Dual-mode variable stiffness actuator using two-stage worm gear transmission for safe robotic manipulators

In this paper, we propose a new type of active variable stiffness actuator (VSA) by using a unique differential worm gear mechanism where a set of compressible coil springs are installed. The stiffness of the VSA is varied depending on the compressed amount of the coil springs; large compression results in an effectively stiff joint, and vice versa (compliant mode). Interestingly, an ideal rigid joint is also realizable if the springs’ reactive force is made larger than the external load (rigid mode). The VSA is characterized to become compliant rapidly if the external load exceeds the springs’ reactive force, so such a VSA can protect itself in an unexpected overloading condition. Conducted experiments confirm the effectiveness of the proposed VSA.