Further Investigations into the Capacitive Imaging Technique Using a Multi-Electrode Sensor

Abstract

As a novel non-destructive testing technique, capacitive imaging (CI) has been used to detect defects within the insulation layer and metal surface of an insulated metallic structure, that is, pipe or vessel. Due to the non-linearity of the probing field, the defects at different depths in the insulation layer are difficult to compare accurately using the conventional CI sensor with a single pair of electrodes. In addition, the conventional CI sensor cannot provide adequate information to discriminate the defects in the insulation layer and metal surface. In order to solve the above-mentioned problems, the multi-electrode sensor is introduced. The multi-electrode sensor uses multiple quasi-static fringing electric fields generated by an array of coplanar electrodes to obtain extra information about the defects in the specimen. In this work, the feasibility of multiple quasi-static electric fields detecting the defects was demonstrated and the Measurement Sensitivity Distributions (MSDs) of the multi-electrode sensor detecting the defects were acquired using the FEM models. The simulation and experimental results show that the Dynamic Change Rates (DCRs) of the measured values obtained at the center of the defects in the insulator layer and metal surface present different variation patterns, which can be used to discriminate these two different kinds of defects. The reasons for the different variation patterns of DCRs were explained by the changing trends of MSDs with increased electrode separation. In addition, it was demonstrated that the different depths of the defects in the insulator layer can be compared accurately by comprehensive analysis of the detection results from all the electrode pairs.

The semi-analytical model of capacitive imaging (CI) sensors based on the renormalization group method

“…The structure diagram of the CI sensor is shown in Figure 1(a), where a represents the length of the working electrode, b represents the width of the working electrode, d 1 represents the width of the interelectrode shield electrode and d 2 represents the distance between the working electrode and the interelectrode shield electrode. Previous research results have shown that the capacitance value of the CI sensor is related to the geometric parameters that constitute the sensor, that is, the capacitance value of the theoretical model of the CI sensor is a function of the geometric parameters a, b, d 1 and d 2 (Li et al, 2018a(Li et al, , 2018b. The capacitance value of the CI sensor is composed of two parts: one part is a function related to geometric parameters, and the other part is a function unrelated to geometric parameters.…”

“…Capacitive imaging (CI) technology tests a specimen by using the quasistatic edge electric field generated between coplanar electrodes (Yin et al, 2020a). When defects are present in the specimen, they cause disturbances to the quasistatic edge electric field (Li et al, 2023;Li et al, 2018aLi et al, , 2018bYin et al, 2018). Variations in the capacitance value between coplanar electrodes occur as a result of changes in the quasistatic edge electric field.…”

Corrosion Depth Inversion Method Based on the Lift-Off Effect of the Capacitive Imaging (CI) Technique

“…It is well known that for static electric field, the electric field lines and the equipotential lines are perpendicular to each other [16]. If there is a defect in the quasi-static fringing electric field, both the electric field lines and the equipotential lines will be distorted [17].…”

“…It has many advantages, such as noncontact, coupling agent free, real-time and intuitive detection results, cost effective and easy to implement [15]. In theory, the CI technique can not only detect surface corrosion defects of the conducting specimen without an insulation layer but also detect surface corrosion defects of the conducting specimen through a relatively thick (up to 100 mm) insulation layer [16]. The principle of CI technique for defect detection is shown in Fig 1. An AC voltage is applied to the driving electrode of a CI sensor, and the sensing electrode of the CI sensor is connected to a measurement circuit.…”

High Sensitive Capacitive Sensing Method for Thickness Detection of the Water Film on an Insulation Surface

“…Li et al studied the capacitive imaging application using a multi-electrode capacitive sensor. The designed sensor was used to detect defects in the insulation layer and the metal surface, and the penetration depth and the signal strength were discussed [19]. Sheldon et al applied the capacitive sensing technique to measure the permittivity of wire insulation materials, and they presented a new equation to calculate the capacitance.…”