BattleBots Post-Fight Report #4 : Shatter! vs Minotaur

For our final regular-season fight we got matched up against the Bull from Brazil himself, Minotaur! It was quite the honor to be put up against a team as well respected as Riobotz, and even if they hadn't been having the best season in 2019, Minotaur is one of the fiercest bots in the competition. If you haven't watched Episode 12 of Battlebots season 4 yet, go watch it! There will be spoilers ahead, you've been warned! Hit the jump for the full fight report.

We first met the Riobotz team back in 2008, over a decade ago! How time flies... it doesn't seem like so long. Back then Bots FC went by the name Atomic Carp, and our biggest robots were MWs. We never fought Riobotz, despite being at 3 events together over the years prior to Battlebots. The thing I remember most about this trip was going out to Cheesecake Factory as a group. It was ridiculous. Riobotz is one of the nicest teams in the game as well as being one of the fiercest competitors in the arena.

Pictured: Bots FC, Riobotz, and WPI combat robotics team members in San Franciso for Robogames 2008. Also pictured: horrendous style choices all around.

Minotaur wasn't running too hot in its first two fights of the 2019 season, never getting the big new drum going, but it certainly got it dialed in for the Desperado. I won't sugar coat it, we were quite nervous to go up against this robot and team. This was the third vertical spinner we were set to face this tournament, and although our armor proved durable against the previous two... this was a full force Minotaur and you just can't fool around with that. Fun fact: our third fight was originally scheduled to be Falcon, not Kingpin. If that had happened we'd have fought 5 vertical spinners! Ha. 

So for this fight we went with our standard anti-vert configuration. We used the thinner/lighter front plow which allowed us to use the ground scraping fangs, and our heavier steel hammer "ol' rusty." We were worried their big teeth may grab our plastic and toss us immediately, and not dig through like Witch Doctor or Wan Hoo... but we had no alternative so we went in and crossed our fingers.

The big strategy here was to not let them get under us. We hop higher than Minotaur's chassis when we fire the hammer, and our chains stick out the bottom... so we really didn't want them to get under us. They were using a 1/2" thick 6061 (I believe) Aluminum top plate, and we were pretty confident we could mess that up if we got enough hits in, but we couldn't let them get under us so it was going to be a game of cat and mouse. Outside of direct hits to the top, we were thinking maybe we could jam in their drum and use their own power against them. They have zero exposed bits other than that. There is no weapon chain/belt that could be hit, it's all protected. There are also no exposed drive components other than solid rubber wheels. It's a tank. So aim at the top/drum area it is.

We started off strafing, hoping to get them off kilter and line up a hit. 

11 seconds in and we had a nice hit! However there was one little issue... we were facing their drum and they immediately got under us. Whoops. Once they got under us, they drove us into the wall.

Watch this gif closely and you can see a chain sitting on top of Minotaur. That was one of our weapon chains. The weapon uses two motors, and Minotaur knocked the chain off of one. At this point, the weapon is now running at 50% power.

You're probably yelling at your screen and saying "Adam, WHY DID YOU YOU HAVE CHAINS STICKING OUT THE BOTTOM! OF COURSE THIS WOULD HAPPEN!" and the answer is, well, it never happened before in 2 dozen fights... We've fought verts, wedges, horizontals, flippers, lifters... we've been hit by kills saws, screws, narlgalators, pulverizers, floor pistons, etc, and no one ever broke the exposed chains.

That was a design feature carried over from the very first Mega Melvin design 3 years ago. It exists because the sprocket diameter is bigger than the gearbox diameter. We could either raise the gearbox off the baseplate or cut a clearance hole in the baseplate. Raising the gearbox causes a whole cascading series of changes, so we just cut the clearance hole and the chain stuck out a tad past the bottom. Next year, we're going to look into raising the gearbox...

So, now we're at 50% power. 50% power is still a lot! That's still 10kw of hammering... but only for a bit. There is another issue when you only drive it with one motor, and that is our slip clutches. If you've read our build reports you know we use a friction based slip clutch, 1 per motor, to limit the torque output and protect the planetary gearboxes. This is especially important at the end of each swing. When the hammer arm reaches the end, it hits the chassis, and then the slip clutch slips. The motor still spins until we release the trigger on the remote, but the clutch slips keeping everything within the proper torque limits. This can create a lot of heat, but it's manageable. When we only have one motor, the clutch slips during every hammer fire, not just at the end of the swing. Due to the weight of the hammer it will actually slip during the entire swing. This creates a LOT more heat than normal. Similar to brake-fade in racing cars, the friction clutches don't work as well when they get too hot. As the clutch heats up, it slips more and more, and that's why you see the hammer going slower and slower over the course of this match.

That said, this is a better failure mode than what happened in the Witch Doctor fight. Our electrical issue in that fight dropped us to about 5% power before blowing up and breaking the whole weapon. We still had a weapon against Minotaur, so we still had a chance!

In retrospect, maybe we should have kept the hammer in the wheel well and tried to do a bit of control-bot stuff with Minotaur.

As we kept going however we recognized the hammer slowing ever more and had to adjust our strategy on the fly. We thought maybe we could push them, but the ground-scraping fangs broke off almost immediately and we just were not able to get under them enough to push them. On the plus side, the plastic armor worked brilliantly and Minotaur was having a very hard time getting a bite on anything other than the bit of titanium wedge sticking out the sides of the front.

One thing we wanted to do even at the beginning was to get the hammer to jam up their drum. This proved impossible later in the fight however because the drum was spinning so fast and our hammer was swinging so slow. It just deflected the hammer away.

Eric was on point with the timing and accuracy of the hammer blows, landing numerous hits to the top of Minotaur, the drum, the wheels. However with the weapon slowing to a crawl, it just wasn't enough against such a tough opponent. If there had been any exposed belts or chains, we would have hit them, but sadly there weren't any.

At this point we had a choice to make. We could just point the front towards them, have them grind away at the plastic while we slowly swung the hammer, and have nothing much happen the last 30 seconds or so of the fight. That seemed boring. The other option was we could go for the win.

Minotaur had been unreliable early on in the competition. Too much resistance on the drum caused the whole thing to overheat, smoke, etc. The plastic wedge wasn't causing much resistance, but our titanium wedge-corners seemed to be doing the trick. So we started purposefully jamming those corners into Minotaur's drum (I also believe Daniel may have been aiming for them, for a different reason). This is risky, because there was a chance they could catch the bottom edge (what I think Daniel was going for) but it was the only way we thought we could win. If we could damage their weapon, get them smoking, maybe we could take it in the judges. Without them losing the weapon, we'd surely lose the decision. So we tried it.

This strategy made a lot of sparks, it was entertaining. It also seemed maybe to be working, until it didn't.

Eventually they caught a corner of the wedge, and BOOM, we're 6 feet in the air. After we landed upside down you could see some smoke coming from the robot. This wasn't anything electrical. All the electronics were fine. This was the slip clutch friction discs burning up. The motor was actually spinning, but the clutch just didn't have enough friction left. Normally, we'd have popped up almost instantly. Oh well.

But let's look at some damage shots!

This right here was our first hammer hit on their top panel, the only full power blow. Scroll up to the first GIF to watch it again if you don't believe me. It was really, REALLY close to puncturing it. If we could have been laying hits like that the whole fight, it would have been a lot different of an outcome. That first hit wasn't even particularly a "good" swing (sometimes the hammer hits harder than others, this is due to magic brushless demigods that live inside the ESC), a good full power swing could very well have pierced it. Oh well, let's hope for a rematch! (or a Hexbugs rivalry set!)

As for us, it's a bit of a different story. On the plus side, the plastic armor was working super well. Here's a shot of how the front looked after the fight:

Not bad! Much better looking than the Witch Doctor fight, probably reusable even if it doesn't look great anymore. We wouldn't be nearly as nervous going into another match with Minotaur knowing how well the plastic worked.

​It was at this point that everything went downhill in the disassembly. The first sign of a problem was that it didn't sit flat on the ground... maybe the ground wasn't level? No... we wouldn't be that lucky. We got it back to the pits and took off the plastic to assess things a bit further.

The right corner of the wedge was missing... not good, but not terrible. It could still function without that. In fact, maybe that was an improvement, no one can catch an edge if there is no edge (a design note for next year)! We signed the piece that broke off and gave it to Minotaur as a memento.

As we went to take off the front wedge to continue the disassembly, we couldn't. And then we noticed that the front was a bit... crooked.

Those red lines should be parallel. You can see on the other side of the bot that all lines up nicely. Minotaur actually bent the entire wedge, and bent our center weapon frame rail too. Ugh. The wedge was irreparable, there was no way to bend it back. So we left it on our table to collect signatures as a memento for ourselves.

Now, back to assessing damage. It took a lot of brute force to get the wedge off and then we noticed the side was cracked.

Ugh. The side is 1/2" thick Grade 5 titanium, and the front wedge was 0.4" thick in this configuration... both were sheared clean through! Next year we'll spend a bit more time optimizing the profile to get rid of these stress concentrations on the wedge and sides, but still... wow. So then we took off the side and noticed the baseplate was cracked!

Darn stress concentrations! That's about 1/4" thick Grade 5 titanium. I mean, come on! The damaged parts add up to, roughly, $5000 of damage. Would have been nice if Minotaur could have picked some cheaper bits to hit! We were able to get most of it repaired with the help of the team at the on site Lincoln Welding booth... but it will never be quite the same. Next time we won't have any exposed titanium bits on the front for anyone to grab. Lesson learned!

Minotaur is an absolute beast of a robot when it's at 100%, and congrats to them on the win!

With the parts repaired, we tossed them in the spare parts pile and bolted the bot back together with some of the non-damaged spares we brought.

Good as new, well, without the fangs since the backup wedge didn't have fang-slots. Would we get to fight again? Who knows?! Hopefully it's a horizontal spinner if we do! We end the season at 2-2, with 2 dominating wins over rookies and 2 tough losses to top tier vets. Now we wait for the selection committee with fingers crossed.

​In the meantime, we ended up with a nice $5000 memento, worth it?

As a closing thought, if we had landed right side up I think we'd have made it to the end. The electronics were fine, and although the frame was warped and cracked in areas, it wasn't enough to completely destroy it, we'd have just been driving a tad more awkwardly than normal.

Thanks for reading, and thanks for being fans!

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