With the release of VEX Sack Attack, many teams have begun to talk about using transmissions on their drivetrains. This is probably the best game ever for a shifting drive; speed is needed due to the large field and race for the easy to access sacks (by the goals), and torque is needed for the inevitable push battle that will happen at the end of the game when fighting over goals. Of course, a traditional drive cannot have both speed and torque unless many motors are used, but it seems difficult to use more than four, maybe six, motors on drive this year, as the sacks are rather heavy, and a 30″ reach will be needed for most robots this year. Of course, strafe will also be useful so that you can defend your troughs easily, and quickly move sacks from you opponents troughs to yours.
What Cameron (TooMuchStategy) from AURA has built to help solve these problems is a drive that switches between traditional tank drive and X holonomic. It uses four 393 motors in the the torque configuration, but this number can be easily increased if you have motors to spare. It uses four pneumatic pistons to shift between X and tank drive. It gets about 8 shifts per tank – we have two tanks on at the moment because we have not yet added pressure regulators; you don’t exactly need 100psi to open up the wheels.
In tank mode, the drive ratio is a bit on the slow side, at 1:1. This is, with four 393’s, rather powerful however, and will usually be enough to win pushing battles. In X mode, the drive ratio is slightly over 1.4 (Squareroot 2) when traveling forwards or straight sideways, and 1:1 when going diagonally – see the “Why is X-Drive Faster” article by Oliver. This makes it highly maneuverable.
As for navigating over or around sacks, in tank mode, driving over sacks is very effective The wheels can easily climb over the sacks. In X mode, sacks can get dragged along with the robot as the wheels try to push them sideways. This system has an advantage over other holonomic systems in that it can switch to tank mode to drive over sacks, then switch back to holonomic mode.