What makes the Nike + iPod Sport Kit tick?
Boing Boing recently posted a link to Spark Fun Electronics' Nike + iPod Sport Kit sensor autopsy. They claim the device is powered not by an accelerometer, but rather a piezoelectric sensor, essentially making it a sophisticated pedometer. It measures how long your weight is on your foot to determine your speed.
But, Apple's website contradicts this claim: "The sensor uses a sensitive accelerometer to measure your activity, then wirelessly transfers this data to the receiver on your iPod nano." To further confuse the issue, Apple has posted a tech doc that implies that the piezoelectric sensor is an accelerometer: "A sensitive piezoelectric accelerometer monitors your footstrike when you walk or run and determines the amount of time your foot spent on the ground. This contact time is directly related to your pace."
I raise this question because, after yesterday's run, I mapped out my route on gmap-pedometer.com, and found that my shoes are over-reporting my mileage to the tune of about 20%. I recognize that this most likely a calibration issue, but the Spark Fun report has me wondering how accurate the Sport Kit sensor is in the first place.
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As to the sensor internals, EETimes wrote an article in August discussing their findings. From the article:
"In the transmitter implementation, this equates to a low-cost stroke of genius in the form of using a piezoelectric disk speaker–common to inexpensive toys and greeting cards–in reverse mode. By allowing some travel in the plate of the speaker, the piezoelectric effect causes the transducer to function as the pickup of foot action. About the size (and cost) of a dime, the speaker isn’t driven to make sound, but rather monitored for motion-induced electrical signals."
You can read more on my site.