Julien Averseng

Active control of a tensegrity plane grid

Tensegrity systems are selfstressed reticulate space structures. As lightweight frames, they are subject to deformation and vibration issues when faced to natural stimulations such as temperature gradients or wind. Classical passive solutions impose to rigidify components or to add damping in the structure using heavy devices. Active systems, mainly developed in space and seismic fields, are controlled using external energy brought by activators. We describe in this paper a mixed geometric and dynamic active control of tensegrity structures using a robust control design technique. An experiment is carried out on a six selfstress states plane tensegrity grid.

Structural design and control of modular tensegrity structures

Tensegrity systems are self-stressed reticulate structures, composed of a set of compressed struts assembled inside a continuum of tendons. This principle can be at the origin of large, lightweight and transparent structures. In practice, a few structures of this kind were built, partly because they are very demanding in design and analysis. In the wish to contribute to the development of practical structural applications, we propose in this paper a design procedure that combines form-finding and structural dimensioning under static load. To optimise the behaviour in the dynamic domain, we present a general methodology suited for the control of the first vibration modes. The case of a modular tensegrity footbridge is taken for application, taking into account different materials.

Interactive Dynamic Design and Analysis of Tensegrity Systems

In this paper, an implementation of the discrete element method is presented with applications in interactive design and dynamic non linear analysis of tensegrity systems, a class of lightweight reticulate space structures. These systems are simulated efficiently using an explicit time integration scheme coupled to a 3D visualization interface, which brings the possibility to interactively model a structure, to follow its evolution in real time and to perform advanced structural analysis. To validate and justify this particular approach in the case of tensegrity systems, a static analysis comparison with other structural analysis softwares is carried out on a representative example. The benefits and versatility of the method are further illustrated through simulations of planar and deployable structures.

The Non-Proliferative Nature of Ascidian Folliculogenesis as a Model of Highly Ordered Cellular Topology Distinct from Proliferative Epithelia

Previous studies have addressed why and how mono‐stratified epithelia adopt a polygonal topology. One major additional, and yet unanswered question is how the frequency of different cell shapes is achieved and whether the same distribution applies between non-proliferative and proliferative epithelia. We compared different proliferative and non-proliferative epithelia from a range of organisms as well as Drosophila melanogaster mutants, deficient for apoptosis or hyperproliferative. We show that the distribution of cell shapes in non‐proliferative epithelia (follicular cells of five species of tunicates) is distinctly, and more stringently organized than proliferative ones (cultured epithelial cells and Drosophila melanogaster imaginal discs). The discrepancy is not supported by geometrical constraints (spherical versus flat monolayers), number of cells, or apoptosis events. We have developed a theoretical model of epithelial morphogenesis, based on the physics of divided media, that takes into account biological parameters such as cell‐cell contact adhesions and tensions, cell and tissue growth, and which reproduces the effects of proliferation by increasing the topological heterogeneity observed experimentally. We therefore present a model for the morphogenesis of epithelia where, in a proliferative context, an extended distribution of cell shapes (range of 4 to 10 neighbors per cell) contrasts with the narrower range of 5-7 neighbors per cell that characterizes non proliferative epithelia.

Divided media based simulations of tissue morphogenesis

Tissue morphogenesis is a key point remaining weakly understood in development biology. In particular, whatever the species considered (animals and vegetables), most of formed epithelial tissues show analogous cell polygonal distributions (Gibson 2009). Accordingly, one may wonder if there are invariant features governing this process. Among the numerous studies performed up today to answer this question, only few approaches including physical models have been developed (Graner et Glazier, 1993; Martinand-Mari et al., 2009). These models allow cellular reorganisation within tissues already formed but by considering only limited movements and without any growing cells, which does not exactly correspond to biological tissue morphogenesis.