To realize home delivery with the robot and better fit the logistics robot into the express delivery industry, several requirements should be met, as shown in Table 1.  
Stair-climbing is a key function to satisfy the requirements mentioned in Table 1. Although many scholars proposed a variety of applications of stair-climbing robots in reconnaissance [10; 11], emergency [12; 13], corridor cleaning [14; 15], etc., these robots were designed without considering the capability of stair-climbing with goods. Hence, all of them cannot be applied to unattended delivery. To meet the above requirements, a humanoid robot is the first option. Digit from Agility Robotics [16], Atlas from Boston Dynamics, and Asimo from Honda are all humanoid robots with excellent performance [17], but the biggest problem is the high complexity of their structure and control system, which also means high costs. Although some scholars proposed a more simplified humanoid robot structure and control strategy [18], the simplified mechanical structure increases the area of its feet, almost occupying the platform of an entire stair during stair-climbing, which impacts people passing on the stairs. Additionally, there are many bionic-legged robots, such as the four-legged [19] and six-legged [20] robots. Although these robots can climb stairs in the fastest and most flexible ways, their movement efficiency on the ground is low, which is not applicable for actual delivery. Other researchers proposed some stair-climbing robots with new mechanical structures. Reference [21] proposed a quadruped tri-star wheeled stair-climbing robot, and these wheels enable it to move on a flat surface and climb stairs. Nevertheless, such a design also has its shortcomings. Firstly, the tri-star wheels' mobility on the flat ground is worse than that of general wheels. Secondly, the position change of the center of gravity caused by the movement of the tri-star wheels would result in inefficiency, and the goods would be bumped, which influences the safety of the goods. Finally, this kind of design is unable to deal well with stairs of different heights. Therefore, this tri-star wheel is not suitable for unattended delivery. In addition to the above robots' mechanical structures, tracked and hybrid structures are also effective [22]. For tracked robots, the control is simple, but the edges of the stairs suffer from large stress [23] and might be damaged. Furthermore, the tracked robot's running speed on the ground is not high as that of the wheeled robot [24]. As for the hybrid robots, the wheel-legged robot is preferred. In Reference [25], a wheel-legged robot was proposed, and it was able to balance its own platform at all times during climbing. This function meets the safety requirement of cargo. Nevertheless, its driving wheels are installed at the end of the leg, and this kind of installation applies shear stress on the leg with a driving wheel when the robot is driven horizontally. In particular, when the robot is loaded and operates for a long time, the shear stress would bring unrecoverable damage. \begin{table}
\begin{tabular}{c c c} \hline \hline
**From the Perspectives of** & \multicolumn{2}{c}{**Requirements**} \\ \hline Customers & \(\bullet\) & Fast delivery \\  & \(\bullet\) & Ensure the safety of goods during the distribution process \\ \hline Express company & \(\bullet\) & Reduce the costs of manufacturing \\  & & and maintenance \\  & \(\bullet\) & The ability to climb stairs with goods \\ \hline Property management department & \(\bullet\) & Reduce the damage to the stairs \\  & & during stair-climbing \\  & \(\bullet\) & Not get in the way of the crowd \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Requirements for robot to realize home delivery. As mentioned above, there is little research on logistics robots which can realize the entire process of express delivery, even home delivery. Therefore, the robot proposed in this paper is designed to have not only mobility on the ground but also omnidirectional mobility indoors.