Design Principle of Vehicle Detection System in Intelligent Parking Lot

Figure 1 Schematic diagram of parking management systemThe controller design block diagram is shown in Figure 2. The system selects SPCE061A single chip microcomputer as the main control chip. The single chip microcomputer detects the arrival of the vehicle through the level change of card reading signal and phase-locked loop circuit. The DS1302 clock circuit provides the system with accurate time information, and displays the parking space and time information in real time by driving the LED display board. The system has a serial communication interface with the upper computer.Figure 2 overall design block diagram of vehicle detection system in intelligent parking lot

3. Introduction to system principle     3.1 vehicle detection     Accurate vehicle detection is the prerequisite for the normal operation of the system. Through the comparison of various schemes, the vehicle detector of this design adopts the ground induction coil detection scheme. Ground induction coil vehicle detector is a vehicle detector based on the principle of electromagnetic induction. The ground induction coil ll is buried under the road surface and a ring coil with a certain working current is wound by multi turn conductors and buried in the road. The coupling circuit composed of ground induction coil is shown in Figure 3:

Fig. 3 coupled oscillation circuit

T is the isolation transformer, the turns ratio is 1:1, the triode UL and U2 form a common emitter oscillator, and the resistance R3 is the common emitter resistance of the two triodes and forms positive feedback. As an inductive element in the detector resonance circuit, the ground induction coil forms L C resonance together with the oscillation circuit of the vehicle detector. When a vehicle passes through, the magnetic flux generated by the unit current in the coil will increase, resulting in a slight change in the coil inductance value, and then change the frequency of LC resonance. This change in frequency is used as the input signal of the vehicle passing through the ground induction coil. In order to detect this change, the common method is to calculate the number of oscillation pulses per unit time by single chip microcomputer to determine whether the car is coming. In this design, it is necessary to detect the frequency change of two ground induction coils. If the single chip microcomputer is used to measure the frequency change of two signals at the same time, the system is relatively large and the program is complex, which makes the single chip microcomputer bear a heavy burden. A new detection method is introduced here: the PLL audio decoder LM567 is used to detect the change of frequency. The application circuit diagram is shown in Figure 4:

Fig. 4 PLL circuitThe external resistance and capacitance of pins 5 and 6 of LM567 determine the center frequency of VCO in IC, fo = 1 / 1.1rc. Pin 1 and pin 2 are usually connected to the ground to form an output filter network and a loop low-pass filter network. The capacitance connected to pin 2 determines the capture bandwidth of the PLL circuit. The theoretical value of the bandwidth can be calculated by this formula:

When the audio decoder LM567 works, if the input signal frequency falls within the given passband, the PLL will lock the signal. At the same time, the internal transistor of LM567 is controlled on, and pin 8 outputs low level, otherwise it outputs high level. When the input signal frequency is within the passband, the LM567 locks and outputs a low level. Generally, when there is no vehicle, the oscillation frequency of the coupling circuit will remain unchanged within a certain range. When the vehicle passes through the ground induction coil, the oscillation frequency of the coupling circuit will change. Moreover, with the different vehicle models and the uneven iron quality of the vehicle itself, the change of this frequency will also float within a certain range. Therefore, through the experiment, select the appropriate LM567 capture bandwidth value, so that when there is no vehicle, although the input signal frequency changes slightly, the floating frequency is within the passband, the LM567 is locked, and the 8-pin output low level; When a car arrives, the frequency changes dramatically and is no longer in the passband, and pin 8 will output high level. At this time, the detection of whether the vehicle is coming is transformed into the detection of the level. The arrival of the vehicle can be sensed by triggering the external interrupt of the single chip microcomputer, and there is no need to distinguish whether the frequency change at this time is caused by the arrival of the vehicle through a complex program, which greatly reduces the difficulty of programming.      3.2 other control parts     Controlling the rise and fall of the brake rod machine is to control the forward and reverse of the DC motor of the brake rod machine. The high and low levels are output through the single chip microcomputer control port to cooperate with the relay. The forward and reverse can be realized by adding positive and reverse voltages at both ends of the DC motor. For time recording, DS1302 calendar chip is selected here. DS1302 can record the year, month, day, week, hour, minute and second. It can be connected to the backup power supply and can always work continuously when the main power supply is turned off. The single chip microcomputer SPCE061A can read the current time at any time. The communication with the host computer uses the universal asynchronous serial communication module (UART) of SPCE061A, which provides a full duplex standard interface. With the help of the special function of iob port and UART IRQ interrupt, it realizes the com link with the RS-232 serial communication interface configured by the host computer, and realizes the timely transmission of data. In addition, the voice prompt part makes use of the voice processing advantages of SPCE061A. Using SPCE061A voice compression algorithm library and built-in DAC, clear voice broadcasting function can be realized without adding voice chip.     4. Main software design of the system     The main program of the system is mainly responsible for system initialization (including the initialization of each IO port, interrupt initialization and DS1302 initialization), and the time display of LED display board. Using the 0.5s time base interrupt of SPCE061A, read the time information of DS1302 every 0.5s and refresh the LED display. The detection of card reading information and vehicle information of ground induction coil adopts the form of external interrupt trigger, and various automatic controls are realized in the interrupt service program. The program flow of the main interrupt service program is shown in figures 5 and 6:

Fig. 5 flow chart of card reading interrupt service program Fig. 6 ground induction coil interrupt service program

This design uses SPCE061A to realize the automatic detection of vehicles in and out of the parking lot, the automatic take-off and landing of the brake rod machine and the real-time display of parking space information. With the IC card information processing and image processing system, the automatic management of the intelligent parking lot can be realized. The application of ground induction coil makes the vehicle detection accurate and ensures the reliability of the system.     reference:     Luo Feiya, Lingyang 16 bit MCU application foundation, Beijing: Beijing University of Aeronautics and Astronautics Press, 2005     Zhao Hailan. Principle of DS1302 real time display time. Electronic technology, 2004, 1:43-46     Feng Hongmei. Design and implementation of ring coil vehicle detector_ Journal of Qingdao University of science and technology 2oo5, 26 (1): 448-451     About the author: sun Congjun, female, born in 1984, is a graduate student of School of electrical engineering, Southwest Jiaotong University. Her main research direction is automatic control and automation. Zhang XiMao, Duan Guoyan, School of electrical engineering, Southwest Jiaotong University, Sichuan Engineering Vocational and Technical College