Atlas motion platform split-axle mecanum wheel design

The Atlas motion platform was conceptually introduced in 2005 as a 2.90 m diameter thin-walled composite sphere housing a cockpit. Three active mecanum wheels provide three linearly independent torque inputs enabling the sphere to enjoy a 100% dexterous reachable workspace with unbounded rotations about any axis. Three linearly independent translations of the sphere centre, decoupled from the orientation workspace, are provided by a translational three degree-of-freedom platform. Small-scale and half-scale demonstrators introduced in 2005 and 2009, respectively, gave us the confidence needed to begin the full-scale design. Actuation and control of the Atlas full-scale design is nearing completion; however, resolution of several details have proven extremely elusive. The focus of this paper is on the design path of the 24 passive mecanum wheels. The 12 passive wheels below the equator of the sphere help distribute the static and dynamic loads, while 12 passive wheels above the equator, attached to a pneumatically actuated halo, provide sufficient downward force so that the normal force between the three active wheel contact patches and sphere surface enable effective torque transfer. This paper details the issues associated with the original twin-hub passive wheels and the resolution of those issues with the current split-axle design. Results of static and dynamic load tests are discussed.

Heat Transfer in Composite Spheroids

Heat transfer from a composite prolate spheroid under the third-type boundary condition is investigated using a Legendre series expansion. The model is verified against published data on cooling boiled eggs and also against the asymptotic solution of a composite sphere. The impact of Biot number on the heat transfer in spheroids with realistic dimensions and properties, such as eggs and olives, is investigated. The results are also presented for varying conductivity ratios and fractional volume of the inner part of the spheroid.

Two-step Homogenization of Poroelastic Properties of a Limestone

This study aims at deriving the effective poroelastic properties of the oolitic limestones based on the Hashin composite sphere assemblage (CSA) micromechanical theory. The microstructure of oolitic limestones generally exhibits an assemblage of grains (oolites) surrounded by a matrix. Grain and matrix are linked via the interfacial transition zone (ITZ). Pores exist in these three material phases (oolite, ITZ and matrix). A two-step homogenization method is proposed. The first step consists of upscaling the properties of each porous phase (i.e. porous oolite, porous ITZ and porous matrix) in which each phase contains two sub-phases including pore and solid. The differential self-consistent scheme is used for the first step. At the second step, the three different porous constituents (oolite, ITZ and matrix) are assembled in a CSA model. A mathematical analogy between thermoelasticity and poroelasticity is used to obtain the effective poroelastic properties. A comparison between the proposed model and test data on the oolitic limestone from Bourgogne (France) helps to calibrate the model parameters and to highlight the role of ITZ phase.

On the effective viscosity of fresh concrete: A homogenization approach

Two new homogenization schemes together with the classical generalized self-consistent scheme (GSC) and its extension are proposed to deal with the effective viscosity of fresh concrete. All these models exploit the composite sphere morphology of suspension made of a viscous fluid and spherical particles. They differ from each other by the ways the extra fluid zone (EFZ) located in between the composite sphere are treated. The comparison with experimental data shows that the GSC provides very good result for a well arranged particle size scale that allows mapping the whole medium by composite spheres of different size scales. However, the GSC cannot be used for a suspension with a non negligible volume of the EFZ. For such a case, extensions of the GSC those take into account the contribution of the EFZ to the overall viscous behavior of the system is necessary to fit with experimental data. Two of them work very well for non-cohesive particles and the other can be employed for suspension with cohesive particles such as the case of fresh cement paste.

Elastic and inelastic local strain fields in composites with coated fibers or particles: theory and validation

This paper deals with mean field multiscale approaches for coated fiber- or particle-reinforced composites under nonlinear strain. The current work attempts to extend Dvorak’s well-known transformation field analysis for mean field approaches, in which the composite’s constitutive law is split into an elastic and an inelastic part. The classical Eshelby’s inhomogeneity problem considering eigenstrains is revisited in order to address the presence of a coating layer. For this scope, three different methodologies are employed, one for general ellipsoidal inhomogeneities, a modified composite cylinder method for long cylindrical fibers and a modified composite sphere method for spherical particles. After identifying proper interaction tensors for the inhomogeneity and its coating layer, the composite’s overall response is evaluated by extending classical mean field techniques, such as the Mori–Tanaka and the self-consistent methods. Numerical examples illustrate the differences in macroscopic and microscopic predictions between the general approach and the modified composite cylinder and sphere Assemblages.