A Design of Energy Efficient Compact Rotary-to-Linear Transmission for Cable-Driven Robotic Systems

Cable-driven actuation has been employed in robots where compact joint design is essential, using motors combined with rotary-to-linear reducers. It is desirable for these reducers to have a lightweight design for the convenient use of robots. Although such designs can be realized using gearless reducers, their limited stroke constrains the range of motion in robots. To address this limitation, gear-based reducers have been developed; however, additional gears required to increase the transmission ratio pose challenges to lightweight design. Moreover, compound planetary reducers developed for compactness still require multiple gears due to the sun-planet-ring configuration. In this study, we present a compound spur reducer that achieves high rotary-to-linear transmission ratios over near-unlimited strokes using a minimal number of gears. By incorporating a differential mechanism into a spur gear reducer, the cable is efficiently wound around a shaft at the differential speed between gears. High transmission ratios are provided by decreasing this differential speed, while near-unlimited strokes are enabled through radial cable winding. Experimental results demonstrated that the targeted transmission ratios could be maintained throughout the stroke, with efficiency comparable to that of conventional reducers. Consequently, a lightweight and cost-effective design was attained by maximizing the achievable transmission ratio per gear.