Benefits of Tensegrity

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Read here about the benefits of tensegrity structures culled from the engineering and biotensegrity literature, with some discussion.

Tension Stabilizes

A compressive member loses stiffness as it is loaded, whereas a tensile member gains stiffness as it is loaded. Stiffness is lost in two ways in a compressive member. In the absence of any bending moments in the axially loaded members, the forces act exactly through the mass center, the material spreads, increasing the diameter of the center cross section; whereas the tensile member reduces its cross-section under load. In the presence of bending moments due to offsets in the line of force application and the center of mass, the bar becomes softer due to the bending motion. For most materials, the tensile strength of a longitudinal member is larger than its buckling (compressive) strength. (Obviously, sand, masonary, and unreinforced concrete are exceptions to this rule.) Hence, a large stiffness-to-mass ratio can be achieved by increasing the use of tensile members. [1]

Tensegrity Structures are Efficient

It has been known since the middle of the 20th century that continua cannot explain the strength of materials. The geometry of material layout is critical to strength at all scales, from nanoscale biological systems to megascale civil structures. Traditionally, humans have conceived and built structures in rectilinear fashion. Civil structures tend to be made with orthogonal beams, plates, and columns. Orthogonal members are also used in aircraft wings with longerons and spars. However, evidence suggests that this “orthogonal” architecture does not usually yield the minimal mass design for a given set of stiffness properties. Bendsoe and Kikuchi, Jarre, and others have shown that the optimal distribution of mass for specific stiffness objectives tends to be neither a solid mass of material with a fixed external geometry, nor material laid out in orthogonal components. Material is needed only in the essential load paths, not the orthogonal paths of traditional manmade structures. Tensegrity structures use longitudinal members arranged in very unusual (and nonorthogonal) patterns to achieve strength with small mass. Another way in which tensegrity systems become mass efficient is with self-similar constructions replacing one tensegrity member by yet another tensegrity structure. [1]

Snelson is skeptical that tensegrity structures will prove to be more efficient. In one study of dynamically morphing airplane wing shapes, Ramrakhyani found that a tendon actuated wing has a similar mass than a conventional wing for the same design requirements, but allows larger deflections. [3]

Tensegrity Structures are Deployable

Materials of high strength tend to have a very limited displacement capability. Such piezoelectric materials are capable of only a small displacement and “smart” structures using sensors and actuators have only a small displacement capability. Because the compressive members of tensegrity structures are either disjoint or connected with ball joints, large displacement, deployability, and stowage in a compact volume will be immediate virtues of tensegrity structures. This feature offers operational and portability advantages. A portable bridge, or a power transmission tower made as a tensegrity structure could be manufactured in the factory, stowed on a truck or helicopter in a small volume, transported to the construction site, and deployed using only winches for erection through cable tension. Erectable temporary shelters could be manufactured, transported, and deployed in a similar manner. Deployable structures in space (complex mechanical structures combined with active control technology) can save launch costs by reducing the mass required, or by eliminating the requirement for assembly by humans. [1]

Tensegrity Structures are Easily Tunable

The same deployment technique can also make small adjustments for fine tuning of the loaded structures, or adjustment of a damaged structure. Structures that are designed to allow tuning will be an important feature of next generation mechanical structures, including civil engineering structures. [1]

See also tuning in musical instruments .

Tensegrity Structures Can be More Reliably Modeled

All members of a tensegrity structure are axially loaded. Perhaps the most promising scientific feature of tensegrity structures is that while the global structure bends with external static loads, none of the individual members of the tensegrity structure experience bending moments. (In this chapter, we design all compressive members to experience loads well below their Euler buckling loads.) Generally, members that experience deformation in two or three dimensions are much harder to model than members that experience deformation in only one dimension. The Euler buckling load of a compressive member is from a bending instability calculation, and it is known in practice to be very unreliable. That is, the actual buckling load measured from the test data has a larger variation and is not as predictable as the tensile strength. Hence, increased use of tensile members is expected to yield more robust models and more efficient structures. More reliable models can be expected for axially loaded members compared to models for members in bending. [1]

Tensegrity Structures Facilitate High Precision Control

Structures that can be more precisely modeled can be more precisely controlled. Hence, tensegrity structures might open the door to quantum leaps in the precision of controlled structures. The architecture (geometry) dictates the mathematical properties and, hence, these mathematical results easily scale from the nanoscale to the megascale, from applications in microsurgery to antennas, to aircraft wings, and to robotic manipulators. [1]

Tensegrity is a Paradigm that Promotes the Integration of Structure and Control Disciplines

A given tensile or compressive member of a tensegrity structure can serve multiple functions. It can simultaneously be a load-carrying member of the structure, a sensor (measuring tension or length), an actuator (such as nickel-titanium wire), a thermal insulator, or an electrical conductor. In other words, by proper choice of materials and geometry, a grand challenge awaits the tensegrity designer: How to control the electrical, thermal, and mechanical energy in a material or structure? For example, smart tensegrity wings could use shape control to maneuver the aircraft or to optimize the air foil as a function of flight condition, without the use of hinged surfaces. Tensegrity structures provide a promising paradigm for integrating structure and control design. [1]

Tensegrity Structures are Motivated from Biology

The nanostructure of the spider fiber is a tensegrity structure. Nature’s endorsement of tensegrity structures warrants our attention because per unit mass, spider fiber is the strongest natural fiber. Articles by Ingber argue that tensegrity is the fundamental building architecture of life. His observations come from experiments in cell biology, where prestressed truss structures of the tensegrity type have been observed in cells. It is encouraging to see the similarities in structural building blocks over a wide range of scales. If tensegrity is nature’s preferred building architecture, modern analytical and computational capabilities of tensegrity could make the same incredible efficiency possessed by natural systems transferrable to manmade systems, from the nano- to the megascale. This is a grand design challenge, to develop scientific procedures to create smart tensegrity structures that can regulate the flow of thermal, mechanical, and electrical energy in a material system by proper choice of materials, geometry, and controls. This chapter contributes to this cause by exploring the mechanical properties of simple tensegrity structures. [1]

Small Storage Volume

The assemblies of rods and strings can potentially be stored and transported in a small volume. There are several options on how to do this practically. One option would be to store all, or some, of the rods separated from the network of strings. The structure can be erected at the final location by inserting all rods into nodes of the string network. Also active control could be used to change the configuration from occupying a small volume to being fully erected. The advantages with respect to storage and transportation could be critical, for instance in military operations, where trusswork structures are used in temporarily bridges or other infrastructure such as tents. [2]

Modularity Through Cells

These structures are often built using a large number of identical building blocks, or cells. A structure assembled from stable units will be more robust with respect to failure in structural elements than a structure built from unstable units. The modularity facilitates large-scale production of identical units before assembled to get the overall structure, which can potentially reduce building costs. [2]

Stabilizing Tension

A crucial property in the various definitions of tensegrity structures is that the structure should have a stable static equilibrium configuration with all strings in tension and all rods in compression. This is only a minimum requirement, and the level of prestress can be altered depending on the wanted structural stability. The same feature is also recognized in biological systems, such as the human body, where bones are kept in position by tensioned muscles. Some examples are given in Lieber (2002), Kobyayashi (1976) and Aldrich and Skelton (2005). One should notice that stronger elements could be needed in order to increase the prestress, in many cases that would also imply increased mass. [2]

Robustness Through Redundancy

The number of strings should be carefully considered with respect to the intended use of the structure. Using more strings than strictly needed gives increased robustness through redundancy. The structural stability could also be improved by adding strings to avoid the so-called soft modes. The additional costs of using more strings than a strict minimum could be small compared with the potentially increased structural quality and the reduced consequences of failures. [2]

Geometry and Structural Efficiency

The traditional configurations of structures have been in a rectilinear fashion with the placement of elements orthogonally to one another. Researchers have suggested that the optimal mass distribution when considering an optimization of stiffness properties is neither a solid mass with fixed external geometry, nor an orthogonal layout of elements. See Michell (1904), Jarre et al. (1998) and Bendsoe and Kikuchi (1988). The structural strength with respect to mass of tensegrity structures is achieved through highly unusual configurations. The structural elements only experience axial loading, not bending moments. This is in accordance with the general rule that one should strive to have material placed in essential load paths to achieve structural strength. [2]

Analogy to Fractals

The overall mass of a system can be minimized through self similar iterations. One example would be to replace rods with tensegrity structures. Such a procedure can be repeated until a minimal mass requirement, or a maximum level of complexity, is achieved. Another example would be on how to decide the number, and height, of stages in a tower structure, based on the criteria mentioned above. See Chan (2005). [2]

References and Links

This list was based first on a list by Skelton.

[1] "An Introduction to the Mechanics of Tensegrity Structures", by Skelton, Helton, Adhikari, Pinaud, Chan. © 2002 by CRC Press LLC [2] "Modelling and Control of Tensegrity Structures", by Anders Sunde Wroldsen, Doctoral Dissertation, 2007 [3] Morphing skins on aircraft for shape change and area increase by Thill, Etches, Bond et al, written 2007, published The Aeronautical Journal March 2008