# Development of an Anthropomorphic Prosthetic Hand with Underactuated Mechanism
## 1 Introduction  
For finger and forearm amputees, prosthetic and robotic hands of different types have been developed steadily, and human hand motions, such as natural and self-adaptive grasping motions, have been realized by anthropomorphic hands with various mechanism types and designs [1; 2]. For instance, the DLR hand and Robonaut hand were developed with the objective of dexterous manipulation [3; 4]. [PERSON] developed a biomimetic prosthetic hand with the aim of reducing weight and cost [5]. In addition, the i-Limb (Touch Bionics), the BeBionic hand (RSL Steeper), the [PERSON] hand (Vincent Systems), and the [PERSON] hand ([PERSON]) have been made available to the public [6; 7; 8; 9].  
Grasping motion is an important function of the hand, and hands with an underactuated mechanism have a remarkable ability to grasp differently shaped objects adaptively because the control of robotic hands with multiple degrees of freedom (DOFs) becomes simple. Furthermore, when hands using an underactuated mechanism are designed, the description and design to realize the desired motions according to target grasping types become easier.  
In the design of an underactuated mechanism, the torque produced by actuators is transferred to each phalanx using diverse methods such as tendons, gears, and linkages. Most of the developedunderactuated hands with five fingers, e.g., the Cyber hand [10], i-Limb [6], Smart hand [11], and Toronto Bloorview Macmillan (TBM) hand [12], were designed using the tendon-driven method. This has the advantage of producing motions that are closely similar to those of the human hand using minimal parts, and the design is very simple and lightweight. However, fixing the tendons and maintaining their tension is difficult in limited space, and the slack and taut problem of tendons should be solved. Moreover, the assembly to have equal tension in each finger module is difficult, and nonuniform tension can make control difficult. Therefore, in terms of sustainability, this method has a weakness. Hands designed using gears and linkages have advantages in terms of uniform power transmission in each finger, exact position control, and high payload, even if they require more mechanical components and have a large and complex structure. In addition, research related to control contact force with objects in grasping has been performed [13; 14].  
In this research, the focus was on the need for the prosthetic hand to appeal to users in terms of weight and shape (anthropomorphic configuration) with high performance (accurate positioning and uniform power transmission), and we designed the underactuated hand mechanism using a combination of gears and linkages, as shown in Figure 1. Compact design in an underactuated mechanism composed of only linkages is difficult because the linkages have diverse lengths for realizing extension/flexion motions, while a mechanism using a combination of gears and linkages can be designed compactly in limited space. In addition, gear reduction ratios and metal frames can increase the available payload and strength of the mechanism. Therefore, we designed the finger module with two four-bar linkages composed of a combination of gears and linkages, and passive motions are realized by torsional springs. The distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) joints are coupled by gears and four-bar linkages. The suggested hand realizes natural (extension/flexion) and self-adaptive grasping motions including adduction and abduction.  
This paper is organized as follows: The design and operation principles of the suggested robotic hand are described and illustrated in Section 2. In Section 3, the kinematic and finite element method (FEM) analyses are explained. Finally, in Section 4, the prototype hand is represented, and experiments are performed.  
Figure 1: Skeleton structure of the human hand (right) and nomenclature of the proposed prosthetic hand (left).