基于全通滤波网络的土壤含水率传感器的设计  

Design of Soil Moisture Sensor Based on All-pass Filter Networks

作　　者：蔡坤[1] 徐兴[1] 岳学军[1] 洪添胜[1] 黄双萍[1] 

机构地区：[1]华南农业大学南方农业机械与装备关键技术教育部重点实验室,国家柑橘产业体系机械研究室,华南农业大学工程学院,广州510642

出　　处：《科学技术与工程》2014年第22期7-13,共7页Science Technology and Engineering

基　　金：省部产学研结合项目(2011B090400359);国家星火计划项目(2012GA780043);广东省自然科学基金项目(S2012010009856)资助

摘　　要：土壤水是土壤的三大组成物质之一,对植物生长起着十分重要的作用。通过对土壤含水率的实时监测,可以动态掌握土壤墒情,对于农业生产中的节水灌溉具有重要意义。设计了一种基于全通滤波网络的土壤含水率传感器。该传感器包含电极、超高频正弦波振荡器、全通滤波网络、相位补偿电路和信号相位差检测电路5个部分。传感器电极以电容的形式接入全通滤波网络,含水率不同的土壤其介电常数也不同,并导致电极电容发生变化,进而使得全通滤波网络的相频特性发生显著变化。传感器通过感知这种变化确定土壤含水率。为了使得全通滤波网络对电极电容变化的反应最灵敏,通过分析全通滤波网络相频特性函数,建立了寻找最优电阻阻值R1的目标函数。通过对该目标函数的求解得到了R1的最优值为11.25Ω。试验表明,传感器预测土壤质量含水率平均预测误差为1.51%,最大预测误差为4.71%。Dielectric-based methods of soil moisture measurement are widely used due to their non-destruction,efficiency and accuracy.The capacitance of the probe on the sensor is affected by the soil moisture.Therefore the mathematical model can be built between the capacitance of the sensor and the soil moisture.In this paper,a new soil water content sensor based on the all-pass filter network is proposed.The sensor consists of 5 parts,a VHF oscillator,a phase-detecting circuit,an all-pass filter,2 RF buffers,and a probe.The phase-detector AD8302 has 2 inputs,Input A and Input B,and converts the phase difference of 2 inputs to a voltage signal.Input A of the phase detector connects to the all-pass filter driven by the VHF oscillator via a RF buffer to reduce the impact of the output resistance of the oscillator on the all-pass filter.Input B must balance the phase shift brought by the RF buffer in front of the all-pass filter by wiring with another RF buffer with the same circuit with the fomer one to reduce the systematical errors of the sensor.The capacitance of the probe of the sensor acts as the capacitor C of the all-pass filter,varying with the changes of the soil water content.Moreover,the variations of capacitance of the probe will cause a significant change in the phase-frequency response of the all-pass filter,which represents a noticeable latency of the signal in time domain,and causes phase difference between the signals output from the VHF oscillator and the output of all-pass filter detectable.The output voltage signal VPHS of AD8302 represents the phase difference multiplied by a coefficient.Thus,the relationship between the soil moisture content and the output voltage signal VPHS can be built to estimate water content in soil.Compared with existing published works on the theoretical implementation which has low accuracy and sensitivity of the sensor,the proposed sensor is optimized by the following steps：① The phase-frequency characteristic of the all-pass filter is derived from the system function of

关 键 词：全通滤波网络 土壤含水率 传感器 相频特性 

分 类 号：S274.2[农业科学—农业水土工程]