Journal of Mechanical Science and Technology 23 (2009) 2658~2667 www.springerlink.com/content/1738-494x DOI 10.1007/s12206-009-0719-3 Journal of Mechanical Science and Technology Non-sample based parameters design for system performance reliability improvement † Young Kap Son 1,* , Jae-Jung Kim 2 and Seung-Jung Shin 3 1 School of mechanical engineering, Andong National University, 760-749, Korea 2 Reliability analysis research center, Hanyang University, 133-791, Korea 3 Division of Information Technology, Hansei University, 435-742, Korea (Manuscript Received October 17, 2008; Revised April 21, 2009; Accepted May 25, 2009) -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract Design methods for quality generally help to improve quality over time, but do not consider change of system per- formance over time, resulting from degradation in components. As design methods for quality over time (performance reliability), which minimizes effects of unavoidable component degradations as well as component variations on sys- tem performance change, system model-based sampling methods using Monte-Carlo simulations have been used. But, there are main concerns related to computational efficiency and optimization in applying the sampling methods. To overcome the concerns, we propose a non-sample method for quality over time. Based on the proposed method, the process of allocating design parameters, which could minimize the noise effects with the consequence that both quality and performance reliability are optimized, is discussed. Reliability metrics such as mean time to failure and standard deviation of time to failure are optimized simultaneously for reliability improvement. Desirability functions for the metrics are introduced to perform the simultaneous optimization. The proposed method is applied to an electrical sys- tem design and compared to a sampling based design method. Keywords: Component degradation; Design parameter allocation; Quality over time; System model; System perform- ance degradation -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction Degradation in components due to the effects of environmental conditions produces time-variant per- formance in component and system with the conse- quence that system performance varies from the ini- tial over time [1, 2]. What customers notice is a change in quality or performance characteristics over time [1]. Customer satisfaction is more quality-driven than value- or price-driven. And thus, customer satis- faction is a function of both quality and overall cus- tomer expectation of quality [3]. By quality we mean conformance to specifications [4, 5]. Quality over time is often referred to as performance reliability, wherein soft failure indicates that performance meas- ures of a system or component do not conform to its performance specifications. Experience points to the belief that a product of high quality when placed in service might be a reliable product (capable of releas- ing performance degradation over time). However, it is not enough to consider its performance change only due to the variation in component during the manu- facturing process. Design methods for high quality generally help to improve performance reliability, but change of system performance over time resulting from degradation in component is not considered in these methods. Therefore, design methods capable of diminishing unavoidable effects of component degra- dation on system performance have a potential to provide important performance reliability improve- †This paper was recommended for publication in revised form by Associate Editor Jooho Choi * Corresponding author. Tel.: +82 54 820 5907, Fax.: +82 54 820 5044 E-mail address: ykson@andong.ac.kr © KSME & Springer 2009