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A Systems Scientist’s Thoughts on Model-Based Systems Engineering

A Systems Scientist’s Thoughts on Model-Based Systems Engineering. Wayne Wakeland , PhD Systems Science Program Portland State University. Dynamic Connected/coupled Governed by feedback Boundaries are artificial (often permeable) Complex & non-linear History dependent Edge of chaos.

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A Systems Scientist’s Thoughts on Model-Based Systems Engineering

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  1. A Systems Scientist’s Thoughts on Model-Based Systems Engineering Wayne Wakeland, PhD Systems Science Program Portland State University

  2. Dynamic Connected/coupled Governed by feedback Boundaries are artificial (often permeable) Complex & non-linear History dependent Edge of chaos Self-organizing Self-replicating (living systems) Adaptive/evolutionary Characterized by trade-offs Counterintuitive Policy resistant Emergent The Nature of Systems

  3. In complex systems, cause and effect are often distant in time and space We may act to produce short-term benefits and long-term costs; we forget about delay

  4. The solution to one problem may cause another problem (unintended results) Example: The ”Green Revolution” agricultural technologies were introduced into Asia in the late 1960s as a solution for food insecurity. Decades later, they have proved detrimental in terms of biodiversity loss, increased use of agro-chemical based pest and weed control, water logging, salinization and land degradation. Artist Gary Larson Slide adapted from LEAD International and Sustainability Institute

  5. The Iceberg: EVENTS – PATTERNS – STRUCTURE What happened? Events REACT Patterns of Behavior Increased leverage and opportunities for learning What has been happening? ANTICIPATE Systemic Structure Why has this been happening? How can I improve the performance of the system? DESIGN Mental Models

  6. Systems Science Methods • Systems science methods focus on understanding the general properties and behavior of complex systems by creating models and finding patterns in data • We strive to interact effectively with collaborators from various disciplines by using clear and well-annotated graphics and diagrams to present models and data analysis results

  7. Key Concepts and Principles • A system consists of elements & relationships, with specific purpose/goal/function • Whole > sum of the parts • Structure causes behavior • Circular causality • Outputs influence inputs; cannot separate cause and effect • Mental models (often hidden) shape our thinking • Systems Archetypes (common structures & behaviors) • E.g., Fixes that fail, Shifting the Burden, Success to the Successful, Limits to Growth, Tragedy of the Commons • And much more (far too much to cover this morning) • Complex adaptive systems, living systems, open systems, structural coupling, autopoiesis, adaptation, resilience, evolution, …

  8. Systems Thinking • Seeing the forest and the trees • Interconnectedness • Thinking dynamically • Behavior over time • Delayed impacts/consequences • Thinking closed loop (vs. linear causality) • Endogenous thinking (system as cause) • Thinking operationally • How things actually actually work

  9. Useful Communications Tools: Causal Loop Diagrams

  10. Example: Fixes that Fail Archetype • The story: due to budget problems, spending on maintenance decreases, which balances the budget…BUT, over time, breakdowns increase, forcing more spending, which stresses the budget even worse than before!

  11. Iteration in Systems Design/InterventionProcesses

  12. Specific Modeling Methods • System dynamics • Focuses on modeling the underlying feedback structures with differential equations • Equation are solved to simulate behavior over time • Discrete system simulation • Uses a Monte Carlo approach to analyze how the variety/randomness impacts system performance • Often emphasizing business operations and processes, especially in manufacturing and supply chain logistics • Agent based simulation • Used to study how low-level interactions between individual agents influences overall system behavior/performance

  13. System Dynamics Example • Project Management • Brooks’ Law: “Adding manpower to a late software project makes it later” • SD has been used to simulate complex projects and evaluate potential decisions, actions, policies

  14. Typical Project “Disasters” • 􀂄 The Channel Tunnel -- original estimate, • $3 billion; final cost, $10 billion • 􀂄 Boston’s “Big Dig” -- original mid- • 1980’s estimate, $2.5 billion; latest • estimate, $14.5 billion (9/2001) • 􀂄 Aircraft development -- nearly double • initial estimate • 􀂄 New Car Development -- original plan, • 400 person-years of effort; final cost, 800 • person-years

  15. Project Behavior Over Time

  16. Discrete System Simulation • Detailed, step-by-step emulation of the flow of entities through the system • With uncertain arrivals, processing times, and/or "routing" (branching) • The computer monitors each simulated entity as simulated time proceeds • Enter system, move thru, according to the various probability fns. governing timing and sequence of events • The computer also records pertinent data regarding the simulated entities and servers • wait times, throughput, queue lengths, process times, utilization… • Creates a synthetic "sample" of system performance data • Sample data is then analyzed statistically

  17. Types of Problems DSS can Address • Performance issues in existing systems • Long waits, high inventory, poor utilization of resources, low throughput • Need to estimate performance of a system under design • What Might One Learn? • Where the bottlenecks are and how they might be alleviated • How to improve flow, reduce queues and wait times, and increase utilization & throughput • The optimal number of servers, queues, buffers, etc. • Effective operating rules or policies

  18. Examples of Discrete System Simulation • Computer network • Freeway system • Business process (e.g., insurance office) • Criminal justice system • Chemical plant • Fast-food restaurant • Supermarket • Theme park • Emergency Response system • Manufacturing facilities • Bank operations • Airport operations (passengers, security, planes, crews, baggage) • Transportation/logistics/ distribution operations • Hospital facilities (emergency room, operating room, admissions)

  19. Project for Systems Science 527 Class, Spring, 2011

  20. Manufacturing Process Design Drawing

  21. DSS model animation (closely mimics the actual system) The model contains complex logic regarding: A) Different fault occurrences, B) Part filling requirements, and C) Realistic variations seen in complex assembly processes

  22. Model Results • The simulation showed the behavior of the proposed new automated system • It suggested that given expected faults, operator utilization will be 45-55% • Thus, if the operator must load parts, do audits, and perform fault correction, they could not handle two machines, as would be needed to achieve the cost targets

  23. Agent Based Simulation • More of a stretch for systems engineering than the others… • Key Features • Agents • Environment • Rules • Spatial aspects • Can reflect heterogeneity of individuals

  24. Key ABS Concepts • Decentralized control • Bottom up as opposed to top down • Emergence • Self-organization • Evolutionary considerations • Examples • Spread of Forest Fires • Flocking • Crowd behavior • Ants (and how ants can find optima) • Network effects

  25. Crowd Crush Model • The problem: crowd panics • Sheffield, England 1986, 96 dead • Phnom Phen, Cambodia, November 23, 2010: 347 dead • Duisburg, Germany, July 25, 2010,19 dead • This model was developed as class project • Alexandra Nielsen, Systems Science 525, Fall 2010

  26. Crowd Crush: defined • Die of asphyxiation, not blunt force trauma • Can die standing • Warning of a crush • Surrounded on all sides • More than 4 people per square meter • Force to kill • The force of 5 people pushing on one person can break a rib, collapse a lung, smash a child's head

  27. Simulation purpose • Discover why some crowds are lethal and others not • Can crowd deaths occur in non-aggressive crowds? • Does aggression or reactivity (jostling) have a greater impact on crowd deaths? • Is there some combination of factors that is reliably lethal? (So we can avoid it) • What interventions can prevent deaths? • Is there a critical density after which nothing will work? Netlogo model…

  28. Model Testing • Interventions • Opening closing entrance exit • Shortening corridor (simulate smaller crowd) • Panic on seeing another dead • Validate vs. anecdotal evidence • Wal-Mart door rush • Cambodia see a “body” → panic • Opening door in a crowd → death (Barnsley Public Hall disaster)

  29. Applicability and Limitations • Large crowds, single doorway • Love Fest • One entrance of a soccer stadium (Liverpool) • Good for understanding simple crowd dynamics • Limited by simplifying assumptions (extremely simple) • No falling • Only forward motion • No groups, altruism, variability in agents • Forces not vectors, not true physical force

  30. Findings • Don't allow a huge build up, then open a door • Closing the gates before clearing the corridor helps, but not much • Do anything you can to prevent panic

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