Thursday, May 16, 2013

New Project (finished too) - Heavy Weight Combat Robot

I guess there aren't going to be any project updates for this project considering its already done. This is the first amount of free time I have had all semester... and it's finals week. At the beginning of the first semester of this school year a small group of my friends and I decided it would be a good idea for the school to have a combat robot club, with the ultimate goal of competing in the 2013 RoboGames competition. With a little bit of work we were able to obtain adequate funding from the school to do pretty much whatever design we wanted.

Originally we wanted to enter into the 110 pound middleweight competition, however after watching videos we decided this weight class was a little lame (very wrong conclusion, even the 3lb robots are scary and exciting to watch). After some group brainstorming and preliminary weight estimates for our design, we
realized the 110 pounds wasn't going to happen, so we just switch to the 220 pound heavy weight class. Looking back, I kind of regret this decision because it made for twice the work, but I'm also glad we chose this weight class because there's a fairly small group of individuals that have attempted the 220 pound robots. It also really pushed me to put myself to my limit to get this project done while maintaining my grades!

One thing we wanted to do with the design was use components that teams don't currently use because either they're new and untested, or harder to use. These three sets of new components are as follows:

-Brushless motors
-Li-Poly Batteries
-Neodymium Magnets.

The first thing we decided to work on for testing was the brushless motors. These aren't just regular brushless motors, they're RC car brushless motors. The funny thing is that most people wouldn't consider these motors because they're designed to run RC cars that are only a few pounds. In addition, the motors are very small (around 1.5" in diameter, 2"-3" in length, and around 1 pound). Compared to the motors used on most combat robots in the 110 and 220 pound weight classes, these things are toys (most 220 pound combat robots use brushed motors that weigh upwards of 10 pounds and are 4" in diameter) The reason we thought these motors could work is that they had power ratings over 1800 watts, not far off from the big motors.

We decided to make a test platform to determine whether or not these motors were actually able to put out the power they advertised.

The idea for this platform was to measure the torque output over a range of different rotation rates while the motor was provided full power. Normally this is done by connecting the new motor to an existing one that has known parameters. We couldn't do this because we didn't have a motor that could spin 40,000 rpm and handle 2-3 HP. Instead I cooked up a platform of our own that tells the torque and rpm. There is an aluminum disk bolted to the motor. A neodymium magnet is spaced very closely to the aluminum plate. The neodymium magnet is held by an axle that is supported by ball bearings. The axle transmits torque between the magnet and a lever at the end. The lever is then placed on a scale.

The setup works by the same physics that cause a magnet to fall very slowly through a copper pipe. When the motor spins the aluminum wheel, the opposing neodymium magnet generates eddy currents in the aluminum (this is going to be a bad physics explanation, but bear with me). These eddy currents generate a magnetic field that opposes the magnet. The eddy currents turn all of the output power from the motor into heat because the aluminum has electrical resistance. The torque is transferred by the eddy current's magnetic field and the neodymium magnet to the lever arm. This torque causes the arm to press down on a scale. This allows us to measure the torque. The rotation rate is found by counting how many times a black stripe on the wheel passes by a light sensor. (Note that the setup uses some of the electronics from the "anti-gravity" robot that my roommate and I were constructing at the time.)

The system showed the motor was providing similar power outputs to the specifications. One mildly dangerous thing about the test setup was the temperature of the aluminum disk. Since the fixture is 0% efficient, all of the output power goes into heating the aluminum disk. Within a few seconds of testing, the disk was well above boiling (wet paper towels hissed as if they were touching a soldering iron)


video

The video shows a small piece of tape on the end of the motor shaft. The motor was surprising loud. Given that we verified the power output we decided to go with the brushless motors for their higher power to weight ratio. The only down side is that they need to be geared down a ton. ~40,000 RPM is not particularly useful on a combat robot drivetrain.

The weapon for our deign is a spinner. Spinners require massive amounts of horsepower to spin up within a reasonable amount of time. We bought the massive PERM motor. it can run up to 72 volts and have a peak power output of around 34 HP.


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Even with only 12 volts I almost wasn't able to hold the motor down during start-up. We scored a good deal on the motor using ebay. It was sitting in someone's garage for a few years, but it was in perfect condition.

Naturally, the robot was drawn up before machining to ensure the parts all fit together and there wouldn't be any nasty surprises along the way.


I had trouble getting the colors to look good on the full render, so here's a contour render. It makes a great desktop background. (I've been making more of these contour renders because I think they look much better.)

I'll just include a bunch of pictures of the machining process. There were lots of cool parts. Since this was a school club project, we worked to get as many people involved in the production process. I was able to see many students go from having never seen or heard of a mill to being able to operate a CNC and make perfect parts.

video

This is the first time I ever used a water-jet. It made production of the large plates on the robot much smoother. This part was made in January, and was one of the first parts made for the robot.



Here you can see the motor on top of the belly pan. The belly pan greatly increases the shear strength of the chassis. It is also a great mount for the electronics. It is 34" x 22" x 1/8". This also shows how massive the spinner motor is compared to the rest of the robot. The limiting factor on the robot height was the motor.



After being back at school for less than 24 hours (came back from winter break), we already cranked out the transmission plates and some of the axles. In addition we picked up the gear stock for the transmission. One drive motor and motor controller are also in the picture.


We made a will call pickup from McMaster! (You can order on a Saturday and pickup the order an hour later!) Lots of screws. The metal came from Online Metals, but we put it in the McMaster box to keep the dorm room as clean as possible.


Here's our double vice setup for the chassis side rails. There's only one vice per mill in the school shop, so we had to jack the second vice from another mill. There are 4 of these parts on the robot. Lots of tool changes without an automatic tool changer is a horrible pain.



I used a machinable collet to hold the gears. The collet was bored out to the OD of the gear stock. This way the gear teeth were guaranteed to be concentric to the bore. This was much more pleasant than making custom jaws for the 3 jaw chuck.


Here is a finished gear stock. Buying 3 gears was more expensive than buying a whole gear stock, so for the 64 tooth 32dp gears we had lots of spare material.


The cutoff tool was used to remove most of the material, but the horizontal bandsaw was used to cut each blank off of the stock. This way stock could be supported by the live center at all times while inside the lathe. The rest of the facing and boring was done with the custom bore collet.

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Gear stock makes great noises...



I designed the transmission to use as many of the same tooth number gear as possible. This way I could use the gear stock to its fullest. I made a full set of spares just in case some catastrophe happened during competition.


In just about 2 weeks we managed to make it this far. This is amazing considering we all have copious amounts of homework and there is only one CNC mill and lathe. The limiting factor has really been the single CNC mill. The manual machines are in such poor condition that even facing stock turns out poorly. All milled parts on the robot have to pass through one machine.


Thankfully I didn't cause this crash. That was 1.5" diameter stock. The tool holder was a little messed up afterwards, but thankfully there was no harm to the machine.

PRO TIP: Watch where you extend your stock relative to the zero of the machine. The program was written to face off the first .1" of the stock. If the stock is extended past that by lets say.... an inch or two, the machine rapids into the part.



The crash didn't really set anything back (aside from morale). The hubs for the wheels were made in a few hours. We used 3" diameter wheels. This reduced the amount of gearing required for the transmissions. The cool thing is that the chassis is so short that the wheels still stuck up past the top.


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A quick rolling test verified the chassis could roll. We added some cardboard boxes to make it look cooler... not sure if we succeeded.

I flew back home to get the rest of the water-jet parts made. Lots of 3/8" 7075 aluminum plate and 1/2" 6061 aluminum plate.




 This is one of my favorite pictures from all of the machining.


Yeah... the spinner is pretty large.



A couple hours worth of water-jet time made the biggest jump in completeness for the robot.


I got to use my insert endmill again (GMT tools are really awesome in looks, performance, and price). It's pretty boss.


These bearing blocks are pretty large compared to the mill. The bearings are 70mm ID tapered roller bearings. Considering they handle all the damage potential, they need to be pretty strong. I also realized how much the student CNC mill needs flood coolant and an enclosure. removing copious amounts of material takes forever.


The finished wheel assembly looks really awesome. The sprockets were cut on an EDM machine. All of them were cut in one stack, so it wasn't a waste of machine time.


I made chain tensioners, but hoped the chassis wouldn't need them. I lucked out and the chain was perfect. #35 chain is pretty robust and can take some slop without any problems.


This is starting to look like a real battlebot.


A quick drive test greatly boosted morale (morale was good before the test, but it was super high after the test). The robot easily carried 3 people. The acceleration skids the wheels on the floor. The massive rpm of the motor makes the gears scream. It sounds pretty mean. This also proved we didn't screw ourselves by going with the brushless motors. The next priority was to build the rest of the spinner.


The stock was only wider than the 3 parts by .03" and thicker than 2 parts by .01". We had to get creative. This was very material wasteful, but considering the original 27 lb slab of 7075 was from a metal recycling place for only $2 a pound, I was cool with machining away half of the material.


Getting closer to the final assembly. Finishing the robot in time for competition is a little more stressful that we had originally thought. The CNC mill kept needing to be used for class projects, which killed the productivity.


The spinner looks pretty menacing on the robot. Now all it needs are the hammers and tool steel blades.


This shows how the parts interlock. The hammer is locked by geometry to the arm plates. This ensures the bolts don't need to take the shear force from the impacts. The 7075 of the arm corroded some which is why it is so much darker than the 7075 used in the tie rod blocks above the hammer.


I cut the protective lexan shields for the electronics using the CNC router. It was surprisingly easy considering some of the troubles I have had with the router in the past (my longboard).


Carbon Fiber tie rods. These parts connect each of the arms to improve the overall strength. We knew carbon fiber might be a mistake, but we used it anyways because it was light and we had it. It was a mistake, but we didn't find out until competition.


The spinner looks really awesome... mostly because its shiny. This weekend was a major push because testing was scheduled for that Sunday. We got access to a gravel pit across from the school to ensure we could be a safe distance from the robot.


Remember the idea of adding neodymium magnets that I talked about at the top of this post?


Yeah they work... We were able to jump on the chassis and not have it fall from the thin sheet metal door. (The concept of adding additional down force led us to design the "anti-gravity" robot)


We also tested it on top of 1" thick steel plate at the school construction site to make sure it could drive with the additional couple hundred pounds of down force. It didn't really seem to notice the extra force, but we had trouble removing it from the ground.


Greasing the spinner bearings... YUMMY


We finally made the S7 tool steel blades.



And heat treated them too... (had to pull 4 all nighters in a week to get to this point because the robot had to be finished for testing the following day)


We also have a robot name!

Considering this was the first combat robot any of us had ever built, we needed to test it. We needed a safe place to test it where we could control who was going to be near the robot. In addition we wanted no property near it just in case there was flying debris or a catastrophic spinner disintegration. We got permission to use the gravel pit next to the school. It's a giant gravel pit that takes up a few square blocks. No debris could escape.


The tool steel blades look pretty scary. The mac was broken, so it was cool to smash. You can imagine what shape the mac was in after the impact.


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The mac was eaten up pretty quickly.




I like how the CD drive ejected. It isn't visible in the pictures, but there were a number of IC chips that flew off the board. The impact G's were so high the surface mount components flew off the boards! That's pretty cool.

video

Needless to say, the microwave didn't stand a chance.


The microwave got smashed in one solid hit.



The tool steel spikes seem to do their intended job pretty well.

Testing was a great way to prepare for competition, mostly because it was fun.

I'll summarize the competition in another post


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