# Design of Capacitive Paddy Moisture Sensor Based on Electrical Impedance Spectroscopy Analysis
## 1 Introduction  
The paddy moisture content is the main technical and economic indicator to evaluate the paddy quality in grain harvest, storage and transportation [1]. A high moisture content will cause vigorous microbial life in the paddy, which results in mold in the paddy [2, 3]. The weight of the paddy will decrease with the reduction in moisture content, which seriously affects the economic benefit of paddy cultivation [4]. With the rapid development of agricultural mechanization, a large amount of high-moisture paddy is harvested in a short time, which generates a large drying requirement. In order to ensure drying efficiency, control precision and improve the quality of dried paddy, an accurate and stable online paddy moisture sensor is essential. A paddy moisture measurement consists of direct measurements and indirect measurements [5, 6, 7, 8]. The direct measurement involves calculating the paddy moisture content directly by quality difference before and after drying. The indirect measurement involves measuring the paddy moisture content by correlation between the moisture content of the paddy and the resistance or capacitance of the paddy. The direct measurement is accurate but time-consuming, which does not meet the needs of a rapid online paddy moisture sensing. Therefore, the indirect measurement is mainly used at present. As indirect measurement method for paddy moisture, a capacitance method, a resistance method,a near infrared method, a microwave method, a high-frequency impedance method, an acoustic method, a neutron method, and a nuclear magnetic resonance method have been widely studied by researchers. Because of economy and fast detection, capacitance and resistance moisture measurement methods have received great attention from researchers [9; 10; 11; 12; 13; 14; 15]. The capacitance method has the advantages of simple structure, high moisture sensitivity, good dynamic characteristic, and high reliability, which meet the current needs of grain moisture detection [3; 5; 6]. The different moisture contents of paddy have different dielectric properties, for the relative dielectric constant of free water is about 80 while the relative dielectric constant of dry paddy is less than 3 [16; 17; 18]. This provides a theoretical basis for capacitive grain moisture detection. However, in addition to the moisture content, the grain variety, excitation frequency, bulk density, and temperature will affect the detection results of a capacitive grain moisture sensor [7; 9]. So, the accuracy and stability of detection results cannot be guaranteed [19; 20; 21; 22; 23; 24]. In order to improve the detection accuracy of different grain varieties, researchers study their dielectric properties in different excitation frequencies, moisture contents, temperature, and densities [25; 26; 27; 28; 29; 30; 31]. The intelligent control of the paddy drying process is an necessary development trend to reduce energy consumption and improve paddy quality. Further, the accurate detection of the paddy moisture content is the basis for the intelligent control of the paddy drying process. Firstly, in order to ensure the drying efficiency, most of the grain dryers are hot-air driers with high temperatures of 40-80 \({}^{\circ}\)C, but the detection temperature range of the most commonly used paddy moisture detection sensors are 0-50 \({}^{\circ}\)C, such as the capacitive quick grain moisture tester PM-8188-A. So, it is necessary to design experiments to study the electrical characteristics of paddy under high temperatures of 40-80 \({}^{\circ}\)C. Secondly, the electrical properties of the paddy are decided by the paddy moisture and the composition of the paddy. The paddy composition will change in storage. In order to ensure that the paddy samples used in the experiment are as similar as possible to the paddy in the actual drying process, the paddy samples were harvested in the field during the harvest season, cleaned manually, and dried directly to simulate the actual drying process.