A two-speed actuator for robotics with fast seamless gear shifting
Alexandre Girard, H. Harry Asada
TL;DR
This work tackles the conflicting torque–speed demands in robotics by proposing a Dual-Speed Dual-Motor (DSDM) actuator that combines a direct-drive motor with a brake and a high-reduction motor through a 3-port planetary junction. The system operates in two modes: high-force (brake closed) and high-speed (brake open), while exploiting input redundancy via a nullspace-based control framework to ensure seamless transitions even under unknown environmental loads. A lumped dynamic model captures the coupled dynamics, and dedicated controllers enable fast, stable gear-shift transitions and synergistic motor coordination. Experimental validation on a linear actuator prototype demonstrates precise high-force step responses, high-speed motion with impedance control, and smooth mode transitions, highlighting improvements in power density, efficiency, and controllability across broad torque–speed ranges.
Abstract
This paper present a novel dual-speed actuator adapted to robotics. In many applications, robots have to bear large loads while moving slowly and also have to move quickly through the air with almost no load. This lead to conflicting requirements for their actuators. Multiple gear ratios address this issue by allowing an effective use of power over a wide range of torque-speed load conditions. Furthermore, very different gear ratios also lead to drastic changes of the intrinsic impedance, enabling a non-back-drivable mode for stiff position control and a back-drivable mode for force control. The proposed actuator consists of two electric motors coupled to a differential; one has a large gear ratio while the other is almost direct-drive and equipped with a brake. During the high-force mode the brake is locked, only one motor is used, and the actuator behaves like a regular highly-geared servo-motor. During the high-speed mode the brake is open, both motor are used at the same time, and the actuator behaves like a direct drive motor. A dynamic model is developed and novel controllers are proposed for synergic use of both motors. The redundancy of motors is exploited for maintaining full control of the output during mode transitions, allowing for fast and seamless switching even when interacting with unknown environments. Results are demonstrated with a proof-of-concept linear actuator.