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Figure 5-6.
Linking shafts with pulleys and a band
On the other hand, you can use 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 the top of Minerva—just look for the white band. Note that 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.
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Figure 5-7.
Cutaway view of the grabber arm
The key to understanding how the arm works is that it moves up and down for two separate reasons:
1. The cams on the lower shaft push the arm up and let it move back down.
2. When the grabber grips something, the geartrain that controls the grabber locks up. When the main drive shaft continues turning, it moves the entire arm up and down.
Suppose Minerva begins with her arm raised and the grabber open. When the main drive shaft starts turning counterclockwise, the lower shaft turns clockwise. The cams rotate down, allowing the
arm to lower. (At the same time, the rotation of the main shaft closes the grabber a little bit.) As the main drive shaft continues turning, the grabber closes. When it is fully closed, either on an
object or itself, the grabber geartrain becomes stationary with respect to the arm. The main drive shaft keeps rotating, raising the entire arm with the closed grabber.