Electromagnetic vibration energy harvester using magnetic fluid as lubricant and liquid spring

•A vibration energy harvester based on the levitation of magnets in magnetic fluid is proposed.•A liquid spring with variable stiffness coefficient is designed.•Helical coils and planar coils are used together to improve output voltage.•The vibration energy harvester has excellent performance under weak vibration. Using vibration energy harvester (VEH) to achieve self-power supply is an effectively way to ensure long-term use of electronic devices. In this paper, an electromagnetic VEH using magnetic fluid (MF) as lubricant and liquid spring is proposed, the VEH uses a hollow shell with variable internal diameter together with MF to form a liquid spring with variable stiffness coefficient to replace the traditional mechanical or magnetic spring, and planar coils and helical coils are used together to harvest the vibration energy, thereby broadening response frequency, improving energy harvest efficiency, and reducing VEH volume and damping. The influence of MF on the lubrication, liquid spring, and vibration state is studied theoretically, and the induced voltage is simulated. A crank-slider linkage has been built to test the performance of VEH, and the influence of vibration frequency, mass of MF, and coil type on the output performance has been studied during a 7.5 mm reciprocating linear motion of VEH. Results show that an appropriate amount MF can improve the output voltage by 173 % at 3.25 Hz, but too much MF will cause a rapid drop in output voltage. Helical coils generate higher voltage than planar coils, and the output power density of the helical coil is 0.92 mW/cm3 at 8.5 Hz. The energy management module can manage electric energy well, and half wave rectifier with low forward voltage drop may be more suitable for VEH with low output voltage. Besides, the VEH delivers a 1.36 mW/cm3 power density with a 10 Ω load when it is swung by human at 6.4 Hz.