Study On The Micro Electromagnetic Vibration Energy Harvester Based On MEMS Technology

The recent advances in microelectronics technology and the design of ultra low power VLSI systems have been making various electronic devices smaller, wireless, portable and low power consumption. Such advance has led to many new applications such as medical implants, embedded sensors and microsystems, wearable devices and wireless sensor networks. However, how to power these micro devices effectively is becoming a key question which limits their practical application. The power consumption of these devices is very low and even in the level ofμW. Meanwhile, they are often used in very bad environment inaccessible to mankind or completely embedded in the structures without any physical connection to the outside world. So they have very high requirements for the volume, cost, lifetime and performance of power supply. The conventional power supplies of these devices are battery and electrical cable. Battery has a short lifetime, contains a finite amount of chemical energy and is bulky compared to micro devices. And it is hardly possible to periodically replace the batteries for lots of wireless sensor nods. On the other hand, electrical cable limits the application of various embedded wireless sensors and microsystems. As a result, the conventional power supply can not satisfy their power requirements. Energy harvesting technology can convert light, heat and kinetic energy available in the sensor's environment into electrical energy and substitute the conventional power supply for powering various micro devices. So they are attracting more and more attention from academic and industrial field.This dissertation mainly focuses on electromagnetic vibration energy harvester based on MEMS technology. A novel sandwiched electromagnetic vibration energy harvester with air channel is presented in this work; the energy harvester prototype is fabricated by silicon bulk micromachining and microelectroplating technique, and the performance of the prototype is measured and analyzed totally. The main work and the conclusion of this dissertation are in the following:1. The physical model of the electromagnetic vibration energy harvester was build including the physical model of vibration harvesting system and that of energy conversion system. The dynamic response of vibration harvesting system under different vibration excitation was solved and used to analyze the influence of damping ratio and vibration frequency on the amplitude-frequency and phase-frequency characteristics. In the physical model of energy conversion system, the average power dissipated within the damper and the power delivered to the electrical domain was calculated and then used to analyze the relationship between output power and input vibration amplitude, resonant frequency, damping ratio and so on. Some conclusions were drawn from the analysis of physical models, which can be used to resonantly design the electromagnetic vibration energy harvester.2. A micro electromagnetic vibration energy harvester based on MEMS technology was presented which includes an NdFeB permanent magnet, a planar spiral nickel spring on silicon and a double-layer copper coil. The nickel spring on silicon was fabricated by silicon bulk micromachining and microelectroplating technique. This fabrication method not only can overcome the disadvantage of low precision brought by conventional mechanical process technology, but also can overcome the disadvantage of high cost brought by silicon deep etching technology. Meanwhile, the gap between magnet and coil can be controlled precisely by using silicon wafer with different thickness. The method of using microelectroplated nickel to fabricate planar spring and the design of spiral shape of spring's beam are very helpful to decrease the natural frequency of vibration harvesting system. Structure static analysis, modal analysis and harmonic analysis of the magnet-spring system were carried out using finite element analysis software--ANSYS. The influence of structure parameters of the planar spring and the magnet on the vibration characteristics of the magnet-spring system was analyzed. Ansoft Maxwell software was used to simulate the electromagnetic characteristics of the energy conversion system and to analyze the relationship between output performance and the structure parameters of coil and the magnet. The optimized structure of the vibration energy harvester was obtained from the optimization results.3. A novel sandwiched electromagnetic vibration energy harvester with air channel was presented, which includes an up coil, a magnet-spring system and a bottom coil. Its sandwiched structure is clearly different with other published prototypes. The symmetrical arrangement of two coils can make the magnetic field be utilized sufficiently and the air channel in silicon frame can decrease the air damping efficiently, both of which can increase the energy conversion efficiency. Meanwhile, the up coil can protect the magnet-spring system from being harmed. The electromagnetic simulation results show that the output voltage of the sandwiched energy harvester is increased 61% comparing with that generated by the energy harvester with single coil. The influence of the phase difference of induced voltage from up and bottom coil on the voltage generated in series coil was also analyzed.4. The electromagnetic vibration energy harvester prototypes were fabricated using MEMS micromachining technique. Microelectroplating technique and polishing technique of polyimide were used to fabricate double-layer copper coil. The planar nickel spring on silicon frame was fabricated using double-side photolithography, bulk silicon micromachining technique and microelectroplating technique. At last, the NdFeB permanent magnet, planar nickel spring and micro coil were assembled into different energy harvester prototypes.5. The output performance of the energy harvester prototype was measured completely. The testing results of the magnet-spring system show that the spring constant of the planar nickel spring is 145.6N/m and the natural frequency of the vibration system is 228.2Hz. The reason about the difference between the simulation results and the tested results was given. The tested results also show that the input vibration acceleration has great influence on the resonant amplitude and resonant frequency. The electrical output performances of three different prototypes were tested. The maximal output voltage and output power of the prototype with single coil are 101mV and 19.5μW respectively, under the vibration excitation of 0.8g acceleration and 280.9Hz. The output voltage of the sealed sandwiched energy harvester prototype is 125mV which is increased 23.8% comparing with that of the prototype with single coil. The induced voltage of the sandwiched energy harvester prototype with air channel is 162.5mV which is increased 60.8% comparing with that of the prototype with single coil. The tested results show that the maximal output power of the sandwiched energy harvester prototype with air channel is 21.2μW. Meanwhile, the tested results also show that the nonlinear vibration behavior of the spring can increase the bandwidth of the amplitude-frequency curve of the output voltage of the prototype. So the nonlinear vibration of spring can be used to realize the wideband operation of energy harvester.