Application scheme of wireless sensor network positioning system based on short-distance wireless communication technology

Wireless Sensor Networks (WSNs) are widely used in environmental detection, weather forecasting, security, surveillance, distributed computing and target area imaging, and have far-reaching applications in military, medical and civil applications. In the sensor network, the location information is very important to the monitoring activities of the sensor network. The location of the event or the location of the node that obtains the information is the important information contained in the sensor monitoring information. Monitoring messages without location are often meaningless.

Wireless Sensor Networks (WSNs) are widely used in environmental detection, weather forecasting, security, surveillance, distributed computing and target area imaging, and have far-reaching applications in military, medical and civil applications. In the sensor network, the location information is very important to the monitoring activities of the sensor network. The location of the event or the location of the node that obtains the information is the important information contained in the sensor monitoring information. Monitoring messages without location are often meaningless. Therefore, it is one of the basic functions of the sensor network to determine the location of the event or the location of the node to obtain the message, and it plays a key role in the effectiveness of the sensor network application.

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

The main design idea of ​​this system is: firstly, arrange a certain reference node in the monitoring area. The function of the reference node is to play the role of the reference point in the positioning process and not participate in the positioning; the mobile node is the positioning node, and the reference node is used to determine its own Location: On the one hand, the gateway node configures the location information of the reference node, and at the same time receives the location information of the mobile node and sends it to the Web server, and the Web server accesses the network to realize remote access and control. The system block diagram is shown in Figure 1.

Application scheme of wireless sensor network positioning system based on short-distance wireless communication technology

1.2 Hardware Design

1.2.1 Node Hardware Design

A wireless sensor node generally consists of a sensor module, a data processing module, a power supply module and a data transmission module. Because positioning is mainly considered in this design, the sensor module is only for future expansion.

Figure 2 is a schematic block diagram of a general node (as a reference node or a mobile node).

Application scheme of wireless sensor network positioning system based on short-distance wireless communication technology

Since the gateway node needs to be connected to a PC or a Web server, a serial port is required. The principle of gateway node is shown in Figure 3.

Application scheme of wireless sensor network positioning system based on short-distance wireless communication technology

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

(1) Processor chip

CC2431 is a system-on-a-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. Based on the RSSI (Received Signal Strength Indicator) technology, the CC2431 positioning engine accurately calculates the position of the relevant node according to the received signal strength and the known reference node position, and then sends the position information to the receiver. Compared to 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 Scnsirion Company. This chip is widely used in HVAC, automobile, consumer electronics, automatic control and other fields, and has the characteristics of high integration, simple interface, high measurement 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 a cascade of chips, and only this part needs to be designed by yourself. Since the frequency of radio frequency signals reaches 2.4 GHz, the microwave wavelength has reached the same order of magnitude as the length of the transmission line. Problems such as microwave reflection and standing waves at the end of the transmission line cannot be ignored, otherwise they will have bad effects. The key to solving these problems lies in the good matching and control of the characteristic impedance of the transmission line and the impedance of the antenna.

Two types of antennas are used in the system design. One is an external 2.4G antenna, which is connected through an SMA head, and there are mature external antennas on the market; the other is an inverted-F microstrip antenna, which has a very low production cost, and the microwave feeder uses a microstrip transmission line and copper plating. The striplines on the dielectric board of the backplane are the same, and have better microwave transmission characteristics and lower transmission loss.

1.2.2 Web Server Hardware Design

At present, the data transmission of wireless sensor network is mainly realized by multi-hop long-distance transmission, but the instability of multi-hop brings great inconvenience to data transmission, and data transmission using serial port cannot realize long-distance data transmission. In order to realize the transmission and monitoring of long-distance data, the system adopts the method of embedded Web server to access the network to realize long-distance transmission. Figure 4 is a schematic block diagram of the Web server.

(1) Processor chip

The S3C44B0 is a 16/32 bit RISC processor introduced by SAMSUNG, which provides a cost-effective and high-performance microcontroller solution for handheld devices and general applications. The S3C44BOX uses an ARM7TDMI core and is fabricated in a 0.25 μm CMOS process. Its low-power and fully static design is ideal for cost- and power-sensitive applications.

(2) Ethernet interface

The network interface chip uses RTL8019AS, which conforms to NE2000, provides automatic detection of 10BaseT integrated transceiver RJ45, and has 2 working modes: 8-bit DMA (only chip address 0x4000-0x5fff) and 16-bit DMA (available to 0x6000- 7fff). The address here only refers to the address within the chip, not the bus address.

The RTL8019AS network chip needs to simulate a total of 2 internal RAMs of the network chip: (1) 0x0000-0x000B, 12 B, the page number is 0x00, and the MAC address is stored (the MAC address stored in odd and even numbers is the same, the purpose is to facilitate 16 bit DMA read). (2) 0x4000-0x5FFF, 8192 B, the page number is 0x40-0x60 (only 0x5f is used, and 0x60 is the end boundary), which is used to store the data packet buffer sent and received. The specific number 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 it is also the industry’s first product that integrates the functions of USB host and USB device in a single chip, which can provide point-to-point connection. It can act as an independent host or device through software control, or even function as a host and device at the same time. It has a 16-bit parallel input input (PIO) and DMA interface, which can be connected to most RISC instruction set processors, digital signal processors (DSPs) and microcontrollers on the market. It is fully compliant with the USB2.0 specification and is ideal 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 for 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 arbitrarily in the mobile area. After receiving the RSSI values ​​of all reference nodes in the positioning area, each node of the computer calculates its coordinate position through the positioning algorithm. The software flow chart of the positioning node is shown in Figure 7.

1.3.4 Web server software design

The embedded Web server adopts the μClinux operating system, which is an embedded Linux operating system specially designed for the memory management unit (MMU) microcontroller. μClinux is developed based on the Linux kernel, which is free software released under the GNU Public License (GPI). After making changes to the standard Linux kernel, μClinux has formed a highly optimized and compact embedded Linux. Although its size is small, μClinux still retains most of the advantages of Linux: stability, good portability, excellent network Features, complete support for various file systems, and a rich API.

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 the system present more flexible functions, the HTML multi-frame structure is adopted, so that multiple documents can be displayed on the browser, cooperate with each other, and convert each other, as shown in Figure 8. The “System Home”, “About Us”, “Online Help” and “Contact Us” pages are static pages, and there is no data interaction with the CGI program. The “Page”, “System Status Query”, “Location Query”, and “Node Configuration” pages are dynamic pages, which are respectively linked with 3 different CGI programs to receive or return real-time data or information.

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

2 System test

This system is mainly composed of two parts: (1) The positioning system composed of reference node, mobile node and gateway node. (2) The communication between the gateway node and the embedded Web server is realized through the serial port, and the embedded Web server can be connected to the network through Ethernet, and can also be transmitted through USB. Specific steps are as follows:

(1) Arrange the reference nodes in a certain area, and select 8 reference nodes this time;

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

(3) Place the mobile node in the reference node area, and select 4 mobile nodes for testing this time;

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

The position of the mobile node is measured by changing the position of the mobile node for many times, and then compared with the actual position. Tables 1 and 2 take the mobile node 1 as an example to measure the results in different areas.

It can be seen from the above table that the positioning system has high positioning accuracy when the area is not large.

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

The Links:   LTM10C273 2SAR553P5T100