Micro measurements; macro machinery

Mark Willliamson

Micrometer scale optical measurements of paper caliper and refined fiber must be reconciled with the massive, energy-intensive machines they are intended to control. Although there are quite different measurement technology paths used by suppliers, all roads may lead to Rome from a control and quality management point of view.

Micrometer-scale optical measurements are now being used in two diverse pulp and paper applications that require about the same degree of ultra-sensitivity. These microscopic measurements are used to control calenders and refiners -massive rotating metal machinery that consume a lot of energy. It might seem like a sledgehammer type of control based on such fine measurements but the mechanical energy and heat transfer is accomplished with good results.

First off the mark, scanning optical sensors now provide caliper measurements for CD control applications. Long serving magnetic sensors –although remarkably precise- sometimes caused sheet breaks, suffered from sticky buildup and marked sensitive coated or SC sheets. As a result, some magnetic caliper sensors could not be used continuously or were excluded from some applications. With the new optical technology those problems have been solved, opening up the number of viable caliper control applications.

At the fiber supply end of the papermaking process, recently introduced fiber fibrillation and even finer crill measurements are promising to define the fiber to fiber bonding potential during process operating time. This gives operators a new fiber quality target to aim for. The goal is to get the most consistent fiber bonding and paper end-use properties.

A grain of salt

Suppliers have developed specialized and considerably different technologies to meet these needs and are naturally promoting their own unique solutions as the best. Which one to choose can be puzzling. Some of the competitive arguments, although they are logical and scientific, need to be taken with a grain of salt from a pragmatic point of view. Potential customers have to evaluate how they relate to the macro control techniques of the pulping, refining and papermaking process.

Two camps

Optical caliper sensors are now well established, with four QCS suppliers in the market, but the debate continues about the best detection technology for the paper thickness profile. In this regard, the suppliers are divided into two camps; laser triangulation and confocal. The discussion centers around the light scattering effects of the fiber matrix in the paper sheet and its effect on the reading. It's believable that the light scattering effect is a function of the relative surface roughness or topography. On the other hand, it is not relevant to building a representative profile for CD caliper control.

Sub micrometer scale paper surface measurements are being made by optical caliper sensors, but how does that relate to the macro scale papermachine CD controls with calender profiler zone widths of 6 to 7.5 cm?
The scale of the fiber matrix with its complex topography and fiber to fiber bridges is a far cry from the reality of how a caliper profile is built up for automatic control. A CD profile from either a magnetic or an optical sensor is constructed from typically 1 cm wide data boxes to determine what control actions are required across a control zone that is 6 to 7.5 cm wide. The microscopic scattering effects that are debated may be a red herring effect when it comes to practical control needs and what it takes to build a well structured reel of paper.

Lay of the land

To be effective for building a reel, the CD control is based on the broad view "lay of the land" profile of a paper sheet –somewhat like a land elevation survey- taking into account the hills, valleys and ridges, not the detailed and complicating topography of all the trees in the forest. Resolving CD caliper streaks is important, of course, but not at the micrometer level in the cross-direction. The key is to measure micro changes in elevation between the profile points. Using this logic, you may conclude that the laser triangulation and confocal techniques are about the same for CD control. They both demonstrate the required sensitivity to sheet surface elevation changes when compared to the traditional magnetic sensors that are still the standard for comparison. There are more relevant and important ways to compare the performance of the competitive measurements.

The real challenge is to maintain repeatability in the presence of normal mechanical tolerances of a scanning sensor platform and sheet flutter. Even normal variations can be real killers since they are larger than the measurement itself. With these disruptions, it's a hard measurement to make, confides one supplier. But it's being done and there is ample proof. Suppliers make these compensations in different ways, but in the end they can show that optical techniques can replicate traditional magnetic caliper profiles or off-line tests. It looks like all roads lead to Rome.

However, the ultimate test is to maintain that level of performance over the long run and that is where good references are essential to make a decision. There are plenty of references that a potential customer could and should check out. At the time of publication of the article "Optical caliper sensing comes of age" in the May, 2011 issue of Pulp and Paper International Magazine (PPI) there were 85 installations of optical caliper sensors. There are many more three years later.

CD caliper control is based on the hills and valleys of the paper sheet much like the landscape on the left, not the forest of fibers it is comprised of.

Fibrillation vs. crill

More recently, online fiber analysis measurements have the capability to resolve the fine details of fiber peeling or roughening effects in chemical pulp, TMP and LC refined pulp. Like caliper sensors, these measurement techniques have taken two paths unique to the suppliers: high resolution image analysis and an optical phase contrast method that uses both UV and IR light. The first technique measures fiber fibrillation and the second one resolves what is called crill. Crill, which is liberated during pulping and refining processes, can be much finer than fibrillation. It is typically 0.25 microns in width. Both measurements are described in more detail in the recent Paper Advance blog "Do new measurements foretell tailor-made fibers" published on April 23, 2014.

The purpose of these measurements is to give operators some guideline as to how they should adjust the process to get the right fiber properties for end use physical tests. These micro-scale fiber fibrils or crill are created by macro-scale pulping and refining equipment that consume a considerable amount of energy. It is believable that as fibrillation is increased by applying more refining energy crill increases in tandem; the degree of change or gain may be the main difference. It's an open question right now.

These online measurement applications are still being evaluated in mills and the results are just starting to become apparent. Pulp makers hope to gain some control over fiber bonding and perhaps tailor make pulps according to end-user specifications. Whether it's a fibrillation index or a crill index, operators may have another tool to get the right pulp quality as a supplement to freeness. That's the common sense that operators like. While the implementation of automatic control may take some time the measurements from these analyzers may be part of a statistical process control strategy. This application development will be interesting to follow as users gain more experience. Maybe, like optical caliper measurements, both roads lead to Rome.

Micron scale fiber fibrillation or crill content is influenced by the macro-scale refining.
Faster is better

Scanning infrared QCS sensors, although they are not in the same micron precision range, have taken some technological leaps lately so their improved frequency response allows their use a diagnostic and problem solving tools for papermaking pulsations and cyclical instability problems. A sensor's precision and response has a direct relevance to the mechanical instabilities and the fluid dynamics of the paper making process. While not useful for scanning QCS-–based control, many instabilities and maintenance problems can be uncovered and solved.

Optical sensors have an edge over nuclear sensors because the measurement response is inherently fast. In fact, many years ago a Canadian paper company used the analog signal from a first-generation opacity sensor to detect short term basis weight variations offline in their R&D lab. The current generation of infrared sensors can now measure variation in milliseconds. Coupled with spectral analysis now available in QCS', machine direction variations can be attributed to stock preparation equipment, consistency upsets, flow instabilities, paper machine rolls and fabrics. Figure 1 shows a spectral analysis of a moisture sensor signal that was attributed to an out of round coating applicator roll. Looking at the raw signal you couldn't tell what characteristic frequencies are embedded. The analysis reveals the sources.

Figure1: High speed moisture sensor analysis reveals a 23 Hz (43ms period) vibration from an out-of-round coating applicator roll. Source: Metso

In tissue applications all suppliers now make infrared fiber weight and moisture readings simultaneously, so the root causes of fiber flow and water removal instabilities can be determined independently. It's a promising tool to improve papermaking stability. And, there is room for future improvement since faster really is better.