Actuation System Design Optimization for Lower Limb Assistive Robotic Exoskeletons

Assistive lower limb robotic exoskeletons are wearable devices that support people with impaired gaits to perform activities of daily living (ADL) independently. The rapid increase in the mobility disorders in the growing population has a significant impact on the demand of wearable devices. Consequently, the requirement to develop efficient wearable devices that meet the needs of the users has increased. This study is oriented towards developing lightweight, power efficient and powerful assistive robotic exoskeletons with a focus on the actuation aspect. An analysis of the gait pathologies among different categories of patients reveals a significant difference in the spatio-temporal, kinematic and kinetic requirements of the impaired gait subjects as compared to the healthy users. Therefore, suitable powered assistive devices have to be developed to address the demand of the users. Following the assessment of the support requirements of the users, the number of degrees of freedom of the device was identified to evaluate an actuator design solution to determine an optimal motor size and transmission mechanism for a rigid actuation system. Part of this study also includes the elastic actuators that were used in two basic configurations as series and parallel elastic configurations. Elastic element was introduced into the system to reduce the power flow requirements of the actuation system. It was beneficial for the reduction of the actuator size and to increase the efficiency of the system. The spring optimization techniques were explored that lead to a decrease in the kinetic requirements of the system. The analysis showed that a parallel elastic actuator (PEA) was able to reduce significant amount of the power requirements of the system, while series elastic actuator (SEA) has a marginal impact on the overall efficiency of the system. An actuator design solution was developed for the elastic actuators to determine an ideal solution for the motor and transmission mechanism for an elastic actuation system. Furthermore, the redundancy of an actuation system was also analysed in which the output power of the system is distributed between the two motors. The redundant rigid and elastic systems were investigated. The benefits of an elastic system as a variable stiffness actuator in a dual arrangement were assessed. An optimized design solution for a dual system was developed to evaluate an optimal dual actuation system for a rigid and elastic system. Based on the outcomes of the actuator design solutions for a rigid and elastic systems in a single and dual arrangements, the optimal actuation systems were implemented and validated using a virtual prototype of an assistive robotic exoskeleton.