An alternative to dancer arms, single-transducer ultrasonic sensors installed above the web loop monitor web tension.

As people enjoy their morning newspapers and a hot cup of coffee, many have no clue about the critical role that web control plays in allowing them to relax and catch up on current events. Without proper web tension throughout the newspaper print process, blurred images and illegible, wrinkled pages result — and that's no way to start the day.

But such nuisances are minor compared to what paper manufacturers can experience. Here, the results of poor web tensioning can be more than annoying; they can be devastating. Without fast and accurate feedback regarding paper alignment, speed, and tension, serious problems can arise. And at the speeds that webs run, things can get out of hand quickly — with paper rolls unraveling everywhere, ending in volumes of wasted material. Subsequent hours of recalibration and setup result in costly production time losses and missed deadlines. Fortunately, sensing solutions such as edge and breakage monitoring and tensioning and splice detection can help combat common web control problems.

Web tensioning

For years, engineers and technicians relied on mechanical devices such as dancer arms to monitor the sag in a web loop — with the arm's lift-and-lower movement directly proportional to the tension. But such contact-based solutions risk damage to both the dancer and rolled material; their inevitable wear and frequent recalibration requirements encouraged development of many of today's non-contact sensing technologies — specifically ultrasonic and optical methods.

Ultrasonic sensors, which rely on variations in sound propagation for material detection, are suitable for print environments. They are immune to paper dust and their accuracy is unaffected by color variations. Like dancer arms, single-transducer ultrasonic sensors installed above the web loop are an excellent way to monitor web tension. Their analog output tells the controller when the loop sags (tension lessened) or is too taught (high tension) with fraction-of-a-millimeter precision.

Optical sensors possess highly precise detection beams, useful for web break detection.

Another use of analog ultrasonic sensors is monitoring a rolled web stock's diameter (which indicates when a roll is low). Rolls with varying core thicknesses and diameters can be easily controlled by a single, center-mounted device. Because these sensors detect solid and liquid media equally well, continuous ultrasonic monitoring of ink levels is a popular application as well.

Another effective use of ultrasonic sensors in web applications involves the use of thru-beam pairs, which are dedicated emitter and receiver transducers. Ultrasonic technology in this configuration allows extremely fast response times because problems are identified within a few milliseconds, rather than seconds or minutes, to prevent immeasurable problems down the road. In addition to response time benefits, thru-beam models allow a much finer degree of material or process flaws to be detected.

Seamless production

Splices are a necessity in the creation of a bulk paper roll. However, the overlapped splice seam is not something that should be found in the finished, printed product. Splice detection is a web control function best done with an ultrasonic thru-beam sensor. During set up and calibration, paper is run between the thru-beam pairs. The sensor electronics “learn” the paper's acoustic properties during a PLC-controlled calibration procedure. By learning on the fly, the sensor takes into account any variances in paper thickness, as well as material flutter. Remote programming capability also allows the same thru-beam pair to monitor multiple rolls and materials. As a new roll is inserted, the remote teach function is activated and the sensor is immediately configured for the new sheet's properties.

Optical sensors

While ultrasonic sensors are appropriate for many web applications, they have limitations that their optical counterparts overcome. Optical sensors (also called photoeyes) possess highly focused and precise detection beams. The most basic applications solved by photoeyes include detection of sheet edges and breaks. Edge-mounted background suppression (BGS) sensors instantly identify if the web begins to skew or become distorted. The same sensor type, mounted above the web, also quickly notifies a controller if the material tears.

When pinpoint precision at high speeds is required (for example, in booklet counting applications) special fixed-focus photoeyes discern heights as small as 0.1 mm and have no problems with variances in paper gloss or color. To trigger processes such as printing, optical contrast sensors scan the outer paper edge, initiating page prints on detection of preprinted locator marks. Other color-sensing photoeyes are used to scan print marks to verify if a specific color has been applied.

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