Melonscooter 2 Rage-Finish

Last week, I actually rage-finished not one but two projects in progress. First was Colsonbot, whose test came this past weekend at Bot Blast; and second was Melonscooter 2, which has been tested every single day since then already. I’m getting to this update now because, well, Bot Blast. Melonscooter 2 is now done, save for remounting my signature orange basket.

Finishing the construction of Melonscooter was delayed by replacement timing belts and pulleys. After assembling the Epic Pulley, I measured the distance between wheel axle and motor and came up with the nearest timing belt size to order – this was done with the tensioners, which are of the axle-pulling type, in the most relaxed position so I could order the next size belt up. Turned out to be a 700mm-size (140 tooth at 5mm per tooth) belt with the 20 tooth pulley I wanted to use. Both of these were ordered from my next favorite belt and pulleymonger, BB Manufacturing.

Waiting on those parts was what delayed the build for a little while. But when they got here, it was time to take everything to completion in one night.

Here’s everything test fitted out and satisfactory. The overall ratio of this drive is 5.4:1, which is higher than my typical ratios for this size wheel (more like 4:1). For instance, previous Melonscooter had 8″ wheels, only 1″ shorter, but only a 4:1 ratio.

The reason I’m doing this is to extract a little more power out of the system by allowing the motor to spin fast. In my opinion, that’s a more tenable solution to R/C motor powered drivetrains, since R/C motors love to spin *fast*. By allowing the motor to spin fast, you can gear higher and hence end up getting more torque per amp out of the system (faster acceleration, punchier throttle, the classic stuff people want R/C systems to accomplish). This is especially crucial if you can’t have a Controller of Infinite Amps.

In my case, I’m limited to a 50/120A small Kelly Controller – 120 amps is all I get, so why not increase the peak power of the system by allowing the motor to spin closer to the Kelly’s top speed limit? At 38 nominal volts, the 80/85 “short melon” with its 170 RPM/V shoots a bit above what the Kelly can hypothetically keep up with, so it should be no problem when loaded down.

That’s also why you’d be hard pressed to ever get more than about 1000W from even a heavily modified Jasontroller – the top speed limit of the things is so low (about 4700 rpm on the average R/C motor) that the only way you have to get more out of the system is increase the current, which is on shaky component ground after 30-40A.

As BurnoutChibi shows, if you can get away with using a fast motor with a high gear ratio, it’s the way to go for sheer performance. If only drivetrain components were lossless – the high the ratios, the less coasting ability you’ll have since the friction inherent in the motor and power transmission is amplified by the gear ratio. I’ll stick with my almost-lossless timing belts, thank you very much, for daily use where I need other redeeming characteristics besides brute force (oh dear, I’m losing it…)

I broke out a charge point for the battery using a somewhat-shielded XT connector. My favorite for this is historically Deans, but I definitely find it hard to aim those things some times. Since this location is immediately inside the kickstand, getting a good line of sight would be harder, so a connector with a shroud like the XT is better.

Now, at least I’m not using the XT for actual power. I hate those things with a passion.

The utility wiring on this build is pretty sparse – just a jumper from the charge port to the battery side of the switch, and the switch outputs directly to controller. I didn’t even bother with a precharge circuit (inrush current limiter) this time. That’s how much I didn’t care.

(The tradeoff being earlier retirement of this Hella switch from arc erosion, and possibly risking detonating the Kelly from inrush current on ever power cycle. Don’t be like me, kids.)

And no, the batteries aren’t isolated. What this means is the pack is not made of two packs joined in series externally, so you could charge each half on one power supply if connected in parallel. This is to remind myself of the fact if I ever take this pack apart and try to charge it from two power supplies at once.

Flipping everything over now for the main installtion. The KBS48121 is bolted in, as is the battery box through its four side screws. At this point, it’s cleaning up and tying up wiring to keep out of the way of rotating assemblies.

This 80mm motor features a Hall Sensor board and mounting rig by Yours Truly.

A bit hard to see here, but now I’m wiring up throttle cables and the like. I didn’t go for super clean through-the-handlebar installation like this thing came with – it would have taken too much time to thread the noodly cable through the wire channel inside. Instead, I used some spirally-loop-wire-loom-twizzler-things that I stripped from the original wiring. This at least makes it look not like shit unlike Melonscooter 1.

I could ride it like this, I guess. This is the state right before the first all-systems test. The throttle wire runs above the battery pack but below the main frame, in the roughly .2″ gap between the two.

To tune the Hall sensors, I followed my own page about Hall sensor tuning to the letter. It only took 2 wire swaps to get the motor to spin, then I alternated measuring current draw and bumping the sensor board a little each time (since you were not going to convince me to reach my hand into a spinning belt drive) until the current fell to something like 12 amps no-load. Still a bit high for this system, but by then the sensor board was maxed out adjusting in one direction and I did not want to play the wire game any more.

Bundled up and bolted together!

A few test spins around the IDC hallway told me that I should not be riding this thing in the IDC hallway.  First, because it launches fairly energetically, and second, because I came close to tearing off that giant Hella switch on the starboard side several times.

Outside MITERS after some around-the-block street runs. This thing definitely tastes like suicide again. I’ll say that Melonscooter hasn’t been this jumpy and powerful since its original R/C controller based incarnation. I have yet to have someone pace me or radar gun me to get a speed reading – maybe it’s time to break out a GPS speed application, but it’s definitely well north of the 22-23mph that the late models of Melonscooter 1 could hit.

It also rides like a old Cadillac and handles like a battleship. The giant 9″ balloon tires are even better at road damping than the 8″ tires of Melonscooter 1, and the much more solid frame design means it just sort of rolls over everything. One thing I don’t like as much about this frame is its very wide turning radius. Those big shiny front forks hit the folding joint at maybe 35 degrees of steering tilt. You’d never go this far when riding at speed, but it can barely snake its way up the wheelchair ramp entrance because the turn circle is so huge. I would place this build on the very high end of what I would accept as portable – it’s definitely big for my usual tastes, and if it were any bigger I would have trouble with bringing it inside.

The possibly suboptimal motor timing means that people heavier than me who try to ride this thing full throttle some times experiences the “Kelly Cutoff”, where the KBS controllers can’t handle the fast-RL time constant of R/C motors generating high current transients and shut down to protect themselves. I’ve also experienced this once or twice so far, but not under conditions which I can reliably duplicate.

Some times the “Kelly Cutoff” is resolvable just by turning down the maximum current command to less than its max rating; for this case, maybe 100A. If it becomes a consistent problem, I’ll try that first since the acceleration on 120A is pretty satisfactory. To put it mildly. The next level of fix would be actually spending time to nudge the sensors.

For now, I’m hopefully going to get to range test this thing once the Mysterious A123 Cells of Yore get a few cycles on them – I’ll check the balance state of the pack then. Hopefully if any of the cells are substantially weak, I can replace them before moving on.

daily van bro

A few days ago, while riding Melonscooter 2 down a main street in Cambridge, I passed a white van, one like many thousands of others plying the streets here, representing about every possible contracting firm, flower business, or sandwich house.

But something just didn’t look right as I passed it. I wasn’t sure what, but I got this feeling of “this van ended too soon”

I was right. It did end too soon. Someone had brought an honest-to-goodness modern Toyota Hiace all the way up to Cambridge, which means they just saved me an entire trip to Japan just to take pictures of weird cars.

The previous generation Hiace is the type subject to all of those glowworm-porcupine-starfish-Transformer mods that I originally began being into these things for. I’m not very in resonance with the current generation styling, but they don’t look too bad with the right color scheme, kind of like driving your wireless router down the street, or at the least some sort of bagel oven. Either way – holy hell, someone brought one to what was basically my front door. Life is taunting me – just fucking taunting me.

Sadly, it looks like a rental vehicle, and from Mexico at that (which is my best shot at getting one of these that isn’t older than I am). I assume it was some visiting tour group from Mexico. Unfortunately, by the time I took this picture, their parking meter had run out, but I didn’t love it that much as to give them freebies, however. Enjoy yourselves some American parking tickets?

So what do I do when life dangles a carrot in front of me? Go get some celery and fight back. Whatever the hell that means.

VAN BROS

I immediately whipped Melonscooter around and fetched Mikuvan just to take this picture. Why yes, my name is Mr. Va(i)n.

Speaking of Vans Next to Things (…..tumblr.com?), here’s another spontaneous picture which may appeal to a wider audience than just me.

Returning from a Home Depot trip for the summer go-karts session (more on that later), I wondered who the hell was parked like an ass on the busiest throughfare in Cambridge.

Check out this three orders of magnitude span of vehicle value (Mikuvan: $800. Lamborghini Gallardo: $200,000). This is a pretty damn near Jalopnik level of “highest vehicle value contrast” here. You can’t get much lower than 3 figures in price, nor higher than 6, typically. The only way for me to beat this is to find a Bugatti Veyron.

 

Now, about that orange milk crate.

 

Colsonbot Rage-Finish and Bot Blast Recap

Well, here it is! Colsonbot was finished in the intervening 3 days between the previous post announcing its arrival and when we all shipped off to Bot Blast. The event was a blast indeed, and I’m furthermore glad we even got there in the first place! Here’s the whole story.

This is the friction drive module fully assembled. A little printed rim attaches around the motor, around which I sling some stretched-out O-rings. I recommend everyone who is considering O-ring friction drive make sure that the rings are not bought “on-size”, since if they are, at high operating speeds they will stretch out and be more likely to pop out

Tension in the friction wheel mechanism is maintained with a stack of wave washers. I was hoping for enough space to put a very short coil spring, but it ended up that I really only had 4-5mm to play with. So, wave washers it is.

Since this bot is way more unibody than anything else I’ve built, I needed to construct most of the wiring harness first before installing the motors. Here are some of the Pololu 30:1 HP “Sanyo-flavored” gearmotors being prepped with wires. I call these motors “Sanyo flavored” since originally the first on the market in the robot world was the 75:1 Sanyo micro gearmotor.

Other electronics being prepped include two Vextrollers, one of the left over 18A ESCs from tinycopter version 0, and a gutted Hobbyking receiver. Everything had to come out of their packages for this bot since the space was extremely tight.

The wheels are also 3D printed affairs. They’re little duallies made with O-rings. The center bore is pre-flatted to mate with the Sanyo motors with a set screw hole on the flat to keep them secure.

The installation sequence is wheels on motors, then motors into their mounts.

The center shaft of Colsonbot is a single hollowed-out 5/8″-18 fine threaded bolt. The hex head was machined down to 1/8″ thick, and it rests flush with the top of the battery cavity in a hexagonal cavity which prevents it from turning as I tighten the clamping nut.

Wiring on the bot can be described as “ad hoc”, to keep it politically correct. I used no connectors at all – there’s not enough space for the ones I had. To reverse motor directions, etc. meant desoldering.

Most of the wiring is done with 30 gauge wire-wrap wire, with heavier currents such as the brushless controller and two Vextrollers done up in 24 gauge.

First power test! This was just to verify that everything was still working and hooked up the right way.

Starting to add the other snappy-lids to keep things constrained. The electronics are mostly just piled inside the cavities. I had thought of making little exactly Vextroller shaped cavities, but realized I would never have put up with that level of cleanliness. It’s just a little too Apple.

Lids attached. The external wire run is unceremoniously held down by a glob of used Kapton tape harvested from the Hobbyking battery.

Time to put everything together and find out exactly how not a 3lb bot this is. Turns out the answer was not even 2 pounds. Great…

I knew Colsonbot would have a not-enough-weight problem since it could literally be made of lead and be fine. There’s just not enough volume of bot inside the wheel. Colsonbot’s base by itself, at 10 ounces, would actually make an excellent 1lb shell spinner with a 6oz real shell. This is something I’m considering for Dragon*Con, now a little over a month away.

With a second 5/8″-11 jam nut and a bit of thin PETG plastic, I made this direction indicator since otherwise I had no idea which way the bot was facing underneath!

And here is the soft underbelly!

Check out some of the test footage:

Colsonbot being  a roughly square 4WD setup, handles extremely well and it’s also quite peppy. It really does not need this many motors. As can be seen in the video, it “gyros” extremely easily due to the large spinning mass. I originally wanted to put a “mast” up high over the wheel so it could flip back over, but I decided to keep the wheel illusion. If Colsonbot gets flipped, it will tend to stay upside down and wobble like a coin.

This work was done on Thursday evening. I left Colsonbot alone after that to help out with the two new stragglers we inhaled: Jaguar (by the way, check out his excellent Instructable on his even more excellent Orbit Wheel pod things) and Julian. Of these two, Jaguar decided to start a new robot some time around 1AM Friday.

bot brast

Great, now we’ve increased from four to six people. I was really getting worried about Mikuvan’s mechanical integrity at this point – six guys and another hundred pounds or two of robot gear times 800 miles round-trip plus mountains. If I was going to grenade something, it was going to be on this trip!

On Friday afternoon, I gave the engine cavity a more thorough check over. My primary focus was on the timing belt, since I had observed some leaking oil on the front underside of the engine, leading me to think leaking camshaft/crankshaft oil seals, which would throw oil all over the belt and cause quicker deterioration. Otherwise, I had already done a mechanical once-over after the Adafruit Adventure. To my surprise, the timing belt cavity was dry and there were no signs of abnormal wear on the belt, and the tension was still good. So where the hell was all my oil going, then?

That’s the back side of the engine where it mates to the transmission. I see a new sludgebunny colony is starting to form. This is the area I cleaned a few months ago to see where the oil leak was, and… well, this just tells me it’s coming from everywhere. The side of the block is still well-coated in grunge.  I definitely have no idea where this could be originating from. Valve cover gasket? Rear crankshaft seals?  I’ll write it off to built-in underbody corrosion prevention for now.

Since after the New York trip the oil level was low, I topped off this time with some heavier weight oil – some Mobil synthetic 10W-40 (the manual calls for 10W-30). Time to see if those “for high mileage engines” claims have any merit. The slightly more viscous oil might reduce the rate of runoff if nothing else.

Whatever the case, it was time to ship out. Space Battleship Mikuvan was temporarily commandeered to become the MITERchan Party Van. Having only like 105 original HP and surely less available nowdays to push 6 people and robot gear, this was pretty much my most conservative road trip ever – essentially spent all at less than 70mph and almost all in the right lane. If you know me well, then you know this is basically the opposite of my usual style. The van is teaching me some humility.

Somehow, 6 hours later, Waffle House.

Every time I wander out of New England, I must stop at a Waffle House, so this time we met up with aaronbot a hour outside of the event venue at a Waffle House in Scranton, PA.

On site at the event… and there’s more than one Colson! On the left is Agent Colson, built from a 6×1.25 wheel – it’s flatter, but only 2WD, and also direct drive from a small outrunner. Colsonbot was originally going to be a DD from a large diameter (e.g. multirotor motor) outrunner, and that idea is still being entertained.

Check out the lineup of newbie bots. From left to right is Glorified Doorstop, DEL-RAN Bumble (by Dane), and Speed Bump (by Jaguar). Don’t let those eyes and zip ties fool you – the front of the bot is to the right. I’m glad that somehow in the last week we managed to generate an entire Battlebots team.

Motorama should take walk-up registrations like Bot Blast, because hell if you could get MIT students to commit to anything more than 1 week ahead of time. Trust me on this one.

The event was held in a regional mall, nestled between JCPenney and Payless Shoesource. Quite a contrast of venues from the typical grungy warehouse or barn you find these ‘bot events in. Because of the proximity to curious onlookers, the audience was pretty steady and numerous for most of the day.

I don’t have as many pictures of everything this time, since I was either sleeping under a table or filming matches (And I’ve posted random robot pictures dozens of times). But here’s Agent Colson after one of its first matches, where it may or may not have hit the ceiling. Sad…

The box is a nicely build 12 footer which fit beautifully into the mall’s… band pavilion? Free Speech Corner? Not sure, but it sure worked well.

Colsonbot in line for its first fight…

Colsonbot performed flawlessly during this match (of course it didn’t win anything), but afterwards I noticed the weapon motor was shifting in its slidy-mount. The motor was chosen as a compromise – the one I purchased from Hobbyking that was intended for the bot just didn’t fit. I used instead a Hacker A20-50S, which was admittedly undersized.

It didn’t get hot enough to burn out, but it did get hot enough to warp the ABS housing, causing some tension loss in the friction drive. I literally bent it back while the motor was still hot.

Both colsons being serviced in some way…

In my second match, I sort of went for broke at the end and tried to keep spinning. This resulted in the motor melting the ABS to the point where the friction wheel just sort of welded into its mount, stopping Colsonbot. I still had drive ability, though, so spent the next half minute just getting punted around.

Colsonbot suffered no particularly permanent damage at Bot Blast, and was a great first version run. Now that I know how this version performs, I can better build the Next Generation Colsonbot.

Boy, some “joke” this has turned into.

For Dragon*Con, Colsonbot will return. I’m currently thinking this as the list of changes:

  • 2 motors, since 4 was a bit excessive, and O-ring belt drive to mimic the stable 4WD drivetrain
  • Fitting the larger and more robust motor in the bot as a result
  • Possibly switching to tinyESCs or something to kill the electronics volume even more.

Finally, here are Colsonbot’s two matches from the day!

Returning from the event was entirely uneventful, save for a short section of I-84 in southern New York which seemed to climb straight up a mountain for 5 miles. I realized it was hopeless after fully opening the throttle, falling back into 2nd gear, and was still losing speed. That stretch was done at 40mph with the heater fan on full as I watched the engine temperature climb slowly towards hot.

Clearly, I am never going on a cross-country roadtrip with this contraption as is – I will never even make it over the Rockies, and if I take the southern route through Arizona, would just light on fire regardless. By the way, Newburgh’s Stewart Airport which we passed next to is the site of the original Three Legendary Derpy Van showdown.

So now that Bot Blast is over, it’s time to start prepping for Dragon*Con. More on that later. The final thing to take care of is…

build your own colsonbot

Here’s the entire Solidworks 2013 CAD folder for Colsonbot. Also included are the ready-to-print STL files for all the major components. You’ll need:

  • A Hacker A20-50s or similar motor
  • An appropriately small ESC for the motor – I used an 18A Turnigy Plush
  • Four Pololu 30:1 micro HP motors
  • Two Vextrollers
  • A 3 cell lipoly battery, this one in particular is known to fit (but only if skinned)