The lift-off spacing distance between the eddy current sensor and test piece will influence the detected signals and accuracy of the measurement. Various techniques including novel sensor designs, features (lift-off point of intersection, lift-off invariance phenomenon), and algorithms have been proposed for the compensation of error caused by the lift-off effect using the eddy current sensor. However, few of these have directly measured the lift-off spacing distance, particularly for the distance up to 15 mm.In this paper, a lift-off tolerant pancake sensor has been designed. By analysing the sensitive region of the magnetic vector potential change (due to the test piece), the receiver of the sensor is designed as a circular spiral pancake coil with a large mean radius and span length (the difference between inner and outer radius). Experiments on the inductance measurement of three different non-magnetic samples have been carried out using both the designed pancake sensor and the previous triple-helix sensor. From the experiment result, the detected signal of the designed sensor has been proved much larger than that of the triple-helix sensor. Besides, simplified algorithms have been proposed for the measurement of the lift-off spacing and thickness of non-magnetic samples when using the proposed pancake sensor. Results show that the lift-off spacing and thickness can be measured with a small error of 0.14 mm (absolute error under 209.66 kHz), and 1.35 % (relative error, under low working frequencies of 142, 238, and 338 Hz) for the lift-off spacing from 1 to 15 mm.

Previously, a conductivity invariance phenomena (CIP) has been discovered – at a certain lift-off, the inductance change of the sensor due to a test sample is immune to conductivity variations, i.e. the inductance – lift-off curve passes through a common point at a certain lift-off, termed as conductivity invariance lift-off. However, this conductivity invariance lift-off is fixed for a particular sensor setup, which is not convenient for various sample conditions. In this paper, we propose using two parameters in the coil design – the horizontal and vertical distances between the transmitter and the receiver to control the conductivity invariance lift-off. The relationship between these two parameters and the conductivity invariance lift-off is investigated by simulation and experiments and it has been found that there is an approximate linear relationship between these two parameters and the conductivity invariance lift-off. This is useful for applications where the measurements have restrictions on lift-off, e.g. uneven coating thickness which limits the range of the lift-off of probe during the measurements. Therefore, based on this relationship, it can be easier to adjust the configuration of the probe for a better inspection of the test samples.