Wearable robotic assistive devices possess the potential to improve the metabolic efficiency of human locomotion. Developing exoskeletons that can reduce the metabolic cost of assisted subjects is challenging, since a systematic design approach is required to capture the effects of device dynamics and the assistance torques on human performance. Conducting such investigations through human subject experiments with physical devices is generally infeasible.

On the other hand, design studies that rely on musculoskeletal models hold high promise in providing effective design guidelines, as the effect of various devices and different assistance torque profiles on muscle recruitment and metabolic cost can be studied systematically.

In this paper, we present a simulation-based multi-criteria design approach to systematically study the effect of different device kinematics and corresponding optimal assistive torque profiles under actuator saturation on the metabolic cost, muscle activation, and joint reaction forces of subjects walking under different loading conditions. For the multi-criteria comparison of mono-articular and bi-articular exoskeletons, we introduce a Pareto optimization approach to simultaneously optimize the exoskeleton power consumption and the human metabolic rate reduction during walking, under different loading conditions. We further superpose the effects of device inertia and electrical regeneration on the metabolic rate and power consumption, respectively.

Our simulation results explain the effects of heavy loads on the optimal assistance profiles of the exoskeletons and provide guidelines on choosing optimal device configurations under actuator torque limitations, device inertia, and regeneration effects.

The multi-criteria comparison of devices indicates that despite the similar assistance levels that can be provided by both types of exoskeletons, mono-articular exoskeletons demonstrate better performance on reducing the peak reaction forces, while the power consumption of bi-articular exoskeletons is less sensitive to the loading. Furthermore, for the bi-articular exoskeletons, the device inertia has lower detrimental effects on the metabolic cost of subjects and does not affect the Pareto-optimality of solutions, while non-dominated configurations are significantly affected by the device inertia for the mono-articular exoskeletons.

Bonab, A.K. and Patoglu, V., 2021. Simulation-based multi-criteria comparison of mono-articular and bi-articular exoskeletons during walking with and without load. arXiv preprint arXiv:2110.00062.