On the Passive Virtual Viscous Element Injection Method for Elastic Joint Robots

Increasing the viscosity of elastic joints can significantly improve the performance of elastic joint robots during physical human-robot interactions. However, current approaches for injecting viscous elements require an additional damper to be added in parallel with the elastic elements. In this article, we propose a new concept called virtual viscous element injection (VVI), which enables a robot to exhibit viscoelasticity without altering its mechanical structure. VVI relies only on motor-side dynamics reshaping and state feedback. Interestingly, the VVI method allows high-resolution joint torque measurements in elastic joint robots, unlike in physical viscoelastic joint robots, which measure joint torque using higher-order derivatives of the positions. Furthermore, the VVI method is proved to preserve the passivity of robot dynamics, which provides numerous possibilities for the applications of combined passivity-based controllers. Specifically, we first emphasize the impedance control method using VVI. The results demonstrate that the VVI-DF method, which combines the direct feedback method with VVI, addresses the issue of excessive acceleration feedback in the controller. This provides looser constraints for achieving a high-gain torque loop in impedance control. Moreover, this article also provides examples of the application of VVI combined with passivity-based position and torque controllers. Experiments and simulations demonstrate the effectiveness of the proposed methods. The proposed method can be extended to various robots, such as exoskeletons, and collaborative robots.