Re: [geodesic 00036] Re: Simple Machines and Tensegrity
Bob Burkhardt
The definition doesn't say anything about purely compressive members,
just purely tensile ones; and the spokes are all purely tensile in their
function as I see them.
The nonredundancy-in-tensegrity quote Phil pointed out in 700.03 (there
is another at 720.10), has certainly been food for thought. I don't
even know that I can say that the beanbag has redundant components as
part of its concept (though a realization may have more or less
reinforcement): there's nothing that can be left out without spilling
the beans. So maybe subconsciously I have been enforcing this
criterion. Anytime anyone moves to tensegrity construction they are
obviously straining to get the most with the least, so it would be
strange to see redundancy, and it seems putting nonredundancy in the
definition would be redundant. Well perhaps some wayward tensegrity
inventor will stumble on us with an example, and then I can rush to the
definition to add nonredundancy to fend off the offending invention.
Bob
On 25 Sep 2008 15:56:55 -0000 "Doug Milliken" <bd...@bfn.org> writes:
> Hi Bob,
>
> Thanks for chiming in. It's been a few years since I looked at your
> nicely detailed Definition and Classification page.
>
> I'd argue gently that a normal bicycle wheel isn't a
> tensegrity--when the wheel is loaded, the bit of rim at the bottom
> straightens out (bends, deflects) enough to lower the
> tension/preload in the lower spokes. So the rim isn't in pure
> compression. Things might be different if the rim was made of many
> short, straight chords, with a node at every spoke? But then it
> wouldn't be round (a very bumpy wheel!)
>
> I remember the discussion some years back about a minimum-spoke bike
> wheel, but since these still have a curved rim (which will bend when
> loaded) I guess I'd still say they are not.
>
> -- Doug
>
>
> On Wed, 24 Sep 2008 12:59:10 -0400 Robert W Burkhardt
> <bo...@juno.com> wrote:
> > For low-frequency tensegrities, removing one member can have a
> very
> > significant effect, but as the frequency goes up, or the number
> of
> > members go up, removing one of them has less and less effect, at
> least
> > for domes and spheres. I have thought tensegrity might be suited
> for
> > designing suspensions, like in the chassis of a car, and perhaps
> they can
> > be thought of as chassis for living machines etc. Wasn't that
> what
> > Buckminster Fuller called his house, a living machine? It was our
> good
> > fortune that he helped get mechanical engineers interested in
> structures.
> >
> >
> > The only structure in my tensegrity menagerie at
> > http://bobwb.tripod.com/synergetics/tensegrity/index.html which
> seems to
> > exhibit redundancy is the bean bag. I am trying to pursue my
> definition
> > from Fuller's point of view, at least where he seems rational,
> and
> > including non-redundancy (no elements in reserve) would be a
> small
> > concession I think. I don't see a particular need for it except
> Fuller
> > likes the idea, and after all he did invent the word. For myself
> I would
> > prefer to just put redundancy into the list of parameters which
> > differentiate tensegrities, but perhaps it belongs in the list
> that
> > determines the boundaries of the definition -- a bean bag is not
> a
> > tensegrity until it is stuffed so full that all the fibers of the
> > container are tensed and it becomes comparable to a balloon. It
> is
> > something to seriously consider. Like that other boundary
> structure, the
> > deresonated dome.
> >
> > Incorporating tensegrity into robots would help make them more
> resilient
> > and perhaps less lethal in an accident. It would make design
> trickier,
> > but designers seem like they must be ready for that transition.
> It's the
> > next step. That seems where Tristan Sterk is headed and the
> dynamic
> > tensegrity bunch.
> >
> > Patrick, how about linking us into bit.listserv.geodesic? I think
> mostly
> > we would contribute to it rather than the other way around. The
> last two
> > messages have the subject "Chinese antique". Could be relevant I
> > suppose.
> >
> > Bob
> >
> > Phil Earnhardt wrote:
> >
> >
> > On Sep 22, 2008, at 6:11 AM, Doug Milliken wrote:
> >
> >
> > On Sun, 21 Sep 2008 21:43:43 -0600 Phil Earnhardt
> <ph...@FloatingBones.com
> > > wrote:
> >
> >
> >
> > A tensegrity icosahedron is decidedly nonredundant.
> >
> > On the other hand, our body's structural system has a marvelous
> amount
> > of redundancy.
> >
> >
> > .....
> >
> >
> > Your comment about redundancy is a bit of a non-sequitur to our
> > discussion on Simple Machines. Redundancy is not a criteria for
> any of
> > the six simple machines. Were you going to comment about anything
> else
> > I said in the last message?
> >
> >
> >
> > I was actually thinking that the lack of redundancy might be
> > incorporated into a definition of tensegrity.
> >
> >
> >
> > Fuller mentions the nonredundancy in 700.03 of Synergetics.
> >
> > There's an interesting question here: could a mechanical network
> of
> > tensegrity structures be designed so that the network was
> redundant? On
> > the failure of any part, could the network be re-balanced solely
> by
> > altering the tension on the elements in the individual [remaining]
>
> > tensegrity structures? I'd love to hear what Bob has to say about
> that.
> >
> >
> > You keep going on about biological systems, which are obviously
> your
> > specialty.
> >
> >
> >
> > Yes. My biggest reason for contemplating simple machines was to
> find a
> > question that physicists and mechanical engineers could explore
> without
> > getting bogged down in the details of biological systems.
> >
> > I also think that structural specialists can get up to speed
> rapidly by
> > reading Myers's "Anatomy Trains" (16-page summary at
> > http://tinyurl.com/at-summary ), Levin's Papers and DVD (at
> > www.biotensegrity.com ), and working hands-on with Flemon's
> models (at
> > http://tinyurl.com/toms-models ). If you get a model from Tom, I
> > recommend the double-tensioned pelvis model. Anyone who has
> worked
> > through parts of Synergetics knows how hard it is to do that
> without
> > having 3-dimensional models in your hands.
> >
> >
> > These have properties that are not normally found in man-made
> > structures, such as the ability to self-repair.
> >
> >
> >
> > Besides the ability to self-repair, biological systems also have
> the
> > ability to patch their structure on the fly to keep moving when
> they
> > must.
> >
> > I believe that roboticists would benefit from using tensegrity in
> their
> > design. As far as I've seen, Robots today are all hookean. Non-
> hookean
> > designs have two decided advantages:
> >
> > 1. The inherent damping of non-hookean systems lowers
> wear-and-tear.
> > 2. Motion in non-hookean systems is far more energy-efficient.
> >
> > After robots are designed with flowing and resilient movement,
> what will
> > "robot" break dancers call their dancing?
> >
> > --phil
> > ____________________________________________________________
> > Click to make millions by owning your own franchise.
> >
>
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> >
> >
>
>
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--
Bob Burkhardt
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Bob Burkhardt
hub and the ground, but due to the bias, the spokes never go out of
tension, although I imagine, as you say, it decreases for the ones
closest to the part of the rim in contact with the ground. As I
understand what you are saying, the spokes are still always in tension,
even though the level of tension changes momentarily at times -- purely
tensile -- tensegrity to my mind. I am just classifying the wheel when
I call it tensegrity. It does indeed function as a compression
component as far as keeping the bicycle off the ground. Many
tensegrities can do this, that is function as compression components in
larger systems; and if the prestress bias is enough, none of the tension
members will go slack. I feel as if I am talking in circles as well.
Bob
On 26 Sep 2008 15:25:02 -0000 "Doug Milliken" <bd...@bfn.org> writes:
> Hi Bob and All,
>
> Full circle on bike wheels<sorry for the pun>. This is based on the
> work of Jobst Brandt (first place I saw it was his book, "The
> Bicycle Wheel", available at better bike shops). The deflections in
> a bike wheel (when it is loaded) are the same as in a rigid-center
> wheel. As noted earlier, this can be shown by experiment
> (science!), only the bottom spokes change tension under load. The
> idea that there is some kind of self-adjusting tension-field or that
> the hub hangs from the top spokes is not true in reality (except
> perhaps in a very slack wheel, which isn't going to last very
> long).
>
> I'll try again, hopefully with different words. Normally, an
> unloaded wire/spoke can't take much of any compression, it buckles
> under load--we all agree on this.
>
> In a bike wheel the spokes are tensioned (preloaded, strained)
> against each other which effectively offsets their "working-point"
> from zero load or stress. Now, with a nicely tight wheel (all
> spokes give a good tone when plucked), the spokes can be pushed on
> until they go slack, so effectively they can carry a compression
> load over a limited range (until the preload is used up), in the
> context of the whole wheel.
>
> This is roughly analogous to the concept of a bias (offset) voltage
> in analog electronics. Some electronic devices (vacuum tubes for
> one) can only operate with positive (or negative?) voltage relative
> to earth/ground/zero-voltage -- imagine that this makes the vacuum
> tube a tension-only member. In some cases it is desirable to have
> the ability to work both above and below ground (alternating
> current). The solution is to take the initial AC signal, add a DC
> offset voltage to it (the bias) and then process this signal, which
> now does not cross zero voltage. Then after the processing (ie,
> amplification) is complete, a final stage (maybe a large blocking
> capacitor) removes the DC bias. With the correct circuit design the
> tube can be used to provide AC output, even though it can't do this
> by itself.
>
> Hope this helps.
>
> I think I'm redundant to the redundancy discussion for
> now...<grin>.
>
> -- Doug
>
> On Fri, 26 Sep 2008 07:13:03 -0400 Robert W Burkhardt
Bob Burkhardt
The Geiger-Levy style of cable dome that is used to supply roofs for
arenas, and Fuller's aspension constructions, would be examples of this
sort of tensegrity where an arena rim or some other peripheral support
may take the place of the bike rim. Certainly the developers of these
latter sorts of structures had no hesitation about calling them
tensegrity, and I think they were justified, although they are obviously
of a different sort than Snelson's autonomous floating-compression
creations.
The difference in how stress is distributed in response to external
loads is a good point. Even floating compression domes (in this case
not autonomous but rather hemispheres bound to the ground) don't always
distribute external loads evenly as with the example of the snow plow
that ran into one and bent a strut on the other side of the sphere from
the plow. Not exactly how a bicycle wheel behaves, but not evenly
distributed either.
Bob
On 29 Sep 2008 16:56:03 -0000 "Doug Milliken" <bd...@bfn.org> writes:
> Bob,
> Glad to see that we agree on how a pretensioned bike wheel works, I
> like your wording too.
> The simple tensegrities that I've played with seem to distribute any
> load throughout all the members (compression and tension)--I guess
> this was my original reason for not putting the bicycle wheel in the
> same category...but enough is enough.
> Best wishes,
> -- Doug
>
> On Mon, 29 Sep 2008 08:25:26 -0400 Robert W Burkhardt