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DERPDrive: The Assembly; Plus, Some Other Neat Parts: IG32 Gearmotors and Scooter Pulleys

chorl3 Comments

It’s been a little while! My entire previous week was spent preparing and organizing the big summer MIT-SUTD go-kart race which went down on Sunday. Yes, that’s a thing. The summer class is not totally over yet, so that report will come shortly.

In the mean time, I want to pay attention to something which has been a little neglected over the past month as I devoted more resources and time to making sure the class ran on schedule and people were able to finish up. That something is DERPDrive, which, given that I found one of my structural frame tubes being used as a go-kart wheel chock the other day, was pretty damn forgotten. Luckily, it’s now assembled and already through one attempt at a test fit for which I could not conjure enough macho to accomplish. It’s general knowledge that I look just as home in a Miku costume as anything else, and sheer manpower is not one of my notable traits. The test fit will be repeated once I can get a team-lift going.

Here’s what all took place.

After watching the paint dry, DERPDrive was unceremoniously shuffled to one corner of the shop as the students’ build season really started kicking in. Over the course of a week or two, all kinds of mayhem landed in my parts box – from safety goggles to tools to other people’s drivetrain parts. One thing that I keep trying to instill in high-intensity overachieving engineering students is how to pick up your own droppings as you work, and how throwing all your tools from the day into the nearest bin doesn’t constitute cleaning up in the least. This occurs with varying degrees of success.

Since the primary mechanical parts of this thing had already been machined and ready weeks ago, all I needed to do was assembly. This went quickly:

In this configuration it has already been mistaken for a motorcycle jack or some kind of pressing tool.

The next steps in assembly included mounting the astronomically huge bearing blocks. Into these 1.25″ bore blocks will be mounted a solid (unless I feel like saving about 2 pounds out of over 100) 1.25″ steel keyed shaft. The holes next to it are for a 3/4″ steel auxiliary shaft, and holddown method for all of them is 1/2″ fine-thread bolts and grade-mismatched-because-that’s-all-i-could-find hex nuts. These things are surprisingly cheap on the surplus market.

They’re also built for serious slop. I assume the target market is people who assemble entire production machines in a cave, with a box of scraps, because every part of them is adjustable.

The mounts are slots, so they can slide back and forth. And the bearings are captured in spherical housings, so you can have shafts that are just eyeball-aligned. So that’s how Tony Stark did it.

The next degree of freedom to adjust is centering the 11″ go-kart wheel and hub in the center of the swingarms. I mounted the wheel on the hub and eyeball-aligned it, then clamped the hub in place. When the wheel is mounted, I can’t reach the keyway clamping screw , so this adjustment needs to be done beforehand. Normally, these hubs are used on the very end of an axle, not in the middle.

The Big Axle assembled. Notice the overlay on top of the frame jacks – this is 1/16″ 80A neoprene rubber glued to the steel. A thin compliant layer ought to increase the amount of friction generated by the jacking force. That’s the intention anyway – I don’t think I will have “detaching issues” when the wheel is down with a few hundred pounds on it.

Now this thing is getting scary. The 52 pound D&D sepex motor is now mounted, and I’m really, really having trouble wrestling this thing around on the bench. It is just now dawning on me that this might be a little more hardcöre than necessary.

But just think of how awesome of a go-kart it will make had already made! This motor has had an interesting MIT tenure for sure.

The midshaft is now mounted. The little sprocket for the final stage is behind the larger one in the front. Between the two stages, the total gear ratio is 8.66:1, which should yield a top speed of about 15 miles per hour (for a certain high field current of the motor – field weakening, a feature available in the Alltrax controller I’ll be using, will produce an artificially high top speed if acticated).

While the motor is capable of much more power to push the vehicle faster, recall that I’m gearing for a high enough thrust force to get the van out of my parking garage, not to do really awkward silent cruises in front of local night clubs while implying that I have something which hangs down really low.

This whole rig now weighs north of 110 pounds, and getting it down from the bench was probably one of the most precarious situations I’ve found myself in.

It’s time for a test fit…

The crime scene. The plan was to back DERPDrive, facing the right direction, under the two frame rails it will squeeze between, and then hold it in position while I tighten the jack screws.

Alright, I definitely didn’t think this through very hard. Minus the jacks, that’s roughly what the assembly’s going to look like once installed. I accidentally found out that my spring preloading setup works really well when I trapped the swingarm against a jack and was still trying to lower the whole thing.

After a multitude of jacks and bottle pistons and failed attempts at bench pressing the thing up to where it needs to go, I decided to give up before dropping the entire van on myself. This is gonna be a 3-hoodrat effort at the least, and I might need to fiddle my way back onto the lift. I got it almost there – but the whole “hold this thing with one arm while trying to reach the jack screws” thing was just not happening at all. This will certainly be troublesome in the future when I have to deal with 1000lbs of batteries and 200 pounds of Siemens motors. Now we know why I build little things most of the time…

What’s nice, though, is finally getting off the bench. Can you tell which half of this bench I parked one of my project heaps on for a month?

More DERPDrive will come after I secure a test fit!

slightly past unboxing: IG32 gearmotor, timing pulleys

I’m not sure I could call this section beyond unboxing, since there wasn’t much to unbox, so let’s roll with slightly past unboxing then?

It’s August, and there is exactly 1 thing I do in August and that is robots. I need to get my fleet gear for Dragon*Con, which is basically in three weeks including travel time. Yep, I’m in that situation again.

Here’s the situation. Überclocker desperately needs a new top clamp arm actuator since the previous one was so damaged at Motorama (see the bottom of the event recap). The leadscrew got pretty chewed up since it was the first thing to hit an opponent that really sunk deep into the fork, so it would bind at the top of travel. And it had more than enough bottom travel, letting the top clamp actually poke out under the fork, which was just unnecessary. Plus, the oddball chain drive I designed is only getting worse tension-wise, and it really has to go.

I wanted to rebuild the clamp actuator using a stock gearbox (so I can have multiple on standby) and not using those damned chains. It should be much lighter than the current half-a-drill setup I got going on, and not nearly as powerful, because there’s no reason to need a full 550 motor on grabby duty.

I already have a fast-travel (1/2″ per turn!) precision leadscrew and nut, so I’d just need to find a motor with higher ratio to get a manageable clamping speed. Going from 0.1″ to 0.5″ per turn would mean a motor that spins 1/5 as fast, or is 5x more geared down. This puts me in the neighborhood of a 30:1 gearbox with a 25,000 rpm motor (18v drill motor overvolted to 7S lipos, or 25.9 volts).

I took recommendations for reasonably nice gearbox, and one of the candidates (recommended by Jamisong) was the IG series from Super Droid Robots. I’ve seen these before, and had been eyeing them for a while, but never had a reason to buy one since I’d been well-pampered by chopping power tool motors. They seemed like a nice compact solution that I could merge with a 400-class motor for less weight.

So I ordered three for kicks, adhering to my 2n+1 Rule of Procurement for Stuff I Can Afford.  One for using, one for backup, and one for fucking around with!

Here they are! Cute little setups, really. The RS-380 class motor it comes with is quite possibly the mildest wound motor I’ve ever ran. The gearbox itself is constructed from a few die cast aluminum parts, and the ring gear is steel with an aluminum over-sleeve-wrap-thing.

…but that’s all I have to say that’s productive and nice.

What is with you people and plastic gears?

The first stage is bullshit plastic! Ostensibly it’s for ‘noise reduction’, but all I see is cost cutting. I will gladly pay like $5 more for some metal in there.

Well there goes any potential of overdriving the motor significantly, or replacing it.

The difference in gear strength is incredible. So you go from 2.5mm thick plastic gears to 5mm thick steel. That’s way, way more of a torque increase than the plastic can handle. If those 2.5mm gears were steel, too? Certainly, then the progression of torque makes sense. But this is just cost cutting, one that happens in enough gearboxes today to piss me off.

So for $20, I’m not going to argue at all with what you get – I’m sure these things work for exactly what they were designed for. But damn, I’m not going to put up with this for actual robot use. Glad I got three – because my intention for the bot is to combined two of the gearboxes into one that has all-steel gears. The untouched one can be for emergency backup or something where I know I’ll have to go extra sissy on the throttle.

Next up on the list is something interesting I found on TNCscooters while I was putting together a weekly student group order list.

It’s a big timing pulley! Specifically, it’s a 5mm HTD pulley, 80 teeth, that has a center thread which mates with most threaded scooter hubs. These seem to come in three sizes.

Even though the material looks like very crudely cast aluminum, this is still good to see existing because typically, you can’t find HTD drive pulleys in very large sizes from industrial suppliers. At least not in a form which could mate to a purchased wheel in this sector of industry.

Here’s an example of the kind of rim you can thread these pulleys onto. The thing on the left is a typical 8×2 tire rim (example 1, example 2). For non-regenerative vehicles (which do not perform motor braking), you’d probably want a strip of teflon tape or other anti-seize tape on the threads, lest over time these two things become one piece of metal. For regen vehicles, there’s not much choice but to use some light threadlocking adhesive to prevent the pulley from unthreading upon motor braking.

If you aren’t inclined to use the cheesy cast aluminum thread, then those four little hole pilots in the center near the raised hub is the exact same spacing as the four lug nuts on said rims! So, you could do it standoff style by drilling through the pilot holes and bolting it directly to the rim.

This is a typical arrangement of the two parts. On the left side thread, you’d mount a band brake rim or brake disc adapter or similar.  This ought to help those who want a synchronous belt drive like melonscooter but have found that commercial HTD pulleys are a pain to interface to without machine tools.

So concludes another episode of Slightly Past Unboxing! Coming up next are a full report of this summer’s go-kart shenanigans, and a catch-up of what I’ve already been doing with the robots in preparation for Dragon*Con.

 

Melonscooter 2 Rage-Finish

chorl5 Comments

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.