Analysis and design of grain moisture detection system based on microwave

Grain moisture content is a key indicator of grain quality, which directly affects the grain procurement, transportation, storage, processing, trade and other processes. At present, when purchasing grain in China, the moisture content of grain is judged by touching or biting it with hands or teeth or traditional detection methods, which has problems such as extremely unreliable measurement results, long detection time, and waste of manpower and material resources. In order to quickly and accurately detect grain moisture, a microwave-based grain moisture detection system is designed. By detecting the changes in microwave amplitude and phase before and after the microwave signal interacts with the measured grain, the moisture content of the grain is inferred.

1 Microwave moisture detection

Microwave moisture detection is a new non-destructive testing technology developed in recent years. It has the advantages of high detection accuracy, wide measurement range, good stability, easy dynamic detection, low sensitivity to the environment, and can be carried out under relatively harsh environmental conditions. Microwaves, as a very high frequency electromagnetic wave, have strong penetrability. It detects not only the moisture on the surface of grains, but also the moisture content inside grains without damage. The dielectric constant and attenuation factor of water in grains are much higher than the dielectric properties of dry matter, and water as a polar molecule is polarized under the action of microwave fields, showing special sensitivity to microwaves. Microwave grain moisture detection uses the absorption and reflection of microwave energy by water to cause changes in microwave signal phase, amplitude and other parameters to detect moisture content. Microwave moisture detection is gradually replacing traditional moisture detection methods such as capacitance method and resistance method with low accuracy, high sampling requirements and poor adaptability, and has become an ideal grain moisture detection technology. Microwave moisture detection can adopt transmission and reflection detection methods, and its microwave sensor layout is shown in Figure 1.

Generally, when the material thickness is relatively thin, the transmission detection method is used; when the material thickness is relatively thick and the density is relatively high, the reflection detection method is used. Microwave detection is a depth measurement technology, and the measured result is representative of the total volume moisture, which is much better than the surface measurement technology.

2 System Design

2.1 System structure design

Based on theoretical analysis and a large number of experiments, a microwave-based grain moisture detection system was designed, as shown in Figure 2. The system is mainly composed of a microwave generator, a microwave sensor antenna, a temperature sensor, a detection controller, and analysis and processing.

The microwave generator operates at a frequency of 10.5 GHz, and the microwave sensor is arranged in a transmission detection method. The isolator allows the forward-transmitting microwave to pass through without attenuation or with very little attenuation, while the reverse-transmitting microwave has a greater attenuation. The use of an isolator can absorb the reflection caused by load mismatch through the isolator and prevent it from returning to the signal source, so that the signal source can work stably. The detector converts the microwave signal into an electrical signal, which is amplified, filtered and A/D converted by the detection controller. The detection controller communicates data with the computer through a serial bus. The computer can complete data analysis and real-time display. The detection controller can perform operations such as setting system parameters, moisture calibration, and displaying detection results. Temperature compensation is performed through the temperature sensor signal to obtain an ideal linear relationship between the microwave detection signal and the moisture content of the grain, thereby improving the detection accuracy of the system. [page]

2.2 Hardware Design

The grain moisture detection controller is the core part of the hardware design. It consists of an amplifier and filter circuit, A/D conversion, a microcontroller, a keyboard, an LCD display, and a serial bus interface, as shown in Figure 3. The microwave electrical signal picked up by the microwave sensor probe is amplified and filtered before being converted to A/D. The grain moisture estimation result is displayed in real time on the LCD. The keyboard interface can be used for controller parameter setting, moisture calibration, and other operations. Data communication with the computer is carried out through the serial port, and the CAN bus is a cascade interface for multiple controllers.

The microwave sensor analog signal processing determines the moisture detection range and detection accuracy of the entire system. Figure 4 shows the sensor signal processing circuit. The microcontroller uses Microchip's PICl-8F6527. The processor uses nanowatt technology, low power consumption, strong anti-interference ability, and rich peripheral interfaces, such as CCP module (PWM), MSSP module (SPI, I2C), EUSART module, A/D conversion module, etc., which can meet the application requirements of the system. 4 crystal oscillator modes, up to 40 MHz. There is a watchdog circuit inside, which can be programmed online serially (ICSP). The A/D converter uses AD7806, 16-bit sampling, and its reference voltage source 3 V is provided by AD780. The A/D sampling resolution is 45.8μV/bit, which makes the system have a higher detection accuracy. AD7806 has a sampling self-correction function to ensure the accuracy of sampling, and communicates data with the microcontroller through the SPI bus. After power-on initialization is completed, AD7806 provides an interrupt signal to the microcontroller through the pin from high to low, notifying the microcontroller that a new A/D conversion data is ready and can be read. The amplification factor of the signal processing circuit can be adjusted online, which is completed by controlling the digital potentiometer AD5227 through PICl8F6527. The two are connected through 3 data lines, which improves the moisture detection range of the system. The entire system adopts 5 V operating voltage, low power consumption design, and optical isolation with the interface of external devices to reduce external interference and improve system reliability. [page]

2.3 Software Design

The grain moisture detection system software consists of data acquisition, moisture value calibration, moisture value calculation, system sensitivity adjustment and display modules. The system sensitivity adjustment module can adjust the analog signal amplification factor according to the sampled data to improve the system robustness. Figure 5 shows the real-time microwave moisture detection software interface.

Figure 6 shows the software design process. When the detection system is installed for the first time or when the material type to be detected changes, the system moisture calibration and parameter setting are required. Generally, 2 or more data points need to be calibrated, and data fitting processing is performed according to the calibration value. After the system is initialized, the data acquisition module collects a small section of microwave signal each time and performs smoothing on this section of data. The bubbling method is used to sort first, and the middle data is selected for weighted averaging, and temperature compensation is performed on the sampled data. The moisture value calculation module calculates the grain moisture content value through a linear matching algorithm based on the prior calibration value and the result after smoothing preprocessing, and displays it in real time. [page]

3 Results and Analysis

1) Experimental Materials The experimental samples were 40 kg each of 8 different types of wheat with different moisture content (moisture content ranged from 8% to 20%). The materials required for the standard drying method included an electric constant temperature oven, an electronic balance with an accuracy of 0.001 g, an electric pulverizer, an aluminum box, etc.

2) Methods The wheat moisture value was measured on the test platform using a microwave-based grain moisture detection system. At the same time, a certain sample was taken and the standard moisture value of the wheat was obtained using the standard drying method. The system measurement value was compared and analyzed with the standard value obtained by the drying method. The experimental data is shown in Table 1.

The grain moisture detection system has been tested experimentally to have a moisture content range of 8% to 20% and a measurement accuracy of 0.5%. A large number of on-site experimental tests have shown that the system can fully meet the needs of grain moisture content detection.

4 Conclusion

The grain moisture detection system based on microwave can detect the moisture content of grain continuously and accurately, providing a strong technical guarantee for the purchase, transportation and storage of grain. Compared with traditional grain moisture detection methods such as capacitance and resistance, microwave detection is fast, accurate and stable, which solves the problems of long detection time, extremely unreliable measurement results and inability to realize online detection in the current domestic grain purchase. A large number of indoor and outdoor experiments have shown that the system can meet the needs of moisture content detection in the process of grain purchase, storage and processing, and has broad application prospects and economic benefits.