June 20, 2007

Rat Trap Car

Back during first semester of last year my engineering class was asked to build a self-powered car with the following features:

• Had to be self powered
• Had to fit in a 1’x1’x1’ cube
• Had to be able to drive 10’, stop, and then fire a marble 10’ into a garbage can

After building a car to these specifications we then had to

• Render it using Autodesk Viz
• Animate it shooting a marble
• Make a video of the actual car completing a test run
• Film it up close while firing.

Here’s my account:

I had a nifty piece of Plexiglas that I wanted to use for the base because it was light. I drilled holes in the four corners, bolted on some I-bolts, threaded dowels through the bolts, and then attached wheels to the ends of the dowels. The next step was to drill a small hole through the Plexiglas near the front. The plan was to mount two rat traps to the top of the platform: one for power and one for shooting the marble. I’ll explain how each worked.

Power Rat Trap
A small nail was put into the back axel. A piece of fishing line was attached on one end to the rat trap and was tied in a loop on the other end. The loop was placed around nail in the axel. When the axel was turned it pulled the rat trap back, ready to be released. When the rat trap is released, it unwinds the back axel, propelling the car forward. When the line runs out, the loop simply slips off the nail and the car continues to coast forward under its own momentum. Here’s a quick video that demonstrates this whole process:

Marble Shooting Rat Trap
Before discussing the role of the second rat trap it’s important to explain how the car stops after ten feet. A string was attached to the front axle. The other end was threaded up through the small hole near the front of the Plexiglas I talked about earlier. Now I can describe the marble shooter. It consisted of a bottle cap attached to a mousetrap. The mousetrap was set and the marble placed in the bottle cap. When the trap snapped the marble was shot forward. What snapped the trap? The string attached to the front axle. One end is attached to the front axle and the other end is threaded up through the base and tied to the trigger. When the string is pulled taught after wrapping around the front axle it pulls the trigger and fires the marble. I’ll try to draw a picture in Paint that demonstrates what happens.

So now the car goes, stops, and fires marbles. The last step was aiming the marbles at the garbage can. That was up to trial and error, adjusting the angle of the bottle cap. Here’s a video of a successful run:

Here’s the close-up of it firing:

Then it was time to do the computer bit. Here are some pictures of the rendered car.