# Design of a Two-DOFs Driving Mechanism for a Motion-Assisted Finger Exoskeleton
## 2 Design Requirements for a Finger Exoskeleton  
To expand the range of suitable patients, a novel proposed exoskeleton should be easy to attach to a finger, adaptable to a wide range of users, and easily portable for home use. Exoskeletons driven by cables are a common solution in the literature. However, they show a range of drawbacks such as high losses due to friction as well as a high risk of cable failures. Further, cables need to be kept under tension during motion. The cable management system has a negative effect on the portability, as it is often bulky as in [12]. Because of that, servo motors with linkage transmissions are preferred in this work as they can be robust, lightweight, compact, and easy to control. The linkage parts can be easy and cheap to manufacture even with commercial 3D printers. However, the design of such a linkage mechanism requires full understanding of the desired human finger motion assistance. A human hand is composed of fingers, metacarpus, and carpus. The fingers consist of three phalanxes, except for the thumb, which consists of two phalanxes. The metacarpus is connected to the proximal phalanx. On the fingers, the second link is the medial phalanx. The third link is the distal phalanx. A detailed description of the joints and functionalities of a human hand can be found, for example, in [35; 36]. The joints between the intermediate and distal phalanxes are called distal interphalangeal joints (DIP), and the joints between the proximal and intermediate phalanxes are called proximal interphalangeal joints (PIP). The metacarpophalangeal joints (MCP) connect the proximal and metacarpal phalanxes, and the carpometacarpal joints (CMC) connect metacarpal phalanxes and the carpal bones, as shown in the scheme of Figure 1, [36]. In general, flexion reduces the angle between bones or parts of the body, whereas extension increases the angle between the bones of a limb. Abduction is an outward movement of a limb, and adduction is an inward movement of a limb [9]. The MCP joints have two DOFs and they allow flexion and extension as well as abduction and adduction of a finger. The interphalangeal joints PIP and DIP have one DOF each. However, the axis of rotation of these joints is not constant during flexion and extension [10], and the ligaments restrict the movements of the joints. Moreover, DIP and PIP joints cannot be moved independently of each other [35]. A finger exoskeleton can be conveniently designed for motion exercising of the muscles after an injury or a stroke. For this application, the motion of a finger will be assisted by the exoskeleton with a motion trajectory similar to a healthy finger of the same size as proposed in Figure 2. The motion trajectory does not require high accuracy. For example, in [11], a misalignment of a phalanx up to \(\pm 8^{\circ}\) is deemed acceptable. However, unnatural movements of the finger must be avoided by the mechanism to prevent potential injuries. As an additional design requirement for a finger exoskeleton, its installation space needs to be small enough to be attached onto the finger and not interfere with its motion. For the same reason, the total weight of the system should not exceed 500 g as reported, for example, in [7; 13; 17]. This value should be considered an upper bound, since a heavy exoskeleton can limit the effectiveness of motion exercising. Indeed, a high weight makes a patient easily tired and not willing to continue the treatment. Thus, it is advisable to limit the number of motors as they are the main source of weight. Motors need to provide a minimum torque of around 200 Nmm as reported previously [3; 7]. One of the main peculiarities of the proposed novel linkage mechanism design solution is that it can move all three phalanxes of a human finger with two active DOFs. This is mainly achieved by coupling the motion of the last two phalanxes. Accordingly, only two DOFs are needed to replicate whole finger motions.