3D space and the Wii

The Nintendo Wii, with its famous wireless controller, has changed the gaming industry. Now millions of people are playing games where they jump, point or punch in the air. The Wii knows where it is in three-dimensional space, but how? It’s a clever little microchip called an accelerometer. When you move, the forces you apply cause masses to start to move. The bigger the force the faster the mass moves. Newton tells us that force equals mass multiplied by acceleration, so if we have a mass and we can calculate its acceleration we have an idea of the forces involved. That’s what the accelerometer chip does.

On the chip a tiny mass (called a proof mass) is held in place by tiny silicon springs. When you move you’re providing a force, so the mass starts to accelerate in the same direction you’re moving. The proof mass shifts, and as it does it changes the electrical properties of its surrounding. The capacitance, which is the amount of electrical charge something can store, changes as the mass moves over an electrical plate. If you have three plates, each perpendicular to each other, surrounding the mass as it moves, the capacitances of the plates change. You can measure this change, and then extract information that lets you know the direction the mass is moving in. This gives you a electrical signal that shows the direction the accelerometer moved in 3D space, and that’s converted to a signal that allows the console to know what you’re doing with your Wii. Game on.