We added a
scaling mechanism for Championships. At the event, we fought some silly consistency issues that we really should have hammered out at home. The hook had a 2/10 success rate with only
one of those attempts finishing in a successful scale.
After 118's reveal video came out, we began prototyping harpoon style scaling as it made the most sense for the machining resources, robot weight budget, and time available to us. I think we had a
working idea of a surgical tubing powered harpoon shooter around the weekend before our first regional.
Development on a scaler slowed between Los Angeles and Las Vegas as we wanted to focus more on potential improvements to our (never fired in an official match) shooter.
After Las Vegas, we decided that scaling would give us the best chance of getting selected for alliance selections over our shooter, so we began more serious development on the scaling mechanism. We designed
winch gearbox plates that were a direct drop-in replacement for our shooter motor mount plates. This allowed us to free up all of the real estate from shooting for a scaling mechanism. This was the "easy part". After calculating the proper gear ratio to lift ~140lb of robot+battery+bumpers(with some safety factor), this part of the system just worked. We used
1.9mm spectra cable as the winch line spooling onto 1/2" hex shaft. This is really good stuff for general robot use. Even when frayed and knotted, it kept on trucking. We ended up replacing the entire line when we arrived in St. Louis and again after our last qual match.
The next major component of the system was the
firing mechanism. This was one of the more difficult components. We went through 8-9 iterations of this mechanism tuning different variables like barrel length, barrel dia(compared to the hook shaft dia - more on this in a bit), surgical tubing type/length/diameter/tension, and force required to lock the hook into place. We found that with this design, shot consistency increases as you increase the length of the barrel as the surgical tubing seems to have more time to send the hook flying in our desired direction. This component was built with a combination of 1x1 tubing, versaframe gussets, a few bits of 8020 brackets and two 3D printed parts(which could easily be made from delrin or some other plastics or even wood by hand). It's actually very simple. We'd probably do something similar again if we ever need a one-shot stored energy mechanism.
Here's a 1/4 speed video of rev 6 firing.
The final component is the hook. This one saw ~20 iterations with variations in hook dimensions, materials, locking/closing mechanisms, and shaft length. Huge thanks has got to go out to team 842. We saw
their scaling mechanism at Las Vegas and they've very graciously explained how their hook worked. Their mechanism was a HUGE inspiration for our version.
This is the best picture I could find of the hook mechanism, you could also see one of the early revs of the hook in the 1/4 speed video of the firing mechanism. This is another very simple part. The "shaft" is a length of
fiberglass driveway marker with a length of thunderhex shaft pressed onto the end. Early revisions of this component used Andymark churro shaft for the entire length. We moved away from this as it added a lot of unnecessary weight to the hook. This component was the most difficult component of the system. Many of the early issues was getting the hook to close on impact with the bar. Early on, we played with a bar that holds the hook open that gets pushed out of the way, allowing it to snap shut on the bar. We had difficulties with consistency and weight with this concept. After we saw 842's overcentered surgical tubing method, we knew this was the simplest and lightest concept we could implement. The version of the hooks that took the field on Galileo ended up being very similar in geometry and construction to 842's.
For this system, we developed a set of procedures to follow to ensure safe loading and transport of the robot. Without the winch line constraining the hook's flight, the stored energy in the system was enough to send the hook into our shop ceiling, piercing the ceiling tiles 1/2" deep. For robot transport, we NEVER stored the hook while energized. While in queue, we verify that the robot's air pressure is >90% (the locking/release mechanism relied on air pressure) and we don't load the hook until our robot's position was settled on the field. As far as mechanism controls goes, we have implemented a system where the operator had to hold an "enable" button before the firing button could activate the system. This prevented accidental firing on the field.
We learned from 118 and Bane in 2015 and this scaling mechanism in 2016 that stored energy systems should NOT EVER be taken lightly and teams must take utmost care in designing, handling, and operating such systems.
The scaling system was the most intensively prototyped system we've ever created. Ultimately, consistency issues bit us in the translation between the practice frame and the competition bot, but we learned so much about prototyping and which variables to watch for and tune. It was an incredible amount of fun to see this project grow from rev 1 of the firing mechanism all the way to fully scaling on Galileo Quals 60.