Steering

By no means is this an innovative feature of our scooter. What is notable is that excluding hardware, the front steering assembly/thrust bearing is the only off-the-shelf part used. It was easy to extract and re-mount, and also saved hours of painstaking manual labor trying to replicate an already perfect part. The stock handlebars were a little short, so we extended them about 6" and mounted a motorcycle-style twist throttle. So, was it worth the $60 Razor Spark price tag? We certainly think so.

Hubmotors

Two custom brushless DC hubmotors propel the scooter. A hubmotor is a self-contained motor and wheel assembly all in one piece. This works by using a brushless as opposed to a brushed setup, where the stator (stationary part) and rotor (rotating part) are inverted. Since the motor makes direct contact with the ground surface, there are no gear ratios to deal with. At the same time, this meant we had to factor in torque output and maximum rpm when designing the motors. Rough specs per wheel are: 14 neodymium magnets (poles), 12 tooth stator, 30 windings (turns) per tooth, 5" overall outer diameter (including urethane), producing peak 590 watts. Read below for a more thorough explanation and for details on the build process.

Controller

The controller is based on a tried-and-proven design painfully develped during last year's project. This time, it's been repackaged in a more compact, 3-phase version for driving the brushless hub motors. The controller takes advantage of very powerful MOSFETs from International Rectifier (IRFB3207 and IRFB3077), each of which can handle as much as 100A for brief periods of time. Cooling is done with a 60mm fan. The controller detects the position of the rotor using external hall-effect sensors, allowing it to do static torque control (no kick-start required) and full regenerative braking.

Batteries

For batteries, we elected to use 20 (as many as we could squeeze in the deck without looking silly) A123 M1 lithium iron phosphate cells. Although slightly heavier than LiPo equivalents these batteries are less prone to exploding and far more cost-efficient. Here is the spec comparison which lead to our decision:

Building

The motors were nowhere near as complex to build as they look. Especially when you consider all the laminations, made from M19 steel and laser cut for us, for FREE! Thank you PROTOlam!!! The rotors consist of 107 laminations (its just worked, ok?) and the stators 68, each out of .014" steel. That puts our rotors at about 1-1/2" wide and the stators at about 1" wide. Although this makes for a fatter wheel than the original scooter, they will produce more torque. Stacking the laminations was a simple matter of crazy glue. A thorough coat was applied to the perimeter, and the perimeter only, as the windings will constrain them in the z-axis.

Winding the stators was a trial and error process. We used 18 guage magnet wire because 16 was just too unfriendly to manipulate. You may notice that there are red plastic covers sandwiching the steel laminations. In our first winding attempt, we wrapped all sharp edges with electrical tape to prevent scratching the insulation. Electrical tape was not enough. These plastic covers were cut so that the teeth are slightly wider and thus there would be no chance of shorting. It was a two to three man job to count, neatly arrange, and wrap the windings. Every other tooth was wound, 30 turns per tooth, LRK-style wiring.

Assembling the rotors was no easy feat. For ludicrous torque we opted for the neodymium magnets with a remanence of over one Tesla (very high). Recomendation: if you plan on building your own brushless hubmotor, be sure you design the rotors to have shallow indents that accept the magnets. This way, the magnets will eventually snap into place as they have three flat surfaces contraining them. Even with such indents, getting the finger-comfort-oblivious magnets into place is a two-man job. Again, we chose crazy glue to hold the magnets to the rotors. Once all 14 were in place, they probably held each other together because they alternate polarity. But these are NEODYMIUM MAGNETS! and you can never be too careful.

When our rotors had magnets and our stators had windings we were presented with the difficult task of how to put them together. The trick is that you DO NOT want the magnets to snap to the stator. Once this happens good luck ever getting the two apart. Our solution (do I want everyone to know?): place a shaft through the stator and set that in the chuck of a drill press so that it is contrained on two axes. Next, we cut a pair of wood triangles and used clamps to get a grip on the rotor. Finaly, clamp down the triangles - and stator - to the drill press plate and press in the stator. The rotor had a tendancy to fly up to the stator so we had to use multiple clamps. This process is made easier if you make endplates with bearings to lign up the shaft-stator assembly. We also used the bolt pattern on the plates as another way of aligning the stator.

The new controller mounts flush to the deck to free up some much needed foot-space. The main difference is the use of surface-mount components, making the whole assembly more compact. Similar to the previous controller, each motor gets six transistors, two for each phase. One connects it to +33V, one connects it to 0V. This is called an inverter, since it turns DC from the battery into "AC" for the motor. At any given time, the phases are either high, low, or completely off depending on where the rotor is. Meaning, one phase is always high, one is always low, and one is always off, but they rotate based on the sensors attached to the wheel. The "high" phase is actually rapidly flicked from high to low at 15.625kHz to give it speed/torque control and regenerative braking.

From left to right: old controller and new controller

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