This paper formulates the dynamics of 5 degrees-of-freedom (DOFs) robotic hand that consists of an index finger and an opposable thumb like the human hand. This system has two geometrical constraints and five velocity constraints generated during the object manipulation task. The three of the velocity constraints are associated with the joint angles of the robot, and are combined to the proposed system on the basis of observations about the fact that the distal two joints of the index finger are mechanically coupled. Based on the equations of motion including these constraints, we perform precise orientation control of a rectangular object grasped by the 5-DOFs robotic hand in the situation that grasping forces are not defined during the manipulation. The manipulation task given is one-finger control (index finger), for which a serial two-phased (STP) controller that had previously been proposed is modified. In experiments, we demonstrate that a simple integral controller designed by eliminating the second stage of the STP controller works well in the soft-fingered manipulation. In both simulations and experiments, it is clearly indicated that Jacobian matrices and the grasping forces are not necessarily required in the control law for accomplishing dexterous manipulation tasks. In other words, it is implied that a complete modelless and grasping-forceless control can be achieved in the task-space control.
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