Chapter 13 Accelerometry

Abstract

Accelerometers are small lightweight motion transducers that are capable of measuring accelerations less than 0.02 G ( G =9.807 m/s 2 , the static acceleration of gravity). The use of accelerometers in human motion analysis is discussed in this chapter. The Internet is a rich source of additional information pertaining to basic technology and manufacturers, and this chapter contains an incomplete list of useful sites. Several types of accelerometers are now commercially available, but piezoresistive, piezoelectric and capacitance accelerometers are employed most commonly in human applications. These accelerometers are based on Newton's law of mass acceleration (Force=mass x acceleration) and Hooke's law of spring action (Force=spring constant x change in length of a spring). Piezoresistive accelerometers consist of a small mass attached to a semiconductor beam that behaves like a spring. Deflection of the beam is measured with strain gauges that are connected in a Wheatstone bridge. The output voltage of the Wheatstone bridge is proportional to acceleration. Piezoelectric accelerometers contain a mass that is attached to a piezoelectric crystal, which behaves as a spring. Deformation of the crystal produces a small voltage (∼millivolts) that is proportional to acceleration. Capacitance accelerometers contain a variable capacitor, in which the gap among the capacitor plates changes in proportion to acceleration.