Int J Adv Manuf Technol (2013) 67:771–784 DOI 10.1007/s00170-012-4521-5
ORIGINAL ARTICLE
A process-oriented approach to modeling the conceptual design of aircraft assembly lines F. Mas & J. Ríos & J. L. Menéndez & A. Gómez
Received: 4 May 2012 / Accepted: 14 September 2012 / Published online: 27 September 2012 # Springer-Verlag London 2012
Abstract The need to develop methods and software applications to support the design of assembly lines is academically and industrially acknowledged. This paper focuses on the conceptual design of aeronautical assembly lines. A model is proposed to represent the process to design an aerostructure assembly line at the conceptual design phase and the knowledge requirements to support such process. The conceptual design process is documented in an Integrated Definition for Function Modeling model and the knowledge model is documented in Unified Modeling Language. The model provides a starting point in the formalization of the assembly line conceptual design. The objective is to use such model for the development of a knowledge-based application prototype in an industrially used software system. Keywords Aircraft assembly line conceptual design . Aircraft assembly line modeling . Knowledge based application development
1 Introduction The design of an assembly line is part of the industrialization phase of an aircraft. Traditionally, the industrialization F. Mas (*) : J. L. Menéndez AIRBUS Military, Av. García Morato s/n, 41011 Sevilla, Spain e-mail:
[email protected] J. Ríos : A. Gómez Universidad Politécnica de Madrid, José Gutiérrez Abascal 2, 28006 Madrid, Spain e-mail:
[email protected]
at the conceptual design phase has not been supported by knowledge based and predictive technologies to the same extent as other aspects of the aircraft development [1–3]. The relevance of both the conceptual phase and the assembly line design is pointed out by two relevant cost-related facts. First, up to 80 % of the final aircraft cost is determined during the conceptual phase. Second, up to 30 % of the final cost is due to assembly operations performed on conventional aircraft structures built up from parts [4]. At the conceptual level, the design process of an assembly line is considered similar to a product development process by several authors [5, 6]. However, when analyzing industrial cases, it is found that product characteristics, company specific practices and industrial sector practices influence the design process [7]. Several authors have acknowledged the need to develop specific methods, guidelines, and software applications to support assembly line designers [7–9]. Fowler and Rose [10] point out the need to develop formal models of manufacturing systems for evaluation and decision making rather than relying on experience and rules of thumb. Knowledge-based engineering (KBE) comprises the use of software techniques to capture and reuse product and process knowledge in an integrated way [11]. The management of product programs in the aerospace sector is defined in the guidelines “RG Aéro 000 40” [12]. Based on such guidelines, Airbus defined the lifecycle of its products and a concurrent engineering development process. Figure 1 shows the different milestones adapted to the aircraft lifecycle characteristics [13]. The design of the airplane, and its corresponding subassemblies, evolves along the design process, four types of product definition structures are created along the lifecycle: “as specified” (M3), “as designed” (M5), “as designed” and “as planned” (M7), and “as prepared” (M9). The design of a final assembly line is shifted from the product lifecycle, starts in
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Fig. 1 AIRBUS product lifecycle and development milestones
milestone M3, and ends in milestone M9. The design process is also divided into three phases: concept, definition, and development (Fig. 1). During the first stages of the product lifecycle several discipline groups work on different tasks, such as: stakeholder requirements, preliminary product definition, commercial agreements, marketing studies, functional capabilities, aero dynamic studies, industrial capabilities, and industrialization solutions. The Program Office is in charge of coordinating and managing all these tasks, securing cost, and proposing different business cases to be evaluated. At the conceptual phase, the manufacturing engineering group is in charge of the product industrialization and has to evaluate the different business cases against possible industrial solutions for the aerostructures and final assembly lines. Such work is currently carried out manually by highly skilled and experienced personnel making use of CAD and office tools (CAD files, spreadsheets, standards, PERT and Gantt charts, etc.). The process depends heavily on the personnel experience and it is very time consuming. As a consequence, manufacturing engineers can only check a simplified set of cases to release early manufacturing processes and resource requirements. KBE is the approach considered to assist in conducting the conceptual design of the product industrialization. This paper focuses on modeling the conceptual design of aerostructure assembly lines. The first contribution is a proposal for the conceptual design process of aerostructure assembly lines. The process is documented using Integrated Definition for Function Modeling (IDEF0). The second contribution is the proposal of a knowledge model to support such conceptual design process. The knowledge model is documented using Unified Modeling Language (UML). The third contribution is a proof of concept knowledgebased application (KBA) prototype, integrated within a
commercial software system, to help generate possible assembly line design solutions at the conceptual design phase. The following sections present firstly a literature review on modeling of assembly information and secondly the proposed model for the conceptual design of aerostructure assembly lines.
2 Review of assembly processes and lines modeling The modeling of aerostructure assembly lines requires reviewing works dealing with modeling of assembly information, processes, and lines. Balancing of assembly lines is a distinctive research line that provides a different view to the modeling of assembly information. Although it focuses neither on the aerospace sector nor on the conceptual design phase, Khan and Day [8] proposed one of the first KBAs for balancing of assembly lines. The KBA proposed an assembly system configuration defined by the product (single product, multiproduct, or mixed product) and by the processing times (stochastic or deterministic). In a second step, it proposed a cycle time and in the last step executed the line balancing. The information used by the KBA was: demand forecast, tasks, tasks duration, precedence relationships, shifts per day, and working days. An insight into the aerospace assembly process planning and the relevance of capturing its rationale were provided by Rajkumar and Williams [14]. An information requirement analysis is proposed to identify the information used in the assembly process planning for aerostructures. It combines a top-down approach, high level document tracing, and a bottom-up approach, where key documents are identified and analyzed. However, rather than analyzing the information
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needed to model an aircraft assembly line, the work focuses on the tasks carried out by the assembly process planner who creates shop floor operational instructions. Such tasks are typically conducted during the assembly line development phase (Fig. 1). Using the International Standard for Product Model Data Representation and Exchange (ISO 10303—STEP), Liu [15] created an assembly information model. Focused mainly on the product design view, the definitions were grouped into five schemas: nominal_shape, form_features, tolerances, mechanical_part, and assembly. The assembly schema contained the concepts: assembly_model, joint, operational_joint, fastener_joint, primary_part, secondary_part, assembly_label, connection, joint_type, CAM_operational_joint, CAM_fastener_joint, and assembly_operations. The attributes used to define an assembly operation were related to 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. This model addresses the definition of the lowest level of assembly tasks. Based on Liu’s model [15], Zha and Du [16] proposed an assembly process planning tool. The tool used the information related to: parts and connectors involved in the assembly, fit and contact relationships between parts, and fastening relationships. Following the trend of previous works with a product view, Rachuri et al. [17] and Sudarsan et al. [18] proposed an extension to the Core Product Model developed by the National Institute of Standard and Technology to include assembly information. Such information was modeled in the named: open assembly model. The model incorporates kinematics concepts and extensions to model different types of design constraints related to: tolerances, kinematics, and part joints. Although still having a product assembly view, Kim et al. [19] used UML to propose an assembly relation model to represent assembly operations. The concepts defined in the model were: assembly, assembly_feature, mating_feature, mating_bond, joint_feature, mating_pair, mating_condition, spatial_relationship, part, form_feature, and geometric_feature. Based on the model, an ontology is proposed where the concept of joining_process is defined for three types of processes: adhesive_bonding, riveting, and welding. The assembly relations are derived from the relations defined between form features of different parts. The need to improve models to represent assembly knowledge was pointed out by Wang et al. [20]. Following the product view approach, they created an assembly knowledge modeled with three levels: function, structure, and part/ feature. Based on the connecting relationships between pairs
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of parts defined by geometric constraints, mating constraints, and movement constraints, a set of rules were defined to create an assembly plan for the parts involved in the assembly of the whole product. Hongmin et al. [21] enhanced the product view of assembly modeling by incorporating knowledge about assembly tools, tool selection, and planning. Following with the interest in assembly sequence planning and using a product view, Barnes et al. [22], Su [23], and Dong et al. [24], consider geometric constraints, mating joint constraints and precedence constraints to generate feasible assembly sequences. Su [23] also proposed an algorithm to select an optimal assembly sequence from the set of all the feasible ones. Dong et al. [24] proposed the concept of “connection” to model relationships among two or more parts and to compile geometric and nongeometric information to perform assembly sequence planning. As it was shown so far, an important part of the models dealing with assembly has a product view, for that reason, they focus primarily on defining the relationships between the parts that constitute an assembly. However, to address the assembly process and the resources used for its execution, there is a need to consider a process view [25]. Lanz et al. [25] proposed a high-level product–process semantic network that includes process-related concepts: activity, operation, task, action, process, sub action, actor, skill, tool, human, device, resource, area, station, line, factory, cell, machine, and robot. The concepts are not further modeled to specify their attributes and methods, but it constitutes an antecedent in the process view approach adopted in this research. The difficulty in codifying assembly knowledge and the relevance of developing software tools to assist in the design of assembly plans were pointed out by Wang et al. [26]. They also mentioned that process-oriented modeling is considered the best approach to developed decision tools for assembly planning in a concurrent environment. A complementary view of the assembly process modeling derives from the research works dealing with assembly line balancing (ALB) [27]. References dealing with this topic exist to a great extent. Boysen et al. [28] present an extensive bibliographic analysis of publications dealing with ALB. The interest of this work resides in the formalization of the information to model an assembly line for balancing. Such formalization, although intended for a different purpose, is a very interesting starting point to identify the information that so far was used in the definition of assembly lines. According to Boysen et al. [28], an assembly line can be defined by a model composed of three basic information vectors: precedence graph information (α), work stations and assembly line information (β), and information about the objective function to optimize (γ).
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Focused on an aircraft assembly line, Heike et al. [29] formulated four models (linear and nonlinear) to evaluate cycle time and worker allocation. The models consider requirements related to: tasks, work stations, and labor. The requirements used information included in the vectors proposed by Boysen et al. [28]. Also focused on aircraft assembly, Jin et al. [30] proposed an activity time-based metric for evaluating the potential of improvement of an assembly work station. The model requires an activity with a time-related attribute and a qualifying attribute with three possible values: nonvalue added, value added, and nonvalue added but necessary. From the assembly modeling review, it can be concluded that the semantic concepts involved in the conceptual design phase of an aircraft assembly line are not fully taken into account in the identified models with three main views: product, process, and line balancing. The modeling of such conceptual phase demands to integrate and to extend concepts from the three views, particularly from the process view. In the next section, the proposed conceptual model for the conceptual design phase of an aircraft assembly line is presented.
3 Aerostructure assembly line conceptual design model: a process-based proposal The design of an assembly line is comprised in the product industrialization phase. The method adopted to create the model for the conceptual design of aerostructure assembly lines comprises two main inputs. The first input was 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. While 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; 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 [5, 6, 31]. The second input was the creation of an “as is” process model based on the industrial practices from previous aircraft projects. From the analysis of such “as is” model, two main elements were identified: process improvements actions and activities where KBE technology could be used [32]. Based on the two inputs, a “to be” process model focused on the industrialize activity was defined using IDEF0 [31]. As it is already mentioned in literature [7–9], the field of assembly line design requires the development of specific models and software applications for its support. This aerostructure assembly line conceptual design model provides
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an explicit starting contribution to the field. The process model shows the activities to conduct, the flow of information, and helps to identify the concepts involved in the conceptual design process. The concepts are specified in a knowledge model using UML. This section shows firstly the process model, secondly the knowledge model, and thirdly a prototype implementation of the knowledge model. 3.1 Aerostructure assembly line conceptual design process model The production of an aerostructure is a concurrent process that can be subdivided into three top level activities: manage, engineer, manufacture. The activity A2 “engineer” comprises two sub-activities: A21 “design”, A22 “industrialize”. The activity A21 “design” creates the design of the aerostructure in two main configurations: “as designed” and “as planned”. The “as designed” configuration provides a product breakdown structure based on a hierarchy of airplane functional groups and the associated components. The “as planned” configuration represents the industrial breakdown, including work sharing agreements among industrial partners. The “as planned” configuration denotes the grouping of components into assemblies that will be created in the manufacturing units. The activity A21 “design” requires a concurrent work with the activity A22 “industrialize” to get the requirements for the “as planned” configuration. The activity A22 “industrialize” is responsible for defining industrialization requirements and for requesting configuration changes if needed to manufacture the aerostructure [31]. The activity A22 “industrialize” is responsible for designing the assembly line needed to manufacture the aircraft/aerostructure, and it can be decomposed into three main sub-activities: A221 “create conceptual assembly process”, A222 “define assembly process”, and A223 “develop detailed assembly process” (Fig. 2). Each of these activities corresponds to the three assembly line design phases: concept, definition, and development (Fig. 1). The model focuses on the concept phase and thus on the activity A221 “create conceptual assembly process”. The definition of an assembly line comprises: capacity of the line, number of stations, basic technologies to be used, stations order, priority of each station, the input product structure and the output product structure, human resources policy, shared resources and the preliminary lay-out. At the concept phase, the following information is still under definition: work load distribution, technologies to be used in the product and in the process, and main machinery and tooling. The main assembly line characteristics to be defined at the conceptual stage can be grouped into four outputs: “conceptual industrial requirements”, “conceptual jigs and tools requirements”, “conceptual as planned requirements”,
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Fig. 2 Decomposition of the activity—A22 industrialize aerostructure
and “conceptual as prepared configuration”. As indicated in Fig. 2, these elements contain the concepts to be specified in the knowledge model. Such elements are also highlighted in bold font in Figs. 3 and 4. The activity A221 has only control elements and no input elements. The explanation for this modeling decision is the IDEF0 semantic difference between input elements (they are transformed by the activity into output) and control elements (they are required to produce the correct output and they may be transformed by the activity creating output). The control “program management and planning” comprises the business case or scenario defined by management to be used by the manufacturing engineer in the definition of the assembly line. The control “conceptual as designed proposal” is the product view provided by product design. The mechanisms to perform this task are: concurrent engineering tools (PLM system, CAX, and Digital Process Engineering tools) and KBA. This activity is decomposed into three subactivities: A2211 “generate as planned proposal”, A2212 “generate assembly lines”, and A2213 “evaluate assembly lines” (Fig. 3). Ultimately, the subject of this work focuses on the activity A2212 “generate assembly lines”. The generation of feasible assembly line solutions making use of “what-if” analysis is the objective of this activity [33]. In the activity A2213 “evaluate assembly lines”, such feasible solutions
will be evaluated and ranked by using quantitative indicators related to: space usage, times, industrial means, tooling and human resources. Outputs used as input feedback: “conceptual as planned needs”, “conceptual jigs and tools requirements”, and “conceptual industrial requirements”; are represented with a dotted line for clarity of the diagram. The concurrency with other activities affects mainly to the control elements: “conceptual as planned proposal” and “program management and planning”. The main mechanism is: “KBA”. Its development is the ultimate objective of this work. The activity A2212 “generate assembly lines” is subdivided into: A22121 “evaluate as planned” and A22122 “generate as prepared” (Fig. 4). The information flow captured in the IDEF0 model allows identifying inputs, controls, and outputs that comprise the knowledge requirements of the process. Such requirements refer to the semantic definition of concepts and they constitute the Aerostructure assembly line conceptual design knowledge model. These knowledge elements are summarized as follows: 1. Program management and planning. It is a control element to the activity A2212. It is defined by management and it is used to generate scenarios and “what-if” analyses. It comprises: physical space constraints, estimated
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Fig. 3 Decomposition of the activity—A221 create conceptual assembly process
Fig. 4 Decomposition of the activity—A2212 generate assembly lines
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demand curve, estimated delivery plan, budget constraints, and location constraints. 2. Conceptual industrial proposal. It is an output from the activity A22122. It defines feasible assembly solutions to be evaluated in the activity A2213. It comprises: (a) Assembly line logic diagram. (b) Preliminary lay-out of stations: size and location. (c) Stations: precedence relationships, duplicates, times, and capacity. (d) Human resources policy and labor allocation: specialty group, quantity, number of shifts, working time, productivity level, and learning curve. (e) Logistic system: size, storage capacity, technology, type, buffers, location, and JIT. 3. Conceptual jigs and tools proposal. It is an output from the activity A22122 and it comprises: (a) Technologies to be used in each station: family type. (b) Jigs and tooling definition: size, capacity, technology, type, reference, and interchangeable. 4. Conceptual as planned needs. It is an output from the activity A22121 that is concurrently executed with product design. It shows the manufacturing view of the product requirements for its industrialization. It has to be compliant with company standards. 5. Conceptual as prepared proposal. It is an output from the activity A22122. It is estimated that there will be more than one feasible assembly solution, resulting in a set of conceptual as prepared alternatives that comprises this output. For each feasible assembly line solution, a record will be generated containing all its defining parameters. This output will be a control element in the activity A2213 where feasible alternatives will be evaluated and ranked. 3.2 Aerostructure assembly line conceptual design knowledge model Considering the presented knowledge elements, the knowledge model was structured into three interrelated knowledge units (KUs): product, processes, and resources. Each KU addresses a different aspect of the assembly line. Basically, the product KU allows representing a product tree, the processes KU allows representing a network of tasks, and the resources KU allows representing a pool of resource elements. There is a strong interrelation between the three KUs that is modeled in the relationships defined between different classes (entities) via attributes. The product KU defines all the aircraft or aerostructure components, both the functional (as designed) and the industrial (as planned) views, and the definition of all the joints to be assembled. The process KU allows defining a procedure to assemble each joint defined in the product KU in terms of technology,
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sequencing, and resources. The assembly process is materialized in work stations and its corresponding assembly line. All the needs related to technology, sequencing, and resources are collected in work stations. Work stations are grouped into the assembly line to create a consistent assembly solution. Rules and procedures are applied to compile all the information. The resources KU compiles three main types of resources: “Jigs & Tools” relates to the resources whose design is strongly linked to the product design (e.g., a jig to assemble an horizontal tail plane or a numerical control (NC) machine to mill the central wing box); “industrial means” relates to resources that can be purchased from a supplier and are defined by standard or catalog characteristics (e.g., cranes, vehicles, manual riveting machines); and “human resources”. Since aeronautical assembly is a labor-intensive process, personnel are very important to determine and evaluate. 3.2.1 Product knowledge unit The product KU defines the “functional view” and the “industrial view”. Both views use the same layer of components or design solutions. The lowest nodes needs to accommodate the definition of the joints or interfaces between them. The “functional view” is a functional decomposition tree in which designers can navigate across the product to carry out design activities related to the aircraft or aerostructure. Examples of functional nodes are: landing gear, horizontal tail plane, fuel system, wing set, tail section, etc. The “functional view” structure is a company-specific customization of international standards and it is documented in the “as designed” product definition structure. The intermediate product corresponding to a functional node may not exist as a real product. For instance, considering the wing set functional node, it will never exist as such because the physical assembly process is defined as a wing-to-fuselage—SubAssy 2 solution (Fig. 5). The industrialization of a product functional view may have several feasible solutions and each feasible solution has its corresponding product “industrial view” (as planned). The “industrial view” fits with the industrial breakdown of the product. Each industrial node exists in the real world as an intermediate product that is assembled by an industrial partner. Concurrently, design engineering and manufacturing engineering have to agree with the common layer of components or design solutions to define the interfaces between them and to build the industrial view of the product. The ‘Industrial view’ is documented in the “as planned” product definition structure. The proposed model considers the product at its different stages and levels as a functional_node and as an industrial_node. Ultimately, the lowest level of the product tree is defined by the concept of component. Both the functional
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Fig. 5 Product functional view and industrial view in DA08
view and the industrial view are made of a tree of nodes, functional, or industrial, ultimately based on the same final layer of components (Fig. 5). Configuration management is a key activity during the development phase; however, in the conceptual phase, due to its high level of abstraction, it is more appropriate to manage variants. A product model includes variants. Variants are the different configurations of the aircraft or aerostructure. At the conceptual phase, it is necessary to take into account the different product versions or variants offered or required by customers. For example, the same aircraft can be designed with two variants: one with a standard fuselage and another one with a stretched fuselage. The assembly line needs to be designed to accommodate both variants. A component (e.g., front fuselage, center fuselage) is conceptually invariant, but the different versions or variants are defined in a lower level concept named cad_model. This implies that a component may have associated more than one cad_model. An aircraft is identified by an integer number. The cad_model concept has an attribute named “range_of_aircrafts” to define the range of aircrafts where each cad_model will be used to
build up digital mock-ups. The ability to manage variants has been modeled by adding the function select_aircraft_number to the three types of nodes that constitute a product tree: root node (aircraft), intermediate nodes (functional_node, industrial_node), and basic node (component). A full definition, digital mock-up, of any aircraft node could be done both in the functional and industrial view making use of the function ‘compose_cad_model’. The definition of the elements that are joined together is defined at the cad_model level and represented by the concept “joint”. A joint is defined by two attributes named “cad_model_1” and “cad_model_2” pointing to cad_model. The objective of the function named “calculate_geometrical_characteristics” is to determine the bounding box associated to each node in both product views: functional and industrial. To determine the bounding box associated with a product industrial node, it is necessary to calculate the relative location of the different components and the bounding box of each basic functional component. Figure 6 shows a simplified version of the product knowledge unit. At the assembly line design concept phase, the product is still viewed at high level, the joint between a wing and the
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Fig. 6 Product knowledge unit
fuselage central section is not detailed to the lowest level of components that will physically interface and create the joint. The definition of the product KU does not determine the assembly sequence. It only defines elements that are joined together. The product model allows defining the product with all its interfaces or joints and the industrial breakdown or work share. The assembly sequence will be determined in the definition of the processes belonging to the assembly line. 3.2.2 Processes knowledge unit An assembly process is the procedure to combine a set of products into another product, executing the defined joints between them by applying defined assembly tasks and using allocated resources. Joints are defined in the product scope and all the assembly processes are defined to fulfill them. In the aeronautical sector, the use of three definition levels is generally accepted: the first level for the whole assembly line, the second one for the stations, and the third one for the basic processes related to the joints. At each level, a process element has at least two products as input, one product as output and the resources needed to perform the assembly. A process is organized as a network of nodes and links. Nodes represent joints to execute and links represent the precedence relations. Precedence constraints are represented at each level: line, stations, and basic processes.
The first or top level is the assembly line. The concept “assembly_line_process” is a container to collect and consolidate all the information coming upstream. Industrial means, human resources, and cycle times are calculated from the work station level data. The assembly_line_process produces the complete product aircraft using the resource assembly_line_resource. The second level is the work stations level. An assembly line is composed of assembly stations entities with precedence relations between them. The concept “assembly_station_process” is a container to aggregate the individual assembly tasks defined in the lower level. Basic processes are grouped logically according to their characteristics or their use of resources. Using the resource assembly_ station_resource, an assembly_station_process creates an intermediate product node that is part of the “as prepared” product structure. The third or bottom level is the basic_assembly_process. It defines how to assemble two components that constitute a joint. It defines the manufacturing solution to execute a joint between two components. A basic_ assembly_process can be of any of the following six standard types: &
Structure process. It includes the mechanical processes to joint parts or subassemblies, basically with rivets or
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screws. It contains a well-known sequence of tasks related to positioning, drilling, and riveting. Electrical process. It includes the mechanical assembly installation of the electrical harnesses in the aircraft or aerostructure and the electrical connection between them. Furnishing process. It includes the assembly processes of non-aeronautical systems. For instance, assembly processes for seats in a commercial aircraft, air-conditioned installation, galleys, parachutes, and medical set in a military transport aircraft. Systems installation process. It includes the assembly of the different systems of the aircraft or aerostructure. For instance, assembly processes to install the landing gear, flaps, actuators, and other system components. Painting and sealing process. It includes the processes to prepare the surface, application of primer layer, topcoat layer, and the different sealant technologies. Test process. It includes the processes conducted to perform the functional testing of the different systems
Fig. 7 Process and resources knowledge units
of the aircraft or aerostructure: mechanical, electrical systems, equipments, hydraulic, fuel pipes, and any test required by the aircraft systems. Figure 7 shows the process and resources knowledge units. Each process level has linked its own resource container to collect upstream resources from lower levels and to contain its own resources. Each process concept has its own functions to calculate its associated resources and cycle time. The lowest level deals with basic assembly processes and contains jigs and tools, industrial means, and human resources associated to the basic assembly tasks. 3.2.3 Resources knowledge unit The resources KU is a pool of classes: “jigs_and_tools”, “industrial_means”, “human_resources”, “basic_assembly_ resource”, “assembly_station_resource”, and “assembly_ line_resource” (Fig. 7). The relationship between the resources of the three process levels: assembly line, stations, and
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tasks is defined indirectly via the processes. There are three basic classes of resources: &
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Jigs and tools. These are resources with a strong link with the definition of the product. They are specialized resources defined in a concurrent way with the product. Examples of this type of resources are platforms to access the product, devices to position subassemblies, and special NC machines for assembly operations. Industrial means. These are resources not linked or with no strong link with the definition of the product. In general, they are standardized commercial resources such as cranes, stairs, cabinets, and standard tools. They can be selected from providers’ catalogs. A basic definition of the product is needed for their selection. Human resources. Assembly in the aeronautical industry is man labor intensive. Human resources are classified by specialties and labor time is subjected to learning curves.
In addition to the three basic resources, there are three containers to collect resources upstream: &
Assembly_line_resource. It is a container for the all the resources coming from the “station_line_resources” plus those resources that cannot be assigned to any specific station. This kind of resource is shared by stations, for
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instance, a hangar that contains the assembly line and a crane that gives service to several stations. It also contains all the logistics resources shared by the stations. Assembly_station_resource. It is a container for the all the resources coming from the individual resources attached to the “basic_assembly_resource” plus those resources that cannot be assigned to a particular assembly process and are assigned to the station. Basic_assembly_resource. It is a container for all the resources attached to each “basic_assembly_process”.
3.3 Prototype implementation of the aerostructure assembly line model To demonstrate the feasibility of the proposed approach, a prototype implementation was conducted. For the implementation, it was required to use the same software system as the one used in the industrial aircraft projects, in this particular case: CATIA V5. The first prototype comprises a partial implementation of the aerostructure assembly line model focus mainly in the product knowledge unit. A case study was used to test the development [34]. The first implementation step was to map concepts defined in the model into CATIA V5 elements and classes. The container for the three basic concepts: product, process, and resource is a process document (.CATProcess) that
Fig. 8 Mapping of the proposed aerostructure assembly line model into CATIA V5
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integrates three lists corresponding to each type of basic concept. The basic concept in the product knowledge unit is a component. A component is defined in a file of type CATPart and corresponds to the lowest level in a product tree. There are three product trees to represent the three product views: as designed (functional), as planned (industrial), and as prepared (manufacturing). Each product tree has its corresponding nodes: functional_nodes, industrial_nodes, and as_prepared_nodes. The classes AsPlanned and AsPrepared were defined to generate the corresponding product structures. Each product structure is defined in a file of type CATProduct. The nodes of a product tree can point to a CATProduct file (intermediate elements) or to a CATPart file (lowest level individual element). The joint concept is mapped into a Joint class and linked to a CATProduct file. The Joint class has a link to the Basic_assembly_process class. An object of the Basic_assembly_process class is defined in a file of type CATProcess. Figure 8 shows the mapping of the proposed model, mainly the product KU, into classes defined in CATIA (shadowed) and the corresponding type of files where instances are stored. The implementation requires also the development of generic algorithms. The main functions developed for the
Fig. 9 Schematic representation of the case study
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prototype application are: product_tree_navigating function, bounding_box_calculation function, and numeric_type_properties_definition function. The first function analyses the product structure to determine the nodes and their hierarchy. 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 analyzes the relative position of the different elements and determines the corresponding bounding box for each product node. Such information is relevant to estimate space allocation for the possible transportation of elements and for each assembly operation (joint) to be executed in each node of the as prepared structure. The third developed function allows defining numeric type properties which are common to all the product nodes (e.g., cost). 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. The developed functions implement the methods included in the model: calculate_bounding_box and calculate_cost [34]. The generation of an as planned structure (industrial view) depends on the as design structure (functional view) and the joints to be executed. The application guides the
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user to define such structure. The guidance is based on the execution of the function that analyses the as design structure (defined in a CATProduct file) and the joints to be executed. Each joint is defined in a CATProduct file whose name starts with the key word: joint. The definition of a possible as planned structure is an object of the AsPlanned class. Using the information contained in an AsPlanned object, the application guides the user to define a feasible as prepared alternative. The as prepared solution defines the execution sequence of the joints to manufacture the industrial nodes. The as prepared solution is stored in an AsPrepared object. Using the functions previously mentioned, such solution can be evaluated in terms of the needed bounding box and cost for each node. Figure 9 illustrates a schematic representation of the executed case study [34].
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The results obtained in the first developed prototype, mainly focused on the product knowledge unit, confirmed that the proposed model could be implemented in the commercial system used in industrial projects. The reader is referred to Gómez et al. [34] for a full discussion of the case study. The complexity in implementing the entire knowledge model makes it an ongoing process. Future work aims an automatic process planning capability in the form of an algorithm to create the as_prepared alternatives from the information defined in the as_planned structure, the joints to execute and the assembly process information. Acknowledgments The authors wish to acknowledge their gratitude to the AIRBUS Military colleagues for their contribution during the definition of the conceptual design process model presented in this paper.
References 4 Conclusions This paper presents a model for the conceptual design of aircraft assembly lines. The model is composed of a conceptual design process model and its associated knowledge model structured into three units: product, process, and resource. The model is the basis for the development of knowledge based applications in a software system widely used in the industry. The prototype development, focused on the product unit, confirmed the feasibility of the approach. The main contribution is the aircraft assembly line conceptual design model. Literature review points out the interest of this area of work and the need for further contributions. The industrial practices from a particular company and a literature review were the inputs to the process model creation. Literature review shows no evidence of any other company’s process publicly available; therefore, the only external reference were the generic design processes proposed in the engineering design literature. In the definition of the model, by comparing with the generic design processes, particularities were aimed to be removed. The proposed model provides a starting point. The second contribution is the prototype application in an industrially used software system. The main functions developed for the prototype application—product_tree_navigating function, bounding_box_calculation function, and numeric_type_properties_definition function—constitute the basic part of the development. The prototype implements mainly the product knowledge unit and analyses the as_planned product tree defined by the user and the joints to be executed to assist in the definition of the as_prepared alternatives. For each as_prepared alternative, the bounding box dimensions and the cost of each product tree node are calculated. The code of these three basic functions can be adapted for the process and resource trees.
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