History, Present Situation and Prospect of Research on Leg Configuration of Humanoid Robot

The motion performance of humanoid robots has not yet fully reached the level of human beings, which is one of the factors hindering their large-scale industrial application. This limitation stems not only from constraints in control algorithms but also from mechanical structure design, particularly the leg configuration, which largely determines a robot’s dynamic balance, load capacity, and energy efficiency. The study examined the origins and evolution of leg configurations in humanoid robots, both domestically and internationally. Currently, the leg configurations of humanoid robots are primarily categorized into three types: serial, parallel, and hybrid serial-parallel. Their structural characteristics directly influence locomotion performance. The study compared the serial, the parallel and the series-parallel configurations and their performance characteristics. The serial configuration offers a large workspace and high flexibility, but its relatively lower stiffness—due to the extended joint transmission chain—compromises its load capacity. The parallel configuration provides high rigidity and fast dynamic response, yet its range of motion is limited. The hybrid serial-parallel design combines the strengths of both, achieving balanced stiffness and flexibility, which has increasingly made it a key research focus in recent years. Finally, this paper also discussed technical difficulties and hot spots in the study of leg configuration and pointed out the development trend: the leg configuration is developing from single series configuration to parallel and series-parallel configuration, from rigid actuator to elastic actuator and quasi direct drive actuator, from torque control to hybrid force-position control.