Rigid–Flexible Coupling Design and Variable Stiffness Control of Drive Mechanism Components for Ostrich-Inspired Robots

Ostriches are the fastest bipedal terrestrial animals. Their hip, knee, and ankle joints possess inherent shock-absorbing capabilities, endowing their legs with excellent compliance. This biomechanical feature ensures both stability and high energy efficiency during high-speed locomotion. For ostrich-inspired robots, designing the leg drive mechanism and implementing compliance control are crucial for achieving biomimetic locomotion. Therefore, this study focuses on the knee joint drive mechanism of an ostrich-inspired robot as a representative case. First, a rigid–flexible coupled structural design is developed. Second, to realize compliance in this mechanism, the stiffness characteristics of its passive compliant element are analyzed. Furthermore, a hybrid active–passive variable stiffness control strategy is proposed, and the series–parallel stiffness composition mechanism is examined. Finally, the proposed active–passive variable stiffness control strategy was implemented on the knee joint drive mechanism and experimentally evaluated. The results demonstrate that this control strategy enables real-time stiffness modulation of the drive mechanism, achieving a stiffness variation range of 6.6–1586.2 Nm/rad.