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Functional Modeling of Socio-Technical System Dependability

This talk outlines the concept of dependability in socio-technical systems and discusses operational and design theory, principles, and problem domains. It explores the role of system health management in preventing faults and mitigating their effects, considering the technical, individual, and social aspects. Various modeling approaches, such as fault trees, failure modes and effects analysis, and fault dependency analyses, are discussed. The importance of qualitative and quantitative modeling, as well as the relationship between system health management and operational architecture, is highlighted.

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Functional Modeling of Socio-Technical System Dependability

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  1. Functional Modeling ofSocio-Technical SystemDependabilityDr.Stephen B. JohnsonNASA Marshall Space Flight Centerstephen.b.johnson@nasa.gov

  2. Outline of Talk • Dependability • Operational & Design Theory • Principles

  3. Problem Domain – Constellation Program

  4. Crew Exploration Vehicle & Lunar Surface Access Module

  5. System Health Management • SHM = the processes, techniques, and technologies used to design, analyze, build, verify, and operate a system to prevent faults and/or mitigate their effects • Technical, individual, and social aspects • Synonym: Dependable System Design and Operations • “Dependability”

  6. Fault Causes • “Faults” result from • Flaws in the knowledge of the creators • Mismatch in understanding between creators and users, • Simple mistakes (solder flaw, math mistake, typos, etc.) • Cognition, communication, or human fallibility • “Random part failure” usually means we don’t know the cause

  7. Dependability Modeling • Fault Trees (top-down) • Start with problem to be avoided, “tree down” all the possible ways it can happen • Failure Modes & Effects Analysis (bottom-up) • Start with component failure analysis, propagate into a designed system • Fault Dependency Analyses (middle out) • Start with conceptual design, insert loss of function, propagate through interconnections

  8. Qualitative vs. Quantitative • All models start on qualitative basis • Physics, psychology, sociology all equally important • Can insert science-based physical data, or probabilistic data to create different kinds of “quantification” • These have differing purposes and goals • For most NASA systems, probabilities are speculative, because systems are few-of-a-kind • Too expensive to run sufficient tests to gather definitive probabilistic data

  9. SHM Operational Functional Relationships • Circular, “closed-loop” relationships • Hints at the physical architecture

  10. System Health Management Operational Architecture

  11. Typical Functions, Mechanisms, and Characteristic Times

  12. Physical Mechanisms and Response Capabilities

  13. Conclusion • Modeling of dependable system operations requires modeling of technical, cognitive, and social factors on the same basis • Can be extended to include political, economic, and other factors • Can be extended to include probabilities • Functional models define relationships in which probabilities can be inserted if there is a need for probabilistic estimates

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