Brief analysis of the design of moisture gradient measurement system based on CC2480

introduction

For a long time, soil temperature and moisture have been the focus of agricultural research. As the two basic properties of soil, slight changes in soil temperature and moisture will have a great impact on the growth of crops. Many studies have shown that in many research fields such as soil and water conservation, agricultural water-saving irrigation, soil fertility allocation, large-scale local climate change and ecological environment protection, the temporal and spatial changes in soil temperature and moisture are also two extremely important reference factors. Therefore, in many research fields such as agriculture, environmental science, and meteorology, soil temperature and moisture are regarded as the basic objects of research and observation.

Due to the complex geographical environment in my country and the uneven data observation levels in various regions, the data sources of soil temperature and moisture are relatively scarce, and data aggregation is difficult. The soil temperature and moisture data obtained by traditional measurement methods are far from meeting the current high-precision, networked, and intelligent measurement needs in terms of measurement accuracy, data collection volume, and reliability. At the same time, traditional soil temperature and moisture measurement instruments can only measure a single temperature and moisture data of the soil surface, and lack methods and instruments that can fully, real-time, and automatically continuously measure soil temperature and moisture in a large area and in the vertical gradient direction of the soil.

With the continuous advancement of modern industrial automation technology, the development of ZigBee wireless communication technology has become increasingly mature and has been widely used in many fields such as wireless sensor measurement networks, automatic weather stations, intelligent transportation, and smart homes. ZigBee wireless communication technology has the characteristics of low power consumption, short distance, low cost, and flexible network layout, which is very suitable for measurement occasions that require automatic and continuous data collection, local distribution measurement, and large-scale network data processing. Through the ZigBee wireless network, it is easy to realize the decentralized layout of multiple soil temperature and moisture sensors, so that the collection and processing of soil measurement parameters can be easily realized.

1 System design principle and structure

The front-end data acquisition of the system includes several groups of soil temperature and moisture sensors, which are determined according to the size of the measurement area. Each group of sensors is arranged in sequence with 7 to 8 sensors at intervals of 20 cm in the vertical gradient direction of the soil to be measured. A cylindrical pit with a depth of d ≥ 1.5 m is vertically excavated in the soil area to be measured. At the same time, the sensors are connected through a card holder similar to

It is fixed in a stainless steel round tube with a diameter smaller than the pit. When burying the stainless steel round tube, first bury the soil outside the tube, and finally inject soil into the stainless steel round tube. The gradient burial of the sensor is shown in Figure 1.

The soil temperature and moisture sensor signals are sent to the analog-to-digital conversion channels on each sensor node for A/D conversion through the front-end signal amplification and sampling circuits. In order to achieve multi-channel soil gradient temperature and moisture measurement, the sensor node controls the multi-channel analog switch through the microcontroller pin signal and automatically selects the channel to be converted in real time.

Each group of sensor nodes automatically establishes a network, and the entire wireless network topology uses a star network structure. This network structure is convenient and reliable, and the central collection node can complete the data aggregation of the surrounding sensor nodes. After the self-established network is completed, the sensor node establishes a binding relationship with the collection node and periodically sends data to the collection node. When the sensor node does not receive a response message from the collection node within a fixed time, it can automatically reorganize the network and find a new collection node again. At the same time, the full-function routing node can be used to realize data relay transmission to expand the entire data collection range. Finally, the collection node stores the data internally, performs relevant correction processing on the obtained data, improves its measurement accuracy, and obtains ideal and reliable real-time data. The data is modulated in accordance with the unified data transmission format of the industry specification, and finally transmitted to the data display terminal through the GPRS module or RS232/RS485 communication interface for observation and analysis. The system structure diagram is shown in Figure 2.

2 System Hardware Structure

The hardware part of the system mainly includes two parts: the front-end signal acquisition and amplification circuit and the data communication circuit. The system hardware structure block diagram is shown in Figure 3.

The hardware structure of the system includes the main controller MSP430F149, CC2480 coprocessor, battery power supply, multi-channel soil temperature and moisture sensor circuits and sampling and amplification circuits. The main controller MSP430F149 is a 16-bit low-power processor from TI. It has up to 5 low-power modes suitable for designing devices that require dry battery power supply. It integrates excellent peripheral modules on the chip, and has 60 KB of Flash and 2 KB of RAM on the chip. The ZigBee coprocessor CC2480 completes data transmission and collection through the 4-wire SPI interface and the communication with the main control MCU. The front-end signal acquisition is completed through the PT100 platinum thermal resistor and multi-channel FDR soil moisture sensor that are suitable for measuring soil temperature and moisture buried in the soil. In addition, the weak current signal measured by the platinum thermal resistor needs to be amplified and raised by the low-power instrumentation amplifier AD8226. The multi-channel FDR soil moisture sensor directly outputs a voltage signal, which can be used through simple resistance conversion sampling.

2.1 Sensor Circuit

The soil temperature and moisture sensor uses a three-wire PT100 platinum thermal resistor suitable for soil measurement. Its outer package is suitable for long-term burial in the soil layer. The value of the PT100 platinum thermal resistor changes with the temperature. It has good linearity within the normal temperature measurement range, high precision, good stability, and strong impact resistance. Its resistance value and temperature satisfy the following relationship: when -200℃≤t≤0℃, Rt=R0×[1+At+Bt2+C×(t-100)×t3]; when 0℃≤t≤850℃, Rt=R0×(1+At+Bt2). A, B, and C are temperature coefficients; Rt is the resistance value at t℃; R0 is the resistance value at 0℃.

The length of the wire of a two-wire platinum thermal resistor will increase as the distance of use increases. The additional error caused by the wire resistance makes the measurement result error larger. The three-wire platinum thermal resistor connects one wire to the power supply end of the bridge, and the other two wires are connected to the corresponding bridge arms. When using a full-arm bridge, the effect of the change in wire resistance on the measurement result is almost negligible, and the measurement distance is far, so it is mostly used in industrial sites. The four-wire platinum thermal resistor is connected to a constant current source through the wires at both ends, and the platinum thermal resistor value is directly measured through the other two wires. The measured resistance value is very accurate and is not affected by the wire resistance at all, but the measurement distance is short and the cost is high, so it is mostly used in experiments. [page]

After comprehensive comparison, the three-wire PT100 combined with the bridge solution is adopted. The three-wire PT100 realizes the extraction of temperature signals through the bridge circuit, which can not only affect the measurement results by changing the length of the lead wire, but also avoid the influence of temperature on the temperature measurement circuit. The differential signal measured by the bridge is connected to the input of the low-power instrumentation amplifier AD8226. This instrumentation amplifier comes from ADI and is specially used for multi-channel, low-power front-end micro-signal amplification. It has excellent common-mode rejection ratio, extremely low bias current and rail-to-rail output. The amplification factor is adjusted by the external precision resistor RG to meet the measurement amplification requirements. Its positive power supply is connected to 5 V voltage, and the negative power supply is grounded. In order to reduce interference, a 0.1μF decoupling capacitor is connected.

After the original signal is amplified, it is raised to a reference voltage range suitable for digital-to-analog conversion by the Vref (1 V) of the AD8226 and then input to the front-stage external multi-channel low-power analog switch ADG758. The 8-to-1 multi-channel analog switch ADG758 is designed for low power consumption. It is connected to the MSP430F149 main controller through the pins A0~A2 of the ADG758 to realize three-line decoding selection to control the selection of each sensor channel. The output terminal D of the analog switch ADG758 is connected to the built-in high-precision 12-bit analog-to-digital converter of the MSP430F149, saving additional analog-to-digital conversion chips, thereby reducing costs and providing the possibility of realizing large-scale sensor networks to measure soil gradient temperature and moisture parameters. The sensor temperature measurement circuit is shown in Figure 4. After calibration in the constant temperature box, the required soil temperature range changes to -40~80℃, and the measurement error is ±0.4℃.

The soil moisture sensor is a FDR (frequency domain reflectometry) type soil moisture sensor. Compared with other soil moisture measurement methods such as drying weighing method, neutron meter measurement method, and TDR, this measurement method has the advantages of fast, accurate, and continuous measurement without disturbing the soil. At the same time, it can automatically monitor soil moisture changes, has excellent performance, is relatively inexpensive, and has no radioactive pollution. The FDR soil moisture sensor outputs a 0-5 V voltage signal, samples the signal through a high-precision resistor, sends it to a multi-channel analog switch, and converts it into a digital quantity through A/D. The FDR soil moisture sensor sampling circuit is shown in Figure 5.

2.2 Wireless Data Communication Circuit

CC2480 is a RF chip produced by TI that supports ZigBee protocol. It has low power consumption and only consumes less than 0.6μA in standby mode. Similar to its predecessor CC2430 chip, the difference is that CC2480 comes with ZigBee protocol stack and supports 10 Simple APIs of TI. It can communicate with any main control chip through SPI/UART interface. It has strong flexibility in use, greatly reduces the complexity of system development, and can better support the realization of multi-sensor intelligent network. CC2480 can act as terminal device node, routing node, and coordinator node in ZigBee wireless network. It has strong versatility in the network and a wide range of applications. The CC2480 interface circuit is shown in Figure 6.

3 System Software Design

The design of the system software is mainly divided into several modules according to the functional blocks. The main loop is to call each function to complete the data acquisition, processing, wireless communication and transmission of the system. The entire software is written in C language, which is highly flexible, readable and portable. The development and final debugging are completed in the IAR for MSP430 integrated development environment.

The main functions include the main function, temperature measurement, moisture measurement, temperature measurement linearization correction, data transmission format processing, wireless data transmission and other functional blocks, as well as RS232/RS485 bottom-level drivers. The temperature measurement function block implements the analog-to-digital conversion of the PT100 bridge temperature measurement circuit and stores the conversion results; the moisture measurement function block is responsible for converting the corresponding voltage signal into the actual moisture value and storing it; the temperature measurement linearization correction function block improves the accuracy of temperature measurement by querying the linear correction table of the platinum thermal resistor; the data transmission format processing function block completes the packaging of soil temperature and moisture data; the wireless data transmission function block mainly completes the wireless transmission of data by calling the control function and protocol stack of the CC2480 coprocessor. Each sub-function maintains its own independent integrity and can be seamlessly called in the main function.

In order to adapt to unattended field use, the watchdog timer should be set appropriately. At the same time, in order to save energy and extend battery life, it is necessary to make full use of the low-power control mode of MSP430F149. When performing A/D conversion, a low-frequency clock can be selected and the CPU can be turned off, or the ADC can be turned off when the CPU is processing data. When measurement is not required, the system can enter an extremely low power mode to save energy. The measurement node program flow is shown in Figure 7.

Conclusion

This soil temperature and moisture gradient measurement system uses a special soil gradient method to lay soil temperature and moisture sensors to achieve three-dimensional soil temperature and moisture measurement. Inexpensive, reliable and excellent performance sensors are selected to meet the requirements of large-scale deployment. Nonlinear errors are eliminated through corresponding software correction, and the measurement accuracy is improved to a relatively high level within a certain range, meeting the design requirements. The front-end multi-channel soil sensor signals are sequentially selected through low-power multi-channel analog switches and sent to the 12-bit A/D conversion channel of the low-power and high-performance MSP430F149 for A/D conversion. Each sensor node automatically forms a network with the data acquisition node to finally complete the wireless transmission of the measured data. By cooperating with the low-power mode of the MSP430F149, each low-power device realizes the energy consumption control of the overall system, and also provides a guarantee for long-term battery power supply in the field without supervision. This system can be applied to a variety of occasions such as continuous automatic monitoring of soil temperature and moisture in large-scale field unattended conditions and agricultural soil environment testing.