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Humans can perceive not only the magnitude but also the direction of force applied on the fingertip. When we grasp an object, the weight of it is felt through force that is parallel to the skin of the fingertip, which is how the object can be grasped without slipping. Focusing on this point, we have developed a tactile sensor that can measure a distribution of force vectors. The measurement method is as follows. The tactile sensor consists of a transparent elastic body, blue and red markers inside the elastic body, and a color CCD camera. An applied force on the elastic body results in movements of the markers, which are acquired by the CCD camera. The distribution of force vectors is calculated using this information. This paper reports experimental evaluation results concerning accuracy of determining position of markers, determination of magnitude and direction of force, spatial resolution, and calculation timing.
Kamiyama et al. (2004) studied this question.