外文文献原文：夹具设计过程形式化的功能方法.pdf

A functional approach for the formalization of the fi xture design process R. Huntera, J. Riosb,*, J.M. Pereza, A. Vizana aDepartment of Mechanical and Manufacturing Engineering, Escuela Tecnica Superior de Ingenieros Industriales, Universidad Politecnica de Madrid, Jose Gutierrez Abascal, 2, 28006 Madrid, Spain b Currently in Enterprise Integration (Bldg 53), Cranfi eld University, Cranfi eld, MK43 0AL, UK Received 14 January 2005; accepted 14 April 2005 Available online 26 August 2005 Abstract The design of machining fi xtures is a highly complex process that relies on designer experience and his/her implicit knowledge to achieve a good design. In order to facilitate its automation by the development of a knowledge-based application, the explicit defi nition of the fi xture design process and the knowledge involved is a prior and a fundamental task to undertake. Additionally, a fundamental and well-known engineering principle shouldbe considered: the functional requirements and their associated constraints should be the fi rst input toany design process. Considering these two main ideas, this paper presents the development undertaken to facilitate the automation of the fi xture design process based on a functional approach. In this context, the MOKA methodology has been used to elicit fi xtures knowledge. IDEF0 and UML have been used to represent the fi xture design process. A methodology based on the function concept and aiming to formalize such design process is proposed. Fixture functional requirements have beendefi ned and formalized. Functional fi xtures elements havebeen used tocreate a functionaldesign solution, the link of these elements with the functional requirements and with typical commercial fi xture components has been defi ned via tables and rules mapping. And fi nally, a prototype knowledge-based application has been developed in order to make an initial validation of the proposed methodology. q 2005 Elsevier Ltd. All rights reserved. Keywords: Fixture design process; Fixture knowledge modelling, Fixture functional requirements 1. Introduction The main objective of any design theory is to provide a suitable framework and methodology for the defi nition of a sequence of activities that conform the design process of a product or system 1. In general, all of them identify requirements as the starting point in the design process. In fact, the engineering discipline dealing with product design can be defi ned as the one that considers scientifi c and engineering knowledge to create product defi nitions and design solutions based on ideas and concepts derived from requirements and constraints 24. For this research, a relevant issue when considering requirements, taking this as a general concept, is to make explicit the meaning of two main terms: Functional Requirement (FR) and Constraint (C). A functional requirement, as it stated by different authors, represents what the product has to or must do independently of any possible solution, 2,4. A FR is a kind of requirement, and considering some basic principles widely recognized in the fi eld of Requirements Engineering, we could add it is a unique and unambiguous statement in natural language of a single functionality, written in a way that it can be ranked, traced, measured, verifi ed, and validated. A constraint can be defi ned as a restriction that in general affects some kind of requirement, and it limits the range of possible solutions while satisfying the requirements. So, a constraint should be always linked to a requirement, and its purpose is to narrow the design outcome to acceptable solutions. Considering the Theory of Axiomatic Design 4, functional requirements should be defi ned in the functional domain, which brings on the scene the issue of how to defi ne and represent the functionality of a product. The way used to represent it will affect the reasoning process of the designer, and in that sense, the mapping between the functional International Journal of Machine Tools fax.: C44 1234 750852. E-mail address: j.rioscranfi eld.ac.uk (J. Rios). and the physical domains, being the later the one where the design solutions are developed. Several authors have investigated the concept of functionality and functional representation 2,58. Their design approach provides a view based on the Function-Behaviour-Structure frame- work, where function is defi ned using structure and behaviour 6. The objective is to fi ll the gap that allows a designer to progress from FRs to physical design solutions. The approach is that product functions are achieved by means of its structure, which seems to lead to an iterative causal approach, where solutions are sought until the selected structure causes the intended functionality. The approach adopted in the research presented in this paper is based on the defi nition of Fixture Functional Components (FFC), which can satisfy the fi xture functionality, and on the mapping between such FFC and fi xture commercial elements. An advanced approach to the defi nition of requirements and functions comes from the creation of ontologies. The ontological approach pursues the defi nition of the meaning of terms making use of some kind of logic, and the defi nition of axioms to enable automatic deduction and reasoning 9. The ontological approach has got a higher relevance since the representation of knowledge is considered the key factor in whatever engineering process, and it has been recognized as a way to facilitate the integration of engineering applications 10, to describe functional design knowledge 7, and to defi ne requirements 11. Considering a purist approach, if an ontology does not include axioms to enable reasoning then it could be considered more like an information model, and in this sense, this is the approach developed in the work presented in this paper. When considering the methodologies developed for the design of fi xtures, it can be stated that in general they are rational and propose a series of steps to follow. For example, the methodologies proposed by Scallan and Henriksen 12, 13, make use of this approach to describe in general terms the information needed in each stage of the fi xture design process. Basically, the importance of modelling in detail such information, which mainly is related to fi xture requirements, fi xture functionality, fi xture components, manufacturing resources, manufacturing processes, and design rules; resides on the possibility to automate the design process through the development of a knowledge- based application or system. It is relevant to mention that several authors have already aimed to develop knowledge- based applications for fi xture design, none of them based on a functional approach, some of the most recently published works can be found in the Refs. 1419. Inthefollowingsections,thispaperpresentsa methodology to formalize the design process of machin- ing fi xturesbasedontheengineeringconceptsof functional requirements and fi xture functions 20. The formalization of the functional requirements is achieved through the application of a structured way of specifi ca- tion via natural language. Additionally, IDEF0, MOKA methodology, and UML diagrams are used to capture, represent and formalize knowledge, being the ultimate goal to facilitate the automation of the fi xture design process. The IDEF0 method is used to create an activity model of the fi xture design process, allowing the identifi cation of the information used in each one of the different tasks it comprises. UML has been used to complement the IDEF0 model by representing the interaction between the activities of the process. The MOKA methodology together with UML, are used to capture and represent knowledge involved in the fi xture design process. Finally, to validate the proposed methodology, partial results obtained from the development of a prototype knowledge-based application are presented. 2. Analysis of machining fi xtures requirements In Section 1, two terms have been defi ned: functional requirement and constraint. Requirements have always existed, the way in which they are expressed, and how they are integrated in the product design process, are aspects that are addressed from different disciplines, for example: product design engineering and requirements engineering among others. In general, Requirements Engineering refers to the discipline dealing with the capture, formalization, representation, analysis, management and verifi cation of requirements fulfi lment. However, all these aspects need to be integrated in the product design process, and require- ments should lead to the defi nition of the possible product design solutions, which in general demands an integrated view of the requirements issue. It is important to keep in mind that the development of such discipline is strongly related to Software Engineering and Systems Engineering, and in fact much of the research related to requirements come from authors from these engineering areas 2123. When considering the analysis of requirements, prob- ably, the fi rst aspect to think about is how the requirements are represented or declared. As it has been previously mentioned, the way of expressing requirements defi nitively affects their interpretation and the creation of a design solution. In this sense, it is widely accepted, that the use of natural language is the most common way of expressing requirements and in consequence, their writing becomes an important issue. The anatomy proposed by Alexander et al. 24 to write requirements in a semi-structured way is used as basis to declare the functional requirements and constraints of fi xtures 20. In machining, work holding is a key aspect, and fi xtures are the elements responsible to satisfy this general goal. In their design process, the starting point is the defi nition of the machining fi xtures functional requirements and constraints. Usually, a fi xture solution is made of one or several physical elements, as a whole the designed fi xture solution must satisfy all the FRs and the associated Cs. Centring, locating, R. Hunter et al. / International Journal of Machine Tools in this case a KBE application for the design of machining fi xtures; and the second one is the functional requirements of the components subject of the application; in this case machining fi xtures. An example of the former ones for an application developed in collabor- ation with an industrial partner is presented by Rios et al. 28. For this kind of FRs specifi cation, UML seems to be Fig. 1. MOKA Entity form for fi xture FRs. R. Hunter et al. / International Journal of Machine Tools it is independent of the knowledge representation to be used in the implementation, and it does not require from the fi xture designer a deep knowledge of any software modelling technique. Thedefi nitionofthesefi xturefunctionsisafi rststepinthe modelling needed for a KBE application development. For example, considering stability as one of the main constraints affecting the fi xture FRs, any fi xture functional solution should satisfy this constraint. To achieve that, it would be necessary to defi ne a fi xture function (FF) for stability Fig. 5. Fixture design process methodology. R. Hunter et al. / International Journal of Machine Tools & Manufacture 46 (2006) 683697688 evaluation, and this function could be called from the fi xture function clamp (clamp_FF) presented as example in the Fig.6.From ahighlevelperspective,thestability_FF would need as input: part information (i.e.: material mechanical properties, shape, dimensions and tolerances), machining process information (i.e.: machining operations, machining strategies, volumes to remove, cutting parameters, cutting tool parameters), and fi xture functional element information (i.e.: function, constraints, rules, containing volume, point and vector of application). Part of this information will have tobeusedtodetermine somederived parameterslikecutting and allowed clamping forces. Making use of such infor- mation together with an analysis model, for example the one proposed by Liao et al. 32, and optimization methods, for example the one proposed by Pelinescu et al. 33, such stability_FF could be developed and implemented. The complexity in the detailed specifi cation of such stability_FF is extremely high, and demands its own research by itself 32,34,35, but the defi nition of a high level function where all the information needed for its development could be represented,isoneoftheobjectivesoftheresearchpresented in this paper. Phase 3: The third phase, functional design (FD), is aimed to create a set of functional solutions for the fi xture design. A functional solution is independent of any particular commercial fi xture component, and it is rep- resented by means of a set of fi xture functional elements. A fi xture functional element satisfi es at least one of the functions identifi ed as inherent to a fi xture, i.e.: centre, position, orientate, clamp, and support. These elements are represented by means of graphical symbols, also called functional symbols, which apart from the functionality also represent some qualifi ers that affect them. Such fi xture functional symbols are based on the technological elements defi ned in the AFNOR standard NF E 04-013 - 1985 36. Fig. 7 presents their structure, which comprises: kind of Part machining: operations strategies cutting parameters cutting tool parameters volume to remove Fixture functional elements: function constraints rules containing volume Optimization method Analysis model Constraints: Deformation Stability Interference Function Clamp (clamp_FF) F4 Part orientation Part support: support points support vectors support surfaces Part location: locating points locating vectors locating surfaces Part information: mechanical properties friction coefficient raw material shape and dimensions part shape and dimensions tolerances Determine cutting forces Determine clamping surface Determine clamping points Determine clamping orientation Determine clamping forces Determine clamping elements Fig. 6. High-level function template representation. Fig. 7. Structure of the AFNOR fi xture technological elements. R. Hunter et al. / International Journal of Machine Tools & Manufacture 46 (2006) 683697689 technology, state of the part surface, function of the technological element, and the kind of contact between the part surface and the fi xture element. In order to progress from the functional design to the detailed one, which is the next phase, it has been defi ned a mapping table between functional symbols and commercial fi xture elements 20, Table 2 represents an example. For the creation of the possible functional solutions a set of input information, analysis models, optimization func- tions, and rules has to be included in the software functions previously defi ned in the second phase. Basically, the inputs defi ned are: Part information: material mechanical properties, shape and dimensions of the part to be machined, and the associated tolerances. Functional element information: functions, associated restrictions, orientation, containing volume, contact parameters, and location point. Part manufacturing process: sequence of operations, and for each operation: machining strategy, cutting para- meters, cutting tool, and volume to remove. Production estimation of: number of set-ups, set-up times, batch size, production rate, and target cost. Resource information: machine morphology, and machine capacity. Functional design brings benefi ts to design environments where the solution is mainly driven by the satisfaction of quantitative functions, as opposite to environments where subjective aspects like aesthetics has a major relevance. In particular, in the fi xture design environment, the advantage of creating a functional solution derives from not using a full library of commercial fi xture elements but a reduced number of basic functional elements, which can be transformed into the former ones in a second design phase. And this is particularly relevant when some kind of artifi cial intelligence technique is going to be applied in the implementation phase, since many of these techniques are based on the initial generation of a complete design space where the possible solutions are contained, if the design space can be reduced then the determination of the solutions can be done more effi ciently. And with the functional design approach the design space is divided in two subsets, one subset dealing with the functional solution and other dealing with the physical one. Phase 4: The fourth phase, detailed design (DD) comprises the creation of detailed solutions from a functional one. To undertake this phase the mapping tables previously mentioned and the corresponding interpretation rules have to be used. To mention as well, that the fi xture software functions apply in a similar way, but with a different input, which basically is the geometry (containing volume) associated with the fi xture element, this is particularly relevant for the interference checking. How- ever, in this case the space of possible solutions is reduced by the fact that only those commercial elements that can be mapped to the functional ones can be used, and that a point of application and an orientation vector for the elements to be used are data as well. A detailed solution contains the fi nal fi xture commercial elements to be used in the machining of the part and their set-up. Finally, the fi fth phase, validation of the design (FV), is aimed to make a fi nal evaluation and validation of the functional requirements and their associated constraints defi nedinthefi rstphase.However,itisimportanttomention that in addition to a fi nal validation, the functional approach, with the separation of the design spaces in two parts, allows imple