Richer Textural Information in the Horizontal Component of the Contact Force Compared with the Normal Component

Most surface texture displays provide either vibrotactile or variable-friction stimuli to users through finger pads, and they control the changes in the contact force either along the normal or tangential direction. This study aimed to answer a fundamental question: that is, which of the two force components contains the most textural information. We measured the contact force when a fingertip slid over 21 types of materials. The force was decomposed into tangential and normal components to compute individual power spectra. Two classification machines, Fisher's discriminant analysis and a support vector machine, were used to categorize the materials based on the power spectra. They confirmed that the tangential component of the contact force was more helpful in distinguishing the 21 materials, with an average success rate of 29%. In contrast, this proportion was as low as 21% when the normal force was used to train and test the machines. Furthermore, this proportion did not improve substantially, even when the two components were used together. Although these classification machines employ principles different from those of humans, these results suggest that humans may rely more on the tangential component of the contact force than on the normal component to judge textures. These results motivate us to compare the two force components in terms of different aspects in the future.