Force Tracking Control With Adaptive Stiffness and Iterative Position of Hip-Assistive Soft Exosuits

Soft exosuits feature nonlinear, low stiffness, and hysteretic behavior, presenting different mechanical responses during loading and unloading of assistive force. Delivering the desired force to the wearer is a challenge in such a system. To address this issue, this article proposed a novel control strategy with adaptive stiffness and iterative position to optimize the assistive forces of the actuators in compliance with the humanexosuit interface stiffness and relative motion. Specifically, a stiffness model was proposed to describe the force loading and unloading behaviors, further an adaptive stiffness-based force controller was designed to improve the force tracking for whole profile by adaptively adjusting the stiffness parameters related to loading and unloading, and iteratively compensating the position error associated with the force amplitude in the model on a step-by-step basis. This control strategy was implemented on a soft exosuit for hip flexion and extension assistance, and its performance was evaluated through walking tests on six subjects. The results showed that after 4 or 5 iterations of optimization based on the initial parameter settings, the proposed controller could comply with the human-exosuit interface stiffness and achieved an improved force tracking, with a minimal root-mean-square error of 7.5 N in desired force profiles tracking, and a metabolic gain of 14.8%, demonstrating a promising potential of the proposed controller for improving the force tracking and the economy of human walking. Note to Practitioners—Soft exosuits have shown promising outcomes in augmenting human walking and reducing fatigue. However, delivering the desired assistive force accurately to the wearer remains challenging due to the nonlinear and variable stiffness nature of the human-exosuit interaction during walking. This article proposed a control strategy for improving the force tracking performance by optimizing the force-positional relationship of the actuators. Specifically, a stiffness model was established to define the relationship between the force and position of the actuators. This model acts as a virtual spring between the wearer and the exosuit, and stiffness of the spring was adjusted to adapt the wearer. On the Basis of this model, we developed a force tracking controller with adaptive stiffness. It can transmit accurately a desired force trajectory to the wearer by adaptively optimizing the stiffness parameters and iteratively compensating the position error in stiffness model. This controller was implemented on a soft exosuit for hip flexion and extension assistance. The results of treadmill and outdoor walking tests with six subjects confirm the significant effect on optimizing the force-positional relationship, improving the force tracking performance, as well as avoiding force hystereses or friction losses. This research addresses the challenge of force control in human-exosuit interaction with variable stiffness characteristics. The limitation of this work is that it only focuses on hip joint assistance. In the future, we hope to extend this control strategy to other joints to achieve the generality of the approach.