Design Of Nutrient Solution Conductivity Sensor And Its Measurement System

Plant Factory is a highly specialized facilities moderm agriculture developed on the basis of greenhouse.In plant factories,crop cultivation methods are mainly nutrient solution culture.Electrical conductivity(EC)of nutrient solution is one of the important factors for crop growth,too thick or too thin nutrient solution will affect the growth of crops.Conductivity is a basic electrochemical parameters of electrolyte solution,the sensor used in conductivity measurement can be divided into electrode type and electromagnetic induction type according to different measuring principle.Currently measuring solution conductivity widely use electrode conductivity measurement method,this method of measurement has been very mature,however,the electrode for electrode method is usually precious metal like platinum,not only has high cost,but also the circuit structure is more complex,and need regular calibration and replacement of its electrode,seriously affect the life and accuracy of the electrode.Electromagnetic sensor for electromagnetic inductive conductivity measurement method has the advantages of not easy polarization,corrosion resistance,and with simple matching circuit etc,but it also has the disadvantages of weak output signal and low sensitivity,and the sensor is suitable for measuring the solution of high conductivity,generally used in industrial electrical conductivity instrument,if used to measure the crop nutrient solution conductivity will result in large errors,and not conducive to the growth of crops.This paper combined the electrode type with electromagnetic induction type which were two kinds of conductivity measurement methods,develop a combined electrical conductivity sensor and its measuring system based on the principle of electromagnetic induction and electrode assembly to meet the needs of conductivity measurement for crop nutrient solution.The main work done in this paper is as follows:(1)In view of the characteristics of electrical conductivity of crop nutrient solution,a combined electrical conductivity sensor based on the principle of electromagnetic induction and the combination of electrode was designed.Based on the principle of electromagnetic induction,measurement principle of the combined electrical conductivity sensor was analyzed in detail,and a mathematical model of solution conductivity and induction voltage of the detection coil was established.According to the mathematical model,the effects of excitation source,transformer parameters and cell constant on the sensitivity of the combined conductivity sensor were discussed.Combined with the related factors affecting the linearity and sensitivity of the sensor,the magnetic core material and the structure of conductivity cell of the combined conductivity sensor was selected and designed;Using Matlab data fitting function,obtained the relation expression between output voltage of the sensor and conductivity under different parameters,the optimization design for excitation source and coil turns were carried out,finally,the amplitude of the sine excitation source is 14V,the excitation frequency is 10kHz,the turn number of excitation coil is 3,and the detection coil turn is 1000.According to the international electronic transformer design manual,the rationality of the excitation source parameters and the number of turns of the coil were verified theoretically.(2)The hardware part of the nutrient solution conductivity measurement system was designed.A minimum system based on STM32F103VET6 MCU was constructed,which realized the signal acquisition,display and communication with the host computer of nutrient solution temperature and conductivity.Designed system power supply module,using the 220V AC power through transformer,rectifier and regulator output voltages required by each module,bypass capacitor was supplied for each group power,and the analog digital power was isolated to improve the stability of the measurement system;The sinusoidal signal of the DDS direct digital frequency synthesis technology was filtered and amplified,the smooth and stable sine signal was obtained,which satisfied the requirement of driving the excitation coil;The nutrient solution conductivity signal detection circuit was designed,after the detection of the effective value of the sensor output signal,the detection signal was converted by STM32 internal A/D,and achieved the requirements of miniaturization and low power consumption;Influence of temperature on solution conductivity measurement was analyzed,the temperature compensation circuit based on single bus digital temperature sensor was designed,and achieved real-time detection of nutrient solution temperature.(3)The software design of the nutrient solution conductivity measurement system was carried out by using modular programming theory,mainly including signal acquisition program,data processing program,temperature compensation program and LCD display program.Software digital filter technology was adopted in the signal acquisition and processing procedures,namely used arithmetic average value method for anti-pulse jamming to carry out digital filter,reduced the error caused by random interference signal.According to the induction voltage of standard conductivity solution measured at different temperature,the necessity of temperature compensation was explained,and temperature compensation relationship between output voltage of the sensor and change of temperature was obtained,a software automatic temperature compensation method based on STM32 was adopted.In the actual measurement process of nutrient solution conductivity,put the relation of temperature compensation into the relationship between the electrical conductivity of nutrient solution and combined conductivity sensor voltage with implantable microprocessor,realized automatic temperature compensation of software and the conductivity measurement of any nutrient solution,and it had further improved the accuracy of the measurement system.(4)Performance testing of the combined conductivity sensor and its measurement system was carried out:By means of Matlab curve fitting method,the combined conductivity sensor was calibrated by using standard conductivity solution;Configurated different concentrations of nutrient solution and standard conductivity solution as the measured object,measured by combined type conductivity sensor and SIN-TDS3031 portable conductivity meter pen,test the accuracy of the sensor;The standard solution of 1.800mS/cm was used to be observed,the measurement results of the combined conductivity sensor were recorded every 10 minutes,test the stability of the sensor.Experimental results show that the relative error(<1%)of combined conductivity sensor is less than SIN-TDS3031 portable conductivity meter pen,and the measured results are less volatile with time.The variance of continuous measurement of standard solution with conductivity of 1.800mS/cm for 2 hours is 5.6167*10-5,so the measured results of the sensor are less volatile with time.The results show that the combined conductivity sensor has good accuracy and stability,and its measurement system is stable and reliable,which can meet the test requirement of the crop nutrient solution conductivity.