Research On On-line Capacitive Sensor Of Seed Moisture Content

Seed moisture content is the dominant factor that influences the seed quality.Drying seed with a seed dryer is an important means to improving the seed germination percentage and ensuring the security during storage.In order to control seed moisture content within the safe range,the seed dryer is required to adjust operation parameters continuously and dynamically.Therefore,the on-line measurement of seed moisture content is very important,which directly affects the performance and efficiency of the seed dryer.Capacitive sensing method is commonly used for the measurement of seed moisture content because of its advantages of fast response,high sensitivity,low cost and so on.But the traditional capacitive sensor of seed moisture content is usually hard to measure on line,prone of edge effect and has low accuracy.In order to achieve the on-line measurement with high accuracy,this paper proposes an in-depth study on the capacitive sensor and the research work is mainly composed of mechanical structure design,electrode structure optimization and measurement system design.According to the demand of the on-line measurement,the sensor is designed into rotating mechanical structure,which achieves automatic feed and discharge.For the electrode structure,the uniplanar cambered electrode is adopted to reduce the influence of seed distribution and temperature.Based on the electromagnetic field theory,an analytic solution for the capacitance calculation of capacitor with uniplanar cambered electrode is quite hard to obtain,which brings difficulty to the optimization of the electrode structure.In order to solve this problem,a numerical method based on the boundary element method(BEM)is proposed.And by using the commercial software COMSOL,a simulation based on the finite element method(FEM)is implemented to analyze the relationship between sensor’s performance and electrode structure parameters including the electrode number,the electrode size and the gap.Therefore,the optimized structure parameters are obtained,which is instructive for sensor design.For the measurement system,a capacitance measurement circuit based on capacitance-to-digital chip AD7746 is proposed,which features high integration,fast measurement and strong anti-interference capability.And upper computer software based on python programming language is also developed to implement the functions of data transmission and real-time display of measurement information.A novel on-line capacitive sensor of seed moisture content is developed.The sensor of optimized structure has good repeatability and achieves measurement accuracy of ±1% in the range of 0%~20%,which meets demand for application in the seed dryer.