The complexity of the human wrist hampers the kinematic compatibility achievable when interacting with a wrist exoskeleton. Past studies have shown that the implementation of passive degrees of freedom (DOFs) between the human and robotic kinematic chain may compensate for any joint misalignments between user and robot. However, the additional inertia and friction associated with these new DOFs affect the transparency of the device. As such, there is a need to investigate the trade-off between two critical objectives of wrist exoskeleton design: ensuring low endpoint impedance; and minimizing the effects of misalignments.
To investigate this trade-off, we developed a low-impedance 2-DOF wrist exoskeleton featuring a cable-differential transmission: the UDiffWrist (UDW). We developed two versions of the robot to investigate how the implementation of passive DOFs affect system transparency, torque transfer to the wrist, and system robustness to misalignments. The UDiffWrist-Colocated (UDW-C) places the user’s wrist inside the transmission and assumes perfect alignment between user and robot; while the UDiffWrist-NonColocated (UDW-NC) forgoes this assumption, and implements a compensation mechanisms comprised of passive joints.
Through dynamic characterization, we have concluded that while the UDW-NC is significantly more robust to misalignments, it is significantly less transparent than the UDW-C. Further, the torque transmission capability of the UDW-NC is significantly worse than the UDW-C. This suggests that while the use of passive degrees of freedom as a mechanism to compensate for misalignments is valid, the benefits can be outweighed by the negative affects associated with their inertia and friction. We have thus concluded that for small wrist movements (~10 degree movements in the flexion/extension and radial/ulnar deviation axes) it is not necessary to compensate for misalignments in our 2-DOF wrist exoskeleton, and we can instead assume alignment.
This work has been conditionally accepted for publication in IEEE Transactions in Medical Robotics and Bionics.