The summer of 2016 I got swept up into the world of combat robotics after BattleBots S2 in 2016 and I set out to produce my very own combat robot. One of the ideas a few members of what would become Questionable Designs discussed was a "horizontal downforce spinner" that uses the downward force from an upside-down propeller blade seated within an undercutter weapon to increase effective weight during a match. I set out to create the antweight "Foiled!" based on this concept. I've now secured many victories and placed at the following events:
- 1st Place MassDestruction November 2016
- 1st Place MassDestruction January 2017
- 2nd Place Motorama March 2017
- 3rd Place and Rumble Winner COB 2017
I had a few goals with "Foiled!" that drove my design process and implementation:
- As an Application Engineer at Markforged, a 3D printing company that prints in high strength materials like carbon fiber, fiberglass, kevlar, and now stainless steel, I wanted to leverage Markforged technology to create a unique robot that couldn't be replicated with more traditional fabrication methods, including more "standard" 3D printing tech
- I wanted my robot to be iconic and identifiable because I noted that many combat robots trend toward specific shapes and styles
- Living in the Boston area surrounded by tons of brilliant engineers, especially those interested combat robotics, I wanted to stand a fighting change at my first competition
My first design challenge was the weapon. The weapon consists of a double toothed 3/16" thick AR500 steel disk. To get the airfoils in, I 3D printed an insert with the mounting geometry of the motor in the center and a bolt pattern around the edge for the weapon. I designed airfoils into this insert for downward force. The insert bolts directly onto the motor, along with a little nub that the weapon rides on to provide the third point of contact with the ground (apart from the two wheels).
Because of the unique nature of Markforged's fiber reinforcement process, the airfoils needed to be thick enough to fit an appreciable amount of fiber in them, so I could not go with a more traditional propellor profile at this scale. In Markforged's slicing software, Eiger, I aligned the fiber paths to travel along the "airfoil spokes" and thus maximize the strength and stiffness in that direction. In terms of flexural strength, Markforged carbon fiber is comparable to aluminum, so I have never been concerned about the part being too weak.
This piece is extremely lightweight, so all of the mass in my weapon is concentrated in the diameter of steel disk. I get additional stability and inertia when I spin up because the moment of inertia on my weapon is much larger than achievable with a "single material" weapon disk. The steel disc weighs about 3 oz while the insert weighs 1.44 oz. This also helps stabilize when I get tossed in the air - many times Foiled will stay mostly parallel to the ground instead of flipping.
One of the interesting things about this design is that I have almost never had any sort of motor failure despite bolting my weapon disk right to my motor. It's usually a very bad idea to direct drive traditional horizontal spinners because all of the shock from impact goes right into the motor, and it usually destroys itself after a few hits. However, my motors have lasted surprisingly long with no damage. This is because of the carbon fiber reinforced insert. The "spokes" of the insert still have some flex in them due to their tough nylon base material. This means the disc deflects slightly upon impact in torsion, absorbing much of the energy before transmitting it to the motor.
You can see some of the behaviors of the weapon in this fight (including recovery from a premature victory dance), as well as in the many other fights down below:
The frame was also printed on a Markforged Mark Two. It is Kevlar reinforced, which basically means that it is incredibly impact resistant and shock absorbing. Where an aluminum machined frame would crack and break, a Kevlar frame would absorb impact and return to its original shape. Additionally, because it is 3D printed I have the design freedom to make it a fairly customized and complicated piece of work.
The weapon motor mounts to the frame in the front, through a hole in the bottom for easy maintenance and removal. All the electronics fit perfectly inside with a tiny bit of wiggle room for hardware improvements and wires. The drive motors slide in from the side and are held in with very advanced duct tape technology. I was originally going to design a squeeze clamp into the frame, but duct tape works, so I'm sticking with it.
The cover for the electronics pretty much just tapes onto the frame, another thing that could be improved for aesthetics but I haven't gotten around to it.
I had no idea where to start when it came to electronics, but I slowly learned. For the weapon motor I've been running an NTM 28-26s 1200KV short shaft with a 30A Afro race spec mini ESC. I had originally started with the normal Afro 30A ESC but the Race Spec Mini has been just as reliable, if not more so, and smaller and lighter. While most motors are chosen for a good balance between torque and impact speed, I went with a faster motor to increase the downforce on the weapon.
For my drive system, I'm using 16mm FingerTech silver spark motors with a 33.3:1 reduction. They spin at roughly 1000rpm. I've recently found a cheaper alternative of the same size on ebay that runs at about 700-800 rpm. The motors run off of FingerTech tinyESCs, which so far have proved very reliable and have never failed during a fight, and have lasted 5 competitions. These ESCs are a bit pricey, but small, light, and definitely worth it!
I originally used the standard hobbyking RC transmitter and receiver, but have recently switched to Spektrum Dx6 based on recommendations from a few friends. It's been pretty good so far, and I love the interface of the Spektrum controllers. I've had a few weird instances where the lemonX receivers have just stopped working though, which is concerning.
I run the entire system off a Zippy Compact 500 mAh, 35C, 3 cell battery. I had originally used the 850 mAh but realized I didn't need anywhere near that kind of run time.
Things I've Learned and System Improvements
Before my first ever fight, I had maybe 5 seconds of driving experience. As I've learned over Foiled's tenure, driving is a very careful balancing act. Because I have a live axle supporting my weapon off the ground, if I spin my weapon faster, my robot wants to turn more. To drive, I've "mastered" the art of decreasing weapon speed when I want more maneuverability, and increasing it while adjusting my driving tactics when I want to make big hits. When I only have one wheel, I still have control because of the weapon spin and the single wheel, which is why I haven't really cared for wheel guards. You can see my first experience with this in my fight against Ferocious Mk 5:
I've been discussing with Austin implementing adaptive steering, so that the robot can calculate the correct heading and account for the turning forces so that I could maybe actually drive straight. First tests of this system haven't gone well because they jeopardized my ability to safety correctly.
Fun fact - for my first two events (Mass Destruction 5 and 6), I drove Foiled backward the entire time because I wired the motors wrong.
The Dorsal Fin
I'd never gotten flipped over due to the success of the weapon airfoil, but I created a dorsal fin add-on to Foiled just in case. It makes it so that I can easily self-right by maneuvering around a bit and turning my weapon speed up. Turned out, I did need it! I somehow got flipped in my match against Hercules in the Motorama 2017 Finals, but things did not go as expected:
The gyro forces of the weapon were strong enough to lift my wheels off the ground so I was balancing on the tip of the fin, and the spinning blade stabilized the robot upside-down. This was at about 10 seconds left in the match. My weapon takes 20 seconds to spin down. Looks like I need to implement some sort of regenerative braking.
Check Your Motor Screws!
One of the issues I have encountered twice now is that if you are screwing these types of motors to your frame, check, confirm, and triple check that your screws don't go past the aluminum plate on the back of the motor. If the screws are long enough, they can dig into the coils within the motor and mess everything up. In my first event, just to be safe, I switched to a brand new motor before the final rounds. In doing so, I tightened my mounting screws too tight and they dug into the coils Make sure you bring washers or leave sufficient space so that your motors don't get ruined when you're frantically switching motors between matches. Here's what happens:
Additional Exciting Fights
My first ever fight - Victory over Ambisinister MassD Nov. 2016:
Great showcase of some fun airfoil maneuvers - Victory over Puppy, MassD Nov. 2016:
My proudest moment - Victory over DDT in the MassDestruction Jan. 2017 Finals:
Fastest ever knockout - Victory Over Cyclone MassD May 2017:
Just an all around ridiculous match - loss to Hotdog Roller Robogames 2017
COB 2017 Rumble, where I won with one wheel and my weapon spinning like a top for a minute at the end of the match.