ON THE APPLICABILITY OF PRODUCT VARIETY DESIGN CONCEPTS TO AUTOMOTIVE PLATFORM COMMONALITY. Zahed Siddique and David W. Rosen*

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1 Proceedings of DETCí98: 998 ASME Design Engineering Technical Conferences September 3-6, 998, Atlanta, Georgia 98-DETC/DTM-566 ON THE APPLICABILITY OF PRODUCT VARIETY DESIGN CONCEPTS TO AUTOMOTIVE PLATFORM COMMONALITY Zahed Siddique and David W. Rosen* Systems Realization Laboratory The Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, GA Nanxin Wang Ford Motor Company Dearborn, MI 482 ABSTRACT The issue of moving from a mass production operating mode to mass customization, or even limited customization, has many companies struggling to reorganize their product architectures. Enabling the production of several related products for different market segments, from a common base, is the focus of the product variety design research area. In this paper, the applicability of product variety design concepts to the design of automotive platforms is explored. Many automotive companies are reducing the number of platforms they utilize across their entire range of cars and trucks in an attempt to reduce development times and costs. To what extent can research on product variety design apply to the problem of platform commonization? This question is explored by comparing product variety design concepts (standardization, modularity, mutability, etc.) to platform structures and requirements. After assessing the applicability of these concepts, a platform representation and methods for measuring platform commonality are proposed that incorporate key characteristics of these concepts. An application to two platforms is included. Although preliminary, this work has led to insight as to why automotive platform commonization is difficult and how product design variety research can potentially aid commonization. The findings are potentially applicable to product platforms in general. INTRODUCTION The current market place is volatile, where the customers are constantly changing demands. The new shift in the current market has introduced the concept of product variety, in which variety and customization replace standardized products. One of the key elements in product variety is the product platform. A product platform is a collection of the common elements, especially the underlying core technology, implemented across a range of products. (McGrath, 995) One way to achieve mass customization is by developing the product platform carefully and then using different modules to provide product variety. Focusing product strategy at the platform level simplifies the product development process because there are fewer platforms than products and major platform decisions can be made much less frequently than product decisions. A platform approach encourages a long term view of the product strategy. Until recently product platform strategy was not implemented by many automotive companies. Balancing the need to customize products for target markets while enabling the economies of scale of a "world * Corresponding author.

2 car" is a challenge faced by every automotive manufacturer. A proliferation of options and model derivatives leads to increased tooling cost and production line complexity. Careful commonization of platforms can be used to increase product variety while reducing the number of components and the product line complexity. The common product platform and the family of product concepts are linked. Common platforms are required to produce families of products; consequently some of the ideas, characteristics, and concepts developed for families of products apply to common platform design. The purpose of this paper is to investigate automotive platform design and commonization in the context of research on product variety design. We first review different product family design research, highlighting the different product, assembly, and design characteristics of product variety. At first glance, it may appear that automotive platforms are quintessential examples for product variety design research. However, our study has identified significant differences between the variety characteristics of platforms from some of the examples that other researchers have studied. Most of the product family design research is applicable to products that are modular with respect to functions. The automotive platform, on the other hand, is not modular in nature, since the platform accomplishes just one function as a whole. As a result some of the family of product design concepts do not readily apply. In this paper we present some of our preliminary findings related to the automotive platform design problem. Specifically these are: Identification of different product family design concepts and investigation of the applicability of these concepts towards automotive platform design Development of a representation scheme for automotive platform commonization. Development of a scheme to measure commonality for automotive platforms. In the next section we will give a brief overview of some of the product family design concepts followed by an introduction to the automotive platform commonization problem. In Section 4 we will determine the applicability of the different product platform concepts for the automotive platform commonization problem. Representation schemes for modeling relationships among the components and assembly process to measure commonality is presented in Section 5. The conclusion (Section 6) will include a brief description of our future directions for this research. 2 FAMILY OF PRODUCTS Design for product variety is a relatively new research field. The characteristics of product variety range from flexible modular designs (Chen et al., 994) to robust and scaleable designs (Rothwell and Gardiner 990) to standardized, flexible products (Uzumeri and Sanderson 995). In Simpson, et al. (997) the authors identify robustness, modularity, and mutability as the core characteristics of product families. In their paper Uzumeri and Sanderson (995) examine the forces that drive product variety and rate of change as they influence the evolution of product families and models. In the paper the authors stress on the importance of variety and analogies to the biological life cycle. Sanderson (99) points out that rapid technological change and increased competition are forcing firms to compress the time it takes to develop product families and realize innovations among successive family members. Modular design and virtual design are the two approaches of design information management discussed in the paper. Virtual design describes the abstraction hierarchy used to represent product function. The functional elements of a design are decomposed to increase the efficiency of reimplementation of common product functions among a family of products and across product generations. Modularity is the concept of separating a system into independent parts or modules which can be treated as logical units. Ulrich and Tung (99) gave a summary of different types of modularity. The authors also state that modularity depends on two characteristics of a design: () Similarity between the physical and functional architecture of the design and (2) Minimization of incidental interactions between physical components. A complete modularity is achieved when there is a one to one correspondence between the physical and functional architectures. In their paper Newcomb et. al. (996) describe modularity of architectures with respect to life cycle concerns, not just product functionality and structure. The authors adopt the architecture decomposition algorithm developed by Kusiak and Chow (987) for partitioning architectures into modules from each life cycle viewpoint. Two measures of modularity are also proposed in the paper: () module correspondence and (2) coupling between modules. In their book Ulrich and Eppinger (995) describe the different stages of product development. The book also focuses on different aspects of product architecture and how they can be used to develop modular products. According to the authors a modular architecture implements one or a few functional elements in their entirety and the interactions between the chunks are well defined and are generally fundamental to the primary functions of the product. The opposite of a modular structure is an integral architecture. 2

3 spatial (EMIS) interactions between elements are documented and the DSM can be partitioned with respect to any of these viewpoints. Front Structure Front Underbody Figure. Automobile underbody Rear Structure Modular chunks allow changes of a product to be made to a few isolated functional elements of the product. Products built around modular product structures can be more easily varied without adding tremendous complexity to the manufacturing system. Modular product architecture also facilitates component standardization. In a series of studies Ishii et al. (995), Martin and Ishii (996) and Fujita and Ishii (997) discuss the direction toward computational approaches for product variety design. A method that can be used to capture the broadest customer preference in a product line while minimizing the life-cycle cost of providing variety is developed by Ishii and et al. (995). The cost measure utilizes the concept of late point identification which urges standardization early in the manufacturing process and differentiation at the end of the process. A product structure graph is used to represent the product variety in a hierarchical tree. These product structure graphs are useful for products that have a clearly defined dimension of variety that involve discrete choices, but is difficult to apply for complex variety structures. The concept of Design for Variety (DFV) is further developed in Martin and Ishii (996). According to the authors DFV refers to product and process design that meets the best balance of design modularity, component standardization, late point differentiation and product offering. Their research approach focuses on measures of the cost of providing variety, representation of variety and measure of the importance of variety. Fujita and Ishii (997) define the formal representation of product variety design and propose a structure of design tasks involved in product variety. Pimmler and Eppinger (994) described a methodology for the analysis of product design decomposition. Design Structure Matrix (DSM) is used to design organization based upon product structure. The DSM partitioning method is used for two purposes in the paper: ) organization of the development teams and 2) the definition of product architecture. This is accomplished by using components as the row and the column elements. The method involves decomposition of the system into elements, the documentation of the interactions between the elements, and clustering (or integrating) the elements into architectural and team chunks. (Architectural chunks are equivalent to product modules.) The energy, material, information and 3 PLATFORM COMMONIZATION One of the key elements in designing for product variety is the product platform. Ideally, a common platform can accommodate different models without requiring any change. commonization can be helpful in reducing product development cost and time for product families by leveraging additional products from a common core. Success of a product family depends on how well the product platform has been designed. In the case of automobiles the underbody structure is the product platform. One of the main purposes of the underbody is to provide support to the rest of the automobile. According to common classification with respect to function, the underbody is integral, rather than modular. The underbody is mainly composed of sheet metal components which are welded or bolted together into a support structure. The underbody is rarely seen by the user and, as a result, variety is not required from the end user s viewpoint. The variety in the underbody comes from the need of manufacturers to accommodate the different car models with different engines, transmissions, suspension, and body lengths, to name a few. From the perspective of our industry collaborator, Ford, a set of platforms is said to be common if their assembly processes, locators and weld-lines are the same. As shown in Figure, typical unibody platforms are composed of three main sections: front structure (engine compartment), front underbody (floor of passenger compartment), and the rear structure (trunk area). The joints between these three sections are called the weld-lines. An ideal common platform is a platform which can accommodate a set of different car models without any changes in components, assembly process and module interfaces. More realistically a common platform is one which can accommodate different variations required for different car models without requiring changes in assembly process, weld-lines and locators. Automobile platform manufacturing and assembly plants are very expensive to design, build or modify. This gives rise to the problem of developing common platforms for same/similar class cars. commonization can be divided into two segments (Figure 2):. Mapping of different platforms into a common platform to support all the individual platforms. 2. Evaluation of assembly processes and plants, that can be used to assemble the platform, to determine feasibility, possible impacts and minimization of cost for changeover. As mentioned earlier automobile platforms are integral. A one to one correspondence between function and 3

4 components is not readily evident. Although research done in the area of developing mass customized products focuses on modular products some of the concepts do or do not apply to this problem. 4 PRODUCT VARIETY DESIGN CONCEPTS AND PLATFORM COMMONALITY Some of the important concepts related to design for product variety were introduced in Section 2. In this section applicability of these concepts and their limitations towards automotive platform commonization will be discussed. Standardization: The main concept behind developing a common platform is to provide a standard platform for different models and classes of automobiles. Thus, the concept of standardization certainly applies to platform commonality. Standardization of all the components in the platform is desired with the goal that it will be able to accommodate the different variety criteria. To develop a common platform, standardization is one of the required characteristics. In the context of platform commonization, standardization has to be achieved in several levels. We consider three levels: Standardization of components Standardization of module interfaces Standardization of assembly process Standardization of components refers to having standard components in the platform. An ideal common platform is a platform which does not require any variation in components to support different car models. It should be pointed out that one of the objectives of a common platform is to provide underbodies for different cars models without changing the assembly process. As a result it is acceptable to have variations in components to provide the required varieties, as long they do not add complexity to the assembly process. There is a natural trade-off between component cost and assembly cost. While slightly different components can be assembled on the same assembly line, different stamping dies will be required to fabricate different components. How this trade-off is resolved depends upon the relative costs of component and assembly variations. Standardization of module interfaces refers to having standard internal and external interfaces. This can be described as a strategy for providing variety by isolating variations in the module interfaces. Interfaces to external modules need to be standardized so that different modules can be assembled, to produce the required product variety, using the same process. As an example different car models use different types of engines and transmissions. However, the engine/transmission module has standard module interfaces with the automobile underbody to ensure that engine and transmission variants can be assembled without COMMONIZATION s Commonize platform components and relationships Different Car Models Common Automotive Processes and Plants Commonize assembly processes for common platform Plant Figure 2. Commonization changing the platform or the assembly process. For many platforms, a subframe is used as the powertrain-platform interface. That is, the engine, transmission, and often the front suspension are assembled onto a subframe. Then, the entire module is decked into the platform in one assembly operation. The workstations that perform powertrain decking are very expensive; interface standardization is required so that all required powertrain and suspension variations can be accommodated. Standardization of assembly process refers to assembling required variations of the platform without changing tools, assembly sequences, and/or the assembly line. Although the ideal common platform is a platform that uses all standard components, more realistically a common platform will require variations in components to provide the required variety. As a result the assembly process needs to be standardized so that the same assembly line can be used to assemble the variations without any change. Delayed Differentiation: The concept of delayed differentiation, i.e., adding variety at later stages of the assembly process, does not directly apply to platform commonization. The delayed differentiation concept is applied to products that have a well defined correspondence between variety attributes and product modules. In the case of automobile platforms, the correspondence between these variety attributes and different models of the platform is not readily evident. Imagine a scenario where front structure length has to change to accommodate a new engine type. In some cases, assembling platforms with different lengths may not be 4

5 feasible because to increase the length of the front structure, the side structures (aprons) must be lengthened, but the aprons are required to be loaded at the early steps of the assembly process. The only way this could be done is by adding different assembly lines for different lengths. Where these parallel lines are added is not particularly important, within reason. However, delaying their addition until late in the assembly process is not feasible without greatly complicating this process. Modularity: The concept of modularity generally applies to the relationship between functions and structure. In the case of automobile platforms, the concept of functional modules does not apply readily, mainly because the whole underbody is a support structure. The other modularity characteristic, that of minimizing incidental interactions, is difficult to apply since a platform is an integral structure. On a more abstract level, the platform can be divided into front structure, front underbody and rear structure, as described earlier. But because of its integral nature, one to one correspondence among components and functions does not exist. Since platforms are large and geometrically complex, it is necessary to partition them into major sections, then partition these sections into manufacturable and assemblable components and modules. The division into front structure, front underbody, and rear structure is just one method of platform decomposition (albeit the apparent standard for unibody platforms). Other methods include the twin-rail platform commonly used for light trucks. A new platform architecture, for small cars intended for developing countries, has been developed by Chrysler which utilizes only four injection molded components for the entire platform and body framing (Kobe, 997). From a life-cycle modularity perspective, the platform can be divided into modules from different life-cycle view points (Newcomb et al., 996). Using the extended definition of modularity to allow one-to-one correspondence between physical structures and structures of relevance to a life cycle viewpoint, the automobile underbody can be divided into modules with respect to external modules, assembly viewpoint etc. As an example components with physical connections with the engine can be considered a module. Also, groups of components that get welded together, then assembled into the platform are assembly modules. The general lesson appears to be that to achieve both variety and standardization, it is necessary to go beyond the conventional view of functional and structural modules to include assembly and other life-cycle considerations. This broader view of modularity enables the isolation of required variety into appropriate module types (structural, functional, assembly, etc.). Module Interfaces: Another important concept that applies to platform commonization is developing robust module interfaces. The variety in automobiles is mainly achieved through modules that are external to the platform; for example, the shape of the car is achieved through body panels attached to the underbody. Developing robust interfaces that can accommodate a variety of external modules without changes is sought while developing common platforms. The impact of platform commonization decisions on the plant floor is critical, due to the expense of assembly plants. Provided that the major module interfaces can be kept constant across a car model family, changes on the plant floor can be minimal. By measuring the number of different assembly fixtures and workstations required across a family, an indication can be given to the platform designer of the impact of his designs. Additional comments on modular interfaces have been offered in the standardization and modularity sections above. Robustness: Robustness implies an insensitivity to small variations and does not dictate a change in form nor a change in function (Simpson et al. 997). A common platform needs to be robust, so that if there are small changes in the platform requirements the form does not change. Small changes in a platform could be adding a different type of rear seat mount. From the assembly point of view robustness is also desired not only in the assembly process, but also in the assembly fixtures and workstations, such that if there are small dimensional changes in the platform components, the assembly line can still operate without any changes. Mutability: Mutability is the capability of the system to be contorted or reshaped in response to changing requirements or environmental conditions without a change in function. This is the characteristic that enables a platform to be used across several car models. For example, by lengthening the front underbody, an increase in wheelbase helps enable the same platform to be used for coupe, sedan and wagon models. An increase in rear overhang is also needed for wagons. By designing assembly stations to accommodate these different lengths, the same expensive welding equipment can be used across all of these car models. In this manner, a common platform should be designed so that if changes in shape or size are desired, the capability of the platform to meet all its functions and requirements is not sacrificed. 5

6 Bolted Front Structure Weld Product Front Variety Underbody Bolted Weld Rear Structure Engine Suspension Mutability of Components Figure 5. architecture first level (automobile Supported without underbody) any changes by process Module interfaces Figure 3. Mutability of components and its effects on commonization Figure 3 illustrates the relationships among platform commonization, component characteristics, and the effects on the assembly process and module interfaces. Product variety requires a certain amount of component mutability. The ranges of component size and shape variations must be accommodated by the module interfaces, from functional and structural viewpoints, and by the assembly workstations, from the assembly process viewpoint. commonization and its relation to different product variety concepts was discussed in this section. As pointed out standardization of components and module interfaces is essential in developing a common platform (Figure 4). Robustness and mutability are two of the characteristics that are desired in common platforms. Although some of the product variety concepts do not apply to platform commonization, most of them do apply and are related. 5 PLATFORM COMMONIZATION - CASE STUDY One of questions that is being answered in this paper is Can product variety design and product family design be applied to automotive platform commonization problem? To answer this question we develop representation schemes and measures of commonality to apply some of the concepts, mainly standardization, discussed in the previous section. We focus our approach at the configuration design stage of the design timeline. Configuration design is the design stage that falls between conceptual design and parametric design. It is at this design stage that product modules and module interfaces are identified. The design of common platforms has important configuration design elements that involve the layout of components, the specification of interfaces between automotive subsystems, etc. The question posed above is divided into smaller questions that relate to different aspects of the platform commonization problem: Representation Question: How to represent the platform configuration to facilitate commonization? The representation should provide the means to () Identify problem areas (2) Measure commonality and (3) Determine module interfaces. Measuring Question: How to measure commonality? What are the important viewpoints? Components Right Apron Mutability Mutability Right Rail Robustness Standardization Right Radiator Engine Bracket Support Suspension Interface Modules Dash/Cowl Process Left Mutability Bracket Mutability Mutability Robustness Left Rail Robustness Standardization Standardization Left Apron Common (a) Right Apron Figure 4. Common platform characteristics Radiator Support Engine Sub frame Left Apron Right Rail Left Rail Suspension Dash/Cowl (b) Figure 6. Front Structure Architecture - 3rd Level for (a) A (b) B Interface Modularity Measuring Question: How to measure interface modularity? In this section we will present our preliminary findings toward answering these questions. 5. Representation - Graphs graphs, similar to the assembly representation presented in Rosen et al. (996), are used to represent the architecture of platforms. The diagrams are used to illustrate the physical connections among components and subassemblies. The graphs can also be extended to include assembly information for illustrating how the connections among the components/assemblies are achieved in an assembly process. 5.. Representing component connections In the graph representation, physical connections among assemblies/components are represented by labeled arrows. The type of connections (welds, bolted joint, etc.) can also be specified in the graph. For any platform, graph representations can be created at different levels of detail to identify the specific level of detail where differences among different platforms start occurring. The virtual design approach of Sanderson (99) encourages such an exploration of commonality at different levels of detail. At the top level, the graph will represent the connections among the principal subassemblies (Figure 5). At this level all unibody automotive platforms have the same architecture, in a more detailed level, the graph will represent 6

7 component/subassembly connections for a specific subassembly. The differences in the product architectures begin to emerge at these detailed levels. Two possible configurations of front structure at a more detailed level are shown in Figure 6. The connections in the platform architectures not only represent physical connections but also components which are used to connect other components. For example, in platform B front structure the engine is connected by using a subframe Representing assembly sequence In Section 3, we illustrated that automotive platform commonization can be divided into two segments - one addressing the issues related to components and the other related to the assembly process of the platform. In this section we will extend the assembly graphs to organize assembly related information which consists of a platform s assembly sequence and related plant information. In our approach, we superimpose the assembly sequence on top of the platform architecture to illustrate the assembly sequence for that level. This combined assembly sequence and platform architecture gives a more complete model of the platform to allow comparison of assembly sequences for different car model platforms to identify areas where commonization is required (product and assembly). In Figure 7.a and 7.b the sequence of component loading is superimposed on the architecture of A and B front structures respectively. Engine and suspension are not included. As can be seen from Figure 7.a, Apron Front Side LHS/RHS, Panel Body Front and Front Cross Dash are loaded first and welded. Then the Reinforcement Bracket is loaded and joined with the assembly followed by Floor Front Cross Lower and Dash Panel. For platform B, Apron Front Side LHS/RHS, Panel Body Front and Dash Panel are loaded and welded first. Then Floor Front Cross Lower followed by Front Cross Dash are loaded and welded at the second and third station respectively. The architectures are very similar but the assembly sequences are different. Floor Front Cross Lower 3 Panel Body Front Apron Front Side RHS Apron Front Side LHS Front Cross Dash Reinforcement Bracket 2 Dash Panel Measuring Commonality One of the primary questions that arises from designing a common platform is - how good is the common platform? One of the main issues that needs to be addressed before measuring the commonality of the platform is - What are the criteria for evaluation? One possible measurement could be % commonality/reusability of the common platform from different relevant viewpoints. In order to determine the % commonality one needs to know what the index means and how to define and validate the scale for this measurement. Some of the possible viewpoints are: Components Viewpoint: What % of the components of a platform are common to different models? This can also be thought of as how big the common core is compared to the whole underbody structure. Operations Viewpoint: What % of the assembly operations are common? In this viewpoint common refers to not only same operations but also the sequence. Component-Component Connections Viewpoint: What % of connections between the components are common? Locators Viewpoint: What % of the locators are common? Common locators can be defined as locators having same type and same relative positions with respect to the weld line and the locators on the front and rear of the underbody. The measurement of % commonality from different viewpoints can then be combined to determine an overall measurement of commonality for a platform Architecture Commonality One approach to determine commonality between platforms is by comparing the underbody architectures. When comparing underbody architectures of different models, the information can be categorized into () common components (2) unique (vehicle-specific) components (3) external modules. The common set of components makes up a common platform. To increase the commonality of the underbody structure, the number of common components needs to be increased. It is useful to look at the different levels of the platform architecture and identify where the differences occur, because this information might provide insight into how to increase the commonality of platforms. Floor Front Cross Lower 2 Panel Body Front Apron Front Side RHS Apron Front Side LHS Front Cross Dash 3 (a) (b) Figure 7. Component Loading Sequence For (a) Front Structure A (b) Front Structure B Dash Panel 7

8 Two different architectures of two different platforms at the same level have been illustrated in Figure 6.a and 6.b. We can now measure the commonality of the two platforms. One measure is percent commonality of components (C c ): 00 * common components C c = [] common components + unique components The percent commonality of connections (C n ) can also be measured in a similar manner by counting the core connections and the unique connections. 00 * common connections C n = [2] common connections + unique connections front structures at different levels. Components that are common to the core of the platforms are shown in bold. Components italicized represent unique underbody components. The shaded s represent connections that are common, which is accomplished by comparing the architecture of the two platforms (Figure 6.a and 6.b). The common components and connections can be easily identified from the spreadsheet and % commonality can be calculated automatically. The percent commonality of components (Cell C) is calculated by using Equation and the percent commonality of connections (Cell F) is calculated using Equation 2. The representation of platform architecture presented in Section 5. provides a means for visualization and identification of differences and commonality among platforms. To provide a better and easier approach to measure commonality we utilize a tabular scheme to represent the assembly graph (Table ). The first row and column list all the components and subassemblies for the underbody. The s in the table represent that the components/subassemblies are connected and the 0 s represent no connection. Table captures the same information as in the assembly graph and is used to automatically determine percent commonality for different The results from Table can be interpreted as follows. Overall, the front structures have similar sets of components and connections. We know from Figure 6 that the primary difference is in the method of attaching the powertrainsuspension package to the front structures. In our method of counting components, which is consistent with Figure 6, platform A has only 4 unique components, while platform B has 3. Hence, platform B has a slightly higher % commonality measure for components. Regarding the measure of connection commonality, the two front structures have the same types of components and the same components are welded together. Again, the differences Table. Example Spreadsheet used to Calculate Percent Commonality Between A and B with Engine and Suspension Front Structure 3rd Level for A Front Structure 3rd Level for B Right Apron Right Apron Right Rail Right Rail Right Bracket Radiator Support Radiator Support Engine Engine Sub-frame Suspension Suspension Dash/Cowl Dash/Cowl Left Bracket Left Rail Left Rail Left Apron Left Apron A Total No. of Core Common Components: 6 A Total No. of Core Common Components: 6 B Total No. of Unique components B Total No. of Unique components including External Components: 4 including External Components: 3 C Percent commonality of components: 60.00% C Percent commonality of components: 66.67% D Total No. of Common Connections: 4 D Total No. of Common Connections: 4 E Total No. of Unique Connections: 5 E Total No. of Unique Connections: 6 F Percent commonality of connections: % F Percent commonality of connections: % Common Connections - connected 0 - not connected 8

9 Table 3. Comparison Of Attributes Between A and B A B A Number of components with 3 3 common loading sequence B Number of components with 4 3 unique loading sequence C % commonality for loading 42.86% 50.00% sequence D Number of Common Stations 4 4 E Number of Unique Stations 6 0 F % commonality of work 40.00% 28.57% stations arise in the method of attaching the powertrain-suspension package to the front structures. The subframe for B has one additional connection to the front structure than platform A s brackets, giving platform B a slightly lower for B Attributes Commonality In Section 5.2., we illustrated how commonality related to components and connections can be calculated. Different commonality measures related to assembly attributes can also be calculated in a similar fashion. The percent commonality of the component loading sequence (C l ) for the front structure assemblies of platform A and platform B (Figure 7.a and 7.b) can be calculated in a similar fashion. 00* common assembly component loading C l = [3] common + unique There are 3 components that have common loading sequence. There are 4 components for platform A that are unique, while for platform B, 3 components have a unique loading sequence. Using Equation 3, the percent commonality of component loading sequences for platform A is 43% and for platform B 50%. Similarly, the measure of assembly work station commonality (C a ) can be given by Equation 4: 00* common assembly workstations C a = [4] common + unique Table 2 shows a comparison between platform A and platform B to calculate C l and C a. The table is constructed from Figure 7 and additional information about assembly operations and work stations. As can be seen there are 3 components that have the same loading sequences and 4 common assembly workstations. The percent commonality of the loading sequence (Row C) is calculated using Equation 3. Percent commonality of work stations (Row F) is calculated in a similar manner using Equation 4. Conclusions from these results can be summarized as follows. The assembly sequences for the two front structures are considerably different. This is somewhat surprising since their architectures appear to be similar (see Figure 7 and Table 2. Percent Commonality Summary for A and B Import ances A B % Component Commonality % Connection Commonality % Work Station Commonality Overall Commonality (weighted sum) Table ). Approximately one-half of the components in platform B and platform A share similar loading sequences, while the other half are unique to their respective platforms. A has a slightly lower percent commonality measure for loading sequence since it has an additional unique component. However, the significant differences arise in the layout of the assembly work stations because B requires 4 additional welding work stations. Another significant difference is highlighted by the number of unique work stations for both platform A and platform B: each requires 6 unique work stations, while sharing only Combining Commonality Measures The percent commonality measures discussed in the last two sections (5.2. and 5.2.2) can be combined into an overall platform commonality measure in several different manners. We will illustrate one method using a weightedsum formulation as shown in Equation 5. 3 Commonality = I i C i = I c C c + I n C n + I a C [5] a i = I i = importances (weighting factors) C i = % commonality measures from Equations, 2, 4. The importances must be normalized and can be used to model designer preferences. For example, a selection of I c = 0.2, I n = 0.3, and I a = 0.5 indicates that the designer believes that assembly is as important as the combination of the components and their connections. Overall Commonality measures can be presented in a related table, as shown in Table 2, which summarizes the results in Table and 2. The Commonality measures for platform B and platform A are 49 and 54 percent, respectively. 5.3 Measuring Interface Modularity A successful approach to developing independent modules is to standardize module interfaces. Modularity presented in Section 2 pertains to functionality. However, modularity is not limited to this. The viewpoint of assembly plays a key role in identifying platform modules. As an example, in the case of the platform-engine interface, subframes are typically used to isolate the required variability from the common platform as presented in the Standardization discussion in Section 4. Thus, only the 9

10 subframe needs to be changed (if needed) to accommodate various engines. The surface acts as an interface; its value is in separating different assembly modules. Two candidate measures of interface modularity will be introduced to assess the independence of modules. For the first measure, assume that ne different engines are required to be used in a given platform. Typically, there are four engine mounts: two on the subframe and two on each apron. Let s say that each mount must be different for each engine; i.e., the number of unique interface components is me = 4*ne. In addition, there may be other interface components, ce (common engine interface), such as brackets, that do not change across engine types. Since we are trying to isolate variability into the interface between modules, this is probably an acceptable situation since no changes to the aprons or other platform components are required. A candidate interface modularity measure is: Interface Modularity = ce/(me + ce) [6] where (me + ce) is the total number of components required to provide the interface between a platform and its powertrain-suspension module. One extreme would be for ce to be zero. In this case, there are no common interface components. The other extreme is for me to be zero which indicates that no unique components are needed for the interface, a very good situation in that variety is being provided without any impact on the interface. Another useful measure of interface modularity can be the average number of components in an interface. Average number of components = (me + ce)/ne [7] An average of indicates that there is one interface component for each engine. The objective is to reduce the average as low as possible. 6 CLOSURE Design theory and methodology research has focused increasing attention on the area of product variety design. In this paper, we investigated the extent to which this research applies to automotive platform design. Many automakers are undertaking significant efforts to commonize their platforms in an attempt to simplify and shorten car development projects. These efforts should greatly reduce the cost of bringing new car models to market. We summarized the main product variety concepts from the literature, then investigated automotive platforms with respect to these concepts. From the applicable concepts, we proposed a platform representation and commonality measures that seem to capture important characteristics of platform commonality and car model variety. We presented two candidate methods and tools for measuring platform commonality and demonstrated them on two similar platforms. Although preliminary, our work has attracted considerable interest in industry and has led to some tentative conclusions: Product variety concepts of standardization (component, interface, and assembly), modularity, robustness, and mutability are key concepts in platform commonization. The delayed differentiation concept does not appear to apply directly to platform commonization. The graph representation of platform architectures is a useful abstraction to illustrate the overall structure of a platform. The representation can be extended to include assembly information. % commonality measures provide an indication of the degree to which platform commonization has been accomplished. The combination of the above enables an understanding of the relationship between configuration decisions and potential assembly problems. This allows the identification of problem areas in the platform. As mentioned earlier this research is in its preliminary stage. Currently we are looking at: Developing a representation scheme that can be used to capture other aspects of the platform commonization problem. Developing definitions and computational foundations that can be used with the architecture representation scheme(s) for automotive common platform. The definitions are required to represent the relationships among the different components/modules of the platform and the specific attribute being modeled. Computational foundations are required to manipulate and synthesize platform architectures subject to design and commonality requirements. ACKNOWLEDGMENT We gratefully acknowledge the support of Ford Motor Company and the cooperation of several Ford employees in providing information. Also, we acknowledge the National Science Foundation through grant DMI REFERENCES Chen, W., Rosen, D., Allen, J. and Mistree, F. 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