Design of wireless sensor network positioning system based on ZigBee

　　Wireless sensor networks [1] WSNs (Wireless Sensor Networks) are widely used in environmental detection, weather forecasting, security, monitoring, distributed computing, and target area imaging. Their applications in military, medical, and civilian fields have far-reaching significance. In sensor networks, location information is crucial to the monitoring activities of sensor networks. The location of an event or the location of the node that obtains information is important information contained in the sensor monitoring information. Monitoring messages without location are often meaningless. Therefore, determining the location of an event or the location of the node that obtains the message is one of the most basic functions of sensor networks and plays a key role in the effectiveness of sensor network applications.

　　ZigBee technology [2] is a short-range wireless communication technology with unified technical standards. Its PHY layer and MAC layer protocols are IEEE802.15.4 protocol standards. The wireless sensor network proposed in this paper works on the 2.4 GHz ISM (Industrial Scientific and Medical) free frequency band, which is universal worldwide. Its data transmission rate is 250 Kb/s and is divided into 16 channels. Compared with Bluetooth or 802.11, which are also short-range wireless communication technologies, ZigBee technology has inherent advantages. ZigBee devices are low-power devices with energy detection and link quality indication functions. At the same time, the collision avoidance mechanism (CSMA-CA) is used to avoid conflicts when sending data. In terms of network security, an encryption algorithm with a key length of 128 bits is used to encrypt the transmitted data information, ensuring high reliability and security during data transmission. The wireless sensor network composed of ZigBee technology has a simple structure, small size, high cost performance, flexible placement, easy expansion, low cost, low power consumption, and safe and reliable. This emerging wireless sensor network will surely have a wide range of application prospects.

　　1 Design of wireless sensor network positioning system

　　1.1 Design Concept

　　The main design idea of ​​this system is: first, certain reference nodes are arranged in the monitoring area. The role of the reference node is to act as a reference point in the positioning process and does not participate in positioning; the mobile node is the positioning node, which determines its own position through the reference node; the gateway node configures the location information of the reference node on the one hand, and receives the location information of the mobile node and sends it to the Web server at the same time. The Web server accesses the network to achieve remote access and control. The system block diagram is shown in Figure 1.

　　1.2 Hardware Design

　　1.2.1 Node Hardware Design

　　Wireless sensor nodes are generally composed of sensor modules, data processing modules, power modules and data transmission modules. Since positioning is the main consideration in this design, the sensor module is only for future expansion. Figure 2 is a block diagram of a general node (as a reference node or mobile node).

　　Since the gateway node needs to be connected to a PC or Web server, it must have a serial port. The principle of the gateway node is shown in Figure 3. [page]

　　There are three types of nodes in this system: reference nodes, mobile nodes and gateway nodes. The composition of the node system is introduced below.

　　(1) Processor chip

　　CC2431 is a system-on-chip (SoC) solution with a hardware positioning engine launched by TI, which can meet the application needs of low-power ZigBee/IEEE 802.15.4 wireless sensor networks. The CC2431 positioning engine is based on the received signal strength indication RSSI (Received Signal Strength Indicator) technology. It accurately calculates the location of the relevant nodes based on the received signal strength and the known reference node location, and then sends the location information to the receiving end. Compared with centralized positioning systems, the RSSI function reduces network traffic and communication delays.

　　(2) Sensor chip

　　SHT11 is a digital temperature and humidity sensor chip launched by Swiss company Scnsirion. This chip is widely used in HVAC, automobile, consumer electronics, automatic control and other fields, with high integration, simple interface, high measurement accuracy and programmable adjustment, ultra-small package size and high reliability.

　　(3) Antenna

　　Antenna design is the key to system design. The rest of the modules in the system are just cascades of chips, and only this part needs to be designed by yourself. Since the frequency of the RF signal reaches 2.4 GHz, the microwave wavelength and the length of the transmission line have reached the same order of magnitude. Problems such as microwave reflection and standing waves at the end of the transmission line cannot be ignored, otherwise they will have a bad impact. The key to solving these problems lies in the good matching and control of the transmission line characteristic impedance and antenna impedance.

　　Two types of antennas are used in the system design. One is an external 2.4 G antenna connected through an SMA connector. There are mature external antennas on the market. The other is an inverted F-type microstrip antenna, which has a very low production cost. The microwave feed line uses a microstrip transmission line that is the same as the strip line on the dielectric board of the copper-clad bottom plate, which has good microwave transmission characteristics and low transmission loss.

　　1.2.2 Web Server Hardware Design

　　At present, the transmission of wireless sensor network data is mainly achieved by long-distance transmission using multi-hop. However, the instability of multi-hop brings great inconvenience to data transmission. At the same time, the data using serial port cannot achieve long-distance data transmission. In order to achieve long-distance data transmission and monitoring, this system uses the method of embedded Web server access network to achieve long-distance transmission. Figure 4 is the principle block diagram of Web server.

　　(1) Processor chip

　　S3C44B0[3] is a 16/32 bit RISC processor launched by SAMSUNG. It provides a cost-effective and high-performance microcontroller solution for handheld devices and general applications. S3C44BOX uses the ARM7TDMI core and is manufactured using a 0.25 μm CMOS process. Its low power consumption and fully static design are particularly suitable for applications that are sensitive to cost and power consumption.

　　(2) Ethernet interface

　　The network interface chip uses RTL8019AS, which complies with NE2000, provides automatic detection of 10BaseT integrated transceiver RJ45, and has two working modes: 8-bit DMA (can only use chip address 0x4000-0x5fff) and 16-bit DMA (can be used to 0x6000-7fff). The address here refers only to the address within the chip, not the bus address. [page]

　　The network chip to be simulated by the RTL8019AS network chip has two internal RAM blocks: (1) 0x0000-0x000B, 12 bytes, page number 0x00, which stores MAC addresses (the MAC addresses stored in odd and even numbers are the same, for the convenience of 16-bit DMA reading). (2) 0x4000-0x5FFF, 8192 bytes, page number 0x40-0x60 (only 0x5f is used, 0x60 is the end boundary), which is used to store the data packet buffer for sending and receiving. The specific amount of sending and receiving is determined by the driver initialization. As for the size of the sending buffer and the receiving buffer, it can be determined according to specific needs.

　　The Ethernet interface uses an RJ45 interface with a transformer.

　　(3) USB interface

　　ISP1161 is the first generation chip of ISP1161A1, and is also the first product in the industry to integrate USB host and USB device functions in a single chip, providing point-to-point connection. It can act as an independent host or device through software control, or even have the functions of both host and device at the same time. It has a 16-bit parallel input/output (PIO) and DMA interface, and can be connected to most RISC instruction set processors, digital signal processors (DSPs) and microcontrollers on the market. It fully complies with USB2.0 specifications and is very suitable for embedded or converged systems.

　　1.3 Software Design

　　1.3.1 Gateway Node Software Design

　　The gateway node plays an important role in the whole system. First, it needs to receive the configuration data of the reference node, and secondly, it needs to receive the valid data fed back by each node. Its workflow is shown in Figure 5.

　　1.3.2 Reference Node Software Design

　　The reference node is a static node whose coordinate position is fixed and does not participate in the positioning calculation. Its workflow is shown in Figure 6.

　　1.3.3 Mobile Node Software Design

　　The mobile node can move freely in the mobile area. After receiving the RSSI values ​​of all reference nodes in the positioning area[4], each computer node calculates its coordinate position through the positioning algorithm. The software flow chart of the positioning node is shown in Figure 7. [page]

　　1.3.4 Web Server Software Design

　　The embedded Web server uses the μClinux operating system, which is an embedded Linux operating system designed specifically for microcontrollers without a memory management unit (MMU). μClinux is developed based on the Linux kernel and is free software released under the GNU Public License (GPI). μClinux has been modified from the standard Linux kernel to form a highly optimized, code-compact embedded Linux. Although it is small in size, μClinux still retains most of the advantages of Linux: stability, good portability, excellent network functions, complete support for various file systems, and rich APIs.

　　The software design steps are: (1) transplant u-boot; (2) establish cross-compilation environment; (3) transplant μClinux operating system; (4) transplant boa server; (5) write application program.

　　1.3.5 Design of server interface

　　The server interface is implemented using dynamic and static web pages. In order to make the browser window of this system present more flexible functions, the HTML multi-frame structure is adopted, so that multiple documents can be displayed on the browser at one time, cooperate with each other, and convert each other, as shown in Figure 8. Among them, the "System Homepage", "About Us", "Online Help" page and "Contact Us" page are static pages, and there is no data interaction with the CGI program. The "Authentication Page", "System Status Query", "Location Query" and "Node Configuration" pages are dynamic pages, which are linked to 3 different CGI programs respectively to receive or return real-time data or information.

　　All HTML files and image files that need to be linked are also temporarily stored in the /μClinux-dist/vendors/tmpt directory.

　　2 System Testing

　　This system mainly consists of two parts: (1) The positioning system consists of reference nodes, mobile nodes and gateway nodes. (2) The communication between the gateway node and the embedded Web server is realized through the serial port. The embedded Web server can access the network through Ethernet or transmit through USB. The specific steps are as follows:

　　(1) Arrange the reference nodes in a certain area. This time, 8 reference nodes are selected.

　　(2) Configure the reference node, that is, initialize the location information for the reference node;

　　(3) Place the mobile nodes in the reference node area. This time, four mobile nodes are selected for testing.

　　(4) The gateway node communicates with the embedded server and displays the positioning information through the web interface.

　　The location of the mobile node is measured by changing the location of the mobile node multiple times and then compared with the actual location. Tables 1 and 2 are the results of measurements in different areas taking mobile node 1 as an example.

　　From the above table, we can see that the positioning accuracy of this positioning system is very high when the positioning area is not large.

　　After testing, the positioning system has good positioning effect in open areas, but it is also affected by the environment, such as the impact of room walls, glass and metal on the antenna. If you encounter these situations, you should try to use an external antenna, which will have a better effect.

References:

[1]. CC2431 datasheet http://www.dzsc.com/datasheet/CC2431_1135879.html.
[2]. SHT11 datasheet http://www.dzsc.com/datasheet/SHT11_607252.html.
[3]. SMA datasheet http://www.dzsc.com/datasheet/SMA_1054310.html.
[4]. ARM7TDMI datasheet http://www.dzsc.com/datasheet/ARM7TDMI_2.html.
[5]. RTL8019AS datasheet http://www. dzsc.com/datasheet/RTL8019AS_1.html.
[6]. ISP1161 datasheet http://www.dzsc.com/datasheet/ISP1161_410194.html.
[7]. ISP1161A1 datasheet http://www.dzsc.com/datasheet/ISP1161A1_html.