SKYWORX

As part of the Introduction to Engineering Design course, students were asked to design, construct, and test a device in one of three project domains before battling against other teams in competition. My partner and I chose the Mouse Trap Vehicle. We were tasked with building a vehicle propelled solely by the potential energy stored in a standard mouse trap spring. The vehicle must also come to a complete stop at a variable target distance. Vehicle size had to be within a certain range, and a minimum travel distance of fifteen feet was a requirement.

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Having access to a laser cutter allowed us to take full advantage of computer aided drawing (CAD) to define the chassis with sleek curves and a snap-fit system that aided with assembly. It also enhanced the car's alignment, reducing its tendency to drift after release. A laser-cut plywood chassis, wing-nut braking system, extended lever arm, and large rear axle wheels were hallmarks of our vehicle.

 

1/8th inch plywood was used for the parts. Parts were laser cut from a single sheet in a single pass, not exactly a recipe for success if something goes wrong mid-way through. Care had to be taken setting the cut-path routine, albeit after two runs failed due to incorrect hole and cutout locations. It's common for cut parts to "fall through" once they become fully separated from the parent plywood material. That was the case for our run, and caused internal cutouts, like axle holes, to be cut off-position. When the chassis permitter was completed, it fell through, became misaligned from the bed, and the run was ruined. The cutting order was reversed; smaller interior holes first, then outer shape second, for the successful run.

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A wing nut braking system is used to stop the car's momentum. The car can be tuned to stop at any distance (prior to naturally slowing down from friction and drag) by precisely positioning the wing-nut. The forward axle is threaded by design, spinning as the car drives forward. The wing-nut, forced to travel down the axle's length with the help of two rotation-preventing guide rails, eventually reaches the chassis wall, causing the axle to lock-up and the car to abruptly stop.

 

This braking system relies on a calibration equation relating the stopping distance to the number of turns of the axle. The calibration was made by running test trials at various stopping distances and fitting a linear regression to the data. Five trials were enough to give an r-squared value of 0.999. Before release, the operator manually turns the front axle to the required number. Twenty spokes on each front wheel aid in calibration.

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Brass bushings were used between the chassis holes and axles to reduce friction and increase maximum travel distance. The car is rear-axle driven, and large rear wheels are used to increase the distance the car can travel, at the expense of propelling force. The same concept is also applied to the mouse trap itself in the form of a longer level arm. Since the car must travel at least 15 feet - and since the mass of the car is relatively small - these attributes are beneficial. This concept is called mechanical advantage, and is commonly used in the opposite manner to obtain a larger force, at the expense of distance.

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Initial tests proved problematic; the car simply did not move after releasing the mouse trap, and it was theorized that the car was too heavy. To combat this, unnecessary material was removed from our first prototype with the help of a drill press and a large drill bit. However, it didn't help. To reduce weight further, the chassis was re-drawn shorter and with additional cutouts, but it still didn't budge. Before giving up and starting over again from scratch, I thought to reduce the mouse trap lever arm length. This proved to be the solution. The short arm gave it more pulling force to accelerate the rear wheels. And the car made it to fifteen feet.

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We won first place (woo!) by having the lowest average deviation distance from the target center, set at nine feet and nine inches from the launching point.