[ShaneP]

Essentially, instead of having the drivebase turn within the frame, I wanted the frame to turn on the drivebase (ie, the whole frame to be a giant turret). Anyone who thinks about that for a few seconds can realize what an advantage that would have been.

We had the same thought. This was one possibility we wanted to explore, but as you pointed out, the forces involved, and the traction required by the wheels to create enough torque is not there. The moment of the trailer is too large.

Now, I'm curious as to exactly how this performs? What method do you use for turning the wheels within the frame? I'm suspecting you turn the wheels as if they were a standard drive base (run each side in opposite directions) and then rotate the frame on top of them. In other words, the base turns like a normal robot, but the frame could remain in the same orientation compared to the field.

Is that the case? Or is it purely by the force of the turntable that your drivebase is oriented? If so, does the frame move at all, or does it stay stationary (due to the low friction of the field not providing huge resistance)? How does the trailer influence the driving?

With the current configuration, the drive frame is rotated by a BaneBot 545 motor in a 64:1 planetary gear box which is mounted on the octagon drive frame. A rubber wheel pokes out of the frame and picks up the outer ring of the turntable which is mounted to the external frame of the robot. The octagon drive frame sits on the inner ring of the turntable. You can see the rubber wheel at the back of the robot in this photo: http://www.chiefdelphi.com/media/photos/33368

The setup is a standard 6 wheel drive setup, inside the powered turntable. Both sides are controlled separately. Since the planetary gearbox is not easily back driven, the robot drives like a normal 6 wheel drive when the 545 motor is not powered.

Alternatively, we can take off the rubber wheel and then that "frees" the 6 wheel drive (disconnecting the 545 motor with the external frame), allowing it to rotate within the external frame under it's own turning force. This is how the drive was originally designed, because it allowed the drivers to drive the robot without having to worry about handling the trailer appropriately. If the drivers wished to go straight back, they could do so by simply backing up, and the trailer would move out of the way without any effort. Essentially, you could drive straight through the trailer. However, with this configuration you cannot point the upper portion of the robot. As the robot went through design phases, the requirements for the drive changed. Once we settled on picking up off the ground and having a stationary (relative to the external frame) dumper, we needed to control the external frame, and added the motor which powered the turntable.

Although the 545 motor does create an indirect link between the trailer and drive train (thus restricting turning), this is still overcome by the driver. When the turntable is spun and the drive is rotated at the same speed in the same direction, you get virtually the same "free spin" effect, as you would without the 545 motor attached. This allows you to drive straight through the trailer again.

Is there any chance of getting a pic of just the drive train up? I want to see how you guys can spin that bot so fast!

Check out this photo: http://www.chiefdelphi.com/media/photos/33368