| Alternating current field measurement (ACFM) is a main progress of Non-destructive Testing (NDT). This method can size the length and depth of crack on metal surface, which has no effect with lift-off, property of metal material and boundary effect. In this paper, a new sensor is designed by using the basic principle and characteristic of ACFM. A cross rectangular exciting coil in this sensor uses a soft magnetic material as magnetic core, and it has smaller bulk, number of turns and exciting power than an odd rectangular coil. The crack on metal surface in every direction can be detected by using cross rectangular exciting coil. A pick-up coil of the sensor designed can acquire the faint signal of the magnetic perturbation on surface of aluminum plane. According as principal of cross-correlation, the hardware circuit is used to restrain noise efficiently and acquire faint change of crack signal. The system's software is written by using windows' program technology. The test curve is smoothed by digital filter which is realized by software. The relation between test signal with the length and depth of crack is analyzed. In order to eliminate the velocity effect when sensor is moved, the butterfly plot is used to distinguish crack. The method of qualitative analysis and quantitative analysis in sizing crack on surface of aluminum plane is researched. The test system is accomplished, which can test, judge, find crack real time, give an alarm automatically and record data. |
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