Ladder logic remains the dominant PLC programming language in roll forming machines worldwide.
It is used to control:
Main drive start/stop
Hydraulic sequencing
Flying shear timing
Encoder length counting
Safety interlocks
Alarm handling
Stacker coordination
When ladder logic is poorly structured, the result is:
Unpredictable machine behavior
Unsafe restart conditions
Cut length variation
Hydraulic conflicts
Long troubleshooting times
When properly structured, ladder logic provides:
Clear logic flow
Easy fault diagnosis
Stable production
Safer operation
This guide explains ladder logic fundamentals specifically in the context of roll forming systems.
Ladder logic is a graphical PLC programming language that resembles electrical relay circuits.
It consists of:
Rungs
Contacts
Coils
Timers
Counters
Comparison blocks
Each rung executes from left to right, top to bottom.
In roll forming systems, ladder logic replaces traditional hardwired relay control.
Word-Based Example (Start Logic):
[ E-STOP OK ] — [ GUARD OK ] — [ START BUTTON ] — ( MAIN MOTOR RUN )
Contacts represent conditions.
Coil represents output command.
If all conditions are TRUE, coil energizes.
A standard roll forming main drive uses latching logic.
Word-Based Example:
[ E-STOP OK ] — [ STOP NC ] — [ START ] — ( MOTOR RUN )
|
[ MOTOR RUN ]
Explanation:
Press START → Motor Run coil energizes
Motor Run contact seals circuit
Press STOP → Breaks circuit
Latch ensures continuous running until stopped.
Safety hardware (relay) removes power physically.
PLC logic must also prevent restart.
Word-Based:
[ SAFETY OK ] — [ NO FAULT ] — [ START ] — ( ENABLE DRIVE )
If safety input drops, drive enable drops immediately.
Restart requires deliberate start action.
Roofing lines depend on accurate shear timing.
Word-Based Logic:
[ Encoder_Count ≥ Target_Length ] — [ Shear Ready ] — ( Shear Trigger )
Shear trigger must:
Be fast
Be single pulse
Reset after actuation
High-speed counter handles pulse counting outside normal scan timing.
Timers manage:
Hydraulic delays
Alarm delays
Stacker sequences
Example:
[ Shear Trigger ] — ( TON 200ms ) — ( Shear Reset )
Timer ensures hydraulic cylinder completes movement before next action.
Counters used for:
Piece counting
Batch control
Maintenance tracking
Word-Based:
[ Shear Complete ] — ( Counter +1 )
Counter resets when batch target reached.
Complex roll forming lines benefit from state-based programming.
Instead of many scattered rungs:
Define machine states:
STATE 0 – Idle
STATE 1 – Running
STATE 2 – Shear Active
STATE 3 – Fault
Then control outputs based on current state.
This prevents conflicting logic.
Fault rungs should be structured separately.
Word-Based Example:
[ Hydraulic Pressure NOT OK ] — ( Fault_Hydraulic )
Master Fault:
[ Fault_Hydraulic OR Fault_Servo OR Fault_Safety ] — ( General_Fault )
Clear fault grouping simplifies troubleshooting.
Shear trigger must be momentary.
Incorrect logic:
Continuous TRUE output → Servo fault.
Correct logic:
Use one-shot rising edge detection.
Word-Based:
[ Length_Reached Rising Edge ] — ( Shear_Pulse )
Pulse duration controlled by timer if required.
Hydraulic extend and retract must not energize simultaneously.
Word-Based:
[ Shear Trigger ] — ( Extend Valve )
[ Extend Complete ] — ( Retract Valve )
Interlocks prevent both valves active at same time.
Mixing safety and process logic
No separation between state logic and fault logic
Continuous shear output instead of pulse
No interlock between hydraulic outputs
Overusing global bits without naming discipline
No structured alarm mapping
No comments in program
Clarity is essential for long-term serviceability.
PLC executes ladder logic cyclically:
Read inputs
Execute logic
Update outputs
Scan time affects:
Output timing
Shear accuracy
Sensor detection
High-speed roofing lines must:
Use high-speed counter for encoder
Avoid scan-based pulse counting
Rung 1: Safety Check
Rung 2: Start/Stop Latch
Rung 3: Encoder Counting
Rung 4: Length Compare
Rung 5: Shear Pulse
Rung 6: Hydraulic Reset
Rung 7: Piece Counter
Rung 8: Fault Monitoring
Organized layout improves maintenance.
Every rung should include:
Description
Device reference
Function explanation
Example:
Rung Comment: “Main drive enable – requires safety OK and no active faults.”
Uncommented programs increase downtime during service.
Roofing lines:
Continuous running
High-speed shear
Encoder-critical
Structural lines:
Punch coordination
Multiple operations per cycle
Longer cycle time
Ladder logic complexity increases with added functions.
Before production:
Verify start/stop logic
Test safety interruption
Confirm shear single pulse
Test hydraulic sequencing
Check batch counter reset
Simulate fault conditions
Never assume correct logic without simulation testing.
Before purchasing a roll forming machine, ask:
Is ladder logic structured by section?
Are safety and process logic separated?
Is shear pulse implemented correctly?
Is high-speed counter used for encoder?
Are faults categorized clearly?
Is program commented properly?
Is backup copy provided?
Is logic easy for local technicians to understand?
Red flag:
“Program works — documentation not needed.”
Professional systems include structured logic and comments.
It mirrors relay logic and is easy for electricians to understand.
Yes, especially if shear trigger timing is poorly programmed.
No. Use high-speed counter.
Yes, for complex systems.
Yes. Structured fault logic simplifies troubleshooting.
Continuous shear output instead of single pulse trigger.
Ladder logic in roll forming machines must provide:
Clear start/stop control
Reliable safety interlocks
Accurate shear timing
Controlled hydraulic sequencing
Structured fault logic
Documented and maintainable code
Well-structured ladder logic ensures:
Predictable operation
Accurate cut length
Safe restart conditions
Faster troubleshooting
Reduced downtime
In roll forming production, programming clarity is as important as electrical wiring discipline.
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