# A Review of Foot-Terrain Interaction Mechanics for Heavy-Duty Legged Robots
## 2 Supporting Feet of Heavy-Duty Legged Robots
### Spherical Supporting Foot Configurations
#### 2.1.2 Feet with Active Driving Joints  
The mechanisms of feet with active driving joints in heavy-duty legged robots are generally more complex. Drive devices need to be installed to make them more difficult to control movement and more cost-effective compared to feet with passive adaptive joints. They have not been widely applied in current research on heavy-duty legged robots.  
In 1996, [PERSON] [65] from the South Korean University of Science and Technology was inspired by the structure of umbrellas and designed a point contact underactuated robotic foot. The oil in and out of the hydraulic cylinder in the middle of the foot will change the opening and closing state of the toes. The point contact between the feet and the terrain enhances the adaptability of the robot's feet to the rough terrain. This type of foot is applied by researchers to a quadruped robot, as shown in Figure 23a.  
In 2007, [PERSON], [PERSON], [PERSON], and others from the University of Tokyo in Japan designed humanoid robot feet. The robot feet are installed on the UT-\(\mu\)2 robot, as shown in Figure 23b. The characteristic of the robot's feet is that the toe mechanism does not use the commonly used hinge type rotating pair, but instead uses a parallel four-link mechanism. Through analysis, it is shown that the combined moment of force at the joints of the parallel four-link-type toe mechanism is smaller than that of the hinge-type toe joint during most motion periods [66].  
In 2009, [PERSON] and [PERSON] from the University of Novi Sad used motion analysis of human feet to design a humanoid robot foot. The type of robot foot not only has passive toe joints but also active toe joints, making humanoid robots able to walk more smoothly and improving their walking ability [67]. The robot foot with an active driving unit is shown in Figure 23c.  
[PERSON] from the Polytechnic University of Madrid designed a mountain climbing robot called Roboclumber, as shown in Figure 23d. It adds hydraulic cylinders above the feet of the robot. Adjusting the foot-terrain force is carried out by changing the oil inlet and outlet quantities of the hydraulic cylinders. The friction force between the robot's feet and the terrain is adjusted. The mechanical performance of the foot-terrain contact surface is improved [68].  
Figure 23: Some feet with active driving joints: (**a**) quadruped legged walking robot Centaur [65]; (**b**) diagram of foot mechanism with toe joints [66]; (**c**) robot foot with active driving unit [67]; (**d**) Roboclumber robot performs climbing experiments outdoors [68].  
In 2010, [PERSON] and [PERSON] from the University of Delft in the Netherlands designed a robotic foot that could utilize energy more effectively during walking. Because people usually waste a lot of beneficial energy when walking, they developed a micro-driven controlled humanoid foot [69]. Being able to more fully utilize the energy lost in human legs and ankle joints during walking, the energy utilization rate has increased by 23% compared to normal walking. The physical image of the robot foot is shown in Figure 24a. The 3D structure diagram of the robot foot is shown in Figure 24b.  
Compared with robot feet with passive adaptive joints, robot feet with active adaptive joints have certain advantages. They can actively adjust the configurations of their feet based on the shape of the terrain, thereby increasing the contact area between the feet and the terrain. When subjected to normal force from the vertical direction and tangential force from the horizontal direction, adjusting the feet to reach the positions where the force is most evenly applied improves the stability of the robot. Their significant drawbacks are a more complex structural design and high control difficulty compared to robot feet with passive adaptive joints. At present, there is little research on robot feet with active adaptive joints in heavy-duty legged robots.