Method for Determining Displacement and Characterizing Spatial Kinematic Misalignment in a Rigid Exoskeleton

With the increasing use of exoskeletons to reduce physical strain in industrial applications, precise adaptation to the user’s anthropometry is crucial for effective force transmission and user acceptance. This paper presents a method that compensates for axial misalignment between the human joint and the exoskeleton’s axes of rotation to optimize the anthropometric alignment. It further quantifies the resulting displacement, providing instructional recommendations for manual refinement of the exoskeleton’s initial kinematic configuration. The method thereby represents a first step toward a comprehensive investigation of the initial offset’s influence on an anthropometric fit. The mathematical derivation was described using the rigid shoulder exoskeleton “Lucy”. The validation on increasingly complex mock-ups showed an average calculated error of 0.84mm (SD 0.24mm) in 2D and 7.97mm (SD 1.30mm) in 3D, where the errors decreased with smaller initial offsets. A preliminary field study with three participants revealed improved anthropometric alignment but indicated limitations in the exoskeleton’s structural adjustment possibilities, highlighting the need for further modifications. Building on these findings, subsequent studies will involve further investigation of factors such as the migration of the instantaneous center of rotation during motion, soft tissue deformations, and greater population diversity.