# Incorporation of Torsion Springs in a Knee Exoskeleton for Stance Phase Correction of Crouch Gait
## 2 Design and Methods
### SolidWorks: Modeling and Motion Analysis  
Two methods (passive only, and passive with motorized knee angle correction) were used to determine the best options for offloading forces generated in the crouch gaits. The initial conditions were taken from the data obtained from OpenSim, to determine the effectiveness of the current iterations, by conducting a dynamic force analysis in SolidWorks and comparing it to the normal gait cycle. A motor was incorporated into the design and was used to control the knee angle. The first method used data points given from the angular range of motion in the knees of each crouch gait from OpenSim that were extracted and converted to comma-separated value (CSV) files and uploaded into the function builder for the motor in SolidWorks, as indicated in Figure 5. The function builder plotted displacement, velocity, acceleration, and jerk for the exoskeleton, given the crouch gait and normal gait parameters. This was used to model the crouch gait behavior of the leg, in conjunction with the exoskeleton. The tendon forces acting in the knee for each crouch gait were plotted and extracted as CSV files, which were uploaded as force parameters to act on the exoskeleton through the gait cycle. The stiffness values for the torsion spring were adjusted to prevent extreme flexion on the knee during crouch gait. The second method motorizes the exoskeleton itself. Data points acquired from the angular range of motion for the knee of the normal gait were input into the motor, while importing data points for the tendon forces acting from the knee of the crouch gait. \begin{table}
\begin{tabular}{c c} \hline \hline
**Segment** & **Proximal Joint to Mass Center** \\ \hline Thigh & 0.430 \\ Shank & 0.443 \\ Foot & 0.436 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters for proportional distance from the proximal joint to the segmental mass center. Figure 4: An OpenSim generated diagram for the stance and swing phase of the gait cycle. The first two iterations of the knee exoskeleton involve a frame where, when paired with lightweight fabric, the thigh support rests on the posterior of the thigh, such that interference with the quadriceps is minimal, as shown in Figure 6a,b. To limit over-flexion of the knee during the stance phase, torsional springs were used in the knee joint. Flexion of the ankle occurs during flexion of the knee. By reducing the flexion of only the knee, the goal is to naturally reduce the flexion of the ankle; however, the ankle and plantar dorsiflexion have not been addressed by any iteration of the exoskeleton. The parameters for design involved finding the average circumferences for the thigh [28] and the calf, as seen in Table 2, to determine the best fit for the exoskeleton. In addition, adjustments were made to account for the incorporation of fabric, for additional comfort to the wearer. Figure 5: SolidWorks function builder used for angular displacement of the knee for crouch gait 1. Figure 6: Two iterations of the passive exoskeleton where the thigh support should sit behind the thigh, and (**a**) shank support is worn in front of the leg, (**b**) shank support is worn behind the leg. The first iteration was designed solely to be worn around the posterior of the leg. The second iteration was designed to support the back of the thigh, with the portion below the knee to be worn around the shin. Each iteration incorporates torsional springs to inhibit flexion of the knee. After the determination of parameters in OpenSim to be used in the spring and motor components of the exoskeleton, placements had to be determined, and the data points uploaded to SolidWorks. This process was used for both passive and motorized components of the exoskeleton. While motors were used in both, for the passive element, the motor merely simulated the knee rotation of each crouch gait. In the motorized version of the exoskeleton, the motor serves as a corrective component by increasing the range of motion through the angles of the knee in the normal gait cycle.