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The number of vehicles on the road is continuously increasing, and traditional traffic light systems typically rely on fixed-time control. However, since traffic flow is constantly changing, this method often leads to inefficiencies. For example, when no vehicles are present in a particular direction, drivers still have to wait for the green light to turn red before they can proceed. This inflexible system causes unnecessary delays and can lead to congestion in opposite directions. Ensuring smooth urban traffic flow is a key indicator of a city's economic development level. Efficient traffic light control is essential for maintaining safe, orderly, and fast traffic operations. Traffic lights remain one of the most widely used tools for managing vehicle movement. To address issues such as short green light intervals at empty intersections or traffic jams at busy ones, the S7-200 PLC series, specifically the CPU 224 model, is used to control traffic signals at intersections.
**1. Hardware Design of the PLC Control System**
**1.1 Control Requirements**
The traffic light system is controlled using a PLC. The main requirements include:
- Red, green, and yellow lights are arranged in both the north-south and east-west directions.
- They operate in a specific sequence.
- A start switch is used to activate the system, while a stop switch halts it during maintenance.
The timing specifications are as follows:
1. In the north-south direction: Red light is on for 55 seconds, green light for 50 seconds, followed by a 3-second yellow light.
2. In the east-west direction: Green light is on for 30 seconds, then flashes for 2 seconds (on for 0.5s, off for 0.5s), followed by a 3-second yellow light. The red light remains on for 35 seconds.
3. The flashing frequency of the green light is set to 0.5 seconds on and 0.5 seconds off.
4. The system operates in a continuous cycle.
These control requirements are detailed in Table 1.
**Table 1: Traffic Light Control Requirements**
[Image: Traffic Light Control Requirements]
**1.2 I/O Allocation Table**
This system uses the S7-200 PLC’s CPU 224 model, which has 14 input points and 10 output points. Based on the control needs, the I/O allocation table for the traffic light control system is shown in Table 2. The system includes two input points: the start button and the stop button. The start button initiates the system, while the stop button is used during maintenance. The six output points control the traffic lights in both the north-south and east-west directions.
**Table 2: I/O Allocation Table**
[Image: I/O Allocation Table]
**1.3 Principle Wiring Diagram**
Lights of the same color in the same direction operate simultaneously. To ensure synchronization and save output points, the same-colored lights are connected in parallel. The 12 lights in both directions are grouped into 6 outputs. Figure 1 shows the wiring diagram of the PLC-based traffic light control system.
**Figure 1: PLC Control Traffic Light Principle Wiring Diagram**
[Image: Principle Wiring Diagram]
**2. Software Design of the PLC Control System**
**2.1 Timing Diagram**
Traffic light sequences differ between the north-south and east-west directions. In a 90-second cycle:
- North-South: Red for 55 seconds, Green for 30 seconds, Flashing Green for 2 seconds, then Yellow for 3 seconds.
- East-West: Green for 50 seconds, Flashing Green for 2 seconds, then Yellow for 3 seconds, followed by Red for 35 seconds.
The flashing frequency of the green light is 0.5 seconds on and 0.5 seconds off.
**Figure 2: Traffic Light Operation Timing Chart**
[Image: Timing Chart]
This comprehensive design ensures efficient and reliable operation of the traffic light system, adapting to real-time traffic conditions and improving overall urban mobility.
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