Research On The Complex Impedance Measuring Instrument Of Macromolecule Humidity Sensor

With the development of science and technology, the measurement of humidityhas been spread to all areas. They are all closed related with humidity both in humansurvival and social development. However, the humidity measurement depends on thehumidity sensor, in order to meet the requirements of humidity sensor in differentapplication areas, it is necessary to research the material properties and the conductivemechanism of humidity sensor. By measuring of the complex impedance of humiditysensor, which can convert the non-electric physical quantity into electricity quantity,this paper lays a foundation for the study of the conductive mechanism of thehumidity sensor.In order to study the humidity-sensitive characteristics and conductivemechanism of humidity sensor, we need to do some measuring for the compleximpedance of humidity sensor in different humidity condition. According to themeasuring principle of the vector method, this paper designs an effective impedancemeasurement circuit, which can achieve the complex impedance measurement of thehumidity sensor under different humidity or different frequency of signal. It not onlycan get the corresponding relationship between the humidity and the impedance of thehumidity sensor, but also can realize the measurement and study of correspondingrelationship between the impedance and the frequency of signal.Because of the mixture impedance of resistive, inductive and capacitive, it can’tmeasure the impedance of polymer resistor type humidity sensor directly by using theordinary voltage dividing circuit, which can’t get the components of vector voltage orthe vector current in the respective coordinate axes. So we adopt the vector measuringprinciple to calculate the impedance of the humidity sensor, which can get the valueof vector voltage and vector current at both ends of humidity sensor. According to thevector measuring principle, it takes LPC2136microcontroller as the core of the entiremeasurement system, and the hardware parts are mainly composed of the DDS signal source, phase sensitive detection circuit and other necessary circuits, such as flashmemory circuit, signal conditioning circuit, a low-pass filter circuit, buttons and LCDdisplay circuit and so on. It can control the system menu and parameter input by usingthe key input circuit and can achieve the serial communication between LPC2136andPC by using SP3232.After completing the measurement, it can store the results in Flash and send themto PC via the serial port in a certain packet format, then save the document into Excel,Txt or Word format, which can be used for data subsequent viewing and the processin future. At last it can achieve the characteristic curve drawing for compleximpedance by using the Matlab software and display the measurement results in theliquid crystal directly.Finally, the system does the overall debugging. From the experimental results, itcan reach the initial design goals from the stability of the system and the measurementaccuracy side.