Wednesday, February 4, 2015

Battle bots, Part 1!!

So, Motorama is coming up at the end of February and with a crew of Miters personnel going along, I decided I'd head too. Charles suggested that I make a bot for the competition rather than just spectate so i set off on designing a small battle bot!
Despite his and Danes protestations, I decided to keep it small and go with a beetle bot class robot (3 lb) rather than a 25-30 lb bot which they'd be bringing. This was because its cheaper, much easier to transport and less likely to get pummeled and flittered by an inverted lawnmower.
This isn't the first small bot I've made. As a junior back home, I made a mini sumo bot (500g) that actually won the national mini sumo competition that year. The front armor was stainless steel obtained from my dads school who's shape was based on a cardboard mock-up I'd made. 3 ultrasonic rangefinders mounted atop top the front armor provided the heading for the opponent while 2 50:1 or 100:1 micro gear motors provided the drive. A homemade Arduino PCB containing a H-bridge and sensor headers was made and that's about it for the mini-sumo bot. The scoop at the front was a surplus membership card to the Engineering Soc on campus, bent, sharpened and taped to the front, which was given to us by the chap we beat in the final. Needless to say, he was not a happy camper.

Anyway, back to the beetle bot. Soon after deciding to actually build a beetle bot, Charles was in Shenzhen and managed to snag a boatload of gearmotors for next to nothing. In return for some Beantown Taqueria, he gave me 2 25mm gear motors with an output speed of 1200 rpm. With these motors in mind I got some grippy wheels and spares from Robot marketplace. I could have gone with a smaller radius wheel for more torque but I calculated that the larger wheels would be borderline acceptable and fine if I got under any opponent. A Hydra ESC/motor controller was also purchased which has 3 channels (5A cont, 8A peak) and offers mixing. To round out my purchases, I grabbed a RC transmitter/receiver combo, two batteries (2S1P 1Ah) and a hi-torque servo from Hobbyking.

With my electronis selected, it was down to the mechanical/weapon design/selection. I was warned on the onset "Don't be a derp and make a wedge" so I needed some form of active weaponry. Initially I was swinging between a flipper design and a captive bolt/harpoon design. I'd worked out how to reload the harpoon with a third motor to be controlled by the third channel of the Hydra using a mechanism similar to how an electric airsoft gun works. I soon realized however that every time I'd use the weapon, it'd propel me backwards at decent pace and need massive internal support to prevent it nuking my own bot. There was also the slight concern that the captive bolt mightn't stay so captive, which the Motorama people and spectators in general may not enjoy so much. A flipper it was then!!

I decided to make the entire body out of folded sheet metal, namely 6061 or some steel sheet. The choice of folded sheet metal was an opportunity to try out the sheet metal functionality of Solidworks which I've recently stated messing with. In fairly short order, I'd a rough body designed that could house the motors and electronics. The flipper would sit front and center in a wedge shaped body which could still be effective even if the servo is knocked out. Aluminium ribs would strengthen then folded metal and provide and anchor point for the flipper. The top armor is removable to allow easy access to the electronics within. I toyed with making the shell out of steel but it increased the weight excessively so I removed part of the base and replaced it with and Al piece to save some weight. In the end I swapped back to aluminium for ease of manufacture.
CAD of the Bot
The pieces were cut on the Shopbot in the IDC by Charles. Unfortunately, someone had screwed with the setting leading to some terrifyingly fast cutting of the sheet with pretty much friction stirred welded the aluminium out of the way, jamming the end mill in record time.
Friction stir welded edges...

How it should have looked on top.

A 1/16"drill but was also busted trying toe mill slots for bending. After a double checking all the settings, the final piece was cut out with a significantly less gnarly edge.

Down in MITERS I began to bend the shell into shape using some some Al blocks from an old laser, a propane torch ( anneal the Al to prevent cracking) and a hammer. After heating/annealing all of the bend lines, I bent the shell into shape.
 After each 45 degree of bend, I'd run the torch over it again to try anneal the part. This actually seem to work......
Spoke too soon...

Well it worked for most of the shell. I messed up by placing a mounting hole in the corner of the narrow lip leaving the very little metal and repeated bending and straightening caused the metal to eventually fail. Since I'd already folded 90% of the body at this stage, I decided to keep going and just mount the separate to the bottom plate. This actually worked quite well in the end but i'll probably remake the base shell taking into account some of the things I learned. I had motor mounts printed but due to some slight warping in the shell, they didn't quite fit so I just removed them and mounted the motors directly to the shell with no additional support.

The first flight!! Wires, wires everywhere!!
I crammed the electronics into the shell and took the bot for a quick spin around MITERS and the IDC. The steering is super sensitive but with some more practice I'll be fine with it. Quick video below of the inaugural spin around.

 And finally, some extra pictures I didn't manage to fit into the main body.

Top armor attached to one of the Al ribs.
The base shell piece. The small uncut areas are intentional and are used to prevent movement of the work piece while it's still being cut.
The three cut pieces after the edges had been treated.
Busted 1/16" endmill.