Archive for the 'Kitmotter!' Category

 

Kitmotter 0002 Design Files

Aug 04, 2012 in Kitmotter!, Reference Posts

Now that I’ve put a few miles on Kitmotter 0002 in the form of Johnscooter, I’m more confident in releasing the design so people can build their own versions or modify it.

Ride testing has let me measure Johnscooter’s propulsion efficiency at about 29 Wh/mi. This is a little worse than RazEr’s 25 Wh/mi, but besides having the World’s Trashiest Bearings, the fact that Johnscooter is operating almost entirely in 25A constant-current mode (due to the current-limiting nature of the motor controllers) means I’m basically full-throttling it everywhere. RazEr and RazEr Rev had a more “open loop” voltage drive which let me draw brief bursts of current to get over obstacles at a higher speed where Johnscooter would slowly crawl through the same.

The motor itself has been holding up fairly well for being made of particleboard (more properly, hardboard, which I suppose is rather plastic-y as wood goes). The new stator hub has been holding up with no issues so far, and it was the most worrisome part of the current design.

It has a few dings on it from actual use on sidewalks and bike lanes, and I never did drill those through-holes for the wheel mounting flange. So the wheel is loose on the motor a little, but when rider weigh is pressing on everything there’s nothing that feels strange.

The trashy 5/8″ bearings have loosened up substantially over time – both in seal drag and in…. radial runout tolerances. I can now grab the motor and wiggle it very slightly, and the greatest contributor to the noise spectrum when riding is those bearings. If anyone else builds this, remember that even though I have the Trashy McMaster Bearings listed in the Bill of Materials, real precision flanged 5/8″ bearings should still be seeked out if possible.

Speaking of BOMs, the files are located here and this is what the ZIP file contains:

  • Original design files in Autodesk Inventor 2012 format. Unfortunately I’m not quite so motivated as to provide generic/open source models for everything.
  • Premade DXFs of all motor mechanical parts ready to cut on your favorite 2D Rapid Proto gadget. The materials (e.g. “0250_al” meaning .25″ aluminum) are recommendations, but the rotor rings MUST BE STEEL. Otherwise, feel free to cut the 1/8″ spacer rings and 1/4″ endcaps from polycarbonate for that clear case-mod motor effect or aluminum for extra strength.
  • Basic bill of materials with most hardware and mechanical parts. The tools I used to carve out the wheel center are also included in the BOM. There’s things that are missing from the BOM because they are variable with each design, including the magnet wire, output leads/termination methods, and optional Hall Effect sensors.

There’s some fab considerations which I’d like to share for this particular design, as they might make the motor more durable:

  • Please, please make the endcaps from something which is not wood. Pleeeeeease. (There’s nothing that wrong with it mechancially, I suppose, but just don’t ride through rain).
  • The two 1/4″ discs can be made from four 1/8″ discs – this is in fact what my endcaps above are – if it would make it easier and cheaper to fabricate. For instance, I ordered a single panel of 4 discs plus the “0125″ parts from Big Blue Saw.
  • The stator hub is designed to be a loose fit with a standard non-kerf-compensating laser cutter. Waterjet cutters and BBS’ laser services all offset the kerf such that parts come out “true” to dimension. Parts made using this method will be tighter-fitting into the stator and shaft as a result and might need filing or sanding.
  • My final winding is 30 turns (+/- 2) of 9-strand-parallel 28 gauge magnet wire. This is equivalent in area of copper to a single 18.5 gauge wire. Dual stranded 22 gauge wire is also workable and 22ga is probably easier to find. This results in an intrinsic Kt (V/rad/s, or volts generated per rad/s of rotational speed & Nm/A, torque per ampere of phase current) of 0.18 – the Kt will vary a little depending on exactly what drive profile is used (trapezoidal, sinusoidal, sensored vs. sensorless, poor vs. well-timed sensors…). The final line-to-line resistance was 0.31 ohms.
  • Magnet wire is cheaper on Amazon and eBay, but here’s a McMaster roll of 22 gauge.
  • Drilling the wheel flange holes through the tire is probably a good idea. Wood/sheet metal screws, with coarse threads, should be used to fasten wheel to the flange.
  • Really, really crank down the #2-56 axial tie rods (but don’t strip the threads!). #2 hardware will pretty much fail in shear instantly if they’re loose and your weight is on them, which is why I tried to use so many tie rods to compensate. The more these are tightened, the more static friction exists between the layers of the motor, so the more weight they can take from the wheel without shearing the screws.

I expect to see more examples of this construction in the near future. Remember, buildable without big machine tools!

Kitmotter 0002 Rework and Rewind

Jul 22, 2012 in Kitmotter!, Project Build Reports

Last time I left off with Kitmotter 0002 suffering from melty-hub syndrome; during test riding, the stator had gotten so hot as to melt its sintered nylon hub partially, causing it to lose grip on the shaft and shear my wire insulation off, causing a short. It was clear that I would have to replace the hub with something more durable and high-temperature.

As much as I would love to include all of the common rapid prototyping / digital fabrication processes on one device, and have a hub 3D printed out of stainless steel, there are cost and practicality concerns. Since half of Kitmotter is made of MDF already, I elected to continue using the material for now. If I could come up with a hub that works when made of MDF, then it should also work when made of aluminum.

This is the layered hub design. Luckily, having to stick to Z-axis thickness quanta of 1/8″ means that this hub is exactly 1″ long, so a few different materials can be used to finely tune costs. I added more ‘hub features’ which fit the HP Laserjet 8000 motor’s stator, as well as a deeper keyway. This hub will be assembled on the shaft and glued in place first, then the stator slipped over it and glued again. With a multi-piece design like this, I didn’t want to risk relying on a press fit (especially since the shaft is a stock chunk of keyed drive shaft and has no ‘placement features’), hence the added mechanical coupling.

I elected to laser this one in-house for quick results. I got a little off on the tolerances, so this is going to end up a little tight, but that will be fixed in the design files before I release them.

Pressing out the old stator hub, which was totally melted in place, required a little attention to not damage the windings. I used an arbor press with a giant wad of shop towels under the copper windings for the initial shove, then just grabbed it and wiggled the assembly apart. You can clearly see there how the normally powdery surface of the SLS nylon part has been melted.

The outside tolerances were correct, so the stator slipped on properly. I used ultra-thin CA glue to retain it – the stuff wicks into the thin and porous MDF gap very well. A higher temperature solution would be coating both stator bore and hub in epoxy, but this was quicker.

It turns out that the new hub’s projections to grip the stator’s “keyways” did not play well with how the phase windings crossed the stator, so I had to cut the crossing strands and solder in jumper wires.

I repaired the damaged heat shrink insulation (with more heat shrink) and tossed it back onto Johnscooter. I was able to get around campus with it without the motor displaying signs of overheating (though the shaft did get very hot). This thing is still really underpowered.  With Kitmotter 0002 coming in at a dismal 0.41 ohms line-to-line, it means I was (again) losing half of every watt put into the motor as heat.

making it betur

As I mentioned before, the winding on Kitmotter 0002 was kind of bullshit – like Kitmotter 0001, I made it quickly without any intention of it actually producing torque. There’s an immense amount of open space still in those slots. 30 turns of my Chibikart hex-28 gauge produced a reasonable torque per amp, but it came at the cost of high resistance – i.e. not that many amps to be had. For motors of the same size, a higher resistance motor tends to heat up more and run slower just due to the I*R penalty of no-load current draw coursing through it. It also won’t produce as much stall torque because the R limits the stall current.

Well that’s ass. If hex-28 worked, then nona-28 should also work:

I ordered 3 more reels of magnet wire for my “Hobbykinging Rig” that I made for winding the Chibikart motors. 9 28 gauge strands in parallel are equivalent to a roughly “18.5 gauge” winding in terms of “circular mils” of copper. It should reduce my resistance by a third, putting me more in the .28 ohms range. I hope.

That’s much more like it. See those filled slots? 30 turns on this core is at the edge of sanity (and windings falling out sideways – the bad version of “Hobbkinging” you don’t want to experience in your motor). Maybe I could have done 31 or 32 if I pulled harder, but 30 turns is on par with the previous winding. The same torque per amp, but less resistance, yields a motor with more maximum output power. Unfortunately the resistance turned out to be more like 0.31 ohms. I suspect the extra is due to the “end turns” effect, where successive layers of windings need to travel around the end of the coil for a longer distance to reach the active length of the stator again. For this reason, busbar windings and holy-shit-how-did-you-wind-that Crazy German Guy 12 gauge magnet wire windings win over Hobbykinging, still. But hey, reduction of resistance by about 25%.

The stator pictured above is another RH7-1260 stator I had (it’s not the one inside Kitmotter 0002). I performed a “stator swap” after finishing this winding. The result is that Johnscooter doesn’t necessarily have more torque to launch with, since the Jasontroller current limits to about 25 amps, but it can keep accelerating for longer because the lessened resistance means the motor’s back-EMF can keep building (accelerating) further before the sum of it and the I*R voltage drop equals the supply voltage from the controller.

Once Kitmotter 0002 gets a few more miles on it, I intend to upload the design files to my site (the Useful Stuff section really needs more love); both 3D models and ready-to-cut DXFs will be provided.

Hub Motors on Everything: Johnscooter, the Testbed for Kitmotter 0002

Jul 17, 2012 in Kitmotter!, Project Build Reports

This scooter might rank as both my fastest-built and shortest-lived vehicle. It was called into existence solely to test Kitmotter 0002 on the evening of the 14th of July, and lasted about an hour and a half. That should tell you that Kitmotter did not fare well under load testing, but it might not be for the reason you think.

The base of Johnscooter is…. well, a scooter frame left behind by someone named John in MITERS almost a year ago. Occasionally, people drop things off and swear they will come and fix it or upgrade it, but it ends up not happening because MIT eats them shortly thereafter, or they ran away to Nicaragua, or some other life event detrimental to finishing scooters occurs. John, if you want your scooter back, I’m done messing with it.

This frame is quite interesting. A little research led me to find that it was called the FX-1 EZRider, which is probably one of the more hardcore names I’ve ever heard given to something like this. They seemed to have been produced a decade ago and used nickel metal hydride batteries, and oddly enough, sold at Walmart. It’s very compact, with 6″ pneumatic wheels native (though I’ve been unable to find where John left his rear wheel), and weighs a bit south of 20 pounds.

I started tearing down the frame to extract the components inside, and I must say I am a big fan of the design. It’s not cheap and thin like a Razor scooter – there’s 5mm and 4mm aluminum all over the place. The frame comprises a single aluminum extruded tube about 3.5″ wide and 1.5″ tall, with the forks (made of 5mm bent aluminum plate) bolted through the sides. The same bolts retain all the internal electrical components. Up front, there is a solid endcap which seals the aluminum tube from the weather.

I’m also a fan of the combination fender brake. You can actuate it both with the brake lever on the handlebar or by stepping on it.

The batteries are in stick form and they slide right into the aluminum frame tube, surrounded by rubber. I was expecting these to be lithium, and therefore toast if they’ve sat for too long, but after dismantling a small portion of the heatshrink, I found that they’re actually 4.0Ah nickel metal hydride cells, of the 4/3 A size. That’s alot of battery. Each pack (there are 2) is made of 24 cells in two sticks, each made of a 3-long string of 4 cells in parallel. Then there’s two of them. That’s 230 watt hours of battery, a number that is only rivaled in this volume by using prismatic lithium polymer or LiFe packs. Seriously, I could not make an A123 pack this good.

As far as I can tell, each pack is only 14.4v nominal, and they are used in series to run at 28.8v. To my surprise, the whole pack charged right back up and peaked at 33 volts after sitting for Robot Jeebus Knows How Long.

Some times you just can’t beat a little bit of old school technology. Unfortunately, for me to actually but 230Wh of those cells is about $140 (nearest match) if I don’t mind something generic, and a cool $570 if I want Sanyo cells. Nickel is still classy, and now you understand why I consider the advent of Hobbyking batteries to be a pivotal moment for the creation of actually useful small vehicles by hobbyists.

Now, this is actually an aftermarket pack – John mentioned when he visited that he purchased this scooter from a builder in California, nicknamed Deafscooter. I’ve actually snooped around Deafscooter’s work on the EVAlbum back before I built my first EV in 2007. In fact, I am fairly certain it is this exact scooter, because the date of sale matches roughly with John’s description. Something about going full circle…

To mount Kitmotter, I decide to cheat a little and just use the axle hole that was already in the rear forks. This involved end-drilling and end-tapping the shaft. Another solution using only ‘garage tools’ is to drill out the forks’ axle holes to 5/8″, the shaft diameter, then drill and tap set screws perpendicularly. I decided to just go with easy since testing the motor was a higher priority.

I cleaned up the battery pack wiring a little (putting them permanently in series) and then stuffed the now-charged batteries right back into the tube from whence they came.

And on the Kitmotter goes! Very little engineering occurred here: Kitmotter had a 3″ width between shaft end faces, and the scooter fork natively spanned 2.6″. Solution: Mash the bent plate forks in a vise until they became unbent.

A view from the other side, showing the quite sophisticated brake mechanism – the little off center thing at the bottom actually rotates with the brake cable, giving the brake lever variable leverage as it is depressed. Did I mention I love this frame design?

For sheer simplicity and speed of setup, I plucked a spare Jasontroller off the table. Jasontrollers handle hub motors very well (almost like they were designed to do it or something…) and Kitmotter2 is indeed a hub motor – many poles, low speed, high resistance and inductance.The throttle that came with the scooter was a generic thumb lever type that was spliced right into the controller.

And here it is!

I replaced the front 6″ pneumatic wheel with a spare 125mm wheel because it otherwise caused the thing to look very strange, and ride ‘nose up’.

Ride testing of Johnscooter indoors revealed that Kitmotter’s hardboard sides seemed to work just fine. I took it through a few bunnyhops to make sure. The #2-56 studs and nuts seem to take a little time to ‘wear in’ the hardboard surface – more washers and Loctite may be necessary in the future. Even though the wheel was bored manually, it still came out reasonably on center because of the hole saw pilot bearing hack, and the wobble wasn’t noticeable in riding.

It was, however, a little underpowered because I never actually wound it to produce torque, being a demo motor and all. I could have definitely put another 2 strands of 28 gauge in parallel and lowered the resistance, or just put more turns on to attain a higher torque per phase amp. Or both. The stock Jasontroller puts out only 25 amps, limited by hardware, so it wasn’t too much.

Here’s some indoor ride testing of Johnscooter:

Next, we took Johnscooter to Ye Olde Silley Vehicule Proving Grounds, the garage (riding the whole way just to abuse the hardboard endcaps some more, just in case). Since it was comically slow, there’s no test video, but it finished our defined test interval in 120 seconds while consuming 10.9 Wh of battery, for an “action score” of 1308.

This is actually not bad at all given its non-optimized motor. For comparison, RazEr Rev finished the same test in 82 seconds while pulling 11.1Wh, in part because it generates so much more torque per Jasontroller-limited amp. Pneu Scooter offers a more fair comparison, having performed a 99 second @ 13Wh run previously (1287).

Unfortunately, Johnscooter only lasted for one run. After the first, the motor was very unhappy – the heat could clearly be felt through 1/4″ of wood, which is a pretty damn good insulator. After letting it cool for about 20 minutes, I tried a run, but only made it up 3 levels before it totally cut out. After that, the response seemed to be intermittent, which I initially blamed on using the aged battery too roughly. As some of the other guys were riding it around, the motor cut out again and this time it felt like it failed short.

Post mortem analysis once Johnscooter was back in the shop revealed…

…that Shapeways’ “White, Strong, Flexible” is definitely not “White, Strong, Flexible, and Heat Resistant Too”. Basically, it seems the nylon stator hub heated to the point of losing structural integrity, stripping its D-flat against the steel shaft keyway and causing the whole thing to shift. This pinched and shorted the wires against the shaft.

Additionally, melt lines can clearly be seen at the stator interface. This thing definitely got hot – though not hot enough to damage the magnet wire, as it was neither discolored nor smelled funny.

But it’s clear that I can no longer suggest 3d printing a stator hub as a viable solution for anything but the lightest duty motors. I’m thinking now of doing a wood (yes, more wood) laser-cut stacked hub that uses the 3-eared shape of the stator bore fully, and also grips the entire depth of the keyway. Yeah, sure, make fun of it for being more wood, but it wo…..uld have a higher temperature resistance, especially composite wood-like substances like hardboard.

I will put Johnscooter back together – it worked very nicely for what it is, and Kitmotter needs far more abuse heaping before I can confidently tell other people to copy and paste.

Kitmotter 0002: It’s made of wood!

Jul 15, 2012 in Kitmotter!, Project Build Reports

Two weeks ago I said that I would build the redesign of Kitmotter in the next 2 weeks. Well, like any good college student, I did it the day (and night) before it was due. So in another installment of the summer of short, one-day builds, I present the NEW AND IMPROVED Kitmotter, now with 99% more wheel.

Most of the design intent of Kitmotter is centered on making it accessible to people who do not have a shop full of machinery at their disposal, and this design was definitely a leap in the correct direction. It was finished without the use of a mill or lathe…or even a drill press for that matter, since all the holes were laser-cut and did not need finish drilling.  The stator was harvested from a HP Laserjet 8000 series main drive motor (copier motors spreadsheet here), and the wheel was cored manually with a hole saw that had a 5/16″ pilot drill swapped in place of the 1/4″ one. The shaft is a pre-cut chunk of 5/8″ keyed steel drive shaft that McMaster sells to you for one hell of a value-added markup, but at least it’s better than buying 6 feet of drive shaft to use 3 inches of it. The hub that mounts the stator onto the shaft was hired out to Shapeways to be produced in laser-sintered nylon plastic. And the rotor, the hardest part to make conventionally, was hired out to Big Blue Saw (for real this time) as a stack of steel plates based on the original “kitmotter principle” seen in Pneu Scooter and Kitmotter 1. Other than that, there’s no special hardware.

As I mentioned before, Kitmotter 2 is a prime candidate for my next DIY vehicle centered Instructable, but there are some unresolved issues with this version that I might try and take care of with another one. But in the mean time, here’s how it was built.

Over the past few days, my order to Big Blue Saw which consisted of both waterjet cutting in 1/4″ steel and laser cutting in…. wait, is that particleboard?! Yes, in the interest of not making it cost a billion dollars to try the idea out (the minimum charge for waterjetting is about $80, plus or minus), I elected to make the side plates out of wood. None of the plastics were really appealing in terms of mechanical strength, and wood is some times underappreciated as a material. I figured I would make it out of the most plastic-like wood, Masonite/hardboard, or high-density fiberboard. This stuff is pretty fantastic in compression because it’s basically a solid brick of cellulose.

Only downside, I suppose, is that this Kitmotter should never be operated in the rain…

The rotor rings were split in half and arranged together with “sprues” so they were one closed profile and could be cut without falling into the tank or wasting huge swaths of material in the center (one of the downsides of this kind of design). Because there are so many perimeter screws, alignment and concentricity shouldn’t be an issue.

The motor’s design constraints (namely the need to only increment the axial thickness in 1/8″ and 1/4″ steps) meant it had to be about 2″ wide, which is wider than they make #2-56 screws long. I had to make meta-bolts using #2-56 threaded rod chunks and locknuts. Which, by the way, McMaster will also sell to you.

I ‘laminated’ the wood endcap together with slow-setting CA glue in between the layers, using the bearing as a centering jig. Slow-settingness was critical to this build because of the extra time you have to push the parts together, and if needed, align them radially. Slow-setting adhesives also tend to be stronger, and the motor is made of wood.

Putting all of the endcap bolts in forced the wheel mounting flange to be ‘geometrically averaged’ so it was the least out-of-round.

The rotor is built up similarly. The screws are positioned such that they are internally tangent to the rotor’s outer circle. They should never be seeing any cyclic ‘wheel loading’, unlike my through-the-wheel bolting scheme in the original Razermotor.BBS’s waterjetting tolerances are fairly typical of standard waterjetting – one side is on dimension and the other side is usually 0.003″ to 0.005″ bigger. I pre-compensated for this in the size of the slots, so even with standard waterjetting the screws could still pass through.

The “taper free” waterjetting is more expensive but can produce true square ends to better than 0.003″ tolerances.

Notice the irregular spacing of the ‘seam’ between the two semicircles of the rotor. This is once again an exercise of geometric averaging – by rotating where the seam is throughout the stack, I not only make sure there’s not a single ‘weak spot’ in the whole rotor, but also average any inconsistencies the waterjet may introduce. While a professional shop is going to keep their machine running pretty tight, waterjetting is still fundamentally machining something with a wet noodle (especially noticeable on hard corners using non-high-quality settings).

Here’s the stator assembly. I used a RH7-5219 core this time, but that particular hub also fits the Laserjet 8150 motor (RH7-1260). There is a flat in the hub to grip the keyway of the shaft (but leaves the keyway itself open to run wire outside). The whole assembly is pressed together and sealed with CA glue in the middle. A thinner CA was used here so it wicked into the semi-porous top surfaces of the laser-sintered nylon.

The nylon parts are easy to press fit because their outer surface is not smooth and still a bit powdery. Not only does a disturbingly cocaine-like powder fall off them as you press, but the added compression hopefully helps part retention…

(say, can you laser sinter cocaine?)

I’m going to skip over the gory details of winding for now, since the process was the same for this as the Chibikart motors (an Instructable would go a little more in depth about how painful it is). I decided to just use the hex-28 gauge rig I put together for Chibikart, though these motors can definitely stand another strand or two in parallel, which would also cut my resistance. There are 30 turns per tooth (120 per phase).

I did a fairly standard hex-28 termination and ran the wires, heat-shrunk for insulation, straight out of the keyed slot. That’s why I chose this drastically oversized shaft – the stock keyway is enough to pass some real wires through. If I increase the wire size, though, this may no longer be true, so I’ll have to test it out anyway.

Now, how do I install this thing? 5/8″ is too big to grip inside a drill press chuck to safely lower it in. I actually have no good answer for this at the moment, but I know it is definitely not “grab it and try to hold on”.

The best I could do right now was to ‘angle’ it in using  a set of giant channel-lock pliers to grip the shaft, then wiggling it until the other side centered in the bearing and slipped through. Because this motor is a fairly low aspect ratio (pancakey), I could do that given the loose airgaps. Even this was a bit of an adventure.

Now with wheel installed. The other endcap doesn’t support the wheel in any way, but is larger in diameter than its bore, so it will at least stop the wheel from falling off.

And all closed up!

I decided that the little short wire stubs were unsatisfactory, so some 16 gauge noodle wire was used to extend it. The termination is 2mm bullet connectors, my new favorite after 4mm bullet connectors – 4mm is just a little ridiculous for 16 gauge wire.

Alright, now to dyno it so I can find out its properties:

Poor scope.

Basically, RazEr REV2 was acting purely as a speed source here, with the scope on averaging mode to get a cleaner reading. I’d run the motor up to speed for a few seconds, then hit stop to capture the waveform. Hey, this thing was impossible to “lathe-o-mometer”, so I had to think of a way around it. Here’s the resultant waveform:

V peak-to-peak of 16.8 volts at speed 100Hz – that works out to a Kt in V/(rad/s) of right around 0.191. Not bad, and it makes sense given the motor geometry. For comparison, Chibikart’s motor is the same stator height but  73% of the radius (50 vs. 68mm), about 50% more airgap (for a decrease in stator surface flux of 87%) and has 90% of the turns (27 vs. 30). Just direct scaling from these factors alone from this motor gets me (0.19 * 0.73 * 0.87 * 0.9) = 0.11 Nm/A….. which is exactly what I found.

There you have it, the beauty shot. Now I need to verify its durability by shoving it onto a vehicle and riding it, but for the time being, it spins quite nicely I SERIOUSLY PAID MONEY FOR THOSE BEARINGS? Seriously, watch the video and listen for the bearings.

They’re the cheapest, “not rated” grade of bearings found on McMaster. Now, I bought them because I figured if I was going to tell someone else what to buy, the more goods on McMaster the better. I’ve had good experiences with the “Not Rated” bearings before, but it looks like cost-cutting has taken their toll on these things, because holy shit they’re bad. Built-in radial backlash and tons of axial wobble. They’re lawn mower or handtruck wheel bearings, not electric motor bearings – hell, the handtruck wheels we get to harvest the tires from have better bearings.

Unfortunately, I was mostly after these bearings for their flange, which makes installation easy. They don’t seem to make many precision bearings in this size – only for said lawn mowers and handtrucks. I might have to just deal with that.

Stay tuned for the next episode, where I ride Kitmotter in the rain and the MDF sides melt!

Return of the Kitmotter

Jun 30, 2012 in Kitmotter!, Project Build Reports

Kitmotter, the concept that never quite was.

Originally, I built Kitmotter 0001 to be used on a little display stand in order to show the concept of laminated (coarsely-layered, anyway) rotors for potential hub motor or custom BLDC motor applications. The concept is a direct knock of the laminated hub motors of B.W.D Scooter which was the first to explore the idea of using waterjet-cut steel rings with magnet “indents” allowing for easy placement and waterjet-cut plastic (or metal) endcaps – basically allowing a brushless motor to be constructed without any intensive machining work. At least, it gets rid of the need for a large machine to turn the steel rotors that my designs all feature – I can make a whole post about all the random workarounds that we’ve tossed around for building whole motors. The same idea has been used on a few vehicles besides B.W.D – the Pneu Scooter and its close design relative picofahrrad, and a non-motor application, just to name some examples.

Kitmotter-on-a-stick lasted for several demo/exhibition type events until I took it to Singapore…


Poor Kitmotter.

The two little necks of 1/4″ acrylic just did not like a kilogram of motor hanging off it as it was bounced around my luggage. And with that, the base was retired and Kitmotter was relegated to a fairly simple life of “static item I would occasionally grab of the table to show people”.

That is until I dropped it one day.

With its main source of attention payments (from curious freshmen and ambitious motor builders) gone, Kitmotter was forced to live in the slums of my multistorey handcart of parts and stuff for many months (picture from before it suffered its unfortunate luggage incident)


Kitmotter could never afford to live in one of those new parts drawers.

As my cleanups and project purges were happening, so the slums were being cleared, demolished to make way for new expensive high-rise plastic sorty-bin developments. The first tenants of the new development were the fasteners, who pretty much lived on a level above the rest of the parts beforehand anyway (literally). Kitmotter was temporarily forced to stay with friends on another shelf in MITERS. The future seemed bleak for Kitmotter – forgotten, broken, and tossed away, an embodiment of ideas whose time had come to pass.

Until today, when I found 3 sheets of almost pristine 6mm acrylic in the laser cutter scraps pile at the Media Lab. Almost, meaning some UROP most likely took 1 part out of one of the corners then left the rest of the material hidden in the pile hoping nobody would find it because he’s too damned lazy to drag it back upstairs. I swear I haven’t done this before during my undergrad adventures, people.

Well, no name means no claim, so I quickly whipped up a new display case design while a job for the lab was processing:


Yes, white on white. I know.

This box gets rid of the mounting ‘stick’ as well as removing a very obvious pinch point in the original design. Instead the motor is first mounted to a reinforcing cross, then screwed to the case. There’s more acrylic to crack… not that it won’t be any harder, since acrylic. The case is also shorter than the first

Here’s the parts of the new box after cutting. I picked some scrap dark green acrylic to make the spacer rings from this time. The “KITMOTTER” is actually vector-etched into the top plate on a setting just fast enough to break the white plastic coating. Normally all of this is peeled off, but the separated letters remained white, making for a good contrast.

But before any exterior remodeling, I first went in and fixed one thing about Kitmotter that has been wrong since the beginning: THE SENSORS ARE IN THE WRONG SLOTS. Actually, even worse – they have been in the wrong slots for every motor I’ve built which has internal sensors.

I have previously put the Hall sensors into the slot between two teeth of the same phase i e.g. between A and a, or b and B. The rationale being that when a magnet transition (edge between N and S poles) happens, that set of teeth will “pull” the magnets above it into direct alignment.

I’m fairly certain this belief was just carried over from 2007-2008 when i was first learning How Moter, and then never validated or refuted. It took several very brain-twisting discussions and whiteboard sessions before I finally saw the fundamental error in judgement. I still can’t quite explain it in diagrams or short technical sentences – this post by Amy might be the closest thing. Bottom line is, the concept is true (magnets being pulled into alignment) but the magnitude of the movement required is 60 electrical degrees, and it is only possible if the Hall sensors are placed between two teeth of DIFFERENT phases (different “letters” in the conventional notation). It is still possible to find a ‘combination’ of sensor and phase connections which resulted in cyclic commutation, but the timing would always be too far advanced by 30 degrees (or too far retarded). Either way, not good, and it explained why 1. Kitmotter always sounded like a moped engine, and 2. why Tinytroller had issues with running it because it had no ability to compensate for sensor timing.

The fix involved just shifting the sensors over one slot. Because Kitmotter was never built to be actually used in a vehicle, I just heat-gunned the hot-glue-retained sensors and squished them down over another slot. I placed them between the ab, BC, and Ca teeth this time, like I was supposed to.

With its new green trim rings (for green energy and transportation!!!)*, I closed Kitmotter back up again, with fresh and unbent bolts too.

*shoots self

The new case completed. The large hole on the top plate is designed to clear the motor wires, and the motor itself is mounted only to the cross.

Replacing the motor controller was a straightforward deal since the wiring remained the same – another example of my project “case mods” this summer, I suppose. The controller is a sensored Jasontroller – this one I actually bought from Jason himself in Singapore last January. These are, unlike the eBay controllers, sensored-only.

I can already tell that the sensors are actually correct this time – Kitmotter no longer runs shittily in both directions! It’s much smoother, and the current draw is lower at 36 volts. Previously I was getting 4-5 amps of current draw, which I attributed to the bearings (huge and greased) dragging it down…all, you know, 160W of it. The no-load current has now decreased to only 1.3 amps at 36 volts, which makes way more sense. The minimum loaded motor speed before it starts ‘bouncing’ due to the timing error is also eliminated – switching too soon would cause the motor to jump back and forth as it doesn’t quite have enough torque to overcome the load before the phases switch again.

With this new discovery, I now trust sensored commutation a little more again. And all this time I thought it was just sensored being terrible.

Here’s a short video of the new home of Kitmotter 0001!

But wait, that’s not all.

son of kitmotter

Everyone wants their offspring to have a better life than they do, and Kitmotter is no different.

To fit a hub motor in a wheel, the wheel must not have a center. One of the issues that caused the project to stall out initially was the lack of a “ring tire” or definitive way of turning a stock wheel into one. We bounced all kinds of ideas around, such as wrapping urethane tread around the steel can like B.W.D (later attempted by Jedboard), but decided it was very troublesome and difficult to reproduce. The compromise idea seems to be Pneu Scooter’s “sidemotor” arrangement where the hub motor isn’t concentric exactly with the wheel, but offset from it in order to use its existing bearings.

It’s difficult to explain how some times solutions to problems seem to pop up with no warning in your head. I had originally thought of using a hole saw to clear out the center of a wheel a long time ago, but I quickly scrubbed the idea because how the hell are you going to keep it centered without a mill?

What I had forgotten back then was that hole saws generally have pilot drills in the center. I’m used to seeing hole saws being used in strange milling machine fixtures to make “fishmouth” joints for future welded tube frames. In this case, the pilot drill is not used.

It was during a conversation with Jamison about his latest sidemotor build that the thought of using a reduced-shank pilot to use the existing wheel bearings of a caster wheel as the centering mechanism suddenly dawned upon me. Now that I have hindsight, duh, it was so obvious. It doesn’t even need to be a drill bit – there’s nothing to drill. It literally can be a pin that is 1/4″ in diameter on one end, since most hole saw arbors take 1/4″ pilot drills.

Buying a 1/4″ reduced-shank drill on Mcmaster-Carr, though, was the quickest solution.

So here’s everything. Most of the skate and scooter wheels that I deal with have an 8mm bearing. 5/16″ is literally a hair smaller than 8mm – 0.3125 vs. 0.3149 (for normal hairs measuring about 0.0025″ diameter). It would be terrible if it were the other way around. You can reasonably use a 5/16″ rod as an 8mm axle – which is exactly the intention here. The drill bit would not have a very exciting existence during this operation, since it would just be spinning inside a bearing.

The inside “corable” diameter of the 125mm skate wheels I use (“YAK” type 12-spoke wheels) is about 3.25″ or 82.5mm. This is conveniently a size for which they make hole saws.

I replaced the pilot drill with the 5/16″ reduce-to-1/4″-shank drill bit. The alignment seems to be spot on here. Because the pilot drill needs to go past the wheel’s bottom, I elevated the whole thing on a piece of scrap wood. I clamped the edges of the wheel through to the workbench to stabilize it – a similar procedure will probably need to be done for drill press jobs. I’m actually not sure why I went for the hand drill this time – probably due to torque concerns from the ~1HP DeWalt drill versus a wimpy 1/3HP drill press motor, but the alignment and precise feed control could have made alot of difference.

Alot of positive feedback induced jamming later (note to self: drill press.), and  it worked!

After vacuuming out the swarf, the result is quite splendid indeed. The interior finish is clearly not as refined and clean-shaven as a lathe boring job, but who cares?

Basically, my assessment of this process is that one of the last missing pieces of a fully-accessible Kitmotter has been realized. I’m now really kicking myself for not having thought of this earlier. The major problems with Kitmotter were the lack of consistent stators (an issue that is solved by consistent copiers and laser printers), stator-to-bore adapter solutions (solved by 3d printable nylon hubs you can get from Shapeways or other 3DRP vendors), and… tires.

The same type of 12-spoke YAK wheels also comes in 100/110mm size which I’ve confirmed to be borable out to about 2.375 (2 3/8″, or 60mm). Guess what – they make hole saws for that size too. This can get exciting.

The hole saws actually appear to cut a little oversize. There’s two contributions to this (rather massive) overcut. First, the saws themselves are a little bigger than nominal dimension, and second, I could not hold this thing straight at all manually. It seems like a drill press is pretty much mandatory. Each time I twisted the saw, it would take a bigger bite out of one side. It tended to jam on the thin plastic spokes (a finer tooth saw would also mitigate this), so some times the twisting was fairly severe.

However, taking into account a straight cut, the diameter is probably going to be still 3.26″ to 3.27″ anyway – which is pretty much 83mm even. Sloppily using an Inch tool to make a metric dimension….

So, what’s the next step? I need to reduce Kitmotter 0001′s 3.5″ outer diameter down another 1/4″. This is much more difficult than it sounds, because the magnets need to come down in thickness (probably to 1/8″) and the can must get substantially thinner radially, and I could run into trouble with the case fastening screws. The screws will most likely need to be moved ‘external’ to the steel ring, sitting in circular grooves.

Alright, enough of this “future talk”, here is Kitmotter 0002, coming soon to a…nother demo stand? near you.

So what’s going on here?

  • The 7 #4 bolts have been turned into 14 #2 threaded studs. This diameter change was absolutely necessary to thin the can down down to 3.25″ – there was no other way.
  • The radial grooves seat the screws and secure the layered can, while the endcaps have fully enclosed holes to keep the studs on-dimension
  • The magnet thickness is 1/8″ instead of 1/4″, again to bring down the diameter. I’ll definitely lose a little torque from this.
  • The bigger endcap has a flange that is supposed to be used as a guide to drill into the wheel, in order to retain it. I might add more holes for more strength since the threads will be in soft gooey scooter plastic.
  • The axle is a stock 5/8″ keyed shaft of a 3″ stub length that McMaster sells directly. The hub to the stator is 3d printed. All other trimmings are left up to the user.

I’m going to build a prototype of Kitmotter 0002 in the next 2 weeks to validate this model, and I consider it right now to be prime Instructables fodder if it ends up working out. Essentially the last missing link in an “accessible” hub motor vehicle has been solved – where accessible means hypothetically buildable without access to heavy machinery like lathes and mills.