| 摘要 |
<p>Magnetic flux is induced in the tape (10) by applying a magnetic field to the tape. Where there is a perforation (12) or a crack (14) air gaps are created and magnetic flux (15, 18) radiates outwardly from the tape across these air gaps. The presence of the crack (14) is detected by sensing the radiating flux (15, 18) at two distances from the tape. At one distance (30), close to the tape, the total flux is the sum of the flux from the crack and the perforation. At the other distance (32), further from the tape, the flux is solely that produced by the perforation. A coil (36, 38) is placed at each position to sense the flux. The tape (10) is moved, which causes the flux (15, 18) to intersect the coils (36, 38). The intersection produces an output signal (40) from each coil (36, 38). The signal (40) produced by the coil (38) further from the tape is subtracted from the signal (40) produced by the coil (36) closer to the tape; the difference being a signal (58) caused by the additional flux produced by the crack (14). An alarm (94) is sounded and an indicator (88) lights when the signal is present.</p> |
