Hub Motors on Everything: ChibiKart

The year of the tiny, chibi, and cute projects continues!

So here’s the backstory. I’m sitting on top of a veritable mountain   pile small cardoard box full of 100mm-class hub motor parts that I commissioned on a whim last fall to test out the workflow on, of being even lazier than I am already and hiring my machining work out to shady Chinese backwater CNC machine shops (because those exist). I discovered that Chinese job shops seem to be every bit as legitimate as American ones, and will definitely hold the tolerances you are not used to holding for your own parts made with your own drawings. And even though the pricing is inexpensive for such machined parts, I am still just hiding several hundred dollars of shiny precision machined electric motor parts in a box.

Not very productive – I’d rather that money be either put into builds or making me more of itself. That doesn’t mean I’m going to start selling motors left and right just yet – I have none of the other support parts at the moment, including important aspects of a wheelmotor such as the wheel – right now, I’m still just carving the center out of 100mm skate wheels. Add to that the amount of actual completion detail that needs to go into a motor such as proper windings and termination, rotor magnets (those things are getting expensive) and possibly even sensor boards and Hall Effect sensors (which would need to be designed, sent for fabrication, and assembled), all of which comprises several hours of manual work, and the case is still not very strong for my immediate entering into the dubious market of small personal transportation implements. Yet. Product development takes real time commitment, something which I have yet to convince myself is worth pursuing.

But to counter that, I am more inclined now to take custom commissions than I was before, like for your Air Trek skates or electric suitcase or nanosegway or whatever. The main reason is that I lied just a little above – I do have a potential source of stators for both the 100mm-class “skate motor”, and the 125mm wheel Razor scooter sized motor. The stator, as I’ve explained before, is pretty much the hardest thing to get custom made for you if you are buying in volumes with less than like six figure units, and it’s harder than I thought to just “buy a few” stock/premade ones. However, this part investigation is still forthcoming – the other reason is that I’ve found several “islands of stability”, or at least islands of reliable part numbers, in my everlasting quest to catalog the stator sizes of all extant copier motors. Yes, that document exists!  Use it for your own hub motor building endeavours.

I’ve literally spent hours sitting on eBay hunting for new part numbers to buy and catalog. Tearing motors apart to make other motors is not a mass production method, but for my one-offs, it’s an acceptable compromise. This means that I also have an asston of random copier motors hanging around, several of which have stators which fit the original 100mm skatemotor design, and I would be totally unsurprised to learn that all my random eBay binging has resulted in more sunk money.

Poetic waxing aside, the combination of too many motor parts and too many random stators means that I need to build something to use them. I’ve pretty much made a pledge to not build any vehicle that is not propelled by hub motors from here on, since doing otherwise would mean I put off development of the motors to something reasonably resembling a product even more, while my surplus parts continue to build up.


Here it is!

…well that’s not very exciting.

This thing is already going to be comically small – that frame rectangle is only 30″ long by 18″ wide – with even more comically small wheels. Coincidentally, 30 x 18 is the exact dimension of the front half of tinykart. Part of this build is also fueled by go-kart envy. Ever since the venerable LOLrioKart was officially decommissioned, I’ve not had a four wheeled rideable object. Two tracks is kind of okay, but not really quite the same. But for me to just build a copycat kart is not very enlightening. Thus, Chibikart.

Four wheel drive, maybe four wheel steering (those corner pods are symmetric in case I pursue it), and really really small.

…and possibly with only slightly more horsepower than the Razerblades, since I’ll literally be using the same kind of motor.

Alright, maybe now there is a better sense of scale. That seat is not a couch or bus seat or something, but a riding lawn mower seat from Surplus Center. Yep, this contraption will be on the same length scale as Amy’s profoundly awesome SAM, but definitely a bit longer.

The seating position right now is “legs out” with a front bumper or other structure out beyond the 30 x 18 and the foot pedals, and a steering linkage vertical immediately next to the front 80/20 bar. During this session, I discovered how pleasant CADing with 80/20 extrusion was. After I figured out, of course, that there were only two narrow perpendicular faces on each side to constrain to.

The pedals are generic Chinese electric kart pedals, sold around the Internet using some form of the name “Simple Hall Foot Pedal” or similar. I’ve yet to receive the ones I ordered, but luckily TNCScooters had reasonable dimensions on their drawings (interpolation, estimation, and “screen calipering” aided in completion of the CAD model too). One of them will have a linkage extension to actuate cable brakes – the actual mechanical braking method is yet to be designed, and might just end up being scooter fenders with rigged cable linkages. Hey these are scooter wheels.

I’ve made a few changes here. First, the pedals have been moved inboard. I’ve decided on a significantly more “cab-over” driving position, again similar to SAM (but not QUITE that much…). The steering linkage will be planar and located below the main frame rails. Overall ground clearance is slightly under 1.5 inches – the wheels are not mounted on center in the 80/20, more like 3/8″ under, but the linkage will take out some of that.

Through careful ergonomic studies involving sitting on a block of foam  on the floor of MITERS, I added the foot brace bar that crosses in front of the pedals. In practice I’d put most of my weight on that and be able to tap the pedals instead of hovering over them while trying to hold your leg steady.

Most of the front end mechanicals are done now. I added a reverse cowcatcher/bumper/whatever it is – either way, makes the thing look a little less hilarious. There’s some mechanical design (read: lengths of 80/20 to copy and paste) still to come such as the seat mount proper and battery/controller/electronics mounts, but I will hold off until the parts arrive.

What’s going to be powering this thing? The orange battery is an A123 special that will remain generically orange and prism-shaped for the time being, since I’m fairly certain it’s Not Supposed to Exist Yet. Bottom line, the vehicle will have a 32 volt (10s LiFe) electrical system, a fairly chunky battery.

Powering the motors will be four of my most favorite alignments of Chinese manufacturing probability, 350W class Jasontrollers! I’m taking a major risk by going all sensorless with this thing, but the Jasontrollers have proven themselves in being able to start high torque hub motor vehicles. I’m hoping that with four of them there’s never going to be a “twitching equilibrium” moment. The downside is no regenerative braking (and no, the E-STOP wire doesn’t count – tried that already on Melonscooter, almost died), but someone has the ability to fix that if only he’d write a startup routine, right?

Chibikart will be a test to see if the skate hub motors can push any reasonable power. Combined with the Jasontrollers, they will hopefully form a reasonable system which in my mind is more product-able than just discrete parts alone – because what the heck are you going to do with just a motor can or a raw unwound stator?

If it turns out they don’t – well, who knows, maybe I’ll just stuff them back in the RazErBlades somehow, or build an updated version of them – after all, they needed more power.

It’s legi…. wait… nevermind.

There seems to be a disturbing trend in all my recent projects of everything progressing smoothly and going right, then the whole project grinding to a halt at the last possible second before completion and being wholly unrecoverable without spending an additional hefty sum of money. Case in point: TB4.5-SP1 for Motorama had essentially half the robot become crippled the day before I had to leave for the tournament.

New case in point: Snuffles Reloaded half-existed for about 30 seconds before the motor controller failed. The failure is undiagnosed at the moment, but probably revolves around blowing a voltage regulator which then shot the controller logic with 35 volts.

Here’s some build pics from over the past few weeks, and the (95%) complete vehicle, along with the half-existance video.

As the mechanicals were nearing completion, I started playing with the electronics. The R/C airplane controller requires a 1 to 2 millisecond long pulse spaced every 20 milliseconds, which is a hobby industry standard for control signals. However, the electric bike throttle I was going to use puts out an analog voltage from 0.8 to 4.2 volts. This is, again, a different industry standard control signal.

I decided to use a microcontroller to translate between the two instead of the awesome ghettomongous discrete-part boardamathinger that I built for the first scooter. A bit of messing around with blinkenlichten on a MITERS STK500 and I was in business.

After playing with the analog-digital converter, I made the output LEDs act as a throttle display meter of sorts, for kicks. Full throttle, all LEDs lit, and they go out in sequence as I let off the throttle. I might actually implement this using a LED bargraph chip in the future.

Next up was assembling the eight giant lithium-polymer cells without killing myself or burning down buildings.

To solder epic batteries, you need an epic soldering iron. Unfortunately, I couldn’t find the epic soldering iron I used at the Media Lab to assemble A123 cells, which are even more epic than my 4AH lipos. So if you do not have an epic soldering iron, you need an epic soldering tip.

A scavenged rod of copper and some lathe time later, and I had an epic soldering tip which fit a Radioshack 40 watt soldering iron, which I do have. The whole thing is 2.5″ long and is .400″ in diameter at the larger half.

The brass one was for testing and practice, since a rod of copper is actually pretty pricey these days to just fuck around with.

The bottom battery pack completed after some careful iron maneuvering. The epic tip made the whole operation touch-and-go, exactly what you want for soldering batteries. You never want to park the iron on a battery cell for more than a second or two.

This was a bit of a risky operation since I was laying the cells face-to-face to conserve wiring volume. Normally, batteries like this are stacked and the cell tabs folded over one another. One wrong move with the massive solid copper tip and I was probably looking at replacing a cell.

After each joint was made (and its balancer lead installed), it was covered in electrical tape. When all the joints were completed, I slammed the whole thing in a tube of giant heatshrink and parked the heat gun over it.

Giant heatshrink should be a primary structural fastener. When it starts tightening down, everything inside sort of scrambles for the lowest volume configuration, and the end result is a very neat package of parts. The pack was embedded into its mount with some also primary-structural double-sided tape.

This pack constitutes cells number 5 through 8. The balancer lead is a standard 3-pin R/C servo plug, which serves the interconnects between cells 5 and 6, 6 and 7, and 7 and 8.

And the test assembly. The connectors fit into the LASER-cut acrylic endcaps as they should, and some CA glue retains them.

Building the internal electronics bay was more interesting, since I had to fit batteries, large power wiring, a controller, a switch, the charging port, and all associated connectors inside. I cut out a rectangular piece of aluminum as the substrate (way to go, conductive mounting surface?). Originally, it was going to be flanged and shaped with a sheet metal machine to accommodate the parts, but I decided to not get fancy and just mount everything with double-sided tape or epoxy.

The same procedure of soldering and wrapping was done to the 4 internal cells (#1 through #4), with the exception that their balancer leads went straight to the Convenient DB-9 Connector of Cell Balancingâ„¢. An 8 cell pack requires 9 pins to be fully tapped. Guess what has 9 pins?

Three of the DB9 pins went to the rear of this pack, where they met with an R/C servo pigtail which connected the back half of the pack to the balancing port.  Two large power wires also connected to the bottom battery pack, one of which is the 0v  (ground) line, and the other an interconnect between cell 4 and 5.

After everything was assembled, it was time for a test run. Verdict: It moves.

Some shoving and… it fits! To install the thing, I had to take off the folding hinge, cram the assembly through to the back side about halfway, insert the folding hinge nut plate, then slide it in the rest of the way. The nutplate sat snugly above the controller, but not enough as to pinch wiring. This precision engineering part of the build came out (went in?) great.

Before I fitted the internal electronics, I threw together a source of 5 volts for the motor controller. It is an opto-isolated controller, and so needs separate logic and power rails. This was a simple 7805 regulator jammed in the empty space between the controller and switch.

Here’s an assembled-on-the-table test run video (.MOV, 4.8 megs) using the creepy custom throttle interface device. The mechanical noise is from the completely unbolted and unclamped motor and chassis resonating on the table.

Unfortunately, this regulator would ultimately cost me all the work for the past few nights and a good bit of money. Protip: A linear regulator cannot drop such a huge percentage of its input voltage and output any appreciable current. I was most likely hitting the top end rating of the 7805, around 35 volts, and expecting it to output a solid 5 volts with at least 100 or more milliamps of current. Shortly after the video was taken, some things went pop.

My best guess is that the controller logic board was hit with the full 30+ volts of the battery. A switching regulator, or even two stages of linear regulators (inefficient, but hey), even mounted externally, would have prevented this disaster.

Anyways, here’s a pic of the almost-running vehicle.

The ’empty weight’ is probably around 13 or 14 pounds. Yes, I had to remove the rear brake in order to pass the motor cables – perhaps it would have been better to route them externally. But who needs brakes anyway?!

Regardless, there’s 29.6 volts of 4AH lithium polymer cells, a (former?) 100 amp motor controller, and a very chunky brushless motor shoved into the space of a Razor scooter. I think it’s pretty damn awesome just for that.

I will need to get a new motor controller and devise a new solution to get a stable 5 volts out from the battery pack before the vehicle will run. Seeing as how this will easily cost over $100, it might have to wait a bit. Possibly a long time – we’ll have to see.

Here’s a closer shot of the undercarriage, which houses all the interesting bits.

In the mean time, finals! Bot on, folks, while I attend to these…uhh, pressing matters.