A Simple Jacobian Matrix-Based Approach for Calibrating the Sensing Depth of Coplanar Capacitive Sensors

Coplanar capacitive testing (CCT) is a new nondestructive testing (NDT) technique that has been developed in recent years. There is no consensus on the sensing depth of coplanar capacitive sensors (CCSs). The sensing depths of regularly used rectangular, circular, and triangular probes were examined in this work. Using the sample lifting method, the sensing depths of the probes were calibrated using samples of various thicknesses made of wood, glass, and acrylic. The experimental findings showed that the sensing depth of the probe varied depending on the sample material and thickness. A novel Jacobian matrix (JM) method for calculating the sensing depth was presented and tested, and its validity and accuracy were verified by comparing the sample lifting results and analyzing the defect detection results. Further discussion focused on the effect of grid division on JM results and the calibration of JM to probe sensing depths of different sizes and separations. With matched fine grid divisions, JM provided unique sensing depths, which may serve as a reference for the consensus CCS sensing depth.