# Omnidirectional Jump of a Legged Robot Based on the Behavior Mechanism of a Jumping Spider
## 1 Introduction  
Compared with walking [1] and crawling robots [2], the jumping robot can walk, run, and jump [3]. Jumping locomotion has characteristics of isolated footholds, and powerful and explosive jumping force [4], which contribute to the quick and effective jumping locomotion of bio-inspired robots to stride over obstacles that are several times the size of their bodies, or cross a gully that is several times the length of their own step, and avoid danger in time. As for the jumping robot, the key point is usually planning the trajectory of their center of gravity (CoG) to achieve the jumping process reasonably, and realize the multi-directional jumping moment in various environments. The jumping robot can realize the jumping process by controlling the take-off speed, attitude and landing stability. When the jumping robot takes off, the robot achieves certain acceleration because its foot generates enough reactive force by impacting with the ground. Then, the robot adjusts its posture in the air, and finally lands smoothly. Hence, the study of the jumping process is an important part of the jumping robot. The jumping robot realizes the physical design of the robot and the jump movements by imitating animal body structures or biological movement mechanisms. The bionics [5] include the structure bionics, the motion bionics and the control bionics [6]. Nowadays, research on the bionic robot based on a bionic structure is abundant. There are both robots that imitate mammals, such as the bionic cheetah and bionic kangaroo [7, 8], and robots that imitate amphibious creatures, such as bionicfrogs and bionic toads [9; 10]. Additionally, there are robots that imitate arthropods, such as bionic spiders, bionic locusts, bionic cockroaches and imitation water insects [11; 12; 13; 14; 15]. The mammal-like robot has the characteristics of being fast at running and jumping, as well as having smooth motion and a high energy utilization rate. However, there is little research surrounding this because of its large volume and heavy weight. The amphibious robots are almost all bionic frog robots. The robot uses intermittent motion so that it may control its posture, in addition to being able to effectively control the energy accumulation and release. Therefore, this type of robot is characterized by its flexible jumping, powerful explosive force and environmental adaptability [16]. Compared with these types of robots, the arthropod robot has the advantage of fast acceleration, low energy consumption and high energy efficiency in the process of jumping because of its small size, light weight and good bounce ability [17]. The structure model of the jumping robot can be divided into a single-legged model and a multi-legged model. In terms of the single-legged jumping robot, based on the high mobility requirements of jumping, some scholars have proposed a single leg motion mode driven by hydraulics [18; 19]. The influence posture and ground impact of the robot on its structure in the vertical jump motion are analyzed, and the overall stability evaluated, to ensure that the robot can complete the take-off task. [PERSON] et al. [20; 21; 22] proposed a scheme based on the jumping mechanism of the kangaroo. They studied the jumping movement of the kangaroo and simplified their bodies into single-legged models to study and discuss. Then, they proposed three models: a rigid body jumping model, a compliant jump mechanism model and a rigid flexible hybrid model. The motion mechanisms of the three models were analyzed to see which one was most successful in making the robot jump smoothly. However, the single-legged model is a naturally unstable system, which cannot remain in the stationary state. Therefore, the single-legged robot has braced structures to maintain stability. Furthermore, it is necessary to adjust the initial attitude angle to achieve a smooth jump. In addition, the single-legged robots only realized the bionic jumping function, and were less involved in the overall movement mechanism.