Conformity Magazine - February 2009 - (Page 19)

of an IC’s output pins to change state at the same time, and common-mode EMI causes noise on many/all circuit nodes at the same time. Also, EMI and some types of faults can create noise that can be mistaken for valid signals. It is generally assumed that two or more independent faults are so unlikely that only single-fault issues need be considered, but this is a misunderstanding. Where the likelihood of certain faults is high enough (e.g., due to inadequate design or assembly) the possibility that two or more such independent faults could occur simultaneously should be taken into account. When designing a vehicle so that a person can drive it safely, it is also appropriate to use task analysis and human reliability analysis. EM and Physical Specifications Specifications should be written for each vehicle safetyrelated system in order to control their design, manufacture, verification and validation, and the specifications should include EM and physical requirements derived from the above. Specifications for the ESAs to be incorporated in a safety-related system should then be derived from the system’s specification, taking into account any EM or physical mitigation measures employed by the system (e.g., shielding, filtering, surge suppression, anti-vibration mountings, forced cooling, etc.) A Verification/Validation Plan Achieving sufficient confidence when verifying and validating the design and assembly requires a mixture of techniques [33], none of which is sufficient alone, including: • Demonstrations • Checklists • Inspections • Reviews and audits • Independent assessments • EM tests on ESAs and complete vehicles • Validated computer simulations EM tests are most useful when they closely replicate the EM/physical characteristics of the real-world environment(s). It is generally best to base such tests on the standardized test methods, competently modified to better simulate the real life EM/physical environments. HALT (highly-accelerated life testing) is a powerful tool for assessing the lifecycle suitability of design and assembly methods, and of EM mitigation techniques such as shielding and filtering [34]. Appropriate design of test set-ups can make it possible to detect unacceptably degraded EM performance during HALT testing. ESAs for use in safety systems always require some final verification/validation tests, as do the completed vehicle safety systems themselves. These tests should be designed to provide the required confidence without high costs. The EM characteristics of serially-manufactured ESAs and vehicles can be significantly affected by any of the following issues: • Variations in purchased parts (e.g., IC die-shrinks); • Alternative or replacement parts; • Variations in plating, painting and fixing; • Differences in assembly (e.g., wiring); • Design changes and improvements; • Firmware bug-fixes and upgrades, etc. Therefore, all of the build-state issues relevant for maintaining tolerable functional safety risks should be identified during design, and controlled by quality control (QC). QC should use a range of techniques; including quick, easy, low-cost EM checks on delivered goods, ESAs and subassemblies, plus sample-based testing designed to maintain an acceptable quality level. QC should employ competent personnel, backed up by appropriate testing, to assess every proposal for a design change for its implications for EM characteristics and functional safety risks. The Results of Verification and Validation Documents should show how any shortcomings in meeting the specifications were dealt with, and the specifications achieved. Measures Necessary to Maintain EM Characteristics Assumptions originally made about real-life EM and physical environments should be verified during the lifecycle of a model of vehicle and, if they are in error, what appropriate actions were taken. Figure 3: Examples of foreseeable EM disturbances february 2009 Conformity 19

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Conformity Magazine - February 2009 Contents Editor's Note FCC Clarifies Closed Captioning Requirements for DTV Receivers Commission Cracks Down (For the Fifth Time!) On Junk Fax Marketer FDA Requests Comments on Global Harmonization Documents Australia Now Requires Solar Panel Testing and Certification EU Drafting New Directive on Toy Safety EU Commission Revises Standards List for EMC, R&TTE Directives Additional Standards Published For the EU's Medical Devices Directive EU Commission Publishes Additional Standards for Directives on Active Implantable, In Vitro Medical Devices EMC For the Functional Safety of Automobiles ESD Open Forum The ILAC Arrangement: Facilitating Trade Efficient Methodology for the EMI Analysis of Optical Transceivers Including PCBs and/or Flex Circuits The EuP Directive - Implementation, Design Requirements and Issues Chambers, Shielding, and Accessories Buyer's Guide CPSIA Lead Requirements Apply to Existing Inventory, Says CPSC CPSC Clarifies Certifcation Requirements Other CPSC Actions in the News IEC Standards Update Product Reviews UL standards Update Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - February 2009

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