| 摘要 |
<p>An arrangement to determine whether a tennis ball is "in" or "out" on a playing surface which includes both a transmit coil (1) and a receive coil (2, 3) beneath the playing surface and a modified tennis ball (24) to have a non-zero magnetic permeability so that an alternating signal can be transmitted from the transmit coil (1) and re-radiated from the tennis ball which can then be detected by the receive coils (2, 3) and can then be synchronously demodulated and resistive and reactive components of the signal analysed to discriminate the ball from other influences. The receive coils can be wound to avoid electro-magnetic interference from far fields.</p> |
