In this paper, a control strategy is proposed to improve the tracking performance of the lower limb exoskeleton (LLE). The proposed active disturbance rejection control (ADRC) with fast terminal sliding mode control (FTSMC) can not only alleviate the disturbance but also converge to a bounded region fast. Based on the robotics analysis, a dynamic model for the LLE was established. To achieve decoupling control for a coupled system, the virtual control input was introduced, where the system uncertainty and external disturbances were regarded as lumped disturbances. To validate the feasibility of the proposed control strategy, the simulations and experiments were both carried out. The numerical simulation results were shown that the proposed control strategy and ADRC can remarkably reduce the chattering phenomena, which is owing to the estimation ability of extended state observer (ESO). Both the simulations and the experiments results were shown that this strategy was better than the conventional proportional-integralderivative (PID) and ADRC in terms of tracking performance. With the proposed ADRC-FTSMC, the LLE system can achieve higher tracking precision and faster response.INDEX TERMS Lower limb exoskeleton, active disturbance rejection, fast terminal sliding mode control, human gait tracking, finite-time convergence.

This paper describes a system design and hybrid control algorithm of an electrically actuated lower limb exoskeleton (LLE). The system design mainly includes three parts: mechanical structure design, actuation system design and sensor system design. According to the initial state of the joint angle, LLE can be divided into Non-anthropomorphic state (NAS) and anthropomorphic state (AS). The human motion intention (HMI) estimation can be divided into gait phase classification and reference trajectory estimation. The fuzzy logic is used to detect different phases in the gait phase classification. In the reference trajectory estimation, the kinematic model of the LLE is utilized to obtain a continuous joint trajectory, which is used as input of the control law. To make the LLE accurately follow the movement of people and remain stable, a hybrid dual-mode control strategy is proposed in this paper, i.e., the adaptive impedance control (AIC) method is used to improve the stability and resistance to shock in stance phase, and the active disturbance rejection control with the fast terminal sliding mode control (ADRC-FTSMC) method is employed to improve the response speed and the tracking precision in swing phase. Furthermore, in order to solve the torque discontinuity in the switching process, a smoothing method is proposed during the transition. Finally, the prototype experiments were set up to verify the tracking performance and power-assisted effect of the proposed exoskeleton. The experiments results show the LLE can achieve excellent tracking performance and power-assisted effect based on the proposed HMI methodology and hybrid dual-mode control strategy. INDEX TERMS Lower limb exoskeleton, system design, human motion intention, hybrid dual-mode control, tracking performance, power-assisted effect.