| The measurement of soil moisture transfer is of great significance in agriculture and soil physics,and is an important parameter when studying irrigation and drainage,soil runoff,soil leakage,migration of soil chemical substances,and soil physical and chemical properties.The heat pulse technology(HP,Heat Pulse),which has been developed in recent years,is gradually used in the research of soil moisture transfer measurement.However,the hardware equipment of the existing system is complicated,cumbersome to operate,and expensive,and the experimental data processing and calculation processes require a variety of software to implement,which has caused considerable difficulties in the study of soil moisture transfer.Based on the research of soil moisture transfer related equipment at home and abroad,this paper designs a soil moisture transfer measurement system based on the HP(heat pulse)technology.The system is composed of hardware sensor circuit and data calculation and processing software.The hardware circuit simplifies the data acquisition equipment in the previous system as a control and acquisition device.It integrates the probe,acquisition circuit and control circuit to complete high-precision data acquisition and heat control function;the data calculation processing software can communicate with the sensor circuit bidirectionally,and can realize soil moisture transfer calculation and data visualization based on the measurement data.The software can control the sensor to perform manual or timing automatic measurement,and can also receive temperature data from the sensor,and then calculate the soil moisture transfer can store the raw data and calculated values of each measurement at the same time,which is convenient for subsequent research.The details are as follows:(1)Based on the Pyboard development board as the main control unit,a soil moisture transfer sensor was developed.Compared with the thermocouple used as a temperature sensing element in previous research,the probe in this article uses a NTC thermistor with higher sensitivity,and a bridge-type voltage divider acquisition circuit is designed for the thermistor.A 24-bit A/D conversion chip ADS1256 is used.The measurement accuracy of the sensor is further improved.In order to meet the power requirements of other circuits,BAT54C,AMS1117-3.3,LT3461ES6,LP2985-50DBVR,and ADR431 chips were used to design a power circuit with three voltage levels:5V analog power,3.3V digital power,and accurate 2.5V reference power.Use SDD-5HB miniature relay with short switching time and high rated power to control heating time.The sensor control program is designed using Micro Python.The control program uses the timer interrupt function to set the sampling frequency.The interrupt callback function uses the SPI bus protocol to communicate with the ADS1256 chip to implement the measurement function.In the main program,the OTG(On the go)function of the USB is set to VCP(Virtual COM port).Mode to use the USB interface to communicate with the host computer.The hardware sensor circuit integrates the probe and the control and acquisition circuits into one,which simplifies the hardware system,improves measurement accuracy,and reduces system cost.(2)Designed to control the working state of the sensor and automatically calculate the soil moisture transfer calculation software through Python software,and realized 5 functions of data communication,calculation,storage,visualization,and import of historical data.Among them,the communication module uses the Pyserial library to create a serial port in the computer,which is used to send the measurement start instruction to the sensor and receive the raw data from the sensor;the data calculation and storage module uses the Numpy library design to convert and format the received raw data And storage operations,and calculate the soil moisture transfer according to the model;the data visualization function uses the Matplotlib function in the Math library to draw a graph of the original data in real time and display it in real time after the calculation of the moisture transfer is completed;the historical data calculation function can Re-draw historical data and calculate soil moisture transfer obtained from this measurement.(3)Two models of MDTD(Maximum dimensionless temperature difference)method and T_d/T_u method are studied,and the measurement accuracy and model adaptability of the system are tested respectively.The experimental results show that,when the water flux between10.5031mm/h and 119.7417mm/h,the correlation coefficients R~2 between the calculated and actual values of the MDTD method in the three soils are 0.9798,0.9780,and 0.9647,and the root mean square error(RMSE)is 7.9557,5.8914,and 6.0811,respectively.The correlation coefficient R~2 between the calculated and actual values of the T_d/T_u method are 0.9985,0.9989,and 0.9986,and RMSE was 1.5594,1.2795,and 1.1486,respectively.The correlation between the calculated value and the actual value of the T_d/T_u method is better,the error is lower,and the calculation effect is better than the MDTD method.In the case of comprehensive consideration of soil texture and other factors,the system uses T_d/T_u method to calculate better.(4)Based on the analysis of the HP technology principle,it was found through experiments that the T_d/T_u method was used to calculate the moisture transfer in three soil samples.When the sampling frequency is 1 Hz,the correlation coefficient R~2 between the calculated value and the actual value is 0.9794,0.9847,and 0.9799,and when the sampling frequency is 2Hz,the R~2values are 0.9985,0.9989,and 0.9986,respectively,indicating that the use of higher temperature sampling interval can effectively improve the calculation accuracy of moisture transfer.But observing the temperature curve of the probe,it can be seen that when the temperature value reaches the maximum value,the change trend is slower,which also means that as the sampling frequency increases,the impact on the measurement accuracy will become smaller and smaller. |
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