【正文】 CASE STUDY IN CONCEPTUAL DESIGN： MOUSETRAPPOWERED VEHICLES As applied to engineering, the adage well begun is half done reinforces the importance of creative brainstorming in the conceptual phase of the highlevel design procedure. By way of a case study in this section, we trace the progress of a hypothetical team of engineering students as they generate concepts for designing a mousetrappowered vehicle. Small toy vehicles that are powered by household mousetraps are sometimes the subject of design and construction projects in engineering and science courses. Readily visualized and built, these vehicles are a useful means for experiencing the conceptual design process and for gaining an appreciation for the tradeoffs that must be made for a design to satisfy all of its constraints. In our illustrative case study, teams of engineering students are challenged to build small vehicles that travel 10 meters as quickly as possible but that are powered by only the potential energy stored in a mousetrap spring. Each vehicle will be designed, built, refined, and operated by a team of three students. At the conclusion ofthe project, the teams will pete against one another in headtohead races during a tournament, so the final products must be both durable and reusable. In addition to the overall objective of producing a fast vehicle, several other specifications must be met: 1. The mass of the vehicle cannot exceed 500 g. 2. The vehicle must fit pletely within a box at the start of each race. 3. Each vehicle will race in a lane that is 10 m long but only 1 m wide. If any part of the vehicle passes outside the lane during a～， the team will be disqualified from the tournament. 4. The vehicle must remain in contact with the surface of the lane during the entire race. 5. The vehicle must be powered only by a standard household mousetrap. Energy that is incidentally stored by other elastic elements or obtained from a change in elevation of the vehicle39。s center of mass must be negligible. 6. Tape cannot be used as a fastener in the vehicle39。s construction. Each of these specifications constrains, in different ways, the hardware that the teams will ultimately produce. If any single requirement is not met, the entire design will be inadequate, regardless of how well the vehicle might perform relative to the other requirements. For instance, because the racing lane is 10 times longer than it is wide, the vehicle must not only be fast but must also travel in a reasonably straight line. If a particular vehicle sometimes veers outside the lane, then it could be defeated by a slower vehicle (even a much slower one) in a headtohead race. The design teams recognize that the vehicles should not be optimized with respect to only one specification, but rather should be balanced to meet all requirements. We next follow the thought process of a hypothetical team as it begins to brainstorm and identify multiple design concepts. The students document their ideas in a bound notebook, and they use written ments and hand drawings to describe each concept. Subsequently, the team will record progress as prototypes are constructed and the oute of their testing and iteration efforts. In short, the notebook serves as a log to chronicle the team39。s entire design experience. In industrial research39。and development settings, such notebooks are often dated, signed, and even witnessed in order to formally document a product39。s development. With an eye toward your own professional career, you should also begin the practice of systematically recording, revising, and developing your original ideas. First Concept: String and Lever Arm An idea that emerges from the team39。s first brainstorming session is based on using the mousetrap39。s snap arm to pull and unwrap string from a drive axle. Together, the team members sketch the concept shown in Figure . As the trap snaps shut, string is unwound from a spool that is attached to the rear axle and the vehicle is propelled forward. The concept vehicle incorporates a lever～ that lengthens the snap arm, pulls more string from the axle, and changes the velocity ratio between the mousetrap and the drive wheels. Although this concept has the positive attribute of being simple and straightforward to construct, the team recognizes that a number of questions are raised, and they also list these in their notebook. What should be the length of the lever arm extension and the radius of the spool attached to the drive axle? With a longenough string, :the vehicle would be gradually powered by the mousetrap over the entire 10m distance. On the other hand, if the string is shorter, the mousetrap will close sooner and the vehicle would coast after being powered only along the first portion of the race course. The team39。s discussion of this issue prompts the idea for a tapered spool, as sketched in Figure (c), which would enable the velocity ratio between the mousetrap and the drive axle to change as the mousetrap closes. Should the mousetrap be positioned behind, above, or in front of the drive axle? In their concept sketch, the students drew the mousetrap directly between the front and rear wheels. At this stage, however, that placement is arbitrary, and the team has no reason to expect that choice to be better than any other. What should be the radius of the wheels? Like the length of the lever arm extension and the radius of the spool, the radius of the drive wheels influences the velocity of the vehicle. The team noted on its sketch that puter pact discs could be used as the wheels, but the vehicle might post a better race time with differentdiameter wheels. The team records these questions and discussion topics in their notebook, but they leave them 杠桿臂 for future consideration. At this early point in the brains