Spreading and Shrinking Effects of Coplanar Capacitive Sensors for Surface Defects

Abstract

With increasing attention to safety, accurate detection of defects in diverse materials is crucial. Non-Destructive Evaluation (NDE) techniques play a key role in this effort. However, many approaches overlook preliminary signal analysis, which is vital for understanding fundamental sensor signal characteristics and improving advanced techniques. This study aims to investigate the fundamental signal properties from coplanar capacitive sensors (CCS) with square electrodes to gain insights into the relationship between signal properties and actual linear defect size in both conducting and non-conducting materials. The first-order derivatives (1st O.DE) of raw signals obtained from CCS through both simulations and experiments were further analysed, focusing on a circular surface defect. Nine sensor configurations were tested to examine the spreading effect. Experiment and simulation results were in good agreement, showing that the Full Width at Zero (FWZ) of the raw signals of both materials is greater than the actual defect diameter (spreading effect) whereas the signals processed using the 1st O.DE exhibited peak-trough widths greater than the actual defect diameter in non-conducting materials (spreading effect) and less than the actual diameter in conducting materials (shrinking effect). These results underscore that the spreading and shrinking effects are intrinsic characteristics of the CCS, attributed to the behavior of the CCS’s electric field and sensitivity distribution field (SDF) when interacting with different materials. By incorporating these insights into novel and advanced methods—such as imaging algorithms, machine learning approaches, and data fusion techniques—future developments can be effectively guided to enhance the accuracy, reliability, and advancement of defect detection, imaging, and sizing in coplanar capacitive sensing for NDE.