Aaron DeVriesIn the 1957 Disney feature "Mars and Beyond" there is this little segment about media depictions of Mars including the war of the Worlds. The animation is silly but the martian fighting machine walk cycle is fun and interesting.
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swope@Aaron_DeVries
The walk cycle looks fine in 2D, but I'm not quite understanding it in 3D.
The walk cycle looks fine in 2D, but I'm not quite understanding it in 3D.
@swope in 3D it would require the hip section to rotate around in a strange way. It would look clumsy in 3D, it's just neat in 2D.
Isaac Ji Kuo@Aaron_DeVries @swope It is straightforward in 3D if you see the hub as rotating.
IIRC, Wells describes the motion of the tripods to be rotating, or maybe I'm just confusing with The Tripods.
Either way, a rotating hub makes it all work okay.
I think that implies the rotating hub has to have a lateral offset from the body above it (leaning to the side).
The center of mass of the body needs to stay above the line between the two feet on the ground. The lifted leg swings out to the side.
It's essentially wheeled locomotion (unicycle) approximated by 3 legs.
@swope @Aaron_DeVries It is not necessary for the line between the two feet on the ground to be constant.
If you look at the location of the feet when they plant onto the ground, you see them alternating left/right like a biped. But since there are 3 feet, each individual foot switches between left and right with each stride.
Anyway, this doesn't really make a big difference either way. In both cases, there's nothing stopping the feet from passing directly under the hub.
You do not need an
@swope @Aaron_DeVries offset, you just need some lean (or the equivalent of lean, in the form of bending the knees to emulate the lean).
But yeah, the feet pass (mostly) underneath on one side of the rotation, and swing around on the other side of the rotation.
@isaackuo
Alternating left/right foot placement is going to cause a lateral sinusoidal motion of the CoM, like a railroad worker walking with one foot on either side of a rail. That lateral acceleration costs energy.
I think the foot placement of the advancing leg is going to be like countersteering on a bicycle.
Lateral balance is statically unstable (2D inverted pendulum on the lateral-vertical plane, pivoting about the line between the two grounded feet).
@swope @Aaron_DeVries I dunno ... I feel like there are LOTS of bipedal creatures and almost all of them (including ourselves) walk with feet going left-right rather than directly in a line. I feel like this evidence that it's more efficient that way.
But in any case, I'm really talking about replicating what's in the animation, not what is most efficient.
When people walk slowly we have wider foot placements, but when we walk with more speed the foot placements are more narrow. I think it's a trade between stability and efficiency.
I did find this tripod robot that does a sort of coiling motion to unload the advancing foot:
https://youtu.be/ARMJr3zDDP8
I think the 2D animation looks fine, but it would be hard to replicate in a 3D animation without looking unrealistically out of balance.
[Tripedal Robot] Walking (Without feedback. three-legged robot) 삼족 보행 로봇: 단순 걷기 실험
@swope @Aaron_DeVries Hey, I think that motion could match the description of tilting and swaying!
It looks inefficient and ungainly, of course. That's basically the problem with some sort of symmetric 3 legged motion.
(As opposed to a gait with two outer legs moving together, with a central leg opposed.)
But it is possible to go fast and smooth and efficient with a rotating hub. I've actually thought seriously about this as used for a three legged AFV.
Like, imagine a Bradley-like hull with
@swope @Aaron_DeVries a revolving tripod underneath. But the feet are angled perpendicular to the legs, and they can counter-rotate.
It seems like the thing can't walk, just rotate in place, right? But if you tilt the whole thing left (or right), then it can sort of roll forward/back by using the stool as a tilted wheel.
It's weird, but it can potentially have some advantages over tracks or normal wheels.