Evaluating Alternatives.pdf - University of Portland School...

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Criteria and Alternatives 1 University of Portland School of Engineering Establishing Criteria and Evaluating Alternatives Defining criteria and evaluating alternatives are aspects of design that students have had little experience with. The purpose of this document is to help students understand the process of defining a problem (creating and defining criteria) and evaluating solutions based on the criteria. 1.0 Understand the Problem or Need It is essential to understand the problem before developing solutions! This seems obvious, but it is often neglected. The following may define the problem or need: - “customers” - management - personal observation or experience - marketing 1.1 Ask questions - What really is the problem? Why is this a problem? - What do you really need? Why do you need it? 1.2 Write a need-statement or problem-statement Do not solve the problem before defining it!!! A narrowly stated problem may limit alternative solutions; a vague statement does not provide sufficient guidance. So put thought into the statement. Examples: No - I need a new car ( limiting definition ). Yes - I need a reliable, safe, and comfortable way to get to work. No – Develop a new airplane ( vague ). Yes – Develop a commercial airplane to transport 250-310 people 3500 miles. Hard work is valued in our society. We are brought up with the idea that we can accomplish great things by working hard. But hard work is not sufficient! Not only must you work hard, you must work on the right problem. There is a saying in business: “work smarter, not harder.” The following examples illustrate the critical importance of having an appropriate and clearly defined problem statement. There is a story regarding the design of a solar panel dampening system for the Mariner VI space probe (Ullman , The Mechanical Design Process , McGraw-Hill). Engineers were concerned that the panels would be damaged when they were opened in space. The problem statement that was pursued may have read something like: “Design a dampener to slow the motion of the solar panels as they reach their fully opened position .” Obviously, it was the engineer’s responsibility to develop a dampener that could operate in the vacuum of space, slow the panels down during deployment and not otherwise hinder the
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Criteria and Alternatives 2 performance of the vehicle. Literally, after spending millions of dollars and thousands of hours, a design to meet the demanding requirements was still illusive. Engineers decided to analyze the worst-case scenario: total failure of the dampening system. Amazingly, the analysis showed that the panels would open without being damaged! The design effort was abandoned and the vehicle performed perfectly without damping. A more appropriate problem statement might have been: “Prevent the solar panels from being damaged during their deployment .” This would allow for various alternatives to be considered (strengthening the panels, etc.). It would also suggest analyzing the current (no damper) design to determine what could cause damage.
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