In this article, a nonlinear disturbance observer based flexible-boundary prescribed performance controller (NDOPPC) is proposed to achieve outstanding transient and steady-state tracking performance for a lower limb exoskeleton. To estimate and compensate the lumped disturbance consisting of unmodelled dynamics and interaction torques applied by the wearer, a nonlinear disturbance observer (NDO) is utilised. The prescribed performance of the trajectory tracking of the exoskeleton is guaranteed by constraining the tracking error between the preset upper and lower boundaries. The boundaries of the proposed NDOPPC can be asymmetric functions in various forms, which increases the flexibility of the prescribed performance. The proposed NDOPPC is designed based on backstepping and the stability is analysed with Lyapunov theorem. Comparative simulations have been conducted with MATLAB/Simulink. Compared to computed torque controller (CTC) and nonlinear disturbance observer based computed torque controller (NDOCTC), the obtained results of comparative simulations demonstrate the effectiveness and superiority of NDOPPC. Finally, different types of boundaries are considered to demonstrate the flexibility of the proposed NDOPPC.

在本文中，提出了一种基于非线性扰动观测器的柔性边界指定性能控制器（NDOPPC），以实现下肢外骨骼出色的瞬态和稳态跟踪性能。为了估计和补偿由佩戴者施加的未建模动力学和相互作用扭矩组成的集中扰动，使用了非线性扰动观测器 (NDO)。通过约束预设的上下边界之间的跟踪误差来保证外骨骼轨迹跟踪的规定性能。所提出的NDOPPC的边界可以是各种形式的非对称函数，这增加了规定性能的灵活性。所提出的 NDOPPC 是基于反推设计的，并用李雅普诺夫定理分析了稳定性。已经使用 MATLAB/Simulink 进行了比较仿真。与计算转矩控制器 (CTC) 和基于非线性干扰观测器的计算转矩控制器 (NDOCTC) 相比，所获得的比较仿真结果证明了 NDOPPC 的有效性和优越性。最后，考虑了不同类型的边界来证明提议的 NDOPPC 的灵活性。