# A Review of Bioinspired Vibration Control Technology
## 2 Bionic Devices Based on the Woodpecker's Head  
Because of the outstanding vibration reduction performance of their heads, woodpeckers have become one of the biological objects that various vibration devices are eager to imitate. The head structure of a woodpecker is shown in Figure 1a [42]. Firstly, the hard skull of a woodpecker's head can withstand the impact of hitting a tree trunk. Secondly, its head has many developed muscles and dense soft tissue, which can effectively absorb vibrations caused by the impact [43]. Finally, there is a very narrow interspace between the outer meninges and the brain, which can weaken the vibration to a certain extent. Based on the above vibration reduction principle, [PERSON] reported an active vibration isolation system [42]. As shown in Figure 1b, the system was mainly composed of a rubber layer, an air spring, and an actuator made of magnetostrictive material, which were used to imitate the cartilage and muscle in the outer meninges, the interspace between outer meninges and the brain, and nerves and muscle, respectively. The dynamic model of the system is displayed in Figure 2. The bionic system used a neural network algorithm to control the actuator, and the results showed that the system can effectively reduce the impact of vibration caused by ground disturbance or direct disturbance on the target platform. After that, a similar bionic vibration control system with viscoelastic materials was proposed [44]. As shown in Figure 3, the difference between this system and the above system was that an air spring and a damper composed of viscoelastic materials were used to simulate the interspace between the outer meninges and the brain in woodpeckers. The experimental results proved that the system can effectively suppress middle-frequency and high-frequency vibrations. Figure 1: Schematic diagram of woodpecker’s head structure (**a**) and bionic active vibration isolation platform (**b**) [42]. Figure 3: Bionic vibration control system using viscoelastic materials [44]. Figure 2: Dynamic model of the bionic vibration isolation system [42]. where m, c, and k are, respectively, the mass, damping coefficient, and stiffness, and the subscripts of 1, 2, 3, 4, and 5 represent a tree, the beak, the hyoid, the skull bone, and the brain of the woodpecker. According to the Lagrange equation of motion, the system was expressed as  
\[\left[ \begin{array}{cccc}m_{2}&0&0&0\\ 0&m_{3}&0&0\\ 0&0&m_{4}&0\\ 0&0&0&m_{5}\end{array} \right]\left[ \begin{array}{c}\ddot{x}_{2}\\ \ddot{x}_{3}\\ \ddot{x}_{4}\\ \ddot{x}_{5}\end{array} \right]+\left[ \begin{array}{cccc}c_{1}+c_{2}&-c_{2}&0 &0\\ -c_{2}&c_{2}+c_{3}&-c_{3}&0\\ 0&-c_{3}&c_{3}+c_{4}&-c_{4}\\ 0&0&-c_{4}&c_{4}\end{array} \right]\left[ \begin{array}{c}\dot{x}_{2}\\ \dot{x}_{3}\\ \dot{x}_{4}\\ \dot{x}_{5}\end{array} \right]= \tag{1}\] \[\left[ \begin{array}{cccc}k_{1}+k_{2}&-k_{2}&0&0\\ -k_{2}&k_{2}+k_{3}&-k_{3}&0\\ 0&-k_{3}&k_{3}+k_{4}&-k_{4}\\ 0&0&-k_{4}&k_{4}\end{array} \right]\left[ \begin{array}{c}x_{2}\\ x_{3}\\ x_{4}\\ x_{5}\end{array} \right]=\left[ \begin{array}{c}F_{(t)}\\ 0\\ 0\\ 0\end{array} \right]\]  
Figure 4: Schematic diagram of woodpeckers’ head endoskeleton [45]. Figure 5: Sectional view of woodpeckers’ head [48]. Figure 6: Simplified vibration reduction model of woodpeckers’ brains as they hit tree trunks [45]. Based on the above analysis, [PERSON] et al. proposed a new bionic vibration absorption system [45; 48]. The system consisted of two layers of metal material, one layer of viscoelastic material, and close-packed micro-glass, as shown in Figure 7. Metal layers made of steel and aluminum mimicked, respectively, the beak and skull of the woodpecker. The viscoelastic layer corresponded to the hyoid bone. The porous structure formed by the micro-glass spheres served as spongy bones to absorb the mechanical vibrations generated by the micromechanical devices for a short time.