Non-destructive inspection of ferromagnetic pipes based on the discrete Fourier coefficients of magnetic flux leakage

For non-destructive inspection of ferromagnetic pipes using magnetic flux leakage (MFL), a sensor consisting of two coils to measure the discrete Fourier sine and cosine coefficients of MFL is developed. The position of a crack in a pipe can be estimated from the squared sum and the ratio of the voltages induced in the two coils. Each coil has sixteen elemental coils with 78 permalloy cores which are placed along a circle with a radius of 6.3 mm and connected in series. The number of turns comprising an elemental coil at an angle ϕ is proportional to sinϕ in one coil and proportional to cosϕ in the other. It was experimentally shown that a circular crack 4 mm in diameter in a ferromagnetic pipe with diameter 21.6 mm was localized. The proposed structure is suitable for narrow pipe inspection where the sensitivity of our previous sensor composed of two coaxial crescent-shaped coils decreases.