Page 121 - The Unofficial Guide to Lego Mindstorms Robots

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Figure 5-6.
Linking shafts with pulleys and a band

On the other hand, you can u se pulley slippage to your advantage. If part of your robot should have a limited range of motion,
like a trap door that opens and closes, a pulley can be very useful. You can run the motor for longer than it actually takes to
open or close the door—when the door has gone as far as it can go, the pulley band will simply slip.

In the final design, a pair of pulleys is used to link the arm motor to the arm worm gear shaft. You can see this arrangement on
t he top of Minerva—just look for the white band. Note t hat this band does not slip, in general. Its motion is geared down so far
that the arm is likely to break apart before the pulley starts to slip.

Mechanical Design

Several thorny mechanical issues complicate the design and construction of Minerva. In this section, I'll talk about some of
Minerva's more interesting mechanical features.

Grabber Arm

Minerva's grabber arm is operated by a single motor. If you run the motor in one direction, the arm lowers, the grabber grabs,
and the arm rises again. Run the motor in the opposite direction and the arm lowers, the grabber relaxes, and the arm rises once
again. This mechanical sorcery is based on the FetchBot, created by Ben Williamson (see the "Online Resources" section for
details). A cutaway view of the arm is shown in Figure 5-7.

The entire arm is controlled by the main drive shaft. A motor linked to a worm gear drives the 40t gear on the main drive shaft.