AC Magnetic Flux Leakage Testing With Real-Time Liftoff Compensation Using Double Layer Parallel-Cable Probe

During electromagnetic detection, the disturbing magnetic field generated by the defect is far weaker than the background magnetic field generated by the transmitter. Thus, the background field can cause the magnetic field sensor saturation and bring interference signals. Meanwhile, in the actual detection process, the deformation of the material and the vibration of the probe will bring about the fluctuation of liftoff, which seriously interferes with the detection signal. In order to eliminate these two interferences, first, this article proposes a magnetizer using a double-layer parallel cables magnetic bridge circuit, which eliminates the background magnetic field at the measuring element and enhances the magnetic field at the workpiece. Then, the ability of the probe to simultaneously measure liftoff and detect defects is studied, and a dual-channel amplifier and acquisition system are proposed, which can measure and compensate the liftoff and quantify defects at the same time. Finally, the feasibility for compensation of crack signals using measured liftoff is studied, and within the liftoff variation range of 0.2-2.0 mm, cracks with a depth variation of 1.0-2.0 mm can be effectively compensated.