Bah! Direction change is no big deal. Ever look inside an old cassette deck or VCR? The motor always moves in just one direction for forward and reverse. You simply pop a gear over based on a clutch. A center driven gear spins in one direction. You connect a gear to the left side and it rotates in one direction, you move that gear away and mesh a gear on the other side and it rotates in the opposite direction. The cam that decides which gear is meshed is just controlled by the ends of the movement (when it reaches an end, it pushes a lever which disengages one gear and meshes the other.) You don't think a car engine actually rotates in the opposite direction when you put it in reverse, do you? 