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Prototype Version 5 Wins Gold at Regional Science Fair!

Following the end of the construction of the fifth prototype electric skateboard, I decided to enter my project into the Greater Vancouver Regional Science Fair. The experience was absolutely astonishing! Not only did I meet hard-working, dedicated students and their innovative projects, but I also met experts in the field of engineering, who offered creative criticism. And all of this couldn’t be possible without the committed judges and voulenteers, who took time out of their daily lives to set up and monitor this event. Way to go!

In order to make my project more scientific, I decided to run an experimental trial to see how the gear ratio effects the power consumption on the fourth prototype of the electric skateboard. I did this with several gear ratios and weights, and I used the results to improve the skateboard, by choosing the best gear ratio for my weight. I also determined the speed at which the skateboard has the maximum range, which turned out to be 19km at 9km/h. This is just an estimate; I still need to test that.

Bluetooth Logging System:

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I also decided to run an experimental trial on the maximum current and power consumption under different loads. Both of these trials were recorded with a custom-made, Bluetooth Logging Module that I designed, programmed and built. It uses a GPS to track location and velocity, a hall effect current sensor (for current), and the standard voltage divider/reference diode for voltage detection. More on the module in a further post.

Introducing Prototype Version 5:

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Prototype version 5 is the best, most complex prototype I have built so far. I had to learn how to work with carbon-fiber for this board, as I wanted to reduce the weight of the board even further. The deck has a foam-core, and I used wooden supports around the trucks. There were a couple of differences with the gear drive as well; I 3D printed two PLA spacers so that they could more easily hold the gear in place. Additionally, I used a 25 tooth gear that I modified. It used the least amount of power out of all the gear ratios tested.

The electronics side was a bit different as well. The largest change was the addition of a lithium-ion BMS board. I found one on eBay for about $15 for the 6 cell configuration I was using. The major advantage: safe and fast balance charging. I can charge from dead to full in under 60 minutes! I also picked up an inexpensive 24v power supply, which I modified to become a constant current/constant voltage source through the “33R Mod”. More on that in a later post.

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These were some of the major changes that I changed from the fourth prototype skateboard. Currently, I am working on alternative methods of controlling the board; everything from Wii-Nunchucks to Bluetooth Low Energy and Gamepad Controllers.

The development of this board, as well as the write-up and experiments that I did helped me win gold at the Greater Vancouver Regional Science Fair! I also recieved an award from the Canadian Institute of Energy, and I will be travelling to Montreal for the nationals in mid-May!

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Sealing the panel

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A good frame will keep the cells sturdy and should seal the cells in a waterproof material. Common sealing methods include EVA film, a glass face, silicon and epoxy based sealants. Each method has their pros and cons. For this panel I decided to use epoxy because it was easy and was a non-glass method of strengthening the wood. Also, I had a lot of it around from a project that my dad is currently working on.

The main disadvantage of using epoxy is that it yellows, and you never fully know when or how much it is going to yellow. The plan was to give the epoxy 3 days of drying time and then cover it with a clear UV protection coat.

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Applying silicone:

To make sure the cells are properly attached to the base, and do not buckle, silicone is applied in a thin layer underneath the cells. I used a squeegee to spread it inside the area of each cell. Applying the cells now, make sure to apply silicone generously to the two wire leads. Those leads will stick out of the surface a bit, but this is okay since the epoxy will cover them.

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Mixing the epoxy:

I used a marine grade, indoor (ironic), and most importantly, self leveling epoxy. It needs to be self leveling or it will leave air bubbles. Almost all epoxies need to be mixed with a 1:1 ratio (for two part epoxy). In my case I used 400mL or epoxy, so 200mL of hardener and 200mL of epoxy. I mixed the epoxy for aprox. 10 minutes and poured it thoroughly over the cells, starting with the cracks, ending with the surfaces.

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The pour:

The cables and cells are now attached to the back plate now. I leveled the panel out on our table by adjusting several wooden blocks. The leveling of the panel is critical, otherwise the epoxy will pour out one edge.

I ended up using about maybe 400ml of epoxy, I wasn’t quite sure how much is enough so I had to do several pours. During this time the use of a flame such as a propane torch is required. Heat raises the air bubbles to the surface where they pop, leaving a smooth surface.

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Building the frame

Wooden Frame

For this panel I decided to go with wood as my back-plate. Not only is wood cheap, it is surprisingly water-resistant if treated properly. The plan was to spray-paint the wood, apply a layer of epoxy, put the cells on top, then somehow attach wires and cover the surface with epoxy. While it was a nice idea maybe, I found a much better way that will be covered in my next post.

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Cutting the back-plate:

I cut a nice 4mm thick piece of pine (I think) to 61 cm by 40 cm. Then with my dad’s careful assistance and help; I cut a 2.5 mm strip to match the outside dimensions of the back-plate. Then I cut matching 45 degree slants into that strip. One of the strips was marginally shorter than the others so I shaved a bit off one side of the back-plate to compensate.

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Gluing the “margins”:

Now that everything is cut out, I needed to glue the margins on. Doing to ended up a slightly messy, yet solid result. I used wood glue for this, just so you know, and I applied it to the margins in a very very shallow layer. Pressing down with some books now, I let the glue dry for several hours.

In the pictures above you can see that I have put the cells on the unfinished base just for looks really. I also learned that the best way to transfer these cells is by flipping and flipping requires the strings to be taped together.

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Painting:

Painting is simple, just spray an overlapping coat from ~8 inches away, flip and repeat. After the paint dries I put the cells back on and drilled two holes, 1 mm apart between the two top tabs. I made a slight mistake of not pushing extra delicately and ended up with a few splinters near the holes.

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