Engrenage à onde de déformation

Résumé

Un engrenage à onde de déformation — en en anglais, également appelé en d'après le nom d'une société fabriquant de tels systèmes — est un type d'engrenage ayant des propriétés intéressantes par rapport aux engrenages traditionnels (tels les engrenages hélicoïdaux ou bien planétaires), en particulier il présente :

un jeu très réduit ;
des rapports de transmission importants pour un mécanisme compact et très léger (jusqu'à 320:1 tandis que pour le même encombrement, un train épicycloïdal aurait un rapport de 10:1) ;
un couple transmissible important ;
des arbres d'entrée et de sortie coaxiaux. Ce système a été conçu par en 1957. Il a été utilisé en particulier pour la motorisation des roues du rover lunaire et le déploiement des panneaux solaires de Skylab.

Fonctionnement L'animation ci-contre présente trois composantes du système :

en bleu, une couronne extérieure, à denture intérieure, liée au bâti (le bâti n'est pas représenté) ;
en rouge, une couronne défo

Source officielle

https://fr.wikipedia.org/wiki/Engrenage_%C3%A0_onde_de_d%C3%A9formation

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Nonreciprocity in acoustic and elastic materials

Romain Christophe Rémy Fleury, Hussein Nassar

The law of reciprocity in acoustics and elastodynamics codifies a relation of symmetry between action and reaction in fluids and solids. In its simplest form, it states that the frequency-response functions between any two material points remain the same after swapping source and receiver, regardless of the presence of inhomogeneities and losses. As such, reciprocity has enabled numerous applications that make use of acoustic and elastic wave propagation. A recent change in paradigm has prompted us to see reciprocity under a new light: as an obstruction to the realization of wave-bearing media in which the source and receiver are not interchangeable. Such materials may enable the creation of devices such as acoustic one-way mirrors, isolators and topological insulators. Here, we review how reciprocity breaks down in materials with momentum bias, structured space-dependent and time-dependent constitutive properties, and constitutive nonlinearity, and report on recent advances in the modelling and fabrication of these materials, as well as on experiments demonstrating nonreciprocal acoustic and elastic wave propagation therein. The success of these efforts holds promise to enable robust, unidirectional acoustic and elastic wave-steering capabilities that exceed what is currently possible in conventional materials, metamaterials or phononic crystals.

2020

Chargement

On the Kinematic Error in Harmonic Drive Gears

2001

Chargement

Therapies using so called extracorporeal shock waves (Extracorporeal Shock Wave Therapy ESWT) have become current medical practice in orthopedy and traumatology. In order to understand and to optimize the effect of shock waves in clinical applications, medical results must be correlated with well characterized mechanical stimuli. This thesis has an industrial scope. It contributes to the comprehension of the generation and propagation of pressure waves in human tissues with the aim of improving existing ESWT therapies and of providing the industrial partner with tools for the design of a new generation of extracorporeal shock waves devices. The adopted general approach is based on a combination of experimental characterization and analytical and numerical modeling of wave generation and propagation phenomena in a medical treatment device and in biological tissues. Firstly, the characterization of a wave generator is based, on the one hand, on measurements of the dynamic behavior of the moving parts coupled with rigid body simulation, and on the other hand on measurements of wave propagation by means of a Hopkinson bar coupled with finite elements simulations. This characterization has shown that the generator produces very reproducible stress pulses. The simulation technique allows designing a new wave generator with a higher energy range and with well controlled operating parameters. The new design is covered by a patent. Secondly, a measurement technique for generation and propagation of pressure waves in soft animal tissues has been developed that is based on PVDF gages. The applicability of these gages has been qualitatively validated by comparative measurements with a Hopkinson bar. The perturbation effect of the gage, acting as an inclusion in the medium to be characterized, has been evaluated by means of simulations of wave propagation in water. Comparison with measurements in soft tissues suggests that it is negligible for pressure measurement in this type of materials. An independent calibration of the gage could however not be performed. Finally, measurements of wave propagation in pig skin and fat using PVDF gauges showed good reproducibility for a given sample. They highlighted the influence of the supply pressure of the wave generator on the amplitude and on the attenuation of the wave in tissues. Moreover, the dependence between the amplitude of the wave and its propagation velocity suggests a non-linear viscoelastic behavior of soft tissues as well as the need of a constitutive model for high strain rates. Simulations of wave propagation using a known hyperelastic constitutive model highlighted the difficulty of modeling such soft tissues. A viscoelastic non-linear constitutive model based on power laws was considered and is an interesting candidate for future simulations. The simulation technique for wave generation and propagation in a solid (aluminum) and a liquid (water) has been validated by comparing its results with measurements performed in these materials (strain gages and PVDF hydrophone respectively). Simulations of pressure wave propagation validated for solids and liquids showed that they can be applied to biological tissues modeled using a known constitutive model; they are a tool for any other simulation using more complex constitutive models. This work contributes to a broader study aimed at establishing and validating constitutive models for biological materials suitable for use in simulations of wave propagation.

EPFL2009

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EPFL2009