Optimizing system design under degrading failure agents

Taguchi's quality loss function defines a quadratic relationship between deviations from a target values and loss to society. Under this approach, targets of system performance indicators, are set to minimize this loss. Traditionally, robust design engineers make the implicit assumption that failure agents affect systems' technical parameters stochastically, under steady state. Consequently, robust design strategy seeks to minimize societal loss by setting each technical parameter as close as possible to the lowest value on the loss function, usually the mid-point between the lower and upper specification limits. However, on closer examination, it can be demonstrated that many failure agents affect systems (e.g., electronic components, Mechanical elements, Software pieces) in a predictable, dynamic direction and rate. The authors denote such agents «Degrading Failure Agents». The paper describes an optimized design strategy accounting for degrading failure agents. This is done by setting the operating points of technical parameters to counteract the effects of such failure agents. The approach is demonstrated with a cardiac pacemaker case study that considers several types of degradation models including joint models and Weibull bathtub distributions.