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Knowledge-based application to define aircraft final assembly lines at the industrialisation conceptual design phase a
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Fernando Mas , José Ríos , Alejandro Gómez & Juan Carlos Hernández a
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PLM Process & Tool Solutions Department, AIRBUS Group, Sevilla, Spain
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Mechanical Engineering Department, Polytechnic University of Madrid, Madrid, Spain Published online: 17 Jul 2015.
Click for updates To cite this article: Fernando Mas, José Ríos, Alejandro Gómez & Juan Carlos Hernández (2015): Knowledge-based application to define aircraft final assembly lines at the industrialisation conceptual design phase, International Journal of Computer Integrated Manufacturing, DOI: 10.1080/0951192X.2015.1068453 To link to this article: http://dx.doi.org/10.1080/0951192X.2015.1068453
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International Journal of Computer Integrated Manufacturing, 2015 http://dx.doi.org/10.1080/0951192X.2015.1068453
Knowledge-based application to define aircraft final assembly lines at the industrialisation conceptual design phase Fernando Mas
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*, José Ríos
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, Alejandro Gómezb and Juan Carlos Hernándezb
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PLM Process & Tool Solutions Department, AIRBUS Group, Sevilla, Spain; bMechanical Engineering Department, Polytechnic University of Madrid, Madrid, Spain
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(Received 8 September 2014; accepted 29 June 2015) The design of an aircraft Final Assembly Line (FAL) is part of the product industrialisation process. The FAL conceptual design phase is characterised by being time-consuming and depending heavily on personnel knowledge and experience. The need to develop methods and tools to enhance the design of aircraft assembly lines is acknowledged by academia and industry. This work proposes a knowledge-based prototype application to assist and guide designers in the definition and evaluation of conceptual FAL alternatives. A digital FAL is part of the industrial digital mock-up (iDMU) and comprises three structures: product, processes and resources (PPR). The implementation of the proposed application was carried out in a commercial software system supporting the PPR structures and the iDMU. The executed case studies show the feasibility of the proposed approach, which can be considered as a starting point contribution to the field. Keywords: aircraft final assembly line conceptual design; aircraft assembly line modelling; knowledge-based application development; iDMU
1. Introduction An aircraft Final Assembly Line (FAL) is a complex industrial installation that involves assembly processes, jigs, tools, machines, industrial means and skilled human resources. The decisions taken along the product conceptual design phase are decisive in its final design and in its industrialisation solution, and therefore they have a strong influence on the design of the FAL. At the conceptual design phase, several disciplinerelated teams work on stakeholder requirements, preliminary product definition, commercial agreements, marketing studies, functional capabilities, aerodynamic studies, industrial capabilities and industrialisation solutions. A Program Management Office (PMO) coordinates and manages the conducted work, and proposes different business cases to be evaluated with the aim of securing cost. The manufacturing engineering group is in charge of the product industrialisation and has to evaluate the different business cases against possible industrial solutions for the aerostructures and the FAL. The process of generating FAL alternatives depends heavily on the personnel experience and knowledge, includes many design routine tasks and is time-consuming. Industrialisation design engineers use mainly CAD and office tools (CAD files, spreadsheets, standards, PERT and Gantt charts, requirements documents, etc.) to evaluate a set of industrial solutions. Due to the time-intensive nature of the routine tasks, the evaluated set of solutions is limited and in consequence, it restricts a full exploration of feasible alternatives (Mas *Corresponding author. Email:
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2014). This statement might seem confined by the industrial environment, where the research has been conducted, but literature shows that the conceptual design phase is less supported by knowledge-based and predictive solutions than other phases of the aircraft development (Mavris and DeLaurentis 2000; La Rocca, Krakers, and Van Tooren 2002; Choi, Kelly, and Raju 2007; Feng et al. 2011; Verhagen et al. 2012a, 2012b). In consequence, this situation is an issue and raises the question: how the process of conceptual design of aircraft FAL could be improved. Prior to provide an answer to that question, it is important to show its relevance. Two cost-related facts and literature show the significance of the product conceptual design phase and the assembly line design. First, up to 80% of the final aircraft cost is determined during the conceptual design phase. Second, up to 30% of the final cost is due to assembly operations performed on conventional aircraft structures built up from parts (Raju 2003). Literature also shows that considering a top-level approach, the conceptual design process of an assembly line is considered similar to a product conceptual design process (Chow 1990; De Lit and Delchambre 2003). However, product characteristics, company-specific practices and industrial sector practices influence the design process, as the analysis of industrial cases and the review of lower level processes show (Wiktorsson, Andersson, and Broman 2000). Because, specific knowledge is used when conducting the assembly line conceptual design
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tasks, several authors have acknowledged the need to develop specific methods, guidelines and software applications to support assembly line designers (Wiktorsson, Andersson, and Broman 2000; Barnes 2000; Khan and Day 2002; Swift, Booker, and Edmondson 2004; Anselmetti and Fricero 2012). To avoid relying on experience and rules of thumb, literature shows the need to develop formal models that comprise industrialisation design and manufacturing knowledge (Fowler and Rose 2004). At the same time, the rationalisation and automation of design in the conceptual and preliminary design phases is a strong argument in favour of creating models to develop knowledge-based applications (KBAs) (Verhagen et al. 2012b). Knowledge-based engineering (KBE) comprises the use of software techniques to capture and reuse product and process knowledge in an integrated way (Verhagen et al. 2012b). In conclusion, the use of KBE is a possible answer to the previously raised question: how to enhance the conceptual design process of assembly lines. The proposed solution is a software tool, based on conceptual assembly line design knowledge and developed within a commercially used software system, to assist and guide industrialisation engineers in generating and evaluating FAL configurations. This paper presents a proof of concept KBA, integrated within a commercial software system widely used in the aerospace sector, to help industrialisation design engineers to generate feasible FAL design solutions at the conceptual design phase. The proposed approach has two main contributions. The first contribution is a model for the conceptual design process of aircraft assembly lines (Mas et al. 2013a). Such model is the basis for the
Figure 1.
development of the KBA. The second contribution is the proof of concept KBA itself. The paper is structured into five main sections. The first section shows the context of the work within the aircraft assembly line design phases. The second section provides a literature review on modelling of aircraft assembly line design. The third section shows the proposed method. The fourth section presents briefly the proposed model for the conceptual design of aircraft assembly lines. The fifth section presents the developed application and the results obtained when used in a case study.
2. Context: the aircraft assembly line design phases As it was previously mentioned, from a conceptual toplevel view, the design process of an aircraft assembly line is similar to a product design process. It comprises three main phases: concept, definition and development; and is shifted from the product design process as is shown in Figure 1. In Airbus, the design of a FAL starts with the milestone M3 and ends in the milestone M9. The milestone M8 marks the end of an aero structure assembly line design, to allow the integration of aerostructures and components into the FAL (Ríos, Mas, and Menéndez 2012; Mas 2014). The first phase: concept; comprises the creation of the conceptual assembly process or the assembly line definition. At this phase, the definition of the assembly line includes: capacity of the line, number of stations, basic technologies to be used, stations order, priority of each station, the input product top-level structure and the output top-level product structure at each station, human
AIRBUS product lifecycle and development milestones (Mas et al. 2013a, 2013b).
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Figure 2.
Example of FAL at the design concept phase.
resources policy, shared resources and the preliminary layout. At this phase, there is concurrency with the product conceptual design phase. At this phase, both the distribution of the industrialisation work between the different assembly plants and the technologies to be used are still under definition. The ‘as designed’ product structure is used as input, and feasible ‘as planned’ and ‘as prepared’ product structures are initially evaluated. Figure 2 shows a graphical example of a FAL design concept at this phase. The second phase: definition; comprises the definition of the assembly tasks to be executed in each assembly station or the assembly process definition. At this phase, concurrency occurs between product functional design and industrial design to harmonise the aircraft design solutions and the manufacturing solutions for the aircraft industrialisation, as a result an ‘as designed – as planned’ product structure is agreed. As a consequence, feasible ‘as prepared’ product structures are derived from the agreed ‘as planned’ product structure. In this phase, balancing of the virtual workstations and FAL, together with 3D assembly tasks simulations are carried out. Figure 3 shows a graphical example of the FAL Workstation 70 definition at this phase. The third phase: development; comprises the definition of the elementary assembly tasks or detailed assembly
Figure 3.
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process development. The ordered set of elementary assembly tasks constitutes a manufacturing solution. Concurrency occurs with the aircraft functional design development phase. The main tasks deal with documenting the elementary tasks in detail by creating work instructions. Information is extracted from the company manuals of standard processes and times. Process times and personnel allocation are refined and returned to the prior definition phase to carry out a more precise balancing of the virtual FAL. Because of this phase the ‘as prepared’ product structure is finalised. Figure 4 shows a graphical example of the FAL at this phase. Using IDEF0 to define the context of this work, a FAL conceptual design process model was created (Mas et al. 2013a). The Concept Phase of the FAL design process corresponds with the activity ‘Create Conceptual Assembly Process’ (A221) (Figure 5). The IDEF0 semantic difference between input elements (they are transformed by the activity into output) and control elements (they are required and used to produce the correct output) allows the modelling of the A221 activity with only control elements. It represents that functional design owns the ‘as design’ product definition and points out the concurrency with industrial design. The collaborative approach would imply to model such element as an input. The control ‘program management and
Workstation 70 (Empennage Integration) at the design definition phase.
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Figure 4.
Joint (Tail Cone and FS19) partial example at the design development phase.
Figure 5.
Activities of the industrialisation of a FAL and KBA context.
planning’ comprises the business case or scenario, defined by the PMO, and used by the manufacturing engineer in the definition of the FAL. The mechanisms to perform this task are concurrent engineering tools (PLM system, CAX and Digital Process Engineering tools) and the proposed KBA. The A221 activity is in its turn decomposed into three sub-activities: A2211 ‘generate as planned proposal’, A2212 ‘generate assembly lines’ and A2213 ‘evaluate assembly lines’ (Mas et al. 2013a). The outputs from the three activities, A221, A222 and A223, comprise the industrial Digital Mock-Up (iDMU). The information contained in the iDMU evolves and increases from the Conceptual phase to the Detailed Phase, and comprises information (requirements) related to the Product (P), the manufacturing Processes (P) and the Resources (R) used in the processes. That is named as product, process and resource (PPR) structure. An aircraft has different configurations to satisfy the specific requirements of each customer, the link between the 3D design (aircraft Digital Mock-Up) and the aircraft configuration management is the ‘Product Configured DMU’. The cDMU provides the right design data for each aircraft
configuration or variant, and it has a single Product (P) structure (Gabarde and Dolezal 2007; Dolezal 2008). 3. Review on aircraft assembly line design knowledge modelling and related knowledge-based developments The information modelling of aircraft assembly line design and the KB development requires reviewing works dealing with modelling of assembly information, processes and lines. In this respect, and since it is not usually performed at the conceptual design phase, it is important to mention that Assembly Line Balancing (ALB) and discrete event simulation are out of scope of this work. However, the review of a selected set of relevant references was conducted to identify the information concepts involved and to avoid incompatibilities with any further extension of the proposed model. ALB is a distinctive research line that provides a different view to the modelling of assembly information. There is extensive literature dealing with models to support ALB, where given a process net with precedence constraints, simulation aims evaluating cycle times, assignment of resources or other characteristics. As a
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International Journal of Computer Integrated Manufacturing relevant example, since combines KBA with ALB, Khan and Day (2002) proposed an assembly system configuration defined by the product (single product, multiproduct or mixed product) and by the processing times (stochastic or deterministic). The KBA proposed a cycle time and executed the line balancing. The information used by the KBA was: demand forecast, tasks, tasks duration, precedence relationships, shifts per day and working days. Focused on an aircraft assembly line, Heike et al. (2001) formulated four models (linear and nonlinear) to evaluate cycle time and worker allocation. The models considered requirements related to: tasks, workstations and labour. The information used in the models is included in the ALB information vectors: α (precedence constraints) and β (workstations and assembly line constraints); proposed by Boysen, Fliedner, and Scholl (2007). Also focused on aircraft assembly, Jin et al. (2008) proposed an activity time and value-based metric for evaluating the potential of improvement of an assembly workstation. Boysen, Fliedner, and Scholl (2007) present a summary of the type of information contained in ALB models, which was reviewed and taken into consideration in this work. Literature shows that when reviewing the modelling of assembly information, the main approach takes a product point of view, focused on the physical description of the joints and intended for the definition of the lowest level of assembly tasks and sequence planning. One of the earlier works was proposed by Liu (1992), who used the International Standard for Product Model Data Representation and Exchange (ISO 10303 – STEP) to create an assembly information model. The model was structured into five schemas: nominal shape, form features, tolerances, mechanical part and assembly. The assembly schema contained concepts such as: assembly model, joint, operational joint, fastener joint, primary part, secondary part, connection, joint type and assembly operations; among others. The attributes to define an assembly operation focused on the physical description of the joint to be carried out: starting point, reference axis, direction, distance, torque and degree. Based on the type of joint definition: operational, fusion and fastener; basic rules were derived to define if the assembly demanded only a mating operation, both a mating and a fastener operation, or both a mating and a welding operation. Based on Liu’s model (Liu 1992), Zha and Du (2002) proposed an assembly process planning tool that used information related to: parts and connectors involved in the assembly, fit and contact relationships between parts and fastening relationships. With a similar approach and on the basis of the Core Product Model developed by the National Institute of Standard and Technology (Sudarsan et al. 2005), Rachuri et al. (2006) proposed an assembly-related extension to incorporate kinematics concepts and to model different types of design constraints related to: tolerances, kinematics and part joints. Also with a product view approach,
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Barnes, Jared, and Swift (2004) with a decision support tool, Dong et al. (2007) with a KBA and Su (2009) with an optimisation tool, considered geometric constraints, mating joint constraints and precedence constraints to generate feasible assembly sequences. With a slightly wider perspective, Kim, Manley, and Yang (2006) proposed an assembly model considering assembly operations. In addition to product-related concepts such as: assembly, assembly feature, mating feature, mating bond, joint feature, mating pair, part and form feature; the concept of joining process was defined for three types of processes: adhesive bonding, riveting and welding. The assembly relations were derived from the relations defined between form features of different parts. In addition to the product view, Lanz et al. (2008) pointed out that a process-view approach was needed to address the assembly process and the resources used for its execution. Wang et al. (2009) also mentioned that processoriented modelling was considered the best approach to develop decision tools for assembly planning in a concurrent environment. Lanz et al. (2008) proposed a high-level product–process semantic network that included processrelated concepts: activity, operation, task, action, process, sub-action, actor, skill, tool, human, device, resource, area, station, line, factory, cell, machine and robot. Even though, the concepts were not modelled further to specify their attributes and methods, it constitutes an antecedent in the process-view approach adopted in this work. Outside of the assembly domain, Guerra-Zubiaga and Young (2008) took also a process-view approach to design a manufacturing knowledge model, comprising the following main top-level concepts: facility, facility knowledge, process and resource. The detail of the model focused on machining processes (milling and turning) and resources (machinery and cutting tools), and related knowledge, mainly of the types explicit and implicit. In this case, the model did not comprise concepts to support both the functional and the industrial structure of a product, and the process concept was mainly intended for descriptive purposes and to allow having explicit and implicit knowledge linked to it. The explicit knowledge concept was used to define decision tables or procedures to document the elementary tasks needed to execute a particular process (i.e. drilling). With a perspective different from the information modelling and KBA domains, Anselmetti and Fricero (2012) proposed an aid tool for the design of process and aircraft assembly lines at the conceptual phase. They proposed to use a graphical notation, based on the standard IEC 60848 (GRAFCET), to represent the aircraft assembly process in a FAL and to develop a software application to support such notation. The notation represents an operation as a step entity (rectangle), where the input (product/s) is depicted in the upper side of the rectangle and the output (product) is depicted in the
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lower side of the rectangle. The step entity comprises four attributes: description (string), activities (list of elementary tasks), detailed definition (string) and CAD_model (link to a file). This notation, although intended as a tool to describe quickly a process and its associated production facility, produces quite complex multi-level diagrams when aiming to model a real industrial process. As a conclusion from the literature review, it can be stated that the semantic concepts involved in the conceptual design phase of an aircraft assembly line were not fully considered in any of the identified models or works with three main views: line balancing, product and process. Therefore, the modelling of the FAL conceptual phase demanded to integrate and to extend concepts from the three views, particularly from the process view. The next section presents the premises, inputs and tools that characterise the context, where the conceptual design of aircraft FAL is carried out, and points out the differences of the proposed approach in comparison with the reviewed works. 4. Method: premises, inputs and software tools Making explicit the premises, inputs and tools that characterise the context of the aircraft FAL conceptual design allows having a better understanding of the method adopted in this work as an explicit starting point contribution to the field. The join of large fuselage elements (aero structures) using mainly screws and rivets characterises the assembly process in a FAL. The elements to be jointed need to be properly aligned prior to the execution of the union process itself. In the alignment process, specific tooling and jigs are used to support and locate the parts. The technical solutions adopted in the design of the FAL are influenced by the large dimension of the parts, the need to guarantee the assembly tolerances and the automation trend in the execution of the union process. In addition to the joining processes of fuselage elements, i.e. nose, forward fuselage, central fuselage, wings, aft; a FAL accommodates processes dealing with installation of systems and equipment, and tests. In general, the fuselage joining processes determine the configuration of the main part of the FAL into several workstations. Each workstation has its own configuration in terms of: jigs, tools, machines and industrial means; suited to the processes to be executed in it. The number of workstations of the FAL relates to technological criteria rather than to a calculation aiming to minimise the total number of workstations. The FAL is designed for a single product with variants and the cycle time is a given data and equal for all the stations (synchronous line). Tasks are allocated to each workstation manually; the total number of allocated tasks ranges from about 50 to 400, and more than 50% of the tasks have not precedence constraints. Due to assembly technical constraints and the
need of using a specific tooling that is only available in a particular workstation, certain tasks can only be executed in a particular station. Task times ranges from several minutes to several hours. Product variants may imply that certain tasks have a duration time depending on the variant. This situation is addressed by considering a mean task time at the conceptual phase. Each task has human resources allocated defined by their specialty and number. Human resources are considered single specialty. The joined elements remain in each workstation until all the processes assigned to the station are completed (Ríos, Mas, and Menéndez 2012; Mas 2014). These characteristics of an aircraft FAL make that solutions, aiming to enhance the conceptual design of the FAL, differ from the solutions adopted, also for the conceptual definition, in other industrial sectors, e.g. in the automotive sector (Michalos, Makris, and Mourtzis 2012). In addition to the scenario defined by the PMO (Mas, Ríos, and Menéndez 2012), the ‘as designed’ product structure is the main input to the FAL conceptual design phase. Such product structure provides the product functional design view. The allocation of the industrial workload to the different assembly plants is represented in the ‘as planned’ product structure. It provides the industrial view and each node exists physically in the form of a real aircraft sub-assembly. Both structures, although containing different intermediate nodes, must point to the same configuration layer where components, joints and CAD models are defined (Gabarde and Dolezal 2007). As part of the scenario, different industrial workload distributions, i.e. ‘as planned’ structures, must be evaluated. In order to carry out a pilot development and implementation, to prove the conceptual approach adopted in the research, industrially used commercial software had to be selected. The software had to provide an Application Programming Interface (API) as development environment and support the concept of iDMU that comprises: product, processes and resources (Mas, Ríos, and Menéndez 2012; Mas et al. 2013b). The selected software was the Dassault Systèmes’ solution: CATIA/ DELMIA v5. In comparison with the approaches proposed by other researchers, literature review shows no evidence of any other company’s design process publicly available. Only two works address the conceptual design of assembly lines: Anselmetti and Fricero (2012) and Michalos, Makris, and Mourtzis (2012). The former focuses on a graphical notation to represent assembly processes, and its application to a case study of an aircraft FAL. The later focuses on the definition of automotive body in white assembly line alternatives using robotic welding technologies. Regarding the assembly information modelling, most of the works focus on the physical definition of the joints aimed to define the lowest level of an assembly task for sequence planning. Although, two works propose a
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process view, and they can be partially considered as antecedent works: Lanz et al. (2008) and Guerra-Zubiaga and Young (2008). The former proposes a high-level product–process semantic network that includes processrelated concepts, but which are not developed further. The later proposes a model comprising processes and resources, but the process concept is mainly intended for descriptive purposes and to allow having explicit and implicit knowledge linked to it. As a conclusion, what makes this work different is the proposal of an aircraft assembly line conceptual design model supporting: Product, Processes and Resources; which are the basis for the creation of the iDMU (Mas et al. 2013b). The proposed model is briefly presented in the next section, a full description can be found in Mas et al. (2013a) and in Mas (2014). 5. Aircraft assembly line conceptual design model The method to develop the model for the conceptual design of aircraft assembly lines is based on the creation of three main components: the ‘as is’ activity model using IDEF0 notation, the ‘to be’ process model using IDEF0 notation and the knowledge model using unified modeling language (UML) notation and knowledge tables (Mas et al. 2013a). The proposed model is the basis for the development of a KBA to support the activity ‘create conceptual assembly process’ (A221) represented in the Figure 5. The creation of the ‘as is’ model started with two main actions. First, the analysis of the product design process described in textbooks on engineering design and the analysis of the assembly line design process described in textbooks on assembly line design. Most textbooks on engineering design provide a similar general structure for the design process with three main phases: Conceptual Design, System level or Embodiment Design and Detail Design (Pahl and Beitz 1996; Dieter 2000). Textbooks on assembly line design rather than providing a systematic process provide a general view of the elements involved in the assembly line design and tend to focus on the ALB problem (Chow 1990; De Lit and Delchambre 2003). Second, the analysis of the AIRBUS available industrial practices from previous aircraft projects. Due to the difficulty of finding other companies’ publicly available assembly process references, a wider industrial context analysis was not possible. The analysis of the ‘as is’ model allowed to identify two main elements: process improvements actions and activities where KBE technology could be used. The ‘to be’ process model was defined with the focus on the industrialise activity, and aiming to eliminate particularities, due to the confined industrial environment where the research has been conducted (Figure 5). Mas
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et al. (2013a) present a detailed description of such model, which should be considered as a starting point in the field. The knowledge model takes as a reference the concepts comprised in the output elements: ‘conceptual industrial requirements’, ‘conceptual jigs and tools requirements’, ‘conceptual as planned requirements’ and ‘conceptual as prepared configuration’; and control elements: ‘program management and planning’ and ‘as designed proposal’; of the activity ‘create conceptual assembly process’ (Figure 5). To support the creation of the iDMU, the knowledge model was structured into three interrelated Knowledge Units (KUs): Product, Processes and Resources. Relations between classes model the interrelation between the three KUs. Since the evaluation of the conceptual assembly line alternatives relates to estimate: space requirements, transportation requirements, cost and operation time; the different classes defined in each KU comprise the needed attributes and methods for such estimations. Figure 6 shows the main classes of the created knowledge model (white background boxes). 5.1. Product knowledge unit The Product KU supports the definition of all the aircraft components, both the functional (‘as designed’) and the industrial (‘as planned’ and ‘as prepared’) views, and the definition of all the joints to be assembled. The ‘as prepared’ view derives from an ‘as planned’ view, comprises an assembly sequence defining the order of execution of the required joints, and shows a hierarchical precedence structure with possible parallel branches. The product, at its different levels and views, is considered as a ‘functional node’ and as an ‘industrial node’. Ultimately, the lowest level of the product tree is defined by elements of the class ‘component’. The product views are made of a tree of nodes, functional or industrial, ultimately based on the same final layer of components. A ‘component’ (e.g. front fuselage, centre fuselage) is conceptually invariant. A lower level class named ‘cad model’ was defined to accommodate the concept of product variant; for instance, the same aircraft may have a variant with a standard centre fuselage and another one with a stretched centre fuselage. This implies that a ‘component’ may have associated more than one ‘cad model’. The ‘cad model’ class has an attribute named ‘range of aircrafts’ to define the range of aircrafts where each ‘cad model’ will be used to build up the DMU. A ‘joint’ class links two components and has two attributes to define the joint type depending on the type of material of the components to be joined: metal– metal, metal–composite and composite–composite; and the type of components to be joined: fuselage–fuselage, fuselage–wing box and part–part-interchangeable. The class ‘joint’ is linked to the process class ‘basic assembly process’.
Figure 6.
Main classes of the proposed model and mapping into CATIA/DELMIA v5.
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5.2. Process knowledge unit The Process KU supports the definition of the process to execute joints, in terms of technology, sequencing and resources. The assembly process is materialised in workstations and its corresponding assembly line. All the needs related to technology, sequencing and resources are collected in workstations. Workstations are grouped into the assembly line to create a consistent assembly solution. Rules and procedures are applied to compile all the information. The ‘assembly line process’ produces the complete aircraft using the ‘assembly line resource’, collects and consolidates all the information coming upstream from the workstation level. Industrial means, human resources and cycle times are calculated from the workstation level data. The ‘assembly station process’ collects the individual basic assembly tasks. Basic assembly tasks are grouped logically according to their characteristics or their use of resources. An ‘assembly station process’ makes use of the ‘assembly station resource’ to create an intermediate product node that is part of the ‘as prepared’ product structure. The ‘basic assembly process’ defines the manufacturing solution to execute the assembly of two components that constitute a joint. A ‘basic assembly process’ can be of any of the following six standard types: structure, electrical, furnishing, system installation, painting and sealing, and test. A ‘basic assembly process’ of
Figure 7.
Example of process-related knowledge tables.
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type: structure; has a set of tasks to be executed depending on the value of the two attributes: ‘joint component type’ and ‘joint material type’; of the linked joint. The explicit knowledge about the processes was represented by means of knowledge tables (Figure 7).
5.3. Resources knowledge unit The Resources KU supports the definition of three main types of resources: ‘jigs & tools’, ‘industrial means’ and ‘human resources’. ‘Jigs & tools’-type relates to the resources whose design is strongly linked to the product design (e.g. a jig to assemble a horizontal tail plane or a NC machine to mill the central wing box). ‘Industrial means’-type relates to resources that can be purchased from a supplier and are defined by standard or catalogue characteristics (e.g. cranes, vehicles, manual riveting machines). ‘Human resources’-type, the aeronautical assembly is a labour-intensive process, is necessary to estimate the personnel and skills requirements. In addition to these main resource types, there are three container classes to collect resources upstream: ‘assembly line resource’, ‘assembly station resource’ and ‘basic assembly resource’. The ‘assembly line resource’ collects stations and resources that cannot be assigned to any specific station (e.g. a hangar, a crane). The ‘assembly station
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resource’ collects basic assembly resources and resources that cannot be assigned to a particular basic assembly process (e.g. an assembly stand). The ‘basic assembly resource’ collects all the resources attached to each ‘basic assembly process’. 6. FAL conceptual design assistant tool prototype Once the Aircraft assembly line conceptual design model was created, the eventual aim was to implement it in a proof of concept assistant tool, integrated within CATIA/ DELMIA v5, to help industrial designers to generate and evaluate FAL alternatives by defining scenarios and using knowledge rules. The visual integrated development environment comprises three main sections for the definition of: modules, forms and classes. The modules structure the application and contain the coding. The forms define the interface with the user and the events triggered by the user-interactive actions. The classes coding define the data structure (attributes and methods) of the application. The programming language is Visual Basic for Applications (VBA) and the software API. Prior to any coding, the first implementation step was to map the defined classes into commercial software elements and classes.
nodes appear, meaning that the ‘as planned’ view and the ‘as prepared’ view are not exactly equal and so neither the corresponding trees. The classes ‘as prepared tree’, ‘link as prepared nodes’ and ‘as prepared node’ were defined to accommodate this situation. The classes ‘AsPlanned’ and ‘AsPrepared’ were defined to generate the corresponding product structures. Each product structure is defined in a tree and stored in a file of type CATProduct. A CATProduct file contains the 3D representation of multiple components by pointing to the corresponding CATPart files. The nodes of a product tree can point to a CATProduct file (intermediate nodes) or to a CATPart file (lowest level nodes are components that point to their corresponding 3D geometric representation). The ‘joint’ class has a relation with the class ‘component’. Every joint has two components, and is defined in a CATProduct file. The ‘joint’ class has a link to the class ‘basic assembly process’. An object of type ‘basic assembly process’ is defined in a CATProcess file. Due to the hierarchical nature of the process tree, where an assembly line process contains assembly station processes and those contain the basic assembly processes, a single CATProcess file contains the definition of the whole assembly process. A resource has its 3D geometric representation defined in a file of type CATProduct.
6.1. Model mapping into commercial software concepts The data structure of the proposed model, represented by white background boxes in Figure 6, had to be mapped into classes, defined in the commercial software, and into new specifically defined classes. The shadowed boxes in Figure 6 represent some of the most relevant classes of the resulting model. The implementation required the persistent storage of instances in files (Figure 6). The container for the three basic concepts of the model, that constitute the iDMU: product, process and resource; is a process file (CATProcess) that integrates three lists corresponding to each type of the PPR structure. The API provides the classes: ‘ProductDocument’, ‘PartDocument’, ‘ProcessDocument’, ‘PPRDocument’, ‘Activity’, ‘Product’, ‘Resource’ and ‘Item’; with the interfaces to the functions needed to analyse and create elements of the iDMU. A ‘component’ is the lowest level node in a product tree and is linked to the ‘cad model’ class. A ‘cad model’ is defined in a file of type CATPart. A file of type CATPart contains the 3D geometric representation of a single component. There are three product trees to represent the three product views: ‘as designed’, ‘as planned’ and ‘as prepared’. Each product tree has its corresponding nodes of type: ‘functional node’, ‘industrial node’ and ‘as prepared node’. Conceptually, any ‘industrial node’ is an ‘as prepared node’ and the other way around. However, when creating an ‘as prepared’ structure from an ‘as planned’ structure, new industrial
6.2. Description of the assistant tool After the mapping of concepts, the next step was to define the flow of activities of the assistant tool. The developed prototype tool provides a step-by-step guide to the engineer when executing the top-level tasks comprised in the conceptual design of the assembly process alternatives: define ‘as planned’ structure, define ‘as prepared’ structure, create basic process structure, assign locations to assembly stations, assign sub-processes to assembly stations, assign resources to assembly stations, compile assembly line and evaluate alternatives. UML activity diagrams were created to specified such flow; Figure 8 shows the top-level diagram. Defining an assembly process alternative, as proposed in the assistant application (Figure 8), requires using the scenario information and involves fixing an assembly sequence, establishing sub-assemblies associated to the sub-stages of the process, locating them into real industrial plants belonging to the set of available company’s facilities, adding sub-processes depending on the type of joint to be executed (e.g. component type: fuselage–fuselage; and material type: composite–composite) and assigning the resources to be used (e.g.: NC machine, overhead crane). The use of a created process library, programmed decision rules and expert technical staffs’ interaction with the system are required along the process to define the alternatives (Mas et al. 2013b).
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the components or defining it if it is not found. It allows defining numeric type properties, which are common to all the product nodes (e.g. cost). It also calculates the value of the property in all the nodes of the product tree by using the utility to capture its structure and applying an adding upward algorithm. For all the lowest level individual nodes, the value of the property is input. For an upper-level node, the value of the property is calculated as the accumulative value of the nodes under its branch. This utility is used to calculate the cost and time at each node of the product tree.
Figure 8. Top-level activity diagram showing the assistant tool user flow of tasks.
In addition to the user interface defined by forms, the implementation required also the development of generic algorithms. The main functions developed for the prototype application were: tree structure analyser, node dimensions calculator and node properties calculator. The first function analyses any product structure, defined in a CATProduct file, to determine the nodes and hierarchy of the product view structures. This function could be adapted to analyse a resource structure. The second function calculates for each node the required bounding box and its geometric dimensions. It calculates the bounding box of each lowest level individual component. It analyses the relative position of the different elements and determines the corresponding bounding box for each product node. Dimensions of each component are calculated by defining parallel planes and checking if they cut the part. If that condition is false, the distance between planes is reduced. The loop is executed until precision required is achieved. The dimensions information is relevant to estimate the space allocation for the possible transportation of elements, and to estimate the industrial facilities space requirements for each assembly operation (joint) to be executed in each node of the ‘as prepared’ structure. The third function allows finding a property in
6.3. Case study Two case studies were executed to test and validate the assistant tool. Using simple 3D solids, a very simple digital aircraft (DA08) was defined specially for the first case study. It comprised seven basic components and six joints (Mas et al. 2013a, 2013b). The case study comprised two different situations: one with a single level ‘as planned’ structure, and another with an ‘as planned’ structure with two levels. Both cases allowed testing the complete functionality of the developed tool. This case study is explained by Gómez et al. (2013). The aim of the first case study was to evaluate the first version of the user interface and the basic functions of the assistant tool without applying decision rules. The second case study was an industrial approximation, with an ‘as designed’ product structure closer to a real aircraft, in terms of major aerostructures. Using also simple 3D solids, another digital aircraft (XDA10) was defined for the second case study. It comprised sixty-two intermediate ‘as designed’ nodes and fifty-four last level components. The FAL must be designed to execute the assembly of the following five major structures: fuselage, wings, engines and pylons, empennage and undercarriage. In addition to the FAL, each major structure has its own assembly line where the corresponding joints must be executed. The definition of the joints, and the product structures: ‘as planned’ and ‘as prepared’; were carried out with the developed tool. Figure 9 shows screen captures with both XDA10 structures. In this second case study, the aim was to check the definition of the joints, ‘as planned’ and ‘as prepared’ structures with a more complex product, to test the modified user interface, and to validate the implemented decision rules. The programmed decision rules require information regarding the assembly plants, e.g. cost/day, surface, distances between them and availability of transportation means; and the assembly time assigned to the tasks of each joint type (Figure 7) under the column ‘Process’ shows an example of such tasks. Figure 10 shows, in the form of tables, an example of the assembly plant information used in the second case study; both cost/day and plant surface are represented by approximate values.
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Figure 9.
Screen captures of Aircraft XDA10 structures: ‘as planned’ and ‘as prepared’.
Figure 10.
Example of assembly plant information used by the prototype application.
For the second case study, the defined industrial workload distribution for the five major components or ‘as planned’ structure was: fuselage (Bremen), wings (Filton), engines and pylons (Toulouse), empennage (Saint-Nazaire) and undercarriage (Seville); the selected location for the FAL was Seville. The transport of the whole fuselage from the plant located in Bremen to the FAL in Sevilla was not feasible. The dimensions of the
fuselage were larger than the cargo volume capacity of the transport aircraft Airbus A300-600ST Beluga, larger than any by-road available transport means, and there was no alternative means. This was the situation to be reproduced and evaluated with the decision rules of the assistant application. Figure 11 shows the evaluation results of the proposed ‘as planned’ alternative in terms of: dimensions of each major component to be transported from its
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Figure 11.
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Results of the second case study.
assembly plant to the FAL, transport distance to the FAL, transport means, assembly time and assembly cost. The results show that the FAL assembly time determines the total assembly time, since all the assembly times of the five major structures in their corresponding assembly line are lower. The null solution for the transport of the whole fuselage from Bremen to the FAL could be solved by transporting the front fuselage separately. However, in this case the FAL should be modified to accommodate the execution of the joint between the front fuselage and the centre fuselage; as a result, a new ‘as planned’ product structure should be created an evaluated. The time model did not include the transport time between the different plants. If considered, then the transport time should comprise, among other concepts, the uploading and downloading of the aero structure into and from the transport mean and the average speed of such transport mean. The time showed in the table under the column ‘Assembly time (d)’ only takes into account pure assembly time calculated from the time assigned to the tasks that conform each type of joint. Similarly, for the cost model, transportation cost is not considered. The assembly time and cost are calculated even though the component cannot be transported. The execution of the case study confirmed the feasibility of the approach, by making possible to evaluate industrial workload distributions alternatives for the concept design of a FAL, by influencing the product design very early when conducting the industrialisation design process, and by showing a potential time reduction in the execution of some routine tasks. Since the work is a proof-of-concept development, it was not tested under real conditions, of a full conceptual design process, to make a quantitative evaluation of the potential time reduction.
development is based on experts’ knowledge and on the product–process–resource (PPR) to support the iDMU concept. The assistant tool is integrated within a commercial software system widely used in the aerospace sector. The use cases executed with the developed application confirmed the viability of the approach. The assistant tool allows defining different scenarios and evaluating them in terms of: dimensions, transport requirements, time and cost. The evaluation of different alternatives allows creating estimates that help in the FAL conceptual design decision-making process. As a prototype application, it lacks desirable features dealing with check of errors, editing of defined joints and definition of joints from design constraints. Both the proposed model and its implementation into a knowledge-based assistant tool provide a starting point contribution to the field. Future works aim completing the implementation of knowledge rules dealing with resources allocation, implementing a multi-criteria decision analysis for the alternatives evaluation and an algorithm to implement automatic process planning capabilities to create the ‘as prepared’ alternatives using the information defined in the ‘as planned’ structure, the joints to execute and the explicit process information and knowledge. Another work that requires further investigation is the evolution of the iDMU along time to support the integration of the information generated during the three design phases: concept, definition and development.
7. Conclusions This paper presents a knowledge-based solution to improve the process of conceptual design of aircraft FALs. The proposed alternative is composed of an aircraft FAL conceptual design process model and its implementation into a proof of concept assistant tool that guides the industrial engineer to carry out such process. The
Disclosure statement
Acknowledgement The authors would like to thank the AIRBUS Group colleagues for their support and contribution during the development of this work.
No potential conflict of interest was reported by the authors.
ORCID Fernando MAS http://orcid.org/0000-0001-7230-9929 José ríos http://orcid.org/0000-0003-2115-9945
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