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a device for measuring acceleration (g-overloads) created by space vehicles, rockets, airplanes, and other moving vehicles and by testing machines, engines, and so on.
Accelerometers are distinguished by the kind of motion that is measured—linear or angular; by their operating principles—for instance, mechanical or electromechanical, and so on; and by their function—measuring acceleration as a function of time or path. There are also peak-value accelerometers, which measure the time at which the object measured attains a specified acceleration level or the maximum value of the acceleration in a process going to completion rapidly—for example, in an impact process. An accelerometer combined with a recording instrument is known as an accelerograph.
Acceleration is measured in the linear mode (one degree of freedom) by a mechanical accelerometer—a pendulous accelerometer in which the deviation of the pendulum from the equilibrium position occurs in response to the acceleration. (The dimension of the deviation is indicated by an arrow on the scale calibrated in units of acceleration.) In an electromechanical accelerometer, acceleration is measured by a strain gauge which varies its electrical parameter (resistance, inductance, or capacitance) in response to mechanical deformation proportional to the acceleration. Acceleration is detected in a peak-value accelerometer either by a pendulous device which breaks contact in the electric circuit when the object investigated attains a specified level of acceleration, or by a piezoelectric sensor which detects acceleration by generating an electrical voltage in response to mechanical compression caused by the forces of inertia. In electromechanical and peak-value accelerometers, accelerations are recorded on the screen of an oscillograph on which electrical signals picked up from devices sensing the acceleration appear after amplification.
Mechanical accelerometers are used to measure large accelerations when machine parts or moving objects experience vibrations of relatively low frequencies—to 10 hertz (Hz). Electromechanical accelerometers are used for high frequencies of vibrations; peak-value accelerometers are used when the vibrations of the measured accelerations run in the range of frequencies from 10 Hz to 20 kilohertz. Peak-value accelerometers make it possible to measure accelerations from 1 cm/sec2 to 30 km/sec2 (0.001 to 3,000 g, where g is the gravitational acceleration).
Three-component accelerometers used in gravimetry— that is, in determination of the gravitational acceleration at sea level and in the air—record and treat accelerations resulting from the heaving or rolling of a ship or airplane at the point where a gravimeter or pendulous instrument is installed. The accelerometers employed in gravimetry are designed for the range of acceleration measurements around several hundred cm/sec2, and their accuracy is of the order of 1 cm/sec2.
REFERENCESFridlender, G. O., and V. P. Seleznev. Pilotazhnye manometriche-skie pribory, kompasy i avtoshturmany. Moscow, 1953.
Iorish, Iu. I. Izmerenie vibratsii:Obshchaia teoriia, metody i pribory. Moscow, 1956.
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.
accelerometerA 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 also enable a handheld display to be switched between portrait and landscape modes when the unit is turned.
Springs, Bubbles, Capacitance and Crystals
MEMS accelerometers initially used a microminiaturized cantilever-type spring, which converted force into a displacement that was measured. Subsequent MEMS devices use a heated gas bubble with thermal sensors that functions like the air bubble in a construction level (see MEMS). Other types of accelerometers use microstructures that change their capacitance or microscopic crystals that generate a voltage when stressed.
Accelerometers, Gyroscopes and Magnetometers
An accelerometer measures a change in velocity and position, whereas a gyroscope measures rotational changes, and a magnetometer measures compass direction. All three are used in an "inertial measurement unit" (IMU) in airplanes, spacecraft and satellites, and mobile devices use accelerometers and magnetometers.
|Dual-Axis Thermal Accelerometer|
|This MEMS unit works 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. As the device is tilted or accelerated, surrounding thermal couples sense the bubble's location. (Image courtesy of MEMSIC, Inc.)|
|An iPhone Level|
|Because of its built-in accelerometer, smartphones can be turned into a digital level with apps such as this one from PosiMotion (www.posimotion.com).|
|The device at the bottom left with the C-shaped wings is an accelerometer. Built one metal layer at a time, Microfabrica's EFAB system was the first MEMS foundry process to quickly turn customers' CAD files into micromachines. (Image courtesy of Microfabrica Inc., www.microfabrica.com)|