by showing that joint angles are relative to
the position of other joints. As you can see,
when the shoulder joint is moved, the angles
of the other two joints (when referenced to
ground) change dramatically.
When arms are built this way, a
significant amount of mathematics is required
to determine the angle each joint should have
in order to create a desired position and
orientation of the gripper.
Furthermore, if you want to keep the
gripper level during some maneuver, for
example, the elbow and wrist joints have to
constantly be manipulated during shoulder
joint movements.
If the motors are moved off the arm —
as in a man-sized arm we built for another
project (see Figure 2) — the orientation of
each joint becomes independent of the
other joints, and we get the added benefit
that the arm does not have to lift its own
motors. All the joint motors are mounted
behind the shoulder joint, and a belt system
delivers power to each of the joints as
shown in Figure 3.
Easy Programming
If you program an arm built with the
principles shown in Figure 3, you quickly
appreciate its advantages. If you move the
wrist joint to a level position, for example, it
will remain level no matter how you move
any of the other joints. It would be nice if
some manufacturer would offer a similar
design, but for now you will have to build
your own.
We have provided details on the
construction of this arm in the RROS manual
(download at www.RobotBASIC.com), but
we also understand that most hobbyists may
not want to undertake a major construction
project in order to experiment with the
principles we are about to discuss.
For that reason, we also built a much
smaller arm that utilizes many of the same
concepts. It is much easier to build because
it has only one moving joint. What may
surprise you is that it retains much of the
functionality of a far more complex version.
The arm is small enough to fit on the
RobotBASIC RB- 9 chassis as shown in
Figure 4.
FIGURE 1.
FIGURE 2.
SERVO 04.2013 47
