Web processing untangled

April 1, 2010
Understanding tension control — and how to monitor it — is the key to keeping complex web processes in working order.

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.

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.

Continue to next page.

All models have dynamic teach-in functions that allow on-the-fly configuration as roll materials and jobs are changed. Another benefit: In safety-critical areas, where threat of human injury exists, optical grids are available to shut down processes if security is breached.

Emerging web trends

As performance standards continue to rise, data is required from increasingly difficult-to-access locations within machinery. This includes areas with rotating parts, where the movement precludes sensor wiring. As a result, demand for wireless sensors is on the rise. Another area where technology is changing involves roll speed control, a task traditionally accomplished by monitoring pulses from incremental rotary encoders. Mechanically coupled to the roll, rotary encoders do a great job of monitoring rotational speed. However, over time, mechanical linkages loosen and wear, causing inaccuracies.

The up-and-coming technology that promises to solve this problem is the optical mouse, which functions using the same basic principal as a desktop computer mouse. An optical mouse used in web processing monitors rotational roll speed without contact or couplings, for an exceptionally long lifespan. Today's optical mouse isn't yet ready for extremely fast web speeds, but ongoing technology advancements mean high-speed designs aren't too far off.

For more information, call (330) 486-0001 or visit

Magnetic particle clutches, brakes offer clean torque control

Many web tensioning processes and other converting applications require smooth and adjustable torque while maintaining a clean and dust-free environment. For these processes, magnetic particle clutches and brakes are often a suitable solution. These clutches and brakes used for tension control or torque limiting are electromagnetically actuated and consist of input and output members with a cavity between them. Within that cavity are magnetic particles, which are basically very small metallic particles. An electromagnetic coil is built into the input to the unit.

When dc power is applied to the coil, it creates magnetism — thereby causing the magnetic particles to group together within the cavity and thus connect the input and output. The strength of that connection is based on the amount of power applied to the coil and therefore the amount of magnetism generated. For example, full power causes particles to group together as a solid mass, while low power allows particles to slip against each other.

Several features of magnetic particle designs make them a wise choice in web processing applications. Because the particles are contained within the cavity, there are no wear particles as might exist in a clutch or brake with a friction design. Processes for medical products such as sterile bandages or pharmaceuticals benefit from the clean operation of these magnetic particles designs. Processing of electronic or optical products can also benefit from this dust-free feature.

What’s more, the ability to engage across a wide range of input power makes magnetic particle designs excellent for applications such as tension control or torque limiting where a less than fully locked-up condition is an advantage. Frictional units can suffer from a stick-slip behavior when low engagement force and low speed are applied. In magnetic particle clutches and brakes, the ability of particles to slip against each other eliminates this behavior, even down to single-digit rpm’s.

The primary constraint on magnetic particle units is that they must be sized to handle both the torque and heat dissipation of their application. Because many are used in tension applications where a constant slip is occurring, heat can be a constant feature and excessive heat can degrade the magnetic capacity of the particles.

Sizing a unit is a relatively simple process: Torque is determined as it would be for any product: T = hp x 63,025 rpm. In its simplest form, heat can be calculated as Watts = 0.0118 x torque x slip rpm. Selecting a unit that meets both criteria ensures a long performance life. Beyond that, selection is simply a matter of selecting the appropriate voltage of coil and unit bore size for the application.

For more information, contact Greg Cober, tension specialist at Warner Electric, an Altra Industrial Motion company, (815) 389-6423.

Product gallery

Dual rate meter

The PAXDR Dual Rate Meter provides real-time viewing of dual input rates, and sophisticated math functions to measure and display the sum, difference, ratio, draw, or percentage of total between the two rates. PAXDR is a 5-digit dual rate indicator and 6-digit dual totalizer that monitors factors such as unwind and rewind wire tension, feed and tension roll speed, and other production rates. Within its 1/8 DIN housing, the new meter is a single, easy-to-use device that simplifies dual-rate applications — without needing any complex programming that would otherwise be required to perform application-specific calculations.

Red Lion Controls Inc.
(717) 767-6511

Vision software

VisionPro Surface, software for material inspection, combines visual defect detection and classification technology with a simple user interface to enable accurate surface texture assessment during manufacturing processes. Unlike traditional inspection technologies that use signal processing to detect defects, this software works by monitoring the material’s visual appearance. Using statistical analysis, it automatically identifies potential defects in the surface and classifies them into groups based on similarity in contrast, texture, or geometry. During the training phase, users adjust sensitivity for defect detection and assign names or values to distinguish between different defect types. During production, the system automatically classifies each defect according to the designer’s categories.

Cognex Corp.
(877) 264-6391

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