Resonating inductive sensors are increasingly popular for numerous measurement techniques, not least in non-destructive testing (NDT), due to the increased sensitivity obtained at frequencies approaching electrical resonance. The highly unstable nature of resonance limits the practical application of such methods while no comprehensive understanding exists of the resonance distorting behaviour in relation to typical measurements and environmental factors. In this paper, a study into the frequency spectrum behaviour of electrical resonance is carried out exploring the effect of key factors. These factors, known to distort the electrical resonance of inductive sensors, include proximity to (or lift-off from) a material surface, and the presence of discontinuities in the material surface. Critical features of resonance are used as metrics to evaluate the behaviour of resonance with lift-off and defects. Experimental results are compared to results from a 2D finite element analysis (FEA) model that geometrically mimics the inductive sensor used in the experiments, and to results predicted by an equivalent circuit transformer model. The findings conclusively define the physical phenomenon behind measurement techniques such as near electrical resonance signal enhancement (NERSE), and show that lift-off and defect resonance distortions are unique, measurable and can be equated to exclusive variations in the induced variables in the equivalence circuit model. The resulting understanding found from this investigation is critical to the future development and understanding of a complete model of electrical resonance behaviour, integral for the design of novel sensors, techniques and inversion models.
- electromagnetic testing
- inductive sensing