International Journal of Production Research, Vol. 45, No. 2, 15 January 2007, 323–350
Process platform representation based on Unified Modelling Language L. ZHANGy, J. JIAO*y and P. T. HELOz ySchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue 50, Singapore 639798 zLogistics Research Group, University of Vaasa, FIN-65200 Vaasa, Finland
(Revision received January 2006) Today’s manufacturing companies have strived to develop a large variety of customer-specified products in an effort to survive and stay competitive. Process platforms have been well recognized as a means for companies to obtain a stable production and thus the economy of scale. A process platform assists companies in configuring similar production processes for producing families of customized products at low costs through managing product and process variety coherently. Within a process platform, all data related to the product and process families are unified as a common generic structure. To shed light on the various constituent elements and complex relationships inherent in a process platform, this paper emphasizes the structural representation of a process platform. A formalism of process platform representation is developed based on the Unified Modelling Language. It consists of a generic product structure, a generic process structure and an integrated generic routing structure. Also reported is a case study of vibration motors for hand phones. Keywords: Process platform; Variety management; Unified Modelling Language; Generic representation
1. Introduction To survive from intense market competition, manufacturing companies have strived to design a large number of customer-specified products (termed as a ‘product variety’; Ulrich 1995) to fulfil the heterogeneous customer requirements. To produce product variety, a large number of production processes or routings (referred to as ‘process variety’; Jiao and Tseng 2004) must be planned properly. In accordance with the domain framework (Suh 2001), product variety resides in the design domain and is exhibited by enormous product data recorded in the form of bills of materials (BOMs), whilst process variety lies in the process domain and is manifested by various process data in the form of routings. To accomodate changes of design specifications for customized products, operations routings may differ from one another in operations, manufacturing resources and process flows, i.e. operations
*Corresponding author. Email:
[email protected] International Journal of Production Research ISSN 0020–7543 print/ISSN 1366–588X online ß 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/00207540600607135
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sequences. Consequently, enormous changeovers in production, such as changes made to machines, tools, fixtures, and set-ups, occur frequently on the shop floor (Wortmann et al. 1997). Product variety brings competitive edges to companies through offering customized products and more choices to customers (Sanderson and Uzumeri 1997). On the other hand, as observed by Mather (1995), Meyer and Lehner (1997) and Nayak et al. (2002), the companies’ focus on individual customers and products often results in product proliferation, which may confuse customers and lead to increased costs and reduced profit margins. Process variety introduces significant constraints to production planning and control, e.g. preventing make-to-order systems from building up customization capabilities (Kolisch 2000). It thus becomes important for companies to manage product and process variety within a unified framework. Considering the causality between the two different forms of variety, variety management is expected to assist companies in managing product and process variety individually, i.e. product variety in the design domain and process variety in the process domain, and in coordinating them coherently — variety coordination from design to production. Process platforms help companies generate similar routings for producing customized products while achieving mass production efficiency. A process platform entails a well-structured mechanism for effective variety management (Jiao et al. 2003). To provide a holistic view of a process platform along with the basic constructs, mathematical models have been established to describe rigorously a process platform from the architecture point of view (Jiao et al. 2006). In order to facilitate solution development of other process platform-related issues, the present paper emphasizes the structural representation of a process platform, i.e. to represent the various constituent elements and their complex relationships inherent in a process platform. A process platform comprises a family of specific routings in relation to a family of customized products. Hence, a process platform contains all data of the product family and the associated process family. To accomplish the inclusion of a large body of data while avoiding redundancy, generic representation (Hegge and Wortmann 1991, Van Veen and Wortmann 1992) is applied. As a result, the same types of data, be they product types or process types, are grouped into classes and thus form generic items. In this regard, process platform elements are the many specific product and process data and their corresponding classes. Accordingly, the complex relationships among elements are characterized by class-to-class, class-to-member and member-to-member relationships. Modelling such elements and relationships imposes special requirements on the representation tools. The tool should possess the ability not only to model clearly multiple classes and specific member instances in terms of their data structures and characteristics, but also to capture the three kinds of class–member relationships at different levels of abstraction. Unified Modelling Language (UML) was developed based on object-oriented (OO) technology, which is suitable to represent both the internal structures and external interrelations of the elements of a system to be modelled (Rumbaugh et al. 1991, Booch 1994). It excels in expressing the complex software/non-software systems and business models. In recent years, UML has been adopted by Object Management Group as an OO modelling language with formal syntax and semantics (Rumbaugh et al. 1999). UML specification (v.1.4.2) has been accepted by the
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International Organization Standard (ISO) as an ISO specification (http:// www.omg.org/). UML defines 13 diagrams that are tailored to specify and model distinguished aspects of a system. These diagrams can be grouped into three categories, including structural diagrams, behavioural diagrams and interaction diagrams. UML allows one to represent the structural aspect of a system in class diagrams. With an attempt to adapt this general modelling language to specific application domains, a standardized extension mechanism is provided in UML. This research applies UML adopted in this research to represent a process platform with respect to the constituent elements and their relationships. While a UML class diagram can represent accurately classes and their relationships, the extension mechanism allows one to define and model member instances and relationships among them. OO techniques enable the capture of relationships between classes and members apart from modelling of classes and instances. In process platform structural representation, not only product and process data are modelled, but also product and process knowledge are represented as selection rules and planning rules. In the next section, the concept implications of a process platform will be discussed. For a comprehensive description, see Jiao et al. (2003).
2. Process platform A process platform represents a conceptual structure and overall logical organization for a product family and the associated process family. Each routing in the process family is planned to produce the relevant variants in the product family. Due to the inclusion of all product and process data recorded in the two families using generic representation, the common structure entails a well-established mechanism for effective variety management. From an architecture perspective, a process platform involves two aspects: (1) a generic routing structure within which variations in diverse products and processes can be differentiated; and (2) the derivation of routing variants in relation to the corresponding product variants from the generic routing structure. 2.1 Generic routing structure The generic routing structure of a process platform refers to the common structure of all product and process elements that may occur in the associated product and process families. It is formed by unifying a generic product structure and a corresponding generic process structure. Du et al. (2001) defined a generic product structure as a structure common to a set of similar products in a family. Thus, a generic product structure represents all customized products in the family and entails multiple entities and various relationships. These entities along with their relationships convey product data and information, such as items, goes-into relationships, design parameters relevant to variety, i.e. variety parameters, and their value instances that are controlled at the lowest level of the generic product structure (Jiao et al. 1999). From the generic product structure, the individual products of a family, i.e. product variants, can be derived by employing the variety generation approaches and product variant
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derivation procedure (Du et al. 2001). The derivation exhibits the instantiation of the generic product structure with respect to the particular variety parameters and their specific values. During the derivation process, the selection rules representing product knowledge are employed to identify the compatible product items through specifying the circumstance under which a particular variant becomes an instance of an item. The generic process structure is the common structure of the process family corresponding to the product family. Process data recorded in the generic process structure include operations, operations precedence and manufacturing resources such as machines, tools and fixtures, set-ups and cycle times. Based on the generic process structure, the routings can be determined for producing the corresponding products. Similar to the instantiation of the generic product structure to obtain particular products, a specific routing can be derived from the generic process structure through instantiating generic process elements. Instantiation of the generic process structure entails the determination of manufacturing resources and operations. While selection rules represent product knowledge in terms of compatibilities of product items and parameter values, planning rules reflect process knowledge and specify the conditions under which specific manufacturing resources and operations are determined. The correspondence between the generic product and process structures is characterized by the material requirement links inherent in operations, more specifically variety parameters and their value instances. To integrate the generic product and process structures to form the generic routing structure, each component material in the generic product structure is connected to the relevant operation in the generic process structure. The direct mapping between product and process data is established by parameters and their value instances that are used by the two generic structures. 2.2 Variant derivation A process platform reveals how the routing variants are derived from the generic routing structure. The derivation of individual routings is de facto the process of coordinating variety from design to production, which is accomplished by identifying process elements for given product elements using the set of variety parameters and their value instances. Owing to the mapping between the generic product and process structures, the variations in production can be eliminated by deriving routings that are the most similar to the existing ones on the shop floor.
3. UML basic constructs 3.1 Class diagram A class diagram consists of classes and various static relationships between them that together describe the structures of models, the elements that exist, their internal structures and their relationships to others. Figure 1 shows the metamodel of the UML class diagram. Classes are drawn as rectangles, in which the top one stands for the name of the class, the middle one for the attributes and the bottom one for the operations. Both the attribute and operation compartments can be suppressed.
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The relationships between classes are drawn as annotated lines between the rectangles. There are five types of relationships between classes: association, dependency, aggregation, generalization and composition. Association reflects a binary relationship between two classes and it can be drawn as a solid line composed of one or more connected segments with some necessary adornments on it. These adornments include association name with or without a small black solid triangle (the point of which indicates the direction to read the name), multiplicity indicating the cardinalities of instances of the attached classes, arrows and diamonds (optional) at the end of the solid line of the association, and role names (optional) of the attached classes. The association name designates the name of the association. Multiplicities on associations are written as a number at each end, with the number applying to instances of the attached class at that end of the line. It can be shown as a commaseparated sequence of integer intervals representing a range of integers in the format: Lower bound . . . Upper bound: A star character (*) can be used for the upper bound, denoting an unlimited upper bound. For example, ‘*’ indicates 0 or more, ‘1. . .*’ denotes one or many, and a single integer value means the integer range only contains the single integer value. The other four relationships derived from association are generalization, aggregation (shared aggregation), composition and dependency. Generalization is shown as a solid-line path from the specific class (subclass) to the general class (super-class) with a small empty triangle on the general class end of the association. Solid filled and hollow diamonds are placed on the end of composition and shared aggregation at the composite side, respectively. Both indicate a part–whole relationship between the two classes. Unlike the fact that a part in a composition connection can only be a part of one composite, the part of a shared aggregation can be a part of a number of wholes. Dependency indicates that the class (client class) at the tail of the dashed arrow line usually with a keyword on it depends on the class (supplier class) at the arrowhead, i.e. a change to the client needs the change to the supplier.
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To meet the requirements of modelling the particular domain of a process platform, the class diagram is extended to include instances and links among them. The name ‘static structure diagram’ may be better and more appropriate to express this, but ‘class diagram’ is shorter and well established. For simplicity, ‘class diagram’ is maintained in this research. In this sense, another two relationships also may appear in the extended class diagram: instance_of and link. Instance_of represents the connection between a class and its instances by a dashed arrow line pointing to the class with the keyword instance_of on it. Several instances of the same class can share one keyword. Link is the connection between instances. A link denoted by a solid line with/without a keyword is an instance of an association. A class diagram is the core of an object-oriented application model and is used herein to describe the structural aspect of a process platform. Although a number of authors in the literature criticize the fact that UML lacks precise semantics and formal formalization, its language features for defining a class diagram are, however, expressive enough. Therefore, this advantage of UML can be exploited to make an accurate representation of a process platform in terms of entities and relationships without further modification on it. 3.2 Extension mechanism UML provides a standardized extension mechanism to adapt this general modelling language to specific application domains. The standard way of introducing an extension to UML is to add a profile (a stereotyped package), which is defined as a set of stereotypes (classes of metamodel elements), tagged values (keyword-value pairs) and constraints. The advantage of the extension mechanism is that a profile does not introduce any new concepts to the language, but rather than specialize it for certain domain. Therefore, UML can fit the particular modelling needs of different problem domains while keeping its integrity. To represent a process platform more precisely, the stereotype instead of tagged values and constraints is used to extend UML. For the domain of a process platform, some special sets of classes (resource type, constituent types . . .) are defined as a specialization of the UML class. Also defined are links (compatible, select . . .), associations (support, perform . . .), and dependency (succeed . . .), which are useful and necessary for designing the data structure domain of process platforms. Therefore, a UML profile for the domain of a process platform consists of the following elements (not exhaustive) as shown in table 1. Figure 2 shows how the profile for the data structure of a process platform is integrated into the four-layer architecture of UML. In the metamodel, in the third layer (L3) the extensions for the data structure domain are defined. The actual process platform models in L2 correspond to class diagrams; and the routing list in L1 for a given customized product configured from the process platform can be seen as the instantiation of the model.
4. Structural representation of process platform The structural representation of a process platform is approached from three aspects: generic product structures, generic process structures, and integrated generic routing structures for the correspondence between generic product and process structures.
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Process Platform Model Process Platform instance (Product and Process Variants)
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4.1 Generic product structure Product elements can be classified into a number of types, including items, end-products, assemblies, parts and raw materials. These elements along with their class-to-class, class-to-member and member-to-member relationships form the generic product structure.
4.1.1 Entities. The entities representing product elements in the generic product structure convey product data and information from different levels of abstraction starting from end-products and ending with specific values of variety parameters. . Item. An entity of the type item represents a set of one or more product variants, each of which is either a physical existing product or a non-physical concept. It can be the end-product at the top level of the generic product structure. In addition, it can be an assembly or a part at any arbitrary intermediate level or the lowest level, and raw material at the lowest level. While a specific item represents only one product variant, a generic item represents a number of product variants of the same type, i.e. a class. . End-product. An entity of the type end-product represents a set of one or more product variants at the top level of the generic product structure. Entities of the type end-product are built by specifying compatible lower level items according to customer requirements. The determination of items that are
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compatible to others is influenced by the design parameters transformed from customer requirements. Assembly. An entity of the type assembly represents a set of one or more product variants at any arbitrary intermediate level of the generic product structure. They can be assembled together to achieve the higher level assemblies or end-products. An assembly has at least two child items: assemblies or parts. Part. An entity of the type part represents a set one or more product variants at the lowest level or any arbitrary intermediate level of the generic product structure. They are usually used to form assemblies at higher levels or the end-products at the top level. A part entity is either a type of in-house manufactured parts at the intermediate level or a type of purchased parts at the lowest level of the hierarchy. Raw material. An entity of the type raw material represents a set of one or more different raw materials at the lowest level of the generic product structure. Raw materials are used to machine or fabricate into the higher level parts. Parameter. An entity of the type parameter is a special kind of attribute (properties of a product) and relevant to variety. It represents a characteristic of an entity of the type item. It describes the item independent from its context. One item may have a number of entities of the type parameter. For example, an office chair can be described by several parameters such as colour, material and number of wheels. Value. An entity of the type value represents an assignment of an entity of the type parameter. Every such parameter may have a number of values. In the context of mass customization, these parameters are termed as ‘variety parameters’. A particular item has a specific value for each parameter. After all the values of the corresponding parameters of a generic item are determined, a specific item variant is obtained. Parameter value. An entity of the type parameter value defines a relationship between an entity of the type parameter and an entity of the type value. It is the combination of an entity of the type parameter and an entity of the type value. A set of such combinations, named specification, can define a unique item.
4.1.2 Relationships. Following the convention in UML, the multiple class–member relationships in the generic product structures are described as follows: . Instance_of. An instance_of relationship shows the connection between an instance and its class. For example, material can be an instance of the class of parameter, and red is an instance of the class value. . Association. An association denotes a relationship between two classes that involves connections among their instances. For example, the connection between two subclasses (PiV, the group of value instances of the i-th parameter; and PjV, the group of value instances of the j-th parameter) of the super-class parameter value (figure 3a) indicates certain relationships between them. These relationships are reified by the links between their instances. They may be incompatible or compatible. The association between
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parameter value and parameter indicates a number of parameter values can be mapped onto the same parameter, while a one-to-one mapping relationship exists between value and parameter value. . Link. A link represents the connection between two instances. It itself is an instance of an association. There are three kinds of situations in which a link exists. In the first situation, a link connects two instances possessed by two different classes. If one class is the composition of the other, then the relationship between these two instances is ‘consist_of ’ or ‘a_part_of ’ (figure 3c). If the two classes do not have the composition relationship, the connection between the two instances is either compatible (AND) or incompatible (XOR) (figure 3c). Moreover, in figure 3(a), two instances of the subclass PiV are Pi Vij (the j-th value of the i-th parameter) and Pi Vii (the i-th value of the i-th parameter), and two instances of subclass PjV are Pj Vjj (the j-th value of the j-th parameter) and Pj Vji (the i-th value of the j-th parameter). While Pi Vii is compatible with Pj Vji , Pi Vii and Pj Vjj are incompatible. Assume Pi Vii stands for ‘square shape’, and Pj Vjj and Pj Vji for ‘wood material’ and ‘steel material’, respectively. The implication is that no specific item with a square shape can have wood as its material, or when a square item is chosen, the other items with material wood at the same level of the generic product structure are not allowed to be assembled together with it to build their immediate parent. The compatible relationship indicates the reverse condition. In the second case, the instances connected by a link belong to a class. Since they are options of a generic item, the relationship between them is XOR, which means at one time only one instance among all can be chosen to represent the item class to build a parent item. In the third situation, a link exists between the instances of the entity type of item and
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parameter value, parameter or value (figure 3b). Such a link has a keyword has. It indicates an item has certain characteristics. . Dependency. A dependency indicates a semantic relationship between two classes, i.e. the operation of a class depends on the operation of another class. The dependency between class parameter and value implies parameters must be bound to actual values in order to express certain meaning. After this binding, a new class parameter value is created. . Shared aggregation. A shared aggregation is a special kind of aggregation. It represents the relationship between a part and a whole where the part can be a part in a number of wholes. The shared aggregation between part/ assembly and assembly/end-product indicates that a number of assemblies/ end-products can share a set of common parts/assemblies. . Generalization. A generalization is the taxonomic relationship between a more general class (the super-class) and a more specific class (the subclass) that is fully consistent with the super-class and adds additional information. The connection between end-product (assembly, part) and item indicates a generalization connection. For example, in figure 3(c), the connections between End Product and Assemblyi, as well as between End Product and Assemblyj are generalization. 4.1.3 Selection rules. The construction of the generic product structure depends on a set of rules, namely selection rules. Representing product knowledge, selection rules specify the circumstance under which an item is a constituent of another item or a particular variant becomes the instance of an item. They are defined to present to customers with only feasible options. The general form for selection rule is as follows: ðconsequentÞ IF ðantecedentÞ, where the relationship OR (_) and AND (^) can be applied to both antecedent and consequent. For example, Pj Vji , Pi Vii and Pj Vjj : fIi jIij g ^ fIj jIji g
IF fPj VjPj Vji g ^ fPi VjPi Vij gfPj V j Pj Vjk g ^ . . .
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is the y-th instance of the x-th generic item; PxV where Ix is the x-th generic item; Ixy is the group of values instance of the x-th parameter; and Px Vxy is the y-th value of the x-th parameter. Formula (1) indicates when Pj Vji or Pi Vjk and Pi Vij are specified, two item variants, Iij and Iji , can be selected together to build their parent item. Formula (2) means the only condition for selecting Pj Vji is that Pi Vij is specified. Selection rules are defined with consideration of a set of constraints due to technique restrictions, economic factors and limitations according to production processes. In fact, these rules describe the compatibility of various item variants and parameter values. Therefore, in collaboration with product items and relationships, selection rules can determine a particular product variant.
4.1.4 Representation of generic product structure. The generic product structure represented using UML-based formalism is given in figure 4. The symbols explained in the legend are consistent with these in the previous text. It shows an item may be
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Representation of the generic product structure.
a raw material, a part, an assembly or an end-product at the hierarchy. An endproduct consists of parts and assemblies, each of which may contain its lower-level items, e.g. a raw material for a machined part. An assembly may be formed by joining its child parts and/or assemblies. In the two subclasses of part class, the class of machined parts requires a relevant raw material class. A number of the same items (raw materials, parts or assemblies) can be consumed by a set of one or more parent items at higher levels. One or more items can have a set of common parameter values; and the differences among such items lie in the optional parameter values specific to each item. No item can assume two parameter values that are incompatible with each other. In addition, at the same level of the generic product structure, two items with incompatible parameter values cannot be assembled together to form a parent item.
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4.2 Generic process structure A generic process structure conveys process data, information and knowledge. The various process elements in the process family along with their relationships form the generic process structure. 4.2.1 Entities. Representing process elements that occurred in the process family, entities in the generic process structure are described as follows: . Operation. An entity of the type operation represents an aggregation of a set of sequenced manufacturing steps, which in turn represent a production job. These manufacturing steps are performed either by labour, by a machine, or a combination of both. An operation can achieve an item’s transformation from one stage to another stage. The same as a generic item, a generic operation consists of a number of specific operation variants. While a generic operation corresponds to a generic item, an operation variant of it is to fulfil a specific variant of the generic item. A generic operation can be described by its properties including the generic input materials, the generic output product, generic cycle time, generic manufacturing resources, including generic machine, generic tool, generic fixture and generic labour. . Machine. An entity of the type machine performs a part or the whole of an operation assigned to it. To complete the operation, tools or fixtures may be used. The selection of machines is affected by properties of items, e.g. materials. . Tool. An entity of the type tool is a support device for an entity of the type machine or labour to carry out an entity of the type operation. A particular tool may be shared by different entities of the type machine or labour. . Fixture. Unlike an entity of the type tool, an entity of the type fixture is a fixed auxiliary device for an entity of the type machine or labour to execute an operation. Usually entities of the type fixture are fixed at a certain place. . Labour. The task for an entity of the type labour that can be either human operators or robots is (a) to perform a part or whole of one particular operation using a number of tools and fixtures; and (b) to execute the preparation activities named set-up before the actual operation commences. Complying with manufacturing practice, in this research labour refers to the number of labour. . Set-up. An entity of the type set-up represents a set of activities prior to actual operations. It is necessary to perform these activities before an entity of the type operation begins. Usually, such set-up activities are performed by entities of the type labour. Examples of an entity of the type set-up are material preparation, changing of tools and fixtures, etc. . Cycle time. An entity of the type cycle time records the duration time of an entity of the type operation from the starting point to the ending point. Each operation must have its own cycle time. However, the cycle times for several different operations may be the same. . Material-handling system. An entity of the type material-handling system (MHS) consists of a set of one or more separate automated guided vehicles (AGVs), articulated arm robots, conveyors, gantry robots and buffers, or a
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combination of some of them. Its main function is to transport materials and finished/semi-finished parts to the designated place for further operation. Usually, an entity of the type material-handling system is shared by a number of entities of the type machine. 4.2.2 Relationships. The many relationships in the generic process structure are described as follows: . Dependency. As mentioned above, a dependency indicates a semantic relationship between two classes. The dependency relationship between operations reveals the starting of one operation depends on the finishing of its preceding operation (assuming all needed manufacturing resources and materials are available and sufficient). A dependency can also be found between an operation class and a set-up class. It suggests the required set-ups must be ready before the operation starts. . Instance_of. The same as that in the generic product structure, an instance_of relationship between entities in the generic process structure represents the connection between a class and its instances. For example, an operation variant for producing an item variant is an instance of the generic operation in relation to the generic item of the item variant. . Generalization. A generalization relationship shows the connection between the more general class and the more specific class of same type process elements. The relationship between a super-operation (machine) class and its suboperation (machine) classes indicates generalization. . Association. An association indicates a certain relationship between two classes of process entities. For example, figure 5(a) shows entities of the type operation are performed by entities of the type machine with the support of an entity of the type MHS, each operation has a cycle time. Figure 5(b) shows entities of the type labour do set-up, use tools and fixtures, control machines as well. . Link. The same as that in the generic product structure, a link in the generic process structure represents the connection between two instances. The connection between machine (labour) instances and tool instance,
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machine (labour) instance and fixture instance describes links, which have a keyword use. It implies that in order to fulfil the specific operations, machines (labour) may use tools (fixtures) to complete the activities. 4.2.3 Representation of generic process structure. Figure 6 presents the UML model of the generic process structure. Symbols are described in the legend. It shows that operations are performed either by machines or by labour, which use tools and/or fixtures. The operations are executed according to a certain sequence determined by the characteristics of the end-products. In other words, one operation starts only after its preceding one is completed. The required set-up activities such as material preparation, changing of tools and fixtures must be finished by entities of the type labour before its succeeding operation commences. Each operation with the aid of MHS will last a period time, which is referred to as a cycle time. The cycle times for several operations may be the same.
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Representation of the generic process structure.
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4.3 Generic routing structure While the formation of the generic routing structure is accomplished by connecting the generic product structure with the generic process structure using material requirement links, the specific unification of product and process data relies on the set of variety parameters and their value instances. In this regard, planning rules are defined to specify process elements for the corresponding product elements. 4.3.1 Correspondence in between. The correspondence between the two generic structures is embodied by a set of mapping relationships, which are classified into association between classes and link between instances: . Association. The association between product entity classes and process entity classes reflects certain meaningful connections. For example, the connection between an operation class and an item class indicates items specify the required operations that are used to produce or process them. The connection between parameter value and labour (machine, tool, fixture) implies different parameter values may select the same/different labour (machines, tools, fixtures). However, parameter values may or may not have an influence on the selection of machines, tools, fixtures and labour. For example, two same types of item variants with a difference in their colors, e.g. color_red and color_black, may require the same manufacturing resources. As opposed to this, in most situations different materials and different shape may select different resources. Figure 7 shows the association between parameter value and machine, labour, tool and fixture. . Link. Connections between product entity and process entity classes are embodied by the specific links between their respective instances. For example, in figure 8, Pj Vji (the i-th value instance of the j-th parameter) — an instance of parameter value (PV) class — selects Lj (the j-th labour instance), Fi (the i-th fixture instance), Ti (the i-th tool instance), and Mii (the i-th instance of the i-th type machine), Mji (the i-th instance of the j-th type machine). 4.3.2 Planning rules. Planning rules are defined in accordance with a set of constraints, e.g. technique restrictions, economic factors and limitations of
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Links between product and process entities.
production process. They enable production personnel to select the right manufacturing resources according to a set of specific parameter values of the items to be processed, and to determine the operations in terms of type and sequence. Both the difference among product elements and the variations among process elements are characterized by variety parameters and their value instances. Thereafter, planning rules tie products and their corresponding processes coherently. The general form of planning rules follows the same format of selection rules. 4.3.3 Representation of generic routing structure. The UML model of the generic routing structure is shown in figure 9. Symbols are explained in the legend. Within a process platform, product items specify the required operations. Parameter values of product items may not have an influence on the selection of tools, fixtures, machines and labour. The completion of operations leads to new items produced. Manufacturing resources, such as labour, machines, tools and fixtures, collaborate to carry out the specified operation. After the completion of operations, the MHS transports the produced new items (raw material as well) to the next operation, and thus keeps the production moving smoothly.
5. Case study The adopted case is the high-variety production of vibration motors for hand phone products. The main parts of a vibration motor are a rubber holder, weight and the mainbody. The mainbody in turn consists of an armature assy, a bracket assy and
Figure 9.
Representation of entities and relationships in a process platform.
Process platform representation based on UML 339
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L. Zhang et al.
a frame assy. A motor and its bill of materials (BOM) structure are shown in figure 10. Table 2 lists its BOM data records in a traditional BOM file. The production process for a vibration motor involves six assembly operations (Avm, Amb, Aaa, Aca, Afa, Aba) and five manufacturing operations (Mt, Mba, Mbb, Mf, Mc): shaping, machining, stamping, forming and other types. While BOM data convey the product-related information for a vibration motor, the process data listed in table 3 indicate a motor’s process information. All the parts of a vibration motor including the purchased items can be customized to meet individual customer requirements. Therefore, these parts can be defined as generic items. Each generic item has a number of options providing all possible selections to customers. The customer selects the specific options to build their expected products. Thus, the enormous number of possible configurations of vibration motors with a basic product structure appears on the shop floor. For simplicity, we only show in figure 11 the generic product structure for a bracket
Vibration Motor Weight
Magnet
Frame Bracket A&B Weight
Mainbody
Rubber Holder
Shaft Frame Assy
Bracket Assy
Armature Assy
Coil Frame
Tape
Magnet Shaft
Coil Assy
Bracket A
Bracket B
Terminal
Terminal Commutator Coil
(a)
Tape Commuter
(b)
Figure 10. Vibration motor and the product structure.
Table 2. Hierarchy level 1 1 1 .2 .2 .2 . .3 . .3 . .3 . .3 . .3 . .3 . .3 . . .4 . . .4 . . .4
BOM data of a vibration motor.
Parent item Vibration motor Vibration motor Vibration motor Mainbody Mainbody Mainbody Bracket assy Bracket assy Bracket assy Armature assy Armature assy Frame assy Frame assy Coil assy Coil assy Coil assy
Component item Mainbody Rubber holder Weight Frame assy Armature assy Bracket assy Bracket A Bracket B Terminal Shaft Coil assy Frame Magnet Coil Tape Commutator
Quantity per 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Mainbody assembly
Armature assembly Coil assembly Coil fabrication Frame assembly Frame fabrication Bracket assembly Bracket A fabrication Bracket B fabrication Terminsal A fabrication
40
30 20 10 20 10 20 10 10 10
* Same sequence numbers indicate parallel operations.
Vibration motor assembly
Operation
50
Sequence number
Machine
Fcaulking Mc. Ainserting Mc. Sinserting & Soldering Mc. ... Cwinding Mc. FMpressing Mc. Fstamping Mc. Bfusing Mc. Binjection Mc. Binjection Mc. Tcutting Mc. ... ... ... ... Die Binserter Badjustor Badjustor Die
...
...
Tool Wsitting jig Wcaulking head Caulking blade Bracket holder Supporting holder pallet Guiding jig Tray Fholder, Mholder Holder Bpressing jig Blocator Bprealignment jig Tholder
Fixture
Production process data of a vibration motor.
Wcaulking Mc.
Table 3.
1 1 1 1 1 1 1 1 1
1
1
Labour number (Mc.)
5.30 5.18 4.52 4.48 5.07 4.04 5.25 5.25 4.93
9.25
9.25
Cycle time (s/item)
Process platform representation based on UML 341
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L. Zhang et al.
assembly family (BAssy), each variant of which is composed of a bracket A (BA), a bracket B (BB) and a terminal (TL). In figure 11, ‘BAssy (I0)’, ‘Bracket A (I1)’, ‘Bracket B (I2)’ and ‘Terminal (I3)’ indicate that BAssy, BA, BB and TL are the 0-th, 1-th, 2-th and 3-th generic items, respectively. In the proper level of granularity, Ix is replaced with the particular name ’and ‘TL3j ’ of the generic item. For example, in the item instance level, ‘BA1i ’, ‘BB2k ’ and are used to represent item variants rather than ‘I1i ’, ‘I2k ’ and ‘I3j ’. ‘BA1i ’, ‘BB2k ‘TL3j ’ denote the i-th BA variant, the k-th BB variant, and the j-th TL variant, respectively. The parameters of generic items are written in the format of ‘parameter name (Pxy)’ as shown, wherein x and y are indices of the generic items and the parameters of the generic items. For example, ‘Color (P11)’ means the first parameter of the first generic item (i.e. BA); ‘Shape (P31)’ denotes the first parameter of the third generic item (i.e. TL). Each parameter has several value instances, e.g. ‘L’ is coded for a particular shape of TL variants and there are three different shapes including ‘L’, ‘T ’ and ‘U ’. Further, all parameters are bound to their possible value ) as shown. In Pxy Vxyz , x, y and z instances and thus form parameter values (Pxy Vxyz are indices of the generic items, parameters and value instances, and thus Pxy Vxyz indicates the z-th value of the y-th parameter of the x-th generic item. The specific can be found in table 4 describing generic operations for values of each Pxy Vxyz producing BA, BB, TL and BAssy. It shows that operations to produce BA, BB, TL and BAssy are characterized by paired parameter and value sets, variety machine (M), tool (T), fixture (F), labour (L) and cycle time (CT). The particular tools, fixtures and cycle times are not exhaustive in this table for illustrative simplicity. In figure 11, the AND and XOR links show the compatibility and incompatibility of parameter values. For example, among the specific parameter values of BA (representing shape L) is compatible with P13 V132 (denoting width variants, P12 V123 6 mm). This means a particular BA variant can be designed to take the L shape with the 6 mm width. Figure 12 shows a configuration result for a BAssy in terms of the selection of items with compatible parameter values. The selection rules employed are listed as follows. The specific values of each symbol are given in table 4: g fBAjBA13
IF
fP11 jP11 V112 g ^ fP12 jP12 V123 g ^ fP13 jP13 V132 g
g IF fBBjBB26
fP21 jP21 V212 g ^ fP22 jP22 V223 g ^ fP23 jP23 V232 g
g IF fTLjTL34
fP31 jP31 V312 g ^ fP32 jP32 V321 g
fBAssyjBAssy2 g
IF
fBAjBA13 g ^ fBBjBB26 g ^ fTLjTL34 g
Figure 13 shows the generic process structure of BA family. The symbols in this figure are described in table 4. The cycle times presented in this figure are the average cycle times of all existing motor variants. Figure 14 shows part of the motor’s process platform in terms of BA. The set of variety parameters and their values are employed to select the right manufacturing resources. A set of planning rules is used to guarantee the right selection. The particular operation selection results in terms of M, T, F, L, and CT for the specific BA, BB, TL, and BAssy in figure 12 are listed in table 5. The employed planning
Figure 11. Generic product structure of vibration motors.
Process platform representation based on UML 343
Bracket A {I1}
Bracket B {I2}
Bracket B fabrication (Mbb)
Generic item {Ii}
Bracket A fabrication (Mba)
Generic operation {O} M11 ¼ BinjectionMc06
M12 ¼ BinjectionMc04
¼CB P11 V111
P11 V112 ¼CR
Blue ðV111 Þ
Red ðV112 Þ U ðV121 Þ T Þ ðV122 L ðV123 Þ 5 ðV131 Þ 6 Þ ðV132 7 ðV133 Þ
M22 ¼ BinjectionMc04
¼CR P21 V212
Red ðV212 Þ U ðV221 Þ T ðV222 Þ
Shape {P22} P22 V222 ¼SU
P22 V221 ¼ST
M21 ¼ BinjectionMc06
P21 V211 ¼CB
Blue ðV211 Þ
P13 V133 ¼W 7
P13 V132 ¼W 6
¼W 5 P13 V131
P12 V123 ¼SL
P12 V122 ¼SU
P12 V121 ¼ST
Variety machine fMik g
Variety PV fPij Vijk g
Variety value fVijk g
T22 ¼ BadjustorVII
T21 ¼ BadjustorA
T12 ¼ BadjustorVII
T11 ¼ BadjustorA
Variety tool fTik g
Generic operations of bracket Assy.
Colour {P21}
*unit: mm
Width {P13}
Shape {P12}
Colour {P11}
Variety parameter {Pij}
Table 4.
F23 ¼ BprealignmentjigIII
F22 ¼ BprealignmentjigII
F21 ¼ BprealignmentjigI
F13 ¼ BlocatorIII
F12 ¼ BlocatorII
F11 ¼ BlocatorA
Variety fixture fFik g
L22 ¼2
L21 ¼1
L12 ¼2
L11 ¼1
Variety labour fLik g
CT22 ¼ 6:04
CT21 ¼ 5:25
CT12 ¼ 5:25
CT11 ¼ 5:50
Variety cycle time fCTik g
344 L. Zhang et al.
Terminal {I3}
Bracket Assy {I0}
Terminal fabrication (Mt)
Bracket assembly (Aba)
Width {P32} *unit: mm
02
Shape {P31}
7
5
11
7
6
5
*unit: mm
Width {P23}
L
ðV322 Þ
ðV321 Þ
ðV312 Þ
ðV311 Þ
ðV233 Þ
Þ ðV232
ðV231 Þ
Þ ðV223
P32 V323 ¼W 7
P32 V321 ¼W 5
P31 V312 ¼ S 11
P31 V311 ¼ S 02
P23 V233 ¼W 7
P23 V232 ¼W 6
P23 V231 ¼W 5
P22 V223 ¼SL
T01 ¼ BinserterV
T02 ¼ BinserterS
M02 ¼ Bfu sin gMcII
T32 ¼ DieII
T31 ¼ DieLS
¼ Bfu sin gMcI M01
M32 ¼ TcuttingMcII
M31 ¼ TcuttingMcI
F02 ¼ Bpres sin gjigBO
F01 ¼ Bpres sin gjigAI
F33 ¼ TholderIII
F32 ¼ TholderII
F31 ¼ TholderI
L02 ¼2 *number/mc.
L01 ¼1
L32 ¼2
L31 ¼1
CT02 ¼ 4:25 *s/item
CT01 ¼ 4:04
CT32 ¼ 4:45
CT31 ¼ 4:93
Process platform representation based on UML 345
346
L. Zhang et al.
a_part_of
BAssy*2
BA*13 has
TL*34 has
P11V *112
P32
has
P13V *132 P12V *123
BB*26
V*
P21V *212
P23V *232
321
P31V *312
P22V *223
Figure 12. Specific BOM structure derived from the generic product structure.
Figure 13. Generic process structure of vibration motors.
rules are listed as follows: For BA : fMjM12 g ^ fTjT11 g ^ fFjF11 g ^ fLjL11 g IF fP11 jP11 V112 g ^ fP12 jP12 V123 g ^ fP13 jP13 V132 g For BB : fMjM22 g ^ fTjT21 g ^ fFjF21 g ^ fLjL21 g
IF
fP21 jP21 V212 g ^ fP22 jP22 V223 g ^ fP23 jP23 V232 g
For TL : fMjM31 g ^ fTjT32 g ^ fFjF33 g ^ fLjL31 g IF fP31 jP31 V312 g ^ fP32 jP32 V321 g For BAssy : fMjM02 g ^ fTjT01 g ^ fFjF01 g ^ fLjL01 g IF fBAjBA13 g ^ fBBjBB26 g ^ fTLjTL34 g
347
Process platform representation based on UML specify
1
perform
P13V *131 P13V *133 P12V *122
P11V *113
P12V *121
: instance
1..*
: class * PxyVxyz :
P11V *123
keyword
Legend:
P13V
P11V *112
P11V *112
P12V
P13V *132
CT *12
use Machine
Tool
L*11
L*12
Fixture
control
M *12
use control
M *11 use
T *11
F *12
T *12 use
use
F *13
select
F *11
select
: link
CT *11
1
P11V
1
control Labour
1
Parameter value
BA*1i
Cycle Time
1..*
BA*i
has Operation
1..*
1..*
Bracket A
1
BA*1i
: generalization
keyword
: association : instance of
: shared aggregation
the z-th value instance of the y-th parameter of the x-th generic item PxyV :a set of values of the y-th parameter of the x-th generic item
* * Fxy* / Mxy /Lxy/ Txy*/ CTxy*: the y-th fixture/machine/labor/tool/cycle tim variant of the x-th generic item
Figure 14. Partial process platform of vibration motors.
The same selection process is applied to other parts of the motor variant as shown in figure 12. The process elements specification result for a particular motor is given in table 6.
6. Conclusions Effective representation of a process platform needs to convey explicitly the complex relationships among constituent elements within and between the generic product structure and the generic process structure. These relationships are characterized by the class-to-class, class-to-member and member-to-member types. Incorporating OO techniques, UML lends itself to represent a process platform in terms of the constituent elements and their relationships by taking advantage of the class diagrams and the extension mechanism. While a class diagram models accurately classes and their relationships; the extension mechanism specifies and models instances and the relationships among them. OO techniques allow the capture of relationships between classes and member instances. The structural representation of a process platform is approached from three aspects: the generic product structure embodying a product family, the generic process structure representing the process family, and the integrated generic routing structure for the correspondence in between. While this paper represents the various constituent elements and their complex relationships inherent in a process platform; it is not enough for a good understanding in terms of semantics modelling, i.e. how a process platform works for production management through coordinating variety from design to production. Therefore, the possible future research may focus on the dynamic modelling of process platforms using proper modelling schemes.
* Same sequence numbers indicate parallel operations.
Bpressing jigAI
Fixture
Bracket A fabrication Binjection Mc04 BadjustorA BlocatorA Bracket B fabrication Binjection Mc04 BadjustorA Bprealignment jigI Terminal fabrication Tcutting MCI DieII TholderIII
BinserterV
Tool
10 10 10
Bfusing MCII
Machine
Bracket assembly
Operation
1 1 1
1
5.25 5.25 4.93
4.04
Bracket A Bracket B Terminal Raw material Raw material Raw material
Bracket A Bracket B Terminal
Bracket assy
1 1 1 n.a. n.a. n.a.
Cycle time Quantity Labour per (number/mc) (s/item) Material item Product item
Specific operation derived from the process platform.
20
Sequence number
Table 5.
348 L. Zhang et al.
Mainbody assembly
Armature assembly Coil assembly
Coil fabrication Frame assembly Frame fabrication Bracket assembly
Bracket A fabrication Bracket B fabrication Terminal fabrication
40
30
10
10
DieII
BadjustorA
Binjection Mc04
Tcutting MCI
BadjustorA
BinserterV
Die
...
...
...
...
...
...
Tool
TholderIII
Bprealignment jigI
BlocatorA
Bpressing jigAI
FholderI MholderVII Holder01
TrayII
Supporting holderI PalletII Guiding jigIII
Caulking bladeII Bracket holderL
Wsitting jigI Wcaulking headI
Fixture
1
1
1
1
1
1
1
1
1
1
1
Labour number (mc)
4.93
5.25
5.25
4.04
5.07
4.48
4.52
5.18
5.30
9.25
9.25
Cycle time (s/item)
Production process derived from the process platform.
Binjection Mc04
Bfusing MCII
Fstamping McI
FMpressing Mc.
Cwinding McV5
Sinserting & Soldering Mc. ...
Fcaulking McHS Ainserting McV
Wcaulking McI
Machine
* Same sequence numbers indicate parallel operations.
10
10
20
10
20
20
Vibration motor assembly
Operation
50
Sequence number
Table 6.
Bracket A Bracket B Terminal Raw Material
Magnet Frame Raw Material
Weight Rubber holder Mainbody Assy Armature Assy Frame Assy Bracket Assy Coil Assy Shaft Coil Tape Commutator Raw Material
Material item
Terminal
Bracket B
Bracket A
Bracket Assy
Frame Assy Frame
Coil
Armature Assy Coil Assy
Mainbody Assy
Vibration motor
Product item
n.a.
n.a.
1 1 1 n.a.
1 1 1
1 1 1 1
1 1 1 1 1 1 1
Quantity per
Process platform representation based on UML 349
350
L. Zhang et al.
References Booch, G., Object-Oriented Analysis and Design, 1994 (Benjamin/Cummings). Du, X., Jiao, J. and Tseng, M.M., Architecture of product family: fundamentals and methodology. Concurr. Eng.: Res. Appl., 2001, 9, 309–325. Hegge, H.M.H. and Wortmann, J.C., Generic bill-of-material: a new product model. Int. J. Prod. Econ., 1991, 23, 117–128. Jiao, J. and Tseng, M.M., Customizability analysis in design for mass customization. Comput.Aid. Des., 2004, 36, 745–757. Jiao, J., Tseng, M.M., Ma, Q.H. and Zou, Y., Integrated product and production data management based on generic bill of materials and operations to support mass customization production. In Proceedings of the 1999 ASME Design Engineering Technical Conferences, (Las Vegas, NV, USA), 1999. Jiao, J., Zhang, L.F. and Pokharel, S., Process platform planning for mass customization, in Proceedings of the 2nd Interdisciplinary World Congress on Mass Customization and Personalization [CD-ROM Proceedings], Munich, Germany, 6–8 October 2003. Jiao, J., Zhang, L.F. and Pokharel, S., Process platform planning for variety coordination from design to production in mass customization manufacturing. IEEE Trans. Eng. Manag., 2006 (in press). Meyer, M.H. and Lehnerd, A.P., The Power of Product Platforms: Building Value and Cost Leadership, 1997 (Free: New York, NY). Nayak, R., Chen, W. and Simpson, T.W., A variation-based method for product family design. Eng. Optimiz., 2002, 34, 65–81. Rumbaugh, J., Blaha, M., Premerlani, W., Eddy, F. and Lorensen, W., Object-Oriented Modeling and Design, 1991 (Prentice-Hall: Englewood Cliffs, NJ). Rumbaugh, J., Jacobson, I. and Booch, G., The Unified Modeling Language Reference Manual, 1999 (Addison-Wesley: Reading, MA). Sanderson, S.W. and Uzumeri, M., Managing Product Families, 1997 (Irwin: Chicago, IL). Suh, N.P., Axiomatic Design — Advances and Applications, 2001 (Oxford University Press: New York, NY). Ulrich, K., The role of product architecture in the manufacturing organization. Res. Pol., 1995, 254, 419–440. Van Veen, E.A. and Wortmann, J.C., Generative bill of material processing systems. Prod. Plan. Contr., 1992, 3, 314–326. Wortmann, J.C., Muntslag, D.R. and Timmermans, P.J.M., Customer-Driven Manufacturing, 1997 (Chapman & Hall: London).
