# A Review of Foot-Terrain Interaction Mechanics for Heavy-Duty Legged Robots
## 2 Supporting Feet of Heavy-Duty Legged Robots
### Plantar Patterns of Supporting Foot of Legged Robots  
The legged robots mainly rely on the friction force between their feet and the terrain when walking. When the friction force is high, the sliding phenomenon of the robot's feet will be reduced. Designing and installing some structures on the bottom of the robot's feet is carried out to improve their adhesion to the terrain. In current research, it has been found that the research on the foot patterns of heavy-duty legged robots is not yet in-depth enough. [PERSON] [70] from the Harbin Institute of Technology conducted an analysis of the equivalent adhesion coefficient of typical foot patterns. The model of robot foot-terrain interaction attachment has been established. The equivalent adhesion coefficient of different foot configurations (flat foot, nail foot, single-baffle foot, and multi-baffle foot) can be calculated. The different feet used for testing in the experiment are shown in Figure 25. The higher the coefficient of adhesion, the better the adhesion characteristics of the robot. The conclusion is that the multi-baffle foot has the best adhesion performance, followed by the single-baffle foot, nail foot, and flat foot. [PERSON] [71] from the Dalian University of Technology designed a new plantar pattern. The middle part adopts horizontal stripes perpendicular to the robot's forward direction, while the remaining parts use 45\({}^{\circ}\) diagonal stripes to enhance the anti-slip ability of the foot. The design of the plantar pattern structure is to first establish a simulation model for the terrain action and an evaluation method for adhesion performance. On that basis, the plantar adhesion performance is the optimization objective, and the structural parameters of the plantar pattern are the optimization variables. The response surface method is used to optimize the design of the foot pattern. Simulation and experimental verification are conducted. Compared to the reference offroad tire design, the adhesion coefficient of the  
Figure 24: Foot with energy recovery [69]: (**a**) physical image; (**b**) 3D structure diagram of foot. plantar pattern is increased by 7.41%. The optimized plantar pattern is shown in Figure 26a. The original plantar pattern is shown in Figure 26b. [PERSON] [72] from the Dalian University of Technology designed the rubber feet of the heavy-duty robot. The plantar patterns of the feet adopt a mixed pattern design, with a crisscrossing pattern in the middle. The offroad patterns with wider grooves are evenly arranged around at an angle of 15\({}^{\circ}\). The rubber feet can be designed with existing vehicle tire patterns as the research background. Considering the combination of the plantar patterns and the feet, it is equipped with patterned blocks, patterned grooves, and base glue. The calculation of the adhesion and climbing angle of the plantar patterns has also been carried out. [PERSON] [73; 74; 75] conducted in-depth research on the effects of plantar pattern depth, groove width, and pattern direction on the friction coefficient. The results show that on the wet terrain, the deeper the pattern, the higher the friction coefficient. Wide patterned grooves have a higher coefficient of friction. The friction coefficients of horizontal and 45\({}^{\circ}\) patterns are relatively high. Their anti-slip performance is good. The different groove designs of the plantar patterns are shown in Figure 27a-c.  
Figure 27: Groove designs of plantar patterns [74]: **(a)** groove orientation 0\({}^{\circ}\); **(b)** groove orientation 45\({}^{\circ}\); and **(c)** groove orientation 90\({}^{\circ}\). Figure 26: Comparison of plantar pattern shapes [71]: **(a)** optimized plantar pattern; **(b)** original plantar pattern. Figure 25: Some different feet [70]: **(a)** flat foot; **(b)** nail foot; **(c)** single-baffle foot, and **(d)** multi-baffle foot. The heavy-duty legged robot relies on the reaction forces provided by the terrain to move.