Improving Stability In Physical Human-Robot Interaction By Estimating Human Hand Stiffness And A Vibration Index

Purpose The purpose of this paper is to eliminate instability which may occur when a human stiffens his arms in physical human-robot interaction by estimating the human hand stiffness and presenting a modified vibration index.Design/methodology/approach Human hand stiffness is first estimated in real time as a prior indicator of instability by capturing the arm configuration and modeling the level of muscle co-contraction in the human's arms. A time-domain vibration index based on the interaction force is then modified to reduce the delay in instability detection. The instability is confirmed when the vibration index exceeds a given threshold. The virtual damping coefficient in admittance controller is adjusted accordingly to ensure stability in physical human-robot interaction.Findings By estimating the human hand stiffness and modifying the vibration index, the instability which may occur in stiff environment in physical human-robot interaction is detected and eliminated, and the time delay is reduced. The experimental results demonstrate significant improvement in stabilizing the system when the human operator stiffens his arms.Originality/value The originality is in estimating the human hand stiffness online as a prior indicator of instability by capturing the arm configuration and modeling the level of muscle co-contraction in the human's arms. A modification of the vibration index is also an originality to reduce the time delay of instability detection.