ICEngineering Subjects

Saturday, March 28, 2015

Electric Longboard!

Before building this thing, I couldn't ride skateboards at all.

Like, as a young kid, elementary school or so, I was pretty sure the cheap plastic skateboard we had was absolutely impossible to stand up on, and my sister and I just would put a knee on it and scooch around.  Even then, the thing would hit the smallest piece of gravel and throw us off anyways.  I ruined a shirt and for the longest time had a scar on my shoulder from trying to take it down the neighbors' driveway hill...

But still, I couldn't get the awesome but entirely impractical idea of an electric longboard (fuelled by my recent RC dabbling, a dude I saw cruising on one in California, the recent assault of 'E-board' startups, and the great forum at out of my head. 

So I chose slightly upsized components, and geared it for 26 mph.  Good idea, right?

This thing is a blast.  It's not perfect, and I have a short list of improvements for if I make another one (!), but it's effective.  And it's so much fun: cruising down the road on such a low-profile piece at road-bike-pace feels miraculous and awesome.
For better or worse, pretty much everybody who sees it also thinks it's awesome.  With this board I've met just as many people on my street in the past week as I have in the past 1.5 years of living here.  However, choosing to ride it somewhere is a very conspicuous activity, and I feel pretty self-conscious about it.

And 26 mph is way too fast.  I think the fastest I've gone so far is just under 20, and the board can wobble pretty badly when going faster (though I think the higher speeds are easier with more experience).  In the video below, I'm only doing 14-16mph.  This summer, I plan to ride it to work and back (6 miles each way) with a battery change probably needed 3/4 of the way there.  That'll be a pretty silly adventure.

As far as building the board, what you see is what it was.  I just purchased parts and attached them.  The only important fabrication for function is mounting the motor to the truck, and a drive sprocket to a wheel (this board is 1-wheel-drive).  Once this is done solidly, the board should work just fine.

1) Drive sprocket to wheel: Parts from spd-si (cool and easy, a tad expensive): used a 9mm width HTD timing belt. Used hand tools only on this board (quick and dirty) - pressed the sprocket into the bearing race of the original wheel (lucky fit!) and added 2 additional ball bearings to keep the sprocket concentric to the truck axle.  (bolts added later for torque transfer because epoxy really didn't even pretend to work)

 2) Motor mount: the board had threaded holes in the trucks (don't know why).  So I used these, with a simple bar across (to get two threaded holes engaged) and a plate up to the motor.  I chose not to use slots, instead drilling multiple bolt patterns for indexed ctr-ctr (more robust when rolling down the street, but much more of a pain to get the center distance correct)

 3) Component selection and mounting to board:
So, I chose to go with a single motor and 1-wheel drive.  I figured I could always upgrade from there, and buying just one would be less risky than buying two of everything if it turned out the parts were poorly chosen.
Test fit (gorilla tape assembly)
Total cost was $330 (thanks to free batteries and a cheap-ish $110 longboard)
Motor: Hobbyking SK3 6364 $70.  I got the 214kv version because that's what was in stock and I was impatient, but would have preferred a lower kv model.
Controller: Hobbyking 150amp car ESC $70.  Some concerns about matching to the motor regarding poles (actually I still don't understand what the SK3 is regarding poles...) - but I wanted something with braking, and that could handle 6S.
--> This portion ended up not working satisfactorily, keep reading!

Receiver/transmitter: the cheapest that hobbyking had (works great)
--> Update June 2015: I'm getting pretty tired of carrying this really huge, attention-grabbing controller when I'm just trying to get groceries...I'm going to look into a modified knockoff wi-mote

Batteries: Would use 6S Lipo (cheapish, chargers are readily available for 6S), but my friend had a load of A123 LiFe cells that he wasn't using, and let me borrow them with no return date... thanks dude!  I modified his 12s (!) packs to 6s, and made a pack connector to go 2p.  With the 6s2p it should be 4600mAh, which seems to be about 4 miles of range.
6S is to balance power vs cheap controller and charger availability.
"This pack can weld metal"  (12s A123 cells)

Taking apart, check out the classy label indexing

It was scary taking an angle grinder to a bus bar

I used XT90 connectors, I like them a lot

 Then I made a few stainless steel mounts.  I used stainless because I had some had seemed like a good idea a while ago for a shifter kart exhaust mount, and I bought some metal (plates and tubes), a tubing bender, and some stainless welding wire...and then didn't use any of it.  So I had it around and still hadn't tried it out.
My unfortunate conclusion is that I should avoid stainless if possible in the future.  It's really difficult to cut and work (the metal work hardens: I also needed to buy some pretty expensive carbide jigsaw blades to cut it effectively), and the welds are functional, but messy when using the C25 gas I have on my mig (I didn't want to spring for tri-mix on this experiment, which means the welds will probably rust in the future! not really stainless anymore...)
Anyways, it's together and solid now!  And I'm really pleased with the battery mount especially, which holds the battery firmly but without any sharp/metal things to abrade the packs.

Stainless hoops for mounting and bottoming protection.  Velcro battery hold-down ties battery down to 1-sided adhesive foam on the board, allows quick change, secure mount, and no battery rubbing against sharp edges.

- 1 wheel drive is functional, but I would choose 2wd for more natural feel next time
- The motor and ESC combo is not perfect.  At big mismatches between throttle and rpm (like, big throttle from a standstill, or trying to rip full throttle from medium speed) the motor makes an electronic screaming noise, and the torque cuts to almost nothing.  I think this is because the motor is sensorless, and I think the ESC is lighting up the next circuits ahead of the motor position, resulting in a not-functioning condition.  The board is still capable of huge torque (pulled me up some pretty long, steep hills at a very good speed), but it's very frustrating from a user feel point of view.
(while-writing update: maybe I can add the position sensor to this motor! oof long thread, hehe, maybe I'll start here... Oh!  wow, there are pcb 'external sensor' boards available already made specifically for this motor.  I think this will be going on the board very very soon!)
-Gearing: on a longboard without wearing pads, 20mph is plenty.  I'll change the gearing for this, and hopefully greatly improve that motor 'scream' condition also.
-Timing belt: is functional, but the belt is showing wear.  I expect to have to change it in another couple weeks
- Since building this I've ridden a real quality (unpowered) longboard, with great trucks, more traditional (but still big, 90mm, and mid-durometer) skateboard wheels, and a nice flexible deck.  The quality board really made a big improvement in handling and feel.

So, researching while writing, maybe an add-on hall effect sensor system to the motor will make the board entirely satisfactory to me.  But if it doesn't, maybe next year, I'll build a nicer deck and trucks with a pair of sensored 1100-1500w motors (if the sensored hardware becomes a bit cheaper).  Cool.

UPDATE 4/14:
I added the position sensor piece which is sold, linked from the instructable above. (It was really difficult to solder...for me at least).  I wired it into that Hobbyking 150A controller, and got the wires all plugged up correctly.  It was AWESOME.  Huge torque, it actually layed a patch of rubber on my basement rug.  Torque steer became a problem.

However, then I started tuning it: I changed the motor timing, softened up the acceleration...and then tried reverse.  It didn't like it.  Reverse and brake didn't work well, strange behavior...and then some smoke came out...  Something is damaged in the ESC...
Back to unsensored...still not working correctly (no effective torque, cogging, etc)... I'll open up the ESC to see what's cooked, but I'm going to say it's torched.  SOOOO close to working great...maybe there is a setting combination which would work without ESC damage, but I'm not going to burn a couple more controllers to find out.

Now I think I'm going to find a way to run a Kelly controller, or similar, with the RC receiver...

Butt-boarding into the sunset.  This thing is a heck of a toy!

Update May-June 2015:
After still not being thrilled with power density of non-RC controllers, and not finding any better sensored controllers than what I had (plus learning about the hall effect sensor latching, and being concerned about RC brake logic ever working correctly with the external sensors)...AND reading this guy, and this post I decided to chance it on another sensorless controller.  I chose the same Trackstar 200 amp controller he talks about in those two linked posts from his blog/site.  I was especially convinced by his youtube video showing the controller logic to change the operating frequency at low speeds and high loads.

   Side note, that guy is awesome.  But you probably already knew if you've found this site by now.  He's the one who made the sensor board I had already tried on the previous controller.  He's made a lot of fun electric motor stuff, to put it lightly...

The Trackstar 200amp controller was an excellent decision.  It can't get the board going from a standstill, but give it the smallest amount of forward speed and the controller grabs on.  Once it's locked it, it DOES NOT let go.  Full throttle from any speed works.  It works so well that I've probably still only pulled full throttle about 10 times, even though I've gotten about 5 more charges worth of riding on the battery packs by now.  The controller dumps enough power to warm up the motor, which the first controller could only do when sensored.

Also, the controller is '8S lipo', (8*4.2v max cell voltage) and actually the capacitors on it say 35volts, so if I ever get bored I can make a small 'booster battery pack' to run in series...maybe go 8 or 9S total for waaay too much power.(the A123 cells are LiFe, max 3.6v each cell, 9*3.6 = 32.4v) 

I've used the board with the new controller: 
-to get groceries 3-4 times
-to ride to a friends house across town to show off
-to ride to party to show off (and share it with other people...hehe)
-brought it to work to show off to/share with my coworkers (and crashed while full-throttle butt-scootching in front of my managers, pretty embarrassed about that...)
-in my engagement photos (yea, really)
-to ride downtown here in Ann Arbor 2x for dinner/errands
-to share with my family and friends when they visited town for my wedding
-on my honeymoon around Asheville NC (hehe, TMI here?) to dinner and breweries
-near the Appalachian Trial when my knee was injured again and I couldn't walk far  (hehe, seriously)

Super fun.  Best toy of 2015.  I didn't know it was going to be this awesome when I started. (humble, yea?)