Human Hand as a Parallel Manipulator

Abstract

When a human hand grasps an object the hand can be viewed as a parallel manipulator. In general, the mathematical analyses of the human hands and multi-fingered robot hands (Murray et al. 1994) are similar. In particular, concepts developed in robotics such as contact models, e.g. soft-finger model, grasp matrix, form and force closure grasps, internal forces, etc. can be applied to analyze the performance of the human hands. Multi-finger prehension is an example of a mechanically redundant task: the same resultant forces on the object can be exerted by different digit forces. People however do not use all the mechanically available options; when different people perform a certain manipulation task they use a limited subset of solutions. Studies on human prehension deal with four main issues: 1. Description of the behavior: What are the regularities in force patterns applied at the fingertip-object interfaces when people manipulate objects? 2. Are the observed patterns dictated by the task and hand mechanics? The mechanical properties of the hand and fingers are complex, and it is not always evident whether the findings are direct consequences of the mechanical properties of the hand or they are produced by a neural control process. 3. If the observed facts/phenomena are not of mechanical origin are they mechanically necessitated? In other words, can the task be performed successfully in another way? 4. If reproducible phenomena are not mechanical and not mechanically-necessitated, the question arises why the central nervous system (CNS) facilitates these particular phenomena. This is a central question of the problem of motor redundancy in general: Why does the CNS prefer a certain solution over other existing solutions? The present chapter briefly reviews some specific features of the human hand and the involved control mechanisms. To date, the experimental data are mainly obtained for the socalled prismatic grasps in which the thumb opposes the fingers and the contact surfaces are parallel (Figure 1). The contact forces and moments are typically recorded with 6component force and moment sensors. Experimental ‘inverted-T’ handle/beam apparatus commonly used to study the prismatic precision grip. Five six-component force sensors (black rectangles) are used to register individual digit forces. During testing, the suspended load could vary among the trials. The load displacement along the horizontal bar created torques from 0 N⋅m to 1.5 N⋅m in both directions. The torques are in the plane of the grasp. While forces in all three directions were recorded the forces in Z direction were very small and, if not mentioned otherwise, were