Design optimization issues in off-line quality engineering systems

Taguchi's off-line quality control is a cost-effective set of design methodologies to improve product quality at the early stages of product or process design. Although researchers and practitioners have proposed a multitude of design methodologies pertinent to individual design modules, such as system design, parameter design, and tolerance design, there is still much room for improvement. Taking a closer look at each design module, this research proposes design optimization models based on a variety of economic considerations by utilizing hands-on optimization techniques, probabilistic modeling, statistical estimation, and response surface methodology. Determination of product specifications and process characteristics, such as mean and variance, is first scrutinized for economic system design. Important design issues are then evoked to further enhance the current practice of parameter design. Moreover, the reasoning behind parameter design is extended to the case of multiple quality characteristics. Tolerance design has received relatively little support mainly due to the heuristic nature of investigation. In this regard, more systematic approaches to tolerance design are also proposed to further extend its application areas. A defining aspect of this research is an extensive investigation and inclusive considerations on various economic aspects of individual design modules by establishing several practical and meaningful results central to off-line quality control. To step forward, individual design modules are investigated and analyzed within an integrated framework to enhance the implementation of the off-line quality control concept. It is recommended that three design modules be conducted in a sequential manner in spite of the interdependence among individual design modules, which in turn calls for an iterative optimization procedure. Design optimization schemes within an integrated analysis framework are proposed under different circumstances and their cost-effectiveness is also discussed. Finally, design optimization issues in inspection procedures are also investigated based on economic considerations as a means for further improving product quality. Inspection procedures have become an integral part for improving product quality in the sense that the decisions regarding the conformance to specifications are usually made on the basis of the realization of inspection procedures. Industrial case studies and numerical examples are presented to demonstrate the applicability of individual design issues and their magnitude of potential cost savings. The significance of this research is to provide an applicable set of alternatives to traditional design methodologies in the context of off-line quality control and inspection procedures so that design engineers may benefit from this research when establishing practical design guidelines.