Kinematic compatibility of a wrist robot with cable differential
actuation: effects of misalignment compensation via passive joints
Abstract
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.