Benjamin Schleich

Skin Model Shapes: A new paradigm shift for geometric variations modelling in mechanical engineering

Geometric deviations are inevitably observable on manufactured workpieces and have huge influences on the quality and function of mechanical products. Therefore, many activities in geometric variations management have to be performed to ensure the product function despite the presence of these deviations. Dimensional and Geometrical Product Specification and Verification (GPS) are standards for the description of workpieces. Their lately revision grounds on GeoSpelling, which is a univocal language for geometric product specification and verification and aims at providing a common understanding of geometric specifications in design, manufacturing, and inspection. The Skin Model concept is a basic concept within GeoSpelling and is an abstract model of the physical interface between a workpiece and its environment. In contrast to this understanding, established models for computer-aided modelling and engineering simulations make severe assumptions about the workpiece surface. Therefore, this paper deals with operationalizing the Skin Model concept in discrete geometry for the use in geometric variations management. For this purpose, Skin Model Shapes, which are particular Skin Model representatives from a simulation perspective, are generated. In this regard, a Skin Model Shape is a specific outcome of the conceptual Skin Model and comprises deviations from manufacturing and assembly. The process for generating Skin Model Shapes is split into a prediction and an observation stage with respect to the available information and knowledge about expected geometric deviations. Moreover, applications for these Skin Model Shapes in the context of mechanical engineering are given.

A Comprehensive Framework for Skin Model Simulation

The need for geometrical variations management is an important issue in design, manufacturing and all other phases of product development. Two main axioms cover geometrical variations, namely the axiom of manufacturing imprecision and the axiom of measurement uncertainty. Therefore, this paper reviews common models for the description of non-ideal geometry (shape with geometric deviations) and shows how the random field theory can be applied to create more realistic skin models (a model which comprises these geometric deviations).Furthermore, methods to estimate and to express the underlying random field from a sample population are shown. These can be used to create and simulate random shapes considering systematic and random deviations observed through measurement or gathered from manufacturing process simulations.The proposed approach incorporates given information from manufacturing process simulations or prototypes. Based on these information, skin model samples are created which can represent the “realistic” part in assembly simulations or other geometrical analyses. This can help to identify the optimal tolerance sets within every stage of the product development process. The efficiency of the introduced approaches is shown in a case study.Copyright

Approaches for the assembly simulation of skin model shapes

Even though they are weakly noticed, geometric part deviations accompany our everyday life. These geometric deviations affect the assemblability and functional compliance of products, since small part variations accumulate through large-scale assemblies and lead to malfunction as well as decreased product reliability and safety. However, the consideration of part deviations in the virtual modelling of mechanical assemblies is an ongoing challenge in computer-aided tolerancing research. This is because the resulting assembly configurations for variant parts are far more complicated than for nominal assemblies. In this contribution, two approaches for the relative positioning of point based models are highlighted and adapted to the assembly simulation of Skin Model Shapes, which are specific workpiece representatives considering geometric deviations. The first approach employs constrained registration techniques to determine the position of variant parts in an assembly considering multiple assembly steps simultaneously, whereas the second utilizes the difference surface to solve the positioning problem sequentially. The application of these approaches to computer-aided tolerancing is demonstrated, though their applicability reaches various fields of industrial geometry. Skin Model Shapes are digital part representatives comprising geometric deviations.Approaches for the relative positioning of point-based Skin Model Shapes are proposed.The approaches ground on algorithms from computational geometry and computer graphics.Applications for the assembly simulation in tolerancing are given.

Contact and Mobility Simulation for Mechanical Assemblies Based on Skin Model Shapes

Assembly modelling as one of the most important steps in the product development activity relies more and more on the extensive use of CAD systems. The modelling of geometric interfaces between the components of the assembly is of central importance in the simulation of mechanical assemblies. Over the past decades, many researchers have devoted their efforts to establish theories and systems covering assembly modelling. Although the product form or shape have been extensively investigated considering the nominal CAD geometry, inevitable limitations can be reported. Computer Aided Tolerancing systems provide simulation tools for modelling the effects of tolerances on the assembly but still lack of form deviation considerations. The skin model concept which stemmed from the theoretical foundations of Geometrical Product Specification and Verification (GPS) has been developed to enrich the nominal geometry considering realistic physical shapes. However, the digital representation of the skin model has been investigated only recently. This paper presents a novel approach for a skin model based simulation of contact and mobility for assemblies. Three important issues are addressed: the geometric modelling of the contact, the contact quality evaluation, and the motion analysis. The main contribution to computer aided tolerancing can be found in the analysis of the effects of geometric form deviations on the assembly and motion behaviour of solid mechanics, which comprises models for the assembly simulation, for the contact quality evaluation, and for the motion analysis. A case study is presented to illustrate the proposed approaches.

How to determine the influence of geometric deviations on elastic deformations and the structural performance

Geometric deviations are observable on every manufactured part since manufacturing processes are inherently imprecise and measuring processes always involve uncertainties. These geometric deviations have influences on the function and the quality of the product. Therefore, allowable limits for these geometric deviations, depicted as geometric tolerances, have to be set. They define the allowable geometric deviations of a part for which the product function is guaranteed. In this context, elastic deformations and the structural performance are key aspects. However, the manufacturing-caused geometric deviations have an influence on the elastic deformations during use and the structural performance of the parts. In other words, there are interdependencies between geometric deviations and elastic deformations which affect the dimensional accuracy of a loaded part in an assembly. Therefore, this article proposes an approach for determining the impact of geometric deviations on the structural performance. This approach employs techniques such as sensitivity analysis, analysis of robustness and evaluation of reliability, and aims at providing information about the expectable elastic deformations and structural performance to geometric tolerancing. It can be concluded that geometric deviations can have a big impact on the structural performance, and thus, not only parametric deviations but also geometric deviations regarding the form and shape of a part itself should be taken into account within the structural performance analysis and tolerance simulations.

How does testing affect the availability of aging software systems

This paper proposes an approach to examining how testing affects the operational behavior of aging software systems. Such an approach requires models for the testing phase and the operational phase that explicitly account for crash failures due to both aging-related and non-aging-related bugs. We develop appropriate semi-Markov models and derive expressions for computing the respective transient and steady-state probabilities needed. Our numerical examples suggest that disregarding the effects of non-aging-related bugs can result in wrong conclusions about the testing phase and the operational phase. Moreover, we show how to combine the two models for a joint analysis in which metrics of interest concerning the operational phase, such as the optimal rejuvenation rate, are random variables whose distributions are influenced by the potential outcomes of testing.

The Implications of the Skin Model Concept for Computer Aided Tolerancing

During product origination many deviations occur which manifest in unreliable functional behaviour and reduced product quality. Thus, the observable geometric deviations have to be limited by geometric tolerances of function-relevant part features. In this context the standards for Geometrical Product Specification and Verification (GPS) provide helpful tools for geometric tolerancing. A basic concept within these standards is the skin model concept which can be understood as a model of the physical interface between the workpiece and its environment. Recent research tries to translate this concept into discrete geometry and focuses upon the technical aspects related to generating skin model shapes, whereas the implications of this concept and emerging simulation possibilities for the geometric variations management process stay disregarded. Therefore, this paper highlights the consequences of the skin model concept and modern simulation tools for the computer aided tolerancing process and the management of geometric deviations during product development.

Skin Model Shapes: Offering New Potentials for Modelling Product Shape Variability

The modelling of nominal product geometry by computer-aided design tools has gained immense attention in industry during the last decades and is nowadays an integral part of the virtual product development process. However, the established geometry representation schemes for CAD imply severe drawbacks regarding the modelling of geometric part deviations, which are inevitably observed on every manufactured artefact. As a response, the concept of Skin Model Shapes, which stems from international standards for geometric product specification and verification, has been developed as a novel approach for the consideration of product shape variability. It employs discrete geometry methods and computational techniques, such as point clouds, surface meshes and geometric processing, to model shape variability and to facilitate the communication of geometric product information throughout the product design, manufacturing, and inspection processes. This paper highlights the foundations of this concept, demonstrates its potentials for the representation of product geometry considering geometric variations along the product lifecycle, and illustrates main applications in the context of computer aided product and process development. In this regard, a focus is laid upon recent results and contributions, such as contact modelling, tolerance analysis, and motion tolerancing based on Skin Model Shapes. Furthermore, challenges for future research, such as the application to complex shapes and compliant parts, as well as the consideration of novel manufacturing processes, are discussed.Copyright

Novel approaches for the assembly simulation of rigid Skin Model Shapes in tolerance analysis

Abstract While the modelling of nominal assemblies is sufficiently solved in modern computer-aided design tools, the assembly simulation for parts considering geometrical deviations is still an important research issue. Particularly in tolerance analysis, the assessment of the effects of geometrical part deviations on the part assembly behaviour is of central importance. Though established assembly simulation approaches for parts with geometrical deviations cover a moderate range of real assembly problems, they are not suitable for all assembly simulation problems. To overcome this shortcoming, a general framework and new approaches for the assembly simulation of Skin Model Shapes for the use in tolerance analysis are presented. The application of these assembly simulation approaches is highlighted in generalized case studies and recommendations for their use in tolerance simulations are derived.

Cost Optimality in Testing and Rejuvenation

In this paper, we extend models for the test phase and the operational phase with cost aspects, to explore cost optimality in testing and rejuvenating software systems that can age. For example, we find that under limited resources it may be advisable not to focus on the removal of aging-related bugs in the test phase, rather, the adverse affects of such bugs should primarily be prevented via software rejuvenation during operations. Our model-based analysis thus helps give a further theoretical foundation for the importance of software rejuvenation.