Explaination of Actuated Tensegrity

Bodies behave as whole systems held in connective tension.

Tensegrities are special case structures where the play of these two forces is visible in the design

the elegance and power tensegrities have to describe and illustrate the behaviour of whole systems as fractals

“Tensegrity is a portmanteau of tensional integrity. It refers to the integrity of structures as being based in a synergy
between balanced tension and compression components.”2 Synergy refers to the observation made first by R. Buckminster Fuller that in any system the whole is always greater than the sum of its parts. The behavior of tensegrities is a visual demonstration of this.

How does Fuller define it? From Synergetics 700.011: “The word tensegrity is an invention: it is a contraction of tensional integrity. Tensegrity describes a structural relationship principle in which structural shape is guaranteed by the finitely closed, comprehensively continuous, tensional behaviors of the system and not by the discontinuous and exclusively local compressional member behaviors.”3 Easy for him to say… Fuller went on to design the
largest enclosed domes ever built utilizing tensegrity principles.The sculptor Kenneth Snelson who made the first tensegrity structures in 1948 called them something different. In an interview he said, “Tensegrity,

the word, has become so confusing through multiple uses that it calls any definition into question. This is the reason I've long advocated Floating Compression… (It) describes a closed structural system composed of a set of
three or more elongate compression struts within a network of tension tendons, the combined parts mutually supportive in such a way that the struts do not touch one another, but press outwardly against nodal points in the
tension network to form a firm, triangulated, prestressed, tension and compression unit. Why triangulated? The reason is that it's possible to build such a structure whose network is non-triangulated. Such structures are
flaccid and decidedly not firm.”4 In the art world Snelson is well known and his floating compression sculptures are found in galleries, private collections, and museums around the world. (http://www.kennethsnelson.net
/faqs/faq.html) A tensegrity requires at minimum three conditions to fit either Kenneth
Snelson's or Buckminster Fuller's definition.

1. A continuous connective tensioned network supports discontinuous compression struts. Snelson insists that struts must be free floating in a web of tension and not touching. A geodesic dome, which Fuller considers to be
tensegrities, has multiple compression struts meeting at central hubs but they are discontinuously connected, that is, they do not transfer compressive loads.
In these domes it is the tension forces that travel along the outer edges of the struts that are continuous. Similarly, if anatomical structures operate as tensegrities, then in most orientations the bones do not pass a direct load
across the joint– rather the tension members; ligaments, tendons, and fascia transfer loads and the bones float in this tension matrix.


2) All tensegrities are prestressed under tension; they are self–supporting and independent of gravity. But the weight of the structure also adds to the prestress. As you increase the weight load the tensegrity tightens and gets
smaller. The heavier the structure is, the greater the tension, and the less the range of motion. This presents real design problems when trying to model living systems that have and use joints with multiple degrees of freedom. My
models for example can emulate biologic movement because I use elastic tension nets that are taut enough to maintain the shape of the model yet have enough residual elasticity to move through a wide range of positions. When
the size and weight of a model increases, so does the prestress. It is always surprising to discover how high the tension levels climb when building large tensegrity structures. In some of Snelson’s largest sculptures (50’–100’) the
tensile cables carry thousands of pounds of force. To make human scale tensegrity models that articulate and are prestressed is not a trivial challenge.

3) Tensegrities are self–contained non–redundant whole systems. All components are dynamically linked such that forces are translated instantly everywhere; a change in one part is reflected throughout. These features
distinguish tensegrities from all other tension structures, e.g. a radio mast or a sailboat’s mast is fixed at the base and needs that fixed point to keep it upright. The boat does not need the mast for it’s integrity but the reverse is
not true. Every part in a tensegrity is reliant on the entire structure for its continued existence. In terms of living forms, a discontinuity in a structure marks the boundary or interface between separate tensegrities. Also,
molecules within cells within tissues within organs within bodies, and bodies within environments are all synergistically linked tensegrities in a hierarchical cascade from the smallest wholes to the largest.


it is a description of the most efficient way that all form is organized, in terms of most economical use
of energy and material,
Tensegrities
have no lever arms or fulcrums in the classical sense. Forces are transferred
globally across the entire structure
The only way to fully
stabilize and constrain any structure is by triangulating surfaces or cavities in
compression and/or tension in all three dimensions
Tensegrity structures on the
other hand show the forces acting upon them by differentiating out tension
and compression vectors into separate components

In living structure, stability is paired with mobility and objects that are
adapted to allow movement possess degrees of freedom and are not fully
triangulated. Degrees of freedom refer to the number of different ways in
which a rigid object can move in three dimensions (six). They are: movement
up and down (heaving), movement left and right (swaying), movement
forward and backward (surging), angling up and down (pitching), turning left
and right (yawing) and tilting side to side (rolling). A mechanism or linkage
connecting more than one object may have more than the degrees of freedom
for a single rigid object. The human arm for example is considered to have
seven degrees of freedom, three at the shoulder, one at the elbow and three at
the wrist. Controlling degrees of freedom means increasing the stability of an
object and in any joint all other unwanted degrees of freedom are constrained
by a combination of bone geometry and connective attachments.

Because tensegrities are never completely rigid, they have varying degrees of

freedom whose range of motion is determined by their triangulation. In this
respect they are superficially more similar to plants than to mobile beings.
They can flex and accommodate to vectors of force by slightly altering shape.
They bend rather than break. But the peripheries of the body demonstrate
wide ranges of motion that vary in each joint. For tensegrities to emulate
anatomy there must be an increase in ranges of motion without sacrificing
stability or degrees of freedom.