Page 313 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

You will notice that as a sequence (script file) is continually re-
peated, the position of the robotic arm will drift from its original
position. There are a number of reasons for this. As the battery
power supply to the robotic arm becomes depleted, the reduction
in electric power delivered to the DC motor reduces the torque
and speed of the motor. So during a timed function, the motors will
not move as far or lift as much with old batteries as they would
with fresh batteries. But this isn’t the entire case. Even with a reg-
ulated power supply, how many times the DC motor shaft spins in
a given length of time is neither counted nor controlled. So the
number of turns the motor spins in each timed sequence will vary
by a small percentage. This causes the position of the robotic arm
to drift. If that wasn’t enough, the gears used in all the motor gear-
boxes have a certain amount of slop (or play) that isn’t taken into
account. All these factors taken together go a long way in explaining
why the position of the robotic arm repeatedly performing a script
file will drift over time.

Finding home

To enhance this project, positional feedback from the robotic arm
could be implemented so the computer could determine absolute
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position of the arm. With basic positional feedback, the robotic
arm can be located in precisely the same position every time at the
beginning of a script file (sequence) run.
There are many approaches one can take. One basic method doesn’t
employ positional control but instead uses limit switches to find a
“home” or starting position. The limit switches determine when
the arm reaches one absolute, or home, position. To accomplish
this, a series of limit switches (momentary contact lever switches)
will close when the arm reaches its limit of travel in that particular
direction. For instance, one limit switch would be mounted to the
base. This switch would close only when the robotic arm was
turned completely in a CW direction as far as it could go. Other
limit switches would be mounted on the shoulder and elbow. They
would close when the respective joint was fully extended. Another
switch mounted to the wrist would close when the wrist was rotated
to the furthest CW position. The last switch mounted to the gripper
would close when the gripper is fully opened. To reset the arm to its
home position, each function is activated in the direction of travel
needed to close a limit switch until that limit switch actually closed.
After all the functions are in the home position, the computer would
then know the absolute position of the arm.

Team LRN
Chapter fifteen