Multi-Electrode Coplanar Capacitive Probe With Various Arrangements for Non-Destructive Testing of Materials

The coplanar capacitive sensing method is considered a relatively novel electromagnetic technique in NDT. The size of the electrodes and the separation distance between them significantly affect the penetration depth, electric field strength, and sensitivity of the measurement. To achieve different penetration depths and electric field strengths, separate sensors with various sizes $/$ separations can be employed which is time-consuming. In order to reduce the costs and time, a multi-electrode sensor is designed and introduced in this paper. This type of sensor uses coplanar electrodes to generate the multiple fringing electric field to inspect the specimen. In this sensor, it is possible to have multiple driving and sensing electrodes at the same time, which provides a variety of different sizes and separation distances leading to various depths of penetration and electric field strengths. In the framework of this paper, the principle of the multi-electrode coplanar capacitive probe was explained. A 3D Finite Element Modelling (FEM) was employed to simulate and illustrate the electric field distribution from a multi-electrode coplanar capacitive sensor with different arrangements of driving and sensing electrodes, and how this field may be altered by changing the arrangement. The multi-electrode probe was manufactured and several sets of experiments were conducted under various conditions. The Measurement Sensitivity Distribution (MSD) was applied to characterise and interpret the variable output from the physical scan of the specimen. The case study on an acrylic sample comprised of defects with different depths demonstrated the feasibility of defect discrimination using a multi-electrode capacitive sensor.