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Uncertainty Evaluation in Automotive Emc Immunity Tests
Published May 23, 2004 by Society of Automotive Engineers of Korea in South Korea
Modern automobiles incorporate increasing amount of electronic systems on board, most of them with vehicle safety concerns (like ABS control, airbag, etc.). These electronics elements work in a harsh electromagnetic environment (generated by other electronics inside the vehicle and from other elements outside it like transmitters). So the vehicle components immunity to electromagnetic interferences is fundamental. There are standards like ISO 11452-x and SAE J1113- x for automotive EMC immunity testing and also automobile manufacturers have their own requirements regarding EMC immunity test. Although electromagnetic immunity is a mandatory test, the uncertainty related to the determination of immunity level is not considered in these standards. American Association for Laboratory Accreditation (A2LA) automotive EMC program requires the laboratories to demonstrate the compliance with ISO/IEC 17025. This standard for laboratory accreditation considers the evaluation of measurement uncertainty as an important point.
Immunity test requires applying a determinate level of electromagnetic interference to the equipment that it must withstand with not significant degradation of its functions. Uncertainty of immunity setup must be determined and regarded in the assessment of interference level that equipment support. Each immunity test has its own particular setup but the methodology for the uncertainty assessment is similar for all the tests. The first step is to find all the measurement set-up elements that would contribute to the uncertainty. It is important not only to think in the recording instruments, antennas probes etc., but also in all the interconnection elements like wires, cables, attenuators, directional couplers, termination loads, etc., and in its position in the test site. The contribution to the uncertainty of each one of these elements must be assessed, using either a type A or type B uncertainty evaluation following the recommendations of the ISO guide to the expression of uncertainty in measurements.
The proposed methodology for uncertainty assessment follows the test chain, and considers the contribution to the uncertainty of all the chain links. For example, bulk current injection (BCI) test using current monitoring probe method involves the following elements: current monitoring probe, EMI receiver or spectrum analyzer, connection cable between current monitoring probe and receiver, current injection probe, directional coupler, power meter and probes, broadband RF amplifier and RF signal generator. The accuracy of elements, mismatches, harmonics content, coupling effectiveness, attenuations, etc., must be considered when evaluating the uncertainty. All this information can be obtained from calibration reports or from manufacturer specifications. The total uncertainty of a determinate test should be used to modify the test levels to assure the electromagnetic immunity of the tested element.