Franklin Institute Competition Post Mortem

I have been looking to compete in a combat robotics competition for a while, and my schedule and robot state finally lined up for the Franklin Institute NERC event (FI) this year. I spent the week before the event on the CNC and lathe making spare parts in anticipation of some likely failure modes.

My father and I took a train from Boston to Philadelphia Friday night before FI, and arrived at 4:30 am Saturday morning in a fairly sleep deprived state.  After waiting around in the train station for a few hours, we walked to the venue (about a mile) and checked in.

Event Horizon went through safety inspection without issue and weighed in at 3 lb. 0 oz. Over the course of the next few hours, the pit area filled up completely. All in all, around 80 bots were present in the various weight classes.

Match 1: Silent Spring vs. Event Horizon

When the bracket was posted, I found that EH was against Jamison Go's Silent Spring for its first match. Silent Spring is an exceptionally well built and high energy undercutter and for all intents and purposes, it is indestructible. I knew this would be a hard match, and that it would likely result in a significant amount of damage.

The match started with a few very violent hits that sent the bots to opposite sides of the arena. During the first of these hits, Silent Spring's disk sliced through the bottom of EH's left front armor brace, leaving it touching the ground, and eliminating my ability to drive straight. Despite this, I was able to keep the drum facing Silent Spring for two more large hits. Looking back on the match video of this match is interesting as there is a clear difference between the hits where the majority of energy came from EH's drum and the hits where the energy came from Silent Spring's disk. 

Damaged front armor brace

In the end however, Silent Spring hits the corner of EH's front right armor support plate, ripping it off. This spins EH slightly, and Silent Spring's disk tooth connects with the inside of the now unsupported side plate, causing it to buckle at the point where the first screw connects the bellypan to the side plate, which lines up with a large cutout for the center armor plate support, causing a huge concentration of stress. It appears that the screw that holds the drum to the side plate came loose due to a combination of vibration and a small number of threads being engaged, so when the side plate buckles, the drum is unseated. The still-spinning drum then hits the floor or Silent Spring's blade, causing it to jump back, dig into the frame cross brace, and be shot from the robot due to its large stored energy. I was quite confused about exactly how this happened until watching the video repeatedly in slow motion.

While this looks like fairly complete destruction, I found that many vital parts of the robot, such as the motors and electronics, were still fully functional. Since I had CNCed an entire spare set of frame members and brought the old drum, the repair process was actually fairly straight forward.

Silent Spring damage

Aside from the obvious complete disassembly, the drum itself sustained a significant amount of damage, with one screw ripped out, one shattered, and one bent at a severe angle. Moral of the story: Aluminum is too soft for this application.

Matches 2&3: <3 Winning by Default

I was able to complete the reassembly process before my next match because of the large number of bots at the event and the resultant long break periods. It turned out that this did not matter much as EH's opponents for its second and third fights did not show up, so it won by default.

Match 4: Hard Drive vs. Event Horizon

My next actual match was against Hard Drive, a bot with some sort of low power vertical spinner and a wedge.

Hard drive has exposed wheels and I was able to knock one off fairly quickly. Before I could knock the second one off, however, the set screw that holds on the back (driven) wheel on one side came loose, and left me without one drive side. EH won the match, but not as spectacularly as I would have liked.

After the match, I put Loctite on the set screws and then put the drive train back together.

Match 5: Gemini vs. Event Horizon

After beating Hard Drive, EH was up against Gemini, a double sided wedge bot. The match started off good, although I accidentally spun up the weapon too fast and flipped the bot over. I was able to get one good hit before the screw that holds the weapon on came loose and the wires spun around, pulling one of the connectors out and turning off the weapon. I tried to push Gemini around after that, but the robots are pretty much matched in pushing power and EH lost to a judges decision.

Thoughts

EH's issues in the last two matches were relatively minor and can be easily fixed by proper fastening methods (pins) in the next version.

The structural issues uncovered by Silent Spring are more troublesome as they necessitate a significant redesign of the frame. Since the bot is at weight, this is difficult.

The drum easily has enough energy to be effective and might very well have been the highest energy drum at the event.

The frame did a good job of taking impacts dealt by the drum.

The robot did not drive particularly well. It was not fast enough and the turning was too sensitive.

The drum tooth system needs a major redesign to make it multi-use.

Planned Changes

I am going to make the frame much stronger by eliminating stress concentrations and move the drive base to brushless motors to free up more internal space. This should make it possible to reduce the length of the bot and thereby reduce weight. If I can, I will use an un-geared brushless motor mounted on the outside of the frame rails to maximize internal space for electronics. I will also extend the belly pan forward on the sides to provide additional bracing.

Regardless of what I eventually do with the motors, I am going to move all wheels to dead axles.

I also need to add full front armor to better protect against spinners with sharp teeth.

On the drum, I plan to add nut strips on the inside to anchor the screws and switch to alloy steel flat-heads. The nut strip will also double as a keyway to prevent all drum pieces from spinning. The motor hub piece will be pinned to the frame so that it can not rotate.