Development Of Cross-coil Band Magnetostrictive Torsional Guided Wave Sensor

Magnetostrictive guided wave detection technology has the advantages of long-distance detection and unreachable area detection with single-point excitation.It has been widely used in stainless steel pipe inspection in recent years.At present,the torsional guided wave sensor for stainless steel pipe detection mainly uses a pre-magnetized magnetostrictive belt to provide a bias magnetic field.This form of magnetic field is unstable,and it is difficult to ensure the consistency of the excitation signal during detection.Based on the analysis of the band magnetostrictive torsional guided wave sensing method,the influence of bias magnetic field on the coupling efficiency of the cross-coil sensor is studied,and the sensor is designed and fabricated.The sensor realizes the regulation of the bias magnetic field,so that the excitation signal has good consistency,and at the same time can provide an optimal magnetic field to obtain a large conversion efficiency of the sensor.This sensor supports stainless steel pipe inspection.Firstly,based on the analysis of the principle of magnetostrictive torsional guided wave sensing,two kinds of band magnetostrictive torsional guided wave sensors,cross-coil and oblique winding,are proposed.The feasibility of exciting and receiving torsional mode guided wave in stainless steel tube is verified by experiments.The cross-coil sensor provides a static bias magnetic field from the toroidal coil,which can be varied by adjusting the magnitude of the current in the toroidal coil.The oblique wrap sensor requires only one AC input,but due to the frequency multiplication effect,the sensor has low efficiency and poor defect detection capability.Secondly,the influence of bias magnetic field on the coupling efficiency of the cross-coil sensor is studied.Based on the Torsional magnetostriction effect,the influence of bias magnetic field strength on the coupling efficiency is analyzed and verified by experiments.The results show that the excitation process and the receiving process have their respective optimal bias magnetic field regions to maximize the coupling efficiency.Further research shows that when the sensor is operating in the optimal bias magnetic field,the effect of the initial residual magnetic field on the coupling efficiency in the magnetostrictive strip is negligible.Finally,combined with the above research on the bias magnetic field,considering the relationship between the coil wire diameter and the rated current,the influence of the structural parameters of the toroidal coil on the transducer coupling efficiency is analyzed.Based on this,the cross-coil sensor was designed and fabricated.The experimental results of parameter analysis show that when the bias magnetic field is fixed and the bias current meets the rated current of the coil,the smaller the loop diameter of the loop coil and the lower the coverage,the higher the coupling efficiency of the sensor.The manufactured sensor can detect the groove defect in the stainless steel tube.