Actuators play a crucial role in robotics, determining the force and speed capabilities necessary for varied tasks, directly affecting the performance of the robotic system. With the growing reliance on robotics in both industrial applications and daily life, innovative actuator research has expanded significantly. Despite advances, traditional actuators encounter limitations in performance and operational range due to inherent physical constraints. To address these challenges, variable transmission mechanisms (VTMs) have emerged over the past decade as one of the alternative solutions, enhancing the adaptability and efficiency of robotic systems. However, there is currently a lack of survey articles that comprehensively cover the mechanisms and working principles of VTMs in robotics. This review article fills this gap by offering an extensive analysis of VTM applications in robotics. It categorizes VTMs based on their mechanisms and principles, presents case studies on both commercial and experimental VTMs, and provides insights into future prospects.

This letter presents a design framework and novel control strategy for a compact coaxial magnetic-gear-based actuation module suitable for small-to-mid-sized mechanical and robotic applications. The proposed actuation module adopts a non-contact magnetic coupling mechanism to transmit rotational power with a predetermined gear ratio, in contrast to traditional mechanical gear-based transmissions. This approach offers several advantages such as enhanced backdrivability, hardware safety, and transparency when compared to conventional contact-based transmissions. Furthermore, the magnetic coupling effect provides a spring-like characteristic that can be utilized to implement a series elastic actuation enabling sensorless torque control. The design of the magnetic gear was optimized using a differential evolution method, and a dynamic model was formulated to specify its dynamic characteristics. Finally, a composite disturbance observer-based torque control algorithm was developed, which capitalizes on the features of the magnetic spring. The proposed control algorithm was validated through several experiments.