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Y (rank) movement
The head moves forwards and backwards along a track to provide movent along ranks of the board.
For accuracy, six 8 tooth gears move the head along two long runs of gear racks.
X (file) movement
The two runners themselves are mounted on two more runs of gear tracks fixed to the base and a compact drive unit is used to move the whole head and rank-track along files of the board.
 
The x drive moves on skids that lock under the lip of the x tracks on the base to prevent the drive slipping. A similar, unmotorised skid is used on the other side of the board to provide stability (and measure distance with a fibre-optic unit used as a rotation sensor).
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Andy's LEGO® Mindstorms® Page
