PLC Programming Exercise – Traffic Light Control System

PLC Programming Exercise – Traffic Light Control System

Introduction

In this exercise, we will design and implement a PLC-based traffic light control system using a ladder logic program. The goal is to control a three-way traffic light system with Red, Yellow, and Green lights, simulating a real-world intersection.

Learning Objectives

By completing this exercise, you will:

• Understand basic PLC programming concepts and ladder logic.
• Learn how to use timers and counters in PLC programming.
• Develop logic to control a sequential system.
• Simulate and test a traffic light control system.

Tools and Components

To complete this exercise, you will need:

• A PLC with a programming environment (e.g., Siemens TIA Portal, Allen-Bradley RSLogix, or any compatible software)
• PLC simulator (if no physical PLC is available)
• Input/output (I/O) devices including three output lamps (Red, Yellow, Green) and one start push button.

Background & Definitions

• PLC (Programmable Logic Controller): An industrial computer used to automate electromechanical processes.
• Ladder Logic: A graphical programming language that represents electrical control circuits.
• Timers (TON/TOF): Used to introduce delays in the execution of logic.
• Counters (CTU/CTD): Used to count the number of events or cycles.

Step-by-Step Guide

Step 1: Define System Operation

The traffic light should follow a predefined sequence:

• Green ON for 10 seconds
• Yellow ON for 3 seconds
• Red ON for 10 seconds
• Repeat cycle continuously The system should start when a push button is pressed.

Step 2: Assign I/O Addresses

• Start Button: I0.0
• Green Light: Q0.0
• Yellow Light: Q0.1
• Red Light: Q0.2

Step 3: Implement Ladder Logic

1. Initialize System: Use a push button to trigger the cycle and a memory coil (M0.0) to latch the system ON.
2. Green Light Timer (T1): Set a TON (Timer ON Delay) with 10 seconds duration. When the timer expires, activate the yellow light.
3. Yellow Light Timer (T2): Set another TON with 3 seconds delay. Once expired, switch to red light.
4. Red Light Timer (T3): Set a TON with 10 seconds delay. When completed, reset to green light, restarting the cycle.

Step 4: Ladder Logic Code

| I0.0      M0.0 |
|----| |--------( )----|

| M0.0      T1 |
|----| |--------TON T1, 10s----|
| T1        Q0.0 |
|----|/|--------( )----| (Green ON)

| T1        T2 |
|----| |--------TON T2, 3s----|
| T2        Q0.1 |
|----|/|--------( )----| (Yellow ON)

| T2        T3 |
|----| |--------TON T3, 10s----|
| T3        Q0.2 |
|----|/|--------( )----| (Red ON)

| T3        M0.0 |
|----| |--------( )----| (Restart Cycle)

Step 5: Simulation and Testing

1. Run the program in the PLC simulator.
2. Observe the timing of each light and verify the correct sequence.
3. Press the start button to initiate the cycle.
4. Adjust timer values to test different timing scenarios.

Common Issues & Troubleshooting

• If lights are not changing, check if timers are triggering correctly.
• If the sequence stops unexpectedly, verify the latching condition of M0.0.
• If timers are not working, ensure correct addressing and time settings.

Extensions & Modifications

• Implement a pedestrian crossing button that extends the red light duration.
• Add a flashing yellow light during off-peak hours.
• Integrate a real-time clock (RTC) for automatic mode switching.

Conclusion

By completing this exercise, you have learned how to implement a basic traffic light control system using PLC programming. This exercise enhances your understanding of sequential control logic, timers, and ladder logic programming.