Friday, October 15, 2010

Pneu Scooter: Δt5

Pneu scooter large tasks:

1. Stator and rotor. Δt1
2. Deck fabrication. Δt2
3. Batteries. Δt2
4. Wind motor. Δt4
5. Motor-to-wheel adapter fabrication. Δt3
6. Encoder. Δt5

Yes, all the large tasks are done. No, you can't ride it yet. There are still a large number of small tasks to be accomplished, such as mounting the front wheel, adding connector and wires, and tweaking/tuning the controller. But I did make some progress. First, I terminated the motor:

It's a "distributed LRK" winding, but split into two half-motors [AabBCc] and [aABbcC] on opposite sides of the stator. Connecting the two half-motors in parallel and running wires out through the center shaft completes the winding of the stator.

Next, a part that wasn't really on the "large tasks" list but is still fairly important: the motor end cap. Since the end cap is not part of the load path from wheel to deck, it wasn't as critical as the wheel adapter. And the motor would spin fine without it thanks to the two wheel bearings. But I thought it would be good to just get it done so that the motor itself is a completed unit.

I went with the trademark transparent polycarbonate sides of the BWD motors, but added a chamfer to the edge (the way Max originally drew it...sorry Max). Also, a trick I learned from BWD: using a large screw and nut as a lathe fixture:

Nuts have six sides. Lathes have three jaws. Commence happiness.

Moments later, this:

Okay, I may have skipped a few steps on the mill. But it's the same hole pattern as in the outside rotor spacer, so I already had the X/Y coordinates I needed. I wound up taking a bit of extra material off the inside surface just to make sure the windings would clear without rubbing on the end cap. Here is the finished Pneu Scooter motor:

The end cap actually straightened the rotor out a bit. Not that it really matters since the rotor skew is on the order of 1mm from one side of the motor to the other. After final assembly I re-tested on the lathomometer and found the back EMF constant to be 0.206V/(rad/s) - which is also 0.206Nm/A - assuming sinusoidal commutation. That's pretty much right on target using any of the prediction methods I've tried. It's roughly the same constant as BWD's front motor, but since it will be able to pull more Amps, it will have higher peak torque. (Though admittedly not as much as BWD's two motors combined.)

I still needed a commutation encoder, though. I decided against internal Hall effect sensors and was geared up to pursue a reflective optical track with IR sensors. I even printed out a set of tracks and laminated it. But then I had second thoughts. Such an encoder would be sensitive to changes in ambient light, dirt, water droplets, all things I'm likely to encounter. Hall effect sensors are nice because they are immune to most of that. But internal Hall effect sensors are impossible to adjust without disassembling the motor, and also they may interact with stator flux. As an encoder of absolute rotor position, the sensor should be picking up rotor flux only. (In a PM motor like this, the difference might be minor.)

BWD used external Hall effect sensors, mostly because we didn't know any better. But they actually turned out to work very well, picking up the fringing field from the huge, overhanging rotor magnets through the polycarbonate end caps. This time, though, the magnets don't overhang very far and the field on the outside of the rotor is not enough to reliably trip the sensors. Somewhere on this train of thought I was browsing on McMaster and stumbled on P/N 3651K4, a flexible magnet strip that's sort-of like an industrial refrigerator magnet. I wound up just cutting this strip into 14 equally-spaced pieces and gluing them to the outside of the rotor can.

Total elapsed time for thinking of this idea, ordering magnets, thinking it is probably a bad idea, doing it anyway, and finding out that it works fine: three days. Yes, it will pick up metal junk from the ground. But so did BWD. It's good for cleaning the shop floor / finding small hardware.

The companion for this sense magnet strip is an external Hall effect sensor board, which I made from convenient polycarbonate:

It also happened so fast that I didn't have time to second-guess it. The surface is courtesy of a 5" boring cutter, which is probably the most dangerous thing I've ever stuck in a mill. The sensor spacing is 17.1º, which is 360º/7/3. (Seven pole pairs per revolution, three sensors per pole pair.) This board mounts to the inside of the rear fork, with a small gap between the sense magnet strip and the Hall effect sensors. The sensor cable from the controller plugs right in to the connector on the sensor board.

And that just leaves the big question: Does it run? Well, as it turns out, I have not quite met my goal of producing a drivable scooter before I leave for a month-long trip to Singapore (during which I will certainly be posting). But the point, maybe, was just to motivate me to actually work on it since I find that grad school generally makes me slow and unproductive as I sit and ponder theoretical possibilities or work on prep work for 2.007. Besides the handlebar and front fork/wheel, there is still a lot of wiring to be done. I did manage in the last bits of time I had left to get the controller operating in six-step commutation mode (noisy mode, I call it), and the motor spins quite nicely. This means that even the hand-cut sense magnet strip is good enough to trigger effective commutation. But it will take some more debugging to get it running in full field-oriented control mode. And I am flying out in a few hours. Hrmmm, what to do...

 To be continued.....


  1. Big fan of the chamfered polycarb end caps!

  2. Hey, did you know that in HongKong, electric bike are illegal! You need flasher, brake light, etc. But in main land China, they are not and there is billions of them. I will go again in China and try to find some. We told me there are all hub motor on the front wheel.

    Have a good trip and have fun!!