Analysis of the Lift-off Effect of Phase Spectra for Eddy Current Sensors

Abstract

This paper presents an analytical model which describes the inductance change when a double air-cored coil sensor is placed next to a conducting plate. Analysis of the analytical model reveals that the phase signature of such sensor is virtually lift-off independent. This finding is verified by numerical evaluations. This paper also finds that the phase signature of a ferrite U-cored sensor can be approximated by that of a double aircored sensor of similar size, and therefore possesses similar lift-off independent property. Measurements made with a sample U-cored sensor next to plates of non-magnetic and magnetic materials verified the theoretical results .

Eddy current evaluation of air–gaps in aeronautical multilayered assemblies using a multi-frequency behavioral model

“…Indeed, in practice, since the sensor is placed in contact with the layered structure, the variations of the distance between the sensor and the layered structure can be assumed to be very small compared to the sensors dimensions, and hence, the lift-off effect can be neglected [15]. However, in order to simulate some variability in the sensor positioning, an additive Gaussian noise of adjustable intensity may be added to the computation results [16].…”

A simplified model for non-destructive thickness measurement immune to the lift-off effect

“…The mesurement method based on the original comprehensive model demands a large amount of computations, and hence time-consuming. The method that stems from equation (10) needs to measure the whole phase spectra [11]. Besides, it is indirect and not objective to estimate the thickness by using the comparison between the measured and precalculated phase spectra.…”

“…A variety of coil designs have also been investigated. Recently, it has been reported that it is possible to significantly reduce the lift-off effect by using the phase signature abstracted from multi-frequency impedance measurement [10].…”

“…FEM) and analytical solutions [12] can be used to calculate the real and imaginary inductance spectra, from which the phase signature can be derived. It has been found that phase signature abstracted from impedance or inductance spectroscopic measurement is virtually lift-off independent if the pole distance is much larger than the radius of the coil [10]. Therefore, the characteristic of non-magnetic metallic plate, such as thickness and electromagnetic properties, could be measured and immune to lift-off effect.…”

Measurement of decarburisation of steel rods with an electromagnetic sensor using an analytical model

“…Non-destructive methods such as magnetic Barkhausen noise (MBN) emission [1,2] and eddy current [3][4][5] measurements have been attempted; however, no quantitative relationships between signal output and decarburisation depth have been established. Previously, we have developed electromagnetic sensors for a range of general steel measurement applications [6][7][8][9][10][11]. However, this is a specific contribution on decarburisation measurement.…”