Game Development Reference
Micro-scale accelerometers are not that much different from the machine previously
described but generally use a cantilevered beam instead of a spring. To track more than
one axis, sometimes three discrete accelerometers are placed out of plane with respect
to one another. Alternatively, more complex models use elements that can sense all three
directions within a single integrated sensor. These generally give better results.
The only important difference from the aforementioned examples, besides MEMS being
thousands of times smaller in scale than the mass and spring, is how to measure the
deflection of the test mass. There are three common methods employed in accelerom‐
eters. For most game devices where extreme accuracy isn't required, the deflection is
usually measured as a change in capacitance. This is somewhat the same way that ca‐
pacitive touch screens work, as described in Chapter 20 , and is shown in Figure 21-2 .
Figure 21-2. MEMS cantilever accelerometer
The beam deflects under the influence of the external accelerations of the test mass and
brings two charged plates farther or closer together. This changes the capacitance of the
system. This change can then be calibrated to the imposed acceleration.
Other methods include integrating a piezoresister in the beam itself so that the deflection
of the beam changes the resistance of the circuit. Although this ultimately gives better
results, these are harder to manufacture. For the most demanding applications, there
are accelerometers using piezoelectric elements based on quartz crystals. These are very
sensitive even during high-frequency changes in acceleration but are generally not used
in sensing human-input motion.