Design of soil moisture sensor based on differential signal

Abstract: 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 180o. 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 0.325 and 0.516V, respectively. In contrast, differential signal control circuit could be applied to improve harmonic distraction where the voltage amplitude of two-order harmonic was 0.097 V and that of three-order harmonic was 1.161 V, respectively. Experiment on the proposed sensor was organized as follows. Firstly, the sensor was calibrated by utilizing two types of soils including loam and sandy soil. Secondly, the sensor was evaluated in the soil samples with different soil moisture content. The result showed that the total harmonic distortion of this sensor of output signal was lower than traditional one. The maximum absolute error of soil moisture was less than 4.89% and the error of soil resistance was less than 2% when soil moisture was between 5% and 30%. The absolute error of soil resistance was less than 10% in the condition that soil moisture was in the interval of 30% and 47%. The study provides an effective method for design of soil moisture sensors.