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As illustrated in the Fig. 8, the proposed high-speed MFL inspection system that is employed in online evaluation on pipelines, tubes and rail tracks is equipped with three sensor arrays. In addition to the sensor array 2 which is applied in traditional MFL system and deployed within the polarities of ferrite core, sensor array 1 and sensor array 3 are supposed to be set up behind and prior to the magnetization device which comprise excitation coil and ferrite core, respectively. Such layout of sensor arrays is determined, for the sake of those: Sensor array 2 will measure the magnetic field for MFL signals which is modulated by defects and moving speed; Sensor array 1 will obtain the eddy current modulated magnetic flux leakage signals in the region where eddy currents in specimen distribute and will be perturbed by any axial defects; Since residual magnetic field is believed to exist in front of the magnetization device [23], sensor array 3 is deployed in this area for measuring residual magnetic field which can be modulated by any defects as well. According to the numerical simulations, the three sensor arrays should be high-sensitivity and highbandwidth and the sampling frequency for signal acquisition should be high enough with respect to online high speed measurement. By integration of three signals, sample defects can be identified.

Fig. 7. Magnitude of Bx vs. X-axis against depth of surface defect.