# Development of an Anthropomorphic Prosthetic Hand with Underactuated Mechanism
## 4 Results and Discussion  
### Prototype  
We developed a prototype of the prosthetic hand mechanism, as shown in Figure 7. The size of the hand is similar to that of a normal male human hand, as shown in Table 2. The weight including cases and actuators is about 475 g. In comparison with the existing high-end commercial prosthetic hands such as i-Limb and Bebionic, the weight of the proposed one is decreased by approximately 23% (i-Limb) and 12% (Bebionic). To improve the grasping force, cases made of rubber material were attached on parts that come into contact with an object. Moreover, we used custom-built spur gears, miter gears, CV joints, and torsional springs.  
Figure 6: (**a**) Stress distributions and (**b**) deflections in the finger module when external forces are applied along the \(y\)- (left) and \(x\)-coordinate axes (right) at the fingertip.  
_Appl. Sci._**2020**, _10_, 4384  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Weight** & \multicolumn{2}{c}{475 g} \\ \hline \multicolumn{2}{c}{**Overall size**} & \multicolumn{2}{c}{[175 mm, 82 mm (palm), 44 mm]} \\ \hline \multicolumn{2}{c}{} & \multicolumn{2}{c}{Thumb} \\ \cline{2-3} \multicolumn{2}{c}{} & Index finger & \\ \cline{2-3} \multicolumn{2}{c}{} & Middle finger & \\ \cline{2-3} \multicolumn{2}{c}{} & Ring finger & \\ \cline{2-3} \multicolumn{2}{c}{} & Little finger & \\ \hline \multicolumn{2}{c}{**Degrees of freedom**} & \multicolumn{2}{c}{20} \\ \hline \multicolumn{2}{c}{**Active motions**} & \multicolumn{2}{c}{5} \\ \hline \multicolumn{2}{c}{**Passive motions**} & \multicolumn{2}{c}{15} \\ \hline \multicolumn{2}{c}{**Joint coupling method**} & \multicolumn{2}{c}{Gears and linkages} \\ \hline \multicolumn{2}{c}{**Adaptive grasping**} & \multicolumn{2}{c}{Yes} \\ \hline \multirow{4}{*}{**Ranges of motion**} & \multirow{2}{*}{Extension/Flexion} & DIP joint & 0–90\({}^{\circ}\) \\ \cline{3-3} \cline{5-5}  & & PIP joint & 0–90\({}^{\circ}\) \\ \cline{3-3} \cline{5-5}  & & MCP joint & 0–108\({}^{\circ}\) \\ \cline{3-3} \cline{5-5}  & \multicolumn{2}{c}{Adduction/Abduction} & 0–10\({}^{\circ}\) \\ \cline{3-3} \cline{5-5}  & \multicolumn{2}{c}{Passive motion of thumb} & 0–50\({}^{\circ}\) \\ \cline{3-3} \cline{5-5}  & \multicolumn{2}{c}{Adaptive grasping in PIP and MCP joints} & 0–25\({}^{\circ}\) \\ \hline \multirow{2}{*}{**Finger/Grasp speed**} & DIP joint & 152.28\({}^{\circ}\)/s \\ \cline{2-3} \cline{5-5}  & PIP joint & 304.56\({}^{\circ}\)/s \\ \cline{2-3} \cline{5-5}  & MCP joint & 487.30\({}^{\circ}\)/s \\ \hline \multicolumn{2}{c}{DIP: distal interphalangeal, PIP: proximal interphalangeal, and MCP: metacarpophalangeal.} \\ \end{tabular}
\end{table}
Table 2: Specifications of the developed prosthetic hand.  
Figure 7: Prototype of the developed prosthetic hand and size comparison with a human hand.  
The white and black linkages in Figure 7 were manufactured from aluminum 6061 and steel, respectively, considering strength and deflection. To satisfy the desired specifications, the diameter of the coil and internal diameter of the torsional spring were 1 mm and 6 mm, respectively. Furthermore, the actuator offered by Maxon Motor has a gearbox with 256:1 ratio, and the output rpm and torque are 25.43 rpm 0.39 Nm, respectively. We controlled each actuator at its current mode control. Because of the use of the actuator with 25.43 rpm, the distal, middle, and proximal phalanges move at 152.28, 304.56, 487.30\({}^{\circ}\)/s, respectively. The grasp speed (152.28\({}^{\circ}\)/s) was designed to be over the minimal acceptable speed (115\({}^{\circ}\)/s) suggested in [2]. The specifications of the developed prosthetic hand are summarized in Table 2.