Design and implementation of active RFID positioning system

With the rapid increase of data services and multimedia services, people's demand for positioning and navigation is increasing. However, due to the limitations of positioning time, positioning accuracy and complex environment, the relatively complete positioning technology cannot be well utilized at present. In view of these problems, after analyzing the characteristics and shortcomings of several typical positioning technologies, an active REID positioning system is designed and implemented. The system takes REID technology as the core and uses a reader to locate the tag. The analysis shows that the system has the characteristics of high positioning accuracy, strong anti-interference ability, large positioning range, etc., and is suitable for multi-tag situations.

Positioning system refers to the positioning and tracking of property and personnel in a limited area, such as within a company, campus, port, warehouse, etc. With the rapid increase of data services and multimedia services, people's demand for positioning and navigation is increasing. It has become an emerging industry and one of the hottest research fields in the 21st century. At present, commonly used positioning technologies include infrared, ultrasonic, GPS , Wi-Pi, etc., but these technologies have defects such as small positioning range, poor anti-interference ability, and low positioning accuracy. In view of these shortcomings, this paper designs and implements an active REID positioning system, which makes up for these defects well and is suitable for more occasions.

1. Positioning technology analysis

Infrared positioning technology is only suitable for short-distance transmission and is easily interfered by fluorescent lights or lights in the room, so this positioning technology has great limitations in positioning range and positioning accuracy.

Although ultrasonic propagation positioning technology has a long range, it is greatly affected by multipath effects and non-line-of-sight propagation. Therefore, this positioning technology has strict requirements on the environment and is not suitable for indoor positioning.

GPS positioning technology is currently the most widely used outdoor positioning technology. It is a satellite navigation and positioning system developed by the United States for military purposes in the early 1970s. It mainly uses the measurement data of several satellites to calculate the location of mobile users. It has a large coverage area, but the positioning signal is weak when it reaches the ground and cannot penetrate buildings. Therefore, this positioning technology is only suitable for outdoor positioning and is not suitable for indoor positioning.

Wi-Pi positioning technology is used for small-scale indoor or outdoor positioning, and the cost is low. However, whether it is used for indoor or outdoor positioning, Wi-Fi transceivers can only cover an area within a radius of 90 m, and are easily interfered by other signals, thus affecting their accuracy, and the energy consumption of the locator is also high.

After analyzing the shortcomings of existing technologies, a positioning technology based on RFID technology is proposed. Compared with existing positioning technologies, RFID technology not only has cost advantages, but also has low environmental requirements and is less affected by the environment. It has high positioning accuracy and a large transmission range. At the same time, it can also read a lot of information about the object from the positioning target.

2. System composition

The active REID positioning system designed in this paper consists of four parts: reader, tag, communication network and background server, as shown in Figure 1.

Figure 1 System composition

Each reader stores its own location information and can send it to tags entering the area through wireless radio frequency communication. The tag and the reader can measure the pseudo-range of radio transmission through radio frequency communication , and calculate its own location information based on it, and then report it to the reader. The communication network can transmit the information received by the reader to the background server, and the background server can also control each reader through the network .

After the system is installed, the tag can determine its own location through radio frequency and upload it to the backend server through the communication network. The backend server collects tag information and provides network services for tag location.

3. Hardware structure

The tags and readers of this system have the same hardware structure. The system design is divided into the following parts: main controller , wireless RF transceiver and ranging module, antenna, and power supply system. The system principle block diagram is shown in Figure 2.

Figure 2 System Schematic Diagram

To meet the needs of high-speed data processing and network communication , the system uses Atmel's Atmega64 as the main control chip. The ATmega64 microcontroller adopts the Harbard structure, has a single-cycle RISC instruction system, has a hardware multiplication circuit inside, and has a fast data processing speed; the I/O port can directly drive a large current load; it has read-write and address latch control pins, which is convenient for expanding and using external interfaces and external storage space; it supports online programming (ISP) and online application programming (IAP), which is convenient for on-site modification and debugging of programs; it has an SPI serial communication interface that supports master/slave mode, which can easily connect serial communication units in master/slave mode. In order to meet the needs of communication and high-speed data processing, this system uses a 16 MHz crystal oscillator.

The wireless RF transceiver and ranging module uses Nanotron's NanoPAN module. This module uses broadband linear frequency modulation spread spectrum (CSS) technology and is adopted by the IEEE 802.15.4a standard. The transceiver is a 2.4 GHz ISM band wireless device that can flexibly provide data transmission rates in the range of 31 25 Kb/s to 2 Mb/s. Its point-to-point ranging accuracy is within 1 to 2 m, and it can also provide reliable data communication with an excellent transmission range. By using a MAC controller, the requirements for microprocessors and software can be reduced, and high-level system design can be easily completed.

The antenna part adopts the design of direct matching antenna. Due to space limitations, the wireless transceiver module is directly connected to the antenna through wires, and ferrite shielding and electromagnetic shielding are used in the design. Ferrite shielding is used to reduce the impact of metal on the antenna, and electromagnetic shielding is used to reduce the magnetic field generated by the antenna coil itself. In order to make a shielded antenna on the PCB board, at least 4 layers of boards are required, and the top and bottom layers must have non-enclosed shielding loops. Such a loop provides electromagnetic shielding and improves electromagnetic compatibility.

Since the reader and the tag need to transmit wireless signals to space, they need to consume more electricity , so the system uses its own power supply and selects a battery system of appropriate capacity according to the actual power consumption, so that the use of the overall system is not affected.

4. Software Structure

The tag and the reader have the same software structure, as shown in Figure 3. The system uses Atmel's AVR Studio as the development platform, and the platform is programmed in C language. In the software system, the tag first sends a request to receive a broadcast packet and waits for the reader's response. When more than 3 (including 3) reader responses are received, the tag starts to measure the distance of the received reader. After the distance measurement is completed, it calculates its own position coordinates based on the reader's position information and uploads them to the reader through broadcast, starting a new round of distance measurement. The program flow chart is shown in Figure 4.

Figure 3 Tag and reader software structure

Figure 4 Program flow

5. Test Results

In the system test, the positioning system was applied to the positioning of personnel in the school laboratory building. The tags were positioned in real time through three fixed readers and displayed in real time on the PC . As shown in Figure 5, the solid points are the locations of the readers and the hollow points are the tags. Finally, the statistical curve graph of Figure 6 was obtained through multiple actual tests. The system has a high positioning accuracy within a range of more than 4 m.

Figure 5 Personnel positioning

Figure 6 Statistical curve

This paper introduces the design and implementation of the active RFID positioning system. It provides a hardware platform structure design scheme, and explains the system positioning method and software workflow. The active RFID positioning system implemented according to this scheme has the advantages of high positioning accuracy, strong anti-interference ability, and large positioning range.