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!

Monday, July 25, 2016

Hexrotor FPV Racer

It took a few months, but I got to the point of being machine limited when FPV racing. When I say machine limited, I mean I basically don't make mistakes and basically run at 100% throttle until the battery dies. The quadrotor also got to the point where it just couldn't keep up with the other racers. When I first started most quads ran with a 3S battery (3 lipo cells, or 11.1V nominal). In the past few months that trend changed to most people flying 4S batteries (14.7V nominal). The extra voltage makes the motors spin faster and provide more thrust. Most of the 4S quads have thrust to weight ratios of about 6:1, with some approaching 10:1. My 3S quadrotor... probably 3:1.

I first looked at retrofitting my quad to run 4S batteries, but I needed new motor controllers. The bigger battery would also shift the center of mass, making it unbalanced. Given the cost and crappy performance I figured I might as well dump some more money in and build my dream machine.

The first thing I wanted to do was correct my complaints with the typical frame designs. Mainly I wanted the battery protected from crashes. During the races a number of people would smash their batteries in a crash. LiPos are pretty volatile and I wanted to mitigate the risk of a battery fire. In addition to the battery armor I wanted the arms to be replaceable. It is much cheaper and faster to replace a single arm instead of the entire base frame plate. My quad has a cracked arm, and I didn't want to spend the money on a new frame plate (I fly more gently). Finally, I wanted the frame to be unique.

This led me to designing my hexrotor. There's one or two hexrotor frames available for purchase (small compared to the hundreds of different quadrotor frames). I also made it radially symmetrical, which is not something I could purchase. The hexrotor factor took care of the unique and cool requirement.

The rest of the hexrotor design focuses around protecting the battery. Most frames have the battery held (exposed) on the top or bottom of the quad with a few velcro straps. I worked to design the hex frame so it wraps around the battery. I split the hex into two portions, a base plate that handles power distribution (battery connector, motor controllers, and all of the wires associated), and a top deck that holds all of the electronics (flight controller, video camera, video transmitter, RC signal receiver, on screen display, and signal LEDs). The battery sits between the electronics deck and the frame. This ensures the battery is safe from damage... or at the very least I'll have already broken all of my electronics before the battery gets damaged. The battery placement also provides a very even weight distribution. The overall center of mass ends up perfectly in the propeller plane, meaning the hex is very well balanced.

I wanted the hex to be similar in size to a standard quadrotor, so I had to pull the props in closer to each other. This led to a problem... the FPV camera would see the props. Thankfully the top deck design eliminated this problem. The deck sits high enough that a highly tilted camera (necessary for these faster 4S racing quads) wont see the props.

Here is a printout of my hex in a 1:1 drawing next to my quadrotor. I like to make these 1:1 drawings to get a better feel for the design and to see problems I might miss on the computer. I guess I'm kind of old school for this kind of thing.

Without shop access I sent my parts out to be machined. I found a site that did custom carbon fiber parts specifically for FPV racers. The price was better than what I could find for just the raw carbon fiber plate. I ordered enough parts to make two frames as well as spare arms for each frame. The two frames and spares came out to the same cost as the one frame for my quadrotor...

The frame is a sandwich of arms between two main base plates. Most existing frames with replaceable arms use 4 screws, some of which are close to the edge of the arm. These arms always break at the screw holes. I used two screws for each arm, directly in the center. This minimizes the loss in strength of the arm from the screw hole placement. I also made sure that the arm will always fail before the base plate. What's the point in having replaceable arms if the base plate cracks instead?

The max center to center distance on the hex is 270mm, which is only slightly larger than my quadrotor frame which is 250mm.

I 3d printed all of the electronics mounts. This time I used tough resin. The tough resin is a newer formulation from Formlabs (the brand of my 3d printer). It can take significantly more impact before shattering. It also tends to be softer and more flexible.

I temporarily put the electronics deck on top of the frame to see how it would look. The frame looks very compact until the deck is raised to make room for the battery.

I couldn't find a power distribution board (they were all built specifically for quadrotors), so I had to just make a wire bundle. It isn't the most elegant solution, but I made it pretty clean.

Dealing with all of the wires and motor controllers was a huge pain. Mounting it to the frame took a lot of care to avoid getting things tangled, or putting something in the wrong place.

My desk was a disaster during the build. I should probably get a dedicated table to work on instead of my computer desk. The good thing is it makes me motivated to finish the project faster, otherwise I basically can't use my computer.

Here is the completed hex next to my quadrotor. It isn't significantly bigger, but it definitely has the cool and unique factor going on.

The battery used is a 1800 mAh 4S (14.8V nominal) 75C LiPo. 75C means the battery can be discharged  in 1/75 of an hour. Maximum continuous current is 135 amps! (1800 mA*75).

I added some addressable LEDs to the back. Each one can be set to any color. I have it set up to act as a turn signal as well as change color depending on the throttle and state of the hex (if the flight controller is armed or not).

The hex stacks up pretty nicely against other quads. The extra thrust is more or less canceled by the extra weight. The hex has no advantage over the quads except for some redundancy (in theory I can lose a prop and still fly). The hex is more dense than the other quads, which makes it slightly less vulnerable to wind. It sounds amazing and has a pretty intimidating effect on the other racers.

I ended up buying 8 batteries. This easily keeps me flying continuously as long as i have my 4 port battery charger in the field with me.

The LEDs really saved me in this crashed landing. The grass grew pretty high in the field I race in, and the hex buried itself in. Without the LEDs I might have permanently lost it.

I also later purchased a gopro and mounted it (5-28-2017). The new mount also came with a bunch of repairs and maintenance. I mostly just had to clean the electronics and replace an antenna that had broken in a nasty crash. It has two antennas (simultaneously receiving), so it didn't cause any issues.

Overall the hex has been a fantastic build for me. It met all three of my design goals. The arms are replaceable, but I have yet to break or even damage one. I think the sandwich design has enough flex to reduce the stress during an impact. It tends to cartwheel in crashes, which also reduces the forces on the frame. The battery armor design has worked FANTASTICALLY! I have flown into so much stuff, and the battery has been fine every time. As for my uniqueness goal... It really hits the nail on the head. The look and sound really get heads turning. The robustness and mass have also made it a bit terrifying in races. I have had a few mid-air head to head collisions, leaving the other quad needing repairs.

I will probably build an upgraded frame in the future as new electronics come out (maybe I'll make my own from scratch), or as the racing trends shift. For now, I am happy with the hex and It will be quite a while before I am once again machine limited and require a faster - higher performing machine.