Engineering guide for surface-sensitive coil
Bottom line: leveler stop marks are transverse witness lines or roll impressions that appear when thin or surface-sensitive strip remains stationary between loaded leveling rolls. Continuous laser loop sensing can reduce repeated production stops by converting loop-depth change into a gradual VFD speed correction. It cannot guarantee a mark-free surface or replace full-loop, tight-loop, fault and emergency-stop functions. First identify the mark, then verify the control mode, usable loop capacity, roll condition and actual material acceptance standard.
A mark that is acceptable on structural steel may become visible after polishing, anodizing, painting or high-gloss inspection. The risk increases with thin or soft strip, polished stainless steel, aluminum, copper, pre-painted coil, laminated material and parts whose visible surface cannot be reworked.
The customer task is not simply to โadd a laser.โ It is to determine whether the defect is created by stationary roll pressure, speed mismatch, a damaged contact point or contaminationโand then decide whether the material loop and control response can absorb normal downstream speed changes.
Identify Leveler Stop Marks Before Changing the Line
Leveler stop marks are also described as set marks, witness marks, leveler roll marks or roller impressions. They usually run across the strip width and appear at positions where production paused. Their spacing often follows the stop history rather than the roll circumference.
A stop mark is not automatically a scratch. When strip stops between alternating upper and lower rolls, the same contact areas remain under load. Thin, soft or highly finished material may retain a shallow transverse impression even when the rolls are clean. A short speed mismatch during stopping or restarting can add a rubbed appearance, but dwell under pressure is a separate mechanism.
| Observed defect | Typical pattern | First checks |
|---|---|---|
| Stop or set mark | Transverse line at recorded stop positions | Stop log, roll penetration, dwell time and material hardness |
| Slip or scuff mark | Short rubbed patch or directional smear | Roll/strip speed difference, VFD ramps, drive and traction |
| Continuous scratch | Long line in the feed direction | Guides, burrs, trapped chips, dirty rolls and hard contact points |
| Repeating roller pattern | Regular repeat linked to rotation | Roll contamination, surface damage, runout, bearings and drive condition |
Use a simple correlation test: mark the strip position at a commanded production stop. If the transverse defect repeatedly appears at that location, investigate dwell pressure and stop frequency before treating it as a general scratching problem.
Understand Why the Mark Appears During a Stop
Stationary strip remains under alternating bending load
A precision leveler reduces coil set and shape defects by repeatedly bending the strip above and below its yield point. During continuous movement, each strip location passes through the bending sequence. During a stop, fixed locations remain loaded between the rolls. Softer material, thinner strip, higher roll penetration and longer dwell increase the chance that the contact pattern remains visible.
Binary loop control repeats the same stop cycle
A mechanical contact arm or one photoelectric switch commonly creates a run/stop command. If the usable loop is short relative to line speed, the leveler reaches the switch limits frequently. Each cycle adds another opportunity for dwell pressure and a transient roll/strip speed difference.
The material path can create a second defect
A contact sensing rod can touch a polished surface. Tight guides, an incorrect entry angle, a burr, a trapped particle or a damaged roll can create a scratch that remains even after loop control is improved. Record whether the mark is transverse or longitudinal, stationary or repeating, and whether it appears before or after the leveler.
Compare Loop-Sensing Methods by Control Behavior
โNon-contactโ describes how the sensor observes the strip. It does not describe the resulting speed command. The important distinction is whether the controller receives one switch point, several zones or a continuous distance value.
| Method | Signal | Typical command | Engineering consequence |
|---|---|---|---|
| Contact arm or gate | Discrete switch | Run/stop or fast/slow after contact | Simple, but may cycle frequently and touch the surface |
| Single photoelectric switch | Discrete switch | Non-contact ON/OFF | Removes mechanical contact but not necessarily cycling |
| Multiple photoelectric sensors | Several zones | Fast/slow/stop steps | Smoother than one switch but still step-based |
| Laser or ultrasonic distance sensor | Continuous distance | Proportional VFD reference | Best suited to gradual correction when tuned and validated |
Laser sensing is not automatically correct for every strip. Highly reflective, dark, perforated or transparent surfaces may require a different mounting angle, filtering, target arrangement or an ultrasonic alternative. Confirm the measuring range, spot position, response time and controller-compatible output using the actual material.
Convert Laser Distance into Stable Leveler Speed
The laser does not directly read downstream machine speed. It measures loop position. The controller compares that position with a target, filters the signal and converts the error into a leveler speed reference.
The laser or ultrasonic sensor reports continuous loop distance within its verified range.
The PLC or loop controller compares the distance with the target and applies deadband and filtering.
The VFD changes leveler speed with controlled acceleration, deceleration, minimum and maximum limits.
When downstream consumption rises, the loop tends to become tight and the leveler speeds up. When consumption falls, the loop becomes deeper and the leveler slows down. A useful control band avoids reacting to every small strip vibration. Too little filtering can make the drive hunt around the target; too much filtering delays the response and consumes more buffer.
Size the Slack Loop as a Speed Buffer
The slack loop separates intermittent downstream consumption from the slower response of the upstream leveler. If a press feeder or cutting process pauses, the leveler may still deliver strip while its control system detects the change and decelerates. The loop must store that temporary difference without reaching the full-loop limit.
Required temporary storage โฅ โซ (leveler speed โ downstream strip speed) dt
Evaluate the interval from the downstream speed change through sensor response, control delay and leveler deceleration.
For a first planning check, record the leveler speed at the moment of the stop, the deceleration time and the downstream pause duration. Then verify the resulting strip reserve against available distance, loop depth and the materialโs allowable bend radius. The final geometry must be proven because strip stiffness and gravity change the usable shape.
There is no universal distance that prevents marks. Increasing spacing can add storage, but excessive sag, floor contact or an overly tight return bend creates new handling risks. Thick or high-strength strip may not form the same free loop as thin aluminum. Use the minimum bend radius and the actual material combination when defining the operating window.
The related HENLI guide on motorized decoiler soft start covers a different motion problem: limiting abrupt payoff acceleration and thick-coil surface rubbing. This page remains focused on stop impressions created in and around the leveler.
Define Normal Slowdown, Controlled Stop and Safety Stop
During a normal downstream pause, the preferred sequence is gradual: the loop becomes deeper, the distance signal changes, the controller reduces leveler speed, and the line enters a low-speed region or controlled stop before reaching its working limit. When downstream consumption resumes, the leveler accelerates within the approved ramp.
If the downstream stop lasts longer than the usable buffer time, the leveler must stop. A laser sensor reduces unnecessary full-speed-to-zero events; it does not allow the leveler to run indefinitely.
Independent limits are still required for tight loop, full loop, sensor loss, out-of-range signal, downstream-not-running faults and controlled restart. Emergency stops, guards and safety-rated devices must place the equipment in the state defined by the line risk assessment. Normal loop regulation must not bypass or weaken those functions.
Commission the System on Actual Production Material
Commissioning should prove the material, control response and finished-surface acceptance together. A stable sensor trace on a test strip does not prove that a painted or polished production coil will meet the customerโs visual standard.
Set the target loop position, working limits, sensor scaling, deadband, filtering and VFD speed range.
Run the real production rhythm, including the longest expected normal pause and controlled restart.
Correlate stop logs with strip marks and inspect after the customerโs relevant finishing or lighting condition.
Record the loop trace, leveler speed, downstream state and mark location on the same time basis. If marks remain while the leveler no longer stops, investigate roll penetration, roll finish, contamination, guides and drive condition rather than increasing control gain.
Use the Symptom to Select the Next Engineering Check
| Production symptom | Likely contributor | Evidence to collect |
|---|---|---|
| Mark only at leveler stops | Dwell pressure, excessive penetration or frequent cycling | Stop log, mark position, roll setting and material sample |
| Loop repeatedly reaches both limits | Insufficient buffer, wrong target, aggressive gain or speed mismatch | Loop trace, line speeds, ramps and machine distance |
| Loop oscillates around target | Too little deadband/filtering or delayed mechanical response | Raw distance signal, filtered signal and VFD reference |
| Longitudinal scratch remains | Guide contact, particle, burr or damaged roll | Inspect the material path from coil to exit |
| Sensor value jumps on one material | Reflectivity, perforation, angle or spot-position problem | Raw signal test using all approved surface conditions |
Prepare the Data for a Leveler and Loop-Control Review
Send the following information when reviewing an existing line, specifying a retrofit or requesting a new precision sheet and coil leveler. Unknown values should remain open engineering items instead of being replaced with a generic sensor or machine distance.
- Material grade, temper and yield-strength range
- Minimum and maximum thickness ร width combinations
- Dry, oiled, polished, coated or laminated surface
- Allowed mark standard and inspection condition
- Current leveler model, roll layout and penetration setting
- Normal and maximum line speed
- Downstream cycle, stop frequency and maximum normal pause
- Distance between leveler and downstream equipment
- Available loop depth and minimum strip bend radius
- Sensor type, mounting position, range and output
- VFD acceleration, deceleration and minimum-speed settings
- Tight-loop, full-loop and sensor-fault interlocks
- Photos or video showing the loop during a stop
- Strip samples marked with defect and stop locations
The leveler must also align with the real material entry and downstream pass line. HENLIโs guide to press-feeder material line height explains that separate alignment task. For a complete system, the decoiler, leveler, loop and NC servo feeder should be reviewed as one material path.
Frequently Asked Questions
Can laser loop control completely eliminate leveler stop marks?
No single device can guarantee a mark-free surface. Continuous loop control can reduce repeated stops and abrupt speed changes, but roll condition, penetration, guides, contamination, material properties and loop capacity must also meet the acceptance requirement.
Is a photoelectric sensor the same as laser loop control?
Not necessarily. One photoelectric switch normally provides a discrete ON/OFF signal. Continuous laser loop control measures distance across a working range and uses that value to adjust VFD speed proportionally. Multiple photoelectric sensors can provide staged fast, slow and stop zones.
Why must the leveler still stop when a laser sensor is installed?
The loop has limited storage. A long downstream stop eventually reaches the full-loop limit, while a high downstream demand can reach the tight-loop limit. Fault and emergency conditions also require defined stops independent of normal loop control.
Which materials benefit most from continuous loop control?
Typical candidates include thin aluminum, copper and stainless strip; polished or bright material; pre-painted or plated coil; laminated strip; and visible parts. Suitability must be verified using the actual finish, thickness, width and production rhythm.
Can an existing leveler be retrofitted?
Often, but the existing motor, VFD, controller, I/O, sensor mounting space, loop geometry and interlocks must be reviewed. A successful retrofit includes control tuning and validation; it is not only a sensor installation.
Reduce Stops Only After You Prove the Mark Mechanism
A reliable leveler stop-mark project starts with evidence. Correlate the defect with the stop position, distinguish pressure impressions from scratches, identify whether the current sensor is ON/OFF or continuous, and calculate whether the loop can absorb the expected speed difference.
Then tune the sensor and VFD as one control loop, retain independent operating and safety limits, and validate the finished surface on actual production material. This sequence gives the customer a defensible setup window instead of an unsupported promise that one sensor will remove every mark.
Need a Leveler and Loop-Control Review?
Send HENLI the material range, surface requirement, line speeds, stop pattern, loop dimensions, sensor information and a short operating video. Our team can review the leveler and control scope around the actual production window.
