## 1 Introduction  
Currently, small mobile robots are widely used in unstructured environments. Mobile robots can assist or replace humans when performing actions, such as target detection and safe and efficient search and rescue [1]. Mobile robots with complex terrain adaptation capability is a current research hotspot in the field of mobile robotics [2; 3]. To design robots with high terrain access performance and motion flexibility for complex terrain, most scholars and research institutions focus their research on new mobile mechanisms for robots [4]. Common mobility structures are broadly classified as wheeled, tracked, and legged [5]. By modeling and studying the body structure and movement mechanisms of living beings, walking structures for mobile robots, such as wheels, legs and feet, have been designed. It is worth mentioning that although they have been used successfully in some cases, the application of robots in rugged and complex terrain is still limited by obstacle traversal performance and control systems. The emergence of multi-motion mode robots and transformable wheel structure robots has prompted innovative ideas [6]. The multi-motion mode robot mainly adopts the design method of the wheeled mobile mechanism mounted on the mobile platform of the leg-type robot [7]. [PERSON] designed a new type of mobile robot with transformable wheel legs in Figure 1a [8]. By combining the parallelogram mechanism with the Chebyshev mechanism, a single-loop closed chain 2 RP3R metamorphic mechanism that can switch between multiple motion modes of the robot, such as four-legged, eight-legged, and wheeled, has been proposed. It has strong obstacle-crossing performance and anti-toppling ability. [PERSON],developed a wheeled quadruped robot ANYMAL in Figure 1b[9, 10], with excellent flexibility and dynamic movement ability. Using the 3D ZMP method, it can perform a variety of actions to overcome obstacles or stand up after falling. The design concept of transformable wheel legs is terrain adaptability. When moving on a flat surface or obstacle below the height of the wheel radius, the robot is in a round-wheeled movement mode. When the robot encounters an obstacle, the transformable wheel undergoes a structural transformation and the robot changes to wheel-legged movement mode [11]. Based on the walking gait of the cockroach, the \"Whegs\" series robot [12] adopts an open, three-spoke rotating wheel-leg design, and each spoke can be alternated periodically as support for contact with the ground when walking, showing a strong terrain-passing ability in Figure 1c. Combining the characteristics of the stable movement of insects in nature, [PERSON] [13], designed a wheel-legged hexapod robot that can pass through complex terrain without any terrain sensing and with an independent, unconstrained, and flexible drive in Figure 1d. Using a unique switching mechanism, [PERSON] [14] designed a novel four-legged/quadruped mobile robot to switch the morphology of wheels and 2-degree-of-freedom legs under the same drive system in Figure 1e. With the help of external friction between the wheels and the ground, [PERSON] [15, 16] designed a new wheel-leg hybrid mobile robot with a new triggering mechanism to improve the transition success rate, and the robot is able to climb over obstacles of 3.25 times its wheel radius in Figure 1f. Using a sea turtle as a bionic object, [PERSON] [17] designed a wheel-track-leg composite bionic robot and analyzed its gait planning and obstacle-crossing performance in Figure 1g. Combined with the passive deformation principle, [PERSON], and his colleagues developed a robot that determines its mode of operation through physical contact force in Figure 1h [18]. The above literature study demonstrates that different levels of research have been conducted on mobile robots' ability to cross obstacles in unstructured terrain.