The first step makes sure that the right design requirements are derived from the customer demands. The properties the product must have to satisfy present and future market demands are defined by analysis of competition and customer requirements.

In the second step, functions that fulfill the demands and their corresponding technical solutions are identified. There might be several technical solutions to fulfill a specific function, but only the most appropriate technical solutions with regard to customer needs and other company-relevant criteria are chosen.

In the third step, the core in the MFD method, the technical solutions are analyzed regarding their reasons for being modules. The results of the first two steps of the MFD method are essential in supporting the decisions made when using the Module Drivers to evaluate the technical solutions. Module concepts are then generated and the interface relations of the modules derived are evaluated in step four. In addition, economic forecasts are made and the expected effects of the modularization are calculated.

In the final step, a specification is established for each module. The specification contains technical information about the module as well as cost targets, planned development, description of variants, etc. From here on, the modular concept can be improved by focusing separately on each module. Depending on the module’s characteristic, tools such as design for manufacture (DFM) and design for assembly (DFA) may be successfully applied.

The presentation of the MFD method follows an ideal working manner from step one to five. However, design work very seldom starts from the first specified step in a method, continues through every single step, in the right order, and ends with the final step. Starting points vary and several iterations might be needed before a satisfying result is reached.

STEP 1: Define customer requirements The first step in any method for product design has to ensure that the appropriate design requirements are derived from the customer/market needs. This implies a thorough understanding of the market situation and customer identity. Before anything else, the product strategy, including brand image, must be defined. Some of the important questions demanding answers are: What is our product vision of the future? What is the profile/image of this product on the market? Who are the most important customers? Who are the most important competitors?

The customer requirements must then be defined so that a specification of the product to be designed can be formulated. A simplified version of the well-known method, quality function deployment (QFD), has turned out to be well adapted for this task. Much has been written about the QFD method and is available in the literature.

Considering the objectives here, the usual QFD matrix is modified by putting “modularity” directly in as the first “how” (design requirement) as shown in Figure 2. This is preferable to establish the right “mindset” of the project team members.

When the most important customer requirements have been identified, they are translated into product properties for the design engineers. The relations between customer requirements and product properties are visualized in the QFD matrix (Figure 2). Measurable target values for the product properties are then set up to guide the design engineers’ work.

STEP 2: Select technical solutions The design requirements derived from the first step will have a strong market focus. To proceed with the product design, a more technical view is needed. Looking at the product from a functional point of view does this. Functions and subfunctions that fulfill the requirements from step one are identified and the corresponding technical solutions, or function carriers, are selected. This breaking down of the product into functions and their corresponding technical solutions is normally referred to as a functional decomposition. Also, by going through the functions for all the parts contained in the product, a mutual understanding of how every part contributes to the whole is achieved within the design team.

A prerequisite to achieving an optimal modular design is functional independence. Functional independence makes it possible to achieve robust modular design where interactions between modules are minimal. The stand-alone modules can then be treated independently from each other. During the functional decomposition, several technical solutions for a certain function may be found and a choice must be made. Experiences have shown that a matrix is a good way to structure and represent alternative solutions. A Pugh matrix is not a mathematical matrix, but a way to express and clarify the advantages and disadvantages of different options, as shown in Figure 3. Evaluation criteria can be collected from step one of the MFD method, together with internal company considerations, such as production goals, part number count, and future development potential.

Step two will result in a functions-and-means tree for the product that visualizes the product’s functional structure and selected technical solutions from which the product platform should be built. The top levels of a schematic function-and-means tree are shown in Figure 4.

STEP 3: Generate module concept In the third step, the technical solutions selected in step two are analyzed regarding their reasons for forming modules. The criteria used are the Module DriversTM described in the article, “What drives product modularity?”

In the Module Indication MatrixTM (MIM), each technical solution is assessed against the Module Drivers. The MIM, shown in Figure 5, constitutes the core of the MFD method. Every technical solution is weighted on a scale where nine points correspond to a strong driver, three points a medium driver, and one point a weak driver, according to the importance of its respective reason for being a module. The irregular scaling is used to support the identification of the really strong driving forces. Many and/or unique module drivers, highly weighted, indicate that the technical solution in question has a complicated requirements pattern and is likely to form a module by itself, or at least, the basis for a module. A unique module driver pattern also indicates that the technical solution should be kept single as long as possible.

Few and/or low weighted module drivers, on the other hand, indicate that the technical solution in question might be easy to encapsulate or group together with other technical solutions. Integration should be executed provided there is a match in the module driver pattern, or at least that there are no contradictions. For example, a carryover should not be grouped together with planned product changes because it would disable the possibility to stepwise develop at low cost.

There is an ideal number of modules to look for, in which there is balance between the time required for the assembly of modules and the time required for assembling the finished modules to each other in the main flow. The value is calculated based on the assumption that each module is concurrently assembled with the others and delivered to the main assembly line where complete modules are assembled to each other. Experiences show that an average “best practice” assembly operation time for parts is about 10 seconds and an average final assembly operation between modules varies between 10 and 50 seconds. A rough estimation is therefore that minimum lead time is achieved when the number of modules equals the square root of the number of assembly operations in the average product, as shown in Figure 6. When the average final assembly time is longer than the average assembly time for parts, the ideal number of modules will decrease.

A suitable number of module candidates, technical solutions with the highest Module Driver scores, are picked out. Then, through pattern recognition in the MIM, lower weighted technical solutions are evaluated as to the possibility of integration with the module candidates. At this stage, a creativity phase can take place in which a number of different module concepts are proposed. The concepts should contain some rough dimensioning and form. One, or a few, of the concepts are then further worked on.

STEP 4: Evaluate module concept Once a modular concept has been generated, questions arise. What are the effects on production and product development? How much better is the new modular concept compared to the existing design?

For a modular design, the interfaces between modules have a vital influence on the final product and the flexibility within the assortment. So, an examination of the interface relations will be an important part of the evaluation work. An interface between two modules might be, for example, fixed, moving, or media transmitting. Fixed interfaces only connect modules and transmit forces. Moving interfaces may transmit energy in forms of rotating or alternating forces. Media can be fluids or electricity.

A good overview of the interface relations can be achieved with an interface matrix. The modules are entered in expected assembly order and their interrelations are marked with G for geometry, E for energy transmitting, and so forth. From an assembly point of view, two ideal interface principles can be identified: base unit assembly and “hamburger” assembly. These are marked with arrows in Figure 7. It is clear that the two ideal assembly principles are beneficial from many other standpoints besides assembly. They facilitate simultaneous development, provide easier process planning, and allow greater freedom in workshop organization, among other advantages. The matrix serves as a pointer to interfaces that have to be given special attention and, eventually, improved. All markings located outside the arrows indicating the preferred assembly principles should be subject to further consideration.

In addition to the interface evaluation, several economic factors must be considered. Normal economic accounting, including activity-based costing (ABC) analysis, does not value all of the advantages and effects that a modular product assortment produces. Also, no existing design for manufacture and assembly (DFMA) method evaluates the advantages on the assortment level. So there is clearly a need for a new evaluation tool.

A rough estimation can be carried out. For some of the effects, a metric or a rule can be defined and used to evaluate the concept (see Table 1). The relative importance of the required effects has to be assessed and fixed at the beginning in every case.

During a development process, many choices need to be made and important strategic actions are taken. An evaluation, such as the one shown in Table 1, will document such action and serve as feedback to earlier phases in the process.

STEP 5: Optimize modules In the final step, a specification is written for each module containing technical information, cost targets, planned development, description of variants, etc. The module specifications constitute the backbone of the product platform. From here on, the modular concept can be improved by focusing on each module separately.

The MFD method should not be considered as a replacement for, or competitive with, design improvements on the component level. It is important to emphasize the necessity of such work within every single module to secure the final result. The MIM now serves as a pointer for what is important for each individual module. For instance, a module that is chosen mainly for service and maintenance reasons should be designed for ease of disassembly.

As discussed earlier, product design improvements may take place at different levels: the product range level, product level, and component level. The work on the product and component level has traditionally been extensive and only one aspect will be treated here, namely the number of different parts used to build a product or product range.

The number of different components in a product has been identified to be an important driver of costs in a company. These costs are, however, difficult to capture. Material, labor, and tooling are the most visible costs, but they only represent a part of the total true cost for the company. Many other costs are driven by the part number count and do not show up in traditional calculation systems since they are overhead costs.

In the same way as the number of parts drives costs in a single product, the part numbers drive costs within the entire product assortment. Thus, when using the DFA technique on a single product variant, the entire assortment of products must be taken into account. With a modular product design where the DFMA technique can be applied per module, design improvements on the component level can be carried out with optimal results.

This article was taken from the book, “Controlling Design Variants: Modular Product Platforms,” by Anna Ericsson and Gunnar Erixon. It was published by the Society of Manufacturing Engineers.