# A Low-Cost Soft Robotic Hand Exoskeleton for Use in Therapy of Limited Hand-Motor Function
## 2 Materials and Methods
### Actuation Method  
The design presented here utilizes a jointless structure that relies on the natural movement and joint constraints imposed by the patient's skeleton. This actuation method is not suitable for injuries or ailments that have compromised the skeletal structure of the fingers. Our design also does not allow for controlling the rotation of the thumb's carpometacarpal joint. This is achieved typically in other designs with hard linkages, which is not appropriate for soft-robotic designs such as ours. Our design allows for grasping motions, keeping the carpometacarpal joint in a natural range of motion for such tasks. The removal of the mechanical and structural linkages greatly simplifies the mechanical design and assembly process. Actuation was achieved by tensioning artificial tendons connected to the fingertips on the palmar side of the hand, as seen in Figure 1. Elastic artificial tendons on the dorsal side of the hand were used to provide the opposing force needed for abduction. The elastic and inelastic tendons were designed to mimic the correct functioning of the human hand in grasping motions. Each finger was actuated using one custom-designed actuator mechanism. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Requirement** \\ \hline
**Design** & simple mechanical and electrical \\ \hline
**Manufacturing** & 3D-printed with off-the-shelf components \\ \hline
**Mobility** & untethered mobile \\ \hline
**Price** & \textless{}5200 \\ \hline
**Weight** & \textless{}1 kg \\ \hline
**Battery Runtime** & \textgreater{}1 h \\ \hline
**Communication Interface** & wireless \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Requirements for the robotic hand exoskeleton. Figure 1: Un-actuated (**left**) and actuated (**right**) finger showing artificial tendon attachment points and fingertip force diagram. To achieve natural motion of a finger, the kinematic design borrowed heavily from the anatomy of a healthy human hand. The actuation force was provided by a tendon running down the palmar side of the fingers. The bone structure and musculature naturally constrain joint angles \(\theta_{\text{M}}\), \(\theta_{\text{P}}\) and \(\theta_{\text{D}}\) to a range of approximately 0-90 degrees. These joints are further constrained to a single plane of motion. The underactuated design relies on controlling the parameter \(\Delta\)L, which translates to the length of the palmar tendon. As discussed later in the paper, \(\Delta\)L is adjusted to allow for full extension of the finger at its greatest value (\(\theta_{\text{M}}\), \(\theta_{\text{P}}\) and \(\theta_{\text{D}}\) all equal to 0). When sufficient force is applied to the tendon \(\theta_{\text{M}}\), \(\theta_{\text{P}}\) and \(\theta_{\text{D}}\) begin to move and \(\Delta\)L becomes smaller, moving the tendon attachment points (shown in red in Figure 2a) closer to each other. The finger is considered fully actuated once \(\theta_{\text{M}}\), \(\theta_{\text{P}}\) and \(\theta_{\text{D}}\) have hit their natural joint constraints. The drawback of an underactuated design is that \(\theta_{\text{M}}\), \(\theta_{\text{P}}\) and \(\theta_{\text{D}}\) are not directly controllable. This, however, does not pose a significant issue due to the fact that the system is designed to mimic the operation of a healthy human hand. The focus of this design was to impart a force normal to the fingertip that was consistent with that of a healthy human hand. The mathematical model was derived to help calculate the actuator torques needed to achieve this force. As shown in Figure 1; Figure 2b, the force normal to the finger changes with the angle of the distal phalanges. Thus, our determination of proper actuator torque was taken when the finger was at full extension, the worst-case scenario. Figure 2b shows the relationship of the angle between the attachment point of the palmar tendon and the center of rotation of the DIP joint, both measurements obtained empirically from dimensions taken from the test subjects and the force.