Methodology for Assessing Safety/Mission Critical Systems

J. Wysocki, R. Debouk, T. Fuhrman, and K. Nouri (USA)


Safety Auditing Methodology, Safety Critical Systems.


We present a simple methodology to evaluate a variety of architecture designs for safety critical systems. Our approach involves including all the appropriate components in a representative fault tree. Evaluating the fault tree will result in the cutsets, the system availability and the system reliability. We next apply a Monte Carlo simulation to the fault tree and include parameters such as mean time to repair, repair time distributions, etc. These parameters affect the total downtime of the system and impact the system usability. Tracking the parameters and the associated levels is done using a Design Of Experiments (DOE). For our approach we can adjust the inputs for the Monte Carlo simulation from the elements in the DOE matrix and evaluate the results for those variables. Using the DOE methodology allows the results to be ranked according to their impact on the responses (number of failures, downtime, relative costs, etc.). Using these responses we assign a figure of merit to each design. The figure of merit will be used to determine the optimal design to implement. The paper will use an example of a simple sensor-controller-actuator system that is designed with multiple levels of redundancy to illustrate the methodology.

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