MicroLogix 1400 Pusher Sequencing Logic for Case Packing

Application Overview

This article covers PLC programming logic for upgrading an SLC 150 to MicroLogix 1400 on a school milk carton caser machine. The application requires coordinating two pneumatic pushers fed by two product lines, controlled by a 2-position Directional Control Valve (DCV), with a total of 5 pushes per tier cycle.

I/O Assignment Strategy

Effective sequencing requires clear input/output mapping. The following table provides a recommended I/O structure:

State Machine Design

The application requires a state machine architecture to manage the complex sequencing. Each pusher operates in distinct phases:

1. WAIT - Pusher at forward position, awaiting product backup
2. DETECT - Product backed up to photo eye for 2+ seconds
3. RETRACT - DCV retracts to allow product entry
4. ADVANCE_HALF - Pusher advances to mid-position switch
5. PUSH - Push product toward case
6. RETRACT_PUSH - Retract for next product
7. WAIT_HEAD - Extend past mid-switch, wait for head to lower
8. HEAD_GRAB - Head descends and grabs product
9. SYNC_RETACT - Both pushers retract while head lowers
10. FINAL_PUSH - Complete remaining pushes

Ladder Logic Structure

The following structured text pattern (compatible with RSLogix 500) implements the core sequencing:

// Main Sequence Controller
// Cycle Counter: N7:0 (total pushes)
// Push1 Counter: N7:1, Push2 Counter: N7:2

// State Register: N7:10
// 0=WAIT, 1=DETECT, 2=RETRACT, 3=ADVANCE_HALF
// 4=PUSH, 5=RETRACT_PUSH, 6=WAIT_HEAD, 7=HEAD_GRAB
// 8=SYNC_RETRACT, 9=FINAL_PUSH, 10=CYCLE_DONE

// Line Selection Logic
IF I:0.0/6 AND I:0.0/7 THEN  // Both lines running
    Push1_Target := 3;
    Push2_Target := 2;
ELSIF I:0.0/6 OR I:0.0/7 THEN  // Single line
    Push1_Target := 5;
    Push2_Target := 0;
END_IF;

// 2-Second Photo Eye Timer
TON(Timer=PTW, Preset=20, Input=I:0.0/0);

// State Transition Logic (Pusher 1 Example)
CASE N7:10 OF
    0: // WAIT
        IF PTW.DN AND I:0.0/1 THEN  // Product detected, pusher forward
            N7:10 := 1;
        END_IF;
    
    1: // DETECT
        O:0.0/1 := 1;  // Retract solenoid
        IF I:0.0/3 THEN  // Fully retracted
            N7:10 := 2;
        END_IF;
    
    2: // ADVANCE_HALF
        O:0.0/0 := 1;  // Extend solenoid
        IF I:0.0/2 THEN  // Mid-switch made
            N7:10 := 3;
            N7:1 := N7:1 + 1;  // Increment push counter
        END_IF;
    
    3: // RETRACT_PUSH
        O:0.0/1 := 1;
        IF I:0.0/3 THEN
            IF N7:1 < Push1_Target THEN
                N7:10 := 1;  // Return to DETECT
            ELSIF N7:1 >= Push1_Target AND N7:0 < 5 THEN
                N7:10 := 4;  // Wait for head
            ELSE
                N7:10 := 10;  // Cycle complete
            END_IF;
        END_IF;
    
    4: // WAIT_HEAD
        O:0.0/0 := 1;  // Extend past mid-switch
        IF I:0.0/1 THEN  // Forward switch made
            N7:10 := 5;  // Wait for head down
        END_IF;
    
    5: // HEAD_GRAB
        IF I:0.0/5 THEN  // Head down switch
            N7:10 := 6;
        END_IF;
    
    6: // SYNC_RETRACT
        O:0.0/1 := 1;  // Retract while head passes
        IF NOT I:0.0/4 THEN  // Head not up yet
            N7:10 := 7;  // Continue to mid-push
        END_IF;
    
    7: // FINAL_PUSH
        O:0.0/0 := 1;
        IF I:0.0/2 THEN
            N7:1 := N7:1 + 1;
            N7:10 := 6;
        END_IF;
        IF N7:1 >= Push1_Target THEN
            N7:10 := 10;
        END_IF;
    
    10: // CYCLE_DONE
        // Reset for next cycle
        N7:10 := 0;
        N7:1 := 0;
        N7:2 := 0;
END_CASE;

Key Programming Considerations

Debouncing Photo Eye Input

The 2-second timer (TON instruction with preset of 20 for 100ms base) provides debouncing for the photo eye. Adjust the preset based on actual product spacing and conveyor speed. Product must consistently block the PE for the full 2 seconds before triggering retraction.

DCV Solenoid Overlap Prevention

Ensure extend and retract outputs never energize simultaneously. Implement interlocking in the ladder logic:

// Interlock Rung
XIC(O:0.0/0) OTL(O:0.0/1);
XIC(O:0.0/2) OTL(O:0.0/3);

Head Synchronization Timing

The "head up" switch (I:0.0/4) must be monitored during the SYNC_RETRACT state. The pushers must complete their single mid-position push and retract before the head fully raises. Add a failsafe timeout:

// Head timeout - abort if head takes too long
TON(HeadTimer, Preset=50, Input=(N7:10=6));
IF HeadTimer.DN AND N7:10=6 THEN
    // Fault - halt production
    O:0.0/0 := 0;
    O:0.0/1 := 0;
    O:0.0/2 := 0;
    O:0.0/3 := 0;
END_IF;

Single Line Operation Mode

When only one line runs, the active pusher must complete all 5 pushes before cycling. This requires dynamic target adjustment:

// Single line push distribution
IF (I:0.0/6 XOR I:0.0/7) AND N7:10 <= 3 THEN
    Push1_Target := 5;  // All pushes on active line
    Push2_Target := 0;
END_IF;

// Two-stage push for single line
IF N7:1 = 3 AND N7:10 = 3 THEN
    // First 3 pushes complete - wait for product backup again
    N7:10 := 0;  // Return to wait state
END_IF;

Debugging Tips

• Use Project Properties → Controller Tags to monitor N7:10 (state) and counter values in real-time
• Force outputs sparingly during testing - use momentary override with immediate reset
• Single-step through CASE statement using Debug → Toggle Breakpoint
• Verify all switches are properly wired with LED indicators at the cabinet

Before You Start

FAQ