Game Development Reference
amount of strain. Once the pressure that causes an amount of strain is known, it is
possible to determine the amount of load. Now you might be wondering how the strain
gauge measures the amount that the Wii's legs compress when you stand on them.
One of the most common electronic strain gauges is the piezoresistive strain gauge . The
simplest example of a piezoresistive strain gauge would be a single wire. If you were to
elongate a wire from its rest length, the cross-sectional area decreases. This causes a rise
in the electrical resistance of the wire. After measuring the rest resistance, you can use
the difference to determine how much the wire has elongated. Knowing the mechanical
properties of the wire, you can also determine how much force it takes to stretch the
To make strain gauges sensitive without having long linear wire elements, the conductive
material is often arranged in a strain-sensitive pattern, as shown in Figure 23-3 . This
looping back and forth of the conductor allows for great sensitivity without increasing
the physical space the sensor occupies. Here the rest length would be 18 times longer
than the physical length of the sensor.
Figure 23-3. Typical strain-sensitive pattern
Center of gravity
The board has four legs, each of which houses a load sensor. The board uses strain gauges
similar to those discussed earlier. These gauges elongate when a force is applied to them.
The elongation changes the electrical resistance of the circuit of which the strips are a
part, and this is reported back to the controller. Figure 23-4 shows two sensor outputs.
The first is with the user standing so that her center of gravity is over the center of the
board. The second state shows what the board's sensors would measure after the user
has shifted her center of gravity.