Human and Passive Lower-Limb Exoskeleton Interaction Analysis: Computational Study with Dynamics Simulation using Nonlinear Model Predictive Control

Forward dynamics simulations have the advantage of assessing performance of novel exoskeleton designs at a low cost. For developing a new passive lower-limb exoskeleton, the simulation needs to represent the sitting posture in which the wearer performs working tasks while maintaining balance with the whole body. The present study constructed a forward dynamics simulation for analyzing and developing a new passive lower-limb exoskeleton; the validity of the simulation was investigated using experimental data. The present method computes the interactions between the exoskeleton and wearer, such as reaction forces, physical posture, and physical load, based on the forward dynamics simulation driven by nonlinear model predictive control (NMPC). The NMPC cost function consisted of the physical load and the fitness of working task with constraints to evaluate balance. As a result, the present simulation represented the characteristic posture when sitting on the exoskeleton in which the wearer performs the working task while maintaining balance with the whole body. However, the simulation computed an upright posture of the lumbar joint that differed from the experimental results and needs to be improved. In future work, the simulation will be modified for representing the valid physical posture when wearing the exoskeleton, such as simulating the physical motion of the same working task as in the experiment, and modeling the interaction between the human, exoskeleton, and ground.