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Why Does the Print Mark Sensor Miss, Double-Trigger, or Cut in the Wrong Position?

A printed non-woven line can run correctly at setup and drift after speed increases, a new roll is loaded, or the background color changes. Operators often move the photoelectric eye or adjust bag length without identifying whether the fault comes from mark contrast, sensor teach-in, web flutter, encoder scaling, trigger timing, electrical noise, or print-repeat variation. The result is random cuts, wasted printed fabric, incorrect handle positions, and repeated stops. More sensitivity is not always the answer; it can make the sensor detect artwork, wrinkles, or background texture as another mark. A reliable diagnosis separates the optical signal from web mechanics and control logic. The Manufacturer or Supplier must test the actual printed roll at production speed and record the approved sensor, mark, mounting, and PLC parameters for each OEM or ODM recipe.

A stable registration system needs a high-contrast mark, correct sensor teach-in, fixed focal geometry, controlled web tension, and a verified PLC reading window. Zhengxin validates photoelectric tracking with the customer’s printed non-woven roll across the agreed speed and roll-diameter range.

This troubleshooting sequence helps application engineers distinguish a bad optical target from unstable material transport, electrical interference, or timing logic. It follows the signal from printed mark to PLC and cutting station, then converts the verified correction into a repeatable Customizable recipe instead of another undocumented operator adjustment.

Confirm Mark Contrast, Size, Color, and Quiet Zone

A contrast sensor detects the difference in reflected light between the registration mark and its background. SICK registration-sensor guidance describes selecting the color channel with the greatest remission difference. The practical target is therefore not simply a black mark; it is a stable optical contrast under the installed light source and sensor technology.

• Keep the mark free from logos, text, halftones, folds, seams, and variable artwork.
• Provide a clean background area before and after the mark.
• Specify mark width in the web direction so detection time remains adequate at maximum speed.
• Control ink density, pinholes, gloss, and treatment variation across print batches.
• Approve mark polarity and color using the actual sensor, not a visual judgment alone.

Laminated, glossy, textured, or translucent materials may require a different optical arrangement. Send printed samples to the Manufacturer before final artwork is released.

Clean, Align, Focus, and Teach the Sensor Correctly

Begin with the hardware. Isolate the machine safely, clean the lens with an approved method, inspect the mounting for vibration, and restore the specified sensing distance and angle. A reflective surface may need the sensor angled to prevent direct glare. Then complete the exact mark-and-background teach-in procedure for the installed model.

Record the sensor model, light mode, teach values, distance, angle, bracket position, and output behavior. A Supplier replacement sensor is not equivalent unless its optical and electrical characteristics are verified.

Separate Optical Detection from Web-Handling Error

If the sensor output is clean but cut position moves, inspect unwind brake, tension controller, web guide, dancer, nip pressure, roller runout, slippage, wrinkles, edge weave, and printed-repeat tolerance. A photoelectric eye reports where a mark passes; it cannot correct an elastic or wandering web by itself.

1. Mark the web and rollers to identify slippage.
2. Trend cut error against roll diameter and line speed.
3. Compare left and right web tension when the material skews.
4. Measure several consecutive print repeats before changing PLC compensation.
5. Check whether the fold or gusset frame pulls the mark laterally after detection.

Test an unprinted fixed-length mode where safe and supported. If mechanical length remains unstable without mark control, repair web transport before tuning the optical system.

Verify PLC Reading Window, Encoder Scaling, and Compensation

Modern controls do more than read a digital edge. They may open a registration window, ignore signals outside that window, filter short pulses, calculate phase error, and apply limited correction. Incorrect encoder pulses per unit, roller circumference, acceleration compensation, or mark-to-knife distance can produce a consistent offset or speed-dependent error.

Back up the PLC and HMI before changing protected parameters. OEM or ODM logic changes should be approved by the Manufacturer because registration interacts with cutting and safety sequences.

Check Wiring, Grounding, Output Type, and Electrical Noise

Verify sensor supply voltage, PNP or NPN output compatibility, connector pinout, shield termination, input common, and cable condition against the drawing. Route low-level sensor cables away from motor and ultrasonic power conductors where required. Inspect for damaged flexible cables and loose terminals that fail only during vibration.

• Compare the sensor output LED with the PLC input status.
• Measure supply at the sensor during machine acceleration.
• Check grounding and shielding without creating unauthorized parallel paths.
• Inspect connectors for ink, oil, fiber dust, and moisture.
• Use diagnostic traces or a suitable instrument only through trained personnel.

If the sensor switches but the PLC does not, the problem is electrical or logical rather than optical. Follow applicable machinery electrical procedures, including the principles of IEC 60204-1.

Validate the Recipe Across Real Production Conditions

Run the lightest and heaviest approved printed fabrics, minimum and maximum bag lengths, and the contracted speed range. Include the beginning and end of a roll because diameter and tension change. Measure cut-to-print position rather than recording only passed or failed.

Save the accepted mark artwork, sensor settings, mechanical position, timing values, and signed bag samples as one controlled production recipe.

Send Zhengxin your printed roll, mark artwork, repeat length, bag drawing, sensor video, and speed data. Our application team can diagnose the signal path and build a Customizable photoelectric-tracking recipe for your OEM or ODM production line.