Study Of Soil Moisture Detection By Parallel-plate-capacitor Based On Frequency Response Characteristic

Soil moisture is one of the most important physical parameter of soil.Throughout the water cycle on the earth,soil moisture is the key factor in the material exchange and energy cycle between land and atmosphere.Humidity parameter has great effect in the agricultural engineering,plant growth,ecological environmental protection,weather forecasting,hydrologic research,disaster protection,civil engineering and many other fields.Therefore,detecting soil moisture is of great consequence.There are kinds of soil moisture detecting technics,however,Compared to other methods,dielectric method is the most wildly used.The principle of dielectric method is that soil's dielectric permittivity is mainly determined by the moisture content,by measuring the soil's relative dielectric constant or physical quantities associated with it we can indirectly get soil moisture.Being one of the soil moisture detecting technics based on the principle of dielectric method,capacitance method has advantages over other members like TDR,FDR and SWR,because it's working circuit is simple and cost less,the measurement is accurate,portable and easy to operate,the capacitance probes can be modified as needed to a variety of different soil textures.Thus,capacitance method has great potential value for widely used,its core lies in the design of the detection probe and detection circuit,and the determination of the corresponding relationship between the soil moisture and the output signal.Accordingly,based on capacitance method,the paper designed a parallel plate capacitive soil moisture sensor which is sustainable with high cost performance after analyzing soil's dielectric properties.And a series of experiments were carried out to test the sensor's performance.The main achievement of this study are as follows:(1)The paper analyzed dielectric model of soil,and selected the model of a resistance and a capacitance in parallel.The concept of transfer function was introduced into the detection of soil's equivalent circuit,on this foundation,we proposed a detecting way for soil moisture based on the frequency response characteristics of soil transfer function.According to the transfer function equations of the soil electrical model,the capacitance and resistance components of the model can be solved directly and accurately,the method effectively deduced the risk that the measurement result was easily influenced by its error of the output signal,the mathematical models are more stable and reliable,and the results are more accurate.(2)This research designed and made a parallel plate capacitance sensor device,which was made up of probe part and circuit part.Parallel plate type structure probe has much great convenient to insert into the soil,and has low requirement on the surface flatness of soil,the single detection range is larger and the maintenance is simple.The plates were made up of of stainless steel,which is sturdier and durable and can be applied in different texture soil.The sensor can easily measure 2.5×3.5×13cm3 volume of soil's average water content once,the sensing circuit is simple and cost low.(3)Multiple tests were done by using ethanol solution and sand as the medium,As an equivalent medium of soil,ethanol solution can be used in laboratory instead of soil material.It can greatly reduce the difficulty of sampling soil moisture sensor in laboratory and save the experimental time.The test selected frequency multiple frequency signal repeatedly scanthrough the medium,according to the verification of the measured medium dielectric model experimental data,the first-order RC parallel network can better represent the soil dielectric properties.(4)The edge effect of the parallel plate capacitor cannot be neglected when measuring the theoretical capacitance value,this paper presents a simplified mathematical description of the theoretical capacitance,which takes into account the marginal capacitance.We used COMSOL software to analyze the edge electric field of parallel-plate capacitor,the simulation results show that the simulation capacitance has a proportional-similarity relation with the testing capacitance,means that they have the similar function relation with water content,these proved the good feasibility of the device we designed and method we proposed.In view of the deviation between the theoretical calculation of the capacitance value and the capacitance value of the test,this paper presented a method that multiply the theoretical calculation capacitance value by a proportionality coefficient(the value of proportionality coefficient related to the material of dielectric)to calibration the sensor's measurement results.After calibration,the theoretical capacitance value is fit well with the experimental capacitance value.This research has a good reference value for the development of parallel plate capacitive soil moisture sensor with low cost,high accuracy and strong antiinterference ability.