This paper adopts the SLRC400 chip and introduces the binary search algorithm into its software programming to identify collisions by bit, which constitutes the core of the radio frequency identification (RFID) monitoring system for mining personnel. The hardware structure and software design of RFID system are briefly described.

How to prevent major accidents in coal mine production has always been a hot spot of people’s attention. In this paper, SLRC400 chip is used to form an RFID system to monitor personnel safety. Equip all personnel underground with radio frequency cards. When underground personnel pass the monitoring point, the reader recognizes the radio frequency card and transmits the radio frequency card number and location information to the upper computer for processing through the data bus. Once an accident occurs, the information of the trapped personnel can be quickly inquired. This minimizes casualties.

System hardware structure

The structure of this system is shown in Figure 1. Among them, the core component of the radio frequency identification system adopts the reader composed of SLRC400 chip to monitor the location of underground employees.

Electrical Characteristics of SLRC400

SLRC400 is a non-contact IC card reader working at 13.56MHz, it supports the S015693 protocol, and can drive the antenna to transmit to a longer distance (1.5 meters) under passive conditions. Its main features are: the digital part has a CRC check function; it has a parallel interface, which can be directly connected to any 8-bit microprocessor, providing greater flexibility for the design of readers and terminal circuits; highly integrated demodulation and coding simulation Circuit; flexible interrupt processing; programmable timer; unique serial data; user-programmable startup structure: independent power supply for digital, analog and transmission parts; external RS-485 and other chips.

Peripheral circuit design of SLRC400

The hardware design of the system includes the hardware circuit design of the reader composed of SLRC400 and the design of the CAN bus communication part. Among them, the peripheral circuit of SLRC400 includes EMC low-pass filter circuit, receiving circuit, antenna matching circuit and antenna.

EMC low-pass filter circuit

The operating frequency of SLRC400 is 13.56MHz, as the signal of the SLRC400 clock is generated by the oscillation of the quartz crystal, and it is also the basis for driving the 13.56MHz carrier frequency of the antenna, which not only leads to the emission of 13.56MHz energy, but also transmits higher order frequencies. harmonics. International EMC regulations specify the magnitude of emitted energy over a wide frequency range. Therefore, in order to meet this requirement, an appropriate filter is added.

receiver circuit

The internal receiving circuit of the SLRC400 works when the RF card is within the range of the reader. When the input is connected to pin RX, the internally generated VMID is used. In order to provide a stable reference voltage, the ground capacitor C3 is connected to VMID. The receiver part of the reader requires a voltage divider between RX and VMID.

Antenna coil inductance selection and impedance matching problem

It is not practical to accurately calculate the inductance value of the antenna coil, but it can be estimated using the coil inductance value formula. The actual capacitance and inductance values ​​of the antenna depend on many parameters, such as the structure of the antenna (type of PCB), the thickness of the wires, the distance between the windings, the shielding, the metal or ferrite in the surrounding environment.

The size of the capacitance value will seriously affect the performance of the reader. To determine the capacitance value, software or hardware methods can be used. A simple method is: SLRC400 has a SIGOuT pin. When the reader sends a certain command, it can be observed through an oscilloscope. The output signal of this pin constantly changes the distance and C2 value between the card and the reader, the oscilloscope will output different waveforms, and the best performance of the reader can be determined according to the different waveforms.

software implementation of the system

System software design

The system software structure is shown in Figure 2. The server, client and database use Windows 2000 Advanced Sever, Windows 2000 operating system and SQL Sever2000 respectively. There are many software for developing database, but VC++ has become the preferred tool for this design because of its WYSIWYG interface design, efficient code execution and extremely fast compilation speed. Among them, the internal microcontroller of the radio frequency identification system adopts the C51 high-level language, and the SLRC400 adopts its standard program. In addition, the system also includes the application program design of other circuits of the reader.

SLRC400 Application Algorithm

The binary search algorithm consists of a set of command and response rules specified between a reader and multiple RF cards. The purpose is to select any one of the multiple cards to realize data communication. In order to select one from a group of radio frequency cards, the reader sends a card reading command to consciously guide the data collision when the serial number of the radio frequency card is transmitted to the reader, that is, the reader judges whether a collision occurs. The algorithm has three key elements: select the baseband code that is easy to identify the collision; use the unique characteristic of the serial number of the radio frequency card; design a set of effective instruction rules to realize the card selection efficiently and quickly.

The instruction rules used in this system are: Inventory Request-Request (serial number): request a response from the reader; Select (SNR)–select (serial number): use a (predetermined) serial number as a parameter to send the RF card. If the serial number of a radio frequency card in the field is the same as this parameter, this radio frequency card is selected and responds to other commands, while the radio frequency cards of other serial numbers only respond to the Inventory Request command;

Stay quiet–quiet state: cancel a pre-selected radio frequency card, the radio frequency card enters the quiet state (inactive), and does not respond to the received Inventory Request command. In order to reactivate the radio frequency card, you can first move the radio frequency card out of the range of the reader antenna and then enter it to achieve reset, or receive the select and Reset to Ready commands.

What determines the reliability of the system function of the binary search algorithm is that all radio frequency cards need to be accurately synchronized, so that the occurrence of collision can be judged bit by bit. In order to prevent the collision of many radio frequency cards, the Inventory Request command needs to be repeated.

SLRC400 application programming

The behavior of reading and writing chip SLRC400 is determined by executing the internal state of specific 9 commands. Statements or data required to execute a command are exchanged through a FIFO buffer. Start up command to reset and initialize; IDLE switches SLRC400 to inactive state; Transmit transfers data from FIFO buffer to RF card; Receive command activates the receiving circuit; Transceive transfers data from FIFO buffer to RF card; WriteE2 command puts slave buffer The data obtained by the device is written to the EEPROM; ReadE2 places the data read from the EEPROM into the FIFO buffer; LoadConfig reads the data from the EEPROM and initializes the register; CalcCRC activates the coprocessor.

All registers are initialized before execution, and then the reader cyclically sends the Inventory Request command at a certain time interval to monitor whether there is a radio frequency card within the reading distance. If so, the RF card responds to the Inventory Request command and sends the card number and CRC check value to the reader. If there is a communication error or no collision, use Transmit and WriteE2 to send it to the microprocessor through the data bus, and then send the Stay quiet command to make the RF card just enter a quiet state. If there is a collision, the anti-collision routine is called, and the search area is narrowed down with a binary search algorithm until one RF response remains.

Epilogue

This system can realize 1.5 meters of personnel identification, and transmit the radio frequency card number and location information to the data center station through the data bus, and use this system to grasp the location of underground employees in time. But the recognition distance of this system is relatively short, how to improve the recognition distance of the reader is the focus of future work.

Edit: jq

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