Achieving Mechanical Transparency Using Fusion Hybrid Linear Actuator for Shoulder Flexion and Extension in Exoskeleton Robot

Recently, the importance of mechanical transparency in human-assistive robots has grown. Traditionally, its primary goal was minimizing interaction forces during assistance. However, under this conventional definition, mechanical transparency was not considered when an interaction force was required during assistance. This research focuses on achieving mechanical transparency within the context of shoulder motion in upper extremity exoskeletons for rehabilitation. Our primary goal is maintaining interaction forces at target values, even with motion disturbances. To this end, we developed a shoulder actuation testbed for exoskeletons, incorporating a fusion hybrid linear actuator distinguished by high back-drivability, robust torque generation capability, and safety features. To attain mechanical transparency, we created a model for calculating the required joint torque, accounting for gravitational dynamics, and subsequently determined the necessary actuator output. The system characteristics were evaluated based on the joint torque generated by the actuator. The actuator utilized pneumatic pressure to generate force and compensated for kinetic friction using electromagnetic forces. The results showed that the compensation by the electromagnetic force reduced the root mean square error of the torque to less than 60% in relation to pneumatic pressure alone. This demonstrated the ability to generate consistent torque with high robustness to motion disturbances.