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Kinematic compatibility of a wrist robot with cable differential actuation: effects of misalignment compensation via passive joints
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  • Haider A. Chishty ,
  • Andrea Zonnino ,
  • Andria J. Farrens ,
  • Fabrizio Sergi
Haider A. Chishty
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Andrea Zonnino
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Andria J. Farrens
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Fabrizio Sergi
University of Delaware

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We present the UDiffWrist (UDW), a low-impedance 2-DOF wrist exoskeleton featuring a cable-differential transmission. To investigate the effect of different design strategies for achieving kinematic compatibility, we developed two versions of this robot: One version (UDW-C) achieves kinematic compatibility only in the case of perfect alignment between human and robot joints. The second version (UDW-NC) connects the human and robot via passive joints to achieve kinematic compatibility regardless of alignment between human and robot joints. Through characterization experiments, we found that the UDW-NC was more robust to misalignments than the UDW-C: the increase in maximum interaction torque associated with misalignments was greater for the UDW-C than the UDW-NC robot (p = 0.003). However, the UDW-NC displayed greater Coulomb friction (p < 0.001). Further, Coulomb friction increased more for the UDW-NC than the UDW-C in the presence of misalignments between the human and robot axes (p < 0.001). We also found that torque transfer was more accurate in the UDW-C than in the UDW-NC. These results suggest that for the small (10 deg) 2-DOF wrist movements considered, the advantages of the UDW-NC in terms of kinematic compatibility are likely overshadowed by the negative effects in friction and torque transfer accuracy.
Nov 2021Published in IEEE Transactions on Medical Robotics and Bionics volume 3 issue 4 on pages 970-979. 10.1109/TMRB.2021.3123528