Study On A Human Body Thermal Energy Powering Wearable Sensor System

The rapid development of communication technology and sensor fabrication technique have enabled the Wireless Sensor Networks(WSNs)to be applied to monitoring multiple types of objects gradually,such as the medical application,intelligent home,materials detecting,military application among others.The successful application of WSNs greatly facilitates people's life and promote the development of related technology fields.However,WSNs still face several obstacles of development.The sensor nodes of traditional WSNs are powered by the small sized batteries,and thus the operating condition of a node is determined by the power supply state of the battery.Hence,the working time of an entire network will be limited by the capacity of the battery of a single node.When the battery power is exhausted,it is necessary to replace the battery manually.But for some deployment nodes in the field environment,it will greatly increase the maintenance cost.The proposal of Energy Harvesting(EH)technique provides a favorable solution to this issue.EH utilizes the natural environmental energy,such as solar energy,vibration energy,heat energy,biological energy,etc.Such environmental energy can be converted into low electrical energy by means of energy harvester.Through the design of post-level circuit system,the produced electricity can be collected and used to power the sensor node.Since the energy of natural environment can be considered as several kinds of infinite energy sources,the electrical energy generated from energy harvester can also be recognized as the infinite electricity source.Therefore,in theory,the self-powered WSNs that use the EH technique to power the sensor node can operate ‘permanently'.Wireless Body Area Networks(WBANs)can be considered as a type of application forms of WSNs.Through using the wearable sensors,human physiological parameters can be detected.Similarly,in order to address the same issue of sensor node power supply of WBANs,EH technique can also convert the related energy of human body into electricity,and power the wearable sensor devices applied to human body and realize self-powered WBANs.In this thesis,a human thermal energy harvesting powering wearable sensor system is designed.Considering the temperature gradient between the human skin and external environment,the Thermoelectric Generator(TEG)is utilized to convert the human thermal energy into the electricity.A Power Management System(PMS)oriented toward human thermal energy harvesting is proposed as well.Using off-the-shelf commercial power management chips,LTC3108 and BQ25504,a power management circuit with the ability of accepting more than 20 Mv input voltage is developed.Combining the features of two different chips,the proposed PMS circuit can operate in three periods: Start-up period,High temperature difference period,Ultra-low input period.When the input voltage of system is between 20 mV to 150 mV,system operates in the ultra-low input period and it works in high temperature difference period while the input voltage is higher than 150 mV.The proposed PMS system omit the usage of micro-controller and thus reducing the size,improving the wearability of system and the harvested energy's storing rate as well.Moreover,for validating the effectiveness of proposed thermal energy harvesting system,the SHT20 digital temperature sensor is adopted as a wearable sensor,and the sensor node is formed with CC2530.The relevant wireless network composed of Zigbee communication technology is tested to verify the functionalities of temperature detecting and data transmitting and receiving.Finally,the experimental verification is carried out in this paper as well,including testing the output performance of TEGs used in system and testing the functions of designed PMS.Furthermore,the system is also applied to the practical application of human body based.When the temperature difference is 10.2? and the input voltage is 104 mV,the system is able to effectively power the sensor node to transmit the data intermittently and the storage of converted energy can be realized as well.The conversion efficiency is 26.5% and the rate of energy storing is 89.7%.Additionally,other working periods of developed PMS have also been validated.