An Interdigital Capacitive Sensor for Nondestructive Evaluation of Wire Insulation

A capacitive sensor has been developed for measuring the permittivity of wire insulation materials. The sensor consists of interdigital electrodes deposited on a thin polyimide substrate that conforms to the surface of the insulated wire or cable. The test piece was modeled theoretically by assuming a perfectly conducting cylindrical rod coated with two concentric dielectric layers. Constant potential difference V between the electrodes was assumed. The method of moments and Green's function solution to the cylindrical Poisson equation were utilized to determine the charge distribution on the electrodes and hence the total charge Q, from which the capacitance was calculated from the relation C = Q/V. To validate the model, calculated capacitance was compared with that obtained from benchmark experiments performed on a set of three brass rods insulated with acetal copolymer, PTFE, and acrylic. The permittivity of these three polymers was measured independently and provided to the model. The mean difference between calculated capacitance and that measured experimentally was found to be <2%. Furthermore, a handheld prototype sensor suitable for clamping to aircraft wire was designed and measurements of capacitance were made on M5086 aircraft wire. This consists of tin-coated copper strands insulated with flexible PVC, a glass fiber braid, and finally coated with Nylon 6. After exposure to various common aircraft fluids (cleaner, deicer, distilled water, hydraulic fluid, isopropanol, and jet fuel), the measurements showed significant changes in measured capacitance, correlating well with the results of accelerated aging experiments on pure Nylon 6.