Wireless plateau frozen soil monitoring system based on IPv6

introduction

It is necessary to monitor the meteorology, changes in frozen soil and the stability of engineering buildings in the frozen soil areas in the hinterland of the Qinghai-Tibet Plateau, and to obtain basic data for the establishment of a database and systematic analysis. However, it is very costly to obtain these data manually in the plateau area. This system utilizes microcontroller (MCU) technology, wireless sensor network technology, GSM/GPRS mobile communication technology and IPv6 technology. The use of IPv6 technology allows a network to support more sensors, which can greatly reduce the number of aggregation nodes and reduce system costs.

1 Wireless plateau frozen soil monitoring system architecture

The plateau frozen soil monitoring system mainly detects soil moisture, temperature, carbon dioxide concentration and nitrogen concentration. The soil moisture sensor monitors the soil moisture 20 cm below the surface; the temperature sensor monitors the frozen soil temperature; the carbon dioxide concentration and nitrogen concentration monitor the atmospheric environment of the plateau. The data of the four sensors are aggregated to the aggregation node through the wireless sensor node and IPv6 network, and the aggregation node transmits the data to the monitoring center through the GPRS network.

The wireless plateau frozen soil information monitoring system based on IPv6 mainly consists of three parts: wireless sensor nodes, aggregation nodes and monitoring center. The architecture of the wireless plateau frozen soil information monitoring system is shown in Figure 1.

2 Design of wireless plateau frozen soil monitoring system
2.1 Design of wireless sensor nodes

Wireless sensor nodes are the direct carriers of sensors and are mainly placed in the wild. The analog signal output by the node sensor is sampled by A/D and sent to the aggregation node through the wireless network. The hardware structure of the wireless sensor node is shown in Figure 2.

2.1.1 Low power consumption and system power supply design

Wireless sensor nodes are powered by batteries, without external DC power supply, and have certain requirements on the size of nodes, so highly integrated, low-power chips must be used. The design uses Freescale's MC13224 integrated chip system. MC13224 is a low-power, high-performance chip system based on the IEEE 802.15.4 protocol designed and produced by Freescale. The system integrates a 32-bit ARM7TDMI-S microcontroller core, a 2.4 GHz IEEE802.15.4 standard RF transceiver, two 8-channel 12-bit ADCs, four 16-bit timers, etc. Only a few external devices are needed to realize a complete signal acquisition and transceiver system. [page]

MC13224 is a low-power series product designed for wireless sensor network nodes. It has three power modes that can be selected separately, including a low-power sleep mode. The sleep current is at the nanoampere level, and it supports an operating voltage of 2 to 3.6 V, which makes it have excellent power consumption performance on battery-powered devices. The power consumption mode can be adjusted when needed through software programming to achieve the best power consumption performance. In particular, the operating current of MC13224 in its power down mode is only 500 nA, and its sleep time can be accurately controlled by an external high-precision 32.768 kHz crystal oscillator.

2.1.2 Sensor interface design

There are four types of sensors, namely soil moisture sensor, temperature sensor, carbon dioxide sensor and nitrogen sensor. The sensor power supply can be switched on and off by switching transistors and MCU to achieve energy saving.

The response time of the four sensors is greater than 150 ms, which is much greater than the minimum sampling time of the ADC integrated in the MC13224, so the three sensors can share one sampling channel. Here, the sensor is set in channel 1, and its reference voltage is set to VREF = 0 V, VREF + = 2.5 V. This can make the A/D sampling data more accurate.

2.1.3 RF interface design

MC13224 integrates a 2.4 GHz IEEE 802.15.4 standard RF transceiver with excellent receiving sensitivity and anti-interference ability. Its programmable output power is +4dBm. Only a very small number of external components are needed to complete the wireless transceiver function. Figure 3 shows the RF antenna interface of MC13224. In order to facilitate development, PCB microstrip antenna and external antenna of SMC interface are designed at the same time. E1 is PCB microstrip antenna and J1 is external antenna interface.

3. Aggregation node design

The aggregation node is the core of data exchange between the sensor network and the monitoring center and the corresponding monitoring center instructions. Its main function is to receive data from the sensor nodes through the wireless sensor network and send collection instructions to the sensor nodes; at the same time, it realizes the two-way GPRS and SMS communication functions of the monitoring center. The hardware structure of the aggregation node is shown in Figure 4.

3.1 IPv6 Wireless Sensor Networks

The design of IPv6 wireless sensor network mainly implements the core part of IPv6 protocol, namely, message fragmentation, reassembly, header compression and address automatic configuration; the network topology adopts the simplest star structure. Since TCP requires three-way handshake, the transmission efficiency is low in wireless environment, so it is not suitable for IPv6 transmission requirements. This design mainly adopts UDP and ICMP message interactive communication. The new TinyOS2.0 system can support IPv6 system. Here, TinyOS2.0 system is used as the operating system platform of WSN.

TinyOS is an open source embedded operating system developed by the University of California, Berkeley, and is mainly used in wireless sensor networks. Its program adopts a modular design, and its core program is often very small. Generally, the core code and data are about 400 bytes, which can break through the limitation of the small storage resources of the sensor system, allowing TinyOS to effectively run on the wireless sensor network and perform corresponding management tasks. The process of receiving and sending UDP data packets in IPv6 wireless network under TinyOS is omitted - Editor's note.

3.2 GSM/GPRS module

At present, the actual data transmission rate of GPRS is about 50kb/s, which can fully meet the data transmission rate requirements of the system. GPRS transmission has low power consumption and is suitable for outdoor power supply environments. Although it is necessary to conduct two-way communication with a monitoring center thousands of miles away, the GPRS transmission equipment only needs to communicate with nearby mobile base stations when working. Its overall power consumption is equivalent to that of an ordinary GSM mobile phone, and the average power consumption is only about 200 mW, which is less than that of traditional data transmission radio stations. Therefore, the GPRS transmission method is very suitable for outdoor occasions where solar power or battery power is used. The use of the GPRS module requires the serial port and module to be initialized first, and the data is transmitted through the GPRS network using the protocol.

3.3 MCU’s Gateway Function in the System

The main function of the gateway is protocol conversion. The specific implementation process of the MCU gateway function is that the system completes the unpacking of IPv6 data in MC13224, transmits the valid data to MC35 through the UART interface for GPRS encapsulation, and then transmits the data to the monitoring center through the GPRS network.

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Both IPv6 and GPRS protocols refer to the OSI seven-layer reference model, and the division of responsibilities of each layer is very clear. When data is transmitted between layers, the data adds headers and tails from top to bottom and removes headers and tails from bottom to top. The protocol data conversion process between GPRS and IPv6 gateways is shown in Figure 5.

The GPRS and IPv6 wireless gateway protocol conversion mainly realizes the bidirectional conversion between IPv6 data messages and GPRS messages. Figure 6 shows the conversion process between GPRS messages and IPv6 messages. PAYLOAD is the effective data load in the message. When the wireless gateway radio frequency part (PHY layer) receives the data message, it extracts the effective load from the PHY to the IPv6 frame header according to the IPv6 communication protocol, and then loads the effective load into the TCP (UDP)/IP message format and hands it over to the terminal device that meets the GPRS communication protocol for processing, thereby realizing the transmission of the wireless received information to the GPRS network.

3.4 Hardware Design for Harsh Environments

Lightning is the strongest natural electromagnetic interference source. Lightning damage and electromagnetic interference are often multi-directional and multi-channel. Most early electrical equipment has the characteristics of simple structure, large size, and relatively high insulation level. Although it is also subject to electromagnetic interference in a strong electromagnetic field environment, the electromagnetic compatibility problem is not very prominent because the signal it transmits is strong. Therefore, how to prevent the electromagnetic pulse in lightning from affecting the components of the node itself and the signals between components is a hot research topic in today's society.

To this end, we use the following methods to reduce the electromagnetic interference of nodes on printed circuit boards.

① Place 0.1μF and 1.0μF decoupling capacitors at each power pin of the device, as close to the chip as possible; place a 100μF large-capacity decoupling filter capacitor at the power input of the entire system.
② Ground the unused space on the PCB board, which is the so-called large-area copper coating.
③ The external crystal oscillator is as close as possible to the external oscillator pin of the component.
④ Use the shortest connection to avoid the creation of an "antenna".
⑤ Use a 1~4.7 kΩ resistor to pull /RST to a high level. Set a 0.1ΩF decoupling capacitor between the /RST trace and the ground.
⑥ Connect the TMS, TCK, and TDI pins to a fixed level.
⑦ When connecting to the system cable or other signals on the circuit board, they should be properly filtered at the connection point of the PCB.

After the above processing, the impact of the external electromagnetic environment on the device can be reduced to a minimum.

3.4.1 How to deal with high and low temperature environments

The Qinghai-Tibet Plateau often experiences extremely high temperatures in the summer, which can often reduce the performance of microelectronic systems or even cause the systems to crash.

In order to solve the problem of high temperature, the system first uses a combination of heat sink and intelligent cooling fan. High temperature environment usually occurs during the summer day. In this environment, there is usually plenty of sunlight and the output current of the solar panel is large. The temperature sensor built into the system is used to measure the temperature and set a certain temperature threshold. When the system temperature is lower than this temperature, the heat sink is used to dissipate heat. When the system temperature exceeds the preset temperature, the cooling fan is turned on to cool the system.

The lowest monthly average temperature on the Qinghai-Tibet Plateau is between -15 and -10°C. Therefore, special measures must be taken to solve the problem of node operation under such low temperature conditions. For this purpose, an intelligent heating system that can be controlled by MCU is designed. The copper heater is embedded in the aluminum radiator through a copper heat pipe. At low temperatures, the fan on the aluminum radiator does not work. At this time, the aluminum radiator acts as a heating plate to evenly distribute the heat of the copper heater to each IC. The copper heater is controlled by the MCU. When the MCU detects that the system operating environment temperature is lower than the set threshold, the heater is automatically turned on.

3.4.2 How to deal with humid environment

During operation, too much moisture can cause the circuit to short-circuit and burn out, so we have made special treatments for the system in humid environments. The specific treatment method is to apply a layer of conformal coating to the PCB. Its features are:

① Fast curing speed, good adhesion to various circuit boards.
② Conformal coating has good high and low temperature resistance; after curing, it forms a transparent protective film with excellent insulation, moisture-proof, leakage-proof, shock-proof, dust-proof, corrosion-proof, salt spray-proof, mildew-proof, aging-proof, and corona-resistant properties.

Finally, a stable external packaging box is designed to prevent the system from being affected by severe weather such as strong winds and heavy rain in an open environment.

3.5 Monitoring Center Design

The main function of the monitoring center is to analyze, manage and display the data uploaded by the data aggregation nodes.

3.5.1 Data Function

The monitoring center monitors many aggregation nodes, and different formulas must be used to invert the physical quantity for each sensor's uploaded data. Different individuals of the same type of sensor also have differences, and the monitoring center needs to set different correction coefficients for different sensor individuals. Because a large amount of data is involved, SQL Server is required to manage the sensor data. The area detected by the monitoring center is very large, and each sensor node is marked on the map in an intuitive way, so that users only need to select the location of their own sensor node when selecting data. Here, the open source geographic information system (GIS) of Map Server is selected, which can combine data and maps and upload the results to the Internet.

3.5.2 Control functions

In addition to being able to perform a series of operations on data, the monitoring center can also control the aggregation node and notify users. The GSM/GPRS Modem in the monitoring center can not only realize GPRS network data transmission, but also send and receive text messages (SMS) and send commands to the aggregation node via SMS. After the aggregation node parses the command, it responds differently to different commands.

Conclusion

This paper designs a wireless plateau frozen soil monitoring system based on IPv6. Through on-site debugging, the wireless sensor nodes can accurately collect the physical values ​​of temperature, humidity, carbon dioxide, and nitrogen, and realize the wireless sensor network based on IPv6. Through this network, each sensor node can accurately transmit the data to the aggregation node. Through the GPRS network, the aggregation node can transmit the fused data to the monitoring center in real time. The monitoring center will analyze the data and display it using the GIS system, and can also use the SMS system to control the aggregation node to better guide environmental protection and construction work.