A biologically inspired motion control method is introduced to ameliorate the flexibility and multijoint autonomy of assistive walking devices based on human-robot interactions (HRIs). A new HRI-based coordination control system consisting of a hip central pattern generator (CPG) control, a knee hierarchical impedance control, and a hip-knee linkage control is also investigated. Simulations and walking experiments are carried out which demonstrate that (i) the self-oscillation and external communication characteristics of the CPG are capable of realizing ideal master/slave hip joint trajectories. In addition, symmetrical inhibition in the CPG unit is essential for maintaining the antiphase motion of the left and right hip joints. (ii) High and low hierarchical impedance control laws allow appropriate knee joint torque to be calculated to maintain posture during the support and swing phases as walking proceeds. (iii) A hip-knee joint linkage mechanism which incorporates a hip joint CPG control and knee joint impedance control allows natural and relevant hip-knee trajectories to be realized. The stability of the HRI-based coordination control method is also confirmed using Lyapunov stability theory.
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