Structure and Actuation System Design of a Paediatric Lower Limb Assistive Robotic Exoskeleton

Interest has grown in the use of robotic exoskeletons to address mobility disorders, to either rehabilitate or assist people with mobility issues. However, most of the available research focuses on developing systems for adults, and the existing literature on both assistive and therapeutic robotic exoskeletons for children is sparse. Therefore, this study focuses on designing a paediatric lower limb assistive robotic exoskeleton system for children aged between 9 and 12 years old, with particular emphasis on the structure and actuation system, to address this gap.

In an attempt to address the unique challenges associated with designing a paediatric robotic exoskeleton, different mobility disorders that affect children are identified, and some generic and specific system requirements are established. Based on these requirements, the targeted population is determined, and a unique lightweight structure design of the thigh segment with height adjustment mechanism is created for this targeted group. To address some of the limitations related to actuation systems of robotic exoskeletons, a cable-driven actuation system is developed, and this actuation system is examined with both single and dual motors to evaluate an optimal actuation system strategy using Matlab Simscape environment.

The biologically inspired frame design merges soft and rigid structures to take advantage of both types of structure while mitigating the limitations of each type. This innovative approach combines 4 longitudinal rods per thigh segment, each of Ø4 mm Al6061 hollow tube, within a soft cover made of silicone to be wrapped around the thigh. In order to accommodate the rapid growth of children, two different lengths of height adjustment mechanism using telescopic tubes are developed. These adjustment parts could be used interchangeably and in conjunction with each other to meet the desired user height. This design approach results in a thigh segment frame that weighs just 539 grams.

The actuation system design for the paediatric exoskeleton knee joint includes a Bowden cable-driven transmission system operated by rotary electric motors in conjunction with harmonic drives for speed reduction. This actuation system is investigated for both single and dual motor strategies. Matlab Simscape simulations reveals that the employment of dual motor provides a heightened degree of control over the kinematics of the robotic system by producing a more refined and precise motion. Furthermore, the dual motor actuation system reduces the overall weight of the knee joint module by 12% compared with single motor actuation.

This research signifies a substantial step towards delivering a paediatric lower limb robotic exoskeleton with a unique hybrid supporting structure. The designed knee joint module weighs just over 2 kg, which is significantly lower than the corresponding weight for the currently available paediatric robotic exoskeletons in the literature. This research also propounds the use of dual motor with a cable-driven actuation system, designed particularly for paediatric lower limb robotic exoskeletons. This system propounds a favourable solution for enhancing the quality of life for children with diverse mobility needs.