# A Review of Foot-Terrain Interaction Mechanics for Heavy-Duty Legged Robots
## 4 Further Research
### Parameterization Research of Soil Characteristics in Extreme/Dynamic Environments  
The foot-terrain interaction behavior involves the configuration of feet and the dynamic terrain characteristics. In the future, in addition to optimizing the structural design of the foot, the various indicators of different soils can also be studied. Clarifying the properties of soil in an unstructured environment will be more conducive to the research on the mechanical properties of heavy-duty legged robots. The performance parameters of different soils on Earth are shown in Table 8.  
The composition of Martian soil includes many complex organic compounds. The composition and performance vary from location to location [107; 108; 109]. Due to the limitations of rocket launch loads, current Mars rovers do not carry dedicated equipment for measuring the mechanical parameters of Martian soil, making it impossible to accurately obtain the mechanical parameters of Martian soil in real time. According to the theory of vehicle terrain mechanics and the mathematical models of the wheel-soil interaction, the mechanical parameters of the Martian soil under the wheel and around the rover can be identified. The Viking probe mainly includes two landers: Viking 1 and Viking 2 [110]. Viking 1 landed on the Chryse Planitia (22.48\({}^{\circ}\) N, 49.97\({}^{\circ}\) W) on 20 July 1976, and Viking 2 landed on the Utopia Planitia (47.97\({}^{\circ}\) N, 225.74\({}^{\circ}\) W) on 3 September 1976. The mechanical performance evaluation of the weathered materials at Viking 1 and Viking 2 landing sites is shown in Table 9. The captured images are shown in Figure 40. The properties of the Martian soil are indispensable for studying foot-terrain mechanical models in the Martian environment. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Terrain Mechanical** & **Dry Sand** & **Sandy Loam** & **Clayey Soil** & **Snow** \\
**Parameters** & & & & \\ \hline \(n\) & 1.1 & 0.7 & 0.5 & 1.6 \\ \(c\) (kPa) & 1.0 & 1.7 & 4.14 & 1.0 \\ \(\varphi\) (\({}^{\circ}\)) & 30.0 & 29.0 & 13.0 & 19.7 \\ \(k_{\text{c}}\) (kPa/m\({}^{\text{n}-1}\)) & 0.9 & 5.3 & 13.2 & 4.4 \\ \(k_{\text{\phi}}\) (kPa/m\({}^{\text{n}}\)) & 1528.4 & 1515.0 & 692.15 & 196.7 \\ \(K\) (m) & 0.025 & 0.025 & 0.01 & 0.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Performance parameters of different soils on Earth [106]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & \multicolumn{2}{c}{**Viking 1**} & \multicolumn{2}{c}{**Viking 2**} \\
**Sandy Flats** & & **Rocky Flats** & **Bonneville and Beta** \\ \hline Bulk density (g/cm\({}^{3}\)) & 1 to 1.6 & 1.8 & 1.5 to 1.8 \\ Particle size (surface and near surface) & 1 to 100 \(\upmu\)m (\%) & 60 & 30 & 30 \\
100 to 2000 \(\upmu\)m (\%) & 10 & 30 & 30 \\ Angle of internal friction (\({}^{\circ}\)) & 20 to 30 & 40 to 45 & 40 to 45 \\ Cohesion (kPa) & - & 0.1 to 1 & 1 \\ Adhesion (kPa) & - & 0.001 to 0.01 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Estimation of mechanical properties of weathered materials at Viking 1 and Viking 2 landing sites [110]. On the surface of the moon, due to the impact of meteorites and micro-meteorites, continuous bombardment by cosmic rays, solar wind, and changes in temperature differences between day and night, the lunar rocks undergo thermal expansion, contraction, and fragmentation, resulting in the formation of lunar soil with an average thickness of approximately 6-10 m. In addition, the lunar environment, which is completely different from that on Earth, such as anhydrous and biotic environments, low gravity, and almost zero atmospheric pressure, also results in significant differences in the physical and mechanical properties of lunar soil and Earth's soil [112]. Currently, the analysis of various physical parameters and mechanical properties of lunar soil mainly relies on two methods: in situ return and in situ detection. In situ detection is mainly achieved using uncrewed detection equipment.