A technique for measuring the electrical impedance of mechanical joints in electrically conductive structures

• Main Author: South, G.
• Published: University of Central Lancashire 1992
• Subjects: 530.41; Electrical engineering
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315374

The impedance of a circular coil placed above a conducting surface is calculated using three mathematical models. The first considers the interaction of the test coil with a uniform, homogeneous and electrically thick conducting sheet. The model quantifies the interaction of the coil and the surface by calculating the field around the coil in the frequency domain. The sheet is included by using reflection coefficients, calculated from defined physical properties, to modify the field components surrounding the coil. Cylindrical symmetry is used to reduce the problem to two dimensions. The second model is a finite difference solution in the frequency domain of the differential equation describing the magnetic vector potential. Cylindrical symmetry is used to reduce the problem to two dimensions. An approximation allowing a mechanical joint of finite physical dimensions to be included is described. The third model is a finite difference, time domain solution of the differential form of the field equations. This is a complete three dimensional analysis of the problem, including the effect of fasteners and sealants. An algorithm allowing the inclusion of materials with anisotropic electrical conductivity is described. Fourier analysis is used to transform the time domain results into the frequency domain. The results calculated from the three models are compared with practical measurements. Two instruments are constructed and practically evaluated. The first is suitable for frequencies up to 50kHz. It is based upon two test coils in a bridge arrangement and the effect of the joint is measured in terms of the out-of-balance bridge voltage. The second instrument is based upon the effect of the joint of interest upon the resonant frequency of a test coil, measured using a modified Q meter circuit. Frequencies up to 70MHz have been considered.