Abstract: The uneven lay length leads to the early retirement of the wire rope. The lay length detection provides the design bases for improving the production technology and enhances the safe use of the wire rope. The wire rope in motion can not be detected by the traditional lay length detection method. The imaging recognition method for measuring the lay length is limited by the external light and the attachment on the surface of the wire rope. In view of the above problems, the lay length is detected by the eddy current method. To enlarge the covering area of the magnetic field, the differential bridge composed of the rectangular coil is utilized. The finite element analysis of the lay length of the wire rope with the differential bridge is conducted. The curve of the real part of the differential voltage shows that the distance between the adjacent peaks or throughs of the scanning signal is the gap between the two adjacent strands. In the range of a lay length the number of the peaks or throughs of the scanning signal is related to the strands number of the wire rope. The eddy currents induced by the current element in the upper side and lower side of the rectangular coil cancel on the wire rope surface. The weaker the cancellation effect, the higher the detection sensitivity of the lay length. To extract the useful information from the experimental results, the scanning signal is post-processed by the ensemble empirical mode decomposition (EEMD) method. As the correlation coefficient is large, the distance between the adjacent peaks or throughs is the lay length. As the correlation coefficient is small, the number of the peaks or throughs in a lay length is the same with the simulated result. The results show that the relative error of the wire rope lay length obtained by this method is less than 2.38%.

Key words: lay length, rectangular coil, eddy current testing, finite element method, post-processed