Design and analysis of an adaptive cable-driven knee unloading exoskeleton based on a rhombus linkage mechanism

Abstract Biomechanical intervention on lower limb joints using exoskeletons to reduce joint loads and provide walking assistance has become a research hotspot in the fields of rehabilitation and elderly care. To address the challenges of human-exoskeleton (H-E) kinematic compatibility and knee joint unloading demands, this study proposes a novel rhombus linkage exoskeleton mechanism capable of adaptive knee motion without requiring precise alignment with the human knee axis. The exoskeleton is driven by a Bowden cable system to provide thigh support, thereby achieving effective knee joint unloading. Based on the screw theory, the degrees of freedom (DOF) of the exoskeleton mechanism (DOF = 3) and the H-E closed-loop mechanism (DOF = 1) were analyzed, and the kinematic model of the exoskeleton and the H-E closed-loop kinematic model were established, respectively. A mechanical model of the driving system was developed, and a simulation was conducted to validate the accuracy of the model. The output characteristics of the cable-driven system were investigated under varying bending angles and bending times. A prototype was fabricated and tested in wearable scenarios. The experimental results demonstrate that the exoskeleton system exhibits excellent biocompatibility and weight-bearing support capability. Compatibility tests confirm that the exoskeleton does not interfere with human motion. Through human-in-the-loop optimization, the optimal Bowden cable output force profile was obtained, which minimizes gait impact while achieving a peak support force of 195.8 N . Further validation from wear trials with five subjects confirms the system’s low interference with natural human motion (maximum lower-limb joint angle deviation of only $8^\circ$ ).