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Accelerometers By virtue of Newton’s second law (F = ma) a sensor may be made to sense acceleration by simply measuring the force on a mass. At rest, acceleration is zero and the force on the mass is zero. At any acceleration a, the force on the mass is directly proportional given a fixed mass. This force may be sensed in any method of sensing force but, again, the strain gauge will be representative of direct force measurement.

Accelerometers - principles Methods of acceleration sensing start with the mechanical model of a mass. The mass, moves under the influence of forces, has a restoring force (spring) and a damping force (which essentially prevents it from oscillating).

Accelerometers - principles Under these conditions, and assuming the mass can only move in one direction (along the horizontal axis), Newton’s second law may be written as: Assumes that the mass has moved a distance x under the influence of acceleration, k is the restoring (spring) constant and b is the damping coefficient. Given the mass m and the constants k and b, a measurement of x gives an indication of the acceleration a.

Accelerometers - principles Therefore, for a useful acceleration sensor (often called accelerometer) it is sufficient to provide a component which can move relative to the sensor’s housing and a means of sensing this movement. A displacement sensor (position, proximity, etc.) can be used to provide an appropriate output proportional to acceleration.

انواع شتاب سنج ها مکانیکی پیزوالکتریکی، پیزومقاومتی MEMS

Accelerometers - Capacitive One plate of a small capacitor is fixed and connected physically to the body of the sensor. A second plate serves as the inertial mass of the sensor is free to move and connected to a restoring spring. Three basic configurations are shown in Figure 1. The restoring force is provided by springs (Figure 1a,c) or by a cantilever’s fixed end (Figure 1b).

Capacitive accelerometers Figure 1

Strain gauge accelerometers The mass is suspended on a cantilever beam Strain gauge senses the bending of the beam

Variable inductance accelerometers A rod connected and moving with the mass links to a coil. The inductance of the coil is proportional to the position of the mass An LVDT may be used

Hall element magnetic accelerometers Acceleration changes distance to hall element Hall element output is calibrated as acceleration The magnet may be on the hall element side (biased hall element)

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