Friday, February 6, 2015

Attrition: Final Push for Motorama 2015

Last post you saw a glimpse of the tragic history of my first and only Hobbyweight (12) combat robot. At the end, I declared I would try my hardest to take the title this year and have implemented extensive upgrades to make it happen. It goes without saying that I did a complete teardown of the robot and its systems but here are the main points to remember moving forward.

Super-awesome new drums from Westar Bot Shop (Team Whyachi)

Last year the drum from 2008 finally gave up. Usually this is the point where builders will reevaluate their design and decide to build an entirely new bot all together. However seeing as how the chassis was nearly undamaged, there wasnt a point in rebuilding that so we will just build the drum again but better.

For this, I had the drums sent to a professional machine shop. Team Whyachi has been providing fabrication services for years and resulted in years of super happy customers. You can find their page hyperlinked numerous times above. I sent them some relatively simple drawings  and Clint Ewert immediately went to work. I started conversing with him maybe Dec 31 2014 and I had the drums back to me 1-2 week of January. Talk about quick turnaround!

The drum is still the same dimensions as the previous one (3" OD, 2.75" ID, 3.625" T-to-T) but the materials are now compatible. 4130 steel all around, hardened afterwards. I asked for 50-55 C Rockwell but some metals treatment literature say 4130 can only be hardened up to 45 C. I'll take the spare to the Rockwell tester to see how it actually came out.

BTW, I bought 2! Two drums for a total of $650 including shipping. Whyachi paid for materials, did machining and services, and a batch of heat treating which makes this a phenomenal deal! Plus, all dimensions are within 0.005" tolerance specification which is ABSURDLY GOOD for welding and HT. Would recommend again!

The bigger issue now is the fact that there was some internal damage to the motor components. While the Whyachi drums were on the way, I cracked open the drum to harvest the relevant components.

Oops. Isnt that supposed to be attached? This was the worst problem among the shattered bearings and dented races because that motor isnt cheap by far. I evaluated at this point if I should purchase a Turnigy equivalent to replace it.

What I ended up doing was JB welding the motor to the shaft. While this make it an incredibly permanent solution, I recognize that the state is fully encapsulated by the can and drum; It should be simple enough to salvage if things go completely bad. I think I am fortunate that none of my motor wires sheared. This gluing process was done with the guts in the frame. Because the center races are all in compression, it locates all the components securely.

I decided not to paint this drum. Partially because I was impatient but also because spraypaint would have fouled my tolerances inside the drum. I may consider painting the spare if the first drum gets damaged.

Revamped drive system

The majority of work was actually put into redoing the drive train. As mentioned previously, I lost a good majority of matches due to drive faults. The first modification to take place was the center double pulley. Last year the FDM pulley had far too much torque at the shaft and displaced the set screws threaded into the pulley.

This year, I borrowed some design concepts from Charles Guan. I started with some 5/8" aluminum hex and turned those into intermediate shafts. The hex would go over the nominal gearmotor shaft providing me a meaty aluminum interface for set screws and the like as well as a hex cross section for torque transfer. Snap rings on either end would prevent the pulley from sliding off the ends.

The pulleys themselves were selective laser sintered (SLS) nylon from Shapeways. They were my usual 20T XL pulleys but designed to go over the hex. I got them in black and red; the red set being the test set. I wasnt too certain about the durability of SLS parts so I got the red set to test part durability. If at any time during a test I saw red  powders on the bot I would determine they pulleys not combat grade. Black would have been much harder to spot.

The entire setup comes together like this:

Pretty neat eh? Initial test drives with the old motors were promising. No red flakes to be seen, nor belt slipping or tooth shearing. We'll run it later in the week with the weapon for some harder durability tests.

Next was the motor. Currently, I use "long can 400" motors or "480 sized" motors. I used to use Banebots 395 motors but they were too slow. See the difference between last year and the year prior. That shows nearly 4 mph of difference in speed! There are a number of 480 sized motors on the market but they are usually used for direct drive prop airplanes which means they are typically low voltage, hot wound, and timed. The trick for me would be to find something mid voltage, mild wind, and neutral timed. This search proved to be near impossible.

I ended up getting four of these 480 motors from RC parkflyers. But testing proved that they were actually 6V native motors and I quickly burned one out. Ugh.

A second option was to modify the mechanics of the gear reduction. The gearboxes are largely custom inside Attrition so this would not be easy. Essentially I needed a way to replace the existing stages with low reduction stages only. Some searching on Alibaba for 32mm planetary gearmotors turned up some interesting concepts. I found my 19:1 gearmotor model but the vendor actually sold a 14:1 and even a 11:1! What did this mean? This meant other stage combinations existed and this modification was in fact possible.

I did my research on gearmotors. Servo City sells 32mm gearmotors as well but their spec sheets are incomplete. Super Droid Robot Land provided the manufacturer's specification sheet which clearly indicated either 11T ot 17T sun gears. BINGO! That means there are only two types of reductions and they are either 3.7:1 or 5.2:1 as previously mentioned.

Then I combed through their standard reductions and factored their values by the two stage types. From this I can accurately tell what stages and how many will be inside each gearbox. I bought a 51:1, which is composed of three 3.7:1 stages.

Looks like the first stage is plastic. Not a problem since the other two were metal. Now I could use the slower but mild wind Banebots 395 motors to give me about 8.3 mph. It isnt as fast as last year but should accelerate faster. Plus, I can actually buy spares! While I was at it, I decided to replace the worn components in the gear motors. New shafts, new bearings, new steel shims!

Actual thought put into front attachments

Now this isnt a primary point of failure, but while I was putting my all into retrofitting the robot I might as well address this as well. Typically, I would run with or without wedgelets which were little slivers of titanium held on with stacks of UHMW shims that acted as spring loaded hinges. It was a neat idea for awhile but the ultimate lack of durability in the UHMW was a failure point in that they were easily sheared off.

After fighting designs like Thomas Kenny's "Shame Spiral" and Pete Smith's "Isotelus Rex", I realized there is a clear winning design to fight the "Solaris" drum bot design and that magical snout was the trick. To counter battle, I would play the arms race against their ramming plows.

I had these modeled in previous models but never took the time to make them. Now that I had "MegatRON" parts being made from Big Blue Saw, I decided to throw on some small parts for Attrition. Now I have a number of wedgelets in various lengths to handle general purpose wedges and specialty designs.

The one type of spinner Attrition has somehow managed to avoid are massive horizontal bars. For these designs, I have some additional defenses in the works, but no need to reveal them yet ;)

Loads and loads of spare parts

This time, I will not lose from lack of preparedness! I am bringing all this and more in case I have failures. These are the expected points of failure so it is possible other things can happen. However, should any of those parts take damage, the bot probably deserves to be retired. Lets hope I predicted correctly!

At this point, we are more/less ready to rock. I'll be running some durability tests with Charles between now and then so be sure to check out those vids when them come out. Cheers and bot on!

Saturday, January 17, 2015

Attrition: The Unspoken Hobbyweight

Before I get into this long long post, I want to mention the existence of my Facebook page. There, I make smaller posts on more incremental work (which is where I've been more recently).

So if you want to see more consistent updates, I suggest you go there and follow :P

Long long ago, I built a hobbyweight with the help of fellow bot people like Brian Schwartz and Casey Kuhns. This was back in 2008 which for me was high school and my resources were limited so I would like to take yet another moment to thank those mentioned above. However, I did not blog about it because my blog did not yet exist. Later versions of the robot were also gone unmentioned (such as my Motorama '13 and Motorama '14 posts). I find it now necessary to log the development of this power-dense combat robot.

This robot was a drum spinner (as was the flavor of the month) and was a wonderful blend of scavenged robot parts. It used a set of 5:1 'mabuchi' gearmotors I scored from ebay, a set of BB 12-45 escs I bought off a leaving bot member, and batteries given for doing a favor for another bot member. I bought a 40 mm copier stator to wind the brushless motor and had machining help as mentioned above. Furthermore, I had machining services donated by local companies to make my drum, an odd combination of 4130 for the drum body and A2 tool steel.

Please for heavens sake do not weld those steels together. EVER. Looked okay but was difficult to heat treat you might imagine...

Either way, this was my first big bot I made and it debut at Motorama 2008 where it went 3 wins and 2 losses to Igoo and Surgical Strike.

It had many notable flaws:

  • Banebots escs overtemp shutoff from the high current draw mabuchi gearmotors
  • poor detail mechanical design and material choices allowed for bent frame and lockup
  • weight distribution made for poor inverted driving (and poor ability to self-right)
  • additional precautions to secure connectors also required

 It was also the first robot I built featuring a custom made brushless-in-weapon system. I'll attribute the original idea to Corona and Skittlebaru but none have gone as far to put the esc inside. Will bales "Fluffy" came out later utilizing my old 5" drum idea having everything except the drive motors inside the drum. Lets just say there was a reason I didnt finish that version of Attrition. The things learned from this robot would carry in designs to come. Additional pictures from long long ago can be found here:

By now I was in college at Georgia Tech. My activities at local events and Dragon Con had interested enough friends to justify the "Georgia Tech Fleet" to drive from Atlanta to Harrisburg for Motorama 2013. I dug Attrition out from storage and decided now was time to address its shortcomings.

I started from scratch. Time to apply new knowledge on mechanical design, materials, and even 3D modeling (since by then I had my SolidWorks professional certification). We would decrease the footprint to save weight and increase the durability of outer armor. For the same goal, we would also downgrade the weapon assembly. The gearmotors would have to go; Instead I picked up four higher reduction planetary units with 'long can 400' motors attached. This new version went by many names such as Tofu or War Machine but decided to keep it as Attrition to indicate its lineage.

Let's do some planetary gearbox math:
A single stage of a planetary gearbox can be calculated by:
(Ring + Center)/Center:1

Pinion :11 T
Sun: 17 T
Ring: 46 T

Stage 1: (46+17)/17:1
Stage 2: (46+11)/11:1

Overall: 19.24:1
This number was good according to my calculations should yield ~10 mph with my long can 400 motors.

To make everything compact, the keyword was 'integration'. I built the gearboxes into the frame rails to save space. The electronics were repackaged into a small cube to reduce wire space needed. I was able to find  the perfect battery to sneak into the remaining space.

Drive test showed the robot was fairly nimble. I dont remember if I had switched to Banebots RS395 motors yet or not. Either way this video showed that I could not use round belts like I usually favor. On high acceleration moves like direction changes, the belts would slip. This made driving with the drum impossible as I later discovered. Later i switched to waterjet machined XL timing pulleys.

The drum was simple; I cut the original drum from Attrition reducing its length from 8" to 5.625". The motor was a bit more challenging. To simplify the design I kept the esc out of the drum. This was a decision made based on 'Cake's matches where the drum would break free and eat its signal or power wires. I also decided to not route wires inside the shaft. This was so I could preload the inner race bearing assemblies in the shaft for higher rigidity and high frequency performance. As such, the wires would exit with a new concept I had involving a large ring bearing on the motor side.

The original concept was to build the internals from several stacks of waterjet material. However the drum was a bit warped so any precision machined parts would not readily fit inside. I ended up wrecking a motor in an initial test and remachined the entire assembly from a single round of aluminum. Some fully assembled pictures.

Now, time for the money shots! The LED vag was a nice touch too...

This version debut at Motorama 2013 where it faced a number of tough opponents including Zandor and the undefeated multi-time champion Cataclysm who won 2nd and 1st respectively. Attrition went 3 wins and 2 losses for 3rd place, going undefeated for 3 matches until he met Cataclysm on Sunday.

Somehow, Attrition came out in good shape. Aside from blemishes on the top and bottom plates, the majority components including the drum were fully operational. The source of my demise was actually lack of spares. After my match with Zandor in the quarter-finals, I was down one drive motor and went into the Semi-finals against cataclysm with only one drive side. This put me at a huge disadvantage and lost decisively. Shortly after, I lost the losers bracket finals to Zandor because of my lack of drive side. Originally the culprit was a broken magnet. The replacement given by another builder ended up failing also because of pinion slip.

Losses at Motorama 2013 were disappointing to say the least. This year's emphasis would be into the drivetrain. I wanted faster and more reliable. Now at MIT, I lost a valuable resource at the Georgia Tech Invention Studio. However my research afforded me new capabilities I would try and leverage when possible. In addition to remaking many outer armor plates, I also swapped the Banebots RS395 motors for the original long can 400 motors and remade the center timing pulleys from 3D printed material. I believed that high starting torque was the culprit for pinion slipping.

Another decal with lewd LED placement was needed.

As fate would have it, I was wrong. At Motorama 2014, Attrition went 3 wins and 1 loss in a round robin format for 2nd place. Agility was useful in my first match against the pneumatic flipper Jumbonator but I lost a drive side again after a win on Upchuck. This was a fatal blow against the strong drive of Isotelus Rex as I could not maneuver for a good hit. The FDM material actually split and rendered the center pulley useless. This was from the high stress concentrations near the 6mm drive shaft to the ~25mm center pulley. This could easily be fixed with a larger diameter intermediate made of metal.

However, another defect surfaced that I had not predicted. The A2 teeth, after years of battle, finally broke during the match against Isotelus. I suspect this was due to fatigue as nothing on Isotelus could have done that. However I note that points were given to him for 'damage' to my robot as the judges pointed out when Attrition had finally spun down after the match. Closer inspection of the welds show little to no penetration into the wall; it was no wonder they had broken.

Present Day:
No place in 2008. 3rd place in 2013. 2nd place in 2014. This year we pull out all the stops. At Motorama 2015 we will go for the prize!

I here conclude this post as a documentation of my first 12-pound combat robot. In another post, I will summarize the efforts to prepare Attrition for Motorama this year which includes professionally made drums and extensive drive modifications.

Video of all Attrition's matches can be found as a playlist on my Youtube. I have placed the direct link below.