Hierarchical Frequency-Based Energy Separation in Variable Structure Control for Robot-Mediated Human–Human Interaction

The concept of “variable structure passivity control (VSPC)” has been proposed as an umbrella terminology for a family of stabilizers designed for a wide range of physical human-robot interactions. In this article, a new variant of VSPC is proposed for stabilizing (tele)robot-mediated human–human interaction taking into account the frequency composition of interactional energy. It should be noted that low-frequency haptic information during human–human telerobotic interaction contains imperative information regarding desired and voluntary haptics-based communicative behavior. The proposed adaptive nonlinear stabilization scheme is named frequency-separation VSPC (FS-VSPC). The goal is to reduce the conservatism of haptic rendering at the lower frequencies while maintaining minimal stability at full spectrum through energy decomposition followed by a hierarchical dissipation of the highest to lowest frequency energy exchange between human biomechanics and the robot. The controller also takes into account the varying dynamics of human biomechanics to adaptively and on-the-fly modify the force, further enhancing the performance, transparency, and stability. The proposed adaptive iterative stabilization scheme is evaluated through a systematic grid simulation in addition to rigorous experimental validation in the presence of a wide range of variable time delays of the network connecting the two robots. The comparative study shows that the proposed stabilizer improves the force reflection ratio by 22.2% and linear force tracking by 11.6% while prioritizing low-frequency force content over high-frequency force content compared with existing methods.