PLC Program for Traffic Light Control

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.

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.