# Structural Design and Kinematics Simulation of Hydraulic Biped Robot
## 1 Introduction  
In complex environments such as fields, mountains, hills, and battlefields, biped robots can actively adjust their body height to adapt to complex changing terrains and different ground conditions, and cross obstacles and ravines. Compared with the wheeled, crawler, and multi-legged robots, the bipedal robots have stronger environmental adaptability. In recent years, humanoid robots have achieved great progress, and researchers have successively developed different types of biped robots and designed gait planning and balance control based on solid models. The WABOT-1 robot was developed by researches from Waseda University in 1973, which was the world's first biped robot that can visually recognize the position, direction, and distance of objects, and can receive commands through hearing. In 1986, Honda started to develop humanoid robots with intelligence and mobility. Since then, a series of humanoid walking mechanisms had been successively proposed. Honda developed seven biped robots which were designated E0 (Experimental Model 0) through E6 [1], and ASIMO [2, 3], respectively. As robots have been developed from the \"automatic machines\" to the \"autonomous machines\" with decision-making capabilities, they can provide increasingly more practical applications in an environment where people can coexist with them. The HRP biped robot was developed by Kawada Industries from Japan, and the motor drive methodwas used in this product [4; 5]. Each generation has shown new and exciting advances. Similarly, KAIST developed KHR-1/2/3 (Hubo) [6], and Waseda continued its long and successful tradition to build many different models such as Wabian-2R [7]. In the Italian Institute of Technology, the iCub robot [8; 9; 10; 11] was designed based on children, and it has 53 driving degrees of freedom. The LOLA robot was developed by a team in the University of Munich, Germany [12], and this robot contains actively driven toe joints, which has improved the stability of the robot's feet on and off the ground. Despite the progress achieved in the design of humanoids/bipeds, there are still some significant barriers such as how to prevent robots (physical structure, materials, actuation, and sensing) from emulating the physical performance of the human body. These limitations are particularly noticeable during the physical interaction and impacts, and mechanical robustness. The vast majority of biped robots are driven by electric motors, but a large number of practices prove that the motor-driven method has some disadvantages which are difficult to overcome, such as low power density, weak resistance to electromagnetic interference, and lack of adaptability to work in the field environment. The hydraulic drive has the advantages of simple structure, large output force, and high power-to-weight ratio, so it is used as one of the driving methods of biped robot. Because the hydraulic drive has a large output force, it doesn't need to use the deceleration device, so it has a very simple drive structure, which can avoid the return clearance and other error caused by deceleration device in motor drive and is beneficial to the force servo control. A research team from Carnegie Mellon University in the United States developed a Sarcos Primus hydraulically driven biped robot based on the underlying force servo control [13], and in this robot, the flexibility of force servo control is utilized to improve the balanced capacity of biped robot movement. Boston Dynamics successively developed two hydraulically driven biped robots PETMAN [14] and ATLAS [15], which have excellent stability and highly dynamic performance. Atlas has been chosen as a working platform by more than ten teams in the Robotics Challenge held by the Defense Advanced Research Projects Agency (DARPA) in the past few years [16], and it has demonstrated a strong capability of dynamic balancing and whole-body walking control in the competitions. In China, [PERSON] et al.