A single motor drives the grabber and the arm. Here's how:
A touch-switch is closed when the arm is at
the top of its travel. The grabber is fully open
in this position.
A pair of cams under the robot lower the arm
onto the block, while the grabber closes.
When the grabber meets the block and
can close no further, the arm moves with
the axle that was driving the grabber...
...and the arm is raised holding the block
until it closes the touch-switch at the
top of its travel again.
Click on the FetchBot below to view a larger animation. (135kB)
More photos of the FetchBot:
Everything is driven by the big gear: when it rotates counter-clockwise (with respect to the photos above) the cams push the arm up while the grabber opens wide. When it rotates clockwise, the grabber closes until it can't anymore. When the axle that moves the grabber can't rotate, the arm becomes locked to the main axle (the one with the big gear) and the whole arm lifts up. This puts a fair bit of strain on the small gears where the arm axle and the main axle meet, which is why there is a lot of reinforcing at the base of the arm. Otherwise the arm axle just pops out of the arm!
The FetchBot files:
Copyright © 1998 Ben Williamson