Stiffness-Switchable Hydrostatic Transmission Toward Safe Physical Human–Robot Interaction

A lightweight and compliant manipulator design has been considered crucial in safe physical human–robot interaction. Remote actuation relocating the massive parts to the robot base and transmitting power to the distal joint minimizes the actuator inertia and provides series elasticity to the actuator. Rolling diaphragm hydrostatic transmission (RDHT), one of the remote actuation, has recently been studied in physically interacting robots, which can tackle the remaining issues in hydraulic actuation, such as low backdrivability and fluid leakage. However, existing RDHTs are challenging to achieve the desired safety and control performance simultaneously due to their fixed stiffness. This article presents a stiffness-switchable hydrostatic transmission (SwHST) consisting of an RDHT and valve-controlled pneumatic springs. The SwHST has a wide stiffness range of 15–290 N
$\cdot$
m/rad and a fast response in stiffness transition of less than 50 ms without any complex stiffness tuning mechanism. It is one of the most efficient transmissions in remote actuation and stiffness adjustment. Its static friction is less than 0.4% of full-range torque, and the stiffness-switching module consumes only 6 W of power when valves are open. The dynamic characteristics of the SwHST are experimentally scrutinized under various operational conditions. Safety performance is verified in unconstrained and constrained collision tests, demonstrating that the SwHST can effectively mitigate the clamping force of more than 50% for both the tests. Control performance is evaluated on position tracking tests. We foresee the proposed SwHST being utilized in human–robot collaboration without jeopardizing control performance through a rapid and efficient stiffness-switching mechanism.