  Human kinematics is of fundamental importance for rehabilitation and
assistive robotic systems that physically interact with human. The wrist plays
an essential role for dexterous human-robot interaction, but its conventional
kinematic model is oversimplified with intrinsic inaccuracies and its
biomechanical model is too complicated for robotic applications. In this work,
we establish an improved kinematic model of the wrist. In vivo kinematic
behavior of the wrist was investigated through noninvasive marker-less optical
tracking. Data analysis demonstrated the existence of measurable dynamic axes
in carpal rotation, justifying inevitable misalignment between the wrist and
robotic representation if using the conventional wrist model. A novel wrist
kinematic model was then proposed with rigid body transformation in fusion with
a varying prismatic term indicating the dynamic axes location. Accurate and
real-time estimation of this term has been achieved through coupled wrist
angles with nonlinear regression. The proposed model is not only accurate but
also conveniently applicable for translating anatomical behaviors into robotic
implementation and functional assessment for precise and dexterous human-robot
interaction.
