accelerometer

A depiction of an accelerometer designed at Sandia National Laboratories.

An accelerometer measures the 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.

Physical principles

An accelerometer measures the acceleration it experiences relative to freefall. This is equivalent to inertial acceleration minus the local gravitational acceleration^[1][2], where inertial acceleration is understood in the Newtonian sense of acceleration with respect to a fixed reference frame, which the Earth is often considered to approximate.

As a consequence, quite counter-intuitively, an accelerometer at rest on the Earth's surface will actually indicate 1 g upwards along the vertical axis. To obtain the inertial acceleration (due to motion alone), this gravity offset must be subtracted. Along all horizontal directions, the device yields acceleration directly. Conversely, the device's output will be zero during free fall, where the acceleration exactly follows gravity. This includes use in an earth orbiting spaceship, but not a (non-free) fall with air resistance, where drag forces reduce the acceleration until terminal velocity is reached, at which point the device would once again indicate the 1 g vertical offset.

The reason for the appearance of a gravitational offset is Einstein's equivalence principle^[3], 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.

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).

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^[4], 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 external force such as gravity, the mass is displaced until the external force is balanced by the spring force. The displacement is translated into 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 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^{[citation needed]}. Useful numbers like 0-60mph, 60-0mph and 1/4 mile times can all be found using accelerometers.

Accelerometers can be used to measure vibration on cars, machines, buildings, process control systems and safety installations. They can also be used to measure seismic activity, inclination, machine vibration, dynamic distance and speed with or without the influence of gravity. Applications for accelerometers that measure gravity, wherein an accelerometer is specifically configured for use in gravimetry, are called gravimeters.

Notebooks equipped with accelerometers can to contribute to the Quake-Catcher Network. QCN is a BOINC project aimed at scientific research of earthquakes^[5]

Accelerometers are also increasingly used in the Biological Sciences. High frequency recordings of bi-axial^[6] or tri-axial acceleration^[7] (>10 Hz) allows the discrimination of behavioural patterns while animals are out of sight. Furthermore, recordings of acceleration allow researchers to quantify the rate at which an animal is expending energy in the wild, by either determination of limb-stroke frequency^[8] or measures such as Overall Dynamic Body Acceleration^[9] Such approaches have mostly been adopted by marine scientists due to an inability to study animals in the wild using visual observations, however an increasing number of terrestrial biologists are adopting similar approaches. 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,^[10] fans,^[11] rollers,^[12] compressors,^[13] and cooling towers,^[14]. Vibration monitoring programs are proven to save money, reduce downtime, and improve safety in plants worldwide by detecting conditions such as shaft misalignment, rotor imbalance, gear failure^[15] or bearing fault^[16] which can lead to costly repairs. Accelerometer vibration data allows the user to monitor machines and detect these faults before the rotating equipment fails. Vibration monitoring programs are utilized in industries such as automotive manufacturing,^[17] machine tool applications,^[18] pharmaceutical production,^[19] power generation^[20] and power plants,^[21] pulp and paper, ^[22] food and beverage production, water and wastewater, hydropower, petrochemical and steel manufacturing.

They are also used to warn of loosening anchors, preventing drifting. An accelerometer is encapsulated in a waterproof casing housed inside a bronze outer case 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^[23].

Building and Structural Monitoring

Accelerometers are used to measure the motion and vibration of a structure that is exposed to dynamic loads^[24]. 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.^[25][26]

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.^[27][28]

Transport

Accelerometers are used to detect apogee in both professional^[29] and in amateur^[30] rocketry.

Accelerometers are also being used in Intelligent Compaction rollers. Accelerometers are used alongside gyroscopes in inertial guidance systems.^[31]

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.^[32] 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.^[33]

Consumer Electronics

Accelerometers are increasingly being incorporated into personal electronic devices. Some smartphones and personal digital assistants contain accelerometers for user interface control. Prominent examples include the Apple iPhone, Samsung Omnia, Nokia N96, Nokia 5800, Sony Ericsson W910i, the Palm Pre, Blackberry Storm, HTC Touch Diamond^[34], the Apple iPod touch, the Apple iPod Nano 4G, the Google G1 and the new Microsoft Zune HD, scheduled for release in the fall of 2009.

Gaming devices like the Wii Remote ^[35] for Nintendo's 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.

The Sony PlayStation 3 uses the DualShock 3 remote which also uses a six-axis accelerometer to make steering even 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^{[citation needed]}.

For example, Apple uses an LIS302DL accelerometer in the iPhone, iPod Touch and the 4th generation iPod Nano allowing the device to know when it is tilted on its side. Third-party developers have expanded its use with fanciful applications such as electronic bobbleheads.^[36]

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 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 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^[37], 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

• 4-20 mA loop power
• Integrated Electronic Piezoelectric (IEPE) accelerometer
• Direct piezoelectric 'charge mode' output, for connection to an instrument with a charge amplifier input ^[39]
• Recording Accelerometer

See also

Wikimedia Commons has media related to: Accelerometers

• g-force
• Gyroscope
• Inertial navigation

External links

• Thinking About Accelerometers and Gravity by Dave Redell, LUNAR #322
• Practical Guide to Accelerometers
• How to Design an Accelerometer
• Different types of Accelerometer

References