Packaging

PLC Packaging Machine Programming: Cam Sequencing, Reject Systems & OEE in Structured Text

High-speed packaging demands precise timing, reliable product tracking, and zero-tolerance reject systems. Build production-grade packaging machine logic with cam sequencing, tracking arrays, and real-time OEE in Structured Text.

· 20 min read

  • Packaging
  • Cam Sequencing
  • Product Tracking
  • OEE
  • Structured Text

🏭 Packaging Machine Architecture

A typical packaging machine combines continuous motion (film transport, conveyors) with discrete events (seal, cut, fill, label) that must happen at precise positions in the machine cycle. This is fundamentally different from process control — timing is measured in degrees of machine rotation, not seconds.

The Machine Cycle Concept

Machine Cycle (0° to 360°)
│
├── 0°–30°     : Film advance / index
├── 30°–60°    : Product drop / fill
├── 60°–120°   : Cross-seal (jaws closed)
├── 120°–150°  : Seal cooling
├── 150°–180°  : Jaw open / package release
├── 180°–270°  : Label application
├── 270°–330°  : Inspection / vision check
├── 330°–360°  : Reject / accept gate
└── 360° = 0°  : Cycle repeats

📐 Cam-Based Sequencing

Instead of timers, packaging machines use virtual cam switches — outputs that turn on/off at specific encoder positions:

TYPE CamSwitch :
STRUCT
    OnPosition   : REAL;       // Degrees where output turns ON
    OffPosition  : REAL;       // Degrees where output turns OFF
    Output       : BOOL;       // Current state
    Enabled      : BOOL;       // Master enable
    SpeedComp    : REAL;       // Advance ON position at high speed (degrees)
END_STRUCT;
END_TYPE
FUNCTION_BLOCK FB_CamController
VAR_INPUT
    MachineAngle    : REAL;       // 0.0 to 360.0 from encoder
    MachineSpeed    : REAL;       // RPM for speed compensation
    Enable          : BOOL;
END_VAR
VAR
    Cams            : ARRAY[1..16] OF CamSwitch;
    NumCams         : INT := 8;
END_VAR
VAR
    i               : INT;
    adjustedOn      : REAL;
END_VAR
IF NOT Enable THEN
    FOR i := 1 TO NumCams DO
        Cams[i].Output := FALSE;
    END_FOR;
    RETURN;
END_IF;
FOR i := 1 TO NumCams DO
    IF NOT Cams[i].Enabled THEN
        Cams[i].Output := FALSE;
    ELSE
        // Speed compensation: advance ON point at higher speeds
        // to account for actuator response time
        adjustedOn := Cams[i].OnPosition - (Cams[i].SpeedComp * MachineSpeed / 100.0);
        IF adjustedOn < 0.0 THEN adjustedOn := adjustedOn + 360.0; END_IF;
        
        // Handle wrap-around (e.g., ON=350°, OFF=10°)
        IF adjustedOn < Cams[i].OffPosition THEN
            Cams[i].Output := (MachineAngle >= adjustedOn) AND (MachineAngle < Cams[i].OffPosition);
        ELSE
            // Wrap-around case
            Cams[i].Output := (MachineAngle >= adjustedOn) OR (MachineAngle < Cams[i].OffPosition);
        END_IF;
    END_IF;
END_FOR;

Typical Cam Setup for a VFFS Machine

// Vertical Form Fill Seal (VFFS) cam configuration
CamCtrl.Cams[1].OnPosition := 0.0;     CamCtrl.Cams[1].OffPosition := 30.0;    // Film pull
CamCtrl.Cams[2].OnPosition := 35.0;    CamCtrl.Cams[2].OffPosition := 55.0;    // Product fill
CamCtrl.Cams[3].OnPosition := 60.0;    CamCtrl.Cams[3].OffPosition := 120.0;   // Cross-seal jaws
CamCtrl.Cams[4].OnPosition := 70.0;    CamCtrl.Cams[4].OffPosition := 115.0;   // Seal heater
CamCtrl.Cams[5].OnPosition := 180.0;   CamCtrl.Cams[5].OffPosition := 260.0;   // Labeler
CamCtrl.Cams[6].OnPosition := 270.0;   CamCtrl.Cams[6].OffPosition := 320.0;   // Vision trigger
CamCtrl.Cams[7].OnPosition := 330.0;   CamCtrl.Cams[7].OffPosition := 355.0;   // Reject gate
// Speed compensation: seal jaws need 5° advance per 100 RPM
CamCtrl.Cams[3].SpeedComp := 5.0;
CamCtrl.Cams[4].SpeedComp := 5.0;

📦 Product Tracking Array

On high-speed lines, multiple products are in the machine simultaneously at different stages. A tracking shift register follows each product:

TYPE ProductRecord :
STRUCT
    Present      : BOOL;        // Product exists at this station
    Weight       : REAL;        // Filled weight
    SealOK       : BOOL;        // Seal integrity check passed
    LabelOK      : BOOL;        // Label present and readable
    VisionPass   : BOOL;        // Vision inspection passed
    RejectCode   : INT;         // 0=good, >0=reject reason
    BatchID      : DINT;
END_STRUCT;
END_TYPE
FUNCTION_BLOCK FB_ProductTracker
VAR
    Track       : ARRAY[1..20] OF ProductRecord;   // 20 stations in machine
    NumStations : INT := 12;
END_VAR
// Shift register: advance all products by one station
// Called once per machine cycle (at 0° or cycle-complete signal)
VAR_INPUT
    AdvancePulse : BOOL;
    NewProduct   : BOOL;         // Product entering station 1
    NewWeight    : REAL;
END_VAR
VAR
    i : INT;
END_VAR
IF AdvancePulse THEN
    // Shift everything forward (station N → station N+1)
    FOR i := NumStations TO 2 BY -1 DO
        Track[i] := Track[i-1];
    END_FOR;
    
    // Load new product at station 1
    Track[1].Present := NewProduct;
    Track[1].Weight := NewWeight;
    Track[1].SealOK := FALSE;
    Track[1].LabelOK := FALSE;
    Track[1].VisionPass := FALSE;
    Track[1].RejectCode := 0;
    Track[1].BatchID := CurrentBatchID;
END_IF;

Quality Checks at Each Station

// Station 4: Checkweigher
IF Track[4].Present THEN
    IF Track[4].Weight < MinWeight OR Track[4].Weight > MaxWeight THEN
        Track[4].RejectCode := 1;    // Weight out of tolerance
    END_IF;
END_IF;
// Station 7: Seal inspection
IF Track[7].Present THEN
    Track[7].SealOK := SealInspectionSensor;
    IF NOT Track[7].SealOK THEN
        Track[7].RejectCode := 2;    // Seal failure
    END_IF;
END_IF;
// Station 9: Vision system
IF Track[9].Present THEN
    Track[9].VisionPass := VisionSystem_Result;
    Track[9].LabelOK := VisionSystem_LabelOK;
    IF NOT Track[9].VisionPass THEN
        Track[9].RejectCode := 3;    // Vision reject
    END_IF;
END_IF;

🚫 Reject Station Logic

The reject system must be 100% reliable — a missed reject means a bad product reaches the customer:

FUNCTION_BLOCK FB_RejectStation
VAR_INPUT
    Product       : ProductRecord;
    StationIndex  : INT;          // Which tracking station is the reject at
    RejectConfirm : BOOL;         // Sensor confirming product was ejected
END_VAR
VAR_OUTPUT
    RejectCmd     : BOOL;         // Activate reject mechanism
    RejectActive  : BOOL;
    MissedReject  : BOOL;         // CRITICAL: product wasn't ejected
    RejectCount   : DINT;
    RejectByCode  : ARRAY[1..10] OF DINT;  // Count per reject reason
END_VAR
VAR
    tmrReject     : TON;
    tmrConfirm    : TON;
    WaitingConfirm: BOOL;
END_VAR
RejectCmd := FALSE;
MissedReject := FALSE;
IF Product.Present AND Product.RejectCode > 0 THEN
    // Product needs rejecting
    RejectCmd := TRUE;
    RejectActive := TRUE;
    WaitingConfirm := TRUE;
    RejectCount := RejectCount + 1;
    
    // Track reject reasons
    IF Product.RejectCode >= 1 AND Product.RejectCode <= 10 THEN
        RejectByCode[Product.RejectCode] := RejectByCode[Product.RejectCode] + 1;
    END_IF;
END_IF;
// Verify reject actually happened
IF WaitingConfirm THEN
    tmrConfirm(IN := TRUE, PT := T#500MS);   // 500ms window to confirm
    
    IF RejectConfirm THEN
        // Product confirmed ejected
        WaitingConfirm := FALSE;
        tmrConfirm(IN := FALSE);
    ELSIF tmrConfirm.Q THEN
        // MISSED REJECT — critical alarm
        MissedReject := TRUE;
        WaitingConfirm := FALSE;
        tmrConfirm(IN := FALSE);
        // Stop the line or activate secondary reject
    END_IF;
END_IF;

📊 Real-Time OEE Calculation

OEE (Overall Equipment Effectiveness) is the gold standard KPI for packaging lines:

OEE = Availability × Performance × Quality
Availability = Run Time / Planned Production Time
Performance  = (Ideal Cycle Time × Total Count) / Run Time
Quality      = Good Count / Total Count


FUNCTION_BLOCK FB_OEECalculator
VAR_INPUT
    MachineRunning    : BOOL;
    PlannedProduction : BOOL;     // Shift is active
    CycleComplete     : BOOL;     // One product completed
    ProductGood       : BOOL;     // Product passed all inspections
    IdealCycleTime    : REAL;     // Seconds per unit at rated speed
END_VAR
VAR_OUTPUT
    OEE_Pct           : REAL;
    Availability_Pct  : REAL;
    Performance_Pct   : REAL;
    Quality_Pct       : REAL;
    GoodCount         : DINT;
    TotalCount        : DINT;
    RejectCount       : DINT;
    DowntimeMinutes   : REAL;
END_VAR
VAR
    PlannedTimeSec    : REAL := 0.0;
    RunTimeSec        : REAL := 0.0;
    scanTimeSec       : REAL := 0.01;
END_VAR
// Accumulate time
IF PlannedProduction THEN
    PlannedTimeSec := PlannedTimeSec + scanTimeSec;
    
    IF MachineRunning THEN
        RunTimeSec := RunTimeSec + scanTimeSec;
    END_IF;
END_IF;
// Count products (rising edge)
IF CycleComplete THEN
    TotalCount := TotalCount + 1;
    IF ProductGood THEN
        GoodCount := GoodCount + 1;
    ELSE
        RejectCount := RejectCount + 1;
    END_IF;
END_IF;
// Calculate OEE components
IF PlannedTimeSec > 0.0 THEN
    // Availability
    Availability_Pct := (RunTimeSec / PlannedTimeSec) * 100.0;
    DowntimeMinutes := (PlannedTimeSec - RunTimeSec) / 60.0;
    
    // Performance
    IF RunTimeSec > 0.0 THEN
        Performance_Pct := ((IdealCycleTime  DINT_TO_REAL(TotalCount)) / RunTimeSec)  100.0;
        IF Performance_Pct > 100.0 THEN Performance_Pct := 100.0; END_IF;
    END_IF;
    
    // Quality
    IF TotalCount > 0 THEN
        Quality_Pct := (DINT_TO_REAL(GoodCount) / DINT_TO_REAL(TotalCount)) * 100.0;
    END_IF;
    
    // OEE
    OEE_Pct := (Availability_Pct / 100.0)
             * (Performance_Pct / 100.0)
             * (Quality_Pct / 100.0)
             * 100.0;
END_IF;

OEE Benchmarks

| OEE Level | Rating | Typical For | | 85%+ | World-class | Best-in-class automated lines | | 60–85% | Good | Well-maintained production | | 40–60% | Average | Opportunity for improvement | | < 40% | Poor | Significant losses — investigate root causes |

Summary

| Concept | Implementation | | Cam sequencing | Position-based outputs with speed compensation | | Product tracking | Shift register following products through stations | | Quality inspection | Station-by-station checks updating product records | | Reject system | Confirmed ejection with missed-reject alarm | | OEE calculation | Real-time Availability × Performance × Quality | | Registration | Print mark detection with correction offset |

Packaging machine programming is where precision meets speed. Every millisecond of timing, every degree of cam position, and every product tracked through the machine directly impacts your customer's bottom line.