Page 105 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

86 Making Things Move

FIGURE 4-10 Graffbot original concept (credit: Mike Kelberman)

A motor and controller drive each spool, so working together, the spools can move
the spray paint can in any shape possible on the wall below them. In order to figure
out how hard the motors must work, let’s draw a free body diagram of the paint can
platform, as on the left side of Figure 4-11.

The only forces in this free body diagram are the weight of the paint can platform (W)
and a force pulling up from each rope (F , F ) to the motor spools. We can use what
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we know to find these forces so we can size the motor properly. So we need to use
SOHCAHTOA to get all the forces pointing in straight lines.

On the right side of Figure 4-11, each rope forms a triangle with an imaginary
horizontal line. If the can is hanging at 45°, part of the rope force is pulling sideways,
and part is pulling up. By breaking the rope force into horizontal and vertical
components, we can see that the two sideways forces (F and F ) are equal and
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