Accelerometer

accelerometer

Instrument that measures acceleration. Because it is difficult to measure acceleration directly, the device measures the force exerted by restraints placed on a reference mass to hold its position fixed in an accelerating body. The output is usually either a varying electrical voltage or displacement of a moving pointer over a fixed scale. Specially designed accelerometers are used in varied applications: control of industrial vibration test equipment, detection of earthquakes (seismographs), and input to navigational and inertial guidance systems.

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accelerometer

A device that detects acceleration and tilt. Built using MEMS technology, accelerometers detect impact and deploy automobile airbags as well as retract the hard disk's read/write heads when a laptop is dropped. Digital cameras employ them in their image stabilization circuits. They are used in washing machines to detect excessive vibration and in pedometers for more accurate distance measurement. They enable a handheld display to be switched between portrait and landscape modes when the unit is turned.

Springs and Bubbles
MEMS accelerometers initially used a microminiaturized cantilever-type spring, which converts force into a displacement that can be measured. Subsequent accelerometers use a heated gas bubble with thermal sensors and function much like the air bubble in a construction level. When the accelerometer is tilted or accelerated the sensors pick up the location of the gas bubble. See MEMS.

MEMSIC Accelerometer

MEMSIC's dual-axis thermal accelerometer works conceptually like the air bubble in a construction level. The square in the middle of the chip is a resistor that heats up a gas bubble. The next larger squares contain thermal couples that sense the location of the heated bubble as the device is tilted or accelerated. (Image courtesy of MEMSIC, Inc.)

Microfabrica Accelerometer

Microfabrica's EFAB system was the first MEMS foundry process to accept CAD files as input, turning customer designs into micromachines much faster than traditional methods. EFAB builds the devices one metal layer at a time. In this image, the device at the bottom left with the C-shaped wings is an accelerometer. (Image courtesy of Microfabrica Inc., www.microfabrica.com)

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Accelerometer

A mechanical or electromechanical instrument that measures acceleration. The two general types of accelerometers measure either the components of translational acceleration or angular acceleration.

Most translational accelerometers fall into the category of seismic instruments, which means the accelerations are not measured with respect to a reference point. Of the two types of seismic instruments, one measures the attainment of a predefined acceleration level and the other measures acceleration continuously. In one version of the first type of instrument, a seismic mass is suspended from a bar made of brittle material which fails in tension at a predetermined acceleration level.

Continuously measuring seismic instruments are composed of a damped or an undamped spring-supported seismic mass which is mounted by means of the spring to a housing. The seismic mass is restrained to move along a predefined axis. Also provided is some type of sensing device to measure acceleration.

The type of sensing device used to measure the acceleration determines whether the accelerometer is a mechanical or an electromechanical instrument. One type of mechanical accelerometer consists of a liquid-damped cantilever spring-mass system, a shaft attached to the mass, and a small mirror mounted on the shaft. A light beam reflected by the mirror passes through a slit, and its motion is recorded on moving photographic paper. The type of electromechanical sensing device classifies the accelerometer as variable-resistance, variable-inductance, piezoelectric, piezotransistor, or servo type of instrument or transducer.

There are several different types of angular accelerometers. In one type the damping fluid serves as the seismic mass. Under angular acceleration the fluid rotates relative to the housing and causes on two symmetrical vanes a pressure which is a measure of the angular acceleration. Another type of instrument has a fluid-damped symmetrical seismic mass in the form of a disk which is so mounted that it rotates about the normal axis through its center of gravity. The angular deflection of the disk, which is restrained by a spring, is proportional to the angular acceleration.