## 2 Mobile Robot Structure Design  
As the degree of urbanization continues to increase, the urban environment becomes increasingly complex and diverse, posing a huge challenge to the mobility of large equipment. Therefore, mobile robots have gradually become an indispensable choice. Mobile robots can adapt well to various urban environments and better meet the various needs and challenges of modern cities. However, in the continuous changes of the urban environment, ground mobile robots also face challenges. Mobile robots must have the ability to explore natural terrain and artificial environments, such as stairs, steps, slopes, etc., while having flexible mobility and portability to adapt to the diversity of urban environments. There are currently many wheeled and tracked hybrid mobile robots available, such as the VIPER robot developed by Israeli Elbit Systems, the Lizard intelligent robot, developed by the Shenyang Institute of Automation, and research robots cited in the literature [9; 10; 11; 12; 13]. These robots take advantage of the benefits of combining wheels and tracks, providing good obstacle traversal capabilities and adaptability to various complex terrains. These robots have widespread application fields, including military reconnaissance, exploration and surveying, medical care, logistics delivery, and other areas. The structural design of robots is adjusted and optimized according to different application requirements. To ensure that our multi-jointed tracked-wheeled compound mobile robot performs reliably in complex terrain, we considered potential real-world imperfections that could impact its functionality. We recognize that the terrain can be varied and unpredictable, and imperfections, such as sensor noise [14] or dynamic effects [15] can destabilize the robot's control performance [16]. Therefore, during our design process, we incorporated contingencies to account for these challenges, such as developing two distinct locomotion modes for the robot to climb stairs and establishing kinematics/dynamics equations for its land movement. In this paper, we focus on urban reconnaissance robots. The mobile system of the wheel and shoe composite mobile robot designed in this paper has certain climbing and obstacle surmounting ability, flexible movement ability, and good passive adaptability to terrain. The design of the mobile system should mainly meet the following design objectives based on the characteristics of the urban environment and the task requirements of the mobile robot (good portability, mobility, and ability to cross obstacles). (1) The climbing slope is greater than 30\({}^{\circ}\), and it can overcome obstacles higher than 300 mm and deep ditches more than 250 mm wide. (2) The mass of the mobile subsystem shall not exceed 15 kg, and the speed shall not exceed 15 km/h. (3) The robot has strong passive terrain adaptation ability and is easy to control. Integrating the characteristics of wheeled and tracked walking mechanisms, a multi-joint wheel-track composite mobile robot has been designed. It consists of four wheels, two intermediate tracks, and four swing arm tracks, which can ensure both the obstacle-crossing capability and rapid driving on flat ground. The mobile robot's ability to cross barriers is related to the mechanism's size, and the wheeled robot's ability is limited by its wheel diameter. To overcome this limitation, the design idea of a folding and deforming mechanism is adopted to allow the robot to fold in a non-working state for storage and transportation. When it is in a working state, the robot unfolds, expanding in size, and thus achieving its obstacle-crossing function. The wheel-tracked composite robot is shown in Figure 1.  
Compared to the design in the literature [10], our robotic platform has undergone significant improvements. One of the major changes we made was to adopt a pendulum wheel design with different sizes between the front and rear.