The Great Curl

Introduction

Curling as a sport has a number of interesting techniques and problems involved with winning, and our version of engineering curling dealt with all of those and a number of others more specific to this project.  In traditional curling, one must slide a stone a set distance and get it to stop, and the closest one to a set point, called the “tee”, is the winner. The hardest part about this is simple in theory, but much more difficult in practice: how hard to slide the rock?  This issue in curling is solved by practice, and with enough of it one can be fairly accurate in how they slide their rock. We dealt with a similar issue in this project, how much power do we need to give to our vehicle to get it to travel the correct distance, too far or too short and we would likely lose this competition.

    Another issue, we would come to realize, was not going to be our own vehicles, but the vehicles we were competing against.  If we could get one vehicle to consistently land near or on the target point, that would still not be enough to secure the win, we would have to also make sure nobody else's vehicle got closer, or bumped us out of position.  Since we were allowed two vehicles for this competition, and the winner is whoever has the closest single vehicle, we decided to take a unique approach and make one vehicle which would be our “rock” and another which would be our “bully”.  Our first vehicles goal was to get as close to the tee as possible, while our second vehicles goal was to remove the competitors vehicle from a potentially winning position.

Procedure

Our initial design used a VEX Robotics Minotaur made by HEXBUGS, which we were actually under the impression came with its own motor, however that did not turn out to be the case. Upon completing construction of the vehicle, we realized this was meant to just be pushed around by hand, and didn’t even have a means of attaching a motor to power the wheels or “bumper”.  Once we realized this, we started experimenting with different methods of powering our vehicle. The first method we tried was affixing a length of Figure 1.1 - HEXBUGS Minotaur           rubber band to the rear or the vehicle, and wrapping it around the axle so that when pulled taunt and released, it would move forward.  In trying to do this, we came across our first issue, that the resistance was far too great to power our vehicle. To address this issue we tried a number of things, first being to try the same method with larger and more powerful rubber bands, and once we saw this did not solve the problem we also increased the distance between attachment points, so we could load more potential energy into our rubber bands.  After much trial and error, we realized this was not going to work, even the largest of our rubber bands would only manage to get the vehicle to move a couple feet; far from our almost thirty foot goal.  
After toying with different ideas on how to power this vehicle, we decided on a small motor.  We knew it had to be less than six volts, so we found a small fan that had a motor we could disassemble and use on our vehicle.  Once disassembled, we saw this motor had a simple rod protruding out of the side, that was our sole attachment point for how we were going to power this vehicle.  With a little reconfiguring, and a lot of removal of unnecessary parts, we were able to assemble the vehicle in such a way that we could power the center axle with the motor using a rubber band, essentially forming our “drivetrain”.  The next issue we faced was how to attach the battery pack and motor, luckily with all the parts we had removed we now had plenty of extra components to assemble a base for the batteries to sit on, and a housing in which to insert the motor. Our final issue to solve with this vehicle was actually caused by our solution to the second issue, mounting our   Figure 1.2 - Rebuilt Minotaur battery pack on the rear of the vehicle.  This caused an excess of weight in the rear, resulting in a lot of drag.  Luckily we had at this point neared completion on our second vehicle, and were able to remove parts of it that we were not using to modify our first vehicle.  Taking a couple mounting brackets and a wheel, we were able to fabricate a third wheel in the rear, which supported the extra weight of the battery pack. Once this was completed, our vehicles only testing to be done was to make sure we could aim it properly, and make sure everything would function properly.
Our second vehicle was a much simpler approach, because we constructed it after building our first vehicle, and already had a much better understanding of what we needed the vehicle to do, not to mention a wealth of knowledge of how to work on these type of vehicles.  After deciding we wanted to have something small and driven by rubber bands, we found out that HEXBUGS, the same company Figure 1.3 - HEXBUGS Gear Racer that made the first vehicle we purchased, also makes a rubber band powered “single gear racer”. This was what we ended up using as the starting point for our second vehicle.  After construction of this second vehicle, we again realized we had a lot of excess parts that we wouldn’t be needed. Because the rear wheels are what are being powered, we were able to remove the front axle, construct a different mount for a single front wheel, and use the other wheel for our first vehicle. Another thing we were able to change was the gearing in the rear of the  Figure 1.4 - Rebuilt Gear Racer     vehicle.  Initially it came with two different gear options, a high                                                                                                                                                                                                speed gearing, and a high torque mode. After some initial testing, the high torque mode was not going to work for what we were trying to accomplish; it ended up spinning the wheels too fast, and they lost traction on initial release. This in turn caused the vehicle to not travel a far enough distance, so we removed this gear ratio and went with the high speed gearing, with which we were able to cover an appropriate distance.  The only other thing to do was test how much we had to wind up the vehicle in order to go our set distance of about 30 feet.