# Mechanical Design and Gait Optimization of Hydraulic Hexapod Robot Based on Energy Conservation
## 1 Introduction  
Over the last decades, bionic legged robots have been developed to increase transportation adaption in mountainous areas, fieldwork, military activities, and other non-structural environments [1]. In hydraulics, advanced walking robots have been developed in succession, for example, Boston Dynamics' Bigdog [2] and Atlas [3], IIT's HyQ [4] and HyQ2 Max [5], Shandong University's SCalf-III [6], Delft University's TU Delft [7], NTUA's HexaTerra [8], and Ritsumeikan University's TaeMu [9]. In addition to these advanced high-tech robots, the actuation of hydraulics has the significant advantage of high power-to-mass ratio, fast dynamic response, and large force output with no gears required compared with electric actuators [10]. However, the hydraulic walking robot is also less efficient [11], which limits its load-carrying capacity and working hours for field applications. Therefore, it is of great significance to reduce the energy consumption of hydraulic walking robots. A variety of approaches have been proposed to reduce the energy consumption in terms of hydraulic driving system, mechanical structure, and motion planning algorithm. From the perspective of the hydraulic system, valve control systems [12] are mostly used in the hydraulic walking robot with the advantage of fast response and high precision control. [PERSON] et al. [13] proposed a separate meter-in separate meter-out (SMISO) hydraulic control system, and the actuators' energy consumption was reduced by 45% without noticeable motion control deterioration. [PERSON] et al. [14] proposed a novel hydraulic system based on the two-stage pressure source, and the energy efficiency was increased by 41.5% compared to the single-stage hydraulic system. [PERSON] et al. [15] proposed aload-prediction based method to reduce valve throttling energy loss, in which the supply pressure was varied to track the force required by any actuator branch. Experimental results showed that hydraulic power savings of up to 70% were achieved. Most of the improvements in hydraulic driving system are too complicated to apply, and more studies are focused on structural design and gait optimization. [PERSON] et al. [16] presented a structural optimization method to change the arrangement of the planar redundant actuator, and the energy consumption of the parallel hydraulic mechanism was reduced by 10%. [PERSON] et al. [17] presented a general theorem for designing a mechanism based on a template with biological relevance for a wide range of tasks. The design of the mechanical structure improved the walking energy efficiency by more than 50%. [PERSON] et al. [18] reduced the hydraulic flow of the quadruped robot, and particle swarm optimization algorithm (PSO) was used to optimize the mechanical design parameters of the robot. Some scholars optimized the mechanism design from the aspect of hydraulic actuators [19; 20], and the elastic load suspension mechanism was also used to improve energy efficiency [21; 22]. In the study of energy consumption of hydraulic walking robots, the intelligent motion planning algorithm is determined to reduce energy consumption [23]. [PERSON] et al. [24; 25] established an energy model including the mechanical power and heat rate and optimized the quadruped robot foot trajectory from the aspects of step height, step length, standing height, gait cycle, and duty cycle. The total joint energy consumption in simulations and experiments dropped by 8.02% and 7.55%, respectively. [PERSON] et al. [26] presented a method to minimize the energy consumption by 19.35% by providing an integrated strategy of motion planning subject to velocity and acceleration constraints. However, few of these studies reduce the total energy consumption of the hydraulic driving system of the legged robot, and the mechanical structure and motion trajectory are rarely optimized at the same time. The optimization of the hydraulic legged robot has multiple parameters, objective functions, and restrictions.