# 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  
Spheres, ellipsoids, and rectangles have been found to be the most common shapes of feet [15]. Common configurations such as cylindrical feet, semi-cylindrical feet, spherical feet, hemispherical feet, square feet, and special feet are summarized. Cylindrical Supporting Foot Configurations  
The Tokyo Institute of Technology has developed the TITAN series of robots. In 2002, the latest generation model machine called TITAN XI was developed, and the robot is shown in Figure 2a. It is a hydraulically driven quadruped robot. The robot can walk  
Figure 1: Overall framework of article. steadily and continuously on slopes covered with reinforced concrete frames. It can achieve intermittent crawling gait based on map information. Terrain adaptive gait makes the robot's motion more stable. The robot's feet are cylindrical [16]. TITAN IX is a quadruped robot with cylindrical foot shapes for humanitarian landmine detection missions [17], TITAN III is a quadruped robot with cylindrical feet [18], as shown in Figure 2b. The COMET-IV robot developed by Chiba University is a heavy-duty hexapod robot based on hydraulic drive, as shown in Figure 2c. The weight of the robot is approximately 2120 kg. Its load-bearing capacity is approximately 424 kg. The overall size is approximately 2.8 m \(\times\) 3.3 m \(\times\) 2.5 m. Each leg has four degrees of freedom (DOFs). It can walk on uneven terrain. The robot's feet are cylindrical [19]. The Dante II robot, designed for exploring planetary surfaces, operates semi-autonomously and is equipped with eight legs. It weighs 770 kg, has the capacity to carry a 130 kg payload, and is capable of navigating slopes up to 30 degrees. The feet of the robot are cylindrical [20]. The NMIIIA robot was successfully developed in 1985, as shown in Figure 2d. It is a crewed hexapod robot developed by the former Soviet Union during the implementation of lunar exploration activities. It is used for star surface exploration and load bearing. The robot has a mass of 750 kg, a load-bearing capacity of 80 kg, and a moving speed of 0.7 km/h. Its feet are cylindrical [1]. The SILO4 robot developed in Spain also has cylindrical feet [21]. The passive joint of the foot contains three rotational degrees of freedom (DOFs). The three-axis force sensors are installed on the robot's feet. The outdoor experiment and ankle joint are shown in Figure 3.  
[PERSON] developed a terrain electric-driven hexapod robot with a high load ratio, ElSpider. Six supporting legs of the robot are uniformly distributed on the body of the central symmetric structure. A single leg adopts a structure with three active and four passive degrees of freedom. It has a weight of approximately 300 kg. The rated load of ElSpider is greater than 155 kg, as shown in Figure 4a. Its overall size is approximately 1.9 m \(\times\) 1.9 m \(\times\) 1.0 m. It can cross obstacles with a height greater than 0.3 m, cross trenches with a width greater than 0.3 m, walk regularly at a speed of 0.16 m/s, and climb a maximum slope of 35\({}^{\circ}\). The robot's feet are cylindrical [22; 23; 24]. The six dimensions  
Figure 3: SILO-4 robot [21]: (a) outdoor experiment; (b) ankle-integrated sensor system. Figure 2: Robots of TITAN series, COMET-IV, and NMIIIA: (a) TITAN XI [16]; (b) TITAN III [18]; (c) COMET-IV [19]; and (d) NMIIIA [1]. force sensors are also installed on the feet to measure the foot-terrain forces, as shown in Figure 4b. A P-P structured hexapod Octopus robot was developed by Professor [PERSON]'s team from the School of Mechanical and Power Engineering at Shanghai Jiao Tong University. The robot's body adopts a symmetrical design. It always maintains three supporting legs to support the body during walking, with good stability and maneuverability. Its speed is 1.2 km/h, and its load-bearing capacity is 200 kg. The Octopus robot adopts cylindrical configurations at the feet.