Design Of Quartz Crystal Microbalance Low-Humidity Sensor And Wireless Sensing System

In today’s society, the demanding of the measurement accuracy in humidity is increasing, especially people starts to pay attention to low-humidity measurement, such as the humidity measurement of SF6 environment in breaker of transformer. However, due to the low content of water molecules in low relative humidity environment, the measurement of most electronic humidity sensors in low relative humidity is insufficient.This paper uses mass sensitive principle of quartz crystal microbalance as the theoretical basis, utilizes the characteristic that QCM sensor can detect nanogram quantities in mass change, and makes the QCM low-humidity sensor, which based on Nafion materials that has good adsorption properties of water molecule. By comparing the curves of different coated Nafion volume, the paper gets the proper coated volume with Nafion based on the sensor’s stability and sensitivity. Then the paper measures the frequency-humidity characteristic, response characteristics and sensitivity of this Quartz Crystal Microbalance low-humidity sensor coated with proper Nafion volume. The result shows that the humidity sensor coated with Nafion has good linearity and repeatability when the humidity varies from 1～10%RH, and its R2 is about 0.98. In addition, the humidity sensor could tell 1%RH humidity change and the sensor’s response time is short. At the same time, this paper also studies the QCM humidity sensor in other relative humidity range. The result shows that the sensor has both good short-term stability and good humidity response in the range of 11.3～97.3%RH.In the meanwhile, this paper designs a wireless humidity sensing system based on FPGA, uses QCM as the sensor of the system, designs the QCM sensor resonant oscillator circuit, and designs and realizes the data collection of temperature and humidity with FPGA. The paper uses D triggers programmed in Verilog-HDL hardware description language in FPGA to eliminate error of temperature drift. Then the paper designs the frequency meter to measure the differential frequency signal’s frequency and it uses the MATLAB to conduct both linear fitting and multi-fitting of the experimental data. And the paper uses SOPC technology to build NiosⅡ core and realizes the conversion of differential frequency to the relative humidity by fitting function in the core, and designs key to realize measurement transition between low and high humidity. Then the paper programs in NiosⅡ to control the LCD screen to display the relative humidity which generated in sensor system, at the same time the NiossⅡ core controls the Bluetooth to transmit the data to the system host which is developed by VS2010, and the system host can conduct functions including serial communication control, real-time dynamic data display, humidity overproof warning, storage and query of historical data and export of Excel files.The test result shows that the system is able to do relatively accurate measurements in low humidity environment. At the same time, the system could also be used in other relative humidity range. In addition, aiming at the impermeability of low humidity environment, this paper conducts wireless transforming tests and SF6 humidity measurement simulation, the result shows that this system could work in mental enclosed environmnet. In general, this system realizes real-time detection of lower relative humidity, and it could work in high relative humidity as well. In the meanwhile, it broadens the range of humidity sensing system in low humidity measurement, it simulates some specific applications such as SF6 humidity measurement, and it basically reaches the expected goal.