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Vibration-based Assessment of Tensegrity Structures
KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.ORCID iD: 0000-0001-8064-5463
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also  investigated. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , 158 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:05
Keyword [en]
Tensegrity, Pre-stress, Vibration, Health monitoring, Buckling, Temperature effect, Vibration health monitoring VHM, Optimization
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-185789OAI: oai:DiVA.org:kth-185789DiVA: diva2:923877
Public defence
2016-05-17, F3, Lindstedtsvägen 26, Stockholm, 13:46 (English)
Opponent
Supervisors
Note

QC 20160429

Available from: 2016-04-29 Created: 2016-04-27 Last updated: 2016-11-16Bibliographically approved
List of papers
1. Natural frequencies describe the pre-stress in tensegrity structures
Open this publication in new window or tab >>Natural frequencies describe the pre-stress in tensegrity structures
2014 (English)In: Computers & structures, ISSN 0045-7949, E-ISSN 1879-2243, Vol. 136, 162-171 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the effect of pre-stress level on the natural frequencies of tensegrity structures. This has been established by using Euler–Bernoulli beam elements which include the effect of the axial force on the transversal stiffness. The axial-bending coupling emphasizes the non-linear dependence of the natural frequencies on the pre-stress state. Pre-stress is seen as either synchronous, considering a variable final pre-stress design or as tuning, when increasing pre-stress is followed in a planned construction sequence. It is shown that for a certain tensegrity structure, increasing the level of pre-stress may cause the natural frequencies to rise or fall. This effect is related to whether the structural behavior can be seen as compression or tension dominant. Vanishing of the lowest natural frequency of the system is shown to be related to the critical buckling load of one or several compressed components. Modes of vibration show that when the force in the compressed components approaches any type of critical buckling load, this results in lower vibration frequencies. The methods in this study can be used to plan the tuning of the considered tensegrity structure towards the design level of pre-stress, and as health monitoring tools.

Keyword
Buckling, Health monitoring, Pre-stress, Resonance spectrum, Tensegrity
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145176 (URN)10.1016/j.compstruc.2014.01.020 (DOI)000336703700013 ()2-s2.0-84899953269 (Scopus ID)
Note

QC 20140514

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved
2. Influence of Temperature on the Vibration Properties of Tensegrity Structures
Open this publication in new window or tab >>Influence of Temperature on the Vibration Properties of Tensegrity Structures
2015 (English)In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 99, 237-250 p.Article in journal (Refereed) Published
Abstract [en]

Vibration health monitoring methods use the sensitivity of the natural frequencies to structural damage. Natural frequencies are sensitive to damage, but are also affected by environmental conditions like temperature changes. It is important to be able to distinguish between the effects of these different factors when using the vibration properties as a monitoring tool. This paper discusses the impact of damage and environment temperature changes on the natural frequencies of tensegrity ("tensile-integrity") structures, in particular noting that component bending is a prominent vibration mode, which motivates a use of non-linear beam elements with axial-bending coupling. The model considers not only thermal expansion effects, but also the change of the elastic modulus with temperature. Changes in natural frequencies produced by environment temperature changes are shown to be similar to the ones produced by damage. The geometry of tensegrity structures, the support conditions and the materials are found to be important factors. The sensitivity of the natural frequency to temperature changes is found to be dependent on pre-stress level.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-145177 (URN)10.1016/j.ijmecsci.2015.05.019 (DOI)000358625300023 ()2-s2.0-84937559648 (Scopus ID)
Note

Updated from "Manuscript" to "Article". QC 20150901

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved
3. Optimization of modular tensegrity structures for high stiffness and frequency separation requirements
Open this publication in new window or tab >>Optimization of modular tensegrity structures for high stiffness and frequency separation requirements
2016 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 80, 297-309 p.Article in journal (Refereed) Published
Abstract [en]

Tensegrities are cable-strut assemblies which find their stiffness and self-equilibrium states from the integrity between tension and compression. Low stiffness and coinciding natural frequencies are known issues. Their stiffness can be regulated and improved by changing the level of pre-stress. In vibration health monitoring, the first natural frequency is used as an indicator of better stiffness, but coinciding natural frequencies will be an obstacle in measuring and analysing the correct resonance. In this paper, the above two issues have been considered for modular tensegrity structures. The finite element model used considers not only the axial vibration of the components, but also the transversal vibration where non-linear Euler-Bernoulli beam elements are used for simulations. A genetic algorithm is used to solve the optimization problem, with a multi-objective criterion combination. The optimum self-stress of the tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others. Two approaches are used to find the optimal self-stress vector: scaling from a base module or considering all modules at once. Both approaches give the same optimum solutions.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Tensegrity, Self-stress, Frequency separation, Optimization, Vibration health monitoring
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-182844 (URN)10.1016/j.ijsolstr.2015.11.017 (DOI)000368204500025 ()2-s2.0-84955750596 (Scopus ID)
Note

QC 20160224

Available from: 2016-02-24 Created: 2016-02-23 Last updated: 2017-11-30Bibliographically approved
4. Reducing effects from environmental temperature on the natural frequencies of tensegrity structures
Open this publication in new window or tab >>Reducing effects from environmental temperature on the natural frequencies of tensegrity structures
(English)In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568Article in journal, News item (Other (popular science, discussion, etc.)) Submitted
Abstract [en]

n vibration health monitoring, dynamic properties such as natural frequencies and mode shapes are used as tools for assessing the structures health condition.~They are, however, also affected by environmental conditions like wind, humidity and temperature changes. Of particular importance is the change of the environmental temperature, and it is the most commonly considered environmental variable that influences the vibration health monitoring algorithms.~This paper discusses how the tensegrity structures can be designed such that some of their lowest natural frequencies are less sensitive to the temperature changes. A genetic algorithm is used to solve the optimization problem. In the form-finding stage, an asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution, particularly when a symmetric self-stress vector is chosen.

Place, publisher, year, edition, pages
Elsevier
National Category
Applied Mechanics
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-186273 (URN)
Note

QC 20160509

Available from: 2016-05-09 Created: 2016-05-09 Last updated: 2017-11-30Bibliographically approved
5. Vibration health monitoring for tensegrity structures
Open this publication in new window or tab >>Vibration health monitoring for tensegrity structures
2017 (English)In: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 85, 625-637 p.Article in journal (Refereed) Published
Abstract [en]

Tensegrities are assembly structures, getting their equilibrium from the interaction between tension in cables and compression in bars. During their service life, slacking'in their cables and nearness to buckling in their bars need to be monitored to avoid a sudden collapse. This paper discusses how to design the tensegrities to make them feasible for vibrational health monitoring methods. Four topics are discussed; suitable finite elements formulation, pre-measurements analysis to find the locations of excitation and sensors for the interesting modes, the effects from some environmental conditions, and the pre-understanding of the effects from different slacking scenarios.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Cable-bar structures, Dynamic stiffness, Frequency response function, Pre-simulations, Tensegrity structure, Vibration health monitoring
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-186274 (URN)10.1016/j.ymssp.2016.08.039 (DOI)000389095400040 ()2-s2.0-84995514089 (Scopus ID)
Note

QC 20160509

Available from: 2016-05-09 Created: 2016-05-09 Last updated: 2017-11-30Bibliographically approved

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