Investigation of the touch processing model in human grasping based on the stick ratio within a fingertip contact interface

Transition of a fingertip contact state during manipulative tasks of human is considered to have essence of skillful force control. In this paper, a scalar parameter representing the contact state of a fingertip concurrent with incipient slip is derived. Also, the grip force control hypothesis based on the stick ratio is proposed. Using the proposed hypothesis, proper grip force can be realized just maintaining a fingertip contact state as similar as possible regardless of the grasped object. The validness of the hypothesis is confirmed by simulations in which the stick ratio sensibility of human is investigated, and psychophysical experiments in which transition of the stick ratio and the grip/load force during lifting tasks is measured.