A method for systems design using precedence relationships: an application to automotive brake systems

A tool called the Design Structure Matrix, developed by Steward [1981], is applied to the design of an automotive brake system. The methodology is used to order and organize the design activities for the brake system design process on the basis of the information flows involved. Because it discriminates between decisions which may be made independently and decisions which constitute a "simultaneous subsystem", the methodology is useful for both engineering and organizational aspects of design. Once simultaneous subsystems have been identified, a system can be decoupled by "tearing" or guessing a sufficient number of parameters to iterate through the design once. Multiple iterations may still be needed for final solution. In the case examined, 103 parameters were ordered in a design structure matrix and a simultaneous subsystem of 34 parameters was found to exist. This system could be solved by tearing 12 of the 34 parameters. Several methods for solving simultaneous subsystems defined by the design structure matrix are presented as alternatives to the tearing procedure. These include . axiomatic design, designed experiments, fuzzy set theory, shunt diagrams (developed by Steward [1981]), and other weighted function methodologies using a priori knowledge and input/output relationships. September, 1990 -~~I _ ~ __ _1_1111____~ 2 1.0 Introduction/Motivation This paper addresses the task of designing complex mechanical systems. The difficulties in designing such systems do not arise simply from their engineering complexity, but also from the organizational complexity necessary to manage the system design process. A methodology for the design of complex mechanical systems is applied which addresses both the engineering and the organizational factors to shorten design cycle time and improve the design itself. The implications of this methodology are analyzed and improvements for systems design are proposed. 1.1 Brake System As An Example This research was conducted at an automotive brake system component supplier, and that setting is used to explore the complex relationships and design dependencies in systems design, and discuss the systems design methodology. The automotive brake system affects vehicle motion by controlling the resistance to wheel rotation. Also, it provides the interface between the wheel and suspension; and between the steering and final drive. A typical brake system has front disk brakes and rear drum brakes. The brake system contains about 28 principal components [see Black, 1990]. Among these are the pedal, master cylinder, control valves, disc brake assembly, drum brake assembly, and parking brake. A power booster (or just booster) is often added to increase the power input to the master cylinder. Within the brake assembly exist such components as the caliper, rotor, pads & lining, and steering knuckle for disk brakes; and the wheel cylinder, shoe & lining, drum, and drum backing plate for drum brakes. 3 See Appendix A for illustrations of various brake system components. 1.2 Problem Setting/Description Engineering Issues This work focuses on interdependencies among the individual components in the brake system. Design of systems is primarily a process of discovering, analyzing, and resolving tradeoffs, and very often involves the simultaneous determination of several variables. Brake design involves such a process of tradeoffs among design parameters. As an example, consider the design of a brake system for higher torque which can be accomplished by using a larger diameter rotor, a smaller master cylinder, larger wheel cylinders, a larger booster, or a higher pedal mechanical advantage. Efficient and effective brake system development requires that designers be informed of parameter changes and their ramifications throughout the system. At the brake component supplier we worked with, very little systems thinking is undertaken by the individual component designers. It is commonly (but usually incorrectly) accepted that the best individual component will make the best brake system. In fact, the company uses a process similar to the "envelope approach" referred to by Robertson [1990]: designers at all levels wait for the proper envelope space constraints to be handed down from a previous level of design with little consideration and understanding of "the big picture". In comparison, a systems approach structures the process so that component interactions can be understood easily. Such a systems approach analyzes interfaces and 4 specifications in terms of the effects on an entire system. As will be discussed, the design structure matrix (DSM) shows when the envelope approach is sufficient and when a systems approach would be more useful. The DSM helps to make this distinction through the identification of "blocks" (interdependent design decisions). Organizational Issues in Design Organizational management challenges arise from the complexity of managing and coordinating the systems design task. A complex system typically requires a significant number of designers. At the brake supplier we studied, design activities are difficult to coordinate due to geographical dispersion of the forty designers (among four separate locations), and the component complexities discussed above. -For any particular brake system, responsibilities for the components typically reside in and shift among different locations over the design life cycle. As evidence of the coordination difficulties, we found that several designers with prime responsibilities in different areas (and located in separate plants) would claim responsibility concerning a particular brake system parameter. In addition, they sometimes disagreed over the information necessary to determine that particular parameter. This occurred with about 20% of the original 103 brake system parameters we identified. Clark et al (1987) found evidence that product development projects in the auto industry have shorter overall lead times and require fewer total engineering resources when organized with a "heavyweight" project manager, as opposed to a functional 5 organization. As our research project commenced, brake system development at this company was changing from a functional structure to a heavyweight subsystem manager structure. That is, a "heavyweight" manager is now responsible for all brake system development, but each development project does not have its own project manager. Rather, the subsystem manager serves as the project manager for each individual project. The most widely used tool (and conceptual model) available to such project managers is that of PERT/CPM (Project Evaluation Review Technique / Critical Path Method). (See, for example, Wiest and Levy (1969). ) However, for the brake system design challenges at the company we worked with, this tool has two shortcomings. The first of these, addressed by the methodology of this paper, is that many individual activities in product development projects are temporally interdependent so that, for example, in a group of activities, each contains the others as precedent activities. That is, rather than activity A strictly preceding or following activity B, for example, A and B are interrelated activities that must be performed simultaneously (typically by iteration). PERT/CPM models do not allow for such relationships. The second shortcoming of PERT/CPM for use by the brake system product development manager is that it cannot analyze the capacity and scheduling problems that arise when more than one brake system are simultaneously in the processing network. In particular, if several brake systems are simultaneously being designed, then they may compete for and queue up for certain engineering resources. For such a case, activity time estimates must include queueing time, which is typically nontrivial to calculate. Although such analysis is beyond the scope of this paper, Harrison (1990) has pointed 6 out that the model of Nguyen (1990) might be used to help analyze this problem. See also Varma (1990). This rest of this paper proceeds as follows. Section 2 discusses related literature, section 3 describes our methodology, section 4 presents our analysis and results, section 5 contains a discussion of some additional issues, section 6 discusses a few ideas for further research, and section 7 contains concluding discussion. 2.0 Related Literature Much literature exists on approaches to design for manufacturing. Nevins and Whitney et al [1988] discuss methodologies for concurrent product and process design. They also give a broad framework for systems design and apply various rules to improve the design's ability to be both manufactured and assembled. Sub [1990] postulates two axioms which underlie all design processes. The first axiom relates to the independence of functional requirements in an optimal design. The second axiom depends on axiom 1 and allows a designer to choose a design with minimum information content between functionally uncoupled designs. Suh uses the axioms to develop a design framework. With this framework, designers can systematically create more efficient designs. Sub [1990] and Stoll [1986] develop some rules for the design process which take manufacturing considerations into account. These rules may be adapted or included in the design structure matrix described below (for fuller treatment, see Black [1990]). Clausing and Hauser [1988] survey and consolidate a large volume of literature to