being accelerated, will read "zero." This includes use in an earth orbiting spaceship.

When held stationary or moving at constant speed in a gravity field by a support, the output of an accelerometer has an offset due to local gravity. The force applied by the support means that, perhaps counter-intuitively, the accelerometer will indicate 1 g along the vertical axis away from the Earth. The accelerometer is simply measuring the inertial acceleration due to this external force (which thus points in a direction opposite its weight, and compensates for its weight so as it keep it motionless in the gravitational field.)

In (non-free) fall with air resistance, drag forces reduce the net acceleration until terminal velocity is reached, at which point the device would once again indicate 1 g upwards, since air resistance pulls the object upwards.

In general, under accelerating motion, to obtain the acceleration relative to the Earth's surface this offset must be subtracted. However, along all horizontal directions, the device yields acceleration directly.

In practice, for finding the acceleration of objects with respect to the earth, such as for use in an inertial navigation system, the correction due to gravity along the vertical axis is usually made by calibrating the object at rest, and may involve the use of dead reckoning to allow for the reduction of gravity with increased altitude.

Structure

Modern accelerometers are often small micro electro-mechanical systems (MEMS), and are indeed the simplest MEMS devices possible, consisting of little more than a cantilever beam with a proof mass (also known as seismic mass). Mechanically the accelerometer behaves as a mass-damper-spring system; the damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not result in a lower sensitivity.

Under the influence of external accelerations that are transmitted via the casing the proof mass deflects from its neutral position. This deflection is measured in an analog or digital manner. Most commonly the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple and reliable; it also does not require additional process steps making it inexpensive. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process steps are needed during the fabrication sequence. For very high sensitivities quantum tunnelling is also used; this requires a dedicated process making it very expensive. Optical measurement has been demonstrated on laboratory scale.

Another, far less common, type of MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor.

Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.

Micromechanical accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of g's. The designer must make a compromise between sensitivity and the maximal acceleration that can be measured.

Applications

In engineering

Accelerometers can be used to measure vehicle acceleration. 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 to 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.

Recreational Boating

A recent application in this market has been the development of an Anchor Alarm (Anchor Alert) by Swiss developer Deep Blue Marine. The accelerometer is encapsulated in a waterproof casing housed inside a bronze outer that is attached to a boat's anchor. If the accelerometer senses anchor movement a radio transmitter sends the information to a remote transponder mounted on the boat's hull. More information can be found at: http://www.deepbluemarine.ch/english/marine/anchor_alert.html

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.

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.

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

Electronic devices

Many laptops feature an accelerometer, such as Lenovo's (formerly IBM's) Active Protection System, and Apple's Sudden Motion Sensor, which is used to detect drops. If a drop is detected, the heads of the hard disk are parked to avoid data loss by the ensuing shock.

Accelerometers are increasingly being incorporated into personal electronic devices such as media players and gaming devices. Some smartphones and personal digital assistants contain accelerometers for user interface control. The Wii Remote for the Wii game console contains a three-axis accelerometer from Analog Devices to sense movement which complements its pointer functionality. This provides more realistic game control.

A number of modern notebook computers feature accelerometers to automatically align the screen depending on the direction the device is held, i.e. switching between portrait and landscape modes. This feature is relevant in Tablet PCs and some smartphones and digital cameras.

For example, Apple uses an LIS302DL accelerometer in the iPhone, iPod Touch and the 4th generation iPod Nano allowing the device to know when it is tilted on its side. Third-party developers have expanded its use with fanciful applications such as electronic bobbleheads.

The Nokia 5500 sport features a 3D accelerometer that can be accessed from software. It is used for step recognition (counting) in a sport application, and for tap gesture recognition in the user interface. Tap gestures can be used for controlling the music player and the sport application, for example to change to next song by tapping through clothing when the device is in a pocket. The Nokia N95 and Nokia N82 have accelerometers embedded inside them. It was primarily used as a tilt sensor for tagging the orientation to photos taken with the built-in camera, later thanks to a firmware update it became possible to use it in other applications. Some other devices provide the tilt sensing feature with a cheaper component, which is not a true accelerometer.

The HTC Touch Pro, HTC Touch Diamond, Sony Ericsson G705, Sony Ericsson W595, Sony Ericsson W910, Sony Ericsson W902, Sony Ericsson K850i and Sony Ericsson C905 also have an accelerometer built inside the phone that enables Track Switching on music player known by users as the Shaker Feature but the W910, W595, W902 and K850 can use the motion sensor feature in gaming, Picture UI AutoRotation and many other applications that require the feature and can be accessible via J2ME application. The first phone from the company to feature an accelerometer was the Sony Ericsson W705.

Camcorders use accelerometers for image stabilization. Still cameras use accelerometers for anti-blur capturing. The camera holds off snapping the CCD "shutter" when the camera is moving. When the camera is still (if only for a millisecond, as could be the case for vibration), the CCD is "snapped". An example application which has used such technology is the Glogger VS2, a phone application which runs on Symbian OS based phone with accelerometer such as Nokia N95. Some digital cameras, such as Canon's PowerShot and Ixus range contain accelerometers to determine the orientation of the photo being taken and also for rotating the current picture when viewing.

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

Main article: gravimeter

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 gravimeters are susceptible to all vibrations including noise, that cause oscillatory accelerations. This is counteracted 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 accelerometers

• 3-Axis Accelerometer data logger - SENSR[1]
• Piezo-film or piezoelectric sensor - PCB Piezotronics, IMI Sensors
• Shear Mode Accelerometer — PCB Piezotronics IMI Sensors,Connection Technology Center,
• Surface Micromachined Capacitive (MEMS) — Analog Devices, Freescale, Honeywell, PCB Piezotronics, Systron Donner Inertial (BEI)
• Thermal (submicrometre CMOS process) — MEMSIC
• Bulk Micromachined Capacitive — VTI Technologies, Colibrys
• Bulk Micromachined Piezo Resistive
• Capacitive Spring Mass Based - Rieker Inc
• Electromechanical Servo (Servo Force Balance)
• Null-balance
• Strain gauge - Endevco, PCB Piezotronics
• Resonance
• Magnetic induction
• Optical
• Surface acoustic wave (SAW)
• Laser accelerometer
• [DC Response] - PCB Piezotronics, Endevco
• High Temperature - PCB Piezotronics, Connection Technology Center
• Low Frequency - PCB Piezotronics, Connection Technology Center, Endevco, Kistler
• High Gravity - Connection Technology Center
• 4-20 mA Loop Power - PCB Piezotronics, Connection Technology Center
• Triaxial - PCB Piezotronics, Connection Technology Center, Endevco
• Modally Tuned Impact Hammers - PCB Piezotronics, IMI Sensors, IMI Sensors
• Seat Pad Accelerometers - PCB Piezotronics, Larson Davis
• PIGA accelerometer - Pendulating Integrating Gyroscopic Accelerometer.

See also

• g-force
• Gyroscope
• Inertial navigation