Research Of Self-Powered Piezoelectric Ambient Energy Harvesting System For Wireless Sensor Networks

With the rapid development of wireless sensor networks,the numbers of ultra-low power smart sensors increase dramatically,so that the power supply system for wirless sensor nodes is required to have the features of being self-powered,maintenance-free,low cost,compact and light weight.The self-powered energy harvesting system,which could effectively convert the weak ambient energy into electricity energy,will replace the battery and become one of the most important ways to power wireless sensor nodes in the future.Since the piezoelectric materials can convert the mechanical energy into electrical energy and have the special features such as small size and high energy density,the piezoelectric energy harvesters are widely used in a variety of ambient energy harvesting systems.This disseration is devoted to the research and realization of the self-powered piezoelectric ambient energy harvesting system for wireless sensor nodes,and especially focuses on the efficient piezoelectric energy collection and conversion technologies from the low-frequency and irregular ambient energy.1.Main research works?1?The piezoelectric energy harvesting technology for the intermittent and random mechanical energy.According to the characteristics of the intermittent and random piezoelectric energy,a low-power energy harvesting circuit with sleep mode is proposed and designed to improve harvesting efficiency.In order to improve the conversion efficiency of the piezoelectric harvester for vehichles passing through the speed bump in the roadway,a novel structure of impact-type energy harvester based on a cantilever beam is proposed.Based on the above research results,a piezoelectric energy harvesting system applied to the vehicles passing through the speed bump is designed and implemented.The measured results show that when a vehicle passes through the speed bump with the speed of 5 to 20 km/h,the average output electrical energy of the piezoelectric energy harvester is 30.0 mJ,and the efficiency of the energy harvesting circuit is about 74.0%,which can be used to power a commercial temperature sensor for 37 minutes.?2?The impact-type piezoelectric energy harvesting technology for the continuous mechanical energy.According to the characteristics of the continuous and impact-type piezoelectric energy,a quick start-up circuit composed of the passive components such as resistors and capacitors is proposed to implement the function of being self-powered and cold-start in energy harvesting circuit,and an alternerting resistance impedance matching technology is also proposed to improve the harvesting efficiency.Based on the above research results,a micro-wind piezoelectric energy harvesting system is implemented.Depending on the speed of micro-wind,a fixed resistance impedance matching circuit for fast-speed wind,and an alternerting resistance impedance matching circuit for slow-speed wind are designed and realized,respectively.The experimental results show that the efficiency of the whole nicro-wind energy harvesting system with alternating resistance impedance matching is 4.6%at the wind speed of 2.0 m/s.And when the wind speed is more than 1.5 m/s,the designed micro-wind piezoelectric energy harvesting system can provide sustainable and steady power supply for a commercial temperature sensor node.?3?Research and design of digitally-controlled high-frequency DC-DC switching converter.The function of the power management circuit is to convert and adjust the output voltage of the piezoelectric energy harvesting circuit into the stable power supply voltage required by the wireless sensor nodes.Based on the requirements of wireless sensor node,the power management circuit should have the features of the digitally-programmability,the fast-transient response speed,and the compact and light weight.Thus,a digitally-controlled high-frequency DC-DC switching converter is researched and designed in this dissertation.Aiming at the difficulty in the design of digital controller for DC-DC switching converter in the high-frequency application,a design method based on?-operator for the digital controller is proposed.In order to improve the transient response speed of the digitally-controlled DC-DC switching converters,a digitally-controlled V² algorithm adopting the Adjacent Cycle Sampling?ACS?strategy is proposed.Based on the above theoretical researches,a Buck-type DC-DC switching converter with the switching frequency of 2MHz is designed and implemented.In the design of the digital controller with dual controlling loop,the outer controlling loop is designed using the D-PID algorithm based on?-operator,and the inside controlling loop is designed using the digitally-controlloed V² algorithm based on the ACS strategy.The measured results show that this DC-DC switching converter has an overshoot of75 mV and the transient response time of 3?s,for the load variation of 30%.2.Main innovations proposed?1?A low-power energy harvesting circuit with sleep mode is proposed.Considering the characteristics of the piezoelectric energy with the features of being imtermittent and random,a low-power energy harvesting circuit with sleep mode is proposed.When the energy harvester generates electrical energy,the energy harvesting circuit match the resistance impedance to implement obtainment and conversion of the maximum energy.When no electrical energy is generated by energy harvester,the circuit enters into the sleep mode to reduce its own power dissipation.Compared with the energy harvesting circuit without sleep mode,the energy harvesting circuit with sleep mode improves the efficiency of 18.6%.?2?A quick self-start up circuit is proposed.In order to implement the functions of being self-powered and cold-start up in the energy harvesting circuit,a quick self-start up circuit composed only by passive devices such as resistors and capacitors is proposed.The self-start up circuit can obtain the energy from the input side to power the entire energy harvesting circuit,so it is not necessary to use any external power source.And even if the energy storted in the battery is completely exhausted,as long as the energy harvester starts to generate energy,the energy harvesting circuit can quickly startup.?3?An alternating impedance matching technology is proposed for the impact-type piezoelectric energy harvester.In view of the low-frequency excitation of impact-type piezoelectric energy harvester,an alternating impedance matching technology is proposed to improve the efficiency of energy harvesting system.The energy harvesting circuit detects the impact period and the vibration period of the piezoelectric energy harvester,and then correspondingly changes the input impedance of the energy harvesting circuit to match the output impedance of the energy harvester which has different impedances in the impact and vibration period,thus the maximum energy can be transferred to the energy storage device in both the impact and vibration period.The efficiency can be effectively improved by using the alternating impedance matching technology,especially in the case of low-frequency excitation such as low speed micro-wind.Compared with the fixed impedance matching technology,by using the alternating impedance matching technology,the efficiency of the energy harvesting circuit can be increased by 9.8%.?4?A design method based on?-operator for the digital controller of the high-frequency DC-DC switchingconverter is proposed.For the digitally-controlled high-frequency DC-DC switching converters,the traditional design methods using the shift operator(z^-1)have the issues such as the poor convergence between discrete and continuous system,poor system stability and the effect of the finite word length.The design method of digital controller based on?-operator is proposed in this dissertation,which has the advantages of good system stability,high control precision and insensitivity to finite word length,espacialy at the high switching frequency.?5?A digital V² controlling algorithm based on the Adjacent Cycle Sampling?ACS?strategy is proposed for the digitally-controlled high-frequency DC-DC switching converters.For the proposed V² controlling algorithm,the digital controller consists of the inside and outside control loops,the inner control loop is based on the ripple control of the output voltage,and the outer oneis based on the errorcontrol of the output voltage using D-PID algorithm.Compared with the existed voltage/current dual-loop control algorithm,the digital V² control algorithm has the advantages of fast transient response speed and the simple circuit structure in which there is no need to detect the inductor current.The simulation results show that,compared with the digital voltage/current dual-loop control algorithm,by using the the digital V² control algorithm,the recovery time of the system is decreased by 3 times as the load is step-changed by 3 times.The simulation results show that,compared with the digital voltage/current dual-loop control algorithm,by using the the digital V² control algorithm,the recovery time of the system is decreased by 3 times as the load is step-changed by 3 times.Based on the proposed piezoelectric energy harvesting technologies,the specific self-powered energy harvesting systems and power management circuits for the speed bump and micro-wind are designed to power wireless sensor nodes.And the achievements obtained in this disseration have important theoretical significance and pratical values for developing the self-powered system harvesting the weak piezoelectric ambient energy and the digitally-controlled power managementsystem.