Electric Assist Unicycle Build

justin_le

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Hey Guys, not too much time to write much now, but here is a photo collection for one of the projects I finished and have with me in SF for the Maker Faire this weekend, which might be of interest to some on this forum. The goal was an electric ASSIST unicycle made from a direct drive hub motor. A unicycle that you can ride normally, but with the advantage of power going on the uphills and regen on the downhills to save your knees in both cases.

The implementation should be pretty self explanatory from the CAD section view:
U Uni Hub CAD Section.gif
 
Side covers started off as 9" x 9" by 1/2" thick aluminum plates from Metal Supermarkets, and are mounted to the rotary table for machining:

UA Aluminum Plates.jpg

Bulk of metal removed with a large diameter hogging bit:

UB Hogging Out.jpg

And press fit cavity for ball bearing too:

 
Inside corner of side plate rounded with a ball nose end mill:

UD Rounding Inside.jpg

All the spoke holes and mounting holes are drilled out on the spoke flange, which is part of the side cover plate rather than part of the rotor.

UE Spoke Holes.jpg
 
After this, the inside surface of the side cover has been finished, so it gets taken off the table, flipped over, and put back on for machining the outside surface.

View attachment 4

UG Second Side Starting.jpg

The flange is milled down 2.5mm thick, exposing all the spoke holes, and the rest of the cover is faced off just for aesthetics.

UH Surface Facing.jpg

Outside is finished with a 3/8" corner radius bit for a nice rounded edge:
UI Outside Fillet.jpg

And finally, the side completed plate is parted off with a small 3/16" end mill:

UJ Parting Cover.jpg
 
The spindle for the cranks started off as an off the shelf cotterless unicycle hub from UDC:

UK UDC Hub.jpg

One of the flanges was machined right off, but the other flange left in place as a means for coupling the pedal cranks to the motor casing via the right side cover.
UL Uni Spindle Machining.jpg

The spindle diameter in the middle just happened to be exactly 20mm already, which was the size of ball bearing that I had going over it.
UM Spindle Complete.jpg
 
A second side plate was machined just like the first, only this one was made to fit a much larger 45mm ID thin section ball bearing to go over the aluminum stator support:
 
The stator was taken from a Nine Continent hub motor that was having hall issues. My plans had the stator bored out to a 46mm hole. However, you can see that if this is done, it would completely eliminate the welding on the center tube that holds the two stamped steel plates together. In order to keep things structurally sound, I first cut some small steel tubes and welded them to three of the "speed holes" that are punched out of the support plates in order to hold the halves together after the center was cut out:

UO Stator Rings.jpg
UP Tig Welding.jpg

Then the original axle hole was drilled:

UQ Stator Boring.jpg

And then bored open to exactly 46mm:
 
The next step was the aluminum stator support, which plays the important roll of preventing the stator from rotating by being affixed to the unicycle frame, and of holding the stator centered in the hub even with the spindle on which it rests can rotate.

Starts off with 3" diameter 6061 aluminum from a scrap yard:
US 3 inch Aluminum Stock.jpg

A fair bit of machining to get her down to size:
UT Turning Al Down.jpg

Close to the final shape:
UU Close to Parting Al.jpg

And the completed unit after parting off. The 6 holes are to bolt to the actual Nine Continent stator and lock it from rotating.
View attachment 1

Need wire slots for phase and hall cables too, so these are milled into the 45mm bearing support:
 
The metal rotor ring and magnets were also made from a returned Nine Continent motor, in this case one that had suffered water damage.

UW1 Water Damaged Rotor.jpg

The stock Nine Continent rotors are a little heavy, with a 7mm thick steel ring for the side cover bolts, and a pretty thick aluminum casting over top of that for the spoke flange. Since I had the spokes coming off the side plate, none of this was needed and I could shave off a lot of superfluous metal:

NC Rotor being machined.jpg

In the end, I took the steel ring down to about 2.5mm thick, reducing the original rotor weight of 1.78 kg down to just 830 grams:
UW2 Rotor Ring Weight.jpg
 
Here is what all the bits and pieces looked like after quite a few late nights in the shop:

UX All the Pieces.jpg

Stator is pressed on and bolted to the machined stator support:
UX2 Stator Pieces Bolted, end view.jpg

Spindle is inserted into the right hand side plate, and original spoke flange bolts to a small aluminum spacer ring so that turning the cranks turns the hub in 1:1 ratio:
UX3 Spindle in Side Plate.jpg

Steel rotor ring with the magnets is snapped onto this side cover and then held snugly down with 9 bolts clamping the edge. This is needed to hold it secure against the magnetic forces with the steel stator is inserted. Otherwise the 2.5mm steel ring isn't strong enough to stay round, and the force of attraction causes it to wrap around the stator and deforming into an oval shape.

UX4 Rotor RIng Secured.jpg

Even then, it was a bit tricky to get the stator in:
UX5 Rotor and Stator Inserted.jpg

After that, remaining side cover plate with the 45mm ball bearing is pressed in place over the wires.
 
Here's a couple shots of the built up wheel, after lacing into a wide 700c rim:

UZ Wheel Build Complete.jpg

UZA Side view of 29 inch wheel.jpg

I've got the wheel mounted in a Kris Holm 29" Frame with a big apple tire. The little battery rack at the back was made from angle aluminum and holds a pair of Hobbyking 5Ah LiPoly packs and an infineon 25A controller. Handlebar off the seat is home-made and has the CA, a throttle, lumenator light, and ebrake lever all attached to it:

UZB Complete KH 29 Electric.jpg
 
WOW... Thats amazing Justin, quite a post after what seems like a long-time-no-see!
How does it ride so far?

Brilliant design, and I can't wait to see how the stabilizer circuits you are probably about to design for it turn out :D
 
So nicely done and so cool! I can't wait to see the videos!
 
Was the Honda electric unicycle a source of any inspirations for your project?


[youtube]I6gz3_ez2lk[/youtube]
 
nah, bet it stems all the way back to wishing for one of these when he was a kid.
bc2.gif
be careful or u might get sued.


correct me if i'm wrong, but it looks like ur doing all the machining by hand, no cnc.
i found every step useful & directly applicable to various projects, none of which are a unicycle.
it'll save me a lot of frustration having ur pictoral roadmap for a guide, it's greatly appreciated.
12011123_web1_Stark1.jpg
 
What, no panniers? :lol: Just kidding--every time I think I'm creative I see stuff like this. :)

Those mountain passes are gonna be easier without the rest of all that bike weight this time around, right Justin? :p
 
Very nice job, Justin! Amazing what you can do with a 9C! It would be so easy to enlarge phase wires with that large bearing!

Dave
 
Super sick man!! Video with burnout please!! :twisted:

I've gotta make it to one of the Maker Faires sometime, great to have someone like you representing the ES scene. Thanks for sharing.
 
Toorbough ULL-Zeveigh said:
correct me if i'm wrong, but it looks like ur doing all the machining by hand, no cnc.

That's correct, there wasn't any CNC used here, just a standard milling machine with a rotary table. I installed a digital readout system on it a few months ago though which was invaluable though, especially for getting all the ball bearing fits on the first pass. The first few side covers I experimented with by turning them on a lathe, but for something of this large in diameter it was a bit tricky and the rotary table with large milling bits was faster and perfectly accurate.

i found every step useful & directly applicable to various projects, none of which are a unicycle.
it'll save me a lot of frustration having ur pictoral roadmap for a guide, it's greatly appreciated.

Well glad it could be of use. Will be curious to see what these various projects up your sleeve are!

Justin
 
dequinox said:
WOW... Thats amazing Justin, quite a post after what seems like a long-time-no-see!
How does it ride so far?

So far, with just a throttle and an ebrake for regen this one works surprisingly well! You don't go especially fast on a 29" unicycle, my cadence tops out around 20-22kph, so with the electric assist I am only averaging just over 1 Wh/km, and for every amp-hour I use going up hill, about 0.5 Ah goes back into the pack on regen from the downhill. So that little 36V 5Ah LiPoly battery is good for over 150km at this rate. I'm at 70km right now and haven't charged it yet.

The control loop with a thumb throttle controller PWM isn't ideal though, and it will be much better when I finish a custom controller that is motor phase current regulated. That way between the throttle and the brake you can dial in the exact torque on the hub which will stay constant regardless of speed, rather than always adjusting up and down on the throttle as your speed changes in order to maintain the same torque. On a unicycle, you have other things to think about, and so having to hold and focus on the throttle for every change in the road or change in speed is a bit consuming.

Brilliant design, and I can't wait to see how the stabilizer circuits you are probably about to design for it turn out :D

So, you are right about this too. In the original plan, I thought it would be great to have self balancing circuitry on the electric assist uni - no throttle and you lean and pedal to accelerate or brake. I also thought you could use it as a 'trainer' device, to make it easier to learn, and then gradually turn down the amount of electronic balancing as you get more comfortable riding. That was the idea at least.

The circuitboard was made to fit in the same enclosure box we use for the CA production, running a PIC18F series chip to take the signals from a rate gyro and accelerometer and then use that to drive a 4 quadrant controller:

Tilt Circuit.jpg

I struggled for several days with the code trying to get this to work, but found that to have both human operated pedals AND a balancing control circuit running at the same time resulted in the two always fighting each other. The harder you pedaled, the more you injected a disturbance that the motor would correct by braking, and if you tried to stop or pedal backwards then the motor would kick in high gear going forwards. It was a really strange experience, like riding through a viscous fluid where the harder you worked the more resistance you faced.

As somebody who rides a unicycle, the only way I could make it work was either to turn the self balancing gain way down, so that I was basically able to overcome the motor and keep it steady with my legs, or I could turn the balancing gain way up, remove the pedals and just rest my feet and let the electronics do everything. As soon as you had pedals, and the motor, and the balancing circuitry, it was chaos.

I did bring a 2nd orange 24" electric unicycle down to the Maker Faire which had the balancing circuitry to show people though and to see if maybe someone else could figure out the right human/electric mix with it.

Mark on euni.jpg

But again it was most rideable with the pedals removed, and since my intent was electric assist this wasn't the end that I wanted. That's why in the larger 29" uni, I left the balancing purely to the rider, and the motor is purely there as an assist and a brake, and that seems to work much better. At least for now, there is still a lot to explore here. -Justin
 
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