# A Review of Bioinspired Vibration Control Technology
## 3 Bionic Devices Based on Animal Leg  
The ostrich, horse, cheetah, and other animals can run quickly in complex terrain environments without being harmed by the vibrations caused by the impact of the ground. Their unique leg structure plays an indispensable role in vibration reduction [40]. As shown in Figure 8a, there are usually X-shaped and Z-shaped structures in the legs of birds. Additionally, an X-shaped sponge structure is also found in the skull (Figure 8b). Inspired by the above-mentioned animal leg structures, [PERSON] et al. proposed an X-shaped bionic vibration control device [49]. In order to better achieve the vibration reduction effect, the animal body structure was simplified. The part of the body above the leg was considered a mass. The leg bones were simplified into rods and retained their characteristic of varying lengths. Springs with different degrees of stiffness and different directions were used to imitate the developed muscles of the legs (Figure 9). Then, the influence of each structural parameter on the system stiffness, loading capacity, and vibration isolation effect was systematically analyzed. Through mathematical modeling and experimental analysis, the bionic device was shown to have the same nonlinear  
Figure 8: (**a**) A bird skeleton and (**b**) cranial bone [49]. Figure 7: Bionic vibration absorption system [45]. characteristics as the animal leg, and its nonlinear stiffness and damping coefficient were able to be adjusted by adjusting the number of layers, the length of the connecting rod, and the installation angle [41; 50]. Compared with the quasi-zero stiffness vibration isolation devices, the bionic device not only had low resonance frequency, but also had excellent loading capacity. After this, [PERSON] et al. also proposed two novel X structures and analyzed their static stiffness, the vibration isolation effect, and the nonlinear law of vibration frequency under three different constraint conditions [39]. In recent years, they have also developed a six-free vibration isolation platform based on the X structure, which has been successfully applied in many fields such as aerospace and machinery [51; 52; 53; 54]. When animals or people move, the angles and shapes of the joints of the legs change (Figure 10a). These changes may be closely related to the damping effect of the entire leg [55]. Inspired by the above phenomenon, [PERSON] et al. designed a multi-joint bionic vibration control device with different stiffness and joint parameters [56]. In contrast to the X-shaped device described above, small parallelograms with springs of different stiffness initiated the functions of each joint of the leg (Figure 10b). Compared with the traditional QZS system, this system had better static load capacity and lower resonance frequency. Cats are known to be able to fall tens of meters through the air without being hurt [57; 58]. As shown in Figure 11a, cats have three leg bones that allow them to adjust their posture flexibly during a fall to minimize damage. Based on the above analysis, [PERSON] et al. developed a bionic vibration isolation device with a polygonal structure [37]. Based on the overall skeleton structure of the cat, a symmetrical quadrilateral structure with four springs was designed (Figure 11b). As shown in Figure 11c, a three-rod structure mimicked the three long bones of the cat's legs. The bearing platform was designed at the top and had a chute to adjust the distance between the legs. The bionic vibration isolation platform displayed excellent suppression effects on the three forms of vibration (periodic, scanning, and random excitation). Figure 9: Bionic schematic diagram (**a**) and structure diagram (**b**) of X-shaped bionic vibration control device [49]. In order to protect itself, the frog can effectively suppress vibration during jump. Inspired by the M-shaped limb structure of the frog, [PERSON] proposed a bionic QZS structure. The bionic QZS structure was mainly composed of plates, torsion springs, and connecting rods (Figure 12b).