Papers by Xavier Lachenal
International Journal of Solids and Structures, 2014
ABSTRACT This study is concerned with the stability characteristics of helix shaped structures ma... more ABSTRACT This study is concerned with the stability characteristics of helix shaped structures made of anisotropic, pre-stressed, thin flanges arranged in such a way as to enable and develop multi-stability. Previous research on similar structures assumed the structural response of the flanges to be one-dimensional due to the narrow width of the pre-stressed members in comparison to their length. In this work, a refined two-dimensional model of the flanges is employed to model the influence of transverse curvature as well as the membrane strain energy associated with the non-zero Gaussian curvature deformations. While longitudinal curvature changes and twist are inherent to the geometry of the helices; the transverse curvature results from a consideration of boundary effects and the minimisation of the (expensive) membrane elastic energy. A qualitative study of the changes in transverse curvature reveals ways of simplifying the two-dimensional model into a simpler, closed form, one-dimensional version applicable to helices with relatively narrow flanges. Correlation is found between experimental results, finite element modelling and analytical predictions for the two models.
2013 IEEE 13th International Conference on Rehabilitation Robotics (ICORR), 2013
A novel actuator is introduced that combines an elastically compliant composite structure with co... more A novel actuator is introduced that combines an elastically compliant composite structure with conventional electromechanical elements. The proposed design is analogous to that used in Series Elastic Actuators, its distinctive feature being that the compliant composite part offers different stable configurations. In other words, its elastic potential presents points of local minima that correspond to robust stable positions (multistability). This potential is known a priori as a function of the structural geometry, thus providing tremendous benefits in terms of control implementation. Such knowledge enables the complexities arising from the additional degrees of freedom associated with link deformations to be overcome and uncover challenges that extends beyond those posed by standard rigidlink robot dynamics. It is thought that integrating a multistable elastic element in a robotic transmission can provide new scenarios in the field of assistive robotics, as the system may help a subject to stand or carry a load without the need for an active control effort by the actuators.
Wind Energy, 2013
With increasing size of wind turbines, new approaches to load control are required to reduce the ... more With increasing size of wind turbines, new approaches to load control are required to reduce the stresses in blades. Experimental and numerical studies in the fields of helicopter and wind turbine blade research have shown the potential of shape morphing in reducing blade loads. However, because of the large size of modern wind turbine blades, more similarities can be found with wing morphing research than with helicopter blades. Morphing technologies are currently receiving significant interest from the wind turbine community because of their potential high aerodynamic efficiency, simple construction and low weight. However, for actuator forces to be kept low, a compliant structure is needed. This is in apparent contradiction to the requirement for the blade to be load carrying and stiff. This highlights the key challenge for morphing structures in replacing the stiff and strong design of current blades with more compliant structures. Although not comprehensive, this review gives a concise list of the most relevant concepts for morphing structures and materials that achieve compliant shape adaptation for wind turbine blades.
Journal of Intelligent Material Systems and Structures, 2014
A classic structural component of mechanical engineering, the I-beam, is re-designed by adding a ... more A classic structural component of mechanical engineering, the I-beam, is re-designed by adding a morphing twist functionality to the high bending stiffness inherent to the geometry of the component. The beam, as with its conventional counterpart, is made of two flanges separated by a web. Here, bi-stability is introduced from a combination of flange pre-stress, web geometry and material properties, yielding a highly non-linear twist morphing structure while keeping the low weight and the high bending stiffness of the beam. The present case study offers two twisted stable shapes and can be morphed from one stable configuration to the other by applying a snap-through twist moment to the ends of the structure. Correlation is found between tests results, finite element model data and analytical predictions, validating the modelling assumptions. A sensitivity study is also performed to understand the influence of the design parameters of the beam and loading condition on the stability of the structure. As a result, the three conflicting requirements of adaptive structures are met in a single structural entity: low mass, load-carrying capability and compliance.
Journal of the Mechanics and Physics of Solids, 2013
We present a cylindrical lattice structure that mimics the behaviour of the virus bacteriophage T... more We present a cylindrical lattice structure that mimics the behaviour of the virus bacteriophage T4 in having two (or more) stable states which differ in their radii and length. While the virus achieves bistability through molecular mechanisms we use composite materials to exploit the interplay between pre-stress, material properties and structural geometry. We demonstrate (computationally) that multi-stability is a robust phenomenon. We also show (analytically) that it is possible to choose the design variables so that the energy is independent of the radius, thus resulting in every state of the structure being stable.
Uploads
Papers by Xavier Lachenal