I didn't see it posted elsewhere, but being a design EE I did a quick search and found the background information on the gyro used in the cart. It's a quick, but interesting read that explains how it works in the cart, and in other devices such as Nikon Coolpix cameras.
http://www.nec-tokin.com/english/product/piezodevice2/ceramicgyro.html
http://www.nec-tokin.com/english/product/piezodevice2/ceramicgyro.html