Design of wireless identification system based on radio frequency technology

    Radio frequency identification is a contactless automatic identification technology that automatically identifies the target object and obtains relevant data through radio frequency signals. Radio frequency identification does not require human intervention, is contactless, has a fast reading speed, is wear-free, is not affected by the environment, has a long life, and is easy to use. At present, radio frequency identification technology has developed very rapidly abroad, with a wide variety of products. It has been widely used in many fields such as industrial automation, commercial automation, transportation control and management, such as automobile, train and other traffic monitoring; automatic toll collection system for highways; parking management system; item management; warehouse management: vehicle anti-theft, etc. Due to the late start of radio frequency identification technology in my country, in addition to being used in the automatic identification system of vehicle numbers for China Railway, it is limited to the application of radio frequency bus cards. This article gives a way to realize a simple radio frequency identification system. The reader and transponder are both included in the single-chip control system. Using the ASK modulation and demodulation circuit and the matching network circuit, the effective identification distance of the entire system is about 8.3 cm, which has certain use value.

　　1 Overall design

　　The radio frequency identification (RFID) system is composed of a transponder, a reader, and application support software. The transponder is powered by a DC power supply, and is mainly composed of an encoding circuit, a carrier oscillation circuit, a modulation circuit, and a transmitting circuit. The principle is shown in Figure 1. This solution is simple and easy to implement, and the circuit is simple. However, this transponder must be powered by a power supply, otherwise the circuit will not work.

　　

　　Considering the transponder as an active transponder, in the reader design part, the received weak voltage signal is amplified, and the useful signal is taken out by the demodulation circuit, and then the decoding chip is used after passing through the discrimination circuit, and finally the display control circuit is used to display the data received by the reader, and its principle is shown in Figure 2. As shown in the figure, the circuit design of this scheme is simple, easy to implement in hardware, and has good feasibility.

　　

　　2 Theoretical analysis and calculation of circuits

　　2.1 Matching theory of coupling coil

　　As a transmitting device of electromagnetic energy, the coupling coil must be matched. The coupling coil shows impedance ZL in the operating frequency range of the wireless identification system. In order to achieve power matching with the system, impedance conversion must be achieved through a passive matching circuit so that power can be transmitted to the coupling coil without reflection. A small number of components can be used to achieve a matching matching circuit. In real applications, there are many different 13.56MHz wireless identification systems that use matching circuits as shown in Figure 3.

　　

　　This design uses the matching circuit to achieve impedance matching. To determine the parameters of the matching circuit, it is necessary to measure the inductance LS of the coil and the ohmic resistance RLS of the wire.

　　2.2 Analysis of the Transmitter’s Transmitter Circuit

　　In the transmitter part of the transponder, a signal of the required operating frequency is first generated by a frequency-stable quartz crystal oscillator. The oscillator signal is fed to a modulation stage controlled by a signal-encoded baseband signal. This baseband signal is a keyed constant voltage signal, where the binary data is represented in the form of a serial code. Depending on the type of modulator, ASK or FSK modulation of the oscillator signal is performed. At this point, the baseband signal is fed directly to the frequency synthesizer, which then uses power amplification to bring the modulated signal to the required level, and then the modulated amplified signal output is coupled to the primary coil.

　　2.3 Reader receiving circuit analysis

　　The reader receiving circuit consists of a coupling coil, an amplifier, a demodulator, a decoder and a display. The voltage signal obtained by the coupling coil is amplified by the amplifier, demodulated by the demodulator to obtain a carrier signal, and then decoded by the decoder and the display circuit to obtain the data sent by the transponder. [page]

    3. Design and calculation of programs and circuits

　　3.1 Design and calculation of reader circuit

　　The reader circuit designed this time consists of a coupling coil, an amplifier circuit, a demodulation circuit, a decoding circuit and a single-chip display circuit. The coupling coil and amplifier circuit design is shown in Figure 4. In order to increase the coupling efficiency of the reader coil, an adjustable capacitor can be connected in parallel with the coil to make its resonant frequency consistent with the operating frequency of the transponder, so that the reader coil works in a resonant state. The resonant frequency of the parallel resonant circuit can be calculated by formula (1):

　　

　　Where L is the self-inductance of the coil, which is tested to be 12.60 μH. f is the operating frequency of the transponder, which is 13.56 MHz. Since the voltage of the reader coil with a parallel capacitor increases significantly when it is excited at the resonant frequency of 13.56 MHz, the operating frequency of the transponder is selected to be 13.56 MHz, and C is theoretically calculated to be 10.9 pF.

　　

　　Figure 4 is a high-frequency small signal amplifier. The typical fT value of S8050 is 200MHz, so the current amplification factor is approximately:

　　

　　The digital recovery circuit uses LM393, as shown in Figure 5. Its function is to restore the demodulated output analog signal to a digital signal for the decoder to recognize. The decoding and single-chip display control circuit is shown in Figure 6.

　　

　　3.2 Transponder Circuit Design Calculation

　　The encoder design consists of a dip switch and an encoder chip VD5026, and the information is generated by the dip switch. The circuit diagram is shown in Figure 7.

　　

　　The carrier oscillator uses a ring oscillator composed of 74HC14, which has low power consumption and low minimum operating voltage, and is suitable for 3V battery power supply. A 13.56MHz crystal filter is connected to the oscillator feedback, and the carrier frequency is highly stable.

　　The modulator is composed of high-speed CMOS device 74HC00, which realizes ASK modulation. The modulation waveform is shown in Figure 8. The modulated output signal is directly sent to the resonant circuit after being inverted.

　　

　　3.3 Programming

　　When VD5027 decodes correctly, pin 17 outputs a high level, and 4-bit data is output from pins 10, 11, 12, and 13. Therefore, the MCU is set to interrupt mode, and pin 17 of VD5027 is connected to the interrupt 0 entry of the MCU after being reversed. The main program is in a dormant waiting state. When there is a transponder and the decoding is correct, it responds to the interrupt service subroutine and displays the corresponding information. The process is shown in Figure 9. [page]

　　

　　3.4 Overall circuit diagram design

　　According to the previous analysis and design, after correct installation and debugging on the breadboard, the printed circuit board was soldered and the test results were correct.

　　4. Workflow diagram of identification device

　　The working process of the identification device is shown in Figure 10. This wireless identification device is manually input with information, encoded by the encoder, and uses ASK modulation with a carrier of 13.56MHz, which is sent through coil coupling. The reader amplifies the received ASK signal, detects it through the diode envelope, sends it to the digital recovery circuit, and then decodes it. When the decoding is correct, the result is displayed by the single-chip microcomputer.

　　

5 Test plan and test results

　　5.1 Inductance Coil Measurement

　　Measuring equipment: QBG-1A high frequency Q meter.

　　Test results: The number of turns of the inductor coil is N = 10 turns, and the diameter of the inductor coil is D = 6.9 cm.

　　Reader inductance coil Ll: 12.73μH, transponder inductance coil Ll: 12.60μH,

　　Analysis: The two inductor coils have the same number of turns and diameter, but different inductances. This is mainly because the inductors are wound by hand, resulting in different tightness and gaps.

　　5.2 Encoder VD5026 Measurement

　　Test equipment: digital oscilloscope DS5062M.

　　Test results: The output is a square wave signal with a good waveform.

　　Oscillation frequency f=22.872kHz, amplitude Vpp=3.00V, Vmax=2.24V, Vmin=0.00V.

　　5.3 Carrier Oscillator Measurement

　　Test equipment: digital oscilloscope DS5062M.

　　Test results: Frequency: 13.56MHz Amplitude Vpp = 1.40V

　　Waveform distortion analysis: The oscillator is a ring oscillator composed of a NOT gate with a gate delay time. LC devices can store energy, so the LC oscillator waveform is better.

　　5.4 Modulation output waveform measurement

　　Test equipment: digital oscilloscope DS5062M.

　　Vpp=4.12V, Vmax=2.24V, Vmin=-1.88V.

　　

　　5.5 Identification Measurement

　　5.5.1 Bit Error Measurement

　　Test method: Set 0000-1111 through the DIP switch on the transponder, and use 4 LEDs and a digital tube to display 0-F on the reader.

　　Test result: The transponder dials 0000-1111, the reader displays 0000-llll accordingly, and the digital tube displays 0~F. The test result is correct, and the bit error rate is 0.

　　5.5.2 Transmission Delay Measurement

　　Testing tool: mobile phone stopwatch

　　Test method: The time interval from the time the transponder information changes to the time it is displayed on the reader.

　　Test results: 0.79s＜1s.

　　5.5.3 Identification distance measurement

　　Test tools: ruler.

　　Test method: Change the distance between the inductive coil of the transponder and the reader, and observe whether the information displayed by the reader is the same as that of the transponder.

　　Test results: stable transmission distance 5.5cm, maximum recognition distance 8.3cm.

　　5.6 Power consumption measurement

　　Test tool: Tianyu TY360 multimeter.

　　Test method: Use a multimeter to measure voltage U and current I, then power consumption P=UI.

　　Test results: a. Reader: U=5V, I=50 mA, P=UI=5x50=250mW. b. Transponder: U=3V, I=8.5mA, P=UI=3x8.5=25.5mW.

　　The above test data show that the design is feasible.