# Cable-Driven Exoskeleton for Ankle Rehabilitation in Children with Cerebral Palsy
## 2 Exoskeleton Development and Control Methodology
### Novel Ankle CDPR Module
#### 2.2.1 Mechanical System  
The novel ankle module is also modeled as a CDPR (see Figure 5), with a prismatic rectangular aluminum shape with dimensions of 1020 \(\times\) 580 \(\times\) 680 mm covering the usual length of a child between 6 and 12 years old (450 and 600 mm) [44]. The new module is classified as a 1R2T CDPR, meaning that it is designed to actuate 1 rotational degree of freedom (R) and 2 translational degrees of freedom (T). This configuration adheres to the following relationship:  
\[m+1=n \tag{1}\]  
where \(m\) is the number of DOFs to be controlled and \(n\) is the number of wires required to do it [45]. Therefore, to control 3 DOFs, we will need four wires arranged in a sagittal plane. A key design consideration of the novel module was the suppression of undesired abduction/adduction (lateral) ankle rotations. To achieve this, a mechanically redundant configuration with eight cables actuated by only four motors was implemented. Each motor simultaneously regulates the length of two mechanically linked cables, which increases the system's stiffness in the lateral plane and improves directional control (see Figure 6). An additional advantage of this redundant configuration is that it preserves the complete symmetry of the exoskeleton. This symmetrical layout ensures that the mechanical and kinematic principles governing Cable-Driven Parallel Robots (CDPRs) remain fully valid and applicable throughout the workspace, facilitating accurate modeling and control. This is accomplished using custom-designed PLA mechanical components (Table 1). Each motor drives a pair of cable drums that are mechanically linked via a centrally aligned PET-printed shaft (180 mm in length), which ensures synchronized rotation and improves structural robustness. This assembly is rigidly mounted to the aluminum profile frame, resulting in a stable and modular mechanical foundation for the ankle module (see Figure 6). Figure 5: Structure of the test bench where the new ankle control module is implemented. The image on the left shows the different mechanical and electronic elements of the device; while the image on the right shows a user using the exoskeleton in a rehabilitation session. Another alternative considered to the 1R2T configuration--featuring four motors controlling eight cables in a redundant setup--was the implementation of eight motors, each actuating a single cable. This would result in a full 3R3T configuration, enabling independent control over 3 translational and 3 rotational degrees of freedom. While potentially useful, this approach would provide control over DOFs that are not critical at the current stage, according to clinical expert review. Moreover, it would significantly increase the mechanical and computational complexity of the system, which led to its dismissal in favor of the simpler and more targeted 1R2T configuration. To transmit motion to the patient's ankle, a set interchangeable custom orthosis was developed in the form of a sole that interfaces directly with the foot. The sole includes four dedicated anchorage points for cable attachment, enabling effective control of the ankle's degrees of freedom. To withstand mechanical loads, the sole was fabricated in PET, offering greater strength than the PLA used for other structural parts (excluding the central axis). This component serves as the system's end effector and is illustrated in Figure 7. \begin{table}
\begin{tabular}{c c c c} \hline
**Part** & **Diameter (mm)** & **Width (mm)** & **Height (mm)** \\ \hline Motor Housing & 75 & 80 & 113 \\ Motor & 63 & 120 & 63 \\ Motor Cover & 75 & 32 & 113 \\ Drum Cover & 45 & 4 & 45 \\ Drum & 17 & 22 & 45 \\ \hline Drum Housing & 55 & 35 & 55 \\ Bearing & 35 & 10 & 35 \\ Bearing Arm & 55 & 15 & 58 \\ Axis & 17 & 180 & 17 \\ Union Arm & - & 16 & 52 \\ \hline \end{tabular}
\end{table} 
Table 1: Dimensions of each of the parts of the mechanical system obtained by 3D printing.