Before revision:  In this study, we extend the one-port passivity analyzes provided in the literature and provide the necessary and sufficient condition for two-port passivity of SEA under velocity-sourced impedance control  . Based on the newly established conditions, we derive non-conservative passivity bounds for a virtual coupler .
After revision:  In this study, we provide the necessary and sufficient condition for two-port passivity of SEA under velocity-sourced impedance control . Based on the newly established conditions, we derive non-conservative passivity bounds for a virtual coupler .