Application of wireless sensor networks in field measurement

In the field of industrial measurement, long-term, large-scale, multi-channel data measurement systems are often required. In the field of field environmental monitoring, due to the special environmental conditions, the monitoring system is often difficult to deploy effectively due to factors such as power supply and long-distance wiring. Wireless sensor networks are particularly suitable for field measurements in the industrial field due to their low power consumption, self-organizing routing, and no wiring required.

This article will introduce a wireless sensor network case deployed in a coastal city in southern my country. After a small modification, the system can meet many industrial measurement needs.

The city has a large number of mountainous landforms and a large population, which requires that the land must be used at a high rate, so a large number of buildings and roads are located near the mountains. The rainfall in the area is high all year round, especially in the rainy season in summer. Unstable mountainous landforms are prone to landslides after being eroded by rain, posing a huge threat to the safety of life and property of residents.

The local authorities have tried to deploy multiple wired monitoring networks to monitor and warn of landslides. However, since the monitoring areas are often in remote mountain areas with few roads, field wiring and power supply are limited, making it very difficult to deploy wired systems. In addition, the wired method often uses dataloggers deployed nearby to collect data, requiring dedicated personnel to regularly go to the monitoring points to download data. The system cannot obtain real-time data and has poor flexibility.

In response to this, after many exchanges with geographical monitoring experts and several field visits, Crossbow proposed a complete wireless solution for landslide monitoring based on wireless sensor networks.

Basic measurement principle

The monitoring of landslides mainly relies on the functions of two types of sensors: liquid level sensors and inclination sensors. In areas where the mountain is prone to danger, multiple holes are set vertically along the mountain, as shown in Figure 1. A liquid level sensor is deployed at the bottom of each hole, and several inclination sensors are deployed at different depths. Since the landslide phenomenon in the area is mainly caused by rainwater erosion, the depth of the groundwater level is the first indicator of the danger of landslides. The data is collected by the liquid level sensor deployed at the bottom of the hole and sent by the wireless network.

The movement of mountains can be monitored by tilt sensors. Mountains are often composed of multiple layers of soil or rock. Different layers have different movement speeds due to different physical compositions and erosion levels. When this happens, tilt sensors deployed at different depths will return different tilt data, as shown in Figure 2. After the wireless network obtains the data from each tilt sensor, professionals can determine the trend and intensity of the landslide and determine its threat level through data fusion processing.

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System design and implementation

The overall system architecture is shown in Figure 3. The products Crossbow used for this project include the new Mote node IRIS, MDA300 data acquisition board, and Stargate base station; the MoteWorksTM software environment includes the Xmesh protocol stack (IEEE802.15.4 compatible), Xserver middleware, and MoteWeb visual management platform.

The data detected by the sensor node is transmitted to the base station through the Xmesh wireless multi-hop self-organizing network, or transmitted to the base station through the relay Mote. Mote is the basic node of the wireless sensor network, composed of a processor and an RF chip. It is small in size, so it is called "Mote". The base station is a gateway device used to communicate the wireless sensor network with the existing IP network.

The base station transmits this data to the central server. After being parsed by the Xserver middleware, users can monitor it through the IT system application software. At the same time, the data interface is fully compatible with the customer's original information management system. Users can flexibly add new sensor data to the original information management system, thereby monitoring the physical world information in real time through the IP network.

In actual deployment, Crossbow adopts a hierarchical network architecture. The wireless sensor nodes in each target monitoring area form a subnet. The nodes in the subnet rely on the Xmesh wireless multi-hop self-organizing protocol to transmit data to the Stargate base station through multi-hop. After pre-processing the data, the base station sends the data back to the central server over a long distance through the GPRS network, as shown in Figure 4.

Each target monitoring area is composed of about 10 to 20 nodes (adjusted according to specific circumstances), and the entire project consists of several monitoring areas. Due to the powerful functions of Crossbow's Xserver middleware server, the system structure is flexible and adjustable (including the number of subnets and the number of nodes in the network). The distance between adjacent nodes is about 20 to 100 meters, and the data collection interval can also be flexibly controlled by the central server. In the dry season, it can be adjusted to collect and transmit data once every 24 hours, thereby saving energy and avoiding a large amount of redundant data. In the dangerous period of the rainy season, the collection interval can be as dense as once every 2 minutes, thereby ensuring real-time monitoring and early warning functions.

The system supports two-way data transmission. All data is collected at the base station and connected to the upper-level IT system for data integration, which facilitates management and query.

Sensor Node

Each sensor node contains a level sensor and tilt sensor element, an IRIS wireless sensor network node, an MDA300 data acquisition board, and a battery pack.

MDA300 provides 8 ADC channels, 8 digital channels and I2C interface for external sensors. In this project, the tilt sensor voltage output is 0~5V, which can be easily connected to the 0~2.5V ADC interface provided by MDA300 through the resistor divider network reserved by MDA300. The liquid level sensor has a 4~20mA current output. Through the external battery pack simulated ideal voltage source, and then using the resistor divider network 124? redundant resistor, 4~20mA can be converted into a 0~2.5V voltage signal that can be collected by ADC.

The MDA300 is configured as 1 level sensor channel and 6 tilt sensor channels.

Relay Mote

The hardware structure of the relay mote is exactly the same as that of the mote, except that it is not connected to the sensor . Unlike ordinary motes, relay motes are not powered by batteries , but by wires, and they always remain in working condition to ensure the communication efficiency of the entire network. The relay mote transmits data from the node to the base station through the mesh network. When a mote fails, the other related motes will automatically reselect the route. After the fault of this mote is eliminated, it will rejoin the mesh network and continue to work. [page]

Base Station

The base station consists of a Stargate gateway and a Mote. The Stargate gateway contains an Intel PXA255 main processor, an Intel SA1111 coprocessor, 64MB RAM, 32MB FLASH, and a 51-pin interface, a PCMCIA interface, and a CF interface.

In this project, Stargate connects to an IRIS node through a 51-pin interface and relies on the Xmesh self-organizing protocol to obtain subnet data; it uses a PCMCIA external GPRS card to obtain long-distance communication capabilities through the GPRS network.

The processing power of the base station itself is used for data preprocessing, and the CF interface is connected to a 512MB FLASH card to store at least 7 days of local data. The actual picture of the base station is shown in Figure 5.

MoteWeb

MoteWeb is a B/S architecture visual monitoring software that supports wireless sensor network systems under the Windows platform. It can directly access WSN data through a Web browser and has a friendly interactive interface. The data of all nodes in the wireless network are parsed by the Xserver middleware and stored in the PostreSQL database. MoteWeb can read and display this data from the database, and can also display the data received by the base station in real time. Based on MoteWeb, managers can quickly organize, search or view the data information of each node through direct data, charts or node topology structures. MoteWeb can also provide alarm information in the form of mobile phone text messages and emails according to the settings of the manager.

Key issues and solutions

Communication distance

In the process of applying wireless sensor networks to this project, the biggest problem encountered was how to ensure that the Mote nodes could still communicate normally under heavy vegetation coverage. Before developing the project, Crossbow sent people to conduct field investigations several times, and conducted detailed discussions and analyses, and finally determined that 2.4GHz was most suitable for this environment.

Table 1 shows the signal attenuation under different conditions. It can be seen that both heavy vegetation and heavy rain will attenuate the wireless signal. 433MHz has better diffraction performance due to its longer wavelength, and performs better in the rain. 2.4GHz has better penetration due to its shorter wavelength, and performs better in heavy vegetation environments. According to the above table, the attenuation caused by heavy vegetation is thousands of times that of heavy rain, and the system should work in a rainy environment for less than 50%. Therefore, 2.4GHz should be more suitable for use in this environment.

In addition, considering the spectrum environment, most of the commercial 2.4GHz devices currently in use, such as WiFi and BlueTooth, are short-range devices, so the 2.4GHz band is relatively clean and has less interference, while the 400MHz and 900MHz bands have relatively more interference.

Although 2.4GHz has a relatively good performance, heavy vegetation and rainfall still cause significant attenuation of wireless signals. Crossbow launched the latest IRIS node in 2007, which uses the new AT1281 + RF 230 chipset and modular design and production. IRIS has greatly improved the communication distance index and reduced its power consumption to a certain extent.

Energy consumption

Each node is powered by a battery. Under Crossbow's ELP (Extend Low Power) power management mechanism, the battery power can keep the node working continuously for more than 4 years.

The battery voltage is monitored at all times. Once the voltage is too low, the node will send the voltage data to the base station. After the data is successfully sent, the node will be in deep sleep mode. After the manager receives a warning that a node voltage is too low, he can carry out system maintenance work purposefully. When the node is replaced with a new battery, it will automatically work normally.

IT system design

The concept of middleware makes the design of the background IT system of wireless sensor networks extremely easy. Xserver provides common data interfaces including database interface and XML interface, which converts the physical information in the world of wireless sensor networks into formats acceptable to various servers. Users can easily add the data of wireless sensor networks to the original information management system.

The wireless sensor network technology of Crossbow Technology Company in the United States has greatly improved the efficiency of landslide monitoring. Wireless sensor network technology not only makes each node easy to install and deploy, eliminating the cumbersome process of wired access and reducing costs, but also the Xmesh-based network can work stably, reliably and continuously for a long time, ensuring data storage and timely update. The working mode of the entire system can also be changed at any time through the network to flexibly adapt to different environmental conditions.