Monday, August 15, 2011

Mini-rhino - we're driven

One of the interesting things about the Gakken Otona no Kagaku "beest" kits is that with the Strandebeest, which is normally wind-driven, one of the suggested mods is to add a Japanino-driven motor to make it remote-controlled. While with the Rhinoceros, which is man-powered, the Gakken version uses a fan and there's no suggestion in the attached mook for adding a motor. When I had my mini-Strandebeest, I tried adding a motor, but the movement of the legs was so stiff that the motor wouldn't spin and the wires overheated. So, I was a little hesitant to buy the mini-rhino. Fortunately, the legs are geared down so much that the fan propels it with just a moderate breeze. Naturally, the next step was to figure out how to automate it.

I was able to find a small electric fan for 100 yen ($1.20 USD), which runs on 2 AA batteries. After a little experimenting, I came up with a mounting for it that consists of 4 thin sheets of cardboard glued together. If I was still in Akihabara I'd just get a sheet of perf board, but the cardboard, at least, is free. Also at the 100 Yen shop, I got a small package of nuts and bolts, and some tie-wraps. I hand-bored holes in the body of the kit for mounting the cardboard panel, and just tie-wrapped the fan battery casing and the motor in place. Luckily, the little white gear on the squirrel cage fan spindle turns out to fit on the motor spindle perfectly. One reason for mounting the motor on the panel, instead of hot gluing everything to the kit body is that it lets me switch between the motor and the squirrel fan as I like.

Initially, I wanted to mount the panel so that it brought both arms of the kit together a bit for added structural integrity, but this caused the gear threads to pull apart from each other and disengage. Seems that the kit was designed for the arms to be pushed apart instead. But, at that point I'd already drilled the holes on one side of the panel too close to the edge and the corners tore off. If I were to do this again, I'd go with the perf board and just slot the two far-end holes to make adjusting the arm tension (and gear spacing) easier. Another issue was that even with the gearing ratio as it is, the motor ran so fast that the legs thrashed about extremely hard and threatened to self-destruct. Because I don't have access to a 1-battery holder, a soldering iron, or a potentiometer right now, I pulled out a good battery and replaced it with one that was nearly dead. Coupled with the extra workload associated with having to carry the motor, plastic case and batteries, the resulting walking speed is much more sedate and acceptable.

Link to Youtube video

In summary, I give myself a B for the final product. It make not look all that great, but it works as well as I could hope for. If I had my Japanino, I'd get a second motor, split the mini-rhino in two, and turn it into a proper self-sufficient programmable robot. Maybe another day.

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