Shape optimization of grasped object for improving ease of grasping

Improving ease of grasping is one of the significant factors for enhancing the value of industrial products because most of products are handled by human hands. In this study, shape of grasped object was optimized to improve its grasping ease with considering the interaction of multiple shape parameters and the individual difference. The shape parameters used in this study were the upper diameter dU, lower diameter dL, ellipticity e, and inclined angle of central axis θ. First, two levels factorial experiment was performed with five male subjects. They were asked to grasp the grasped objects for 10s, and the subjective scores of grasping ease were measured. Multi-way ANOVA was conducted for the measured subjective scores, and the result showed that the main effects of the upper diameter, ellipticity, and inclined angle are statistically significant, and the significance probability of the lower diameter was relatively low (p = 0.085). In addition, the interaction of upper diameter and inclined angle and that of ellipticity and inclined angle had significant effects. The interaction effects of upper diameter and ellipticity was almost statistically significant (p = 0.05). The four parameters were assigned to the L27 orthogonal array so that the three combinations (i.e. dU×e, dU×θ, e×θ) would be considered, and the subjective scores were measured for the twenty-seven conditions. In the second experiment, six male and six female subjects participated. The cluster analysis was performed for their subjective scores so as to evaluate the individual difference. The result of cluster analysis showed that the subjects formed mainly two clusters according to their feeling on the grasping ease, but the clusters were not related to the anthropometric dimensions of the subjects. The response surface of the grasping ease for all subjects and two clusters were approximated, and the optimum shapes were determined.