Electrical Impedance Analysis of Vehicle Structural Components and Assemblies

The objective of this paper is to propose that the automotive engineering “legacy” approach of assuming that vehicle electrical return paths using conductive structural components have zero (0) ohms of impedance, needs to be reviewed. The process in this review consists of the development of an equivalent circuit composed of both real (“resistive”) and reactive elements (due to inductance and capacitance). This issue needs to be addressed due to the increased complexity of electrical and electronics systems in vehicles and the use of the vehicle structural (such as chassis, frame, or bracket) components to provide a path for the return of electrical power and/or signals as a way to eliminate additional wiring manufacturing issues, cost, and weight contribution to a vehicle. The effects of both real and reactive impedances in the return path can lead to “ground shift” conditions, data signal corruption, and/or electromagnetic compatibility (EMC) conditions that may impact the performance of the vehicle systems or the ability to meet vehicle level EMC requirements. The methods used to illustrate these issues include evaluation of the geometric shapes of representative component assemblies, the material properties (including electrical characteristics), the conductivity of interface surfaces, and the resulting current path topology. The data that is shown consists of representative mechanical assembly details, equivalent reactive component values, and their circuit representation. Additional information is presented regarding the effectiveness of various techniques that have been used for vehicle EMC compliance, such as the application of “ground straps”, and/or requirements for electrical bonds. Next steps in this work are proposed as electrical impedance measurements of representative on-vehicle structural elements and then these values would be compared with data developed from analysis and modeling work. It is anticipated that additional work can then accomplished to develop guidelines for use of structural components as electrical connections or portions of electrical paths.

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See papers presented at SAE World Congress & Exhibition, April 2008, Detroit, MI, USA, Session: Electromagnetic Compatibility

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