中文摘要：

研究土壤含水率的测定对于农作物生长,灌溉及农业自动化发展具有重要意义。该文针对传统含水率传感器电极输出信号谐波失真较大的问题,设计了一种差分信号控制的土壤含水率传感器。鉴于传感器电极输出的信号失真是由于土壤非线性因素引起的,该文利用集成时基计时器设计差分输入信号控制电路,减少输出信号的总谐波失真度。此外,建立相应的数学模型,得到土壤阻抗与信号周期变化关系。构建传感器硬件结构,通过微处理控制器测量信号周期得出土壤含水率变化数值。试验表明,传感器输出端的信号总谐波失真较传统结构减少12.56%。土壤质量含水率在5%~30%时,土壤含水率测试最大误差不超过4.89%,土壤阻抗测试误差不超过2%。

英文摘要：

Measurement of soil moisture is important for water management in crop cultivation, design of irrigation schedule and development of agricultural automation. Harmonic distortion of soil moisture sensor is high so that the measurement of soil moisture content by using those sensors has a large deviation. Therefore, a new design of soil moisture sensor based on differential signal was proposed in this study. The sensor consisted of six parts, which were circuit based on differential signal, correcting circuit, waveform generator, microcontroller unit, power conversion module and electrode module. Since the output signal of senor electrode had harmonic distortion due to the nonlinear factors of soil, a new circuit scheme was proposed using integrated time-based timer to design differential signal control circuit. The relationship between soil resistance and signal period was analyzed by mathematical models. The soil moisture could be obtained by constructing the hardware structure of sensor and measuring the signal period on microprogrammed control unit（MCU） controller in terms of the relationship. The two ends of input waveform in differential signal control circuit were with equal amplitude and phase difference of 180 o. Thus these electrodes were almost equal to differential signal. Energy of even order harmonic distortion reduced, while odd order harmonic distortion energy increased at the same time. Moreover, considering the probability of mismatch to electric characteristic parameters, the variance ratio of period of signal in the output was less than the proposed circuit. To verify harmonic distortion of the proposed sensor, frequency spectrum was obtained by utilizing the software of electronic design automation for the proposed sensor with two ends of input-output and classical sensor with a single end of input-output. Total harmonic distortion of traditional circuit for a single end of input-output was 47.40% where the voltage amplitude of two-order harmonic and three-order harmonic was as high as 0.32