Why is he named Clearance? Because we figured we should have some clearance somewhere, but the only available space was in the name.
I am extremely proud to present an offseason project the CyberCavs have been working on. Myself and several other students and mentors poured a huge amount of time and effort into this robot. We only had 15 days to build, wire and program the robot, but we pulled it off. It was incredibly rewarding to see Clearance limp, walk, then eventually run (and occaisionally tip) at Robots@CNE this weekend.
The Inspiration
This season I was impressed by several simple but very effective robots. At UWaterloo, 9098’s robot was just a drivebase and an intake, but it was incredibly fast and effective and they captained the 3rd alliance (as a rooky team). At IRI I watched team 1923 zip around the field scoring cubes incredibly quickly. After seeing these robots I thought: can we do that but smoler and swervier?
The answer was yes.
The Robot
I wanted to make the drive base as small as possible. Since we were going to use 4678’s backup MK4is (which are 7.5" square) that meant the robot would be at least 15" square. However, I realized that if I made the robot a bit longer I could fit the battery in between the swerve modules to help keep the CoG low. Naturally this meant we were in for rather painful bottom battery changes but the low-ish CoG was worth it.
The frame perimeter I chose was 15"x16", which isn’t as small as Project T.A.D.P.O.L.E, but to my knowledge, it’s the smallest FRC swerve robot ever fielded.
The spark maxes were attached to a plate that sits just above the battery and between the swerve modules. Wiring was a nightmare and we had to use a ridiculous number of zip ties to hold all the wires away from the swerve gears and out of the way of the battery. It would have been easier to fit everything if we were willing to shorten the motor and controllers’ leads, but we didn’t want to do that for an off-season project. Considering the packaging demands of this robot, falcons would have been a useful upgrade… If only Vex could make reliable products and keep them in stock.
The intake was designed with 3" stealth wheels, 4" compliance wheels, lots of tapped thunder hex, and 1/4" plate. The plate is probably overkill, but we didn’t want it to break so we erred on the side of chonk. If we remake them for a future off season we will likely swiss cheese the plate more and weight reduce the gears to save weight and keep or CoG farther back. Big thanks to 1706 and @Emerson1706 for the dimensions and spacing of the intake.
Since the robot was so light, I knew we would have enough torque to spin the wheels of the L2 MK4is we had (and even L3s would have more reduction than we needed). Instead of letting this extra power go to waste, we modified the swerve drives to have an exceptionally aggressive gear ratio. Instead of the 6.75:1 L2s, 6.12:1 L1s, or even the 5.14:1 L4s, our custom modules were geared at 2.74:1. We achieved this by replacing the pinion->idler->double gear arrangement of the drive side of the MK4i with a pulley->pulley/gear combo. We used GT2 3mm pulleys, 3D printed gears, and lathed out gear stock to make these parts. We calculated this should be good for a theoretical 11 m/s but haven’t yet had time to get an empirical number. We are very pleased with how well these parts held up.
It weighs about 75lbs (including battery and bumper, about 60lbs without) in CAD. Unfortunately, I haven’t had time or a scale to weight it yet. Given the slightly lacking acceleration performance we saw at the competition I expect fasteners took it a good bit over 75lbs.
Here are some more photos of the robot being assembled:
We even managed to put together some very sharp bumpers.
The Competition
We finished wiring the robot at 2am on the morning of the competition and didn’t power it on until we after loaded in. When we did, there were (of course) CAN errors. Fortunately for us, the first day of the competition was entirely practice matches, because it took us to 4pm to find a CAN wire that was connected green-to-yellow and fix it. We played 3 three practice matches and quickly discovered that if we could pop a wheelie and tip over by accelerating (unforeseen consequences of a 10.75" wheel base). We were having some problems turning but, given the circumstances things were going pretty well.
On the second day of the competition we programmed our arm (only shearing one gear in the process) and played two matches before lunch. The turning issues persisted, and at lunch we figured out one of our swerve modules worked while translating but was 90 degrees off while rotation. We painstakingly checked over our code, but it took us multiple hours to find a stray minus sign that was causing all of our nightmares. Our next match was amazing. We drove, we turned, we scored 4 game pieces, and we even shot a cube mid in auto.
On the third day we worked on moving in auto. We left the community once, overshot an auto balance once, and docked once (it might even have been an engage if our alliance partners hadn’t jammed the charge station down - no hard feelings). We were the second pick of 7th alliance and lost our first match to the eventual event winners (3683, 4678, and 8089). In our second (and final) elims match something went wrong with our electrical system and something sparked (caught on camera) knocking us out for the match. Despite the unfortunate ending to our event, we were very pleased with our performance and hope to be able to go to another off-season event.
Main Takeaways
- Mini bots are hella cute.
- Very small wheel bases make you quite tippy.
- If you don’t CAD the fasteners things might not fit once you add them.
