Human Arm Stability in Relation to Damping-Defined Mechanical Environments in Physical Interaction with a Robotic Arm

This paper presents an experimental study that investigated how humans interact with viscous, damping-defined mechanical environments and quantified the lower bounds of robotic damping that they can stably interact with. Human subjects performed posture maintenance tasks for different arm postures while holding a robotic arm manipulator simulating unstable (negative) damping-defined environments and applying rapid perturbations to disturb the arm posture and challenge arm stability. The results of this study demonstrated that the lower bound of robotic damping for stable physical human-robot interaction was more than twice as low in the anterior-posterior (AP) direction than the medial-lateral (ML) direction, with lower limits of -50.3 Ns/m and -21.6 Ns/m in the AP and ML directions, respectively. The results further showed that the human arm is less capable of adjusting to the unstable environments when it is close to the body and laterally displaced for the AP and ML directions, respectively. Secondary analysis on the kinematic response in the phase space also demonstrated that arm stability in the unstable environments can be more easily achieved in the AP than ML direction. The outcomes of this study can be used to design less conservative robotic impedance or admittance controllers that utilize a wider range of robotic damping up to a certain extent of negative damping but do not compromise coupled stability of the human-robot system, which could improve the overall performance in physical human-robot interaction by achieving more agile operations and reducing user effort.