Abstract: The lay length is one of the important parameters of the wire rope. The measurement of the lay length can indirectly reflect the stress state and the surface damage state of the wire rope. 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. The magnetic flux leakage method requires the wire rope to be magnetized. In view of the above problems, An eddy current method to detect the lay length using the double pancake coils differential bridge is proposed. Firstly, the governing formulations based on the A_r,V-A_r for the eddy current response analysis are established. Combing the formulations, the distribution of the eddy current in the single strand is simulated. It forms vortex on the strand surface just under the pancake coil. The distributions of the eddy current on the curved surfaces of the two sides of the strand are symmetrical. Secondly, the medium radius of the pancake coil is optimized. When the medium radius equals to or is smaller than the gap between the two adjacent strands, the detection precision of the lay length is high and the detection sensitivity is also high. Thirdly, the experimental platform is set up to test the lay length. The experimental results and the simulation results show that the distance of the adjacent peaks or throughs of the scanning signal is just the gap between the two adjacent strands. The number of the peaks or throughs of the scanning signal in the lay length is related to the strands number of the wire rope. Finally, in order to effectively extract the useful information from the experimental results, the ensemble empirical mode decomposition (EEMD) method is utilized. The results show that the relative error of the wire rope lay length obtained by this method is less than 0.13%.

Key words: wire rope, lay length, eddy current testing, differential bridge, finite element analysis