## 1 Introduction  
With the development of science and technology, robots have become indispensable automation equipment in human society by assisting or replacing humans in various situations. Among mobile robots, hopping robots show a stronger ability to overcome obstacles in the fields of planetary surface exploration, unstructured terrain search, etc. [1; 2; 3; 4] when compared with wheeled robots [5], legged robots [6], and tracked robots [7]. The hopping robot was first proposed by [PERSON] and [PERSON] [8] in 1967. Then the one-leg jumping model proposed by [PERSON] [9] became the theoretical research basis for later single-leg jumping robots. As time goes, the NASA three-generation hopping robot [10] and the MIT MICRBOTS [11] significantly progressed the hopping robot research. According to hopping characteristics of the hopping robot, hopping robots can be classified into two categories: continuous hopping robot and intermittent hopping robot. A continuous hopping robot can recharge its hopping energy in the flying/landing phase, and immediately hop again after contacting the ground, while the intermittent hopping robot needs to realize its energy recharge and hopping attitude adjustment in another ground phase. Currently, continuous hopping robots usually adopt hydraulic [12; 13] and pneumatic [14] actuators to realize continuous actuation, which can enable the continuous hopping robot to have high power density and fast drive-response. However, it is still difficult to realize miniaturization and multi-drive coordinated control for them. The intermittent hopping robots usually use their motor as their actuator and use springs as an elastic components [15; 16]. Then, by slowly recharging and locking the energy of elastic components by the motor and the hopping mechanism, the intermittent hopping robot can realize hopping once the elastic energy is released. Due to the limitation of the motor and the complexity of additional energy locking and releasing mechanism in the intermittent hopping robot [15; 16; 17], the preparation time that is required to complete a hop will become relatively longer than continuous hopping robots. The energy locking and releasing mechanism of the hopping robot can be divided into two categories:  
1. Take advantage of the characteristics of the actuator, either by increasing the number of actuators or by using the forward and reverse rotation of the actuator. For instance, NASA's second-generation frog-inspired hopping robot uses an additionally actuated latching mechanism [10], while the NPU kangaroo-inspired hopping robot additionally uses an actuated ratchet-pawl mechanism [15; 16] to realize energy locking and releasing. In addition, the locust-inspired hopping robot designed by [PERSON] et al. [18] and the integrated jumping-crawling robot designed by [PERSON] et al. [17] realizes energy release by reversing the actuating motor. The energy storage and release of these aforementioned robots are all distributed; therefore, it is difficult to absorb the landing impact energy for the next hop. 2. Using special devices and irregular component contours such as eccentric cam [19; 20; 21], incomplete gear [22; 23; 24; 25; 26; 27], etc. to achieve the conversion between energy storage and release. For example, a miniature jumping robot proposed by [PERSON] et al. [28] uses a quick release detent ball mechanism to hold the spring, and it uses the lever as a strike mechanism to strike the push shaft to release the stored energy. [PERSON] et al. used two eyebolds located at opposite ends of the rear leg and the fishing line to hold energy for the spider-inspired hopping robot [29], which can release the energy stored in the spring by cut the fishing line. In addition, quadruped robot with jumping ability uses the ratchet and pawl [30], while the first-generation MSU Jumper [31] and the second-generation micro-robots uses one-way bearings. Moreover, the cylinder-shaped robot [32] uses the latch and the hook to realize the conversion of energy state.