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Abstract Humans can physically interact with other humans adeptly. Some overground interaction tasks, such as guiding a partner across a room, occur without visual and verbal communication, which suggests that the information exchanges occur through sensing movements and forces. To understand the process of motor communication during overground physical interaction, we hypothesized that humans modulate the mechanical properties of their arms for increased awareness and sensitivity to ongoing interaction. For this, we used an overground interactive robot to guide a human partner across one of three randomly chosen paths while occasionally providing force perturbations to measure the arm stiffness. We observed that the arm stiffness was lower at instants when the robot’s upcoming trajectory was unknown compared to instants when it was predicable - the first evidence of arm stiffness modulation for better motor communication during overground physical interaction. 

Many anticipated physical human-robot interaction (pHRI) applications in the near future are overground tasks such as walking assistance. For investigating the biomechanics of human movement during pHRI, this work presents Ophrie, a novel interactive robot dedicated for physical interaction tasks with a human in overground settings. Unique design requirements for pHRI were considered in implementing the one-arm mobile robot, such as the low output impedance and the ability to apply small interaction forces. The robot can measure the human arm stiffness, an important physical quantity that can reveal human biomechanics during overground pHRI, while the human walks alongside the robot. This robot is anticipated to enable novel pHRI experiments and advance our understanding of intuitive and effective overground pHRI. 

We present a new design method that is tailored for designing a physical interactive robotic arm for overground physical interaction. Designing such robotic arms present various unique requirements that differ from existing robotic arms, which are used for general manipulation, such as being able to generate required forces at every point inside the workspace and/or having low intrinsic mechanical impedance. Our design method identifies these requirements and categorizes them into kinematic and dynamic characteristics of the robot and then ensures that these unique considerations are satisfied in the early design phase. The robot’s capability for use in such tasks is analyzed using mathematical simulations of the designed robot, and discussion of its dynamic characteristics is presented. With our proposed method, the robot arm is ensured to perform various overground interactive tasks with a human.