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MIT ESD 342 - Assumptions and Biases

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Assumptions and BiasesThings That Are ImportantThings We Usually Don’t Think About Because They Won’t HappenOther ThoughtsWhat’s Basic to MechE8/24/2006 Background © Daniel E Whitney 1997-2006 1 Assumptions and Biases• Every design and system has multiple phenomena operating at the same time• This has several consequences– Parts must be designed and tested separately and then tested together– Analytical models and simulations will not be able to encompass all the important things that could happen or must be understood– A “digital” plug-play or modular approach will not work and may not even lead to a good design8/24/2006 Background© Daniel E Whitney 1997-2006 2Things That Are Important• Geometry, geometric relationships, and visualization• Mass and space occupancy• Motion (dynamic space occupancy, acceleration loads)• Forces, loads, load paths• Tight coupling that’s unavoidable: propagation of loads, heat, fluid, vibration, fatigue - generally linkage of effects and time-driven effects• Ever-present constraints – Generic: the laws of physics– Specific, often enterprise-driven: space, weight, cost in this particular design8/24/2006 Background © Daniel E Whitney 1997-2006 3Things We Usually Don’t Think About Because They Won’t Happen• A theory that will tell us the right way to design something, tell us how far off the optimum we are, or tell us what to do to get to the optimal design• The idea that it will be right the first time• The idea that we will have time to be sure it is right• The idea that you can get it done without someone who really knows what they are doing8/24/2006 Background © Daniel E Whitney 1997-2006 4Other Thoughts• The “right way” usually is found and after a while a search is no longer needed (airplane wing, car engine); no canonical architectures exist, such as trees, but consensus architectures for specific kinds of things emerge• No single part is the hero that makes the product work; the product’s architecture, the way the parts work together, is the key thing• Sometimes no agreement or convergence occurs (car door design)• More design and testing time are devoted to finding and mitigating side effects than in assuring achievement of basic function and performance8/24/2006 Background © Daniel E Whitney 1997-2006 5What’s Basic to MechEA. Analytical1. Need to consider basic physical phenomena as part of every design exercise2. Must know the limits of the models3. Must do geometric reasoning4. Each phenomenon has its own detailed models5. “Stochastic world” not a high priorityB. Design1. Main functions and failure modes involve multiple interacting phenomena and constraints in every design2. Side effects operate at high power3. Good designs are “elegant”4. “System view” not a high priority in education or most practice areas - combination of good components will do5. Integrated models are unreliable or do not


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