Modeling and precision force control of a waist assistive exoskeleton based on holonomic constraints

Waist assistive exoskeleton is a wearable robot used to alleviate muscle fatigue and prevent lower back injury for workers during repetitive load lifting. Multi-joint human-robot coupling, the effect of human control and the model uncertainties like load variations make the robust controller design of waist assistive exoskeleton become challenged. Most existed control methods are based on a simplified dynamic model and neglect model uncertainties, which leads to a limited control performance. This paper focuses on the dynamic modeling and high-performance force control of waist assistive exoskeleton. In order to obtain a dynamic model which is accurate as well as suitable for controller design, a 5-DOF human-robot rigid body dynamics is established first. Then holonomic constraints are proposed to describe the control effect of the wearer, which helps convert the 5-DOF dynamics into a 1-DOF dynamics. Based on the established 1-DOF dynamics, adaptive robust force control strategy is proposed to effectively address various model uncertainties and disturbances. Comparative simulations and experiments indicate that the proposed control method can realize accurate and robust force control performance under different loads.