Controlling the interaction forces between a human and an exoskeleton is crucial for providing transparency or adjusting assistance or resistance levels. However, it is an open problem to control the interaction forces of lower-limb exoskeletons designed for unrestricted overground walking. For these types of exoskeletons, it is challenging to implement force/torque sensors at every contact between the user and the exoskeleton for direct force measurement. Moreover, it is important to compensate for the exoskeleton's whole-body gravitational and dynamical forces, especially for heavy lower-limb exoskeletons. Previous works either simplified the dynamic model by treating the legs as independent double pendulums, or they did not close the loop with interaction force feedback. The proposed whole-exoskeleton closed-loop compensation (WECC) method calculates the interaction torques during the complete gait cycle by using whole-body dynamics and joint torque measurements on a hip-knee exoskeleton. Furthermore, it uses a constrained optimization scheme to track desired interaction torques in a closed loop while considering physical and safety constraints. We evaluated the haptic transparency and dynamic interaction torque tracking of WECC control on three subjects. We also compared the performance of WECC with a controller based on a simplified dynamic model and a passive version of the exoskeleton. The WECC controller results in a consistently low absolute interaction torque error during the whole gait cycle for both zero and nonzero desired interaction torques. In contrast, the simplified controller yields poor performance in tracking desired interaction torques during the stance phase.

翻译：控制人体与外骨骼之间的交互力对于实现力觉透明性或调节助力/阻力水平至关重要。然而，针对无约束地面行走的下肢外骨骼，其交互力控制仍是一个未解决的难题。此类外骨骼在用户与设备的每个接触点安装力/力矩传感器进行直接力测量存在技术挑战。此外，补偿外骨骼全身重力与动力学效应（特别是重型下肢外骨骼）至关重要。现有研究或采用简化动力学模型（将腿部视为独立双摆），或未构成交互力反馈闭环。本文提出的全身外骨骼闭环补偿（WECC）方法，基于髋-膝外骨骼的全身动力学与关节力矩测量，计算完整步态周期内的交互力矩。该方法进一步采用约束优化方案，在考虑物理约束与安全边界的同时，实现期望交互力矩的闭环跟踪。我们通过三名受试者实验评估了WECC控制的力觉透明性与动态交互力矩跟踪性能，并与基于简化动力学模型的控制器及被动状态外骨骼进行对比。结果表明：在零与非零期望交互力矩条件下，WECC控制器在完整步态周期中均保持低绝对交互力矩误差；而简化控制器在支撑相跟踪期望交互力矩时表现较差。