Design Of Low Power Capacitive Humidity Measurement System

Planar capacitance sensor is a new type of capacitance sensor in which the excitation electrode and the induction electrode are in the same plane.Compared with the traditional parallel plate capacitance sensor,it has the advantages of single side penetration,adjustable signal strength,tomographic imaging and so on.It is widely used in nondestructive testing of material properties such as humidity,thickness and porosity.Soil moisture is an important parameter in the study of agricultural drought and crop drought,which is of great significance in agricultural production,the micro moisture in soil determines the growth and development of crops and the absorption efficiency of fertilizer nutrients.In this paper,a low-power soil moisture measurement system based on planar capacitance sensor is developed,which can effectively measure soil moisture,and has the characteristics of low power consumption and wireless communication.By analyzing the working principle of planar capacitance sensor and the actual needs of soil moisture measurement system,a low-power design scheme based on planar capacitance sensor is proposed.The hardware of the measurement system includes MCU core module,excitation module,capacitance conversion module,filter module,amplitude and phase detection module,man-machine interaction module,wireless communication module,power management module,etc.Lithium battery is used for power supply,and the voltage level is simple,and the power supply scheme is efficient.The core processor and other components are low-power devices,which realize the power consumption control of the whole measurement system.The measurement system takes the MSP430F5529 microcontroller as the core.It is responsible for centralizing the operation of other modules and processing and computing the signals.In order to generate high-frequency excitation signal by oscillation circuit,it is necessary to control the cost and power consumption to generate sine wave signal with frequency up to MHz and stable waveform.The capacitance conversion module converts the capacitance change of the sensor into the waveform change of the sinusoidal signal,which is captured by the subsequent circuit.The difficulty of the module is that the input impedance of the sensor is very high,the output signal is very weak,and it is easy to be interfered by the noise signal.In order to solve the contradiction between the high frequency of signal and the low power consumption of MCU,the amplitude and phase detection scheme is designed to obtain the amplitude ratio and phase difference between the excitation signal and the response signal,and the sampling and operation pressure of the core processor is reduced through the module.The wireless communication module is mainly to realize the functions of communication with various platforms and wireless sensor networking.This paper designs the measurement system software,including the main monitoring program,initialization module,data acquisition module,interrupt module,man-machine interface module,wireless communication module,etc.It mainly realizes the system sleep control,data filtering and calculation,result display,wireless communication and other functions.The main characteristics of the algorithm are accuracy and rapidity,the algorithm includes data preprocessing and humidity calculation.Data preprocessing is to remove outliers and filter the mean value of a group of collected data.The calculation of soil moisture value is based on the functional relationship between the amplitude ratio and phase difference obtained from the calibration experiment and the actual soil moisture.According to the actual needs of soil moisture measurement,a probe type planar capacitance sensor is developed and the experiment is designed.The planar capacitance sensor is designed as probe type through flexible copper foil material,and then the designed measurement system is combined with capacitance sensor to measure soil moisture.And also designed the calibration experiment,humidity measurement experiment,power consumption experiment and wireless communication experiment.The experimental results show that: the system has high precision,low power consumption,strong stability,and can effectively achieve the measurement of soil moisture.