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My Sumobot Geoff

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Posts: 22
Posted: 2012-03-30 23:57 
 Post subject: My Sumobot Geoff
Was bored so I've decided to make my first project thread. This is about my sumobot, Geoff(named after the Top Gear electric vehicle), that I built for Science Olympiad which has a 40x40cm size limit and 2kg weight limit. Since this was my first real robot that I've ever actually made I went through a large progression of increasing complexity as I faced new dilemmas and learned more. I also don't have access to CNC or molding machines so everything I've made with hand tools and a drill press. To show the entire build I'll start from the beginning and describe each version, though I didn't document every step with pictures.

It all started in the end of April 2010 when our team got second overall at State and qualified for nationals. That year sumobots was a first time trial event and set to become an official event the next year, so I had yet to do anything towards building it and had a little over two weeks to order parts and build before the national tournament in May. So without time to waste I ordered some bits off the internet; a 1/4" polycarbonate sheet for the chassis, wheels, some sprockets, chain, and two very powerful geared-motors. The motors are one of the few things that have remained throughout the build, they are "BaneBots 64:1, 36mm Planetary Gearmotor, RS-540 Motor" and pack 2527 oz-in of torque each. I also took a trip to the local hobby store for guidance in the radio control and picked up two RC car ESC's and a used remote+receiver. This version was only powered by 6AA's as I had yet to fully understand the concept of amperage, and as a result it was very under powered.

A front view showing the polycarbonate wedge that I spent about an hour drilling some 300 holes into to lose weight and make it under the 2kg limit, which I ended up being rather close to at 1980g. You can also see the solid aluminum axle in the back.

Here you can see the blue ESC and a nice view of the grippy rubber wheels.

Here you can see the motor, gearbox and chain drive which provided power to the rear wheels.

Unfortunately when I arrived at competition in May, I found out that I couldn't use my controller because it was 72mhz and was for airplanes only. So Geoff had become a sitting duck and in hope that I might win a tiebreaker(which are decided by weight) I stripped out the electronics and covered him in sticky duck tape to hinder opponents. My first opponent was much lighter and could't even attempt to move Geoff, and because of the grippy wheels my second opponent, an RC car, had to make several full speed rams just to push Geoff out of the ring.

Over the summer of 2010 I set out to greatly improve Geoff and fix his many flaws. I traded in my radio for a fancier 2.4Ghz system and began redesigning Geoff from the ground up. I decided on a smaller chassis plate out of 1/8" polycarbonate that would be double-decker to greatly reduce flex. I also moved the motors so that they were on opposite corners which gave better weight distribution and allowed for a much smaller overall footprint. I also added individual aluminum axle-blocks to support the individual aluminum axles for the chained-drive wheels. To fix the power issue I switched to using dual RC car 7.2v Nimh batteries, one for each motor.

This is the only record that I have of this version, its a video showing him pushing a total of about 30 pounds with a lot of wheel-spin

The first competition of the year was coming up in November and I had to add a bumper of sorts before then. Having been to the nationals competition and watched many online videos I realized one thing that seemed to happen in many battles was the two bots would come to the center, meet and then kind of stay put as both struggle to push the other. To end the stalemate one of the bots would attempt to turn away but couldn't without reversing first, all because they were rectangles with flat sides. This also would hurt a bot that was attacked on its side, as it couldn't turn to face the opponent without escaping first. To overcome this problem I decided to make my bumper circular so no matter where I was hit all I would have to do is spin to face the opponent and beat them with torque. I built my bumper out of some thin polypropelene which is very light and also very flexible and elastic. However, I still ended up being over the weight limit by more than 100g. As a quick and easy fix I replaced all my steel screws/bolts/nuts with aluminum ones, but I was still over weight. Looking at the heaviest parts, I decided to replace the wheels, which weighed about 90g each, and as you can see in the video I needed more traction. After searching the internet for awhile I decided on some Lite Flite tires which I would mount on custom made hubs. I made the hubs out of some thicker polypropylene sheet, out of which I cut circles, one for each side, and epoxied in pieces of polypropylene tubing, and the sprockets for the chained-drive wheels. I squeezed the tires between the two halves and held them together with four long and skinny screws. The new wheels were much much grippier, because of the greater surface contact, and much much lighter at only 20g per wheel.

Here are two sides of one of the chained-drive wheels

Here is the new bumper mounted on the chassis, minus the batteries which you can see in the bottom right. Though this picture still shows the steel hardware

After only getting 3rd at the competition(horrible judging ousted me from the finals) I set out to prepare for the next competition in December. The only flaw I had found in my design was that it was horribly susceptible to wedges(of which most bots were) because my bumper was a whole centimeter off the ground. I fixed this problem by first lowering it about 5mm and adding little aluminum feet all around the circle. The feet rested on the ground and had a wedged/sharpened edge to keep any other bots' wedges from getting under.

Here's a video of a match where the circular bumper really showed its ability and saved my butt several times during the match, I also have no clue why they decided to run the event on carpet

The next competition would be State in April 2011. Given the large amount of time I set out again to fully redesign and rebuild Geoff. First I made the basic rectangular chassis a little bigger to accommodate the batteries which really didn't fit in before. I also remade the chassis plates out of slightly thinner polycarbonate and added some weight saving holes. Also for desperate weight saving I redid the wiring to rid of any excess, and I replaced the solid aluminum axles with hollow ones. At this point I had just a basic platform, much like version 2.0 but improved. Once again I thought for awhile(about 2 months) contemplating new bumper designs. I wanted to add a wedge yet at the same time I wanted to keep the maneuverability of the circle. Finally I came up with a solution; I put a wedge on both the front and the rear, while on the sides I put curved bumpers. All were made out of polypropylene, but unlike the previous circle which was attached by polypropylene supports(which were entirely too flexible) I opted to use carbon fiber and aluminum brackets. For the wedges, I added a sharpened aluminum edge. The wedges were also mounted on small hinges and I added extension springs to really ensure that the edges stayed on the ground. Throughout the assembly process I was very frugal with everything, like how much tape I used, because I really was cutting it close on weight. In fact at the State competition Geoff weighed in at 1998g, just 2g under the limit.

Basic front view, note the post-State battle wounds on the wedge.




Shot of the carbon fiber and aluminum supports for the bumper

More carbon fiber, and the springed wedges

The aluminum axle block, also remade and drilled with weight saving holes

At state I ended up only getting second. My excuse is that when I fought the final opponent their robot went right up the wedge an got caught on top of mine, however somehow a long screw sticking off the bottom of their bot jammed into one of the tires, which then became stuck. The motor struggled to turn the wheel and shredded the tire against the screw.
Since sumobots was taken out as an event this year(2012) I haven't worked on Geoff at all, though I hope to take him out again and compete sometime in the future. But for now I need to keep working on Robot Arm.

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