Research And Development Of Humidity Detection System Based On Fiber Bragg Grating

Humidity is a very important index in both industrial and agricultural production process as well as people's daily life.Especially in industrial process,humidity often plays a crucial role in controlling the quality and cost of products,even it is related to the safety of production environment.However,in the industrial environment,the measurement environment of humidity often is accompanied by high temperature,which makes it difficult for the conventional humidity sensor to work normally.Taking the setting machine of traditional textile industry as an example,the zirconia humidity sensor commonly used in heat setting machine is difficult to be popularized in the market due to its high price and high maintenance cost.The humidity sensor based on Fiber Bragg Grating(FBG)has low cost and can adapt to the humidity measurement in high temperature environment(up to 90 ?),so it is suitable for use in heat setting machine and other equipment.Therefore,in this thesis the FBG humidity sensor and its detection system were developed which can be used in high temperature environment.The main contents of this thesis are as follows:(1)Fabrication of FBG humidity sensorThe expansion and humidity sensing characteristics of FBG humidity sensitive films were studied respectively in this thesis.A sensing model and scheme of FBG coated with thermoplastic polyimide(PI)was proposed,and the fabrication of FBG humidity sensor was completed.(2)Humidity calibration experiments of FBGIn this thesis,saturated salt solution was used to calibrate the range of humidity sensor.The experimental results show that the sensor has a good linear response from11% RH to 73% RH in the temperature range from 30 ? to 70 ?.The sensitivity of the sensor was about 2pm /% RH,and the measurement uncertainty was about 1% RH.Aiming at the cross sensitivity of ambient temperature to humidity response of FBG,a temperature compensation method was proposed.Because the temperature expansion coefficient and humidity expansion coefficient of PI humidity sensitive film have the characteristics of linear superposition,the temperature compensation coefficient can be obtained by linear interpolation of the functional relationship between the calibration of temperature and the center wavelength displacement of FBG.The experimental results show that the temperature compensation method can effectively expand the working environment temperature of the FBG sensor to 90 ? without reference grating within the temperature compensation range of 20 ?,and this method can ensure the average error of central wavelength within 0.03 nm.(3)Wavelength demodulation method of the FBG humidity sensorIn this thesis,the chirped grating was used as the edge filter to establish a linear relationship between the central wavelength and the light intensity of FBG.The central wavelength variation of FBG was demodulated by detecting the light intensity signal.The FBG light intensity signal detection circuit,the current voltage conversion circuit,the linear amplification circuit,the low-pass filtering circuit and the other signal processing circuit were designed.After the signal processing,the voltage signal was input to the data processing module,where it was inverted to obtain the humidity value and display it.The experimental results shows that the demodulation accuracy is about2 pm,the demodulation range is from 1549 nm to 1551 nm,and the sampling accuracy is 0.01 V.(4)Experiment debugging and testingBy using the aging test box to simulate the high temperature and humidity environment of industrial field,the designed FBG humidity detection system was debugged and tested.From the analysis of the performance of the detection system,the humidity detection system can meet the application requirements of general industrial environment.It can realize the humidity detection range from 30% RH to 70% RH within the temperature range of 30 ? ?90 ?.The experimental data analysis shows that the linearity of FBG humidity detection system is good,and the error is in the range of 5% RH ? 10% RH.