The Effects of the Connection Stiffness of Robotic Exoskeletons on the Gait Quality and Comfort

An increase in the interaction forces/torques at the exoskeleton-human connections due to kinematic mismatches may consequently result in the user discomfort and/or lower performance of the exoskeleton. The stiffness of the exoskeleton-human connection elements plays a key role in this issue. The purpose of this study is to assess the effects of the exoskeleton-human connection stiffness on the user comfort and limb tracking error during normal gait. A biomechanical model of the leg was built and connected to a model of an exoskeleton by elastic connections whose stiffness tensors were identified experimentally for three participants. The effects of the connection stiffness on the gait performance were investigated using two indices: discomfort index (DI) and tracking error index (TEI), which is the difference between the human joint angle and that of the exoskeleton. DI was calculated based on the mechanical energy stored in the elastic connection elements. Although an increase of the stiffness magnitude in every direction results in DI growth, the torsional stiffness of the shank connection was found to be the most sensitive for DI. TEI, on the other hand, showed both increasing and decreasing trends with the stiffness increase in different directions. It was found that there are optimal values for some connection stiffness elements, especially shank connection, possibly due to intricacy of the knee joint, that would improve both DI and TEI. For instance, the decrease of the torsional and mediolateral shank connection stiffness improved DI and TEI by an average of about 50%.