Design of a Small RFID System

　　Nowadays, various intelligent control systems cannot do without the transmission of data information. Among them, wireless data transmission is a new transmission method that is different from traditional wired transmission. The system does not require transmission cables and is low-cost. By matching the corresponding wireless communication interface circuit for the single-chip microcomputer, wireless data transmission between single-chip microcomputers or between single-chip microcomputers and microcomputers can be realized. The commonly used wireless communication interface circuit is a circuit with a wireless transceiver chip as the core. When data is transmitted, necessary anti-interference measures and identification measures are taken in software design to effectively avoid interference and achieve satisfactory communication effects. In this paper, based on the 89c2051 single-chip microcomputer, wireless communication is carried out to identify contactless wireless identification devices. Its application can be embedded in systems such as power management or gas charging, and can also be used as an independent card reader to operate IC cards. It can be applied to different industries with different software.

　　1 How the system works

　　This design uses a single-chip microcomputer as the core of the reader and transponder, and these two parts mainly use LM567. This system is a small wireless identification device with a maximum operating distance of 70mm. The internal structure of the system is divided into a radio frequency area and an interface area: the radio frequency area contains a modem and a power supply circuit, which is directly connected to the antenna; the interface area has a port connected to the single-chip microcomputer, and also has a transceiver/transmitter connected to the radio frequency area, an E2PROM that can store 3 sets of register initialization files with the single-chip microcomputer program, and a collision prevention module and control unit that performs 3 data verification error prevention mechanisms and collision prevention processing. This is the core module for the reader to achieve wireless communication with the transponder, and it is also the key to the design.

　　2 Hardware Circuit Design

　　The wireless identification system device consists of three parts: reader, transponder and coupling coil (i.e. antenna).

　　2.1 Reader Design

　　The basic circuit of the reader is shown in Figure 1. When a transponder is close to the reader, the antenna in the reader forms an LC parallel resonant circuit, whose frequency is the same as the transmitting frequency of the transponder. In this way, under the excitation of electromagnetic waves, the LC resonant circuit resonates, so that the high-frequency signal flows into the input end of the LM567 in the reader for demodulation. The output demodulated signal is the opposite of the above-mentioned microcontroller encoding signal, and finally decoded by the microcontroller and output for display.

　　LM567 has dual functions of modulation and demodulation. The modulated signal can be directly recognized by the single chip microcomputer, and sent to the digital tube for display.

Figure 1 Reader

　　2.2 Transponder Design

　　When the transponder is working, the data is encoded by the 89c2051 microcontroller and then sent to the LM567 to be modulated onto a high-frequency carrier. The antenna coil connected to its output end continuously emits a set of electromagnetic waves of a fixed frequency (145kHz). When a transponder approaches the reader, the reader recognizes and displays it. The transponder hardware circuit is shown in Figure 2.

Figure 2 Transponder

　　2.3 Design of coupling coil (i.e. antenna)

　　Antenna is a transducer. When transmitting, it converts the high-frequency current of the transmitter into electromagnetic waves in space; when receiving, it converts the electromagnetic waves intercepted from space into high-frequency current and sends them to the receiver. Antenna design is an important part of designing a low-power, short-range wireless transceiver for radio frequency identification system. A good antenna system can achieve the best communication distance. There are many types of antennas, and different applications require different antennas. In low-power, short-range RFID systems, a reliable and low-cost antenna system is required, and coupled coil ring antenna is a more commonly used one.

　　2.3.1 Analysis of the equivalent circuit of the loop antenna

　　The voltage and current at the excitation point of the loop antenna are related by the input impedance of the loop, that is, V = ZI0. In order to evaluate the capacitance Z'in for antenna resonance, the input impedance of the loop antenna must be determined; similarly, in order to evaluate the antenna efficiency and radiation impedance, the ohmic losses in the loop conductor and other ohmic losses must also be determined.

　　2.3.2 Antenna Design Parameters

　　The input impedance Zin of the loop antenna can be given by the following formula:

　　Wherein, RR is the radiation resistance; RL is the loop conductor loss resistance; RX is the additional ohmic loss resistance; LA is the loop antenna inductance; L1 is the loop conductor inductance.

　　The loss resistance of the ring conductor is:

　　Where l is the length of the metal ring conductor, p is the circumference of the cross section of the ring conductor, RS is the surface resistance of the conductor, u0 is 4π×10-7H/m; σ is the conductivity of the conductor; the unit of RL is Ω. The additional ohmic loss resistance mainly comes from the equivalent series resistance on the capacitor CP:

　　2.4 Equivalent circuit of reader with antenna

　　The conductor loop required to generate the alternating magnetic field is represented by the coil L1, and the series resistor R1 is equivalent to the ohmic loss of the winding resistor in the conductor loop L1. In order to obtain the maximum current in the conductor loop L1 when the operating frequency of the reader is fTX, thereby generating the maximum magnetic field strength H, a series resonant circuit with a resonant frequency fRES=fTX is formed through the capacitor C1 in series.

　　In Figure 4, the transmitter output of the reader generates a high-frequency voltage u2, and the receiver is directly connected to the antenna coil L1. The total impedance Z1 of the series resonant circuit is the sum of the individual impedances, that is:

　　2.5 Antenna connection matching study

　　Depending on the frequency range used by the reader, different methods are used to connect the antenna coil to the output of the reader transmitter. The antenna coil is connected directly to the power output stage through power matching, or fed to the antenna coil via a coaxial cable. The antenna coil L1 presents an impedance ZL in the operating frequency range of the RFID system. In order to achieve power matching with a 50Ω system, this impedance must be converted to 50Ω through a passive matching circuit, and then this power can be transmitted from the reader end stage to the matching circuit via a coaxial cable with almost no loss and no radiation. [page]

　　3 Debugging and testing

　　3.1 Debugging Methods

　　When debugging the circuit, the oscillation frequency can vary within 0101Hz~500kHz. The oscillation frequency must be adjusted to achieve the best match with the carrier frequency of LM567, otherwise it will affect the demodulation of the subsequent circuit (for example, when the oscillation frequency is too close to the carrier frequency of LM567, it will directly affect the demodulation of the subsequent frequency, etc.), and the oscillation frequency of the microcontroller will also affect the output waveform. If the output is not a rectangular wave, it may affect the microcontroller's recognition of the signal, making the reaction time longer or compiling errors, so it is very important to debug the circuit. LM567 must be debugged to smoothly achieve mutual modulation and demodulation. If the transmitter is working and the receiver cannot decode correctly, the resistance value of the timing resistor R8 should be adjusted to meet the requirements. Since the oscillation frequency (center frequency) of LM567 requires high accuracy, when adjusting the resistance value of R8, the R8 resistor should be replaced with a 10kΩ multi-turn precision wirewound resistor. This resistor changes its resistance value by tens of ohms every time it rotates one circle, and the accuracy is high. If there is no such resistor, it can also be replaced by an ordinary fine-tuning resistor, but it should be adjusted carefully. If conditions permit, you can directly connect the frequency meter between the 5th pin of the transmitter's LM567 and the ground, measure its oscillation center frequency, write down the value, and then measure the frequency of the 5th pin of the receiver's LM567. If the center frequency of the receiver (referring to LM567) is different from the center frequency of the transmitter's LM567, adjust the resistance value of R8 to make the center frequencies of the two audio decoders equal.

　　3.2 Test Data

　　When testing the data, we tested all 8-bit codes from 00 to FF (4-bit is definitely fine), calculated and measured the power supply power many times, and tested the data transmission within the range of 1 to 6 cm for the distance of the coupling coil, which can ensure stable data transmission within 5 cm. The test data list is as follows.

　　Table 1 Test data list

　　Test data list

　　According to the above experimental measurement results, it can be seen that this system realizes all the basic functions and most of the functions required by the question, and has its own unique functions, and its performance is reliable and stable.

　　3.3 Test Results Analysis

　　3.3.3 System waveform data test

　　A top-down debugging method is adopted, that is, each module is debugged separately, then connected into a complete system, and then the overall debugging is performed. The comparison of data sending and receiving signals is shown in Figure 3.

Figure 3 Comparison of data transmission and reception signals

　　3.3.2 Possible errors caused by the system itself

　　(1) External interference factors

　　There is a very prominent drawback of using wireless transmission, that is, it is susceptible to electromagnetic interference and poor transmission effect. Due to external high-frequency signals, many devices such as metal may interfere with electromagnetic waves, so the circuit is bound to produce errors.

　　(2) Distance interference factor

　　When the electromagnetic field propagates in a conductive medium, the amplitude of its field quantity E and H decays exponentially with the increase of distance. From the energy point of view, electromagnetic waves decay very quickly in a good conductor, decaying from the surface of the conductor to 1/e of the surface (about 3618%), that is, the reason why the distance is specified as 5cm in the article is because of this. The garbled and unrecognizable code generated by the distance factor is also one of the reasons for the error.

　　4. Program flow chart

　　The software programming uses the microcontroller assembly language, and the editing software is Keil51. The program flow chart is shown in Figure 4.

Figure 4 Program flow chart

　　5 Conclusion

　　The reader of this system can identify the presence of a transponder within a range of 6cm. If there is a transponder within the monitoring range, it will give a clear indication and read the preset 4-bit code of the transponder, and then display it; in addition, the transponder part can also send a coded signal within the reader's recognition range by setting a 4-bit code through a switch. At the same time, the reader also has the function of writing the transponder's code, which is received and stored by the transponder. The focus of the design is to ensure the accuracy, and the reliability of wireless transmission can be improved by adding a check code in the software.