Accurate evaluation of objects being inspected by the magnetic flux leakage testing (MFL) method relies on a good correlation between the signal characteristics and the discontinuities under actual test conditions. In practical MFL, imperfect shape and position of the products will lead to a changeable spatial location of the sensors and generate different signal characteristics even for the same discontinuity. In this paper, the signal characteristics of the widely used circular induction coil influenced by spatial location are investigated. First, based on magnetic dipole theory, the magnetic flux leakage distribution of a standard discontinuity is obtained. Second, based on Faraday’s law of electromagnetic induction, the signal response model of a circular induction coil at an arbitrary spatial location is built. Then, the influence of the coil spatial location on signal characteristics is analyzed by changing the coil spatial location. It is found that the spatial location influences the signal characteristics greatly, including the signal waveform and the amplitude, which should be taken into consideration in the sensor design and signal analysis. Lastly, MFL experiments are conducted to verify the signal characteristics of the circular induction coil, and the testing results are consistent with theoretical analysis.
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