PLC Control Systems in Roll Forming

1. Definition

A programmable logic controller (PLC) on a roll forming machine coordinates strip motion, forming speed, pre-punch and post-punch sequences, cut-to-length or flying shear actions, and safety interlocks. Unlike a standalone motion controller on a press, the roll forming PLC integrates discrete I/O (limit switches, pressure sensors, guard doors), analog and high-speed counter inputs (encoder feedback), and fieldbus commands to variable-frequency drives (VFDs) and servo amplifiers on one scan cycle.

Modern lines range from compact PLCs running fixed-length stop-and-cut logic to technology CPUs executing cam-based flying shear profiles while managing recipe storage for dozens of profile types. The PLC program is the authoritative source for when the line may run, how fast it accelerates, and where holes and cuts occur relative to strip travel.

2. Control Architecture Overview

Roll forming control architecture layers machine functions from field devices through the PLC to the operator HMI. The table below maps typical components and data paths on a mid-to-high complexity line (punch + flying shear).

Machine Matcher describes modern PBR (purlin bearing rib) architecture as PLC-centered with VFD main drive, servo feed, flying shear sync, punch timing, stacker logic, and separate safety PLC on automated high-speed installations. Simpler lines may omit servo feed and use mechanical stop-and-cut with encoder preset only.

3. PLC as Line Controller

The PLC executes cyclic scans: read inputs, evaluate logic, update motion setpoints, write outputs. Roll forming programs are structured as state machines (Idle, Threading, Run, Fault, Maintenance) with parallel branches for punch and cut subsystems.

3.1 Processing Requirements

• High-speed counter inputs for encoder pulses (1,000–10,000+ pulses per meter depending on wheel diameter and PPR).
• Deterministic handling of cut and punch triggers within milliseconds of length setpoint.
• Recipe storage: material thickness, line speed, length, hole pattern offsets, cut mode.
• Fault latching with timestamp for maintenance screens on the HMI.

3.2 Scan Cycle vs Motion Tasks

Standard PLC scan (1–10 ms) suffices for VFD speed commands and hydraulic valves. Flying shear and servo cam profiles run in motion tasks or technology objects on motion-capable CPUs (e.g., Siemens S7-1500T) or in dedicated servo drives with electronic cam (E-CAM) when the PLC sends enable and length parameters. Splitting fast motion from slower sequence logic prevents missed cut triggers at line speeds above 80–120 m/min on thin profiles.

4. I/O and Encoder Length Measurement

4.1 Discrete I/O

Typical inputs: coil low-level sensor, pre-punch alignment sensor, hydraulic pressure OK, guard door closed, E-stop chain healthy. Outputs: main contactor enable, VFD run forward, punch solenoid, stacker lift, tower lamp stack. I/O density scales with automation level; a servo punch line may add 20–40 digital points beyond a basic cut-to-length machine.

4.2 Encoder Installation

A measuring wheel or shaft-mounted encoder tracks strip travel. Circumference calibration converts pulses to millimeters. Siemens forum example for flying shear roll forming cites 3600 PPR encoder on product drive with external encoder as master for cut length when no separate leading servo axis exists. Resolution target: cut length tolerance 1–2 mm over full panel length under stable speed on modern machines per industry specs.

4.3 Length Control Modes

Encoder slip on oily or painted strip is a recurring error source. Pneumatic or servo pinch roll pressure, knurled measuring wheels, and closed-loop feed correction (comparing command vs feedback pulse delta) reduce cumulative length drift.

5. VFD Main Drive Control

The main roll forming motor runs through a VFD for soft start, adjustable line speed, and overload protection during threading. The PLC sets speed reference (analog 0–10 V or fieldbus word), run command, and fault reset. Acceleration and deceleration ramps prevent strip snap at coil splice or when entering the first forming stands.

5.1 VFD Integration Points

• Speed reference from HMI recipe or analog potentiometer backup.
• Load monitoring: over-torque may indicate jammed strip or tooling crash.
• Safe torque off (STO) wired from safety relay or safety PLC output.
• Fieldbus status words for ready, running, fault code to HMI.

Dallan’s fully electric roll forming reference with Siemens SINAMICS S120 documents common DC-link energy exchange: braking energy from flying shear deceleration can feed other axes, reporting measurable energy reduction versus hydraulic stop-cut systems in published case data (34% savings cited for a 120 m/min comparison project).

6. Servo Punch and Flying Shear Synchronization

6.1 Servo Punch Feed

When hole pitch tolerance or high punch frequency exceeds encoder-only open-loop feed capability, a servo-driven pinch roll indexes strip before each punch stroke. Machine Matcher notes closed-loop servo feeding compensates slip and acceleration effects at elevated line speeds. The PLC coordinates punch press fire with feed-complete signal and blocks shear until punch cycle finishes if collision geometry requires.

6.2 Flying Shear Principle

Flying shear (flying saw) matches cutter carriage speed to strip speed, executes cut while moving, then returns to home for the next cycle without stopping the line. Siemens FlyingSawBasic application supports cut-to-length using external encoder only (leading axis set NULL, masterSelectExtEnc true) for roll forming where induction motor drives forming rolls and servo moves cutter horizontally.

6.3 Delta E-CAM Alternative

Delta ASDA-A2 servo drives implement electronic cam with up to 720 points for flying shear and rotary cut applications per product documentation. In distributed architectures, the drive executes cam profile while a Delta DVP PLC or minimal controller sets length and enable. Food packaging solution literature lists flying shear as a standard E-CAM application alongside DOP HMI and MS300 VFD.

7. Safety PLC and Interlocks

Automated roll forming lines with guarded infeed, punch zones, and flying shear require safety-rated stop functions per machinery directive (ISO 12100) and regional codes. Architecture options:

• Dedicated safety PLC (e.g., Siemens F-CPU segment) running safety program separate from standard logic.
• Standard PLC with safety I/O modules certified for SIL/PL targets.
• Hardwired E-stop chain to contactors and VFD STO, with PLC monitoring aux contacts.

Machine Matcher recommends safety PLC separation on high-speed automated lines so protective logic remains independent of non-safety program changes during recipe tuning.

8. Platform Examples (Delta, Siemens, Mitsubishi)

Multiple vendors supply PLCs used on roll forming equipment worldwide. Selection depends on spare parts regional availability, integrator skill, motion features, and HMI pairing.

These brands appear as examples in machine specifications; they are not exclusive options. Retrofit projects often retain existing motor and drive hardware while replacing obsolete PLC and HMI with current models, preserving I/O mapping where possible.

9. Commissioning and Fault Diagnosis

Commissioning sequence: verify I/O wiring and encoder direction, calibrate pulses-per-meter, jog VFD at low speed, thread strip, single-shot cut at low speed, then ramp speed and tune flying shear cam or servo gains. Document pulse-per-meter value per measuring wheel wear schedule.

Common faults: encoder direction inverted (length counts down), missed cut from scan overrun (move compare to hardware), VFD ground noise causing counter glitches (shielded cable, separate PE paths), hydraulic punch firing outside window from debounce omission. Siemens forum troubleshooting for flying shear roll forming lists encoder signal quality, servo tuning, PLC communication delay, and mechanical backlash as primary length error sources.

Pair PLC diagnostics with HMI alarm history (see related HMI article) so operators report fault codes that maintenance can trace to I/O addresses or drive parameters without opening the cabinet on every stop.