View Full Version : Can tripods effectively walk?
09-28-2003, 03:52 PM
In the book War of the Worlds by H. G. Wells, as well as in some other old sci-fi classics, tripods walk the Earth terrorizing mankind.
How would they do that? And presuming that they could walk, how would you explain to me how the system works?
09-28-2003, 04:08 PM
I can't see why they couldn't. We have only two legs, and we can walk fine. Four-footed animals also have no trouble ambulating. I don't see why a three-legged machine or creature would have any dificulty. I would assume it would move one leg at a time while keeping the other two on the ground.
09-28-2003, 04:10 PM
Yes, In one version of the scanning device in a Scanning Tunneling Microscope is a tripod that walks.
The legs are piezoelectric crystals. One leg is first held in fixed position by a vacuum between the tip and the surface. A voltage is applied to the other two legs so as to lengthen them which moves their ends a smidgen away from the fixed leg. Then the two lengthened ones are held oown by vacuum and the first one is released and lengthened by the same amount. The voltage is then removed from all three legs returning them to their original length with the scanner slightly moved from its original location. Etc.
09-28-2003, 04:13 PM
I'll get back to you with an answer on this if I'm ever cut off at the knees.
09-28-2003, 05:46 PM
If so, expect to see a lot of very surprised photographers.:D
09-28-2003, 06:59 PM
Three-legged dogs are moderately common, and I've seen a three-legged horse, and a couple of tri-ped cats, too. I'll assure you: from these examples, and given a sufficiently sohpisticated processor and articulation; not only can a tripod walk, but they can run, too.
09-28-2003, 07:06 PM
A photo of a three legged creature (http://www.rolfharrissongs.pwp.blueyonder.co.uk/pictures.htm) is on this page.
This one seems to possess effective mobility. IIRC the central leg goes first followed by the outside two, thus avoiding excessive roll in the gait.
09-28-2003, 07:46 PM
Niven's "Pierson's Puppeteers" were 3 legged - two front legs and one central hind leg. They could move with a 2-1-2-1 gait. This is effectively what an amputee quadraped does when running, except the single leg isn't centrally placed.
If you want a more "artistic" gait, moving only 1 leg at a time, how about this:
repeat, in mirror image, moving foot 1 instead of 2. Somewhat mincing, and the third frame leaves the feet rather close together, but there's probably a lot of variants of this sort of thing.
The creature could move two legs at a time, pivoting on alternate legs.
You could have the legs move in a completely symettric fashion, coming to front in sucession, and allow the creature or machine to have some provision for rotating around as it walks. Maybe it has a bearing in the middle. Maybe it has eyes all around.
09-28-2003, 08:00 PM
Didn't triffids have three legs?
09-28-2003, 08:14 PM
In Rendezvous With Rama, the creatures (essentially organic robots) "spin," using one leg as a pivot, rotating until the second leg is in line with the destination, then plant the second leg, pivoting on that one now, and so on. Think like you're rolling a barrel on it's rim. While weird-seeming to our minds, this should work quite well, at least for fairly small creatures.
09-28-2003, 08:19 PM
I thought this was going to be a Man with a Movie Camera question...
09-28-2003, 08:20 PM
So, my Gitzo wasn't stolen.
It left on its own 3 feet.
09-28-2003, 08:24 PM
I'm of the opinion that tripods might even smoother, steadier walkers than we bipeds.
Bipedal locomotion looks like a series of falling forward and catching yourself with the free leg just before you lose balance.
Of course, someone with real credentials will be along shortly to to correct my post. :)
Measure for Measure
09-28-2003, 08:31 PM
One could speculate that some of the dinosaurs were tripods: while at rest, they would be supported by two legs plus a tail. When moving, their tail would be lifted and they would revert to a two-legged system which is "dynamically stable" but not "statically stable" in the jargon of this page on robotics:
Birds (and squirrels for that matter) certainly use their tail as a counter-balance: I'm not sure whether they use them for support.
09-28-2003, 08:37 PM
I can't think of any bird that uses it's tail for support, and squirrels certainly don't. flowbark, you'd do better to use kangaroos as an example, which do sometimes use their tail as support.
Measure for Measure
09-28-2003, 08:40 PM
No Clue Boy:
I don't have any real credentials, but my link notes that "statically stable walking", while very safe, is also very energy inefficient.
09-28-2003, 08:56 PM
Yeah! I just noticed that. Sometimes intuition works, huh?
In order to remain stable, the robot's center of gravity (COG) must fall under its polygon of support. This polygon is basically the projection between all of its support points onto the surface. So in a two-legged robot, the polygon is really a line, and the COG cannot be stably aligned with a point on that line to keep the robot upright.
Balancing a biped, static or moving, requires constant control. For robotics, this site suggests the more legs, the better.
09-29-2003, 02:11 AM
Originally posted by Q.E.D.
I can't see why they couldn't. We have only two legs, and we can walk fine. Four-footed animals also have no trouble ambulating. I don't see why a three-legged machine or creature would have any dificulty. I would assume it would move one leg at a time while keeping the other two on the ground. Actually, some four-legged animals, horses specifically, sometimes have all four feet off the ground at the same time when moving fast (galloping). (This was proven in one of the first applications of photography, many years ago. I believe it was done with a series of cameras along a racetrack, with threads stretched across the track to trigger each camera.)
So we needn't assume that a three-legged animal would move only 1 leg at a time. Probably would do so when walking, but when jogging or running, it's more likely to move several legs at once. At least, that's likely projecting from experience with animals here on earth.
09-29-2003, 02:17 AM
Damn you all for making me check with google that John Christopher had already covered this. I expect a better level of geekery hereabouts.
The Rolf Harris bit was up to par, though.
Carry on. (Yes, it's an invocation of what Sid James would do with the idea of a tripod).
09-29-2003, 07:38 AM
Originally posted by 2trew
The Rolf Harris bit was up to par, though.
Rolf may well be performing 'Jake the Peg' at this very moment in celebration of 50 years on TV (http://www.itv.com/news/100366.html).
..and some song about kangaroos, which ties in (down) with tripod-ambulation.
09-29-2003, 12:58 PM
The martians in Robert Heinlein's Red Planet also "spin" like Clake's Ramans, IIRC. (presumably the young martians in Stranger in a Strange Land do, too, since they're supposed to be set in essentially the same universe).
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