With the increasing number of cars, traditional traffic light control relies on timed control. Since traffic flow is constantly changing, when no cars are passing, oncoming vehicles must wait for the green light to turn red before proceeding. This results in poor responsiveness in terms of time and space, wasting time and causing traffic jams for vehicles in opposite directions. The smooth flow of urban traffic is a crucial indicator of a city's economic development. Solving the problem of highway traffic light control is a vital link in ensuring safe, orderly, and efficient traffic operation. Traffic lights are the most effective and common method for traffic management. To overcome the waste of resources such as no cars passing through intersections with few vehicles when the green light is on, or traffic jams at intersections with many vehicles due to short green light durations, this paper uses the CPU224 from the S7-200 PLC series to control traffic lights at intersections.
1. Hardware design of PLC control system
1.1 Control Requirements
Implement traffic light control using a PLC . The control requirements are as follows: Red, green, and yellow lights are installed in the east-west and north-south directions of the intersection, and they illuminate in turn according to a specific time sequence. A start switch controls the operation of the traffic light system; pressing the stop switch stops the system.
(1) During the time when passage is prohibited in the north-south direction, the red light in the north-south direction will be on for 55 seconds, the green light in the east-west direction will be on for 50 seconds and then flash for 2 seconds, and then the yellow light will be on for 3 seconds.
(2) During the time when east-west traffic is prohibited, the green light for north-south traffic will flash for 2 seconds, then turn yellow for 3 seconds. The red light for east-west traffic will flash for 35 seconds.
(3) The green light flashes for 0.5 seconds on and 0.5 seconds off.
(4) The system operates in a cyclical manner. Specific control requirements are shown in Table 1.
Table 1 Traffic Light Control Requirements
1.2 I/O Allocation Table
This system uses the CPU224 from the S7-200 PLC series. This type of PLC has 14 input points and 10 output points. According to the control requirements, the I/O allocation table of the traffic light PLC control system is shown in Table 2. The system has two input points: a system start button and a stop button. The start button initiates normal system operation, and the stop button pauses the system during maintenance. The six outputs represent the traffic lights for the north-south and east-west directions.
Table 2 I/O Allocation Table
1.3 Schematic Wiring Diagram
Traffic lights of the same color and direction operate simultaneously. To ensure synchronization of traffic lights of the same color and direction, parallel outputs are used, which also saves on the number of output points. The 12 traffic lights in the north-south and east-west directions are combined into 6 outputs. Figure 1 shows the wiring diagram of the traffic light PLC control system.
Figure 1. Wiring diagram of PLC control of traffic lights
2. Software design of the PLC control system
2.1 Timing Diagram
Traffic lights operate differently in the north-south and east-west directions. Within a 90-second cycle, in the north-south direction: the red light is on for 55 seconds, the green light is on for 30 seconds followed by a 2-second flash, then the yellow light is on for 3 seconds. In the east-west direction: the green light is on for 50 seconds followed by a 2-second flash, the yellow light is on for 3 seconds, then the red light is on for 35 seconds. Figure 2 shows the traffic light sequence. The green light flashes for 0.5 seconds on and off.
Figure 2 Traffic light operation sequence diagram
2.2 Ladder Diagram Program
Traffic light control systems are typical timing control systems, where all actions are performed in a fixed time sequence. Pressing the start button initiates operation; the ladder diagram program first initializes and resets all variables during the first power-on cycle. Cyclic control is implemented using the normally closed contacts of the T37 microcontroller. Pressing the stop button stops the entire system. The ladder diagram program is shown in Figure 3.
Figure 3. PLC Traffic Light Control Ladder Diagram
3. Conclusion
Using a single timer T37 to control one cycle (90s) saves on the number of timers. Furthermore, using timers and comparison instructions to control each traffic light makes the entire control system simple and clear. Modifying the on/off time ratio of each traffic light is also very convenient when traffic conditions change and the on/off time of each light needs to be adjusted. Using a PLC to control traffic lights not only reduces the failure rate and improves the reliability of the control system, but also allows for reasonable adjustment of the red and green light times at each intersection based on traffic flow, making the control more flexible, effectively alleviating traffic congestion, achieving optimal control of the traffic system, minimizing vehicle congestion at intersections, improving road traffic utilization, and realizing scientific management of traffic conditions.
