Scholarly article on topic 'Unprecedented Structural Skins. Experiments towards an Intelligent Tensegrity Skin'

Unprecedented Structural Skins. Experiments towards an Intelligent Tensegrity Skin Academic research paper on "Materials engineering"

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{tensegrity / lightweight / biomimetic / exo-skeletal / "3D printing."}

Abstract of research paper on Materials engineering, author of scientific article — Alberto Campesato

Abstract Pin-jointed prestressable structures -known as tensegrity systems- have been largely studied for over six decades, untouched in their complexity and fascination. Prior research has marginally offered interactive toolsets to design tensegrity: none without tedious procedures embedded in their resolution. Restrictions in terms of simultaneous design's topology and optimization - aggravated by lacking effective means to automate manufacturing and specially assembly of such system - have greatly disrupted and cornered their current applicability. We present results of unprecedented processes extending tensegrity systems applicability. Employing a novel approach to design's topology and optimization of arbitrary tensegrity systems, we developed means to automatically design and manufacture them. With advantages in tailoring design and production costs, we explore some speculative scenario. Therefore we present experiments towards novel processes and products aimed to revolutionized the market of lightweight structural systems, both in static and dynamic applications. Moving from tensegrity systems and their intrinsic transportability, material-economy, easy manufacture, customization and assembly -the latest expanded within our research- we propose a new standard for structural performing skin. We aim for an intelligent skin: free-form, lightweight, widely adaptable in use, and capable of embedding complex design features and functionalities. Central in addressing economical solutions to nowadays challenging tasks we present our findings, analyzing production processes and potential challenges further suggesting relevant scenarios of pertinence. Highlights: ▶Tensegrity automated production is possible linking design topology, optimization, manufacturing and assembling. ▶Two automated manufacturing approaches are outlined according to scale, functionality and use.▶Injection moulding is nowadays proofed for structural performances, cost competitiveness, easy multifunctional layer implementation.▶3D printing will favor in future robust automated customized production.

Academic research paper on topic "Unprecedented Structural Skins. Experiments towards an Intelligent Tensegrity Skin"

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Procedía Engineering 155 (2016) 183 - 194

Procedía Engineering

www.elsevier.com/locate/procedia

International Symposium on "Novel Structural Skins: Improving sustainability and efficiency through new structural textile materials and designs"

Unprecedented structural skins. Experiments towards an intelligent

tensegrity skin.

Alberto Campesato*

ACCED, Witte de Withstraat 47a3, Rotterdam 3012BM, The Netherlands *

Abstract

Pin-jointed prestressable structures -known as tensegrity systems- have been largely studied for over six decades, untouched in their complexity and fascination. Prior research has marginally offered interactive toolsets to design tensegrity: none without tedious procedures embedded in their resolution. Restrictions in terms of simultaneous design's topology and optimization -aggravated by lacking effective means to automate manufacturing and specially assembly of such system - have greatly disrupted and cornered their current applicability. We present results of unprecedented processes extending tensegrity systems applicability. Employing a novel approach to design's topology and optimization of arbitrary tensegrity systems, we developed means to automatically design and manufacture them. With advantages in tailoring design and production costs, we explore some speculative scenario.

Therefore we present experiments towards novel processes and products aimed to revolutionized the market of lightweight structural systems, both in static and dynamic applications. Moving from tensegrity systems and their intrinsic transportability, material-economy, easy manufacture, customization and assembly -the latest expanded within our research- we propose a new standard for structural performing skin. We aim for an intelligent skin: free-form, lightweight, widely adaptable in use, and capable of embedding complex design features and functionalities. Central in addressing economical solutions to nowadays challenging tasks we present our findings, analyzing production processes and potential challenges further suggesting relevant scenarios of pertinence.

Highlights: ►Tensegrity automated production is possible linking design topology, optimization, manufacturing and assembling. ► Two automated manufacturing approaches are outlined according to scale, functionality and use. ►Injection moulding is

* Corresponding author. Tel.: +39-347-477-3301; Present address: Contra della Fascina 17, Vicenza 36100, Italy.

E-mail address: aacampesato@gmail.com

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the TensiNet Association and the Cost Action TU1303, Vrije Universiteit Brussel doi:10.1016/j.proeng.2016.08.019

nowadays proofed for structural performances, cost competitiveness, easy multifunctional layer implementation. ^-3D printing will favor in future robust automated customized production.

© 2016 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the TensiNet Association and the Cost Action TU1303, Vrije Universiteit Brussel Keywords: tensegrity; lightweight; biomimetic; exo-skeletal; 3D printing.

1. Introduction

1.1. Tensegrity: past, present and undergoing research

In 1864 Clerk Maxwell anticipated what a century later R. Buckminster Fuller - looking at the work of a young artist called Snelson - would have named tensegrity. Since then, six decades of research have expanded our understanding of such system, stimulated by the aerospace industry's quest for deployable lightweight systems. Again in the last decades new discoveries have flourished and progressed thanks to improvements in numerical methods and symbolical computation. Where Snelson had emancipated the native definition of tensegrity rethinking them as endoskeletal prestressed structures, Motro disengaged the patent based definition from the principle. "Tensegrity system is a system in stable self-equilibrated state comprising a discontinuous set of components inside a continuum of tensioned components".

While traditional trusses structures have quickly encountered technological development and market demand, tensegrities further challenged designing both static and dynamic configurations, sunk in tedious manufacturing and assembling processes. We believe that further achievements in these fields are needed step-stones to reach concrete market applicability.

With renew interest among relevant properties demonstrated by tensegrity, we would like to recall their scalability, unparalleled lightweight and material efficiency [16]. Their intrinsic deployability gained them a central role among deployable structures for space application. Needless to say, a larger investigation from static behavior (Hernandez and Mirats [7]) to dynamic response (Mirats and Hernandez [11]); from the deployable configurability and dynamic control (Mirats and Hernandez [11]; Adam and Smith [2]) to form finding design (Chandana et al. [5]) and design optimization (Masic et al. [10]) ; from biological similarities in modeling natural organism (Ingber [8]) to structural-material efficiency (Lieber [9]), gives just a taste of potential applicability and complexities inherently embedded in such lightweight system (Fig. 1).

Fig. 1. (a) tensegrity model; (b) geodetic tensegrity dome; (c) deployable tensegrity masts for space application.

To clarify the underneath principle, Hanaor distinguished pin-jointed skeletal structures composed of bar and cables in two major classes: not prestressable (statically determinate structure and mechanism) and prestressable

(trusses structure and precisely tensegrity). Tensegrity stiffness depends on the state of self-stress -or prestress-keeping the integrity of the structural system from falling apart. Negligible masses and force fields acting at the nodes of the system do not influence their state of internal equilibrium either their stability. They do not require in general the presence of external forces (e.g. gravity) or any physical environment for the self-equilibrium being stable in any position and geometrically scalable. Therefore higher is the prestress applied, higher is the stiffness of the system responding to external actions. Deformability of elements in tension causes loss in stiffness of the system (Motro et al. [12]). The resolution of the prestressability problem is indeed one of the major challenges in the process of form finding. Further, results in the theory of rigidity and stability of frameworks achieved in the eighties by Connelly [6], Roth and Whiteley [15] helped to define new classes of mathematical objects -tensegrity frameworks- successfully resolving some simplified tensegrity topology using symbolical computational software.

Referring to the work of Motro [13], we understood that tensegrity design has limitation firstly in the design phase: sets of equations are integrated till convergence at an optimized configuration, using dynamic relaxation methods. Secondly in the necessity of "mounting struts" that geometrically locate in space nodes throughout the assembling, preventing an easy free form construction. As outlined in prior works at the IASS 2010 Shanghai [3] and IASS 2015 Amsterdam [4], an approach focused in using an integrated design methodology to form find stable configurations suggest the possibility to avoids or contains such limitations. Therefore, integrating both design topology (form finding innovative configurations) and optimization topology (optimizing an existing one) within a unique design step, we use a combination of two software toolsets. Briefly, the input geometry is engineered into a sounding tensegrity system containing all the requirements for fast construction and assembly. We use a Rhino-script plugin to translate a desired model to a proto-topology input; then a standalone tailored software to optimize it. In this paper we limit our analysis to technologies and experiments employed for the production of such system, addressing further static and dynamic behavior analysis to future papers.

In what follows we may use terms that could slightly differ from others author's definitions. Again, we will recall processes from design to manufacturing using such methodology, presenting some development through study cases.

1.2. A short introduction on living systems, chemistry and their virtual convergence. A prelude to nonlinear solutions: organism & biomimetic

Few know tensegrity has been used within the field of molecular biology to model virus structures (Caspar and Klug, 1962) and their behaviors undergoing external environmental changes. The interest has later moved to cytoskeleton: scaffold contained within the cytoplasm of living cell, that are protein polymer based cellular structures (Ingber, 1993). Surprisingly some mechanism of motion depending on the substrate the living cell moves on, was only understood once the model of tensegrity was introduced and explained. In recent years efforts have aimed to establish links between tensegrity systems, muscular-tendons-ligaments apparatus in vertebrates and mechanisms of locomotion to better comprehend living systems.

A puzzling problem such as the explanation of mechanisms behind human brain folds has been recently linked to simple mechanical instability associated with buckling. The relative expansion of the brain outer cortex with respect to the soft tissue underneath, causes the characteristic folding configuration based on the particular shape (Tuomas Tallinen et al. [17]). Interestingly, such natural folding lines due to mechanical instability occur in the attempt to balance material property limitations, responding geometrically to a buckling problem. Like in origami, folding lines create stiffer discontinuous sets of elements breaking the continuity of the outer -more flexible- layer. Recalling the Fuller definition of tensegrity such as "Islands of compression inside an ocean of tension" it cannot go unnoticed striking resemblances between cortical convolutions patterns and folding lines in tensegrity "envelops" we studied.

Looking at other disciplines we were inspired by the work of Prigogine -Nobel in chemistry in 1977- showing the existence of oscillating chemical reactions, spatial structure of non equilibrium, chemical waves. All conditions generated far from the thermodynamics equilibrium, where instability (and non-linearity) drives the progression of the thermodynamic equilibrium and its final product. Such are "dissipative structures" increasing the production of

entropy -instead of diminishing- in proximity of the equilibrium. Enzymes, ensuring the rich multiplicity of catalytic reactions and enabling life in all forms.

In static analysis otherwise, tensegrity is described and studied by nonlinear sets of equations and inequalities to be simultaneously solved together with the prestressability conditions. Resolving them results in the equilibrium, stability and rigidity of the system, non mentioning the system's existence in the real world (non trivial solutions). Curiosly such nonlinear search -compares to linear ones- leads to richer set of solutions within wider spaces of possible configurations, challenging the notion of equilibrium. In such search-space each member of the system behaves -ideally- linearly: purely in tension or compression with zero momentum at the nodes.

We believe searching new forms and systems that engineers the future, obligatory bring us to explore the "dynamic" equilibrium of systems. Considering systems statically and kinematically indeterminate able to withstand -under particular condition of prestress- stable configurations -such as tensegrities- opens opportunities in further explore the realm of equilibrium. Inspired by the work of Poincare2 and his approach, we have to develop alternative ways to freely design tensegrity. In such perspective searching alternative methodologies to design, manufacture and activate tensegrity become an imperative goal in our studies. In what follows we present results and further speculations hopefully giving some extent of the possibilities at hand.

2. Foundations

2.1. Engineering systems today: smart textile and biomimetic

We were intrigued by the tensegrity inherent ability to contain within the envelop itself the whole structural framework needed for stability, enabling large scale structures with little weight and small size parts. The possibility of drawing a continuous pattern of forces with a minimum discontinuous path of compressed members within a continuum of tensioned members, offered the opportunity to experiment alternatives using latest technologies available. Among others we considered 3D printing and injection moulding manufacturing versus standard construction methods of cables-struts systems, commonly seen in tensegrity structures. Improvements in manufacturing and assembling capability using an integrated methodology -from design to production- virtually brought us to rethink such class of systems. A unique "structural" envelop possibly embedding multiple layers of utility: power supply, actuators, feedback sensors for static and kinematic response. Only recently such opportunity has been more widely accepted with the emergence of e-textile technologies for smart "skin". In such and similar directions Nottingham University together with EPSRC Centre for Innovative Manufacturing is working on multifunctional marketable components.

Where common structural systems need costly and non-trivial processes to move from designing to manufacturing, our approach to tensegrity design is focused in ready-to-build design solutions, with unlimited applicability. Issues imposed by the nature itself of tensegrity are therefore mitigated.

Standard lightweight controllable systems either require specific design to implement kinematic behaviors, or suffer in control problems to safely and precisely maneuver throughout the intended trajectory. Tensegrities on the other side are well accustomed -even if not trivial task- to be activated because their nature of self-equilibrated "mechanism". As system kinematically and statically indeterminate they may offer multiple self-stress states and mechanisms for a given framework: showing stable stiffness configuration where a proper self-stress function is applied to comply stabilization of infinitesimal mechanisms [7]. As such, they can outperform standard systems being able to better fit open-end solutions, or additionally capable to capture into their "envelop matrix" biomimetic principles. For example the ability -how we suggest in the followings- to tune the structural pattern around nodes through a gradient material matrix. A transitions from areas in tension to areas in compression within a unique

2 Poincare in the so called "problem of the three bodies" had to move away from conventional methods to resolve problems limited by the non-integrability of systems of non equilibrium.

envelop skin. We recall that biomimetic systems are engineered apparatus making use of embedded means to achieve higher functionality and superior resilience imitating principles and elements of nature. In such a perspective we think fields such as robotics (ex. Paul et al. [14]), lightweight exoskeleton (ex. ActiveLink-Panasonic with PowerLoader Light Ninja) and anthropomorphic prosthetic, or even responsive architecture (ex. Tristan D'Estree Sterk with ORAMBRA) could benefit of similar approach or systems. Showing some results in such direction we intend to serve as inspiring story of where future structural skins could further adventure.

2.2. A straightforward methodology of designing tensegrity

We define tensegrity a close system, based on an arbitrary discontinuous pattern of elements separately in tension and in compression, jointed in connectivity networks of distributed nodes. They are indeed in self-sufficiency and self-equilibrated dependency, freeing the equilibrium from influences of external force such as gravity. Tensegrity therefore can be described as discrete and systematic structure: ensemble of discontinuous (regular or irregular, referring the assembling methods of basic module, Motro 1992), ordered (recurrent connective pattern), and finite (numbered in a finite set of members in compression and tension) collection of elements linearly behaving. The importance of keenly designing such connectivity network -hence the relation among members in tension and compression- is fundamental in seeking easy solutions for manufacture and assembly, not mentioning the structural stability. Our objective is therefore searching optimum solutions structurally stable and close enough to the intended design input topology to form find, progressively narrowing down the searching space of feasible solutions.

The whole process of design consists therefore of two stages. Firstly an arbitrary input geometry -a free-form envelop- is created and translated into an abstract topology. Such geometry is codified into a network protocol of elements based in known tensegrity units. Specifying shape constraints, physical environment, maximum elements' size or material properties, we define a so called input proto-typology. The seeding topology so obtained will be then optimized by the toolsets we created, optimizing stresses and form-fitting parameters. Converging towards the final configuration the system is inheriting the blueprint network protocol initially defined. A straightforward manufacturing procedure and assembly is thus achieved. The assembled configuration progressively set to the ideal and stable one, joining one module after the other. Lengthening or shortening the relative distance of the two sides of a member in tension, nodes allow post-tuning where needed.

The choice to minimize weight and enhance reconfigurability -for robotic applications- of tensegrity solutions found, conditioned our seeding topology to fundamentally depart from a class-one tensegrity system later optimized in a tailored discontinuous irregular tensegrity (or asymmetrical class-k tensegrity as Skelton et al. could define) for the specific application intended. The system therefore is modeled to work in static behavior withstanding its own weight only. Therefore under external load applied, the system tends to change shape towards yet a stable stiffen configuration (as explained in Masic et al. [10]). Moreover for this class of systems is possible to further stiffen the configuration using or higher prestress, either adding extra elements to remove soft modes. In our case we intend to counteract load scenario -or simply activate the system- by actuating controlled number of members. As shown in the following (Fig. 7 (b)) a rudimentary static analysis under different load conditions can only emphasizes the presence of soft modes where kinematic behaviors have been planned.

In the static configuration, deformations due to material propensities or internal instability are controlled by applying optimum self-stress function; choosing low-elongation materials for the tensioned envelop; tuning stress distribution at the node where compressed member encounter the tensioned envelop; optimizing node design to work as closely as possible to modeled ideal conditions. Then, kinematic behaviors - alternatively load responses-are implemented deforming in a controlled way certain elements, imposing local rigid transformations.

2.3. A market analysis in robotic, prosthetics and lightweight system industry. Potential outcome and opportunities

The industrial robotic market is currently accountable for a +5% growth registered in 2012 with over 168.000 units globally sold in 2013 only (+5% compared to 2012). The average price per robot is estimated in 60.000 USD.

USA and Japan markets dominate the run for industrial automation in manufacturing. Other markets escalating the economical expansion -Asia- or in moderate recession -Europe- are otherwise pursuing partial recovery thanks to industrial automation conversion. On the other side the marketplace for robotics is opening towards professionals and small businesses, diversifying and aggregating the offer in few larger competitors and small service suppliers.

In parallel the market of prosthetic -with a yearly growth of +3%- accounts a value of which 2.1 billions goes only into products, components and supplies. It is a consolidated market with growing drivers and growing number of players (fragmentation trend likely due to emerging 3D printing technologies) with locked value chain.

Both markets have a trend toward lightweight solutions targeting niche segments of costumers. We believe a subsequent conjunction of the two mentioned markets - industrial robotics automation and prosthetic device- is soon going to generate a new segment of "enhancement personal device" for mobility and productivity. Compared to the existent offer, we think a favorable positioning should aim to lightweight low-cost solutions with limited payload and unparalleled performances. Of particular interest is the sub-segment of industrial robotics for beverage, electronics, medical and pharmaceutical sector (35% of the global market of industrial robots depends on 5 Kg payload handling capacity). In the prosthetic field instead the sub-segment for upper extremity prostheses accounts already 41% among limb amputations. Capturing such opportunity will drive innovation in term of differentiated product, business model and customer relationship ("customer centered" markets). In such perspective a differentiated offer minimizes usual investment costs, associated to lightweight high-end solutions, till today at the expenses of small business competitiveness or costumer affordability. Additionally it positively effects long term impact in non-competitive economies -third country without effective labor policies- that currently struggle with inadequate means widening marketplace opportunities. Offering to present and future customers an affordable new class of lightweight systems -with high performance and low cost- is a priceless asset strengthening core businesses and market expansion [1].

3. Findings

3.1. Preliminary outcomes in straightforward manufacturing of tensegrity

Tensegrity are known to be hardly controllable in form, with inherent skewed outlines breaking the shape. With our methodology we can tune the abstract topology and consequently affecting the final configuration. Without compromising in weight or functionality we can expand the portfolio of configurations -geometry and aesthetic- or even effect local and global performances of the system according to design needs. For example the achieved variable spacing -a gradient modularity controlled in the seeding topology- translates in adapting the rigidity of the body where required for stability or aesthetic appeal (Fig. 2). The rich complexity of the modular structural recurrence -broken unexpectedly- and the natural intricacy flowing through the double-curved surface, beautifully breakout underneath opportunities.

Fig. 2. (a) 3D printed functional prototype of prosthetic leg with variable spacing and rigidity; (b) assembling of the exhibited "flying wing" pavilion at IASS2015 Amsterdam. Testing validity of double curved surface potentials, variable gradient spacing in member distribution and

standardized nodes allowing post tuning.

a b c d

Fig. 3. Details developed for the IASS 2015 pavilion. PET film with post-assembled nodes and struts. (a) (b) nodes prototype with snap-in component and node transition between compressed and tensioned members; (c) top of the final assembled system; (d) interior view of half

assembled model.

From our experience, a recurrent problem in the construction of tensegrity systems -especially with larger scale-is the distribution of local stresses uniformly through nodes: from discontinuous compressed patterns to a continuum tensioned network. When ideal conditions are not closely met, the overall behavior of the system is negatively effected with localized deformations worsening overall performances. Controlling such aspect highly influences construction costs, generally requiring more complex solutions for nodes. The design software developed provides detailed information for each node of the system. A parametric design would enable tailoring each node differently from the others. Nevertheless such approach would require using additive technologies -such as 3D printing- to quickly manufacture such variety. In the attempt of containing costs with respect to the flexibility of the process, alternative solutions from full scale standardization to detailed tailoring have been long explored, further tested and analyzed. We resolved therefore in distinguish two different classes of systems depending on the construction approach. Those systems based on the use of polymer film as tensile network and assembled struts (Fig. 3); and those are developed departing from a unique casted layer - skin like - comprising of jointed tensioned and compressed members (Fig. 4).

a b c d

Fig. 4. Details across prototype models and manufacturing technologies: (a) conventional tensegrity system based on cables and struts with tuning elements; (b) 3D printed compressed members assembled on PET film as tensioned network; (c) 3D printed tensional and compressive network based on PLA layer; (d) urethane injection-casted compressive and tensional network: prototype of structural skin for lightweight arm.

The first approach -based on polymer film- requires to precisely cutout the area of the tensioned pattern from a sheet of material. The compression network made of struts or bar joins in nodes, all separately produced and

assembled. Both can be optionally machined, casted or 3D printed. Keeping in mind the latest does not assure - with current technologies - sufficient mechanical strength, injection moulding technology still remains the most preferable one for strength, production time and associated costs. Unfortunately the use of a tensioned film and struts ask for better design of nodes, being separately manufactured and assembled, deeply effecting stress distribution among components. It is generally difficult indeed obtaining sufficient stress distribution both containing dimensions of nodes either assembly workability. That is especially true handling small scale systems such as in prosthetic or exoskeleton applications. On other hands advantages are in manufacturing and assembly speed, with flexibility in designing interchangeable solutions. Nonetheless node stress distribution is a non trivial inconvenience, often difficult to control (Fig. 3 (b)).

The second approach -based on a casted layer- requires an injection moulding setup to cast the parts both in tension and compression. Simplifying, two step are necessary: first build up the tensional layer; then set the compressed member within such layer. The tensional layer is made out of a silicone-textile matrix using similar production processes used in composite materials. The compression members can be optionally inserted or injected, according with the material used for the compressed struts. Additionally such tensile matrix can be produced out of multiple layers, eventually providing extra embedded functionalities. Such approach is more prone to accommodate small scale systems. Moreover, intrinsically it better handles stress distribution. On other hands difficulties rise on the manufacturing side, often involving complex injection moulding processes and setup with finer detailing. Such hassle reflects in sunk costs, incumbents with long term return investments versus competitive life cycle. Obviously, tailoring is either excluded or affordable in limited degrees. Once overcome initial obstacles, advantages are: greater adherence to ideal working conditions for materials and systems, material choices (variety of materials available for casting technologies), assembling speed, and potentials in integrating farther degrees of utility within multifuctional systems (Fig. 4 (d)).

Both approaches are based on direct design-to-production processes, automatized and departing from the optimized computer model output. Taking great advantage of low cost 3D printing to rapid prototyping functional model and test hypothesis, we could further adventure in alternatives and opportunities moving toward more resilient solutions that better respond to stresses and functionalities involved .

3.2. Structural skin and further developments

The best approach to use, fairly depends on system requirements, budget limitations, and finally manufacturing possibilities at hand. Both in fact have their pros and cons. Desirably initial design choices should not effect the possibility to freely design the static configuration, and later kinematically activate it. In simpler terms: further changes or upgrades should be easier and require few arrangements especially regarding manufacturing and assembling procedures. We found plug&play components best allow open-end solutions, with greater market advantages in future product development of user-oriented controllable lightweight systems (Fig. 5).

The integrated approach to design stable tensegrity systems, readily allows to implement in a later time dynamic behaviors departing from the very same stable static configuration. The software developed need to be only roughly informed where the rigid transformation will effect the system, providing detailed members and nodes behavior enabling the planned controlled motion.

Concretely a power supply system for the actuators enabling rigid motion is of easy physical implementation -in term of manufacturing- as encapsulated casted layer. Nevertheless the minimization of its impact in the structure's weight and envelop functionality strictly depends on the methodology chosen to produce the required conductive path. Tests have been made using highly conductive ink, e-textile, conductive polymer paste or film (Fig. 6 (b)). Whatever the choice, a desirable design is free from "hanging" cables or appendixes to best enhance user's functionality. Touch-less sensing could further offers opportunities in activating and controlling the system. Nodes designed as plug&play components allow easy application of nodal actuator to locally deform needed elements enabling the controlled kinetism (Fig. 6 (a)).

a b c d

Fig. 5. IASS 2015 pavilion: (a) 3D printed master mould preparation of plug&play component; (b) silicone master mould; (c) casted nodes; (d)

casted node in industrial urethane resin for functional test.

As representative lightweight controllable systems we choose to prototype an anthropomorphic arm, with few simple sets of motions. The weight of the prototype casted with a silicone-textile matrix and post-inserted struts is approximately 220 grams for a length of 45 cm . A rudimentary analysis of the static behavior under loading conditions identifies one the soft modes introduced into the design to activate and control the system. A rigid translation is shown (Fig. 7 (b)). A variable load equal to the system weight -from 0 to 200 grams- is applied at one side of the system, deviating from its original position approximately 17 cm in its lowest position. More extensive dynamic analyses will require to fully implement actuators and power supply to correctly study the system behavior. We address such wider discussions to future work. In Fig. 8 it is shown the software design process during the optimization process, and in Fig. 9 the initial free form input modeled in Rhino, later further informed as abstract topology and used by the standalone optimization software resolving into the final optimized configuration.

Fig. 6. (a) 3D printed lightweight prototype arm with test-node actuator for activating kinematic behavior. (b) Casted prototype with embedded

copper layer to test effect on structural skin.

Fig. 7. (a) Casted lightweight prototype arm with detail of weight, interior and node for actuation. (b) Simplified loading test.

Fig. 8. Software screen shots of the running simulation and optimization.. It shows the free form conceptualization and its stress state evolution.

Fig. 9. From the software free form input of the initial design, to the so called abstract topology, and finally the optimized tensegrity

configuration.

4. Discussion

Simplifications in modeling and manufacturing processes have become increasingly important in the perspective to bring tensegrity systems at practical application in current design practice and daily life products. The methodology adopted and case study showed empathize the potential of a completely automated manufacturing process. Additionally we have briefly shown achievable aesthetics and lightweight performances applicability. Common lightweight systems still require higher weight for less flexibility in the designing process. The technologies employed promise vast opportunities in experimenting materials and complex embedded functionalities, beyond our current tests. 3D printing technology still remains a good candidate for tailoring solutions, even if still lacking sufficient affordability and structural performances compare to injection moulding processes, as we discussed.

One of major difficulties we encountered is the overall redistribution of stresses through nodes. Where disruptive local deformations arise - at the cost of the whole integrity- continuous structural envelop with gradient material properties could moderate effects. Spotted area with major stresses can be differently tuned compare to others. Once again, injection moulding has demonstrated more economical and competitive results among others technologies, with proofed decades of market product experience. Nevertheless, user-oriented tendency in product development is increasingly forcing to rethink in terms of mass customization. Where injection moulding shows itself obsolete if confronted with needs such as integrating sudden changes, recent 3D printing technologies offer better compromises. We think additive manufacturing will greatly support the future of automated customized production, specially once material structural performance are improved and multifunctional systems play a more central role in customized product developments.

5. Conclusion

We introduce some fundamental concept for tensegrity system. Spanning from biology to biomimetic we explored some applications across research fields. We establish the relevance of the principle beneath tensegrity systems. We mention few limitation in designing both topology and optimization of tensegrity. Additionally we talked about the lack of concrete means in manufacturing and production. At such we respond with an approach aimed to resolve and mitigate such critical issues, where possible improve. We briefly analyzed and discussed the methodology we adopted. Mentioning cases study we offer our experience in the construction of tensegrity systems. Results show how higher complexity and aesthetic richness can be feasibly obtainable in tensegrity design. The methodology does not require mounting struts for assembly or post-process procedures particularly tedious limiting the overall complexity. We finally explained the motivations of our interest with a brief market analysis. We shortly mention the possibility to easily activate tensegrity. At other works we defer longer treaty of such extensive subject. We briefly mention issues depending on the scale of the system and how problems are differently treated varying the production methodology. Further developments are required to account for scale limitation and develop easier automated processes of production of system with gradient material properties.

Form finding combined with latest techniques to manufacture in real-time customized designs, push farther frontiers of non-standard tensegrity systems. A more competitive use of tensegrity system as a concrete structural alternative demands greater flexibility in designing and manufacturing. Finally, with our methodology we demonstrate intrinsic capability to achieve arbitrary design and unparalleled structural skin.

Acknowledgements

The author gratefully acknowledges Huang T. for priceless review. To my father.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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