Period Motion Tracking of Lower Limb Exoskeleton: A New Time-Independent Control Approach

This article introduces the planar vector field generator (PVFG) for a lower limb exoskeleton to achieve stable and adaptive periodic motion. Unlike traditional methods, which often rely on predefined gait trajectories, the PVFG-based control method leverages the natural dynamics of the system in its phase space, decouples the control from temporal constraints, and tracks the target periodic motion in the form of limit cycles. This article presents the theoretical framework of PVFG, and proposes the PVFG-based time-independent controller (PVFG-TIC) for the lower limb exoskeleton by integrating a master–slave phase coordination strategy. The stability of the proposed controller is proved through extended Lyapunov stability theory. The proposed control method improves upon existing limit cycle-based approaches by imposing fewer constraints on the target limit cycles. Experimental results on different types of exoskeletons validate the effectiveness of the proposed controller, which offers time independence, phase coordination, stability in wide range, and adaptability to switching target periodic motion, highlighting the potential of PVFG in the control of lower limb exoskeleton.