Programable Logical Control (PLC)

Author

Thomas M. Rudolf

Logical Operations with Ladder Diagram

GRAFCET (Graphical Functional Control Step Transition) emerged in the 70s through collaboration between PLC manufacturers and official organizations like AFCET and ADEPA. It is a graphical tool used to describe deterministic control behavior.

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Input and Output Variables, Internal Registers of PLC

GRAFCET allows modeling the sequential behavior of an automated system. It describes process evolution, indicating the actions required and the information triggering them.

Elements of GRAFCET

  • Stages: Each stage corresponds to a specific process step.
  • Transitions: Define the conditions to move between stages.
  • Links: Connect different process parts.
  • Interpretation Elements: Conditions necessary for process transitions.

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Different Components and Instructions of PLC Programming

A sequential process executes an action based on the result of the previous action. Transition conditions must be met to move from one stage to another.

Transition Conditions

  • Active Condition (CT[i] = F1): Activates stage i when signal F1 is active.
  • Inactive Condition (CT[i] = F1’): Activates stage i when signal F1 is inactive.
  • Time Condition (CT[i] = t/i/10 sec): Activates stage i after a time delay.
  • Logical Condition (CT[i] = F1·F2’·F3): Activates when specific signals are met.
  • Unconditional Condition (CT[i] = 1): Always activates the next stage.

Process Evolution Example

  1. Step 1: The part reaches position B (sensor detects it).
  2. Step 2: Conveyor moves part from A to B.
  3. Step 3: Robot picks up the part.
  4. Step 4: Drilling process begins.
  5. Step 5: Finished part is detected and removed.

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Conversion from GRAFCET to Ladder Diagram

  • Each stage corresponds to an internal PLC bit (B3).
  • Stages are activated and deactivated using Latch and Unlatch commands.
  • Left side conditions define activation and deactivation logic.

Example: Traffic Light Control Using Timers

  1. Start Button: Initiates the sequence.
  2. Red Light: Turns on for a specified duration.
  3. Green Light: Turns on after Red Light.
  4. Yellow Light: Activates before Red.
  5. Cycle Restarts.

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Common Errors in GRAFCET Implementation

  1. Incorrect transition conditions leading to improper process evolution.
  2. Simultaneous sequence start/end issues.
  3. Nested simultaneous sequences causing execution conflicts.
  4. Skipping intermediate stages unintentionally.

Practical Exercises

Exercise 1: Convert the Following Sequence to Ladder Diagram

  • Sequence: B+ → A+ → A- → A+ → B- → A-

Exercise 2: Crane Control System

  • Four limit switches: Two for horizontal movement (S3, S4) and two for vertical (S1, S2).
  • Motors: Two dual-direction motors controlling crane movement.
  • Process Flow:
    • Move crane to pickup position.
    • Lift part using vertical motor.
    • Transport to destination.
    • Lower and release part.

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