PhD on the tolerancing of a machine tool with an overconstrained parallel structure (M/F)

Job title:

PhD on the tolerancing of a machine tool with an overconstrained parallel structure (M/F)

Company:

Job description

Offer DescriptionTeam :
Hélène Chanal (MCF-HDR), Pierre Beaurepaire (MCF), Benoit Blaysat (PU), ..
funding :
Three-year doctoral contract with Clermont Auvergne INP, funded by the French National Research Agency (ANR)
Location : SIGMA Clermont – Institut Pascal
partner : LS2NThe aim of this project is to develop a new Exechon-type parallel robot architecture that can be moved for machining and drilling tasks on large parts in the aerospace and naval sectors. This architecture has to meet both stiffness and lightness requirements in order to minimise the energy consumption needed to set it in motion. With this in mind, the originality of the project lies in its focus on robots with an overconstrained parallel architecture (OPKM ); this type of architecture makes it possible to reduce the weight of the moving parts while offering high stiffness performance. The design of OPKMs involves cross-disciplinary and innovative work in scientific areas such as mechanism synthesis, hyperstatic mechanism tolerancing and modelling the behaviour of the robot architecture under the stresses induced by hyperstatism. The challenge is to be able to control the behaviour and manufacturing cost of this type of robot in terms of weight savings and energy consumption.
In this project, we want to develop tools that can be used in industry to design an OPKM to control its cost and behaviour while reducing the mass in motion. The ambition is to be able to propose a new OPKM architecture that will result in energy savings that will offset the additional cost. Such a study does not exist, even though the results of work on Exechon-type OPKMs show their suitability for machining and drilling large parts.Presentation of the thesis topic:
This thesis will focus on the design of a new OPKM. Works will be oncducted to define the architecture and on the definition and quantification of acceptable/tolerable defects at the joints of an OPKM regarding the required machining quality.
The aim is to introduce a new tolerancing method dedicated to each element of an OPKM to guarantee its assembly and final precision. This method should be used for mechanism synthesis. At present, OPKMs are considered too expensive to be a real alternative to machine tools to reduce energy consumption. Controlling their tolerancing cannot be achieved simply by adding clearances and following geometric approaches, as can be done in the literature. In fact, adding clearances is not possible given the precision required and the stresses induced by hyperstatism. Probabilistic approaches coupled with an elasto-geometric model that is quick to simulate must therefore be implemented.
The method for analysing OKPM tolerances will be adapted on the basis of those for overconstrained mechanisms with clearances. However, the defects of each surface involved in the various joints will not be modelled explicitly. This is because there are no clearances to accommodate these defects and the analysis of the surfaces is carried out on a scale that is too small compared with that of OKPM. Work will be carried out to introduce the notion of kinematic joints into the tolerancing method.
In order to be able to carry out all the simulations required for tolerance synthesis, it is necessary to develop an elasto-geometric model dedicated to OPKMs and offering reduced simulation times. To carry out this work, we will rework the vector formalism that we have already developed to integrate the deformations of the links and elements of the architecture of an OPKM (Guyon et al., 2023). The new formalism should make it possible to introduce defects exhaustively while controlling their number. Two types of tolerancing will appear, depending on whether the geometric parameter is constrained by overconstrained. Geometric imperfections will therefore be the geometric defects introduced by the formalism. The imperfections will be modelled by random variables and simulation (Monte Carlo for example) will be used to propagate their effects to the OKPM. The parameters associated with the active links will be modelled as free variables because they do not take a fixed value, but must be able to vary over a predefined interval. An approach based on ∀ and ∃ quantifiers will be implemented to guarantee correct operation for all free parameter values, i.e. throughout the OPKM workspace. Work will be carried out on formalising the requirement, which could be, for example, the torque to be applied to the motor to carry out a movement. Because of hyperstatism, an additional effort must be applied to change the deformation of the robot architecture during movement. As a result, the tolerances implemented will be constrained by the need to assemble and not block the robot, and by the final precision expected of the robot.
The work should make it possible to propose an elasto-geometric model incorporating assembly and manufacturing faults in the OPKM architecture. It will be based on a synthesis of the dimensional tolerances of the robot’s components as a function of the expected precision and stiffness of its architecture. This work will lead to a compromise between the size of the tolerance intervals (and therefore the cost) and the stiffness of the tool. It will also enable the assembly positions of the robot architecture to be identified.
This work will enable us to assess the impact of architecture and stiffness on tolerance intervals. In this way, we will be able to assess whether Exechon-type OPKMs are appropriate solutions for machining large parts.reference:
Guyon, J.B., Chanal, H., Boudon, B., Blaysat, B., 2023. Geometric modeling with small defects of over-constrained Parallel Kinematic Machine. Mech. Mach. Theory 179.RequirementsResearch Field Engineering Education Level Master Degree or equivalentResearch Field Physics Education Level Master Degree or equivalentLanguages FRENCH Level BasicResearch Field Engineering » Materials engineering Years of Research Experience NoneResearch Field Physics » Acoustics Years of Research Experience NoneAdditional InformationWebsite for additional job detailsWork Location(s)Number of offers available 1 Company/Institute Institut Pascal Country France City AUBIERE GeofieldWhere to apply WebsiteContact CityAUBIERE WebsiteSTATUS: EXPIRED

Expected salary

Location

Aubière, Puy-de-Dôme

Job date

Thu, 28 Mar 2024 04:55:17 GMT

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