Design and tests of a three finger hand with 1-DOF articulated fingers

This paper presents the design of a new low-cost, easy operation hand having three 1 dof anthropomorphic fingers with human size. Experimental results on human cylindrical grasping have been used for designing a 1 dof anthropomorphic finger having human size. The mechanical design of this finger has been mainly focused on a suitable articulated mechanism. This mechanism transmits the power to the phalanxes for a human-like grasping motion and it can be embedded in the body of the finger itself. The design of a three finger hand has been presented as based on the designed finger module. Experimental tests on a built hand prototype have been carried out in order to show the feasibility of the proposed design and validate its operation. INTRODUCTION Several researchers have investigated different types of devices for achieving the grasping and handling of objects. Mechanical grippers having two fingers are widely used in industrial application and mostly in industrial robots, [1]. However, multi-fingered robotic devices and hands have been also widely investigated, as reported for example in [2-15]. This type of devices tries to mimic the performances of human hands in order to have high flexibility multi-purpouse devices. Significant examples can be identified in the Stanford/JPL hand, [5], the TUAT/Karlsruhe Humanoid Hand, [6], the DLR's Hand II, [7], the BUAA/Bejing University four-fingered hand, [8], Manus Colobi, [9], the TBM hand, [10], the Barrett Hand, [11] and the hands proposed in [12], [13] and [14]. Most of the available multi-fingered prototypes have a high number of degrees of freedom, a complex control and a high cost. The proposed hand is low-cost and easy-operation that means the new prototype has been built with commercial components and it can be controlled easily. Moreover, the new drive-finger mechanism for a 1 DOF anthropomorphic finger can be imbedded in the finger body and it remains in the finger body during its movement. Since a recent past, design and research activities have been carried out at LARM, Laboratory of Robotics and Mechatronics, in Cassino in order to design a low-cost, easy operation anthropomorphic finger having 1 dof, as outlined in [16-21]. A preliminary prototype that has been built at LARM is shown in Fig. 1. The size of this prototype is about 1.5 times bigger then the average human finger. This paper reports the design and test of a new robotic hand with three anthropomorphic fingers of human size. In the previous papers a different hand of bigger size has been investigated. Experiments with the three fingered hand prototype has been presented in this paper. Special care has been addressed to the design of a suitable transmission system embedded in the body of the finger by using articulated mechanisms. A three finger hand has been also designed and built at LARM by using the designed finger module. Experimental tests have been carried out on human grasping and on the built hand. The comparison of the experimental results can validate the design and operation of the built hand. EXPERIMENTAL TESTS OF HUMAN GRASP For the design of an anthropomorphic hand a suitable knowledge of the human grasp can be very useful. In particular, dimensions of fingers, grasping forces and contact points between fingers and objects have been investigated in human grasping for design purposes of a new low-cost easy operated hand. As first step, several videos of the human hand grasping have been processed. Kinematic models of the fingers have been derived from the analysis of the human grasping as proposed in Fig. 2. The dimensions of each phalanx of index, medium and thumb have been measured for five persons. The measured values and average values have been obtained as reported in Table 1 referring to the kinematic models of Fig. 2. It is worth noting that it has been decided to measure only index, medium and thumb since they are the most used in human grasping. By analyzing several videos of the human cylindrical grasping of a given object it has been observed that there are almost constant ratios between the motions of each phalanx in the approaching motion to the object, as Visiting student at LARM during Academic Year 2002-03 from University of Los Andes in Merida, Venezuela.