Research On Wearable Hybrid Energy Harvesting Technology

For maintenance-free and self-powered wearable device,focusing on the ambient RF electromagnetic energy and vibration energy from human body,a new type of wearable hybrid energy harvesting system has been designed,the whole system includes vibration module,RF module,rectifier module,boost module and energy storage module.Taking into account the wearable feature,the flexible piezoelectric materials PVDF are used as vibration sensor and the patch antenna is used as RF energy sensor,which forms the hybrid energy harvester.This idea breaks through the traditional system that harvests only single energy,it effectively enhances the harvested power of energy and expands the application field.Based on the theory of piezoelectricity and Maxwell electromagnetic field theory,two structural modules of energy harvester and the optimization method of efficiency improvement were studied in this paper.Several antenna structures and piezoelectric devices were designed,the theory and design method of high efficiency rectifier were researched.A low threshold rectifier circuit was proposed and simulated.Finally,two kinds of hybrid energy harvesting systems were constructed and tested.Specific designs and conclusions are summarized as follows:Two piezoelectric structure models are designed to collect vibration energy,one is pedal type structure,which puts the piezoelectric material in the shoes.The structure was tested by human walking state at 2Hz frequency.When there is no load,the output voltage of the structure is 4.13 V,when the resistance is 600 kohm,the voltage across the resistor is 2.58 V and the output power is 11.093μW.The other is cylindrical structure,which is suitable for bending joints,the maximum peak to peak output voltage is measured as 3.52 V at the frequency of 1Hz,when the resistance is 600kΩ,the voltage across the resistor is 1.8V and the output power of piezoelectric device is 5.4μW.Two kinds of RF energy harvesting antenna are designed,one is embedded structure,it can be placed in the mobile phone shell or the arm package for mobile phone,the simulated best resonant frequency is 2.34 GHz and the measured value is 2.36 GHz,the harvested RF power in the ambient is-30.82 dBm,about 0.83μW.When it embedded in the phone shell,it can collect the RF energy is-17.67 dBm,about 17.1μW.The other is the wrist type structure,the best resonant frequency is 2.37 GHz,the harvested RF in the ambient is-32.82 dBm,about 0.52μW.When the phone is close,the harvested power of RF energy is-18.94 dBm,about 12.76μW.A low threshold circuit is designed,including the design of rectifier,filter and boost circuit.In the design of the rectifier,according to the low energy density of the RF and vibration energy in the ambient,a low threshold circuit is proposed,the minimum input voltage is 200 mV,the rectifier efficiency is 64% at the low frequency and it is 46.24% at the high frequency.Boost circuit is divided into two parts: one part is ring oscillator which provides 1kHz oscillation frequency,the quiescent current is 2.84 nA,and the power consumption is 452 pW.The other part is the charge pump circuit,a four-grade two-way charge pump circuit.When the input voltage is 200 mV,the output voltage is 970 mV,the average power of the storage capacitor is 1.14μW and the voltage ripple rate is ±0.5%,the conversion rate is about 10%.Hybrid energy harvester system is built,HSMS2852 is used to make the rectifier bridge then the energy storage capacitor is added.The cylindrical piezoelectric and the embedded antenna structure are connected in series.The max voltage of the system is 1.663 V,the max output power is 2.77 mW.The pedal type piezoelectric and ring type antenna were connected in series.The voltage of the system is 1.36 V,the max output power is 1.84 mW.These tests can be verified that the whole system can supply power to the power wearable device independently.