A Biomimetic Compliance Control of Robot Hand by Considering Structures of Human Finger

In this paper, for an object grasped by a robot hand to work in compliance control domain, we first analyze necessary condition for successful stiffness modulation in the operational space. Next, we propose a new compliance control method for robot hands which consist of two steps. R IFDS(Reso l ved In ter -Finger Decoupiing Solver) is to decompose the desired compliance characteristic specified in the operational space into the compliance characteristic in the fingertip space without inter-finger coupling, and RIJDS(Reso lved In ter -Jo in t Decoupling Solver) is to decompose the compliance characteristic in the fingertip space into the compliance characteristic in the joint space without inter-joint coupling. According to the analysis results, the finger structure should be biomimetic in the sense that either kinematic redundancy or force redundancy are required to implement the proposed compliance control scheme. Five-bar fingered robot hands are treated as illustrative examples to implement the proposed compliance control method. To show the effectiveness of the proposed compliance control method, simulations are performed for two-fingered and threefingered robot hands. 1 I n t r o d u c t i o n Related to grasping and manipulation of robot hands, a coordinated dynamic hybrid position/force control method considering the dynamics of manipulator and object has been proposed for a set of robot arms or a multi-fingered robot hand handling one constrained object [1]. Hasegawa et.al [2] presented a multi-sensors-based manipulation of an object by multi-fingered hand equipped with three-axis 0 -7803 -5886 -4 /00 /$ 10 .00© 2000 IEEE 388 Science, H a n y a n g Univ . , Seoul, K o r e a rch Center , KIST , Seoul, K o r e a Cente r , K I S T , Seoul, K o r e a .hanyang .ac .k r ) force sensor in the fingertip. But, their explicit forcebased fine motion control may practically not be easy because the real force signal is very noisy. It therefore has been pointed out that instead of explicit force signal the stiffness or compliance is an important quantity for characterizing the grasping and manipulation of robot hands in the case that it is specially dominated in approximated linear analysis where low velocities and small relative motions lead to small inertial forces. Many approaches have been reported in the field of grasp stiffness or compliance. The stiffness of objects grasped by "virtual springs" was analyzed in cases of planar and three-dimensional space in [3]. In [4]~ the effective grasp stiffness was analyzed by considering the structural compliances in the fingers and fingertips, servo gains at the joints of finger, and small changes in the grasp geometry that affect the way in which the grasp forces act upon the object. However, the geometric condition for successful implementation of compliance control scheme has not been addressed so far. Reflecting geometric: conditions for successful compliance control, Yi, et.al.[5] proposed that an independent joint-based compliance control via redundant actuation is much adequate approach to modulate the operational stiffness comparing to the case of the kinematically redundant structured fingers or manipulators. In this paper, we first analyze the geometric conditions for successful implementation of the stiffness control in the object operational space. Our proposed compliance control method consists of two steps; R I F D S and RIJDS. To show the effectiveness of the proposed compliance control method, simulations are performed for two-fingered and three-fingered robot hands.