Working Principle and Structural Design Conclusions and Further Work MEMS Piezoelectric Vibration Energy Harvester with Multiple Resonant Frequencies Jing Zhao, Xingguo Xiong, Peiqiao Wu School of Engineering, University of Bridgeport, Bridgeport, CT 06604 Abstract MEMS (Microelectromechanical Systems) vibrational energy harvester can collect vibrational energy from surrounding environment and convert it into electrical energy, so that it can be stored in battery for future use. Due to their small size and quick response, they can be embedded in shoes, under the ground surface of busy streets to harvest energy which otherwise would be wasted. In this poster, a MEMS vibrational energy harvester with multiple nested beam-frame structure is proposed. Due to its nested structure, it has 3 cantilever beams leading to three different resonant frequencies. This allows it to harvest energy at three different frequencies, hence improves its energy output. The device is designed and simulated in COMSOL. Modal simulation is used to find the multiple resonant frequencies of the device. COMSOL piezoelectric simulation is used to verify the piezoelectric response of the MEMS energy harvester at different frequencies. The proposed MEMS energy harvester can collect vibration energy with improved frequency response. Figure 2. Maximal Displacement (Left) and Electric Potential (Right) with the resonant frequencies. a. Freq = 222.69 Hz; DispMax =450 μm b. Freq = 222.69 Hz; Electric potential = 20V c. Freq = 479.57 Hz; DispMax = 250 μm e. Freq = 479.57 Hz; f. Freq = 1224.2 Hz; DispMax = 200 μm g. Freq = 1224.2 Hz; Electric potential = 25V Introduction Internet of Things (IoTs) need many micro sensors to monitor the status of the environment. Such micro sensors are typically powered by batteries. For long-term continuous monitoring of the environment, the batteries require periodical replacement or recharging, which can be a costly and cumbersome task. To overcome this deficiency, efforts have been devoted to harvesting energy from the ambient environment. Such energy transducers are termed as micro-energy harvesters or micro-power generators. There are several different energy sources available, such as solar energy, thermal gradient and mechanical vibration. Among them, mechanical vibration is ubiquitous, and hence it has drawn significant attention. Mechanical vibration can be converted into electrical energy as a power supply via electrostatic, electromagnetic, or piezoelectric transductions. Piezoelectric vibration energy harvester generally consists of a beam and mass structure, with piezoelectric material deposited on the beam surface. When the beam is actuated by the environment vibration, the induced stress in piezoelectric material lead to voltage output, which is in turn used to charge a battery. In this way, the electrical energy can be stored for future usage. Piezoelectric vibration energy can harvest most energy when the external vibration is around the resonant frequency of the structure. Most piezoelectric vibration energy harvester is designed to be sensitivity to one resonant frequency. In this poster, we proposed a novel MEMS piezoelectric energy harvester with nested structure. The proposed device has multiple resonant frequencies, leading to wider frequency band, hence it can harvest more energy from the environment. After given the MENS piezoelectric vibration energy harvester a acceleration (a = 1×g = 9.8 m/s2, the result of frequency domain show in Figure 3, 4 and 5. Base on the result of the piezo effect voltage under different frequency, we got the best frequency range for the piezoelectric vibration energy harvester, as show in Figure 6, the best frequency that products the maximal energy of the proposed piezoelectric vibration energy harvester is around 223 HZ. Working Principle and Structural Design Traditional MEMS vibration energy harvesters are designed with a single vibrational mode. In this way, they are most efficient to pick up the vibration at a certain frequency, if the frequency of the vibration energy harvester was far away from the frequency of the ambient vibration, the efficiency would be very low . In order to pick up more energy from environmental vibration at different frequencies, a wider frequency band is preferred. In this research, we proposed a MEMS piezoelectric energy harvester with nested structure, so that it has multiple resonant frequencies. This allows the device to harvest more energy at different frequencies. The mass of two unit at the beam end is used to adjust the resonant frequency and out put power of harvester. The three beams of the harvester is based on PZT, when the beams vibration at a certain frequency, the piezoelectric material is placed under mechanical stress, meaning that the generation of electric when stress is applied. As shown in Figure 1 respectively. Two units are nested into the third unit leading to multiple resonant frequencies. Using simplified spring-mass model, the resonant frequency of the structure can be estimated as: f=(1/2π)sqrt(k/M), where f is frequency; k is Hooke’s constant of the beam; M is the mass. The Hooke’s constant of cantilever is: k=EWbtb3/(4Lb3), where E: Young’s modulus of material, Wb, tb, Lb: width/thickness/length of cantilever. Figure 3. Potential distribution under frequency of 150 HZ. (a = 1g) Figure 4. Potential distribution under frequency of 250 HZ. (a = 1g) Figure 5. Potential distribution under frequency of 500 HZ. (a = 1g) Figure 6. Total Electric Energy V.S. Frequency. Figure 7. Displacement distribution under frequency of 223 HZ. (a = 1g) Figure 8. Potential distribution under frequency of 223 HZ. (a = 1g) Material Material properties value PZT-4 Young's modulus, Ep (GPa) Poisson's ratio, σp Density, ρp (kg/m3) Piezoelectric charge constants, d31 1012 (C/N) 7.8 0.32 7600 -122 Al Young's modulus, Es (GPa) Poisson's ratio, σs Density, ρs (kg/m3 ) 69 0.33 2700 silicon 160 0.22 2330 Silicon oxidase Young's modulus, Eso (GPa) Poisson's ratio, σso Density, ρso (kg/m3 ) 72 0.16 2200 Figure 1. The design of piezoelectric vibration energy harvester. Table 1. Material properties of MEMS piezoelectric vibration energy harvester. Beam Length(μm) Beam Width(μm) Beam Thickness(μm) Mass Length Mass Width Mass Thickness Unit 1 5600 400 60 4400 400/3400 600 Unit 2 1200 200 40 800/2800 800/1000 1000 Unit 3 Table 2. Geometrics of the energy harvesters Design. PZT Length PZT Width PZT Thickness 5100/5000 10 1000/900 Al Length Al Width Al Thickness 5000 5 900 Table 3. PZT geometrics of the piezoelectric energy harvesters Design.(Unit: μm) Table 4. Al geometrics of the piezoelectric energy harvesters Design. (Unit: μm) Conclusions and Further Work In this poster, a MEMS piezoelectric energy harvester with nested beam-mass structure is proposed. Due to its multi-stage nested structure, the structure has multiple resonant vibrational modes with different resonant frequencies. This leads to wider frequency band and can pick up more energy from the environment compared to traditional energy harvester with single resonant vibrational mode. The working principle of the structural design is analyzed in details. COMSOL modal simulation is used to extract the resonant frequencies of the energy harvesters. We also perform piezoelectric simulation to simulate the voltage output due to input vibration from the environment. In the future, we will focus on manufacturing the MEMS piezoelectric vibration energy harvester. Simulation Results COMSOL simulation is used to extract the interrelated vibration modes for the proposed energy harvester. In the left of Figure 2 shows the results of bending displacement of the design are simulated in COMSOL with given frequency. In the right of Figure 2 show the result of the electric potential under given frequency.