A favorite device for storing potential energy is
the spring. Everything from clocks to catapults make
use of springs. One common use for springs is to
return something to its original position. Another interesting
application is the measurement (and creation of) force, such as springs in a scale. The third use is to store energy. All springs perform all
three functions all of the time, but specific devices are built to
exploit certain functions of the spring.
There are many physical forms of springs, created for specific purposes. The most basic form is the bending bar, shown in Figure 1. The familiar coil spring is just a bar spring in a different form. A rubber band is also a spring that makes use of the elastic properties of polymer materials.
To understand how a spring works, one must zoom in to the microscopic level
where molecules interact. Molecules are held together in rigid
bodies because of electromagnetic forces. Some of these forces are
repulsive, and some of them are attractive. Normally they balance
out so that the molecules are evenly spaced within an object;
however, by bending a bar, some molecules are forced farther apart
and others are shoved closer together, as in Figure 1. Where the molecules have
been spread out, the attractive forces strive to return the
original spacing. Where molecules have been forced together, the
repulsive forces work to return the object to the original shape. A coil spring works in more or less the same
way as a bar spring; when its shape is deformed, molecular forces act to return it to its original shape.
A rubber band is slightly more versatile than a metal spring
because of its flexibility, just as pulleys are more versatile
than their rigid cousin the lever. Using springs in a small robot might take a
small amount of imagination, but rubber bands almost scream to
be used. There are often several small tasks that a robot
performs only once during a game. It would not make sense to
devote an entire motor to such a task. It's not worth carrying
around the extra weight if the task could be accomplished just
as well with rubber bands. Examples include shooting a ball, pulling up a gate, or closing a jaw.
Rubber bands also prove useful in the case of
repetitive motions. Rather than turning a motor forward then
backwards then forwards and so on, one could make use of a cam
and a rubber band to allow the motor to always turn in one
direction. Look at the assembly in Figure Figure 2
for an example.
Rice LEGO Robot Course
