Hub Motors on Everything, Part I: The RazErBlades Contingency Plan

Reason #1 to not engineer things at 5 in the morning: You think that putting hub motors on inline skates is actually a good idea.

Reason #2 to not engineer things at 5 in the morning: You forget how many magnets each hub motor needs, and like a total dumbass, only order half the number you need.

Well, guess who is guilty on both counts. Late in June, I put in a reorder of the custom arc-segment magnets that I got for the first two skatemotors from SuperduperfabulousMagnetGeorge. Each motor takes 7 “north” magnets and 7 “south” magnets, where the designations just describe which pole is on the inside face of the arc. So, I ordered 16 magnets in total, 8N and 8S, so I have 2 spares in case I break something.

wait, what?

If you’re keeping track, the left RazErBlade has 2 motors. That means I only ordered enough magnets to make 3 wheel drive skates. By the time I discovered this minor oversight, it was already two weeks ago, so I hurriedly put in an appended order. The custom magnet service has a minimum turnaround time of 3 weeks, and there was (at that point) 3 weeks left until Otakon. Now there is one, and I’ve been informed that my appended order will ship next Thursday.

You know, when I leave for the con. Clearly, this was not going to work at all.

And so I deployed the backup, pain-in-the-ass-but-it-would-get-them-moving plan, and dropped some more dimes on a set of rectangular magnets for the left side motors.

Using GoBrushless’ excellent rotor magnet placement calculator, I discovered that SMG’s stock 20mm x 5mm x 2mm magnets were a good fit for the can if I doubled them up side-by-side. They would require some spacing games, but I was used to playing that with RazEr anyway.

And here they are! I got the shipment notice 90 minutes after I entered my order – that’s essentially on par with McFaster-Carr. Due to the miracle of express shipping, they were in my mailbox the day after.

I printed out the generated magnet placer graphic to use as an epoxying guide. Step one is to put in the “keystone” magnets, the first 14 of alternating poles. Trying to jiggle too many magnets next to eachother, I have found, always results in unsatisfactory placement and a dent or two in the workbench from my forehead.

After the first 14 magnets set in each motor, I crammed their complements in next to them. Putting 2 magnets of the same pole orientation next to eachother means they tend to force themselves apart. To combat this, I wedged little plastic spacers into the horizontal gaps as I placed each new magnet.

I made the spacers using a handy-dandy sheet metal notcher tool and some strips of thin unknown plastic.

new rimz

Usage reports from friends who actually are good at skating have told me that the 72-78A durometer scooter wheels are too soft to perform most skating maneuvers effectively, such as sliding or otherwise breaking traction.  So I wasn’t totally crazy when I thought the ‘blades handled like bricks – they actually do!

Solution: Hop online and find some harder compound wheels. I decide to upgrade one step and go to 85A wheels. Finding 100mm wheels was actually pretty difficult, since the vast majority of inlines use smaller wheels such as 72 or 80mm. Then came the issue of filtering those 100mm wheels to find the ones which can be hollowed out to 2.5 inches on the inner diameter, which was a requirement not met by most.

I finally located these K2 wheels on and had a pack rush-shipped (By this point, I think express shipping has almost matched the cost of parts for this project).

These wheels have a glossy, blank white tread and a black plastic core. Very plain, yet functional, and I was impressed by the quality and finish.

No matter, they’re going on the lathe NOW. I made a quick mandrel to grip them by their bores, since the urethane was actually too slippery to grip with the outer diameter chuck jaws. A flying pass with a boring bar severed the spokes from the outer part of the rim.

Well, mostly. The bar broke through at a place that was not the outer diameter of cut, so now I have these spoke stubs to contend with. When the shops with bigger machines open again, I’ll just knock those out by virtue of gripping the wheel’s OD in a bigger lathe.

final preps

This weekend (and extending into next week, likely, due to laziness) I plan to re-engineer the Skatroller to allow for manual activation of the DEC modules’ electric braking. My spare force-sensing resistor will be hidden under the original wrist-forward trigger point such that it will detect two possible states – willful activation of braking and the palms-open-oh-shit-i-am-about-to-die faceplant mitigation position.

Which, mind you, may possibly be mutually coupled.

I’m also going to switch the analog op-amp circuit to an Arduino Nano based solution, because it’s much easier to throw some if() statements at the two FSRs than try to play the AND/OR/MAYBE game with logic gates and linear components.

Did I just advocate the use of software? Doom.

Past that, I’m going to refine the power system of the Skatroller to use a single lithium polymer cell with a Lilypad boost converter unit. This ought to net me much more efficiency and subsequently battery life, as well as avoid stressing out the XBee by running non-spec voltages.

Non-straightjacketed Agito, theoretically coming to an Otakon near you. Because I'm totally going to be able to stay upright while 95% blind, without the use of my arms, and with motors attached to my legs. Yeah.

Deathblades/RazErblades: On-the-ground Testing!

i’m still alive

A few days ago, I took the RazErblades into Boston proper in an attempt to gather real “in-city” usage data. Needless to say, as someone experienced in mounting wheels to other things, but a relative amateur to the concept of strapping wheels to yourself, I just barely survived the adventure, but emerged with valuable data and a laundry list of practical improvements to the controls.

Before that, though, I had to actually finish the new frames.

Last time, I showed the frames in their mostly complete state. I was able to make the last of the mounting blocks, so here’s a closeup of the mounting arrangement.

I discovered after trying to crank the mounting bolt with all my might that the thread I thought was metric (M6 x 1) was actually 1/4″-28.

How silly.

So, as the world’s worst and laziest machinist, I just drove a 1/4-28 tap right over the existing M6 threads. Because 28TPI and 25.4TPI (1mm pitch) are almost equal, there was a cool thread harmonic thing going on in the distal parts of the through-hole.

Transferring componentry over to the new frames…

The cavity being 1mm wider this time, the batteries slipped right into place. They seem to be resting on the horizontal T-nuts, so I added a layer of foam rubber between them and the frame for some modest level of shock protection.

A bit of stuffing later, and the ‘blades are ready again. I shouldn’t have problems with the whole thing falling off any more.

ground test

Boston is a city of hills.

And traffic, people, potholes, curbs, and don’t forget the broken-ass narrow sidewalks and whole stretches of ancient unmaintained cobblestone.

If I was going to die anywhere while testing motorized skates, it might as well be in Boston, firmly embedded into the front bumper of a T bus.

When a crew of Putzen decided to go shopping in preparation for Otakon, I tagged along – partially because I also needed material to work with, but mostly because I wanted to get in some IRL off-campus, urban testing of the ‘blades.

The test site was Newbury Street, a famed shopping district of Boston that combines literally all of the aforementioned challenges. It was 90 degrees and the day before Independence Day, so people were everywhere. The start of the street is a long downslope with alleyway curb cuts, seemingly random switches between asphalt and sidewalk concrete, and worst of all, brick pavement. Did I mention there were way too many people?

All my hallway cruising paid off, and there was not a single crash or ditch involved.

Well, technically the test site was along the MIT coastline and across the Harvard Bridge, which presented a lesser challenge to help me tune my own stabilizing loop. I only ended up skating one way – the group had dinner at a little Japanese noodle place.

Bad mistake – afterwards, I figured I could barely keep static balance, nevermind dynamic. So I called it a day then, and walked back.

I wasn’t in the mood to take video or to have video taken, so no video of this test! Sad, I know.


The distance: 1.97 miles

The battery usage, as determined by my charger afterwards: 0.52 amp-hours

I estimate that I was using the electric assist around 25% of the time – mostly because the region was hilly enough such that I didn’t think the motor would actually affect my mobility. Most of the traveled distance was cruising, controlling speed, or just straight out skating like they weren’t motorized or something. New concept, I know.

With this conservative estimate, the “fuel mileage” of the ‘blades is (.52Ah * 22.2v nominal ) / (25% of 1.97 miles) = approximately 23 watt-hours per mile. For reference, RazEr itself seems to hit around 25 Wh/mi.

I suspect that with more electric assist and less me-assist, this number will drop precipitously.

Some observations, lessons, and anticipated changes:

  • Speed control down a hill is a horrible bitch. I could only learn so much from watching Youtube videos, and having to do it myself is no fun… especially when dodging shoppers and tourists.
    • Consequently, I’m going to enable the motor braking on the DEC modules. All this entails is keeping the enable line high such that the controller tries to hold zero speed, fighting my motion.
    • To do this, I’ll have to add another sensor or two to the wristpad controller such that it can distinguish between when I want to accelerate, coast, or brake. I’ll probably make the brake position the old throttle position, which is wrist down, since not only does that trigger when I make the corresponding motion, but as mentioned in the post, also triggers in the palm-open position. You know, like catching an impending faceplant.
  • Electric mode is wonderful for crowd-mingling. I found myself not having enough space to actually keep up the kicking motion to move forward. This was when the electric mode shined – slow, walking pace mingling in close quarters. I definitely received a few weird stares for seemingly moving with no effort while next to someone for 100 or more feet.
  • Electric mode works when skating normally. The sensored motor control means that the controller never has guess where the motor is – it always knows, even if the speed is varying greatly. I haven’t definitively tested if actually using the motors while kicking contributes to speed that much, but it’s something I’d like to get to.
  • Cobblestone and brick pavement is a travesty to anything without pneumatic tires

    • Especially on a downhill section like the Boston end of the bridge.

The motors showing some battle scars from stopping and being accidentally run into a curbside or three!

So what about the new frames? They held up great. I had no issues with loose hardware or flexiness, so the T-nut design proves itself.

the day after

…was July 4th.

Every year, the giant barge of fireworks parks literally right in front of Killian Court, on the river, so MIT gets the best possible view without even trying. The problem comes when everyone tries to crowd the riverfront to watch – and this is hundreds of thousands of people on both sides of the river easily. Roads get closed and blocked off, and the police roll out in full force to make sure nobody drunkenly riots (or if they do, it’s not too outrageous).

Wait, did someone say “roads get closed”?

The three major streets that define the “MITmuda Triangle” were closed down to road traffic – limited to bikes, pedestrians, and…

…small electric vehicles!

We basically rolled out as many small EVs as we could find – the BWD, RazErblades, RazEr itself (before a mysterious controller malfunction grounded it), and the secret MITERS electric pocket bike.

I also unchained LOLrioKart from the ceiling and took it out. As usual, it was a total attention fairy, and I’m sure there are now a few hundreds of pictures of it floating around on Facebook with people making strange faces in them. The rollout was, all things considered, a success, and we even managed to squeeze in a trip to 7-eleven (in full EV regalia) for slurpies, which constitutes the strangest mutation of cruising I have personally witnessed.

We need more small EVs. I get hits from 18.*.*.* all the time, so I KNOW you MIT people are actually reading this site. BUILD MORE SMALL EVs. Swarm and destroy!

Here’s a short video of Shane taking the ‘blades out for a spin near the south end of campus. This is still with 50% motor capacity, by the way.

Once I’m able to get the left side motors magneted up, I’ll gather some people to take real running video. Hopefully, it will be interesting with all 4 motors, and unless something really dumb happens (like I blow all the controllers or… like… die or something), the thinly-veiled-ground-test at Otakon is on schedule.