Design and Validation of a Cable-Driven Joint Actuator for Pediatric Knee Orthoses

Robot-assisted gait rehabilitation is an increasingly common therapeutic intervention for enhancing locomotion and improving quality of life for children with lower-limb mobility impairments. However, there are few systems specifically designed for pediatric use, and those that do exist are largely cumbersome, bulky, and noncustom devices that ultimately reduce therapy effectiveness. This paper introduces the Cable-Driven Joint System (CDJS), a novel approach for pediatric gait rehabilitation that addresses these shortcomings in a lightweight and compact robotic device using the patient's professionally fitted orthosis. The CDJS consists of a 2.1 kg actuation unit that is held by a clinician which delivers assistive torques through a Bowden cable transmission to a 0.3 kg joint mounted to user-custom bracing. This work details an actuator benchtop evaluation, demonstrating a peak torque of 20 N·m, peak velocity of 7.2 rad/s, bandwidth of 9.7 Hz, and a mass moment of inertia of 58.38 kg cm2. An actuator model was developed and evaluated in simulation, showing a strong correlation with the experimental torque data (R-squared = 0.95) and indicating a transmission efficiency of 72%. In-air gait tracking experiments on an emulated subject showed that the CDJS assisted the subject to track a nominal knee trajectory with an average root-mean-squared error of 2.56 deg at a continuous torque of 1.37 N·m. These results suggest that the cable-driven actuator meets the design requirements for use in pediatric gait rehabilitation and is ready for implementation in clinical device trials.