# A Review of Foot-Terrain Interaction Mechanics for Heavy-Duty Legged Robots
## 2 Supporting Feet of Heavy-Duty Legged Robots
### Supporting Foot Configurations of Heavy-Duty Legged Robots
#### 2.1.1 Feet with Passive Adaptive Joints
But unlike the other feet of heavy-duty legged robots, the middle of the cylindrical foot is hollow. Its structure can effectively reduce the mass of the robot's feet. And springs are installed above the feet to provide cushioning and shock absorption [26; 27; 28]. The indoor experiment is shown in Figure 5a. The load-bearing progress is shown in Figure 5b. Researchers from Jilin University have designed a heavy-duty hexapod robot [30; 31; 32]. The feet are designed as ball joint structures, and force sensors are connected in series above the ball joints to detect whether the feet are firmly pressed against the terrain. There are reset springs parallel to the ball joints between the upper and the lower plates of the ball joints. They are used for feet to avoid sticking during the process of stepping on the terrain due to the large deflection angle of the foot bottom. Thick rubber pads are installed at the bottom of the feet to cushion the landing of the feet. The prototype of the heavy-duty hexapod robot and its foot are, respectively, shown in Figure 6a,b. Figure 4: EISpider robot [25]: (**a**) load experiment; (**b**) robot foot. Figure 5: Octopus robot: (**a**) indoor walking experiment [26]; (**b**) load-bearing progress [29]. On the basis of the investigation of the heavy-duty legged robot, researchers from Huazhong University of Science and Technology present a novel foot structure for the heavy-duty legged robot. Its highly adaptable foot system with significant adhesion can be utilized to navigate extreme roads and complex terrains, including mountainous areas and swamps. The foot design, inspired by mountain-dwelling creatures, has been crafted to ensure substantial adhesion and enhanced adaptability [33; 34]. The feet are installed on the heavy-duty legged robot, as shown in Figure 7a. The robot foot and single leg are, respectively, shown in Figure 7b,c. The advantage of cylindrical feet is that they have a large contact area with the terrain. Large contact areas can provide greater adhesion to withstand heavy loads. At the same time, the heavy-duty legged robots equipped with cylindrical flat feet are suitable for long-distance transportation. The disadvantage is that it is necessary to design a swing structure, otherwise it cannot walk stably. The stress distribution model of the cylindrical foot of the heavy-duty legged robots is shown in Equation (1). Then  
\[\left\{ \begin{array}{l}F_{\rm N}(t)=\sigma(t)\times A\\ F_{\rm T}(t)=\tau(t)\times A\end{array} \right. \tag{1}\]  
where \(F_{\rm N}\) is the normal support force of the foot. \(F_{\rm T}\) is the tangential driving force of the foot. \(A\) is the contact area between the foot and the terrain. Semi-Cylindrical Supporting Foot Configurations  
The Big Dog robot [35; 36; 37] developed by Boston Dynamics in the United States is a quadruped robot. The main components of the robot are shown in Figure 8a. It has full flexibility and can stand, squat, and move. The crawling speed is 0.2 m/s, the jogging speed is 1.6 m/s, and the jumping speed can reach 3.1 m/s in laboratory testing. The weight is  
Figure 6: Heavy-duty hexapod robot of Jilin University: (**a**) prototype [30]; (**b**) foot [32]. Figure 7: Heavy-duty six-legged robot from Huazhong University of Science and Technology: (**a**) robot model [33]; (**b**) robot foot [34]; (**c**) single leg [33]. approximately 109 kg, and the size is 1.1 m \(\times\) 1 m \(\times\) 0.3 m. The feet of the Big Dog robot are semi-cylindrical, as shown in Figure 8b. The Big Dog robot can pass through rock slopes up to 60\({}^{\circ}\) and has excellent adaptability to complex terrain. Researchers at Huazhong University of Science and Technology have developed a hydraulic-driven quadruped robot, MBBOT [38; 39]. Each leg of the robot includes four active degrees of freedom. The total mass of the robot is 140 kg. The feet of the four legs are also equipped with three-dimensional force sensors to detect the magnitude of the force between the legs and the external environment.