Design And Development Of Multi-Parameter Detection System For Plant Ion Absorption In Hydroponic Environment

Inorganic ions are important components in the growth environment of crop plants.In the process of plant growth,obtaining the kinetic parameters of ion absorption can provide dynamic and theoretical basis for crop variety selection and efficient cultivation.The conventional kinetic characteristic parameters are usually obtained by manual sampling:first,the ion concentration in the solution is measured off-line by spectroscopy or chromatography,and then inverted according to the ion depletion method,which is time-consuming and inefficient.For this reason,a multi-parameter detection system of ion absorption based on ion selective electrode method is developed in this paper:firstly,the ion concentration in hydroponic solution is obtained by two-electrode measurement system combined with ion correction and measurement model,and then the ion depletion curve is fitted by recursive least square,and the kinetic parameters of ion absorption are inversed by root surface area.（1）Based on the two-electrode measurement system of ion selective electrode method,the Nernst relationship between electrode potential response and ion concentration is deduced.According to the acquisition method of conventional ion depletion curve,the coefficient of ion depletion model was identified by recursive least square on-line,and the kinetic parameters and ion velocity of plant root system during ion absorption were inversed by Hofstee method and improved Michaelis-Menten equation.（2）A linear metrological model of the Nernst equation is investigated to resist the potential drift generated by the electrode under non-theoretical conditions and during continuous detection using correction means.To suppress the nonlinear variation of the electrode response produced by the multi-ion mixed solution,multiple linear regression with support vector regression is used to numerically establish the relationship between the mixed electrode response and the ion concentration by small sample modeling.Finally,the evaluation indexes of the three measurement models were compared and tested by experimental design.（3）The ion absorption multi-parameter detection system takes STM32microcontroller as the main control unit,adopts ion-selective microelectrode array,designs multi-channel signal conditioning and acquisition circuit with precise two-electrode system interface,adds TDS and water temperature detection unit and ESP8266 wireless communication module,designs root water culture observation box with electrode detection,and builds visible light imaging device.The firmware program and hardware driver of the lower computer are written on STM32,and the upper computer is developed based on the operator framework on the Lab VIEW platform,which realizes the functions of multi-sensor signal acquisition,Nernst model correction,root surface area measurement and ion absorption multi-parameter acquisition.（4）The system hardware completed the interface input impedance,measurement relative error and stability test,the input impedance is close to 10¹³Ω,the measurement accuracy is basically within±0.5%,and the maximum absolute difference of 5-hour repeat test is less than 1m V.ion concentration detection test completed the electrode potential response test and electrode slope correction test,the lower detection limit range of the four electrodes is between10^-5～10^-7 mol/L,the correlation coefficient of linear region was 0.99,and the conversion rate of Nernst slope after calibration solution correction was above 80%or 90%,respectively.The root surface area measurement results were verified by comparison with Win RHIZO software,and the correlation coefficient R² was 0.97.Finally,the results of the multi-parameter experimental detection at four ion uptake were demonstrated.