Application of Temperature and Humidity Sensor in Recorder in Agricultural Growth
In modern agricultural informatization, information is collected mainly in temperature, humidity, rainfall, soil moisture, soil temperature, wind speed, wind direction, static radiation, flow, and pressure signals. The effective collection of these information needs to be selected to meet the requirements and be simple. Easy-to-use, cost-effective sensor. This article combines the practical application experience of the Beijing Agricultural Information Technology Research Center, focusing on the measurement and control of air and soil temperature and humidity, and the selection and application of temperature and humidity sensors in agricultural information.
1 The temperature sensor temperature sensor in the recorder includes a thermocouple sensor, a thermal resistance sensor, a thermistor, a transistor temperature sensor, and an integrated module temperature sensor that has appeared in recent years. The temperature data that need to be collected in agricultural production include air temperature and soil temperature. Usually, a thermal resistance PT100 is used to measure the soil temperature, and an all-digital integrated temperature sensor is used to measure the air temperature.
1.1 Soil temperature sensor of the recorder The soil temperature data acquisition uses the thermal resistance PT100 temperature sensor. The advantages of the thermal resistance are good linearity, high precision, good long-term stability, and a wide operating temperature range, as long as appropriate data processing is performed. It can transmit, display and record its temperature output.
The nominal resistance of the platinum resistor at 0°C is 100Ω. It is a standardized device. The long-term stability, reproducibility, fast response, and wide operating temperature range of the platinum metal make it suitable for a wide range of applications. In applications, the resistance and temperature of platinum resistance are not linear, but after a certain temperature compensation, they are basically linear. Therefore, as long as the known current flows through the resistor, an output voltage proportional to the temperature can be obtained. According to the known resistance and temperature relationship, the measured temperature value can be known.
In practical applications, using a three-terminal adjustable voltage regulator LM317 self-made 1mA constant current source to power the soil temperature sensor, you can get the voltage signal output to meet the required range, the principle shown in Figure 1.
1.2 Recorder Air Temperature Sensor The signal temperature of the air temperature sensor is selected using the all-digital integrated temperature sensor DS18B20. DS18B20 is an improved smart temperature sensor recently introduced by DALLAS Semiconductor. Compared with the traditional thermistor, it can directly read the measured temperature and according to actual requirements, through a simple programming to achieve 9 ~ 12-bit numerical readings. The 9-bit and 12-bit digital quantities can be completed in 93.75ms and 750ms, respectively, and the information read from the DS18B20 or the information written to the DS18B20 requires only one port (single-wire interface) to read and write, and the temperature conversion power comes from the data. The bus, the bus itself, can also supply power to the DS18B20 that is attached without additional power. Therefore, using the DS18B20 makes the system structure simpler and more reliable. It has greatly improved temperature measurement accuracy, conversion time, transmission distance, resolution, etc., which brings users more convenient use and more satisfactory results. The typical wiring diagram is shown in Figure 2.
As can be seen from the wiring, using a transmission line can complete the acquisition of the temperature sensor signal, which is a wiring method using external power supply, this type of temperature signal conversion speed is faster, can meet the real-time requirements of relatively high occasion. In the circuit, in order to meet the requirements of long-distance signal transmission, a tri-state buffer 74ALS244 is used for signal driving, which improves the reliability of signal transmission.
DS18B20 can also use internal power supply, but this single-wire connection, the temperature is converted to digital signal speed is relatively slow; DS18B20 makes full use of the unique characteristics of a single bus, you can easily set up a sensor network, improve the system's anti-jamming, so that The system design is more flexible and convenient, and it is suitable for on-site temperature measurement in harsh environments.
2 Recorder Humidity Sensors Humidity sensors include thermal conductivity humidity sensors, electrolytic humidity sensors, field effect tube humidity sensors, refractive moisture sensors, and crystal oscillator humidity sensors. In addition, the humidity sensor in a broad sense should also include a dew point sensor and a moisture sensor. The humidity data that need to be collected in the water-saving irrigation control system includes air humidity and soil moisture. This paper uses the principle of known permittivity to convert the corresponding humidity into the corresponding voltage to measure the soil moisture. In the air humidity measurement, the HS1101 relative humidity sensor is used.
2.1 Soil moisture sensor of the recorder
Soil moisture is also called soil moisture, ie, the volumetric moisture content of a soil, defined as the volume of water contained in a unit volume of soil, expressed as ωs. It is known that the volumetric moisture content of the soil is linearly related to the square root of the dielectric constant of the soil, ωs=aξs+b. Among them: a, b is a constant determined by the soil type. It is also known from the theory of transmission lines that the characteristic impedance Z0 of a coaxial transmission line depends on its geometry and the dielectric constant of the insulating material, ie Z0 = (60/ξ)ln(r/R). Where: r and R are the signal line and shield radius. As shown in Fig. 3, a coaxial line Ls with a dielectric constant εs is connected in series with a coaxial line L0 with a known dielectric constant ε, and the reflection coefficient Ï at the interface J is: Ï =(Zs-Z0)/(Zs+Z0) where: Zs is the characteristic impedance of Ls; Z0 is the characteristic impedance of L0. A sinusoidal excitation voltage is applied to the input of L0 and a standing wave is generated in L0. Assuming that the length of L0 is 1/4 wavelength, the voltage peak of the input point is: Vi=Va(1-Ï), the voltage peak at the interface J is: Vj=Va(1+Ï), where: Va is the excitation The magnitude of the voltage. Therefore, there are: Vj-Vi = 2VaÏ, from which to derive the relationship between ξs and (Vj-Vi) are as follows: ξ s = bZ0 × 1-(Vj-Vi) / 2Va1 + (Vj-Vi) / 2Va, where: b is Ls structure constants.
Based on the above theory, a soil moisture sensor is designed and a 100 MHz sine wave is used as the excitation signal. The valleys and peaks of the standing wave are detected at the input end I and the boundary of the L0, and the dielectric constant of the Ls insulating material is obtained through amplification and adjustment. Changes, which translate into changes in output voltage, indirectly measure soil moisture.
2.2 Recorder Air Humidity Sensor The air humidity can be said to be the most difficult to measure in all data. Because of the instability of humidity, slow response speed of humidity sensor, and troublesome correction, humidity measurement has always been a difficulty in measurement and sensing. In actual air humidity measurement, HS1101 relative humidity sensor is used in this paper. Its characteristics are complete interchangeability, high reliability and long-term stability without calibration, fast response time, patented solid polymer structure, suitable for both linear voltage output and frequency output circuits, relative humidity at 0~ 100% RH range; capacitance from 162pF to 200pF, the error is not more than ± 2% RH; response time is less than 5s; temperature coefficient is 0.04pF / °C.
The HS1101 capacitive sensor is equivalent to a capacitive device in the circuit configuration, and its capacitance increases as the measured air humidity increases. How to accurately change the amount of change in capacitance into a signal that is easily accepted by the computer, two methods are commonly used: First, the humidity sensitive capacitor is placed in a bridge oscillation circuit, the generated sine wave voltage signal is rectified, DC amplified, and then After A/D conversion to digital signal; the other is to put the humidity sensitive capacitor in the 555 oscillation circuit, change the capacitance value to be inversely proportional to the voltage and frequency string signal, which can be directly collected by the computer. This article uses the method of converting humidity into frequency signal in practical application. Humidity conversion circuit shown in Figure 4.
3 Conclusion The above sensors have been applied in the Panggezhuang watermelon base in Beijing Daxing District and Beijing Wuhuan Highway Green Belt. The sensor meets the requirements of timely collection and accurate transmission, and the entire system runs well. It has been verified by practice that it has good use value and promotion value. The choice of sensors should be based on the actual purpose of use, indicators, environmental conditions, costs and other aspects of comprehensive consideration, each sensor should also be selected from different focus. This article did not simply select some powerful but expensive sensors for measuring temperature and humidity in agricultural applications. Instead, it considers existing sensors comprehensively, selects devices with higher cost performance, and after actual application, achieves lower usage. The cost basically meets the needs of measurement.
1 The temperature sensor temperature sensor in the recorder includes a thermocouple sensor, a thermal resistance sensor, a thermistor, a transistor temperature sensor, and an integrated module temperature sensor that has appeared in recent years. The temperature data that need to be collected in agricultural production include air temperature and soil temperature. Usually, a thermal resistance PT100 is used to measure the soil temperature, and an all-digital integrated temperature sensor is used to measure the air temperature.
1.1 Soil temperature sensor of the recorder The soil temperature data acquisition uses the thermal resistance PT100 temperature sensor. The advantages of the thermal resistance are good linearity, high precision, good long-term stability, and a wide operating temperature range, as long as appropriate data processing is performed. It can transmit, display and record its temperature output.
The nominal resistance of the platinum resistor at 0°C is 100Ω. It is a standardized device. The long-term stability, reproducibility, fast response, and wide operating temperature range of the platinum metal make it suitable for a wide range of applications. In applications, the resistance and temperature of platinum resistance are not linear, but after a certain temperature compensation, they are basically linear. Therefore, as long as the known current flows through the resistor, an output voltage proportional to the temperature can be obtained. According to the known resistance and temperature relationship, the measured temperature value can be known.
In practical applications, using a three-terminal adjustable voltage regulator LM317 self-made 1mA constant current source to power the soil temperature sensor, you can get the voltage signal output to meet the required range, the principle shown in Figure 1.
1.2 Recorder Air Temperature Sensor The signal temperature of the air temperature sensor is selected using the all-digital integrated temperature sensor DS18B20. DS18B20 is an improved smart temperature sensor recently introduced by DALLAS Semiconductor. Compared with the traditional thermistor, it can directly read the measured temperature and according to actual requirements, through a simple programming to achieve 9 ~ 12-bit numerical readings. The 9-bit and 12-bit digital quantities can be completed in 93.75ms and 750ms, respectively, and the information read from the DS18B20 or the information written to the DS18B20 requires only one port (single-wire interface) to read and write, and the temperature conversion power comes from the data. The bus, the bus itself, can also supply power to the DS18B20 that is attached without additional power. Therefore, using the DS18B20 makes the system structure simpler and more reliable. It has greatly improved temperature measurement accuracy, conversion time, transmission distance, resolution, etc., which brings users more convenient use and more satisfactory results. The typical wiring diagram is shown in Figure 2.
As can be seen from the wiring, using a transmission line can complete the acquisition of the temperature sensor signal, which is a wiring method using external power supply, this type of temperature signal conversion speed is faster, can meet the real-time requirements of relatively high occasion. In the circuit, in order to meet the requirements of long-distance signal transmission, a tri-state buffer 74ALS244 is used for signal driving, which improves the reliability of signal transmission.
DS18B20 can also use internal power supply, but this single-wire connection, the temperature is converted to digital signal speed is relatively slow; DS18B20 makes full use of the unique characteristics of a single bus, you can easily set up a sensor network, improve the system's anti-jamming, so that The system design is more flexible and convenient, and it is suitable for on-site temperature measurement in harsh environments.
2 Recorder Humidity Sensors Humidity sensors include thermal conductivity humidity sensors, electrolytic humidity sensors, field effect tube humidity sensors, refractive moisture sensors, and crystal oscillator humidity sensors. In addition, the humidity sensor in a broad sense should also include a dew point sensor and a moisture sensor. The humidity data that need to be collected in the water-saving irrigation control system includes air humidity and soil moisture. This paper uses the principle of known permittivity to convert the corresponding humidity into the corresponding voltage to measure the soil moisture. In the air humidity measurement, the HS1101 relative humidity sensor is used.
2.1 Soil moisture sensor of the recorder
Soil moisture is also called soil moisture, ie, the volumetric moisture content of a soil, defined as the volume of water contained in a unit volume of soil, expressed as ωs. It is known that the volumetric moisture content of the soil is linearly related to the square root of the dielectric constant of the soil, ωs=aξs+b. Among them: a, b is a constant determined by the soil type. It is also known from the theory of transmission lines that the characteristic impedance Z0 of a coaxial transmission line depends on its geometry and the dielectric constant of the insulating material, ie Z0 = (60/ξ)ln(r/R). Where: r and R are the signal line and shield radius. As shown in Fig. 3, a coaxial line Ls with a dielectric constant εs is connected in series with a coaxial line L0 with a known dielectric constant ε, and the reflection coefficient Ï at the interface J is: Ï =(Zs-Z0)/(Zs+Z0) where: Zs is the characteristic impedance of Ls; Z0 is the characteristic impedance of L0. A sinusoidal excitation voltage is applied to the input of L0 and a standing wave is generated in L0. Assuming that the length of L0 is 1/4 wavelength, the voltage peak of the input point is: Vi=Va(1-Ï), the voltage peak at the interface J is: Vj=Va(1+Ï), where: Va is the excitation The magnitude of the voltage. Therefore, there are: Vj-Vi = 2VaÏ, from which to derive the relationship between ξs and (Vj-Vi) are as follows: ξ s = bZ0 × 1-(Vj-Vi) / 2Va1 + (Vj-Vi) / 2Va, where: b is Ls structure constants.
Based on the above theory, a soil moisture sensor is designed and a 100 MHz sine wave is used as the excitation signal. The valleys and peaks of the standing wave are detected at the input end I and the boundary of the L0, and the dielectric constant of the Ls insulating material is obtained through amplification and adjustment. Changes, which translate into changes in output voltage, indirectly measure soil moisture.
2.2 Recorder Air Humidity Sensor The air humidity can be said to be the most difficult to measure in all data. Because of the instability of humidity, slow response speed of humidity sensor, and troublesome correction, humidity measurement has always been a difficulty in measurement and sensing. In actual air humidity measurement, HS1101 relative humidity sensor is used in this paper. Its characteristics are complete interchangeability, high reliability and long-term stability without calibration, fast response time, patented solid polymer structure, suitable for both linear voltage output and frequency output circuits, relative humidity at 0~ 100% RH range; capacitance from 162pF to 200pF, the error is not more than ± 2% RH; response time is less than 5s; temperature coefficient is 0.04pF / °C.
The HS1101 capacitive sensor is equivalent to a capacitive device in the circuit configuration, and its capacitance increases as the measured air humidity increases. How to accurately change the amount of change in capacitance into a signal that is easily accepted by the computer, two methods are commonly used: First, the humidity sensitive capacitor is placed in a bridge oscillation circuit, the generated sine wave voltage signal is rectified, DC amplified, and then After A/D conversion to digital signal; the other is to put the humidity sensitive capacitor in the 555 oscillation circuit, change the capacitance value to be inversely proportional to the voltage and frequency string signal, which can be directly collected by the computer. This article uses the method of converting humidity into frequency signal in practical application. Humidity conversion circuit shown in Figure 4.
3 Conclusion The above sensors have been applied in the Panggezhuang watermelon base in Beijing Daxing District and Beijing Wuhuan Highway Green Belt. The sensor meets the requirements of timely collection and accurate transmission, and the entire system runs well. It has been verified by practice that it has good use value and promotion value. The choice of sensors should be based on the actual purpose of use, indicators, environmental conditions, costs and other aspects of comprehensive consideration, each sensor should also be selected from different focus. This article did not simply select some powerful but expensive sensors for measuring temperature and humidity in agricultural applications. Instead, it considers existing sensors comprehensively, selects devices with higher cost performance, and after actual application, achieves lower usage. The cost basically meets the needs of measurement.
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