FIGURE 3.
Constructing the Arm
Most of the structural
components of the arm and gripper
are made from various sizes of craft
sticks, but you can also use Popsicle
sticks and coffee stirs. Figures 5 and
6 show alternate views to provide
additional detail. You can also refer to
Figure 7 of last month’s article which
offers a clearer view since it showed
the arm before the wiring was added.
Since this arm is small and
lightweight, the axles for the joints are
just bolts with plastic
spacers serving as
bearings. The wrist
joint is fixed to the
forearm, and the
forearm maintains
the position
established by the
two pulleys mounted
on the shoulder and
elbow joints.
FIGURE 4.
which maintains the orientation of the
fingers. Actually, each strut is made
from two sticks to increase stability.
The bolts at each end of the
sticks serve as axles and should be
used with nylon nuts so they will stay
fixed without being overly tight. We
added a cross-member that slides
between the strut sticks for additional
stability, but this is not necessary if
the ends of the struts have more
overlap than the base of the hand and
fingers.
The nice thing about constructing
with craft sticks is that they are very
cheap and easy to work with,
allowing you to experiment with
numerous designs.
This one-motor arm is surprisingly
agile. The gripper can be lowered to
ground level to pick up objects and it
can be raised so that objects can be
placed on platforms or dropped into
baskets. When the arm retracts, as in
Figure 4, the gripper and any object
in its grasp are out of the view of the
robot’s primary perimeter sensors,
A small servomotor opens and
closes the gripper as shown in
Figure 6. Notice that three sensors
are mounted on the fingers just as
they were on the ball trap used by
the soccer robot last month. Foam
rubber is mounted on the inside
edges of the fingers to give a better
grip.
Look carefully at the finger at
the bottom of Figure 6. A small
snap-action switch is mounted
under the finger with foam glued to
its lever (you can only see the lever).
Reading the state of this switch
makes it easy to determine when
the gripper has closed on an object.
Creating an
Application
Our next step is to write a
program that will allow the robot to
find small objects and move them to
a beacon in much the same manner
as soccer robot. Most of the
48 SERVO 04.2013
