Design of a Novel Flat Cycloidal-Planetary Series Reducer and Experimental Assessment for Assistive Exoskeletons Actuation-CYPLO

The recent advancements in assistive exoskeletons have highlighted their effectiveness in reducing the users' metabolic consumption, providing functional compensation, and preventing biomechanical overloads. The exoskeletons' transmission mechanisms are essential for guaranteeing their effectiveness, safety, and comfort. Therefore, lightweight, efficient, backdriveable, and minimally bulky reduction gearboxes with low mechanical output impedance have become a widespread choice in modular assistive exoskeletons. In previous works, the value of the cycloid-planetary series transmission has been assessed. However, the previous design turned out to be too bulky in the axial direction. In this paper, we propose a novel CYcloidal-PLanetary cOncentric series combination (CYPLO) that exploits a compact-cam cycloidal architecture nested inside the sun of a planetary stage. This design enables valuable performance regarding key gearbox features such as regularity, friction, and gear play, while maintaining an extremely advantageous compactness in the axial direction and significant backdriveability even for high transmission ratios. A reducer implementing this novel solution has been designed, developed through 3D printing, and examined experimentally to evaluate its performance with respect to the previous designs and state-of-the-art gearboxes employed in wearable devices' actuators. The results show that this transmission architecture represents a valid solution for lightweight assistive exoskeletons, allowing for high reduction ratios while maintaining compactness and backdriveability.