# A Review of Foot-Terrain Interaction Mechanics for Heavy-Duty Legged Robots
## 1 Introduction  
In nature, there are various uneven and irregular terrains that have complex environmental surfaces, such as grassland, desert, mud pools, and mountains. The exploration of unknown terrain is dangerous for humans. Autonomous mobile robots can be the first to enter the detection before humans enter unfamiliar and dangerous environments, which greatly ensures the safety of human life and improves the efficiency of exploration work. With the continuous development of autonomous mobile robot technology, autonomous mobile robots have been widely used in multiple fields to protect human safety and improve production efficiency [1]. Considering the varying modes of contact between mobile robots and terrain, the broad classification of mobile robots includes wheeled robots, tracked robots, snake-like robots, spherical robots, and legged robots [2]. The currently widely researched and applied mobile robots are mainly wheeled robots. Wheeled robots have limitations in their use due to their high requirements for terrain environments, necessitating relatively wide, flat, or smaller rugged terrains. Compared to the harsh requirements of wheeled robots in the terrain environment, tracked robots have to some extent improved the high demand for the terrain. However, the large contact area of the tracks with the terrain also brings newproblems. The terrain adaptability of snake robots has significantly improved compared to the tracked robots, but these types of robots lack a load-bearing capacity and have relatively slow movement speed. They can only achieve the robot's own motion [3]. The spherical robots have good mobility performance. Although the spherical shape can effectively protect the fragile and moving parts of the robots from external damage, the contact mode between the spherical robots and the terrain is point contact, which is not conducive to control movement. The spherical robots have not been widely used [4]. The legged robots often use biomimetic technology to design their structures. They usually imitate humans walking on two legs, and mammals or insects walking on multiple legs. Their structures are more flexible and can maintain relative stability. Compared to wheeled robots and tracked robots, legged robots require only intermittent and discrete landing points to cross obstacles like legged animals. Therefore, walking mechanisms have stronger adaptability to walk on complex terrain. Compared to snake robots and spherical robots, the legged robots have a load-bearing capacity. The research on legged robots has received widespread attention in recent years due to their excellent terrain adaptability and motion flexibility. In fields such as interstellar exploration [5], humanitarian demining [6], logging [7], and nuclear industry [8], the legged robots have unique advantages and have been widely used. Based on their load-bearing abilities, legged robots can be classified into two categories: heavy-duty legged robots and light-duty legged robots. It can be seen that heavy-duty legged robots have three characteristics compared to light-duty legged robots: large mass, large volume, and high payload-total mass ratio [1]. The heavy-duty legged robots may encounter various complex terrains in actual environments, which also makes the robots full of challenges when moving. Compared to the light-duty legged robots, the heavy-duty legged robots are more prone to foot sinkage occurring when traveling in soft and muddy terrains due to the lower pressure-bearing capacity of the soil. With the low adhesion of smooth surfaces such as ice and snow, heavy-duty legged robots are also more prone to foot slips occurring. The heavy-duty legged robots are more sensitive to foot-terrain interactions compared to the light-duty legged robots. When a heavy-duty legged robot interacts dynamically with complex terrain during movement, it is prone to the phenomenon of robot instability. It has a significant impact on achieving rapid robot movement and completing designated tasks.