Evaluating the Effects of Variation on Product Appearance through Visual Robustness

Doctoral thesis, 2011

For consumer products where the product appearance plays an important role in meeting expectations, the visible effects of manufacturing variation can have a negative influence on the product experience. A concomitant of mass production is that no manufactured units are exactly identical, as a result of variation. However, reducing variation is often associated with increased manufacturing and development costs. The main question addressed in this thesis is therefore how to evaluate the effects of variation on product appearance and perceived quality. The product property controlling these effects is referred to as visual robustness. Visual robustness is the ability of a product to stimulate the same visual experience as the nominal product, despite small variations. It depends only on the product appearance in terms of form and colours of visible parts. A key suggestion is therefore that product appearance should be made robust to deviations, thereby dampening the visual consequences of variation. Particular focus is set on visual robustness to geometrical manufacturing variation, visible in the split-lines between parts. However, a product can be visually robust to different types of deviations, such as colour deviations occurring during manufacture, or to the effects of ageing or wear. As visual robustness controls the effects of deviations on the perceived product quality, special attention is given to this relationship. Different types of aspects that influence the perceived product quality have been identified and categorized. The main result of this thesis is a framework describing visual robustness and its relation to the perceived product quality. To explain what makes products visually robust to geometrical variation, the concepts of visual references and amplifying factors are introduced. Two user studies are performed, exemplifying phenomena related to visual robustness, and two prescriptive methods for comparing the visual robustness of early design concepts are presented. The Visual Sensitivity Analysis aims to quantify visual robustness by looking at the amount of visual references between parts in an assembly. The Appearance FMEA is a procedure for enabling detection of different kinds of appearance imperfections, and promotes considerations of visual robustness in early design stages. A number of studies have been also performed in order to describe how aspects related to visual robustness are handled in Swedish industry today.