PLC Program for Traffic Light Control
Control Purpose:
- Enabling the traffic lights to work by Start button X0 and to stop by Stop button X1.
- Setting the time of red light in East-West direction as 60 sec and North-South direction with a heavier traffic as 30 sec.
- The time of red light in East-West direction equals to the time of “green light + green light flashing + yellow light” in North-south direction, and vice versa.
- When yellow light is ON, cars and pedestrians should not cross the road, and yellow light will last for 5 sec for the crossing cars and pedestrians to pass safely.
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Timing diagram of traffic lights in East-West direction:
Timing diagram of traffic lights in North-South direction:
Devices:
| Device | Function |
| X0 | Start button |
| X1 | Stop button |
| T0 | 60 sec timer. Time base: 100ms |
| T1 | 20 sec timer. Time base: 100ms |
| T2 | 5 sec timer. Time base: 100ms |
| T10 | 50 sec timer. Time base: 100ms |
| T11 | 5 sec timer. Time base: 100ms |
| T12 | 5 sec timer. Time base: 100ms |
| T13 | 30 sec timer. Time base: 100ms |
| S0 | Initial step |
| S10 ~ S13 | Controlling the Traffic lights in East-West direction |
| S20 ~ S23 | Controlling the Traffic lights in North-South direction |
| Y0 | Red light in East-West direction |
| Y1 | Green light in East-West direction |
| Y2 | Yellow light in East-West direction |
| Y10 | Red light in North-South direction |
| Y11 | Green light in North-South direction |
| Y12 | Yellow light in North-South direction |
Ladder Program for Traffic Light Control:
Program Description:
- WhenStart is pressed, X0 = ON. PLS instruction will be executed, and M0 will create a rising-edge pulse to set T0. The program will enter the step ladder process.
- · When Stop is pressed, X1 = ON. PLS instruction will be executed, and M1 will create a rising-edge pulse to execute [ZRST S0 S127] instruction. All steps will be reset and all traffic lights will be OFF.
- This example is designed by the application of the simultaneous divergence sequence. The two sequences running simultaneously are East-West direction and North-South direction.
- When the red light of East-West direction is ON, the corresponding state of North-South direction will be the sequence of “Green ON”, “Green Flashing” and “Yellow ON.”
- When the East-West direction sequence is finished (the yellow light is OFF), the North-South direction sequence will be finished as well (the red light is OFF). The program will return to the initial step S0.
- When a step is transferred from one sequence to another sequence, the former sequence will be reset including the step and output point Y.
- The time of yellow light in East-West direction (Y2) is not controlled by a timer because when the red light in North-South direction is OFF, the yellow light in North-South direction will be reset at the same time. In this case, T13 is ON to redirect the program to initial step S0, and the outputs (Y2 and Y10) corresponding to S13 and S23 will thus be reset.
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