A device for measuring the movement of a ball is a flat pad or a frame defining an opening with one or more magnetic field inducing elements mounted thereon. One or more sensor elements are mounted in the flat pad or the frame, and connected to instrumentation so as to record induced, time-varying current...http://www.google.com.hk/patents/US5863255?utm_source=gb-gplus-share專利 US5863255 - Device and method to measure kinematics of a moving golf ball
