I designed and built a quadruped robot dog from scratch this summer for a “Mech Warfare” airsoft battlebots competition. I’ve always wanted to build something with legs, and this seemed like a great opportunity to push the boundaries on my mechanical design skills. This blog covers my design process, stress testing, and final robot that I built :)
The competition
The rules of Mech Warefare are pretty simple:
No wheels, legs only (for the vibes) Airsoft guns and target pressure plates for scoring You can’t see the robots during the competition. It all has to be FPV video streaming “No gimmicks” (learning of this rule immediately threw out most of my ideas)
After watching some videos of past competitions, I decided that my strategy would be to maximize speed so I could attack opponents from the side and back before they realized what was happening. (I also have future ambitions to build an auto-aim stable enough to fire while moving, but haven’t gotten to that yet).
Quadruped Design - Background research
Most contestants use older “spider” like designs, but for speed I wanted to use a more dynamic dog like design, similar to Boston Dynamic’s Spot and the Unitree Stellar Hunter. I started exploring the web for every example of robot leg geometry and known best practices that I could find.
The most common two designs I found were 1) 4-bar-linkages, and 2) parallel linkages. Both of these really just boil down to how to use two motors to place the “foot” at an arbitrary X,Y location.
Now wouldn’t it be simpler to just have one motor at the hip, and one motor at the knee?
That works, but motors are heavy, and moving one around at the knee creates a LOT of extra work for the hip motor. It’s much better to keep the motors as stationary as possible, and use a mechanical linkage to transfer the motion to the knee. I decided on the 4 bar linkage, as the simpler design (and because it’s what Unitree uses!)
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