Servo - September 2007

intimidation factor and also to showcase the advantages of the omni wheels. For the power transmission, we went with a super simple, four wheel direct drive system. Normally, we would like to include some sort of gear reduction for more torque, but a direct drive is so much simpler, and the omni wheels make the lower torque design plenty feasible.

One of the things that makes Vex robots such great rapid prototyping tools is that they are so easy to build — after putting together the drive train, we just had to fasten the microcontroller, transmitter, and batteries to the frame and we were ready to roll. We determined that one last touch was necessary to compete with the coolness of the automotive drive vehicle — some carbon fiber plates to define the front end.

The automotive drive vehicle was a much greater challenge. The rack and pinion steering was difficult to recreate with only Vex parts, but the new bits in the gear set gave us a solid start. The great thing about building an automotive bot was that it would utilize two of the unique parts from the gear set: bits for a differential and a rack.

We thought that the differential would be a good place to start. The pumpkin has room for three beveled gears, but, if you want to utilize one shaft so both wheels turn the same direction, you can only employ two of the three beveled gears. The pumpkin is also designed to rotate, and we had to constrain that degree of freedom for our design. That object was easily achieved by meshing the teeth around the pumpkin with a gear that we fixed to be stationary. The motor drove the back wheels by connecting to one of the beveled gears, and we had our fixed diff drive train.

This design more closely resembles a racing type of differential that is referred to as a spool type rear end since both wheels turn exactly the same amount all the time. A real street car differential has the ability — through more gears — to allow each rear wheel to turn at different speeds when necessary. This can be achieved with this kit by utilizing two axle shafts and the three bevel gears while having

OMNIWHEEKTANK IN PROGRESS.

the whole pumpkin rotate. Since we are all about high-performance, we used the racing design.

Rack and Roller

The back wheels were the easy part. To recreate rack and pinion steering with Vex parts, we had to build pivoting mounts for the wheels that could be slightly rotated by the linear motion of the rack via pivot points and kingpins. We tackled it one bite at a time, starting with mounts for the wheels. Those went together fairly easily, and then we were able to see how the robot might sit given different placements for the wheels. With the biggest Vex wheels available, the robot would sit nicely parallel to the ground with the given height of our mounts.

We moved onto the rack when we saw how things could come together. Rack and pinion steering works by having a fixed pinion (a gear) drive the rack back and forth. For our rack to move back and forth, we had to put together a slider, and branching off of the slider were the pivot arms. The wheel mounts

VEX RACK.

Robot vs. Wild

OMNI WHEEK TANK DRIVE ROBOT.

were structurally attached to the frame of the bot with bits called kingpins — the shafts that the wheel mounts would rotate around.

Getting everything to come together was a challenging undertaking, but we were actually able to build the automotive steering robot completely out of Vex parts with the minor exception of some Nylock nuts. We downloaded a quick Easy C program to the robot to make sure that the controls were as we wanted them, and we were ready to test our creations.

The tank drive vehicle performed as we expected. When we tried to turn while scrubbing the normal wheels, its performance was lackluster, but scrubbing the omni wheels gave us a tight turning radius worthy of a high performance vehicle. The robot was fairly peppy by virtue of its four wheel drive, and even without gear reduction it hopped over obstacles like feet with ease.

The automotive steering vehicle drove well enough, but it did have some trouble making tight turns while moving forward. In reverse, it turned like a dream, but in the forward direc-