Development of UB Hand 3: Early Results F. Lotti, P. Tiezzi and G. Vassura DIEM, Dep. of Mechanical Engineering University of Bologna Via Risorgimento 2, 40136 Bologna, Italy {fabrizio.lotti, paolo.tiezzi, gabriele.vassura}@mail.ing.unibo.it L. Biagiotti, G. Palli and C. Melchiorri DEIS, Dep. of Electronics, Computer Science and Systems University of Bologna Via Risorgimento 2, 40136 Bologna, Italy {lbiagiotti, gpalli, cmelchiorri}@deis.unibo.it Abstract— The first part of this paper describes the devel- opment of a humanoid robot hand based on an endoskeleton made of rigid links connected with elastic hinges, actuated by sheath routed tendons and covered by continuous compliant pulps. The project is called UB Hand 3 (University of Bologna Hand, 3rd version) and aims to reduce the mechanical complexity of robotic end effectors yet maintaining full anthropomorphic aspect and a good level of dexterity. In the second part this paper focuses on the early experiences of the UB Hand 3 in performing manipulation tasks. Index Terms— Humanoid Robot, Hand Design, Soft Finger, Elastic Hinges, Dexterous Manipulation. I. I NTRODUCTION In dexterous robotic hands the reproduction of human hand compliance during the interaction with the objects to be manipulated (as demonstrated in [1], [?], [2]) is fun- damental for grasp adaptability and stability. Furthermore, a continuous soft cover increases the level of protection against external agents and unexpected impacts, thus in- creasing the reliability of the robotic hand. Even if these advantages are widely recognized, few humanoid robotic hands, so far, have been designed specifically to optimize this aspect. In most cases they are covered with thin layers of elastomeric material, capable to provide high surface friction but not enough thick to actually work as real compliant pads. The usual adoption of mechanical finger structures based prevalently on the exoskeletal pattern, which means rigid hollow frames with transmissions or actuation inside [3], [4], makes very difficult to place thick layers of compliant material around the finger structure without increasing the size of the fingers beyond acceptable limits. The UB Hand 3 project addresses alternative design solu- tions in order to substitute the exoskeletal structure with an endo-skeletal articulated frame, aiming to reach the desired external compliance and to simplify the overall mechanical complexity of the hand. The general goal of the project is to test non-conventional design solutions, understanding what may be their advantages and their limits by means of theoretical investigation, practical implementation and testing as well. A strong issue is to test not only the validity of such solutions in terms of theoretical behavior, but also to point out and to evaluate technological aspects related to their application and their compatibility with general specifications, like the adoption of proper sensory equipment or the application of specific control strategies. One of the key choices of the UB Hand 3 project is to investigate advantages and limits of articulated structures obtained with serial compliant mechanisms: a series of different finger architectures have been built and evaluated based on this concept and described in previous papers [5], [6], [7]. The present paper illustrates the hand architecture that is the result of the evolution performed so far: it will probably be improved in the future, but it seems now a valid base to start on-field evaluation of the proposed concepts. Some preliminary results of the hand operating capability are presented in the final part of the paper. They have been obtained with a prototype that only partially implements all the prospected solutions, but already confirm that the proposed approach exhibits very high potential. II. ARCHITECTURE AND KINEMATICS OF THE HAND The updated prototype of the hand is characterized by a modular structure in which four identical fingers and one opposable thumb are assembled on a carpal frame, that will be connected to a wrist. The overall dimensions of the hand are very similar to the human one and in Fig. 1 a direct comparison is proposed. A compliant layer, reproducing the role of human hand soft tissues, covers the endoskeletal structure, as sketched in Fig. 2. The internal articulated structure is designed according the “compliant mechanism” concept so that the mobility of the phalanges is obtained by means of elastic joints (see Fig. 3a). The Fig. 1. The UB Hand 3 in comparison to the human hand.