28 Oct 2006 tbenedict   » (Master)

I'm curious about something for most of the "bug style" walking robots. (This question really doesn't apply to the RoboOne style biped robots.) Almost all of the bug-style walkers have their legs articulated the same way: The entire leg pivots around a vertical rotary axis at the hip joint. Also on the hip joint is a horizontal rotary axis that lets the entire leg move up and down. Past this there's typically at least one other knee joint, also usually oriented horizontally.

Earlier when someone posted about the Boston Dynamics "Big Dog" robot, someone made a comment about how odd the leg joint arrangement was. It's odd for a robot, but not for a dog. The primary joint is a horizontal rotary axis, oriented parallel to the length of the body. This lets the leg swing in closer to the body or out, farther away. Mounted on this is another rotary axis that pivots with the rest of the leg. It's neither always vertical or always horizontal, but with the leg directly under the robot it's mostly a horizontal rotary axis that lets the leg swing forward and back.

It's this hip design (a very good analog to what you see on a cat or a dog) that lets the Boston Dynamics robot have such nimble feet and respond so well to a disturbing force (e.g. a swift kick to the ribs, as you can see on the videos on their web site.)

The only other place I've seen that close an analog between nature and robot was the study done at MIT on the mechanics of cockroach leg joints. It's not nearly as obvious as one might think, and bears little to no resemblance to the hexapod arrangements available from most robotic supply houses. Among other things, the hip joints more closely resemble those of BigDog.

So I'm curious about two things: One, why haven't more people tried this hip orientation? For accelerating from a dead-stop, it places more of the loads along the leg rather than across it (much easier on the joints!)

Second, and more important, did I just ask the "but why?" question that's going to get me out of mini-sumo and into my next phase of exploration in robotics?

In a way I hope the answer to the second question is no. I love building mini-sumo robots. In another way I hope it's yes. Walking robots pose REALLY neat questions I'd like to play with.

I guess I really hope the answer is "both"! It means I get to make walking mini-sumo robots! Though hitting the 500g limit with three or more servos per leg is pushing it...


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