The first model in the Tetrix Prime robotics kit is called
WheelieBot. WheelieBot is a 3-wheel robot that uses a continuous rotation servo motor on the back wheel for motion and a standard servo motor on the front axle for steering, It's a good starter robot for a beginner, because it is quick and easy to put together. But let's face it - it's boring to drive. WheelieBot has no speed and the steering is abrupt and jerky. I can't think of a better opportunity to introduce a lesson in gears.
Let's first tackle the speed issue since everyone likes to drive fast. To increase the speed on the back wheel, I need to use a method called
gearing up. In a gearing up arrangement, a large
driver gear is attached to the motor and a smaller
follower gear is attached to the wheel. I removed the continuous motor and replaced it with a small 40-tooth gear. My intention was to move the motor back just enough to add a large 80-tooth gear, but the wheel kept hitting the motor housing. I could have avoided this problem by putting the gears on the same side of the beam as the wheel, but then the robot would have been significantly unbalanced (being a 3-wheeled robot, it tends to tip as it is). Instead, I added a second 40-tooth gear to act as a spacer. The additional gear doesn't influence the speed, but it has the added benefit of keeping the wheel turning in the same direction as it did without gears (teachable moment: an even number of gears changes the direction of rotation; an odd number of gears, keeps the direction of rotation the same). Since the 80-tooth gear is twice as large as the 40-tooth gear, the speed is doubled. If you watch closely, you can see that every time the driver gear makes one complete turn, the follower gear turns around twice resulting in a much more exciting robot to drive.
Unfortunately the faster speed only compounded the abruptness of the steering often causing the robot to tip over. In this situation, I wanted to slow the steering down giving me more control over the steering. This gearing method is called
gearing down. In a gearing down arrangement, the 40-tooth gear becomes the driver and the larger 80-tooth gear becomes the follower. I removed the beam attachment hub and replaced it with the 80-tooth gear. You need to make sure the set screw on the gear faces up, so that the 2 nubs on the underside of the gear fit into the holes on the cross beam (the beam acting as the axle between the two wheels). This gives your robot much more stability. You'll need to use two shaft collars to attach the two beam sections together. Make sure all set screws are facing forward before tightening otherwise the front wheels will be out of balance when you turn the power on. I made all these improvements using only the elements in the basic Tetrix Prime kit. It would have been nice to have some additional bushings and another 40-mm axle, but my robot seemed to drive fine without them.
