A Board and Its Sensors
The svelte Wii Balance Board weighs in at about 8 pounds (roughly 3.5 kilograms) and can support up to 330 pounds (about 150 kilograms) for in-game functionality. (It can actually support quite a bit more, up to 620 pounds, but games like "Wii Fit" will not work if you weigh more than 330 pounds [source: Casamassina].) It runs on four AA batteries, which Nintendo says can provide up to 60 hours of play time depending on the settings used, and the board shuts off automatically to conserve power when not in use after a short period of time. It has built-in wireless capabilities and communicates with the Wii using the same Bluetooth technology found in the Wii Remote.
Load sensors sit snugly at the bottom of each of the board's four squat, cylindrical-shaped legs. These work together to determine the position of your center of gravity and to track your movements as you shift your weight from one part of the board to another [source: Okamoto et al.]. Each is a small strip of metal with a sensor, known as a strain gauge, attached to its surface [source: Snider].
Strain gauges measure the level of stress applied to an object. A gauge consists of a single, long electrical wire that is looped back and forth and mounted onto a hard surface -- in this case, the strip of metal. Applying a force on the metal by standing on the board will stretch or compress the wire [source: NASA].
Imagine stretching a rubber band: It gets longer as you pull, and it also gets thinner as it stretches. The same thing happens in the strain gauges -- they get longer while their cross-sectional area gets smaller. Because of the changes to length and diameter in the wire, its electrical resistance increases. The change in electrical resistance is converted into a change in voltage, and the sensors use this information to figure out how much pressure you applied to the board, or how much you weigh.
The sensors' measurements will vary depending on your position and orientation on the board; if you're standing in the front left corner, for instance, the sensor in that leg will record a higher load value than will the others. A microcomputer in the board takes the ratio of the load values to your body weight and the position of the center of gravity to determine your exact motion; it then transmits that information back to the Wii at a rate of 60 signals a second [sources: Okamoto et al., Casamassina]. The sensors must be on a flat surface to work properly, so foot extensions -- which come with the board -- provide stability and ensure proper communication with the Wii and the board when it's used on carpet.
The bending in the metal is practically imperceptible to the naked eye: for instance, someone weighing 220 pounds would distort it by less than a millimeter [source: GameSpy]. The sensors are highly accurate and can detect changes in weight of as little as tens of grams [source: Casamassina].
Up next, we'll look at some of the peripheral's known limitations and risks and explain why you can (but probably shouldn't) wear shoes on it.