Discuss this article in the SERVO Magazine forums at http://forum.servomagazine.com
The Futaba had a seven-channel receiver which would be
more than enough to add a mechanism.
Quantum Reach
Now that we had more channels than we could ever
want, we needed to design the mechanism. A classic device
that appeals to the mechanical engineering affinities of
people like us is a robotic arm. An arm would be the
perfect way for our Rover to collect samples of mysterious
rocks, and to discover if the Lunar Rover provided a suitable
base for expansion. We thought this would also be a good
exercise to test the limits of the basic kit.
As we mentioned, all we had was the 3-in-1 Rover
starter kit and a structural expansion set, so we wanted to
see how easily that would translate into an additional
mechanism. If the basic kit and small expansion set offered
plenty of material to allow a tinkerer’s imagination to run
wild, then we would consider the Minds-i Rover to be the
perfect sort of present that would nurture a young
roboticist’s imagination and put them on the track to
become the next JPL or NASA superstar.
We value elegance and efficiency in our mechanical
designs, and we thought it would be a creative constraint
to realize our arm by using only one motor. This means
fewer wires to get pinched in joints, fewer devices to draw
power from the bot’s single battery pack, and fewer things
to program. It also means designing a mechanism that can
essentially perform two motions: closing the claw and
raising the arm. Thankfully, we had some design inspiration
from one of our robots of yesteryear that performed a
similar task.
As that design inspiration, we looked to a previous
project of ours that last played a supporting role in our July
2005 column. Gog VI is a LEGO Mindstorms robot that we
built for the Science Olympiad competition when we were
in high school. The goal of the Robot Ramble event at the
Science Olympiad was to deposit a variety of objects into a
box, with the caveat that the same box determined the
maximum dimensions of the robot.
We wanted to be weight conscious with the arm
mechanism because the LEGO motors had limited power,
and we wanted to figure out a way to articulate the
claw without putting a motor on the end of a long
moment arm.
The final design involved a worm gear driven by the
motor that meshed with a large gear. The large gear was
able to articulate both the end effector and the arm itself
by directly pulling the support connected to the end
effector. When the end effector opened or closed
completely, it would lock against the supports for the arm,
and thus the large gear would lift the entire arm.
LEGOS, of course, are renowned far and wide for their
accessibility and adaptability into any design that you could
possibly come up with. Would the leftovers from the Lunar
Rover starter kit and a bag of extra parts be enough to
build a similar mechanism?
A SEVEN-CHANNEL FUTABA RECEIVER.
Mars Bars
Our first instinct was to use a Savox servo to power the
arm mechanism. We were concerned that scaling up the
design we used on a small LEGO robot would require a ton
of power – a claw on the end of a long arm is not the sort
of endeavor for a servo short on torque.
By a cruel twist of fate, however, our Savox servos were
AWOL from Robot Central, so we had to go to Plan B. Our
next choice was to use one of the motors from the VEX
Robotics design system. A VEX motor is not as torquey as
the Savox servos, but it is much easier to use as the
centerpiece of a mechanism – the square shafts of the VEX
kit are much easier to attach to mechanisms than the
sometimes fiddly holes on a plastic servo horn.
Using a VEX motor also meant it would be easy to
mate to other VEX parts, namely a worm gear from the
VEX gear set. Aluminum VEX frame pieces would also make
for a sturdy bracket for the motor and gears. We were
somewhat apprehensive about mixing kits, though. In our
last project, we added the brain from a Mark III robot to a
GOG VI — A DESIGN INSPIRATION.
SERVO 11.2012 69
