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This is the story of my high school engineering team. The memories from this group and projects are a lot of fun for me to think about. What amazes me most in retrospect is how little we knew or had. We were given a goal and a budget. Our faculty advisor was a physics teacher. She basically wrote us checks as we needed them and kept people from getting to unruly. The team was formed to compete in the yearly National Engineering Design Challenge (NEDC). This competition is trying to become, for engineering, what the Westinghouse Science Awards are for the natural sciences. Schools across the country are given the same problem. It is always some practical device that allows a handicapped person to perform an everyday task with greater ease. The team gives a presentation to a panel of judges, explaining and selling their device.
My junior year was the third year on the team for me. This was a saga. The problem was unbelievable. In ten minutes, the team and its machine must do the following: remove up to 120 lb. of groceries from the simulated trunk of a car, carry these groceries across a pile of wood-chips and up a ramp. It must stop on the way up the ramp for five seconds. It then continues from the ramp across a three foot platform and down two stairs. It must climb back up the stairs (again stopping for 5 seconds), and continue back across the platform and down the ramp. Here's the catch: the operator of the device may not exert more then 7 lb. of force on the machine at any time.
Our device was good. It was remote controlled. The frame was made of zinc bar-stock held together with carriage bolts. Four, 20 inch dirt bicycle wheels were chained to motors (from kiddy cars) with a 7:1 gear ratio (provided by the gears and chains from rollup fire doors). The motors were powered by a wheelchair battery. To be able to stop on the stairs and the ramp we needed brakes. After trying several mechanical schemes, we settled on resistive braking by the motors. In our device, when power was removed from the motor, a set of electrical relays disconnected the motor from the battery and shorted out the motors against themselves. Now the motors were generators trying to generate against themselves. This caused a huge braking force, sufficient to hold the cart on a ramp or stairs. It was tested with 300 lbs. at an incline of 45 degrees and it held strong.
Our biggest problem was climbing the stairs. First, we calculated the coefficient of friction between the tire and the stair. This was important because if the tires couldn't support the weight without slipping, then the machine wouldn't grip enough to climb the stairs. Then we calculated the maximum amount of torque generated by the motors, to see if we had enough power. The data from both test showed that it was theoretically possible. How would we get it to work in practice?
We found that weight distribution and tire pressure were critical factors. Shifting the groceries by as little as half an inch made a difference. The weight had to be as far forward as possible when climbing the stairs (this had almost catastrophic results later). Four days before the competition we had to rebuild the whole frame to lengthen the wheel base to help solve our weight distribution problems.
The groceries bags were placed in a special box, already in the trunk of the ³car². The car we simulated was a Volvo station wagon. This allowed the box with groceries to be winched straight out of the back of the car and onto the machine. Turning was accomplished by having the wheels on one side go forward and the wheels on the other go in reverse. What I found most impressive about our machine was that the only power tools used to build it were an electric drill and soldering iron. All the metal was cut by hand and bolted together. This clearly showed in the rolling junk heap that was our baby.
The one thing that our team could stubbornly not agree upon was the name of our device. Most people advocated the name Shop'n Cart, similar to those ads seen late at night on local television. A small minority (myself included) felt that our creation was worth more then being placed in the ranks of the Chiapet and ThighMaster. Our vote was for "Sisyphus Assist," a name that was more consistent with the dignity that our device rated. I lost. Shop'n Cart will soon be hauling Chiapets in a house near you.
It is the day of the presentation. We have spent the morning hauling Shop'n Cart and all of its related equipment to the Arlington Marriott Hotel. There are 15 other schools here from across the country. We have been looking at the other devices. These big schools with huge welding and machine shops have some very impressive looking and beautifully made devices. Students clearly didn't build some of them, but too late now. God help our mass of metal, wires and wheels.
After sitting through about half of the presentations we are called on to get ready. Order is determined by lottery, and each team gets about 20 minutes' notice. Each team is given 2.5 minutes to set up, then 10 minutes for the presentation.
We set up. We still have a ways to go when the 2.5 minute set up time ends. Stay calm, keep setting up, but hurry. Finally we're ready. However, in our haste Shop'n Cart was not placed in the correct position in relation to the car trunk. We are already behind in our time plan; precious more seconds are lost maneuvering the device into place. We had planned about a 30 second safety margin in our presentation. Would that be enough?
With a whirr, swish, and clank the box of groceries is removed, loaded and secured. All right back on schedule. The judges and the whole auditorium (300 plus people) are watching closely now. With the familiar grinding-whine, and Tommy at the helm, our device maneuvers some more to align itself with the course. I should remind the reader, that for our device to climb the stairs, most of the weight had to be on the uphill side. This side is the downhill side when going up and down the ramp (After running over the course in one direction we didn't turn the machine around. Rather, we just ran it in reverse back over the course).
Shop'n Cart grinds its way across the wood-chips and leaps up the ramp. Tommy, nervous, forgets to stop.
"Tom, back down and stop on the ramp or we'll be disqualified. "Shit," he mutters. The device stops and begins to back down the ramp. Tommy presses stop. The resistive brakes kick in. As a testimony to their braking ability, all four wheels lock up instantly. 120 lb. of groceries located high above the downhill wheels keep moving forward, rocking the device forward with it. Certain disaster, our beloved child is teetering on only two wheels for a second, an eternity. Far too slowly other two wheels begin to drift back down towards the ramp. We, the judges, and the crowd breathe again. Well, it got their attention at least. But how much more time had been lost? The run continues without further incident. It takes a careening dive down the stairs at full power. Then Shopın Cart climbs back up, wowing everyone with is speed up the stairs. It plunges down the ramp again and comes to rest on the correct side of the wood-chips. Then the user friendly judge, a.k.a. the slowest man on earth, waddles up from his chair, picks up the control box. He presses the indicated button, and sits down again.
Now for the presentation. We have the theater's whole attention. The run was solid. I get through my lines about different wheel sizes and frame configurations. I saw the judges following our presentation and connecting with our points. I actually came across clearly, despite being under the gaze of 5 people that mattered and an assorted 300 more. My lines could have been Chinese, because my only thought is TIME. Is there enough after everything that had gone wrong?
We're in the final stretch right now. Elena, Tom, Mark, and I have done our part. Charles is doing his thing about the next generation model of Shop'n Cart. Doing well too, but with the speed of a sloth (in fairness to Charles, he was going at the pace we practiced). Doesn't he see the women looking at her stop watch? Probably not. Hurry up we don't have time for this. "also in our next generation model-" "Maret School, your time for this presentation has expired."
Done. We won third in the nation. We got the most points of all the teams for the actual operation of our machine. We got very little for the next generation section. If we had finished our speech, we would probably have gotten second place. Nevertheless, our school was a thousand dollars richer for our efforts, and more importantly none of the other teams were will to pick up our challenge of a post contest Drag Race.