Sunday, August 21, 2016

Mini Combat Robot Finished

I finally found the time and resources to finish the last few parts on the robot. The first and biggest thing was finishing the spinner. I discovered though testing the polyurethane belts I had originally chosen couldn't handle the rotation rate and would stop transmitting any torque (they also melted from slipping). I opted to redesign the spinner using inside-out timing belt. I chose this for a few reasons... I used the timing belt belt inside-out to make sure it could still slip in an impact situation. This prevents the motor from being damaged by the near instant changes in rotation rate. I chose to use timing belt because it's much easier to find small timing belt sizes than it is to find small flat belts. The timing belt also has low stretch cord (fiberglass or kevlar usually) embedded in the belt.

The new timing belt design required the whole spinner be redesigned and made. Thankfully I had a friend with access to a 5 axis CNC. The new spinner is made from a billet of 7075 aluminum (probably not the best choice because it's more brittle than other alloys) with S7 tool steel teeth. I made sure the design could safely handle 30krpm without ejecting the teeth.

I disassembled the whole robot again, and broke it down into just the spinner assembly for testing. I had to do a bit of rework on the main plates to fit the shaft properly. The new spinner mounted properly the first try. I also put some armor surrounding the motor shaft to make sure debris can't damage the motor.

The belt tensioner can be seen on the underside. The original design didn't need one because I was using stretchy polyurethane belt. In this new design, the timing belt is pulled tight by a few ball bearings pressed onto a dowel pin. The dowel pin mount slides back and is locked in place by two screws. The mounting block has a precisely machined slot to keep the bearings perfectly perpendicular to the mount.

Here is a quick test of the spinner mounted to some random material I had laying around. A friend and I threw objects at the spinner to see how much it could damage it could do. The soda can is one of the most satisfying tests.

I also tried throwing 2x4s at the spinner. I wasn't taking video at the time, but the hit that damaged this chunk of wood threw the 2x4 well above my head. I quit testing at that point before I did something dumb(er) and injure myself.

The robot was pretty much done at this point. I just needed to wire everything up and get the top/bottom plates made from metal (I had some temporary laser cut acrylic plates). Unfortunately I couldn't figure out how to get all of the motor controllers and wires to fit. Everything was just too packed in.

I opted to get new motor controllers for the drive. Hobby brushless motor controllers have come a really long way since I bought the ones for the drive. The old ones were rated for 25 amps each. They also couldn't instantly switch from forward to reverse (dumb "feature" for RC cars). The new controllers (the tiny circuit boards in the picture above) are meant for FPV racing multirotors. They're rated for 30 amps and can instantly switch from forward to reverse (an important feature for multirotors that can fly upside down). I've used these controllers on my hexrotor, so I knew they were good and the ratings were not made up.

A quick test of the drive ended up shattering the old 3d printed spline couplers. I reprinted the parts with tough resin (a less brittle type of printer resin from Formlabs). These parts held up fine and are still in use.

I easily fit all of the electronics in the robot. I also finally mounted the power wires in the robot such that the lid could close and seal the battery in.

A quick test fit showed even the battery could fit. I added some pieces of dense/rigid foam in to keep the battery and spinner ESC in place. The foam has some squish, so it prevents the parts from getting too much force during an impact. I also finally got the top and bottom plates made. I gave up trying to get machine access to make the plates myself. I ended up sending them out to a local shop. They even used the material I had already purchased. The parts ended up really cheap. I will definitely send out any future waterjet parts to this shop. It ends up better quality and even cheaper than me paying for my own machine time at a place like the techshop.

The plates fit nicely and none of the electronics or wires are badly squished. It was nice seeing it finally done after starting the project more than 3 years ago.

It makes a really nice display piece of my bookshelf... I  don't have any competitions to go to, so finishing this was mostly just to have closure on the project.

The robot is pretty quick. The new drive ESC make it significantly easier to drive. It rumbles the floor pretty badly, so I probably won't drive this indoors again (don't want to anger the neighbors below).

A few months after actually finishing the robot I finally got a chance to test it on something. A friend had me pickup some Legos from craigslist for him. He didn't pick them up after the entire summer. Normally I'd be down to play with Legos, but these were kinda gross. They were covered in dirt and who knows what else. They took up room in my living room... something had to be done...

My roommate and I made a video to provide some "motivation" for my friend to pickup his Legos. We made a small house using some of the bricks (only the really common or already damaged bricks). The robot instantaneously disassembled the house (ignore my poor driving).

The spinner took a nice chunk out of one of the bricks. The video also worked well. All of the Legos were picked up within a day of posting on Facebook.

It was nice to finally put this project behind me (actually behind me on a bookshelf). I'll probably never have it actually compete at this point. I probably can't even use it to smash stuff since I don't have an arena or some other safe place to protect me from flying pieces. That being said, it makes a nice desk ornament and could be used if I ever have the immediate need for a small combat robot!

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