Cost function evaluation for optimizing design and actuation of an active exoskeleton to ergonomically assist lifting motions

Abstract

Spinal exoskeletons can reduce the risk of low-back pain by decreasing the back muscle activity and the spinal compression forces of users during heavy lifting tasks. Model-based simulation and optimization are helpful tools to support the design of exoskeletons reducing the number of prototypes iterations and testings. In this paper, we present a comparison of different cost functions based on modeling and optimization techniques to determine proper actuator characteristics for an active spinal exoskeleton supporting stoop-lifts of a 10 kg box. Using motion recordings of five different subjects and additional anthropometric measurements, we created subject-specific musculoskeletal models. A corresponding parameterized model of an exoskeleton with passive and active components was created and combined with the human models. The spring characteristics and the torque profiles of the exoskeleton are optimized for various objectives which consist of a term for tracking the recordings and an additional term from a set of cost functions to reduce the load on the subjects. User comfort is guaranteed by appropriate interaction force limits. The results show that all cost functions reduced significantly the human torque loads. However, they result in different amounts and distributions of the load reduction as well as different contributions from the passive and active components of the exoskeleton. They also yield different actuation patterns of the human model performing the stoop-lift. The analysis of the effects of cost functions in this study highlights the importance of selecting an appropriate cost function for optimization.