## 3 Results and Discussion  
### Sensor Design  
A high-precision and high-reliability capacitance digital conversion chip AD7793 (Analog Devices, Inc., Norwood, MA, USA) featuring a 32 kHZ working excitation frequency, a measurable dynamic capacitance range of \(-4\) to 4 pF, and a minimum resolution of 0.004 pF was adopted as the core of the circuit [27]. The maximum measurable capacitance can be expanded by configuring the offset capacitance (for example, when the offset capacitance is 17 pF, the measurable range is 13 to 21 pF and the maximum measurable capacitance is 21 pF). In addition, the change in the equation of the dielectric constant with corn moisture content can be obtained by fitting the experimental data of the corn dielectric constant with moisture content at a 32 kHZ frequency according to [PERSON] et al. [28]:  
\[\varepsilon_{m}=3.467M-44.582\left(R^{2}=0.989\right) \tag{22}\]  
In this study, the proposed sensor had a measurement range of 0% to35% w.b. and a resolution of 0.1% w.b. To shorten the feeding and discharging period to ensure rapid measurement, a small amount of corn sample (about 600 g) should be filled. Thus, according to the calculation function of corn bulk density [29], the calculated total volume of the sensor to be designed was 0.0007 m\({}^{3}\). Moreover, according to the relative research [22], the corn repose angle is supposed to be 35\({}^{\circ}\). In addition, the cavity was composed of PVC insulating plastic with a dielectric constant of four. Therefore, when the filling corn moisture content was 35% w.b., the distribution nephogram of the sensor capacitance with the thickness of the material and the thickness of the cavity wall was obtained by calculating the above parameters and solving Equation (20), as shown in Figure 5.  
Figure 5 shows the sensor capacitance decreased rapidly with the increasing thickness of the corn and increasing thickness of the cavity wall, and the influence of the thickness of the cavity wall on the capacitance is more significant. Therefore, the thickness of the cavity wall is also an important parameter when considering the appropriate plate spacing in the process of designing a parallel plate capacitance sensor. In addition, \(C_{\text{max}}\) is the maximum measurable capacitance isoline, that is, the maximum capacitance value under the constraints of the capacitance measurement circuit described above, and the measurable region is above the curve. \(R_{m}\) is the isoline whose resolution is less than 0.1% w.b. when the corn moisture content is 10% w.b., and the measurement resolution in the area under the curve met the design requirements. \(C_{dy}\) indicates the capacitance range of  
Figure 5: The distribution nephogram of sensor capacitance under different corn thicknesses and cavity wall thicknesses. Note: the filling corn moisture content was 35% w.b.  
isoline, that is, when the size of the sensor on the line is selected, the difference between the maximum capacitance and the minimum capacitance is the maximum measurable dynamic capacitance range of the measurement circuit (8 pF). In addition, considering the requirements regarding mechanical strength and smooth corn flow, the sensor's minimum plate spacing (\(L\)) should be no less than 50 mm, and the minimum thickness of cavity wall (\(\delta\)) should be no less than 2 mm, as shown in the corresponding mark in Figure 5. Based on the above discussion, the area enclosed by the red dotted line in Figure 5 is the dimension point meeting the design requirements. For the measurement circuit to be full scale at the maximum moisture content, the dimension point should be close to \(C_{dy}\). In this study, the point pointed by the design value in Figure 5 was selected as the actual sensor size, corresponding to \(x_{m}=y_{m}=80\) mm, \(z_{m}=31\) mm, and \(\Delta_{z}=79\) mm. The sensor structure is shown in Figure 6. with this sensor structure and size, the measured capacitance range was 0.17 to 8.17 pF.