Subject:
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Re: Lego AT-AT (secrets revealed)
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Newsgroups:
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lugnet.starwars
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Date:
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Fri, 8 Dec 2000 15:06:03 GMT
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Viewed:
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1083 times
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In lugnet.starwars, David Eaton writes:
> Well, I've gotta come in and clarify. I don't think it's possible to achieve
> a walking model made of Lego parts (not counting future miracle parts) that
> walks as an AT-AT is "supposed" to walk (using all the joints as shown in
> the movie, not necessarily timed the same, but probably not that far off
> from it), while at the same time being both to minifig scale and also having
> a believeable minifig-scale interior.
Well - okay - can't argue with that. But the question remains: SO? I mean,
we've yet to see the interior of an AT-AT in any film (apart from the
cockpit footage in ESB) - and not even George Lucas came up with an actual
walking one; they're all stop-motion animation, and I doubt the movie models
have much of an interior. Is that seriously what you were expecting? How
about the pros and cons of fitting a working Repulsor-Lift in a minifig
Landspeeder then?
> Here's why.
> Problem 1. Leverage. When in minifig scale, the body is HEAVY...
Well, you wrap strings around a large solid drum for the leg. Remember it
only has to bear the weight of the body in a fixed position. Once it's
ready to move, you only have to lift the leg, not the whole body. I'm not
saying you could do it in that space, but it's not impossible.
I don't know how reliable the newer pneumatics are, but that's probably not
worth even trying. You'd need some pretty solid bell cranks linked to the leg.
> Problem 2. Inconsistant rate of speed. At least one of the joints in the
> model needs to work at a varying speed through the walking cycle...
For the accurate walk, you need to move one leg at a time. This means that
each foot steps forward for 1/4 of its cycle, then sweeps back (at 1/3 the
speed) for the other 3/4 of its cycle (as the other 3 feet take a step).
This is how large quadropeds (elephants, hippos) move around slowly, and is
incredibly hard to do mechanically from a single drive. Mine moves two legs
at once - the Mk 2 goes side-to-side, like an elephant's brisk walk. The
new one does diagonal pairs, like a wider animal (I think - anyone got
footage of a hippo running?). In both cases, the rear leg lags the forward
one by a precise amount.
> Problem 3. Space. Well, ok, I've already mentioned it, but the space
> requirement is really tough. Any mechanism to control the movement of the
> lower hip or knee joint has to go THROUGH the upper hip section...
Well, use more bits of string down the leg.
> ...so assuming it WAS possible, you can't fit the mechanism into the
> AT-AT and still leave room for a believeable minifig-scale interior.
Hang on - no, don't tell me, let me guess - does your model have a
detailed interior... by any chance? ':-)
> Anyway, admittedly, my constraints are rather silly.
I'm not going to touch that one with a 10m cattle prod...
> And as is probably obviated, I'm valuing accuracy to the "real thing"
> above all else. After all, I am a modeller :)
Well, first thing I noticed about the ones in the movie was that
they walk. Second was that you don't get to see inside the body
anyway.
> > Well, I've
> > always started with the walking mechanism and built the detail on to that...
> > http://www.joefish.cwc.net/bricks/scifi/atat/
>
> Very cool!
Glad you liked it.
> Actually, the walking mechanism (especially the '2nd' AT-AT
> model) looks remarkably similar to the one used in the 8890 technic idea
> book, although the brace is placed horizontally rather than vertically, if I
> recall correctly.
Don't have this... checking Brickshelf...
Eh? It's nothing like it! Look a bit closer - that one can't even raise
or lower its legs. They're pinned to the body at the top. It must rely on
rocking about to raise a leg clear of the ground. If anything, mine's like the
wobbling dog from the very first 8888 ideas book, but with levers instead of
sliding pivots for the legs.
> Any more insight into how the knees on the '3rd' AT-AT
> model were done? You say it's got "two cranks and spring-controlled knees
> and ankles"... how does that work exactly?
Well, the idea of mine is to extend the upper leg up into the body (where
you don't see it), and control it from there. Then the only job of the
thigh beam is as a pivot point for the main leg. It's not taking the load
in the same way as the AT-AT leg is supposed to, but with the thigh beam
moving up and down and the top of the upper leg hidden in the body, it
looks awfully like the real thing.
The top of the leg is cranked round in a circle, and with the pivot point
only moving up and down it means that the bottom end of the leg moves
round in a longer ellipse (or a sort of fat banana-shape actually).
The upper cranks go round at twice the speed,
raising the whole mechanism as each leg passes directly underneath. This
lowers the body slightly as the leg is at its maximum extent, and also
raises the leg on the far side as it goes forward.
If you look closely at the pictures, you can see a 1x8 technic beam down the
inside of the rear-left knee. The lower leg pivots on the end of this. Just
in front, a suspension strut (look really closely) keeps the two halves of
the leg apart, so the knee is normally slightly bent. As weight is applied,
this gap closes and the leg straightens. As the weight comes off, the
spring extends and the knee bends again. The pivot point is actually
a black peg, to damp the response a bit.
The ankles are pivoted in the middle, with a suspension strut fitted to
the back. This keeps the foot tilted front-up, which stops the AT-AT
dragging its toes when the foot is lifted and brought forward (a big
problem on rough terrain... like carpets). As soon as weight goes
on, the foot levels out.
> ... when one leg is lifted, the weight has to be shifted off of this
> leg. And the easiest way to do that of course is to simply shift to the
> weight to the opposite leg, and potentially even shorten the opposing leg
> (bend the knee more or something) on that stroke so the body actually leans
> away from the raised leg...
Doesn't work like that. Think about it - what you've just described
actually entails lifting two legs at once. Instead, you have to position
three of the legs so that they balance on their own, then you can raise
the redundant one.
> Cool! Minifig scale is roughly 20 & 2/3 inches high, although 14 inches
> would be accurate to the often reported "50 feet" height of the AT-AT.
Yeah, well, I don't buy that for a minute. Mine's small. I can live
with that. What do you use as minifig scale anyway? And I don't mean
in studs...
And as for '60 km/h over level terrain' - that means all four
legs would have to move in a complete cycle every second! Right...
> Can't wait to see it! Walking mechanisms have always fascinated me... I have
> to say, though, that the Dark Side Developer's mechanism (though very cool)
> is kinda silly looking... It'd be interesting to be able to build your
> design and see how you did certain things...
>
> DaveE
Well, it's interesting to see that the DSDK AT-AT is the first walker set
LEGO have done, despite putting one in most of the ideas books (see
Brickshelf, 8888-8891). The dinosaur looks the neatest one. Note that
it only raises the rear legs - the front legs just swing, and it relies on
a rocking motion to raise them clear of the ground. Not one to run across
a thick carpet. A lot of clockwork animal toys use a similar method,
but it's no good if you're carrying passengers. And the one in 8890 was
just a desparate attempt to sell more of those dire Technic figure sets.
Jason Railton
http://www.joefish.cwc.net/
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Message has 1 Reply:
 | | Re: Lego AT-AT (secrets revealed)
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| (...) Um.. I'm not really arguing that an AT-AT not done to my exacting specifications wouldn't be cool or neat or anything, I was just clarifying my position. Actually, throughout this message it seems I've offended you or something, or perhaps you (...) (24 years ago, 8-Dec-00, to lugnet.starwars)
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Message is in Reply To:
| | Re: Lego AT-AT
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| (...) Well, I've gotta come in and clarify. I don't think it's possible to achieve a walking model made of Lego parts (not counting future miracle parts) that walks as an AT-AT is "supposed" to walk (using all the joints as shown in the movie, not (...) (24 years ago, 7-Dec-00, to lugnet.starwars)
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