Page 42 - The Unofficial Guide to Lego Mindstorms Robots

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Each wheel assembly is actually composed of three wheels arranged in a triangular fashion. The robot drives these wheels to
move. When a large obstacle (like a step) is encountered, the entire wheel assembly rolls on its center axis. In essence, the
en tire wheel assembly acts like a large triangular wheel. This large wheel size enables the tri-star design to drive over large
obstacles.

Killough's platform is an interesting variation on the wheels-within-a-wheel concept. It's really too exotic to describe here; the
"Online Resources" lists two web pages that contain photographs and diagrams of this platform.

Bumpers and Feelers

Hank uses the touch sensors to figure out when he bumps into something. But it's not really enough to put a touch sensor just
on the front of your robot, because then it could be activated only in one specific spot. Instead, Hank uses a pair of bumpers to
detect touches across the entire front of the robot.

The idea of a bumper is to make a large area sensitive to touch so that the robot can detect collisions with a wide variety of
objects - chair legs, walls, pets, rocks, trees, and so forth.

Hank uses bumpers that rest lightly against the touch sensors. When the bumper is pressed anywhere along its length, the touch
sensor is then also pressed. A slightly different approach is to make a bumper t hat is held tightly against the sensor. When the
bumper collides with something, the sensor actually turns off instead of on.

T he trick with bumpers is to make them sensitive but not too sensitive. The bumper needs to trigger the touch sensor when the
robot bumps into something. On the other hand, it should not trigger the touch sensor when the robot starts or stops moving
abruptly or when it's driving over a bumpy surface.

Gears

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Gears are clever mechanica devices that can be used to trade speed for power or to translate motion from one axis to another.
A gear, in essence, is a disk with teeth on its edge. It has a space in its center where you can put a shaft. Gears have three
primary purposes:

1 . You can trade speed for power by using a small gear to drive a larger gear. The shaft on the larger gear will turn more
slowly but more powerfully than the shaft on the smaller gear.

2 . The opposite effect—trading power for speed—occurs if you use a large gear to drive a smaller gear. The shaft on the
smaller gear will turn faster than the one on the larg er gear, but with less power.