[1A3] Reducing the lift-off effect in coercivity measurements
R Lyu¹, J Salas Avila², T Meng¹, Y Shao¹, Z Chen¹ and W Yin¹
¹University of Manchester, UK
²Manchester Advanced Intelligent Equipment Research & Innovation Centre Ltd, UK
Magnetic coercivity measurements are used for non-destructive testing owing to their ability to reflect microstructural properties and the structural integrity of metals. Air gaps between the sensor probe and the sample cause errors in coercivity measurements, referred to as the lift-off effect. A new calibration method is developed to reduce measurement errors due to lift-off by incorporating additional inductance measurements. The calibration principle is based on the fact that both the coercivity and inductance change with the variation of lift-off. The proposed method starts by finding how the coercivity varies with air gaps between the sensor and the sample and then derives the coefficients for the coercivity-inductance relationship for different samples. The correction model is then adjusted to predict the base coercivity (ie the coercivity when the air gap = 0) using the inductance and coercivity measurement results at an unknown lift-off. A portable commercial coercivity meter is employed for experimental validation, composed of a measurement and control unit and a sensor probe. The paper describes coercivity measurements based on pulse excitation, inductance measurements of the sensor probe and the proposed calibration technique principles and results. Experimental results suggest that the error caused by air gaps can be reduced from 40% to less than 10%.
