The last two weeks of build season were spent making our robot bulletproof.
During one of week 5’s many driver practice and programming sessions, the rotary potentiometer on the intake arm slipped and migrated away from its home position. As a result, the arm moved past the programmed safety stop limit. The robot’s carbon fiber front cross brace acted as a hard stop and the robot’s chassis and the intake arm itself were unscathed. However, the Banebots 775 motor driving the arm through a 256:1 gearbox and 2.57:1.25 chain reduction had way too much torque and bent the 3/8″ hex shaft about which the arm rotates. After replacing the shaft with a spare, the same situation occurred, but this time the 14 tooth sprocket attached to the motor split into two halves before the shaft had a chance to bend.
We took several measures to beef up the subsystem and make sure these problems would never occur again. First, we replaced the 3/8″ hex shaft with a 1/2″ steel shaft to prevent it from bending. Second, we replaced the 14 tooth motor sprocket with a 21-tooth sprocket, the largest sized one that would fit in the available space. This change reduced the output torque by 33% and increased the ball arm’s speed by 33%. The lowered torque will prevent the originally over-powered system from destroying itself and the increase in speed will help shave valuable seconds off of our autonomous routines. Finally, we removed the rotational potentiometer and replaced it with a string potentiometer. The string potentiometer will provide much more consistent and reliable readings, regardless of what is happening on the rotation shaft, since it measures only the linear distance the arm has traveled. We also added a beam break sensor to detect when the robot has acquired a boulder.
With these changes in place, we were able to finish programming the low bar autonomous mode we started the previous weekend. We then moved on and were able to finish programming autonomous routines to cross the portcullis and cheval de frise and score a boulder.
Meanwhile, the awards and spirit committees were equally as busy as the builders and programmers. They have been working with students from our school’s film and TV production classes to document the season and create a video for the Chairman’s Award. They also emailed alumni and school administrators to set up times to film interviews. The team standards were delivered and mounts were made to fly them above the playing field and in the stands. Keeping with the medieval/Monty Python theme of FIRST Stronghold and our team’s Hawaiian imagery, we also made coconut clappers to help cheer in the stands and make noise when voting for the audience selected defenses.
Construction of the hanging mechanism continued into week 6. The final design lifts the robot at a rate of 24 in/sec with a final speed of 1 ft/sec after a 2:1 pulley reduction. A custom 2 CIM worm gearbox powers the winch through a 3-stage mast. Both the ball arm and hanger have rotation points on the front of the robot. This design will allow us to score a boulder at the last second and then hang without needing to rotate the robot 180 degrees between the two actions. The hanging mechanism will be completed during the final days of build season after the 3D printer finishes the tubing plug blocks and mast pulley cable retainer parts.
To round out the weekend, we finished making new sets of red and blue bumpers for the robot.
Build season officially ends on Tuesday, February 23rd. Our first competition is in Horsham, PA March 4-5th. We’ll have a six hour robot access period to add the final bells and whistles to the robot before then.
P.S. This year’s robot has been officially named Evil Machine 14: The Executioner!