Examination of robotic manipulability indices to evaluate upper limb manipulability in digital human models

Robotic manipulability indices (RMIs) quantitatively evaluate kinematic factors regarding manipulability and can be applied to manipulability evaluations in digital human models (DHMs). However, in the kinematics of human motion based on Euler's angles, the rotational transformation order around three orthogonal coordinate axes is arbitrary, meaning that the coordinate transformation at a shoulder joint is not unique. Because RMIs are calculated from the coordinate transformation matrix, the RMIs of an upper limb may be influenced by this arbitrariness. This study examined the effects of the rotation order of the coordinate transformation at a shoulder joint on the manipulability evaluation of an upper limb. The results indicate that RMIs depend on rotation order. Thus, a modified method that eliminates the influence of rotation order was subsequently proposed. Experimental results indicate that RMIs calculated via this proposed modified method are rotation order independent, verifying that it contributes to manipulability evaluation in DHMs.