Online computation of grasping force in multi-fingered hands

This paper presents a new solution for solving the grasping force optimization problem, fundamental in dexterous manipulation by multifingered robotic hands. Several methods have been proposed in the literature, yielding optimal solutions, with either recursive or non linear programming techniques. However, most of them involve many computations and cannot be used online. Furthermore, they do not offer a smooth solution regarding to possible changes in the contact conditions due to finger rolling or gaiting, or in the desired resultant force to be exerted on the grasped object. The more recent ones are fast and smooth enough for real-time computation but the method we present here is faster, easier to implement and provide very satisfying results, even though the solution is sub-optimal. The method is based on the minimization of a cost function that gives an analytical solution but does not ensure by itself the satisfaction of the static frictional constraints. An associated iterative adjustment modifies this function until the internal forces enter the friction cone. The minimal solution is found within a few iterations. Force determination is therefore included in the simulation of a hybrid position/force controller to prove the effectiveness of such an approach for updating the force references during the grasped object motion.