The drying room communicates with the host computer through the RS 485 bus. The upper computer sets the target temperature and humidity upper limit in the drying chamber, and displays the parameters such as the drying room temperature, humidity, blower speed, and exhaust fan switch status in real time. The embedded system software adopts the FreeRTOS real-time operating system to ensure the real-time and reliability of the system, real-time monitoring and control of the temperature and humidity of the solar drying room. The field application proved that the system is stable, the control precision is high, and the response speed is fast, which proves that the scheme has strong practicability. 0 Preface Solar energy is a clean renewable energy source with a broad application prospect. In recent years, the use of solar energy for the processing of agricultural products and pharmaceuticals has also developed rapidly due to its characteristics of energy saving, short drying time and high drying quality. In order to ensure the quality and drying efficiency of the dry materials, the solar drying equipment needs to monitor the temperature and humidity in the drying chamber in real time during the drying operation. The intelligent solar drying control system based on AT89C51 developed by Wang Shengli and Fu Lisi of Shenyang Agricultural University did not transplant the real-time operating system. The real-time requirements of monitoring and control could not be properly met, and the drying effect of the equipment was also affected. Although the PLC-based solar energy drying control system developed by Xu Mingna of Inner Mongolia Agricultural University is relatively stable, the overall cost is relatively expensive and is not suitable for large-scale application. Aiming at the real-time, stability and promotion design requirements of solar drying monitoring system, this paper proposes a design scheme of real-time embedded solar drying monitoring and control system based on STM32 and FreeRTOS. The scheme adopts temperature and humidity sensor AM2301 for temperature and humidity. The measurement is carried out and transmitted to the PC host computer via the RS 485 communication line, realizing the real-time monitoring of the temperature and humidity of the solar drying room; using the digital PID control blower speed and the relay to control the exhaust fan start and stop to complete the solar drying indoor temperature and humidity Real-time control. The upper computer is written by configuration software, which has the advantages of strong adaptability, good openness, easy expansion, economy, short development cycle, etc. The monitoring and control interface is simple and clear and easy to operate. The test shows that the whole system has the characteristics of stable operation, rapid response and easy operation, which can realize real-time monitoring and control of the temperature and humidity in the drying room during drying operation. 1 Embedded solar drying monitoring and control system design The embedded solar drying real-time monitoring and control system consists of a PC host computer, an embedded ARM processor, an AM2301 temperature and humidity sensor, an RS 485 communication circuit, and a relay control circuit. The real-time temperature and humidity parameters collected by the AM2301 in the drying chamber are transmitted by the embedded ARM processor to the PC host computer via the RS 485 communication line for display and storage. The artificially set drying temperature of the upper computer is transmitted to the embedded processor through the RS 485 communication line as the system control target quantity, and the PID control algorithm is called by using the actual temperature in the drying room as the input quantity. The output of the PID control algorithm is used as the operating frequency of the inverter to adjust the speed of the blower in real time, so that the amount of hot air is sent in real time to achieve constant temperature control of the drying chamber. When it is detected that the humidity in the solar drying chamber is higher than the upper limit set by the upper computer, the relay contact suction control exhaust fan is opened to evacuate the excessive moisture in the drying chamber to achieve the purpose of humidity control. The system structure block diagram is shown in Figure 1. 2 Embedded solar drying monitoring and control system hardware design 2.1 Embedded processor selection and application The main control processor of the embedded solar drying real-time monitoring and control system uses the low-power high-speed industrial grade chip STM32F103VBT6 ( STMicroelectronics). The STM32 family features an ARM Cortex-M3 core designed for high performance, low cost, low power embedded applications with integrated secure clock mode, low power mode with wake-up, internal RC oscillator, and embedded The reset circuit and the like greatly simplify the peripheral circuit design and greatly improve the performance. STM32 series MCU can also realize the transplantation of real-time operating system conveniently, which can meet the design requirements of this embedded solar drying real-time monitoring and control system. 2.2 AM2301 temperature and humidity acquisition circuit design The embedded solar drying real-time monitoring and control system uses the AM2301 humidity sensitive capacitor digital temperature and humidity module to obtain real-time temperature and humidity parameters in the drying chamber. The AM2301 includes a capacitive sensing element and a high-precision temperature measuring element connected to a high-performance 8-bit microcontroller for superior quality, ultra-fast response, high immunity to interference, and cost-effectiveness. Calibrate in an extremely accurate humidity calibration chamber. The AM2301 makes system integration easy and fast with a standard bus interface. Ultra-small size, very low power consumption, signal transmission distance of more than 20 m. The temperature and humidity acquisition circuit is shown in Figure 2. The AM2301 sensor has a temperature measurement range of -40 to 80 °C and an accuracy of 0.1 °C. The humidity measurement range is 0.1 to 99.9% RH with an accuracy of 0.1% RH, which fully meets the design requirements of the system. The AM2301 temperature and humidity sensor has a measurement resolution of 8 bits. The single bus transmission data is divided into an integer part and a fractional part. The complete data transmission is 40 bits. The specific data format is as follows: 32-bit data bits, including 8-bit humidity integer data, 8-bit humidity fractional data, 8-bit temperature integer data, 8-bit temperature fractional data; 8-bit check digit, 8-bit humidity integer data + 8-bit humidity fractional data +8 Bit temperature integer data + 8 bits of temperature decimal data results after 8 bits. The AM2301 temperature and humidity part reading procedure is as follows:
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