Electric Roller Blades Prototyping

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Created: Wednesday, 19 February 2014 04:14

All kidding asside, this is my first post in what will hopefully become a weekly blog. Mostly, I'm starting this because of my most recent project, a pair of electric roller blades, powered by four out runner motors. However, I'm thinking it'll be a good place to put up almost anything I find interesting. If I go see an interesting car show, pictures! If I visit a far away land, stories! And, if I build or design something cool in class or on my own, engineering insights!

So lets get started shall we? The roller blades I just mentioned. Interesting little devices. Really, what I want is wheels instead of feet, but that's not really possible (nor always practical) so a pair of roller blades are as close as I'm ever going to get. That's really as deep as the reasoning went to get me interested, but the engineering challenge really drew me in. Charles Guan provided the inspiration to do this project with his Razer Blades project (link) and I started from there. Somehow though, I wasn't happy with the status quo and decided that suspension was necessary. This started an awkward design phase:

I wasn't pleased with this result. My focus on this design was keeping the wearer's foot low, but on second though, most roller blades don't actually do this. They place the wearer's foot above the wheels, though they also used smaller wheels. The ones pictured here are 8" in diameter. The other problem I faced was knowing which parts I would be using. For this model, I used what I was comfortable with, a set of headway lithium batteries and FIRST Robotics-sourced CIM motors. Clearly then, this needed a redesign and a solid dose of clarity. What form factor would suit this style of device? How should it realistically be powered? A brushless motor would be a huge improvement over the brushed DC CIM and a higher quality battery pack than the headways would also be a boost in overall performance. While I looked into these upgrades, I started development on a re-design.

This design is a clear improvement on the previous design, both from an aesthetic standpoint, and from a structural standpoint. Raising the user's foot allows for a suspension which loads axially, rather than being cantilevered off of the side. However, it is still quite wide and I worried about usability. From my youtube video research, rollerbladers don't always keep their feet close together, but they often move their feet close past their legs and this design may have the battery clipping the other foot's achilles tendon. Either way, I decided to try a vertically oriented battery.

Around this time, I also found an opportunity from Charles Guan to get a hundred or so A123 lithium batteries. While much smaller than the headway cells, they are much more energy dense and stay functional through more charge cycles. These cells were 26650's (26mm diameter, 65mm length) and were rated at 2.5AH. I developed a battery pack for roughly the same size and revised the design.

I was pretty proud of this battery pack design which kept cells isolated from each other and in a plastic casing. Additionally, it made sure each cell was making contact with the bus bar using a wave compression spring with the cross section of an 8AWG wire. The amperage output was much higher than anything the motors could pull and it allowed me to insert plastic barriers keeping bus bars from touching each other while assembling a live pack. Essentially, what you see here is a safer, more reliable pack than my previous proposal. Around the same time, I also decided to use a brushless motor system. This decision came after realizing that the poor efficiency of the CIM motors would negatively impact my range, and might cause some overheating issues. Luckily I found a lovely 7000rpm outrunner on Hobbyking and a tiny 48V motor controller from Kelly that had regen. What a nice surprise! I wasn't expecting to get regen on this vehicle.

However, upon reflection, I decided that even this was as useable as it should be. For one, the whole device looks tall and awkward. While looks aren't my first priority, I do understand that getting someone to like something involves getting them to like how it looks, and I don't want people dismissing my designs based on their ugliness. Still, these are quite tall, with small wheels. So I tried a lower longer one with larger 12" wheels.

Wow, what a looker. Unfortunately, not very usable. Even without all of the hardware in place, this model was showing a weight of 40lbs. Maybe that would work as a single board, but 80lbs strapped to your feet? I don't think so. At this point I was at an impasse. How do you get this thing to be small enough to be practical and keep all of the things that I wanted? I stubbornly said the dual motors per side weren't going anywhere and the suspension was also still a requirement. The only thing left to do was to remove the battery pack and allocate it space within a backpack. I would only feel comfortable doing this with an LiFePO4 lithium cell as I really have no interest in strapping a ticking timebomb onto my back.

With this in mind, I began my final iteraction cycle. Versions 5 and 6 are very similar to each other. I'd like to get into some detail on how they differ and why, but maybe I'll save that for a dedicated suspension post.