I was surprised to learn last Friday while reading The California Tech (Caltech's student newspaper) that my gender has been determined by my participation in the annual ME72 competition! In last Thursday's competition, the devices that students in ME72 had designed and built during the 10-week quarter contended against each other to see which could deliver the most ping-pong balls to a square hole within 35 seconds. A picture of the entire class (29 men and 3 women) appeared on the front page of The California Tech and the caption under the picture read "Today I am a Man."Well, right now I don't feel like analyzing the viewpoint that mills, lathes, and drill presses cannot be used by testosterone-impaired people, so I'll just tell you a little about the contest and my (non-winning but nifty nonetheless) design instead.
First I should explain to you the constraints under which we had to work. Each student was given an identical "bag of junk" containing a few small motors, some sheets of metal and plexiglas and masonite, various tubes and rods made out of metal and wood and plastic, some zip-lock bags, a few o-rings, a sheet of nylon cloth, some nylon string, many small things like screws and springs and rubber bands, enough silicone casting compound to make a few tires, orange-go (can be used as a belt on pulleys), and other similar random parts. Our devices could be made out of only those parts and adhesives. Other constraints included size (no larger than 20 x 20 x 40 cm), weight (no more than 5 kg), and the stipulation that our device causes no ping-pong balls to leave the contest arena (a table approximately the size of a large bed).
The objective of the contest was to deliver as many balls as possible (hopefully more than the opponent) to a "drain" (a square hole in the table). The device had to start in a given starting location with balls already inside the device and somehow get the balls into the drain across the table within 35 seconds. To make the contest more challenging, there was a trough between the starting location and the drain.
Some people built vehicles to drive over to the drain and dump the balls in, others built bridges that allowed the balls to roll above the trough, some people built arms to lift and carry balls to the drain, and there was one machine that projected balls across the table.
The device I made was a vehicle. The design was actually quite simple. The only difficulties came from the fact that it had to be constructed out of the materials found in the "bag of junk."
First I made a frame by cutting aluminum into strips:
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I then made wheels out of plexiglas (2 of them were covered with silicone for traction) and attached them to the frame:
We had two 24-volt motors in our bags of junk. These could be controlled separately by a joystick. I attached these to the frame, one resting on top of the other:
Motors, of course, spin much faster than we want the wheels to spin, so there has to be some kind of transmission connecting the motors to the wheels. The transmission can consist, for instance, of gears, or, as in my machine, of pulleys and belts. I made pulleys out of plexiglas and made belts out of orange-go (a plastic, mildly elastic cord).
That is all that is needed to make a car that is able to drive around and do nothing else. Steering is accomplished by supplying more power to one motor than to the other.
Of course, my car had to be able to carry and dump ping-pong balls as well. The dumping would be controlled by a 12-volt motor and a pulley which I connected together and then attached to the car:
The bottom of the car was made out of nylon cloth. It was cut down the middle and then threaded together with nylon string. The string was attached to the pulley on the 12-volt motor. Turning on the motor pulls the string out of the cloth, thus allowing the two halves to separate and the ping-pong balls to fall through the bottom. When the motor is off, the string is held in place by friction.
Then all that needed to be built were side walls to hold the balls in. There were, of course, many subtleties involved in building the device. Wheels and pulleys needed to have bearings to keep them from being worn out. Hubs had to be made to keep the shafts, wheels, and pulleys in place. Because there were few screws in the bag of junk, other methods had to be used to connect the components. The edges of the nylon cloth had to be melted to keep it from fraying. And so on!
Although I didn't win the competition, I am proud that I was able to make a working device. I learned a heck of a lot this term, so despite the minor cuts, bruises, and burns, I'm very glad I took this course!
