Optimal Design of a Novel 3-DOF Orientational Parallel Mechanism for Pelvic Assistance on a Wheelchair: An Approach Based on Kinematic Geometry and Screw Theory

Pelvis mobility is essential to the daily seated activities of wheelchair users, however it is not yet fully addressed in the field of active wearable devices. This letter presents a novel design and optimization methodology of an in-parallel actuated robotic brace for assisting the human pelvis during seated maneuvers on wheelchair. This design uses human data captured by cameras in conjunction with the knowledge of kinematic geometry and screw theory. The mechanism has full rotational three degrees-of-freedom (DOFs) and also accommodates coupled translation of the human pelvic segment. This type of motion was realized by employing three kinematic limbs that impose non-intersecting zero-pitch constraint wrenches on the platform. Our multi-objective optimization (MOO) routine consists of two stages: (I) platform constraint synthesis, where the geometric parameters of the limb constraints were determined to minimize the pelvis-platform errors of trajectories and instantaneous screw axes (ISAs); and (II) limb structural synthesis, where limb types and dimensions, workspace, transmission performances, singularities, and actuated joint displacements were considered. The optimized mechanism has an asymmetrical [ R RR]U-2[ R R]S architecture. This mechanism can also be integrated into our previously developed Wheelchair Robot for Active Postural Support (WRAPS).