MELSEC Tutorial 05 – Ladder Logic Programming Sequence Instructions

Ladder Logic Programming is a vital skill for developing control systems using Mitsubishi Electric’s MELSEC PLCs. MELSEC Sequence instructions in ladder logic allow you to manage process steps systematically, ensuring precise operations in automation tasks. This tutorial focuses on creating and implementing sequence instructions using GX Works3, offering a clear understanding of how to use these advanced tools effectively.

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Understanding MELSEC Sequence Instructions in Ladder Logic

Sequence instructions are essential for automating multi-step processes. Each sequence step represents a specific action or condition in the system, allowing operations to progress logically and predictably. Sequence programming is widely used in manufacturing systems like assembly lines, conveyor belts, and robotic arms.

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Key Components of MELSEC Sequence Instructions:

1. Step Relays (S bits): Used to define individual sequence steps.
2. Transition Conditions: Define when the system moves from one step to the next.
3. Actions: Perform operations such as turning on motors or activating valves.

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Step 1: Setting Up Your Project in GX Works3

Before starting, ensure your software and hardware setup is ready:

1. Create a New Project in GX Works3 and select your MELSEC PLC model (e.g., iQ-F series).
2. Configure the I/O mapping to match your hardware setup, ensuring inputs and outputs are correctly assigned.

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Step 2: Designing the Sequence Logic

To implement sequence instructions:

Example Process: Conveyor System with Item Sorting

• Step 1: Start conveyor.
• Step 2: Detect item using a sensor.
• Step 3: Sort item based on weight or size.
• Step 4: Stop conveyor after processing.

Writing the Sequence Logic:

1. Define Step Relays:
   • Assign a unique S bit to each step (e.g., S10 for Step 1, S11 for Step 2).
2. Set Transition Conditions:
   • Use input signals (e.g., sensor activations) as conditions to transition between steps.
3. Assign Outputs:
   • Activate outputs (e.g., motors, alarms) corresponding to each step.

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Step 3: Implementing MELSEC Sequence Instructions in GX Works3

Programming Example: Conveyor Sequence

1. Step 1 – Start Conveyor: Start Button --] [-- Step 1 (S10) -- Conveyor Motor --( )--
   • When the start button is pressed, the conveyor motor is activated, marking the first step of the sequence.
2. Step 2 – Detect Item: Sensor Detect --] [-- Step 2 (S11) -- Item Sorting Output --( )--
3. Step 3 – Sorting: Sort Complete --] [-- Step 3 (S12) -- Continue Conveyor --( )--
   • Once sorting is complete, the system continues the conveyor operation.
4. Step 4 – Stop Conveyor: Stop Button --] [-- Step 4 (S13) -- Conveyor Stop --( )--
   • The conveyor stops when the stop button is pressed.

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Step 4: Testing and Debugging

Using the GX Works3 Simulator:

1. Simulate the Program:
   • Test the sequence logic virtually to verify transitions and actions.
2. Monitor Step Relays:
   • Check the activation of each step relay (S bits) during the simulation.
3. Adjust Conditions:
   • Refine transition conditions for optimal system performance.

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Debugging Tips:

• Ensure that step relays reset correctly when transitioning between steps.
• Check input/output wiring for physical connections.
• Verify that timers or counters used in transitions operate as expected.

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Conclusion