Abstract
In cooperative motion of multi-arm robotic mechanisms or in the case of the grasping by robot hands, internal forces which balance inside the mechanisms are generated. The internal forces have some effect on the characteristics of the mechanisms, while they produce no resultant forces which affect the environment. This paper deals with the effect of such forces on structural stiffness of the mechanisms. Firstly it is made clear that the change of the stiffness is caused by the change of lines of the internal forces due to the deformation of the mechanisms. As an example, the structual stiffness of the spring supported system was analyzed. It was pointed out that, when the internal force is in compressive form the mechanisms may have negative stiffness leading to the unstability in grasping operation. It was also cleared for four-bar mechanisms that the stiffness of the mechanisms change due to the internal forces, though the internal force problems have never discussed for such fundamental mechanisms. It was shown that the effect of the internal forces changes according to the configuration of the mechanism and it is preferable to grasp objects with the configuration of the fingers closed to the rectangular form.
