a 200% version of Fluffbug robot

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The parts available to us sometimes drive what we build. When I found some 2g sub-micro servos in my drawer, I decided that my #Fluffbug robot needs a 50% version, and so #Wee Bug was created. Now I found some beefy MG886R servos, so it's time for a 200% (not quite) version.

  • Edward Knifelegs

    deʃhipu07/04/2023 at 22:29 0 comments

    The 2.5mm aluminium sheet arrived, and I spent a weekend sawing it with a hacksaw and drilling by hand and with an electric drill.

    If you plan on doing anything similar yourself, I have two tips for you. Get the sheet attached to something heavy securely, a good vise or clamps can speed up your sawing greatly, and don't arrange the parts the way I did in there, interlocking, without a straight sawing line separating them. It's much easier if you can separate the elements with straight cuts first.

    Then some drilling and a lot of filing, and we have workable legs. Note that it vaguely resembles a knife, mostly because it's made of shiny metal. But all the edges are rounded, and the tip is not sharp either.

    Finished robot. Yes, it does look like hacked by hand, and it would look much more "finished" if I used a cnc machine or some kind of other cutting service. But I happen to be happy with that looks, and I don't plan on making more than this one prototype. I added some RGB LEDs to act as eyes, but they are not connected to anything right now. If I ever replace the wooden body with a custom PCB, I will probably include footprints for those LEDs and connect them to spare pins, but for now I haven't decided if I will ever go for that.

    Finally, a family photo of all three bug robots: #Wee Bug, #Fluffbug and Bigbug all together.

  • Material Alternatives

    deʃhipu06/02/2023 at 00:57 0 comments

    After building and testing the robot, I have some changes to consider.

    First of all, the plywood I used is not great for the legs. Surprisingly, it's not the weight of the robot resting on them that is the issue, but the fact that the parts twist, especially when the robot tilts from side to side to unload its leg before raising it, and the whole thing becomes rather wobbly.

    Now, what are the alternatives?

    I could go with stronger plywood (the one I used was sold together with my scrollsaw, and is rather soft), and perhaps laser-cut it this time. Or I could go with laser-cut acrylic like in Fluffbug, it should still be stronger than this.

    Or I could really go all the way, and use 2.5mm aluminium sheet. It's soft enough to cut by hand with a hacksaw, and should give me plenty of strength. I'm tempted to do it, especially if I can cut it at the hackerspace and avoid the mess at home.

    In either case, I think I will also need to make the legs a little shorter. I was optimistic and really scaled them 200%, but that makes the robot look really tall and narrow, and doesn't help with stability. Shorter legs should make it easier to fine tune the gait, no matter what they are made of, and this is not a race, so I don't really worry about the speed.

  • Initial Decisions

    deʃhipu06/02/2023 at 00:47 0 comments

    Building a smaller robot is easy, as everything is cheaper, relatively stronger, and the only worry is fitting everything together in the smaller package. Building a bigger robot is a challenge. Everything is more expensive and costs more to ship, takes more power and has smaller margins for strength. And inertia rears up its ugly head. So we need to plan carefully.

    Most decisions will be driven by the choice of the servos. The MG996R are almost twice as big as the SG92R I used for Fluffbug, so we will scale the whole thing proportionally. The 23mm width of the SG92R becomes 40mm width of the MG996R, and everything else grows with it.

    I'm not going to use a PCB for the body, it would cost too much, and I'm not sure at this point how big it would need to be to fit all the components, so instead it will be a piece of plywood, cut by hand to fit the servo size. I'm going to use the same hand-cut plywood for the leg parts, at least initially, because getting them laser-cut in China and shipped here would be a bit expensive, and I don't have enough social skill to ask for lasercutter access at my hackerspace.

    For the brains, I think I will go with another Pi Pico clone, because it's easy to add servo connectors to it using just prototyping board – I'm not making a dedicated PCB, at least not just yet.

    The power needs a substantial upgrade. I think I can no longer use a single 1S lipo cell. I will go with two 18650 cells, with built-in protection. That means there isn't going to be a charging circuit or an on-board protection circuit, which simplifies things a bit. On the other hand, I will need to get that 8V to the cozy 5-6V for the servos and electronics, so I will need a buck converter in there, and one that can handle considerable currents. I have some no-name UBEC modules in my drawer, so I'm going to try these, and if that doesn't work, a 9A buck converter is on order.

    That's pretty much all I should need to get the basic walking going. Once I got that, I will see about maybe putting a Linux single board computer on it, or other things.

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