Design of positioning system based on wireless sensor network

Positioning is an important supporting technology for wireless sensor networks and has a wide range of applications. ZigBee technology is a short-range, low-power, low-data-rate, low-cost two-way wireless communication technology that can be embedded in various devices and supports geolocation functions. Applying ZigBee technology to wireless sensor networks is a focus of current research, and the research and application of related positioning technologies have also received widespread attention.

1 Overview of WSN Positioning

1.1 Current status of WSN positioning research

The widespread use of wireless sensors has led to the rapid development of positioning technology. TI has launched a system-on-chip (SoC) solution CC243l with a hardware positioning engine, which can achieve a positioning accuracy of 3 to 5 meters and a resolution of 0.25 meters in typical applications. However, due to the fixed positioning algorithm, its application lacks flexibility. On the other hand, positioning using ordinary wireless transceiver devices combined with software algorithms has received widespread attention.

1.2 Introduction to CC2430

CC2430 is a 2.4 GHz RF system-on-chip launched by TI. The device integrates ZigBee RF front-end, memory and microcontroller. The microcontroller uses the 805l core, with 128 KB programmable flash memory and 8 KB RAM. It also contains A/D converter, timer, AESl28 coprocessor, watchdog timer, 32 kHz crystal sleep mode, timer power-on reset circuit and 21 external programmable I/O ports. It supports the CSMA-CA function specified by IEEE802.15.4 in hardware. CC2430's rich resources, low power consumption and low cost make it very suitable for wireless sensor networks.

2 Positioning system network structure diagram

The system architecture is shown in Figure 1. The wireless sensor positioning system includes three types of nodes: coordinator, reference node and blind node. The position of the reference node is known, and the blind node uses the known reference node information and a certain positioning algorithm to determine its own position and complete its own positioning.

The design process of a complete wireless sensor positioning system includes three aspects: hardware node design, positioning node software design and host computer software design. The hardware design provides the system with a positioning hardware platform, the positioning node software design mainly completes the data transmission process of the wireless transceiver module, and the host computer software receives the wireless collection data, uses a specific algorithm to complete the positioning, and dynamically displays the positioning results.

3 WSN Positioning Node Hardware Design

3.1 Overall design

The hardware design framework of the positioning node is shown in Figure 2. The hardware design is divided into two parts: wireless communication module design and wireless test module design. The wireless communication module provides an interface for wireless data between nodes and is the core part of the node. The wireless test module realizes data transmission between the PC and the coordinator node through the RS232 serial port conversion circuit.

Figure 2 Block diagram of positioning node hardware design

3.2 Hardware Implementation

The wireless communication module includes CC2430 and its related peripheral circuits. Since CC2430 integrates 8051 core and wireless transceiver module, it simplifies the circuit design process, eliminates the design of interface circuit between single chip microcomputer and wireless transceiver device, and shortens the R&D cycle.

The serial port conversion circuit of the wireless test module adopts MAX3232 dual-channel converter with an operating voltage range of 3 to 5.5 V. This circuit is mainly used for serial port communication between the coordinator and the PC.

For RF circuits, it is crucial to solve the problem of interference between devices. It is recommended that the wireless communication module use a double-layer PCB board, with the top layer used for signal line wiring and the bottom layer used for power and ground wiring. A small number of vias are used to connect to the ground in the open area without wiring. In addition, be sure to make the bottom of CC2430 reliably grounded. The size of peripheral components should be as small as possible, and 0402 specification resistors and capacitors can be used. If a PCB antenna is used, in order to reduce the impact of the board on the PCB antenna and achieve the best performance of the antenna, RF4 board can be used, with a board dielectric constant of 4.5, a thickness of 1 mm, and a copper thickness of 0.35μm. There are no special requirements for the PCB production of wireless test modules.

4 Positioning Node Software Design

There are three types of nodes in the wireless sensor positioning network, namely gateway (coordinator), reference node (router) and blind node (terminal). The gateway plays a vital role in the entire system. First, it receives commands from the host computer, opens the network, and waits for other types of nodes to join the network. Secondly, it receives valid data fed back by each node and transmits it to the host computer software for processing. The reference node is a type of stationary node with a known position. Its task is to receive information packets with RSSI (Received Signal Strength Indicator) values ​​and calculate the RSSI average value. Finally, after the blind node packages each RSSI average value, it sends it to the gateway and transmits it back to the host computer monitoring software for processing. The blind node is a type of movable node that can move arbitrarily within the area surrounded by the reference node. The blind node broadcasts the RSSI cluster to the surrounding space and receives the average RSSI value of the reference node within one hop range. After packaging the received average RSSI values, it sends them wirelessly to the coordinator node. Figure 3 shows the overall sensor network positioning communication process.

Figure 3 Sensor network positioning communication network process

Considering that the gateway node can be used as a reference node in addition to networking and serial communication functions, only two node programs need to be written. The unique functions of the gateway node are marked with dotted lines. The workflow of various nodes is shown in Figures 4 and 5.

Figure 4 Blind joint positioning process

Figure 5 Network joint positioning process

5. Host computer software design

5.1 Host computer functions and their implementation

In the host computer monitoring software of the wireless sensor positioning system, the functions related to positioning mainly include two categories: positioning project management and positioning information processing. Positioning project management completes the functions of loading the positioning area schematic diagram and configuring the reference node coordinate information. Positioning information processing completes the parameter collection and data processing functions. Specifically, positioning project management needs to include the following three aspects: 1) Loading the positioning area plan, the user selects the positioning area schematic diagram in bmp, jpg, gif and other formats for a specific positioning scene; 2) Configuring reference node information, the user configures the reference node and sets the reference point number in the project, and marks the reference node position in the positioning area schematic diagram; 3) Saving and modifying the project, saving and modifying the project information at any time. Positioning information processing includes two aspects: 1) Providing an interface between the PC and the coordinator, usually using the serial port to communicate with the coordinator, the PC sends command information downward, and the coordinator reads data upward. 2) Regional positioning, positioning is performed according to the information read from the reference node and the positioning results are displayed and saved.

The upper computer function implementation process is shown in Figure 6. Among them, the system initialization includes: loading the positioning area schematic diagram, setting the area size parameters, setting the reference node position and serial port related parameters, and opening the serial port.

Figure 6 Host computer implementation process

5.2 Positioning algorithm selection

Based on the received signal strength indication (RSSI) algorithm, the relationship between RSSI and distance is used to collect data in a specific positioning environment, fit the data, obtain the correlation curve between the two, and use the ranging-based positioning algorithm to confirm the position of unknown nodes.

For indoor positioning, the most direct way to achieve accurate positioning is to establish a signal strength database of the site to be positioned and determine the position of the blind node by numerical comparison. However, database construction is time-consuming and laborious. Changes in the indoor environment (such as the movement of indoor facilities, changes in the reference node position, etc.) require the re-construction of the sampling database. In order to obtain the highest possible accuracy at a relatively low cost, the two positioning methods can be appropriately integrated. Each algorithm has its advantages and disadvantages, and the appropriate algorithm should be selected according to the specific application needs.

6 Conclusion

This paper introduces the software and hardware design of the wireless sensor positioning system in detail. The positioning system can make full use of software methods to achieve high positioning accuracy. It reduces the requirements for positioning hardware and keeps the wireless sensor positioning system at a low cost. The actual test results show that the positioning system is feasible and easy to implement.