Abstract: Using three meteorological elements—temperature, average wind speed, and relative humidity—measured in real time by an automatic weather station as basic parameters, the critical values for freezing and thawing of the wind sensor at the automatic weather station were determined. The automatic operation of the wind sensor antifreeze heating device was controlled through a PC RS232 communication interface, an ATmega8 microcontroller, and opto-isolated drivers, thereby achieving the purpose of antifreeze protection for the wind sensor of the automatic weather station.
Keywords: Automatic weather station; Wind sensor; Antifreeze; RS232 interface; Microcontroller; Circuit design
Introduction
With the full implementation of my country's atmospheric monitoring automation project, Gansu Province has now built 85 automatic weather stations. However, none of their wind sensors have been protected against frost, hoarfrost, and freezing damage. Under low temperature and calm wind conditions, frost, hoarfrost, and freezing damage frequently cause wind sensors to freeze and become unable to rotate, resulting in missing meteorological observation records. The Huajialing, Wushaoling, and Xifeng automatic weather stations in Gansu Province, as well as other automatic weather stations across the country, have experienced missing records due to frost, hoarfrost, and freezing damage, seriously affecting the normal operation of these automatic weather stations. In 14 instances of frost freezing on wind sensors at the Huajialing and Xifeng automatic weather stations between 2003 and 2005, the longest freezing time was 39 hours, the largest diameter of the frost mixture was 70 mm, the largest weight was 352 g/m³, the minimum relative humidity was 93%, the maximum wind speed at the start of freezing was 4.6 m/s, the longest calm wind period was 6 hours, and the maximum temperature drop from the start to the end of freezing was 6.8℃. Therefore, addressing the impact of rime, hoarfrost, and freezing damage on automatic weather stations has become an urgent problem to be solved in automatic weather observation.
In countries like Finland, automatic weather stations use heating devices with a power of ≤4W for wind sensors. Only the temperature index is considered. When the temperature is ≤4℃, the automatic weather station automatically starts the heating device to heat the wind sensor and melt the frost and rime that freeze the wind sensor. However, in the operation of pilot stations in China, the effect is not very ideal. Therefore, it is not comprehensive to only consider the temperature when solving the problem of frost damage to wind sensors. Yin Xianzhe et al. studied the frost damage of wind sensors in automatic weather stations and believed that the freezing of wind sensors is the result of a combination of meteorological conditions such as temperature, humidity, and wind speed [1] . The frequency of mixed frost and rime is high, the freezing time of wind sensors is the longest, and the damage is the greatest [2-3] . They proposed that the basic conditions and characteristics of severe icing are freezing rain or heavy fog and snow weather with a temperature of -5 to 0℃, an average wind speed of ≤5m/s, and a relative humidity of >80%. Based on the above critical conditions for icing, an automatic heating control circuit for wind sensors was designed, using three real-time meteorological indicators—temperature, average wind speed, and relative humidity—as the criteria for judging whether the wind sensor is freezing or melting. This circuit can prevent or eliminate the freezing of the wind sensor, thus achieving the purpose of anti-freezing protection for the wind sensor in automatic weather stations.
1. Overall Structure of Automatic Control System
The structure of the wind sensor heating automatic control system is shown in Figure 1. It mainly consists of parameter sampling, command control, serial communication interface, ATmega8 microcontroller, opto-isolated drive circuit, heating circuit, etc.
The parameter sampling section uses real-time data measured by automatic weather stations and extracts real-time meteorological element indicators measured by automatic weather stations through self-developed software. The critical values are -5 to 0℃, average wind speed ≤5m/s, and relative humidity >80%. The command control circuit is then used to determine whether to send a command. When the set standard is reached, a command is sent to the ATmega8 microcontroller through the communication interface circuit. Then, the wind sensor antifreeze heating device is started or stopped by the opto-isolation drive circuit [4] .
A resistance heating wire is used as the anti-freezing element for the wind sensor, and it is installed above the bearing sleeve of the inner shell of the wind sensor. A safe 36V AC voltage is used as the heating voltage, with a heating power of approximately 8.6W to ensure safety for personnel and instruments. Heating and thawing can be stopped when water vapor conditions are insufficient for condensation, thereby saving energy.
2 Hardware Design
2.1 Communication Interface Circuit
Because the PC's RS232 serial port uses the RS232 transmission protocol, its high and low levels are -12V and +12V respectively, which are inconsistent with the microcontroller's levels. Therefore, the PC and the microcontroller cannot be directly connected with a cable. An RS232/TTL level conversion circuit must be added between the PC and the microcontroller. That is, the communication interface circuit usually selects a dedicated RS232 interface level conversion integrated circuit, such as MAX232, HIN232, etc. NIH232 and MAX232 can be directly interchanged [5-6] . Here, the NIH232CP chip is selected to complete the serial port interface circuit (Figure 2).
2.2 ATmega8 Microcontroller Control Circuit
The ATmega8 microcontroller is a high-end Flash microcontroller based on the AVR RISC architecture and manufactured using low-power CMOS technology. Its core connects 32 working registers and the instruction set together. All working registers are directly connected to the ALU (Arithmetic Logic Unit), enabling the execution of one instruction in one clock cycle while accessing (reading and writing) two independent registers. This structure improves code efficiency, making the execution time of most instructions only one clock cycle. Therefore, the ATmega8 has a performance close to 1 MI/s/MHz, and its operating speed is 10 times higher than that of ordinary CISC microcontrollers [7] .
The ATmega8 microcontroller integrates a hardware multiplier with an execution speed of 2 clock cycles, 8KB of Flash program memory, 512 bytes of E2PROM, two 8-bit timers with compare mode, one 16-bit timer with compare and capture modes, three PWM outputs with a maximum precision of 16 bits, an 8-channel 10-bit A/D converter, a PI/TWI synchronous serial port, and a USART asynchronous serial port. The numerous system-level functional units integrated into the ATmega8 greatly facilitate the development of the control system. During the design process, hardware circuitry was simplified as much as possible through software programming, effectively shortening the development cycle.
In the application of this system, the three real-time meteorological element indicators of temperature, average wind speed and relative humidity measured by the automatic weather station are extracted by software, and the critical values of meteorological elements for freezing and thawing are determined. When it is necessary to heat the wind sensor, the output command is sent to the ATmega8 microcontroller through the interface circuit, so that the PC0 terminal (pin 23) of the ATmega8 outputs a high level, and the driving circuit is controlled to start the heating device; when the set time is reached or the freezing condition is not met, a stop heating command is sent, so that the PC0 terminal (pin 23) of the ATmega8 outputs a low level, and the driving circuit is controlled to disconnect the heating device, so that the heating circuit stops working. Thus, the purpose of antifreeze protection of the wind sensor of the automatic weather station is achieved [8] . The control circuit of the ATmega8 microcontroller is shown in Figure 3.
2.3 Heating Drive Circuit The ATmega8's I/O port output load capacity is a maximum of 40mA, which cannot directly drive high-power devices. An intermediate drive circuit is required to control the operating status of the power device via the microcontroller. In practical applications, relays or AC contactors are typically used for indirect drive. Because relays or AC contactors have mechanical contact characteristics, they significantly reduce the overall stability and reliability of the control system. A thyristor (SCR) is a power switching semiconductor device that can operate under high voltage and current conditions. It has advantages such as no mechanical contact, small size, and easy installation, and is widely used in power electronic equipment. To avoid the disadvantages of mechanical contact switches, this system uses a fully opto-isolated intermediate drive circuit based on a thyristor. A schematic diagram of the heating drive circuit is shown in Figure 4.
When pin 23 (PC0) of the ATmega8 outputs a high level, the bidirectional optocoupler, through the current-limiting protection resistor R4, is powered on. The gate of the bidirectional thyristor TRIACI is triggered and turned on by the signal from R1, R2, and the bidirectional optocoupler, thus powering on the heating circuit. When pin 23 (PC0) of the ATmega8 outputs a low level, the bidirectional optocoupler is turned off, the gate of the bidirectional thyristor TRIACI is turned off without a trigger signal, and the heating circuit is powered off and stops working. R3 and C2 in the circuit form a RC absorption unit, which can reduce the overvoltage impact on the thyristor caused by the self-induced electromotive force generated by the inductive element in the heating circuit when the thyristor is turned off. R1 and C1 form a low-pass filter unit, which can reduce the impact of false triggering of the bidirectional optocoupler on subsequent circuits. Meanwhile, the use of bidirectional optocouplers completely isolates strong and weak circuits, avoiding interference from high-power devices to the microcontroller.
2.4 Selection of Heating Components
Through repeated screening of the heating effects of various heating tubes, resistance heating wires, and ceramic heating elements, and verification experiments on their impact on the observed data, resistance heating wire was finally selected as the heating element for the wind sensor to prevent frost damage.
3 Software Design
The control program consists of data acquisition, parameter setting, and heating control. The acquisition program reads real-time data files automatically generated by the automatic weather station, extracting data on indicators such as temperature, wind speed, and relative humidity [10-11] . If the set critical parameters are reached, a control command is sent to the ATmega8 via serial port to automatically start the heating circuit. After a certain delay (reaching the set heating duration or not meeting the freezing conditions), a stop heating command is issued to disconnect the heating circuit and turn off the heating. Alternatively, the software can be used to select the "manual start" mode to manually control the start and stop of the heating circuit, achieving the purpose of antifreeze and thawing of the wind sensor in the automatic weather station.
The control program was developed based on Visual Basic 6.0. Using the MsComm control provided by Microsoft effectively avoids the cumbersome programming drawbacks caused by directly calling the Win32 API, achieving the full-duplex asynchronous communication required by this system with less code [12-14] . Users can control the operation of the heating circuit arbitrarily through this software. The software interface is shown in Figure 5.
The control program is typically installed on the monitoring computer of the automatic weather station for easy access to real-time observation data. If installed on another computer, a shared path for the real-time observation data must be configured. If the computer's serial ports are insufficient, a USB-to-232 adapter can be used, but a USB cable driver must be installed, and the serial port number must be correctly configured in the control program. The development process successfully ran using a laptop with the USB cable driver installed.
4 Hardware Installation
4.1 Heating device
The heating device for the wind sensor in an automatic weather station uses a resistance heating wire as the heating element, which is installed inside the sensor. The advantages are: ① Because it does not affect the wind flow field, there is no mechanical friction, and it has no impact on the anemometer's photoelectric counter. Therefore, it does not affect the accuracy of the observation data; ② The resistance heating wire device is easy to replace, maintain, repair, and is inexpensive; ③ The expected lifespan and cycle of the heating device are over 2 years, facilitating regular maintenance of the automatic weather station.
4.2 Heating wires and power supply
The heating device for the wind sensor of the automatic weather station utilizes the space reserved by the manufacturer. The wires run in the same direction as the power supply line of the automatic weather station and are sent to the sensor through the wind tower of the automatic weather station. This does not affect the aesthetics and ensures that it can resist lightning strikes and electromagnetic interference [15-17] .
An AC power transformer is used to convert the 220V AC power supply from the automatic weather station to 36V (a safe voltage) for heating, ensuring safety for both personnel and the instrument. The heating wire has a resistance of 150Ω and a heating power of 8.64W. The entire circuit is compact, weighing less than 1000g, and can be housed within the data logger's chassis.
5. Operational Inspection
This heating device was initially tested in a household refrigerator. At -18℃, the surface temperature was maintained at 5℃ after heating by the heating element. A temperature sensor was installed on the outer surface of the sensor during the test to monitor the temperature. If the temperature exceeded 40℃, the power would automatically cut off to prevent thermal damage to the sensor element. No temperatures exceeding 40℃ were observed during the test.
To ensure the comparability of the experiments, an "Automatic Weather Station Rain, Rime, and Freezing Damage Prevention Observation Experiment" was conducted from October 5, 2006 to June 25, 2008. Both the experimental sensor (with a heating device) and the operational sensor (without a heating device) were located within the observation field. The experimental sensor was mounted atop an electric wind-fed tower at the same height as the operational wind sensor. During the experiment, the most severe low-temperature rain, snow, and freezing disaster in China occurred at the beginning of 2008. From February 28 to March 16, the operational automatic weather station wind sensor (without anti-freezing device) froze for 17 consecutive days, while the experimental sensor (with a heating device) did not freeze even once.
6 Conclusion
Currently, most domestically produced automatic weather station wind sensors employ three-cup anemometers and long-arm single-blade wind direction sensors. In the near-surface layer of the atmosphere, airflow exhibits turbulent characteristics, and the turbulent nature of the wind field structure leads to uneven freezing of frost and hoarfrost on the wind sensors. Based on a comprehensive understanding of the formation conditions of frost, hoarfrost, and ice, this study analyzes and researches anti-freezing technologies under different meteorological conditions. According to the meteorological conditions of severe icing, the critical index for temperature-controlled thawing is determined. Through screening various components and developing a wind sensor protector, the optimal heating power is determined, and a heating control circuit for the automatic weather station wind sensor is designed. This circuit effectively protects the automatic weather station wind sensor from the damage caused by frost, hoarfrost, and ice, ensuring the accuracy of observational data and the normal operation of the automatic weather station.
The heating device for the wind sensor in the automatic weather station is installed inside the sensor, which does not affect the wind flow field, has no mechanical friction impact, and does not affect the accuracy of the observation data. The expected lifespan and cycle of the heating device is more than 2 years, and it can be inspected and maintained along with the automatic weather station's calibration cycle. At the same time, the resistance heating wire device is characterized by easy replacement, easy maintenance, easy repair, and low price.
References:
[1] Yin Xianzhe, Ding Ruijin, Wang Wei, et al. Study on frost damage caused by automatic weather station wind sensors [A]. Proceedings of the 2005 Annual Meeting of the Chinese Meteorological Society [C].
[2] Wu Youxun, Wang Jinbao, Wang Keqin, et al. Climatic characteristics of snow, rain and rime in Huangshan Mountain [J]. Meteorology, 1999, 25(2): 51-52.
[3] Jiang Xingliang, Yi Hui. Icing and Protection of Transmission Lines [M]. Beijing: China Electric Power Press, 2002.
[4] Hu Hancai. Microcontroller Principles and System Design [M]. Beijing: Tsinghua University Press, 2002, 25-66.
[5] Cao Guohua, Gao Yi, Jiang Tao, et al. Principles and Interface Technology of High-Speed Embedded Microcontrollers [M]. Beijing: National Defense Industry Press, 2004.10.
[6] Zhang Qinghui. Automatic detection method and software implementation of serial communication baud rate [J]. Microcomputer Information, 2002, 18(12): 57-58.
[7] Ma Chao, Zhan Weiqian, Geng Degen, et al. ATmega8 Principles and Application Manual [M]. Beijing: Tsinghua University Press, 2003.5-82.
[8] Yan Baozhong, Yu Linghong, Wang Renlong. Research on Web server based on AVR high-speed single-chip microcomputer [J]. Applied Technology, 2006, 33(3); 25.
[9] He Limin. Design of Single-Chip Microcomputer Application Systems [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1992.
[10] Monitoring Network Department, China Meteorological Administration. Operation Manual for Surface Meteorological Observation and Forecasting System Software [M]. Beijing: China Meteorological Press, 2005, 157-192.
[11] Monitoring Network Department, China Meteorological Administration. Meteorological Information Network Transmission Operation Manual [M]. Beijing: China Meteorological Press, 2006, 20-36.
[12] Gao Chunyan, Li Junmin, Zhang Yaoting, et al. Key Technologies and Practical Applications of Visual Basic Database Development [M]. Posts & Telecom Press, 2004, 118-159.
[13] Christoher J. Bockmann et al. Practical Examples for Visual Basic Programmers [M]. Beijing: Electronic Industry Press, 1999, 31-62.
[14] Microsoft. Microsoft Win32 Programmer's Reference [M]. Beijing: Tsinghua University Press, 1995, 68-151.
[15] Hu, Yufeng. Principles and Measurement Methods of Automatic Weather Stations [M]. Beijing: China Meteorological Press, 2004, 27-3.
[16] Yin Xianzhe, Guo Aimin, Lu Huiyun. Comparative analysis of wind speed records from CAWS automatic weather stations and manual observations [J]. Arid Meteorology, 2006, 24(1): 57-59.
[17] Yin Xianzhe, Xu Qiyun, Wang Wei, et al. Electromagnetic pulse protection design for Huajialing automatic weather station [J]. Arid Meteorology, 2004 (Supplement): 68-70.
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Design of antifreeze control circuit for wind sensor in automatic weather station
