# Development of a Lightweight Prosthetic Hand for Patients with Amputated Fingers
## 2 Design of the Prosthetic Hand
### Principles of Operation  
Each phalanx is moved using tendon-driven method, as shown in Figure 3. The joint frames in each phalanx are connected by tendons attached to the tendon-fixing frame. The ends of each tendon are fixed (as shown by the red and green points) on each joint frame, and the tendons on the left and right sides are fixed with a phase difference of \(90^{\circ}\) on the rotational axis of the revolute joint of each phalanx. This fixing method facilitates extension/flexion motions by which the tendons on the left side (green lines in Figure 3) pull the joint frames in flexion motion, while the tendons on the right side (red lines in Figure 3) pull the frames in extension motion. Moreover, because the slack and taut problem associated with the tendon-pulley driving method causes poor power transmission, it should be solved for driving the finger module consistently. In this study, to solve this problem within the limited design space, the paths of all tendons passing the PIP and MCP joints were designed to pass  
Figure 2: Nomenclature of components and frame sizes in (**a**) finger module and (**b**) mounting part.  
through the centers of the rotational axes of these joints. This path design facilitates steady power transfer to the joint frames, regardless of the motions of the phalanxes, as illustrated in the upper and side diagrams in Figure 3. This design method has been proven to be considerably efficient in a limited design space, and it decreases product weight because incidental mechanical parts, such as pulleys and shafts, are not needed for driving. In addition, it simplifies the design owing to the use of a minimum number of parts, improves productivity, and reduces production costs.  
The diameters (\(d_{i}\) (\(i=1,\ 2,\ \text{and}\ 3\)) and \(d_{4}\)) of the joint frames in each phalanx and the tendon-fixing frame were designed equally, i.e., \(d_{1}=d_{2}=d_{3}=d_{4}\). Therefore, extension/flexion motions can be realized by driving the actuator in the range of \(0^{\circ}\)-\(90^{\circ}\).  
In the proposed finger module, self-adaptive grasping was achieved using two linear springs in the middle and proximal phalanxes (Figure 4a,b) and a torsional spring in the mounting part (Figure 4c). Specifically, the torsional springs of the four fingers generate adduction/abduction motions passively. As shown in Figure 4a,b, during natural motion in which external forces are absent, each phalanx is driven from \(0^{\circ}\) to \(90^{\circ}\). However, when the middle and proximal phalanxes are in contact with an object, although the joint frames are rotated continually, the frames are stopped, and the linear springs are compressed. Similarly, the torsional springs are compressed depending on the external forces generated by contact with an object. The torsional spring in the index finger is compressed in the counterclockwise direction, while the other springs are compressed in the clockwise direction (Figure 4c).  
Figure 4: _Cont._  
Figure 3: Connecting points and paths of tendons.  
The finger module and mounting part are composed of 32 and 23 parts, respectively, including the springs, bearings, and shafts, as depicted in Figure 5. The developed prosthetic hand can be assembled easily under the following assembly directions, as suggested in Figure 5.