When designing a robot for human-safety during direct physical interaction, one approach is to size the robot’s actuators to be physically incapable of exerting damaging impulses, even during a controller failure. Merely lifting the arms against their own weight may consume the entire available torque budget, preventing the rapid and expressive movement required for anthropomorphic robots. To mitigate this problem, gravity-counterbalancing of the arms is a common tactic; however, most designs adopt a shoulder singularity configuration which, while favorable for simple counterbalance design, has a range of motion better suited for industrial robot arms. In this paper, we present a shoulder design using a novel differential mechanism to counterbalance the arm while preserving an anthropomorphically favorable singularity configuration and natural range-of-motion. Furthermore, because the motors driving the shoulder are completely grounded, counterbalance masses or springs are easily placed away from the shoulder and low in the torso, improving mass distribution and balance. A robot arm using this design is constructed and evaluated for counterbalance efficacy and backdrivability under closed-loop force control.
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