Design and Control of a Novel Variable Stiffness Series Elastic Actuator

This article expounds the design and control of a new variable stiffness series elastic actuator (VSSEA). It is established by employing a modular mechanical design approach that allows us to effectively optimize the stiffness modulation characteristics and power density of the actuator. The proposed VSSEA possesses the following features: no limitation in the work range of output link; a wide range of stiffness modulation (∼20 N·m/rad to ∼1 KN·m/rad); low-energy-cost stiffness modulation at equilibrium and nonequilibrium positions; compact design and high torque density (∼36 N·m/kg); and high-speed stiffness modulation (∼3000 N·m/rad/s). Such features can help boost the safety and performance of many advanced robotic systems, e.g., a cobot that physically interacts with unstructured environments and an exoskeleton that provides physical assistance to human users. These features can also enable us to utilize variable stiffness property to attain various regulation and trajectory tracking control tasks only by employing conventional controllers, eliminating the need for synthesizing complex motion control systems in compliant actuation. To this end, it is experimentally demonstrated that the proposed VSSEA is capable of precisely tracking the desired position and force control references through the use of the conventional proportional–integral–derivative controllers.