Nested Planetary-Cycloidal Transmissions Design for Compact and Lightweight Exoskeletons Actuators

Recent progress in assistive exoskeleton technologies has demonstrated significant improvements in reducing users’ metabolic cost and mitigating biomechanical overload. A key factor in achieving these outcomes lies in the design of the transmission systems, which must ensure efficiency, safety, and user comfort. Consequently, it is fundamental to develop compact, lightweight, and backdrivable gearboxes with low mechanical output impedance for wearable robotics. This paper presents two configurations of a novel nested Planetary-Cycloidal (PLACY) transmission system, which address the axial bulkiness limitations found in previous cycloid-planetary (CP) series implementations. The proposed architectures integrate a compact planetary stage, whose output element (the outer ring) is employed as an input shaft for two different cycloidal reducers. One implements a double-disk cycloidal design (PLACYd), while the other is realized according to the cycloidal compact-cam variant (PLACYc). These solutions enable high gear reduction ratios while maintaining minimal weights, low encumbrances, and backdrivability. Two 3D-printed prototypes are developed by employing the cycloidal non-pinwheel design and subjected to a comprehensive experimental characterization to assess their employment for wearable robotics actuators. Performance metrics were compared with those of the earlier CP designs and with other state-of-the-art 3D-printed and metal-built gearboxes to validate the developed prototypes’ applicability to real-world assistive scenarios. Experimental results confirm that the proposed PLACY transmissions result in promising solutions for assistive exoskeletons, offering a favorable balance between high reduction ratio, compact form factor, and mechanical transparency.