Notice the logo applique created by Ora Lin!
This part of the poster describes the features of our final robot
The transmission that connects six motor drivetrain to the hanging winch.
This part of the poster describes our design process, starting from "Bot 1" that we built in November
Bot 1
We only had two weeks to develop our
first competition robot. With that
limitation, we decided to focus on stashing BuckyBalls, since they are not
descorable. We also wanted to be able to
go under the barrier and over the bump.
We used a front lever arm and
conveyer belt to lift BuckyBalls and
deposit them into the goals. A motorized
roller with rubber flaps pulled BuckyBalls from the ground into the ball
carrier. The U-shaped chassis left space
for the lever arm and made it easy to line up with the goal. The wheels were attached to legs to gain
clearance for crossing the bump. It was
four wheel drive, using direct drive on the back wheels and geared drive on the
front.
Bot 1 performed reliably, but there
were points that could be improved. It
sometimes took several seconds for the conveyer flaps to transfer a BuckyBall
into the goal. The robot tipped over
easily because the rear wheels were too far forward.
Bot 2
We wanted the robot to deposit BuckyBalls
in the goals more easily, so we redesigned to a scissor lift with a gravity-fed
intake tray that quickly deposited balls into the goals. When the lift was raised, a rack gear pushed
a slide out the rear of the robot, where it acted as an anti-tipback. We added Mechanum wheels for improved maneuverability
and traction. The Mecanum wheels were
driven by chains that meshed with sprockets attached to the motors. These sprockets acted like raised wheels to
help the robot drive over the bump without needing the high clearance of Bot
1. This allowed more vertical space
inside the chassis while allowing the entire robot to fit underneath the twelve
inch barrier.
The new ball carrier was effective,
and the Mecanum drive worked well.
Weaknesses were that the scissor lift took several seconds to reach goal
height, and the robot crossed the bump very slowly. The scissor lift swayed from side to side
because there was no cross bracing between the two sides of the lift. It was clear that the lift, which was powered
by two motors, was not strong enough to lift the robot off the ground for a
hang.
Bot 3 (no photo)
Hoping to make the lift strong enough
to hang, we used four 1:1 motors instead of two. To reduce the tendency to tip over, we substituted
steel beams for aluminum in the lift to lower the center of gravity. We changed to a larger sprocket on the intake
in order to spit out balls faster. To
cross the bump more easily, we elevated the chassis two inches.
Despite these well-intentioned
“improvements,” Bot 3 was a disaster. The
scissor lift was too weak to handle the power of the four motors, and the aluminum
beams bent under the force. The ball
carriage, which was the same one we used in Bot 2, was too wide and got stuck
on the scissor lift when the lift went up. The drive wasn’t very powerful, and
the resistance of the foam game mats stalled the robot.
Bot 4
After the slow scissor lifts of bots
2&3, we wanted to bring BuckyBalls up to goal height more quickly. We decided to change to a six bar linkage lift
driven by four motors. A six bar lift uses parallelograms to keep the end of
the lift in the same orientation as the chassis of the robot. The six bar lift
did prove to be faster.
We also wanted the robot to be able
to spit BuckyBalls across the field, so we could pass BuckyBalls from hanging
zone to allies in the goal zone. We
thought that we could gear our top roller so that it would spin fast enough to
launch bucky balls across the field. The
robot did spit balls out, but not as fast we wanted it to. It turns out that the motors are internally
geared down so that when we geared them up there was too much resistance to achieve
a very high speed.
Another problem was that the Mecanum
drive had too much resistance and not enough torque to adequately drive the
robot, so the robot stalled very often.
It was very slow: It took about 5
seconds to cross the field.
Bot 5
We were satisfied with our lift
design, but wanted to find a better way to throw BuckyBalls, add the ability to
manipulate Large Balls, improve the drive system, and enable the robot to high
hang with a ball. We designed a new ball
carriage made of aluminum and lexan, with a one-piston BuckyBall launcher
mounted at the top of the intake. To
intake Large Balls, we added vertical side rollers with 6-tooth sprockets on
the bottom and 36-tooth gears wrapped in rubber bands on the top. The robot
could score BuckyBalls in the goal, but the rollers meant for large balls were
not very effective. The BuckyBall launcher was also disappointing. It could only fire the top ball of three, and
only about 3 feet in front of the robot.
To improve speed and torque and
reduce stalling, we switched to a six motor drivetrain with omni wheels, geared
1.5:1, that was more compact and powerful than the Mecanum drive. After the changes the robot moved around the field
more quickly, and had extra torque to push other robots and cross the
bump.
To enable the robot to high hang, we
used a pneumatically activated transmission to connect the 6-motor drivetrain
to the lift. The transmission could not
handle the load of the 6 drive motors, though.
We also added a spring-loaded hanging
hook to the back of the ball carraige.
The hook could knock balls off the barrier, but prevented the robot from
crossing under the barrier backwards.
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