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Lego PC Bot Castor
As you will have already noticed the red ball provided with the Lego NXT kit was
used as the laptop castor (or third wheel). We decided not to use the third
motor provided in the kit as the rubber wheel would cause too much friction when
being forced to the left or right. As the robot is meant for home use the plastic
on the ball tends to provide much less friction on a carpet or floor than
the third wheel. We also tended away from the other castor implementations
seen in other Lego NXT constructions as they are too thin to operate reliably
on a thich carpet and tended to cause jerkyness as the robot moved due to the
castor wheel slipping into the current orientation.
We also determined that based on the laptop base a lot of weight from the laptop
would be resting on the castor which would cause movement issues unless that weight
was compensated for by the castor. Thus the red (or blue) ball would work well for
distributing the laptop weight and allow the robot to slip over a thick carpet.
While the ball mainly slips over the carpet when it does move (over hardwood) it provides
a crude omnidirectional wheel.
click on image for larger view
The trick behind the castor is to use the third wheel hub as the main vertical support.
This allows the weight of the laptop to be distributed on the wheel and provides
a good hold on the ball. The supporting lego pieces seen around the ball are used
to hold the ball in position and prevent the ball from simply dropping below the
wheel hub holder when in operation or when simply picked up. The left and right
black bar supports of that sturcture allow for fine tune adjustment to get the holder
into the correct position. With this tuning the ball would either slip out or
would require too tight a hold which would prevent the wheel from turing at all.
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