Research On Key Technologies Of Wearable Ultra-Thin Semi-Active RFID Temperature Tag

Body temperature monitoring is very important in disease prevention and treatment,as early detection of abnormal body temperature can help people seek treatment or preventive measures earlier.With the progress and development of low-power technology,RFID tag have integrated sensing and signal processing capabilities.Using RFID temperature tag to continuously monitor human body temperature in real-time has become an emerging temperature measurement method.However,due to the complexity of the human electromagnetic environment,when RFID tag come into contact with the human body,the impedance characteristics and radiation gain of the tag antenna will be deteriorated,ultimately affecting the communication distance of the tag.At present,researcher have developed a series of wearable RFID temperature measurement tags to achieve continuous monitoring of human body temperature.However,these wearable RFID temperature measurement tags usually only work in the HF frequency band(13.56 MHz)or UHF frequency band(840-960 MHz),which cannot take into account the convenience brought by different RFID systems.In addition,when the current wearable RFID temperature measurement tag is applied to armpit temperature measurement,the communication distance is too short due to the tag antenna being clamped under the human body’s armpit.In view of the above problems,this paper designs three wearable ultra-thin semi-active RFID temperature tags.A HF-UHF dual band wearable ultra-thin semi active RFID temperature measurement tag has been designed based on the equivalent solution model of the human body.This tag uses a small loop antenna as the matching structure to achieve conjugate matching between the input impedance of the ultra-high frequency antenna and the chip impedance.The bent UHF antenna is designed inside the HF antenna to achieve a compact dual band antenna design.In addition,a part of the left and right radiating arms of the antenna and the flexible battery are conformal overlapped to reduce the interference of metal components in the battery to the UHF antenna.The simulation results show that the impedance characteristics,radiation gain,and radiation pattern of the designed UHF antenna are insensitive to the metal components inside the battery.The experimental results show that when the tag is attached to the surface of the human equivalent solution model,the simulation results of the chip port impedance characteristics are basically consistent with the measured results,and the measured distances of the designed tag operating in the HF and UHF frequency bands reach 2 cm and 1.1 m,respectively.An ultra-thin semi-active RFID underarm temperature measurement tag based on wearable differential feed structure is proposed to address the issue of short communication distance when applying wearable temperature measurement tags to human underarm temperature measurement.In the design of the tag,a differential feeding structure circuit model based on ideal coupled microstrip lines was first proposed to solve the impedance mismatch problem caused by transmission line feeding in UHF RFID tags.On this basis,a wearable differential feeding structure was designed to keep the tag antenna away from the armpit environment;Then,based on the designed feed structure,a new type of loop tag antenna was designed.Compared with traditional dipole tag antenna,the radiation gain of this antenna increased by2.32 d B;Finally,the designed antenna is connected to the RFID chip through the wearable differential feed structural join for simulation analysis.The simulation results show that the input impedance of the chip port is close to the conjugate of the output impedance of the chip port at the 920 MHz frequency point.Experimental testing was conducted on the designed tag prototype,and the experimental results showed that when the tag was attached to the surface of the human equivalent solution model,the measured impedance characteristics of the chip port were very close to the simulation results,and the measured reading distance reached 3.3 meters.An ultra-thin semi-active RFID underarm temperature measurement tag based on wearable differential port to single port feeding structure is proposed to address the issue of short communication distance when applying wearable temperature measurement tags to human underarm temperature measurement.In the design of the tag,a differential port to single port feeding structure circuit model is first proposed to convert the differential signal output by the RFID chip into a single ended signal,so that the single ended antenna can also be applied in RFID tag design;Then,based on this,a wearable differential port to single port feeding structure was designed to keep the tag antenna away from the armpit environment;Subsequently,a new monopole antenna was designed based on the designed feed structure,which has stronger impedance regulation ability compared to traditional inverted F antenna;Finally,the designed antenna is connected to the RFID chip through the wearable differential port to single port feeding structural join for simulation analysis.The simulation results show that the chip input impedance at the 920 MHz frequency point is close to the conjugate of the chip port output impedance.Experimental testing was conducted on the designed tag prototype,and the experimental results showed that when the tag was attached to the surface of the human equivalent solution model,the measured impedance characteristics of the chip port were very close to the simulation results,and the measured reading distance reached 2.4 meters.