Center topic:
System composition and working principle
Hardware circuit design
System software design
System test
solution:
Interrogation unit design and signal post processing unit design
System software design
The research of vehicle identification system started in the 1970s, the earliest use of optical and infrared technologies, its environmental adaptability is poor, and the maximum vehicle speed limit of the vehicle is 70km / h, which has gradually been eliminated.
SAW technology is an emerging identification technology. It uses the encoded SAW sensor as an ID tag, without any connection, has advantages such as low energy, strong anti-interference ability, small size, and is particularly suitable for non-contact measurements of high-speed motion vehicles in strong electromagnetic interference environments.
In this paper, a vehicle automatic identification system based on SAW technology can be applied to non-parking vehicle identification, providing an efficient solution for highway vehicle charging systems, vehicle identification distance 10m, maximum speed 120km / h. The identification code length is 32 bits, and the number of vehicles that can be identified is 232. The system is working all day in the field environment.
System composition and working principle
The SAW sensor consists of a SAW sensor label loaded on a high-speed motion vehicle, an interrogation unit with an active antenna, and a signal post processing unit. As shown in Figure 1.
The SAW tag consists of a sensor antenna, a piezoelectric substrate, a fork finger transducer, and a reflective pole encoded by the sensor.
The sensor antenna receives the inquiry electromagnetic pulse signal transmitted by the distant interrogator. The conversion of the fork reflector is converted into a sound surface wave. After the reflection strip is formed, the echo is formed, and the echo is re-converted into electromagnetic waves through the fork reflection. Once again, it is emitted through the antenna. These echo signals form a pulse sequence determined by the number and position of the reflection strip of the crystal surface, which is similar to a barcode pattern, and the time delay of each pulse depends on the SAW propagation speed. The signal post processing unit estimates the pulse delay variation, and the identification code is demodulated in real time.
Hardware circuit design
a. inquiry unit design
Figure 2 is a block diagram of an inquiry unit. It consists of a transmitter, a receiver, a local oscillation module, and the like.
Local Oscillation Module: Three frequencies are required in this system, namely 750 MHz, 330 MHz and 165MHz, which are generated by frequency integrated. The frequency synthesizer adopts AD ADF4127L, which is a current type of dual-frequency compact synthesizer that provides both radio frequency / intermediate frequency, using SPI interface control, configuring its internal register via the FPGA chip.
The output frequency produced by ADF4127L is locked by VCO at 330 MHz and 750 MHz. The 330MHz VCO uses MAX2608, which produces a frequency of 300MHz to 500 MHz; 750MHz VCO uses Mini's POS-1050 type, frequency range of 685MHz to 1025 MHz.
The 750 MHz signal is provided to the transmitter of the upper frequency conversion and the lower end of the receiver front end of the transmitter. The 330MHz signal is provided to the receiver to the receiver, and another 2-point frequency generates 165 MHz frequency. The power distributor selects the LRPS-2-4J of MINI with a very low voltage stationer ratio. It is used to use Microwave's low noise diogram HMC361S8G from MicroWave.
Transmitter: FPGA generates 40ns time window to intercept 165MHz carrier, exhibit a spectral spectrum, generate a center frequency of 165 MHz, a pulse signal having a bandwidth of 50 MHz, and sent to the SAW expander to form a linear frequency range pulse. The SAW expander adopts the surface-angle delay delay line produced by Nanjing 54, with a bandwidth of 20 MHz. After the extended pulse is only 20MHz bandwidth, the expander is attenuated 30dB. After this signal is enlarged by the intermediate frequency amplifier AD8353, the upper variable frequency is performed to 915 MHz, and the extracorporeal clutter is filtered through the RF band pass filter, and then the power preamplifier is placed by the MAX2235, and the query signal is formed. The upper variable frequency chip uses MAX2671, and its RF output ranges from 400 MHz to 2500MHz. The radio frequency bandpass filter uses the F5CE-D2 series SAW bandpass filter, which uses special DMS dual-mode SAW technology, with excellent extracted attenuation and communication flatness, widely used in mobile communications.
After the query signal is completed, the antenna switch immediately hits the receiving location and prepares for reception. The conversion switch uses MINI's KSWHA-1-20, which is a GSAS switch with high isolation and switch conversion speed 3NS.
Receiver: The design scheme of an ultra-equipped receiver is used, as shown in Figure 3.
When the echo signal reaches the front end of the receiver, the filter is first filtered. There are two radio frequency filters in the receiver: pre-select filter and mirror suppression filter. Pre-selecting filter filtrates unwanted signals to prevent saturation of the RF front end and generate intermodulation distortion. Mirror suppression filter inhibits the first mirror, semi-central and intrusive responses. The RF filter selects FAR-F5CE-915M00-D236. After the filter is processed, the signal is enlarged by the radio frequency amplifier MAX2642, which is a low noise amplifier, providing 0 dBm input IP3 under the noise coefficient of 1.3 dB.
After radio frequency filtering and amplification, the signal is converted with the vibration 750 MHz. The lower variable frequency chip uses MAX2682, which uses a double balance mixer structure with an input frequency 400MHz to 2500MHz. After the change is down, the signal enters the intermediate frequency SAW filter for intermediate frequency filtering, and then outputs a compressed pulse by the SAW compressor. The compressed signal is substantially amplified in the AD8353, and the orthogonal discharge is sent, and the I, Q two signals are demodulated. RF2721 is an orthogonal demodulator designed by RF for digital mobile receivers, which recovers the same phase and orthogonal baseband signal from the amplified and filtered intermediate frequency signal.
b. Signal post processing unit design
The principle block diagram of the signal post processing unit is shown in Figure 4.
From the interrogation unit, the orthogonal component q sent, filtered off the high frequency component by the low-pass filter, and then converted to the digital quantity through the 8-bit mode / digital converter AD9058, passed by the FIFO. , Send to the DSP for operation. FIFO selection HBA's FQV251, DSP selection TI's floating-point processor TMS320C33. I, Q signal is a demodulated signal sent by the receiver, which carries the identification information of the identification card, and the format of the identification code is 32 bits. When the clock is synchronized, the associated processing algorithm using soft decisions is used according to the square of the two signals and whether the current bit is 0 or 1.
The FPGA is responsible for the generation of various control signals, selecting the EPM7032 of Altera's MAX7000 Series, 5000 available doors. EPROM selection ST company 27C256.
System software design
Software flow is shown in Figure 5. After the system starts, the DSP initializes, the FPGA configuration frequency synthesizer ADF4127L generates a precise frequency source. The interrogator conversion switch indicates the transmitting position, the transmitting unit transmits the inquiry pulse signal. After a period of time, the switch refers to the receiving position, the receiving unit receives the echo signal, first performs A / D sample, and puts the number I and Q road sampling value square When it is added, if it is greater than the coding threshold, it is determined that the current code is 1, otherwise 0. After the 32-bit identification code is determined, the interrogation unit re-issues the interrogation signal, which is cycled. The destructed identification code is given to the external device via the serial port for processing.
System test
This system has been tested many times in the highway toll station and parking lot. The test results show that the system will work all day, and high-speed sports vehicles can be accurately identified within 10m distances, speeding up the speed of the vehicle through the toll stations, to a certain extent, the cost of transportation is reduced. , Reading the full text, the technology area
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