A Research On The Piezoelectric Energy Harvesting System Based On Vortex Induced Vibration

The ocean monitoring is the basis of the integrated marine management.It detects and forecasts the marine environment quality,and provides information and evidence for the marine resource utilization and conservation.With the development of microelectronics technology,the wireless sensor network(WSN)which consists of a large number of micro sensor nodes has been deployed in the ocean to collect and process information.They have the advantages of low cost,flexible layout,wide coverage,high signal-to-noise ratio,and strong fault tolerance.Hence,the WSN has a strong applicability in the field of ocean monitoring.However,the power supply limits its application.The traditional power supplies include the chemical batteries and cablecasting.Because of the cost,lifetime,and maintenance problems,they are not suitable for powering the widely distributed sensor nodes.In order to power the sensors,a vortex-induced vibration piezoelectric energy harvesting system(VIVPEHS)is studied in this dissertation.The energy harvesting system using a cylinder and a PVDF(polyvinylidene fluoride)cantilever beam is analyzed systematically.The structures of the bluff body and the piezoelectric body are improved to increase the output power of the system.They are respectively placed in parallel to further raise the output electricity.In addition,a novel charge amplifier is designed to test the system performance.The main contributions are stated as follows:(1)A VIVPEHS with a cylinder and a PVDF cantilever beam is designed.The mathematical and simulation models are established with multiple physical fields coupled,which are verified by the experiment.The mathematical model starts from the basic material properties of the fluid,the bluff body and the piezoelectric body.The hydroelastic and electromechanical subsytems are analyzed,respectively.As an elastic and piezoelectric body,the PVDF transducer works in both the subsystems and couples them by the elastic stress.The dynamic simulation of the system is carried out with the flow field,the stress field,and the electric field coupled.The simulation results indicate that the lift and drag forces on the cylinder and the piezoelectric cantilever beam are proportional to the the flow velocity and the cylinder diameter,as well as the the output voltages of the PVDF cantilever beam.The experimental platform is designed,simulated and manufactured to test the system.The experimental results are in good agreement with the simulation data,which verifies the validity of the system.(2)The structures of the cylinder and piezoelectric catilever beam are improved to increase the input vibrational energy and output electrical energy.The vortex streets induced by the cylinders with a slit,a concave back or both the slit and concave back are analyzed and compared.The vortices shed from the cylinder with a slit and a concave back are most stable,and have the highest frequency and vorticity.Therefore,the cylinder with a slit and a concave back is best suited for the energy harvesting system.In the VIVPEHS,since the modified cylinder enhances the input vibrational energy of the system efficiently,the output valtage of the PVDF cantilever beam are raised.Based on the analyses of the electric-mechanical transfer model of the PVDF beam,the thickened piezoelectric film is able to increase the ouput voltage of the system significantly,namely,the output electrical energy.(3)In order to further increase the output power of the system,the VIVPEHS with multiple parallel modified cylinders and PVDF cantilever beams are designed and researched.The vortex fields induced by the double improved cylinders in tandem or in parallel are simulated,whose vortex street characteristics are studied with different distances between the two bluff bodies.The two improved cylinders and two PVDF cantilever beams in parallel raise the vortex shedding frequency and the output voltages by coupling the two vortex streets.The two improved cylinders and three PVDF cantilever beams in parallel therewith have little effects on the vortex shedding frequency.The piezoelectric beam between the two vortex streets does not influence the energy harvesting of the other two beams,meanwhile,it is able to output comparable electricity.The three improved cylinders and five PVDF cantilever beams in parallel further improve the vortex shedding frequency and the output voltages.The parallel structures of the bluff and piezoelectric bodies make the energy harvesting system more complex,but the positive couplings between the vortex streets and the piezoelectric cantilever beams enhance the output energy of the system.(4)Based on the relationship between the output charge,the force and the displacement of the piezoelectric cantilever beam,a novel charge amplifier is designed to detect the output charge of the energy harvesting system.The applied force and the vibration displacement can be obtained then.As the output electric charge of the piezoelectric cantilever beam is very tinny,the structure and parameters of the amplifier circuit are optimized to reduce the the measured voltage noise,the discharge time and the measurement error,which ensures the high measurement accuracy of the circuit in the low-frequency working section of the energy harvesting system.The joint simulation and experiment of the VIVPEHS and the circuit are carried out,and their corresponding results are consistent with the theoretical results.The charge amplifier for the energy harvesting system is capable of evaluating the system performance.