INTER-DIGITAL CAPACITIVE SENSOR FOR EVALUATING CABLE INSULATION THROUGH JACKET

摘要

Interdigital capacitive (IDC) sensors have previouslybeen used to measure cable-age/damage indicativedielectric properties of cable insulation when placed indirect contact with the insulation. Often cable insulation iscovered by a polymer jacket precluding direct contact withthe insulation. As part of a continuing program to developand evaluate nondestructive examination methods that maybe applied to cable condition assessment, an IDC tomeasure the effect of cable polymer dielectric changethrough a covering jacket has been developed. The IDCconsists of two fork-like electrodes facing each other withthe fork tines interspersed and separated by a small gap.On one electrode, the gap is 5 mm while on the other, thegap is 2 mm. Electric field depth-of-field penetrations aresimilar to the electrode gaps. The electrodes are printed ona flexible substrate that can be conformed to the outsidejacketed surface of a cylindrical cable, with tines parallelto the cable axis. The electrodes are connected to a broadfrequency-spectral impedance meter that senses thecapacitance between the narrowly gapped electrode tines.This capacitance is known to vary as a function of thepermittivity of any material in close proximity to theelectrodes. For jacketed cable systems, the IDC responseis dominated by the jacket but, by analyzing measurementsfrom IDC sensors with different depths-of-field penetrationinto the cable under test, the influence of the cable jacketmaterial (primarily influenced by the shallow-depth-offieldIDC) can be separated from the deeper-depth-of-fieldIDC sensing the combined effects of both jacket andunderlying insulation. No other NDE method is known thatis capable of sensing insulation dielectric propertiesthrough an overlying jacket.