Just like cell phones and televisions, the Nike + iPod sensor and receiver communicate using radio waves. The sensor detects every step a runner takes and broadcasts this information to the receiver. The receiver routes the information to the iPod Nano, which relays it to the runner, either on the screen or through the headphones.
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Piezoelectric materials can create electrical impulses when they
The sensor fits into a hole under the insole of a Nike+ running shoe. It detects the runner's footfalls through its piezoelectric accelerometer. Piezoelectric materials produce electrical current when they change shape, or they change shape when exposed to electricity. Piezoelectric transducers, often used in speakers, rapidly change shape when they come in contact with electrical current. Piezoelectric sensors, on the other hand, use quartz, silicon or manmade crystals that produce electricity when squeezed, moved or bent.
These generator-like crystals are often microscopic. Depending on how the crystals are cut, they usually produce an electrical charge when compressed in a specific direction or along a specific plane. For these reasons, piezoelectric sensors can be very small and very accurate. In addition to the Nike+ sensor, tiny piezoelectric sensors provide the motion-sensing capabilities for the PlayStation 3 and Nintendo Wii controllers.
The accelerometer in the Nike+ sensor detects when a person's foot is on the ground. When someone is standing still or walking slowly, his feet spend more time touching the earth than in the air. But when jogging or sprinting, his feet spend less and less time on the ground. The faster he runs, the less time his feet spend in contact with the surface under them. Because of this basic trait of walking and running, a processor can use equations to convert contact time into running speed. The process is the most accurate when runners calibrate their sensors by running a pre-set distance so get a baseline reading.
The accelerometer also acts as the sensor's on/off switch. When the shoes aren't moving, the accelerometer has no footsteps to report -- it stops sending data. In the absence of the accelerometer's output, the sensor eventually puts itself to sleep. But when a runner puts his shoes on and takes a few steps, the sensor generates electrical pulses, and the sensor resumes operation. The sensor also has a physical switch which can turn the unit off; the unit stays off until someone presses the switch again. This preserves the battery's life (the 1,000-hour battery can't be replaced) when someone isn't running regularly or is running in different shoes.
The Nike+ iPod sensor sends information to the receiver using a built-in transmitter and antenna. It broadcasts its data at a radio frequency of 2.4 gigahertz using a proprietary protocol. In addition to transmitting data about a person's running stride, it transmits a unique code that it uses to identify itself. We'll look at how the receiver uses this information next.