軟鋼ワイヤの縦波用多段電磁超音波センサの開発

抄録

Two types of electromagnetic acoustic transducers (EMATs), both for longitudinal wave in steel wire, are developed. In inspection of steel wires throughout a long range, ultrasonic wave will be an effective tool. The EMAT, which consists of a coil surrounding the wire and an electromagnet, enables us to generate and detect the longitudinal wave at an arbitrary position of the wire, and seems to be suitable for this purpose. However, there are two problems to be solved, both can be attributed to the characteristic of the EMAT. First, the magnetostriction of the wire limits the saturation amplitude of the generated wave, though it is required to be high to improve the signal to noise ratio. We develop a multiple EMAT, which effectively amplifies the signal by overlapping the waves excited by some equivalent coils. The other problem is that the EMAT launches the longitudinal wave symmetrically in two directions. To estimate the position of the flaw, the measurement should be done from at least two different locations of the EMAT. We also produce a unidirectional EMAT with two equivalent coils. One of the two directional waves excited by one coil is canceled by antiphase wave from the other coil. Combination of these two techniques is proved to be the solution for the above two intrinsic problems of the EMAT.