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 paper, 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 Variable Structure Passivity Control (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 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 to existing methods.