AlmatyExoElbow: development and biomechanical analysis of a 2-DOF cable-driven elbow exoskeleton

2.1. System architecture

The proposed AlmatyExoElbow is a lightweight modular exoskeleton designed for elbow rehabilitation. The design aims to improve biomechanical compatibility, reduce parasitic loads, and enhance adaptability during rehabilitation exercises.

The system consists of a proximal module, a distal forearm module, linkage elements, and a cable-driven transmission mechanism. Remote actuator placement reduces distal segment mass and inertia, while the modular architecture facilitates adjustment for different users and simplifies maintenance.

Fig. 1 shows the exploded view of the proposed design. The main components include: (1) proximal ring module, (2) central joint housing, (3) elbow ring, (4) linkage bars, (5) protective housing, and (6) actuator mounting unit. These elements provide structural support and force transmission between the arm and forearm segments.

The current configuration represents a preliminary conceptual prototype intended for initial biomechanical and structural assessment. Finite element analysis was used to identify critical stress concentration regions that will guide future structural reinforcement and geometric optimization. Compared with conventional rigid rehabilitation devices, the proposed cable-driven architecture offers improved adaptability and reduced mechanical constraints.

Fig. 1Exploded view of the elbow exoskeleton

Fig. 2Elbow exoskeleton mounted on the arm

2.2. Structural modules and human-interface design

The exoskeleton employs ring and semi-ring modules to improve load distribution, biomechanical compatibility, and user comfort during rehabilitation exercises. As shown in Fig. 2, the proximal module serves as the main support structure and incorporates the actuator mounting and cable-routing elements, while the distal module is attached to the forearm and transfers assistive rotational forces.

Adjustable fastening elements allow adaptation to different user anatomies and provide stable fixation during movement. The cable-driven architecture enables remote force transmission, reducing distal segment inertia and improving motion flexibility compared with conventional rigid rehabilitation devices.

The modular and detachable design simplifies installation, maintenance, and future modifications. The current prototype represents a preliminary rehabilitation-oriented configuration intended for initial biomechanical and structural assessment, while further optimization and experimental validation are planned for future work.

2.3. Cable-driven mechanism and working principle

The proposed exoskeleton employs a cable-driven mechanism to transmit forces from the actuator unit to the forearm module. Remote actuator placement reduces distal segment mass and inertia, improving biomechanical compatibility during rehabilitation movements.

Fig. 3 illustrates the kinematic scheme of the system. The mechanism uses cable routing through guide elements and attachment points (A, B, C). Variations in cable lengths ($L_1$–$L_3$) generate rotational motion of the distal module and assist elbow flexion–extension and forearm pronation–supination movements.

Compared with rigid transmission systems, the cable-driven architecture reduces alignment requirements between anatomical and mechanical joint axes, thereby decreasing parasitic loads and improving motion adaptability. IMU sensors are integrated for motion monitoring and future implementation of intelligent control algorithms.

The assembled prototype is shown in Fig. 4. Assistive torque is generated by regulating cable tension, enabling passive, assistive, or active rehabilitation modes depending on the selected control strategy. The current prototype represents a preliminary structural and functional implementation intended for conceptual validation, while future work will focus on dynamic analysis, cable tension optimization, and experimental rehabilitation assessment.

Fig. 3Cable-driven mechanism scheme

Fig. 4Experimental prototype

3.1. Model parameters and boundary conditions

Finite element analysis (FEA) was performed in SolidWorks Simulation using SI units. The exoskeleton was modeled using AISI 304 stainless steel due to its widespread use in rehabilitation and medical devices. The main material properties were: density 8000 kg/m³, Young’s modulus 1.9×10¹¹ Pa, Poisson’s ratio 0.29, and yield strength 2.07×10⁸ Pa.

A fixed boundary condition was applied to the proximal module, while external loads representing forearm motion assistance were applied to the distal segment. The analysis was conducted under simplified worst-case static loading conditions to identify critical stress concentration regions. Dynamic effects, soft tissue interaction, and cyclic loading were not considered and remain subjects for future investigation.

3.2. Finite element model

The finite element model and boundary conditions are shown in Fig. 5. A refined mesh was applied in critical regions, including joint interfaces and cable-guiding elements, to improve stress distribution accuracy. The analysis was intended as a preliminary structural assessment of the proposed concept.

3.3. Stress analysis

The von Mises stress distribution is presented in Fig. 6. Maximum stresses were concentrated near joint interfaces and load-transfer regions, where geometric discontinuities produced local stress concentrations ${\sigma }_{max} \approx$ 5.20×10⁹ Pa.

The obtained peak stress values exceeded the yield strength of AISI 304, indicating that the current prototype configuration requires structural refinement and reinforcement. These results highlight critical regions for future optimization rather than final mechanical performance.

Fig. 5FEM model and boundary conditions

Fig. 6Stress distribution

3.4. Displacement analysis

The displacement distribution is shown in Fig. 7. The maximum displacement was observed at the distal segment: $U_{max}\approx$135.5 mm.

The relatively large displacement is associated with the lightweight modular architecture and simplified boundary conditions. The results indicate limited structural stiffness under the applied loading scenario and suggest the need for further design optimization.

Fig. 7Displacement results

Fig. 8Strain distribution

3.5. Strain analysis

The strain distribution is presented in Fig. 8, ${\epsilon }_{max}\approx$ 6.82×10⁻³.

The highest strain values occurred in regions corresponding to previously identified stress concentration zones. These areas represent critical structural locations requiring further geometric refinement and stiffness improvement.

Overall, the numerical analysis provided a preliminary assessment of the proposed concept and identified key regions requiring optimization before future experimental validation and rehabilitation testing.