handbook of modern sensors

308 8 Velocity and Acceleration
Fig. 8.5. Exposed view of a piezoresistive
accelerometer.
accelerometers with discrete, epoxy-bonded strain gauges tend to have undesirable
output temperature coefficients. Because they are manufactured separately, the gauges
require individual thermal testing and parameter matching. This difficulty is virtually
eliminated in modern sensors, which use the micromachining technology of silicon
wafers.
An example of a wide-dynamic-range solid-state accelerometer is shown in Fig.
8.5. It was developed by Endevco/Allied Signal Aerospace Co. (Sunnyvale, CA). The
microsensor is fabricated from three layers of silicon. The inner layer, or the core,
consists of an inertial mass and the elastic hinge. The mass is suspended inside an
etched rim on the hinge, which has piezoresistive gauges on either side. The gauges
detect motion about the hinge. The outer two layers, the base and the lid, protect the
moving parts from the external contamination. Both parts have recesses to allow the
inertial mass to move freely [4]. Several important features are incorporated into the
sensor. One is that the sensitive axis lies in the plane of the silicon wafer, as opposed to
many other designs where the axis is perpendicular to the wafer. Mechanical integrity
and reliably are assured by the fabrication of all of the components of the sensor from
a single silicon crystal.
When acceleration is applied along the sensitive axis, the inertial mass rotates
around the hinge. The gauges on both sides of the hinge allow rotation of the mass
to create compressive stress on one gauge and tensile on the other. Because gauges
are very short, even the small displacement produces large resistance changes. To
trim the zero balance of the piezoresistive bridge, there are five trimming resistors
positioned on the same crystal (not shown in Fig. 8.5).