Gait selection and analysis of biped robots on different terrains under unified control

Bipedal locomotion is influenced by knee structure, swing strategy, and terrain conditions, but these factors are often studied separately, making direct comparisons difficult. This paper presents a unified planar X-shape biped model that incorporates two representative knee structure; human-like and avian-like configurations, as well as two swing strategies: reciprocating X (ReX) gait and rotary X (RoX) gait. Under a unified control framework, four gait modes are generated and evaluated on uphill, flat, and downhill terrains. Their performance is systematically compared in terms of gait generation feasibility, specific resistance, and impact energy loss. The results show that ReX gait lower and more stable impact energy loss, whereas RoX gait demonstrates better energetic efficiency and speed adaptability across a wider range of terrain conditions. In addition, the avian-like knee structure is more favorable for ReX gait on uphill and flat ground, while the human-like knee structure is more advantageous for RoX gait under the same conditions. These results provide a useful reference for the morphology and gait strategy design of bipedal robots for locomotion over different terrains.