Automated Assembly Line Control System

An industrial conveyor control system implemented in IEC 61131-3 Ladder Logic using OpenPLC. Models a three-station assembly line with a timed processing halt at the middle station, dual-layer fault detection, and continuous self-resetting operation.

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System Overview

Three sensors track each part through the line. The middle station halts the conveyor for a timed processing cycle — welding, inspection, labeling, or any fixed-duration operation. Two independent fault detection mechanisms monitor for jams and abnormal accumulation. The system runs continuously and resets itself with no operator input under normal conditions.

[SensorIn] ──── conveyor ──── [SensorMiddle / Process T#2s] ──── [SensorOut]
   count++                          jam detection                   count++

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Variable Table

#	Name	Class	Type	Description
1	SensorIn	Input	BOOL	Detects incoming part
2	SensorMiddle	Input	BOOL	Processing station presence
3	SensorOut	Input	BOOL	Exit confirmation
4	StartButton	Input	BOOL	System start
5	StopButton	Input	BOOL	System stop
6	ResetButton	Input	BOOL	Manual fault reset
7	ConveyorMotor	Output	BOOL	Main drive motor
8	AlarmJam	Output	BOOL	Fault indicator
9	TONWait	Local	TON	Processing timer (T#2s)
10	TONJam	Local	TON	Jam detection timer (T#5s)
11	CTUIn	Local	CTU	Incoming part counter (PV=5002)
12	CTUOut	Local	CTU	Outgoing part counter (PV=5000)
13	WaitDone	Local	BOOL	Processing cycle complete flag
14	Jammed	Local	BOOL	TONJam output latch
15	ErrorState	Local	BOOL	System fault latch
16	IODiff	Local	INT	CTUIn.CV − CTUOut.CV
17	DiffAlarm	Local	BOOL	IODiff ≥ 3 flag
18	AutoReset	Local	BOOL	Automatic counter reset trigger
19	ResetState	Local	BOOL	Unified reset signal

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Ladder Logic — Rung Descriptions

Rung 1 — Motor Control

(StartButton OR ConveyorMotor) AND /StopButton AND (SensorIn OR WaitDone)
AND /SensorMiddle AND /ErrorState → ConveyorMotor

Seal-in latch with start/stop logic. Motor holds on its own contact after start. SensorMiddle NC contact halts the belt when a part arrives at the processing station. WaitDone bypasses SensorMiddle after the processing cycle completes so the part can exit.

Rung 2 — Processing Timer (TONWait)

SensorMiddle → TONWait [PT = T#2s] → WaitDone

Starts counting when a part reaches the processing station. WaitDone goes TRUE after 2 seconds, releasing the motor.

Rung 3 — Jam Timer (TONJam)

SensorMiddle → TONJam [PT = T#5s] → Jammed

If SensorMiddle stays active beyond 5 seconds, the part has not cleared the station — jam detected.

Rung 4 — Counters and Difference Logic

SensorIn (P)  → CTUIn  [PV=5002, R=ResetState]
SensorOut (P) → CTUOut [PV=5000, R=ResetState]
CTUIn.CV − CTUOut.CV → IODiff    (SUB block)
IODiff ≥ 3            → DiffAlarm (GE block)
CTUOut.Q AND /DiffAlarm → AutoReset

Counts every part entering and exiting. Computes the live in/out difference each scan. If 3 or more parts are on the line simultaneously, DiffAlarm fires. CTUOut reaching PV triggers AutoReset only when the line is balanced.

Rung 5 — Fault Latch (ErrorState)

(Jammed OR DiffAlarm OR ErrorState) AND /ResetButton → ErrorState

Latches on any fault condition. Requires manual ResetButton to clear — operator acknowledgment is mandatory before restart.

Rung 6 — Reset Logic

AutoReset OR ResetButton → ResetState

ResetState feeds both CTU reset pins. ResetButton also directly clears ErrorState.

Rung 7 — Alarm Output

ErrorState → AlarmJam

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Fault Detection Logic

Layer 1 — TONJam (time-based)

A part remaining at the processing station beyond 5 seconds triggers a jam. Catches mechanical blockages at the station level.

Layer 2 — IODiff (count-based)

Every scan computes IODiff = CTUIn.CV − CTUOut.CV. If the difference reaches 3, parts are accumulating faster than they exit — catches inter-station jams that TONJam cannot see.

Both layers drive ErrorState. The motor stops and AlarmJam activates until an operator resets.

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Automatic Reset Mechanism

CTUOut PV=5000, CTUIn PV=5002. When CTUOut reaches 5000:

• DiffAlarm = 0 (line is balanced) → AutoReset fires → both counters clear simultaneously
• DiffAlarm = 1 (parts unaccounted for) → reset blocked, fault path takes over

The PV asymmetry ensures CTUOut always triggers first. Both counters reset in the same scan cycle — IODiff is preserved across the boundary.

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Tools

• OpenPLC Editor — IEC 61131-3 Ladder Logic IDE
• IEC 61131-3 Function Blocks: TON, CTU, SUB, GE