Tuesday, March 8, 2011

Pneu Scooter: Sensorboard v2

Hrm, all this excitement about snow scooters that don't really work when it turns out that my other electric kick scooter has done a pretty good job this winter. Except it gets really, really dirty:


Especially now that the two-to-three-foot mountains of snow on the side of the road have started to melt, the combination of dirty snow, road salt, and sand has become a bit of a problem. You'd think corrosion or electrical failure would have taken out Pneu Scooter by now. But in fact the only issue I've had is somebody beasting it up and down the hallway so hard that the fuse blew, taking out some critical controller hardware with it. Or some critical controller hardware blew, taking out the fuse with it.

In either case, I essentially had to replace half the active components on the controller, and the IXYS MOSFET brick. One current sensor and all? the gate drivers also ate it. After I rebuilt the controller, things seemed to be working except for one very odd new symptom: For the first 30 seconds or so of riding, one or more of the Hall effect sensors would produce sporadic faults that caused the rotor position estimator to fail and the controller to "click". 

This didn't seem like the microcontroller reset problem that I solved a while back, more like simple sensor faults. My hunch is that the same transient that killed everything else also messed with one or more sensors. And the fact that the problem consistently fixed itself after about 30 seconds of riding a) made me fairly sure it was the type of problem that I couldn't fix using deterministic troubleshooting procedures b) made me not really care. But, in preparation for the Friday Energy Showcase of the MIT Energy Conference, I decided to finally replace the sensors.

Step one was to laser cut a new sensor mount: 

The original sensor mount was made from polycarbonate and I cut the 2.5"-radius inner surface with a giant boring head in the mill. Lacking the courage and time to attempt this again, I opted for the wimpy solution of just throwing files at the laser cutter. The downside is that the first metallic object large enough to get jammed between the sensors and the wheel may just crack the acrylic right off. But the upside is that I can now put little sensor alignment features right on the part, saving me some careful measuring and gluing.

Also, t-nuts.

+ sensors + wiring + Amazing Goop =

Maybe the sensors will also be more protected from dirt and debris now that they are recessed into the plastic and potted with goop. If not, I can just hit print a few more times on the laser cutter and make a bunch of spares. Replacing individual sensors is virtually impossible, but having entire spare boards seems like a good idea given how fragile the sensors seem to be. I've also switched to the simpler, cheaper, and hopefully more reliable ATS177 Hall effect sensor.

Here's how it fits in with the wheel:

The sense magnet strip has held up pretty well. It definitely needs to be cleaned frequently, because it picks up magnetic dirt and debris off the ground, but overall it's not a bad solution for making a sensored motor without the commitment of internal sensors. As a reminder, the rubber strip magnet I used is McMaster P/N 3651K4. Some day soon I will revisit sensorless control, or hybrid control that only uses sensors at low speed. (3ph v4.0???). But for now, things are at least back to a working state.

Here's Pneu Scooter and RazEr rEVolution at the Energy Showcase:

"Does it generate electricity?"
- No, it consumes it.

"Are you selling them?"
- No.

"Do you have a business strategy?"
- No.

"Is it really saving energy if it replaces walking?"
- No.

"Do you have a card?"
- No.