The Sixth–Finger: a Modular Extra-Finger to Enhance Human Hand Capabilities Domenico Prattichizzo 1,2 , Monica Malvezzi 1 , Irfan Hussain 1 and Gionata Salvietti 2 Abstract— Robotic prosthesis are usually intended as arti- ficial device extensions replacing a missing part of a human body. A new approach regarding robotic limbs is presented here. A modular robot is used not only for replacing a missing part of the body but also as an extra–limb in order to enhance manipulation dexterity and enlarge the workspace of human beings. In this work, the model and control of an additional finger, the Sixth–Finger, is presented as a case study of this type of robotic limbs. The robotic finger has been placed on the wrist opposite to the hand palm. This solution allows to enlarge the hand workspace, increasing the grasp capability of the user. An object-based mapping algorithm is proposed to control the robotic extra-finger by interpreting the whole hand motion in grasping action. A four DoFs modular prototype is presented along with numerical simulations and real experiments. The proposed Sixth–Finger can lead to a wide range of applications in the direction of augmenting human capabilities through wearable robotics. I. I NTRODUCTION Wearable robotics is a promising way of integration be- tween humans and robots. Typically wearable robot are intended to be a way to substitute missing parts of the human body, by means of prosthesis [1], [2], or to enhance human body force and precision capabilities, by means, for example, of exoskeletons [3], [4]. Recently, robots integrated with humans have been ac- tively developed as wearable robots by taking advantage of the progress in miniaturization and efficiency of the technological components. The purpose of these new devices is not to enhance the lift strength far above human capability by wearing a bulky robot, but to support human capability within its range by wearing lightweight and compact robots [5]. As example, in [6] the authors presented the Supernu- merary Robotic Limbs (SRL), a wearable robot designed to assist human with additional arms attached to the wearer’s body. Such devices, further investigated in [7] and [8] can closely cooperate with the human, for instance holding an object, lifting a weight, positioning a work piece, etc.. It is very interesting to observe how these wearable robots are potentially able to enhance the strength and the precision of the human users, and also to enrich the skills and tasks that can be performed. In such work the authors furthermore observe that the wearer and the extra–arms work very closely that he/she could potentially perceive them to be his/her own. In this paper we propose to further exploit this approach. While the works previously described are relative to devices that can be applied as extra–arms or extra–limbs, we focus 1 Universit` a degli Studi di Siena, Dipartimento di Ingegneria dell’Informazione, Via Roma 56, 53100 Siena, Italy. {malvezzi, hussain, prattichizzo}@dii.unisi.it 2 Department of Advanced Robotics, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genoa, Italy. gionata.salvietti@iit.it Fig. 1. The first prototype of the Sixth–Finger. on a smaller but paramount part of our body: the hand. The goal is then to integrate the human hand with an additional robotic finger: the Sixth–Finger (Fig. 1). The challenge is that of designing a new finger which plays the role of an additional thumb [9]. The thumb is the finger which lets us manipulate objects in a complex way [10]. The main function of the human thumb is the pincer action that allows to grasp objects, to hold them firmly, to perform complex manipulation (e.g. twisting, rubbing). If we had a second thumb, probably ring and pinkie fingers would be dexterous as the index and middle fingers are. Trained with the second thumb, less-developed fingers will strengthen and gain dexterity. This could give humans the possibility to manipulate objects in a more efficient way, enhancing our hand grasping dexterity/ability. As an extension of the human hand, the second thumb could make certain tasks much easier, and extend our hand’s workspace. The aim of the prototype described in this work is to make human hand more symmetric, so that when the human fingers close (when phalanxes flex), the extra–finger reflects exactly these motions. This extension is going to increase the human hand workspace and its grasping abilities. The difficulties in achieving such improvements lie both in the design and the control of the Sixth–Finger device. For the design aspect, the extra–thumb has to be sufficiently small and light in order to be easily worn and carried by the user. We developed a modular finger that can be worn on the wrist with the help of a rubber band. The structure of the modules is obtained by using rapid prototyping techniques, while the active degrees of freedom (DoF) are realized with servomotors. Concerning the control, we decided to capture the motion of the human hand through a dataglove and map such motion on the extra– finger. This solution differs from classical techniques for exoskeleton controllers which are based on the reading of some bio-signal like EMG [11]. Our approach goes in the