A method for improving the excitation efficiency of the magnetostrictive sensor used in GWT under a wider inspection frequency range is needed.In this paper, we present a method to enhance the excitation efficiency of the magnetostrictive transmitter for pipe inspections by optimizing the axial length of the excitation magnetic field. What’s more, the enhancement can be realized under the frequency range of L(0,2) mode rather than a single frequency.To achieve this objective, a special structure, adding two copper rings beside the transmitter coil, is used here to adjust the axial length. The structure is similar to the one used by Seco et al. [18], however, the focus of this paper is different from the previous paper, in which the authors focused on compensating the hysteresis occurred in the magnetostrictive linear position sensor.

The details of the structure to adjust the axial length of the excitation magnetic field are presented in Section 2. In Section 3, the influence GSK-3 of the axial length of the excitation magnetic field on the excitation efficiency is analyzed theoretically. Section 4 validates the theoretical analysis by experiments. Finally, a brief conclusion is given in Section 5.2.?The Method for Adjusting the Axial Length of the Excitation Magnetic Field2.1. The Axial Length of the Excitation Magnetic FieldFor an alternating magnetic field produced by the transmitter coil in a pipe, there are some parameters could be used to describe the distribution of the field, including the strength, the depth, the axial length, and the shape of the distribution, as shown in Figure 3.

Figure 3.The parameters of the alternating magnetic field produced by a solenoid coil.The strength stands for the maximum of the magnetic field strength of the field. The depth is the skin depth and the axial length is the axial range where the alternating magnetic field exists. The shape of the distribution is the geometrical shape of the magnetic field strength in the whole skin area when the above three parameters are determined. The alteration of transmitter structures may change these four parameters, and the four parameters influence the excitation efficiency. Usually the axial length of the excitation magnetic field is influenced by the material of the pipe and the structure of the transmitter coil. However, the conventional structure cannot meet the requirement of adjusting the axial length conveniently. A special structure is needed to adjust the axial length. A structure, in which two copper rings are added beside the transmitter coil, is used in this paper. The detailed comparison between the conventional structure and the special structure is presented as follows.2.2.