Research On Microwave Humidity Sensor Based On SIW Re-entrant Cavity Resonator

In the intelligent manufacturing system,the sensor is the key component to perceive and obtain the status and changes of the processing environment,and is the key link of intelligent manufacturing.Among them,relative humidity（RH）is one of the important indicators to evaluate the environmental state.The precise detection and control of RH plays an important role in the fields of crop production,mechanical processing,precision instrument manufacturing and biomedicine.With the characteristics of high sensitivity,multi-parameter characterization,strong penetrating ability,fast detection speed,and high integration,microwave sensors has shown unique advantages in the field of RH sensing and detection.It will be a key research direction to develop a microwave humidity sensor with excellent performance and diversified functions.This paper through the resonant cavity perturbation theory,wave guiding and antenna theory and other basic knowledge of learning.Based on the structure of Substrate integrated waveguide re-entrant cavity resonator（SIW RECR）,two kinds of microwave humidity sensors with excellent performance are designed.Research on the sensing application of environmental relative humidity has been carried out,and the specific research results are as follows:（1）A SIW-RECRs differential humidity sensor loaded with PEDOT:PSS film is proposed.The sensing and reference SIW-RECRs are stacked longitudinally to construct a differential structure to suppress cross-sensitivity caused by temperature and other factors.Three different technologies,such as air-filled substrate,metal grid holes（METGH）and sensitive material,are integrated to enhance the humidity sensing response.Firstly,introduction of air-filled substrate and METGH can effectively improve the flowability of the humid air,and can reduce the response time.Secondly,PEDOT:PSS is used as a humidity sensitive（HS）material and deposited in the highly concentrated E-field distribution area of sensing SIW-RECR,and the modulation of the dielectric constant of HS material by humid air can achieve high humidity sensitivity.Furthermore,the proposed differential structure itself can suppress common-mode perturbations to prevent temperature cross sensitivity.Several different sensors are fabricated and a series of experiments are performed to investigate the dependence of humidity sensing response on the device structure and the deposited amount of HS material.When a specific amount of HS material is deposited in the HS area,excellent humidity sensing performances such as high sensitivity,low hysteresis and fast response can be successfully achieved.Among them,the sensor B₁（300-u L HS material and with METGH）demonstrates the best humidity sensing performance in the range of 32.8%to 97.3%RH,and shows high sensitivity of 209.3 k Hz/%RH and low hysteresis of 0.592%RH,and the corresponding response time is reduced to 1/6 of that without METGH.Moreover,the sensor B₁ has excellent suppression on the temperature variation through the actual temperature effect experiment.（2）The FSIW-RECR wireless humidity sensor loaded with orthogonal slot antenna is proposed,and the negative resistance circuit is integrated to realize the high resolution detection of RH.Firstly,the SIW cavity folding technology is used to further miniaturize the RECR structure with four capacitive columns loaded with orthogonal slot antennas,and it has bidirectional radiation characteristics to receive external interrogation signals and transmit response signals outward.At the same time,air filled substrate and METGH technology are used to improve the flowability of the humid air,and can reduce the response time.The radiation characteristics and electromagnetic field distribution of the sensor are simulated and analyzed,after further optimization,when the external standard horn transmit/receive antenna is rotated to make the polarization direction of the electromagnetic wave consistent with that of any pair of arc-slit receive/transmit antennas,the transmission response curve S₂₁ of resonant mode 1 or mode 2 can be obtained to achieve wireless information transmission.In order to improve the resolution of the sensor and realize the high-resolution detection of RH,the structure is constructed into an active sensor with high quality factor Q by integrating a microwave negative resistance circuit,which greatly improves the humidity detection resolution of the sensor in two working modes.Finally,a wireless humidity sensing test system was built to explore the ability of proposed sensors to detect RH changes.The experimental results show that the Q-factor of the active sensor in resonant mode 1 and mode 2 are 5230 and 4073,respectively.Compared with the passive sensor,the Q-factor is increased by 96（mode 1）/75（mode 2）times.The resolution of humidity detection in low humidity environment is5%RH,and that in high humidity environment is 4%RH,showing excellent high-resolution performance.