Study of Lift-Off Invariance for Pulsed Eddy-Current Signals

Pulsed eddy current imaging and frequency spectrum analysis for hidden defect nondestructive testing and evaluation

“…The pick-up units detecting the magnetic field disturbed by presence of a defect [17] are various in detection principle and structure, such as a differential coil [16,18], a three-dimensional coil [19], a magneto-resistive sensor [20], a giant magneto-resistive (GMR) sensor [21][22][23], a Hall-effect sensor [24][25][26][27], or a superconducting quantum interference device (SQUID) magnetometer [28]. The driving unit inducing eddy current is usually a cylindrical coil.…”

Pulsed Eddy Current Non-destructive Testing and Evaluation: A Review

“…At the early stage, LOI phenomenon could only exist when a PEC probe using a coil as the receiver is placed above a nonferrous sample. Tian, et al [70] obtained LOI points by doing the derivative of the output of a Hall-based probe, and Li [71] extended the method to a PEC probe in which a solid-state magnetic field sensor could be placed at arbitrary positions. Further, Lefebvre and Mandache [72] recorded LOI phenomenon by covering a conductive but nonferrous layer on a ferromagnetic substrate.…”

“…Tian, et al found that lift-off invariance (LOI) existed for magnetic sensors -not only for pick-up coils -and proposed new lift-off estimation techniques by using theoretical models [70]. They believe that the techniques may also be used when an array of pick-up sensors are used.…”

Pulsed eddy current technique for defect detection in aircraft riveted structures

“…In PEC testing, the hall-effect sensor can be used to measure the magnetic field signals, for example, in Tian GY's studies [4,9,15]. As shown in Fig.…”

“…Traditionally, the differential signal is worked out directly from the difference of a reference signal from a defect-free sample and a defect signal [15]. To suppress the lift-off effect, optimization of probe is attempted.…”