Design-for-six-sigma for multiple response systems

An important upstream activity in the overall design of a system is the allocation of the means and tolerances. This is a daunting task because of the need to satisfy multiple competing demands that arise from performance, cost and quality policies. Herein, the so-called design-for-six-sigma is adopted for the allocation process whereby the philosophy of zero defects in Six Sigma is now applied to the system performances. Probability-constrained optimisation is invoked. Robustness and cost measures are required. The production cost provides the objective function to be minimised and a maximum allowable system probability of non-conformance (e.g. a system defect rate) provides the primary design constraint. The design of an electro-mechanical servo system with three responses, three control variables and two noise variables is given. Means and tolerances for the three control variables plus system costs for progressively lower product defect rates show the practicality and potential of the approach.