# Design of a Two-DOFs Driving Mechanism for a Motion-Assisted Finger Exoskeleton
###### Abstract
The hand exoskeleton version HX [11] is suitable for both the index finger and the thumb. The exoskeleton has five DOFs and is driven by cables. The weight of the index finger module is 118 g, and the total weight lying on the hand is 438 g. The exoskeleton is made of a 3D-printed titanium alloy. At LARM (Laboratory of Robotics and Mechatronics), a specific research line has been addressing the development of exoskeletons for motion assistance, as reported, for example, in [22; 23; 24]. Moreover, [25; 26; 27] focus on the fundamentals of the mechanics of grasping as well as the design and validation of anthropomorphic robotic hands. The LARM robotic hands are based on a driving mechanism with linkages that remain within the finger body duringthe finger operation, as reported in [28; 29; 30]. The design of such a driving mechanism is the conceptual reference for the exoskeleton solution that is reported in [24] and in preliminary exoskeleton designs, as reported in [31; 32; 33; 34]. The main problem with existing exoskeletons is that they are often not wearable by different patients, as in 4,5], are bulky, and the overall equipment is not easily transportable, such as in [6] or is heavy, such as in [11]. Commercial robots, as in [6], are considered too expensive for home rehabilitation use. Further, the Amadeo device is not able to fulfill a complete grasping movement. The solution in [8] has no defined trajectory to move all joints of the finger in a defined way. Accordingly, the authors believe there is still a need for a design procedure that can lead to novel design solutions as based on kinematic analysis and a proper mechanism synthesis referring to the specific task of finger motion assistance. This paper aims at a systematic design approach towards a novel two-DOFs driving linkage mechanism for a motion assistance finger exoskeleton by presenting a novel design solution for a finger exoskeleton with adaptability to the finger size, as well as cost-oriented design and user-friendly features. The design process is carried out within a specific design procedure. As a first step, the movement of a human finger was characterized by video motion tracking to identify the desired reference finger motions. Then, the relative kinematics of a human finger were obtained based on the acquired data. As a next step, a type synthesis was carried out to identify a mechanism consisting of linkages with two active DOFs as the most convenient solution to assist the motion of a finger along the desired trajectory, as also preliminarily discussed in [33]. This paper also provides FEM analyses that are integrated in the proposed design approach. A graphic/geometric synthesis procedure has been implemented for achieving the dimensional synthesis of the proposed linkage mechanism. Numerical simulations and experimental tests have been carried out and discussed to prove the feasibility and effectiveness of the proposed design solution. The main contribution of this work can be recognized in the design of a proposed novel linkage mechanism that, unlike other existing designs, can drive the three finger phalanxes by using two independent active DOFs that are both driven by rotary servomotors placed on the back of the palm. This configuration allows for a wide range of motion assistance with proper adaptability to a variety of human fingers. The paper content can be outlined as follows: the first section addresses the design requirements for achieving a device for finger exercising/rehabilitation of multiple users; next, the paper deals with the kinematic design of the proposed new device based on a two-DOFs driving linkage mechanism, whose synthesis is described in Section 4; the following section focuses on the mechanical design and construction of a prototype; Section 6 describes an experimental validation with comparisons of numerical and experimental results to assess the feasibility and performance of the proposed device.