ARM Intelligent Traffic Light System

At present, domestic traffic lights generally use fixed-cycle program control technology, that is, the on and off time of traffic lights is mainly determined by experience and previous statistical data. To realize the intelligence of road traffic, it is necessary to introduce variable-cycle traffic light control technology, detect the traffic flow and congestion data at the intersection in real time, and dynamically adjust the traffic lights according to the rules to obtain a more satisfactory traffic rate.

This paper proposes a design of a traffic light control system with distributed characteristics. It uses RFID technology to improve the accuracy of collecting road condition information, uses the current loop long-distance transmission method, and applies artificial intelligence theory to make the system more adaptable and scalable.

1 System Function and Overall Structure Design

In terms of functional features, the intelligent traffic light system should have signal control modes such as fixed time mode, time setting mode, time sensing mode, on-site remote control mode, and remote control mode; it can set date, time, time period, sensing parameters, cycle time, phase difference, green signal ratio and other parameters; it can perform system self-check, green conflict detection, light fault detection, and line fault detection; it has powerful input/output functions, and can realize different phase light control output and detection functions for intersections. In addition, the system provides a friendly human-machine interface, and users can set and control the signal machine through manual switches, keyboards or remote controllers.

The signal machine is the core of the whole system. It consists of five functional module plug-in boards, including LCD screen, control board, light group driver board, switch power supply, button board, etc., as well as power distribution board, terminal block, etc. This system selects the 32-bit embedded RISC processor AT91RM9200 based on ARM core as the signal machine control board processor, which can meet the requirements of signal machine intelligence and make the signal machine a platform for collecting and processing traffic flow data, communication networking and regional coordinated control in the system.

The structure diagram of the intelligent traffic signal light system is shown in Figure 1.

Figure 1 Traffic light system structure diagram

The signal light uses light-emitting diodes, or LED technology, which has high luminous power, strong earthquake resistance, and is energy-saving and stable. All components in the system, including the signal lights, are powered externally and equipped with UPS. The signal light controller does not provide working voltage for various signal lights, but only transmits signals, fully realizing weak current control. The control of the signal light brightness adopts the principle of pulse width modulation, which rectifies the voltage in full wave, divides it by resistors, and converts it into a square wave with a certain duty cycle by an optical coupler. When the power supply voltage increases, the duty cycle will decrease, inhibiting the increase in LED brightness. When the power supply voltage decreases, the duty cycle will increase, limiting the decrease in LED brightness, thereby realizing automatic control of LED brightness.

Traffic lights can be controlled by long-distance communication using current loop signal transmission. This long-distance communication control circuit is simple, low-cost, and has strong anti-interference capabilities. It uses a current loop to transmit three serially shifted signals - data D, clock CLK, and latch STR. Traffic lights convert serial information sent by the controller into parallel outputs to accurately achieve bit control. Different traffic lights can be connected in parallel on the line. In this way, the load capacity of a line output is sufficient to control more than a dozen traffic lights, which can meet the needs of various traffic intersections.

The intelligent traffic light system is both an independent system and a part of the entire regional intelligent traffic light system. Through information sharing, the linkage control of traffic signals can be realized, and effective traffic forecasting and diversion can be carried out. When the traffic flow at the intersection is relatively uniform and stable, the coordinated operation of the traffic lights in the region can realize the "green wave belt" control.

The following is a structural diagram of the regional traffic signal linkage system, as shown in Figure 2.

Figure 2 Regional traffic signal linkage system structure diagram [page]

2 System Hardware Design

2.1 Traffic light master control module

The main control machine adopts the processor AT91RM9200, which is based on the ARM920T core of ARM and integrates a wealth of peripheral function modules. It is very suitable for real-time control and supports real-time operating systems with high computing speed.

The signal machine is powered by a 12/5V power supply, the AT91RM9200 operates at 3.3V and 2.5V, and other devices in the system choose to operate at 3.3V and 5V.

The signal machine needs time synchronization in the internal communication and regional coordination control of the system, so the RTC (real-time clock) is designed for time synchronization. The AT91RM9200 has an internal integrated RTC unit and only needs an external crystal oscillator to work, which is very convenient.

The signal master needs to store the boot program, embedded operating system and its file system and application program, as well as other data that needs to be saved during operation, so the storage space needs to be expanded through external storage units, including SDRAM, NorFlash and NandFlash. NorFlash is mainly used to store the boot program Bootloader and the operating system Linux kernel image. After the system is powered on or reset, the Bootloader runs from the FlashROM, and the Bootloader initializes the hardware and copies Linux to the SDRAM for operation.

NandFlash is mainly used to store applications and data.

To facilitate the operation of the human-machine interface, the AT91RM9200 has a built-in LCD (liquid crystal display) driver controller that can automatically generate LCD drive control signals and can be directly connected to the LCD. The keyboard module expands a 4×4 keyboard matrix through the ZLG7290B, which is connected to the processor through the IIC serial bus.

Considering the size of the signal machine and to facilitate the upgrade and expansion of the equipment, the data bus, address bus and necessary control signals are derived from the signal machine control board, and a unified system bus is designed to dispatch and control various functional modules, such as the traffic information collection module, signal light controller module, infrared receiving module, fault detection module, etc. These modules correspond to the corresponding functional modules one by one and are connected to the signal machine control board through the slot interface.

2.2 Traffic flow information collection module

Radio Frequency Identification, or RFID, is commonly known as electronic tags. It is a contactless automatic identification technology that automatically identifies the target object and obtains relevant data through radio frequency signals. The working process does not require human intervention and can be applied to various harsh environments. A set of RFID equipment includes three parts: radio frequency tags, antennas, and readers. The antenna and reader are placed on the roadside. When a vehicle with a radio frequency tag passes through the road section, the reader will receive the radio frequency modulated signal carrying RFID information through the antenna, and transmit it to the signal master after processing to complete the information collection of road condition data. The relevant data is also sent to the upper computer-the regional signal light system control terminal via the local signal. The traffic information is modeled and calculated in the signal master to obtain the cycle time required for the current signal light.

2.3 Emergency Intervention Module

The remote control circuit is loaded in the system to support emergency intervention with infrared remote control. The 8-channel infrared transmitting/receiving dedicated integrated circuit BA5104/BA5204 is used, which can transmit 6 continuous signals and 2 single signals, has convenient serial port access, low price and strong stability. The infrared receiving end is connected to the signal control board bus, and the remote control key signal can be transmitted to the signal main control end for interrupt processing, and it has the highest priority.

2.4 Fault Detection Module

The working environment of traffic lights is relatively complex, and there may be various unpredictable power supplies, electromagnetic interference, and random obstacles of the signal machine itself. In order to ensure the reliable operation of the system, in addition to adopting software countermeasures, a hardware watchdog reset circuit MAX708CPA is specially introduced. This device has functions such as μP reset, power failure monitoring, and manual reset, which can play a good protective role. In addition, a fault detection circuit containing voltage detection and current detection is used to perform real-time detection of the signal light controller and the signal light, and the returned TTL level signal is transmitted to the signal machine main control board. The signal machine system can choose to continue execution, or alarm upward, or even shut down automatically according to different results.

2.5 Traffic light controller module

The signal light controller is a necessary hardware connection between the signal machine and the signal light. The various light color states of the intersection signal light are converted by the signal light controller through processing the signal machine data.

2.6 Regional linkage module

The local signal system and the regional signal system management end use a local area network (LAN) for two-way communication. The local signal control end packages the data information to be processed through the Web Service application program interface and sends it to the regional signal system control end, which completes the processing and returns the final result to the local signal. Web Service technology makes full use of the computing power of the regional signal system control unit, reduces the processor overhead of the local signal, and makes the signal system more stable and easy to maintain.

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The main program flow chart is shown in Figure 3.

Figure 3 Main program flow chart

The traffic light system program can be divided into the following modules:

The signal main program module is the main module of the whole system. It uses the execution results of other modules according to priority to generate the current signal light cycle.

The timing control program module supports inputting fixed time values ​​through the keyboard to change the currently executed signal light cycle, combining manual experience with the system to reflect the human-computer interaction capability.

The traffic flow adjustment program module uses RFID technology to identify the traffic flow near the traffic intersection, and uses GA (genetic algorithm) and fuzzy logic theory [6] to perform real-time analysis and calculation of the traffic conditions at the intersection, and adjust the signal light cycle based on the results. In this system, this module is also the most intelligent part. In addition, through the extension technology of RFID technology, namely the vehicle automatic identification management (AVIM) system, the traffic flow information is connected with the vehicle information monitoring and management center to form a vehicle information management platform.

The timer adjustment program module is set for sections of roads that have formed certain traffic patterns. You can pre-set different traffic light cycles for different time periods every day. For example, if there is less traffic at night, you can make a timer adjustment to change the traffic light to a flashing yellow light, and when the specified time arrives the next day, the traffic light will automatically return to green.

The yellow flash warning program module can immediately interrupt all traffic lights and change them to flashing yellow lights. It is mainly used during periods of low traffic volume to improve traffic efficiency.

The remote control forced reset program module is designed for traffic police to direct traffic on site. By simply pressing a specific button on the infrared receiving end with the remote control, the phases of all signal lights on site can be forced to reset. It is particularly suitable for regulating one-way traffic jams and for vehicles such as ambulances and fire trucks that require barrier-free passage through intersections quickly.

The linkage processing program module is used to process the instructions sent by the upper computer (the traffic police linkage center control machine). After the traffic lights are connected to the network, the traffic intersections in the entire area can be coordinated and controlled. If congestion occurs at an intersection, the linkage center can send instructions to appropriately extend the passing time of the traffic lights at the adjacent intersections in the direction of the oncoming vehicle, effectively diverting traffic.

The "watchdog" program module can generate an overflow signal when the main program runs abnormally, and send a reset signal to the processor through the pin to restart the main program.

The system detection and alarm program module is set up to deal with signal light failures. It makes judgments based on the returned TTL level signal. Once a problem is found, it will report the error information to the upper level in real time, providing a basis for further system maintenance.

4 Conclusion

The development of intelligent traffic light system can effectively improve the current traffic situation, increase the utilization rate of existing road resources, and save social costs. This system is based on AT91RM9200 processor, application-centric, and uses embedded operating system. The design has the characteristics of low cost, simple operation, convenient expansion, high information sharing, strong flexibility, etc. It has good reference value and high practical value.