Thursday, August 7, 2014

Quadruped Robot: The Beginnings

Recently, I stumbled upon a lot of quadruped/hexapod projects online, and I thought making one would be a neat project to work on. I decided to go with a quadruped both because it would be cheaper (fewer servos) and seemed slightly more challenging in terms of walking gaits.

The first step was to get servos. Turns out, servos can be pretty expensive, especially when you need 12 of them! I was initially going to use those small 9g servos (cheap at ~$3 each from ebay); however a lot of the projects that used those servos seemed to have issues with jittering and seemed a bit too toy-like. I ended up getting the standard sized HK15138s from HobbyKing ($3.50 each). While their torque isn't great, I'm hoping that they should be good enough for my needs.

For the microcontroller, I am using the Teensy 3.0 (I got it a while back, but really used it for anything). Since the servos are rated at 4.8-6V, I decided to use 4 NiMH batteries instead of a dual cell LiPo so I wouldn't have to deal with voltage regulation. I also got a cheap bluetooth module from ebay. Using the module turned out to be ridiculously straightforward, since it just uses a serial interface (compared to the headaches with the nRF24L01...). Throwing everything together along with headers for servo pins on a perf board:

Turns out, the JR-style servo connectors are just slightly wider than 0.1", so I could only put them in groups of 4

For the frame, I decided to go with laser-cut acrylic parts. Since I wanted everything to be relatively solid, I chose a simple design where each joint is supported at two points, not just the servo horn. Essentially, the frame consists of a main body part, a bunch of servo brackets, and leg pieces. I created the parts in SketchUp, which turned out to be pretty intuitive (I no longer have access to Caltech's SolidWorks license).

Since I don't have access to a laser-cutter, I ordered the parts from Ponoko on a 384mm x 384.0mm sheet.
The order was $55 for 3-day making (up to 8 days otherwise) and 2-day priority mail; a little pricey, but not completely unreasonable. I received the parts exactly 1 week later.

That familiar burnt plastic smell though...

Assembling everything was straightforward, although pretty time-consuming (took me 4-5 hours total). I used super glue for bonding everything. I wasn't super careful, so there are a lot of places where I discolored the acrylic from smearing the glue, but I'm okay with the overall result.

First leg complete! Note the janky use of eyelets as spacers - I will fix this once I have something that can cut machine screws easily...

Everything done!
I still have to route the wiring more neatly and mount the electronics / battery pack (probably should have put more mounting holes on the frame), but the hardware is otherwise mostly done! I will start working on the software soon; I plan to start with implementing IK and getting the robot to stand and shift in place. Really hoping the torque of the servos is enough!


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