The Detection Methods Of LC Passive Wireless Sensing

LC(inductor-capacitor)passive wireless sensors rely upon correlating the quantity to be sensed with the resonant frequency,which can be contactlessly measured through inductive coupling.Usually,LC passive wireless sensor is constructed from a planar spiral inductor connected with a variable capacitor.Due to their simplicity,non-contact interrogation and unlimited lifetime in theory,micro-fabricated LC passive wireless sensors are very suitable for the applications in harsh environments such as confined spaces,rotating components.With the flourishing development of micro-electro-mechanical system(MEMS)and the internet of things(IoT),growing attentions have been paid on LC passive wireless sensors,especially the implanted pressure sensors.However,most previous researches focused on the design of sensing elements,while the wireless readout has not received enough attention.For the above reasons,this dissertation studies the LC passive wireless sensing technology,especially on the enhancing the readout of LC sensor and the detection of multi-paramenters.The main work and innovations of this dissertation are described as follows:(1)The fundamentals of LC resonators are introduced,different extraction methods of the resonant frequency are analyzed,and the basic components of the LC sensor are introduced.On this basis,several sensitive principles are analyzed.In addition,the effect of the environmatal media on the signal transmission is tested,and the results suggest that the effect of the capacitive link can not be ignored when the dielectric constant of the media is great.(2)An approach to extend the remote distance of LC passive wireless sensors by using an adaptive repeater is presented.The working principle of the adaptive repeater is introduced,and the theoretical model is built.The critical coupling distance is also calculated to avoid frequency splitting which causes the frequency shift and sensitivity decay.The adaptive repeater is realized by the varactors which are controlled by an external signal source to set the resonant frequency of the repeater cyclic change.In experiments,the adaptive repeater is used to enhance the readout of a humidity LC sensor.The results show that the readout distance is extended to 3 times of the distance without a repeater,while the signal strength and the sensitivity keep the same.On the basis of the adaptive repeater introdecud above,a passive wireless adapative repeater is developed.By replacing the external signal source as wireless power coupling from the readout coil,the repeater also works in a wireless passive way.The experiments show that the readout distance is extended to 3.2 times of the distance without a repeater,while the signal strength and the sensitivity keep the same.(3)A left-handed metamaterial(LHM)with adjustable working frequency is proposed to enhance the readout of LC sensors.The adjustable LHM is realized by connecting a discrete capacitor to the double-layer coil.In experiments,the LHM is used to enhance the readout of a humidity LC sensor.And the results show that the readout distance is extended to 1.7 times of the distance without a repeater,while the signal strength and the sensitivity keep the same.(4)A specific-winding inductor is proposed to suppress the mutual coopling between the stacked inductors.By respectively connecting three stacked specific-winding inductors to humidity capacitor,temperature capacitor,and pressure capacitor,the three LC resonant circuits realize a multi-parameter detecting system to monitor humidity,temperature,and pressure simultaneously.The results show that the mutual coupling between inductors could be nearly zero,three resonant frequencies can response to the corresponding capacitors and can monitor different parameters simultaneously,and the resonant frequencies can hardly affect each other.(5)An LC sensor integrated with switch is presented to realize multi-parameter momitoring.The switch alternates different sensitive capacitors into the LC tank to realize different parameters monitoring.The switch is drived by the wirelss power coupled from the readout coil,making the LC sensor still work in a passive wireless way.In esperiments,the humidy and pressure are both detected by the switch-integrated LC sensor.And results show that the LC sensor with integrated switch can realize an independent monitoring of humidy and pressure.In a word,this dissertation is focued on enhancing the readout of LC sensors and realizing multi-parameter monitoring.The proposed adaptive repeater and the LHM extend the readout distance greatly,and provide a new direction to improve the readout of LC sensors.The intucor model in specific-winding inductors has a certain useful experiments for solving the mutual coupling problems in LC sensors.Besides,the LC sensor with integrated switch provides an easy effective approach to achieve multi-funtion sensors,and presents new ideas on expanding the LC sensors towards a wireless sensing platform.