Force Decomposition in Robot Force Control

It has been recently pointed out that the lack of unit invariance of the usual Euclidean norm on the twist and wrench spaces would render many of the results in robot force control invalid. This observation actually begs the more fundamental question of what force should be controlled. In this paper, we investigate this problem in the context of multiple-arm manipulation. Motivated by a simple one-dimensional example, we show that a reasonable choice for the force to be controlled is the internal force at a specified frame in the payload. The force control problem and the associated stability issues are then discussed.