The evolving role of planetary transmissions in collaborative and humanoid robotics

Abstract

Mechanical transmissions have evolved from primarily power-conversion devices into increasingly integrated elements of robotic actuator systems. Servo-controlled machinery added requirements for accurate motion transmission, while collaborative and humanoid robotics introduced the need for bidirectional power flow, mechanical transparency and physical interaction. This paper examines how these cumulative requirements are reshaping the role of planetary transmissions in robotics. Particular attention is given to the trade-offs between transmission ratio, torque density, efficiency, backdrivability, dynamic behaviour and manufacturability. The emergence of quasi-direct-drive actuators demonstrates the potential of conventional low-ratio planetary gearsets combined with high-torque-density motors, while recent developments in compound planetary architectures provide an alternative route towards high-ratio, compact and backdrivable transmissions. Rather than identifying a universally optimal solution, the paper argues for application-specific optimization of transmission architecture and ratio within the complete actuator system. Finally, recent developments in integrated torque sensing and condition monitoring are discussed as evidence of a further evolution towards observable transmissions, in which mechanical architecture and sensing are increasingly considered together. These developments suggest an expanding role for planetary transmissions as scalable, application-optimized mechatronic systems for next-generation collaborative and humanoid robots.