Design and implementation of intelligent parking lot vehicle detection system

1. Introduction

In modern large parking lots, the intelligent parking management system makes vehicle entry and exit procedures simple and safe, and realizes the functions of automatic detection, billing, statistics, and display of vehicles, which greatly saves human resources and improves work efficiency. In the entire parking management system, the vehicle detection part is the key to the system. This design introduces a parking lot vehicle detection system, which realizes the control of the gate machine and parking space prompts through vehicle detectors distributed at the entrance and exit.

2. System Overview

The schematic diagram of the entire parking management system is shown in Figure 1. The information display board is an LED display screen that displays the current time and parking space information. When a car enters, the driver swipes the card, and the card swipe signal is read by the controller, which controls the gate machine to rise, and a voice prompt "Welcome" is given. When the sensor coil detects that the vehicle has entered, the parking space information is updated, the vehicle image is captured, and the gate machine falls. Similarly, when the vehicle leaves, the driver swipes the card, controls the gate machine to rise, and a voice prompt "Thank you for coming" is given. When the sensor coil detects that the vehicle has left, the vehicle information is captured, the gate machine falls, and the parking space information is updated. The processing of the vehicle's image information and IC card data information will be completed by the host computer in the duty room.

The controller design block diagram is shown in Figure 2. The system uses the SPCE061A microcontroller as the main control chip. The microcontroller detects the arrival of the vehicle through the card reading signal and the level change of the phase-locked loop circuit. The DS1302 clock circuit provides accurate time information for the system, 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 host computer.

3. Introduction to system principles

3.1 Vehicle detection part

Accurately detecting vehicles is a prerequisite for the normal operation of the system. After comparing various schemes, the vehicle detector of this design adopts the ground induction coil detection scheme. The 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. It is a ring coil with a certain working current, wound by multiple turns of wire, and buried in the road. The coupling circuit composed of the ground induction coil is shown in Figure 3:

T is an isolation transformer with a turns ratio of 1:1. Transistors Ul and U2 form a common emitter oscillator. Resistor R3 is the common emitter resistor of the two transistors and forms positive feedback. The ground induction coil is an inductor element in the detector resonant circuit, and together with the oscillation circuit of the vehicle detector, it forms an LC resonance. When a vehicle passes by, 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 changing the frequency of the LC resonance. This frequency change is used as the input signal of a car passing through the ground induction coil. In order to detect this change, a common method is to use a single-chip microcomputer to calculate the number of oscillation pulses per unit time to determine whether the car has arrived. In this design, it is necessary to detect the frequency changes of the two ground induction coils. If a single-chip microcomputer is used to measure the frequency changes of the two signals at the same time, the system is relatively large and the program is relatively complicated, which makes the single-chip microcomputer burdened. Here is a new detection method: using the phase-locked loop audio decoder LM567 to detect frequency changes. The application circuit diagram is shown in Figure 4:

Figure 4 Phase-locked loop circuit

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The resistors and capacitors connected to the 5th and 6th pins of LM567 determine the center frequency of the voltage-controlled oscillator inside the IC, fo=1/1.1RC. The 1st and 2nd pins are usually connected to capacitors to the ground respectively, forming an output filter network and a loop low-pass filter network. The capacitor connected to the 2nd pin determines the capture bandwidth of the phase-locked loop circuit. The theoretical value of the bandwidth can be calculated using this formula:

When the audio decoder LM567 is working, if the input signal frequency falls within the given passband, the phase-locked loop will lock the signal, and the internal transistor of the LM567 will be controlled to conduct, and the 8-pin outputs a low level, otherwise it outputs a high level. When the input signal frequency is within the passband, the LM567 is locked and outputs a low level. Usually, when there is no car, the oscillation frequency of the coupling circuit will remain unchanged within a certain range. When the car passes through the ground sensor coil, the oscillation frequency of the coupling circuit will change, and with the different models and the uneven iron quality of the car itself, the change of this frequency will also float within a certain range. Therefore, through experiments, the appropriate LM567 capture bandwidth value is selected, so that when there is no car, although the input signal frequency has a slight change, this floating frequency is within the passband, the LM567 is locked, and the 8-pin outputs a low level; when a car arrives, the frequency changes dramatically and is no longer within the passband, and the 8-pin will output a high level. At this time, the detection of whether the vehicle arrives is converted into the detection of the level. The arrival of the vehicle can be sensed by triggering the external interrupt of the microcontroller, without the need to use complex programs to distinguish whether the frequency change at this time is caused by the arrival of the vehicle, which greatly reduces the difficulty of programming.

3.2 Other control parts

Controlling the rise and fall of the gate lever is to control the forward and reverse rotation of the DC motor of the gate lever. The high and low levels are output through the single-chip control port to cooperate with the relay. The positive and negative voltages at both ends of the DC motor can realize the forward and reverse rotation. For the recording of time, the 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 it can always keep working continuously when the main power 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 functions of the IOB port and the UART IRQ interrupt, it realizes the connection with the RS-232 serial communication interface COM configured by the host computer, realizing the timely transmission of data. In addition, the voice prompt part uses the voice processing advantage of SPCE061A. Using the voice compression algorithm library and built-in DAC of SPCE061A, a clear voice broadcast function can be realized without the need for an external voice chip.

4. System main software design

The main program of the system is mainly responsible for system initialization (including initialization of each IO port, interrupt initialization and initialization of DS1302, etc.), and time display of LED display board. It uses 0.5s time base interrupt of SPCE061A to 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 sensor coil adopts external interrupt triggering form, and various automatic controls are realized in interrupt service program. The program flow of main interrupt service program is shown in Figures 5 and 6:

Figure 5 Card reading interrupt service program flow chart

Figure 6 Ground sensor coil interrupt service program

5. Conclusion

This design uses SPCE061A to realize the automatic detection of vehicles entering and leaving the parking lot, the automatic rise and fall of the gate 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 the ground sensor coil makes the detection of vehicles accurate and ensures the reliability of the system.