How the Nike and iPod works
Just like cell phones and televisions, the Nike + iPod sensor and receiver communicate using radio wavesThe sensor detects every step a runner takes and broadcasts this information to the receiverThe receiver routes the information to the iPod Nano, which relays it to the runner, either on the screen or through the headphones
The sensor fits into a hole under the insole of a Nike+ running shoeIt detects the runner's footfalls through its piezoelectric accelerometerPiezoelectric materials produce electrical current when they change shape, or they change shape when exposed to electricityPiezoelectric transducers, often used in speakers, rapidly change shape when they come in contact with electrical currentPiezoelectric 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 microscopicDepending on how the crystals are cut, they usually produce an electrical charge when compressed in a specific direction or along a specific planeFor these reasons, piezoelectric sensors can be very small and very accurateIn 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 groundWhen someone is standing still or walking slowly, his feet spend more time touching the earth than in the airBut when jogging or sprinting, his feet spend less and less time on the groundThe faster he runs, the less time his feet spend in contact with the surface under themBecause of this basic trait of walking and running, a processor can use equations to convert contact time into running speedThe 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 switchWhen the shoes aren't moving, the accelerometer has no footsteps to report -- it stops sending dataIn the absence of the accelerometer's output, the sensor eventually puts itself to sleepBut when a runner puts his shoes on and takes a few steps, the sensor generates electrical pulses, and the sensor resumes operationThe sensor also has a physical switch which can turn the unit off; the unit stays off until someone presses the switch againThis 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 antennaIt broadcasts its data at a radio frequency of 2.4 gigahertz using a proprietary protocolIn addition to transmitting data about a person's running stride, it transmits a unique code that it uses to identify itselfWe'll look at how the receiver uses this information next