Abstract：A compliant robotic arm with passive compliant mechanical structure and active compliant control, SoftArm II, is designed to cope with the dynamically-changing work environments and uncertain human-robot interactions. Series elastic actuators (SEA) are introduced between joint motors and links as transmission module, which is composed of linear springs evenly arranged circumferentially. Firstly, the kinematic model, dynamic model and system stiffness model are set up for the compliant 3DOF robotic arm. Then, SEA spring stiffness can be determined using weighted average joint stiffness at several representative poses in the workspace based on the system stiffness model. Position PID (proportional-integral-derivative) control is employed in the robotic arm, while contact forces at the arm's end-point and joint torques are monitored to modify the planned trajectory adaptively. Experiments of the circle trajectory tracking in free workspace, human and the robot pushing along a straight line, and collision imitating, are carried out using the compliant robotic arm SoftArm II. The results show that SoftArm II reaches a good position tracking accuracy in free workspace, compliant interaction between the robotic arm and human is achieved, and collision of the robotic arm is safely avoided. The results indicate that the SEA-based passive compliant mechanical structure and the control approach based on end force and joint torque monitoring have the potential to meet the compliance and safety requirements of a robotic arm used in human-robot coexistence environment.
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