Magnetoresistive Sensor With Magnetic Balance Measurement for Inspection of Defects Under Magnetically Permeable Fasteners

Magnetoresistive (MR) sensors in conjunction with low-frequency eddy current can be used for detecting subsurface defects under fasteners in aluminum multilayer airframe structures. Frequently, the fasteners are made of magnetically permeable material, e.g. steel, which can be magnetized resulting in a strong remanence. Strong residual magnetism can shift the work point of MR sensor outside of its linear range. The limited measurable range of MR sensor presents additional challenges, since the amplitude of the steel fastener signal is much greater than the defect signal. This paper shows that one can employ magnetic balance measurement (MBM) method to inspect defects under permeable fasteners using MR sensor. The MBM method is based on proportional-integral-derivative feedback control mechanism. The operational principle of MBM was numerically mimicked using a Simulink model. Calculation results show that the bias point of the MR sensor in MBM is kept invariant, so the effect of the strong remanence field is eliminated and the operating point of the sensor on the linear region of its characteristics stays constant. Furthermore, the measurement range of the MR sensor was extended 21 times with given parameters without sacrificing sensitivity. A prototype was built and tested on a two-layer aluminum sample with steel fasteners and machined subsurface notches. Experimental results show that MBM improves the performance of MR sensor for defect inspection around magnetized steel fastener.