Quality Control System (QCS)
188.8.131.52 Scanning Measurement
Most paper quality measurements in the paper machine are scanning measure¬ments. In various locations so called “scanners” with usually one upper and one lower measurement head are found (Fig. 9.2). Both measurement heads move synchronously across the paper width. One scan takes about 20 to 30 s. The paper moves much faster than the scanning measurement. While the paper is moving
e. g. 500 m the scanner moves across the web only 7 to 10 m. Thus, the scanning sensors can be seen as a system which measures paper quality in MD and which is moved slowly across the web.
During each scan hundreds of measurement values are collected. The resolution in CD is typically 10 mm. Each scan contains information of MD and CD variation of the measured paper quality. MD and CD information are used for different purposes and have to be separated from each other.
9.2 Quality Control System (QCS)
. • The stable CD profile is given by a weighted average of multiple scans. Typically the displayed CD profiles on an operator screen correspond to an average of 9 scans. Averaging is needed to suppress the MD variation and the residual variation of the process. Thus a CD profile contains the average of the last 3 to 5 min.
. • The MD profile is the time series of average values of the individual scans. Advanced methods use e. g. Kalman filters to generate a best estimate of the current MD value every 5 s, by assuming, that the CD profile has less dynamics than the MD profile.
184.108.40.206 Fixed Point Measurement
Besides scanning measurements there are also a few fixed point measurements for paper properties which are assumed to be constant across the web width. For example color measurements are sometimes fixed point measurements as color should not vary across the web width.
220.127.116.11 Basis Weight
Basis weight is measured in g m–2 and it is the total mass of 1 m2 of paper, includ¬ing all components, like fibers, fillers and water. The sensors consist of a radio¬active source and a radiation detector on the opposite side of the paper. The absorp¬tion of radioactive radiation is a measure of the total mass of the paper. Radioactive sources are usually promethium, krypton or strontium. Promethium provides the signal with the highest sensitivity and can be used for basis weights up to 250gm–2. Strontium is used only in very heavy board applications.
Nowadays detectors are either ion chambers or solid state detectors. Ion cham¬bers are gas filled tubes and require a high voltage to detect electrons. Solid state detectors use the photovoltaic effect to detect electrons.
There are also other possible ways to measure basis weight, such as X-ray tubes to generate radiation, or spectroscopic methods, but currently they cannot compete with the sensors based on radioactive radiation.
The accuracy of a basis weight measurement is typically 0.1 g m–2. To get a better understanding what this figure really means, it is good to know that the surface of the detector is smaller than 10 cm2. Thus, if a sample is fixed between the sensor heads with a size of 10 cm2, the weight of this sample is measured with a precision of 0.0001 g.
Moisture is measured in per cent of basis weight. The measurement is usually performed by analysing absorption of infrared light at three to four different wave¬lengths. Two of the wavelengths match the absorption peaks of water and fiber. The other wavelengths are taken for reference purposes. Modern moisture meas¬urements have internally four detectors to make sure that all wavelengths are measured simultaneously, whereas older measurements used only one detector and a spinning filter wheel.
For high weights above 300 g m–2 infrared radiation can no longer pass the sheet. Therefore in this case microwave radiation is used instead. The measured physical effect is that moisture slows down the speed of microwaves, which can be detected by e. g. evaluating the phase shift of the signal.
The best achievable accuracy of moisture sensors in a PM today is 0.25 % relative moisture content.
Fillers or “ash” mean the amount of inorganic material within the paper. The measurement output is either g m–2 or % of basis weight. Typical inorganic com¬ponents are:
. • clay
. • calcium carbonate (CaCO3)
. • titanium dioxide (TiO2)
Filler content is measured by absorption of radioactive radiation or by absorption of X-ray radiation. Thus the principle of filler measurement is similar to basis weight.
Radioactive radiation is monochromatic and stable. The radiation has one ex¬actly defined wavelength. Clay, CaCO3 and TiO2 have different absorption coeffi¬cients for radiation of the given wavelength. Thus,
. • for measurement with radioactive radiation, the percentage of each filler compo¬nent has to be known beforehand. With deinked pulp (DIP) an additional meas¬urement of the ratio of the different components is required. This can be carried out by using XRF (X-ray fluorescence)
. • for X-ray radiation the spectrum of the radiation source has to be tuned by filters to compensate for the different absorption characteristics of the different fill¬ers.
9.2 Quality Control System (QCS)
The accuracy of ash measurement today is typically 0.1 g m–2, similar to basis weight.
Caliper is measured in micrometers. There are three main ways to measure cal¬iper:
1. Contacting caliper measurement: Two measurement fingers contact the paper from both sides. The distance between the fingers is measured using electromag¬netic fields. This method is used when accuracy demands are high. For high speed paper machines the contacting measurement has many disadvantages:
. • At the contacting point the paper becomes polished. A small glossy stripe may be visible on the paper surface.
. • If the stock is not of best quality and if for example a small hard or sticky piece happens to be at the paper surface, the contacting sensor will tear it out. This may result in a hole in the paper or even in a web break.
. • The fingers are subject to wear as minerals in the paper grind the measurement fingers when passing by at high speed.
The accuracy of this method depends on the application and is between 1 and
1. 0.5 microns. The relative accuracy, which is needed to measure the shape of a cross profile, can be better than 0.25 microns.
2. Air bearing caliper measurement: The sensors are mounted onto flat meas-urement plates with a diameter of roughly 60 mm. Those plates are drilled, with pressurized air coming out of the tiny holes. The plates are pressed towards the paper from both sides. The distance of the plates from the paper is controlled by the air flow. The distance between the plates is measured using an electromagnetic field.
The thickness of the paper is calculated by subtracting the thickness of the air bearings from the distance of the sensor plates. As the thickness of the air bearings is not known precisely, this method is mainly used as a relative measurement to detect MD and CD profile variations of very sensitive papers, where a contacting measurement is not applicable. But also the relative accuracy is much less than the accuracy of the contacting sensor.
2. 3. Optical caliper measurement: Due to the deficiencies of the air bearing sensor and the need for a precise noncontacting caliper measurement, most sensor man¬ufacturers currently push the development of optical noncontacting measurement methods. They consist of three sensors:
. • An electromagnetic distance measurement which measures the distance be¬tween the two sensor plates on opposite sides of the paper.
. • Two optical distance measurements which measure the distance between each sensor plate and the paper.
The difficulty here is, that the paper surface is optically not well defined. The outer layer of the paper is less dense or may even be transparent. Thus optical measure¬ments “see” the paper surface as being about 5 microns beneath the real physical surface. Therefore, the accuracy of the existing optical measurements is for most applications still not sufficient.
18.104.22.168 Coat Weight
Coat weight measurement usually requires two measurements, one before and one after the coater, which are subtracted from each other. These measurements can be basis weight measurements or ash measurements. Coat weight is meas¬ured in g m–2.
Nowadays in some cases single sided infrared coat weight measurements are used, as both sides of the paper are coated simultaneously, and the paper maker wants to know the amount of coat on each side. Such infrared coat weight meas¬urements are hard to calibrate, as the infrared beam penetrates through the coat¬ing color into the paper, and the paper also contains minerals and pigments. Thus the accuracy of this kind of coat weight measurement varies a lot, depending on the application.
Color measurement is a collection of optical measurements. The basic measure¬ment is color itself and is presented to the user in a color coordinate system. A common color coordinate system is for example CIELAB, where the parameters a* and b* define the color space and L* defines the brightness.
Paper color is usually defined as the reflection of the surface of a pile of paper in defined light conditions, where all paper sheets in the pile have the same color. Thus the following difficulties arise in an on-line measurement:
. • In a paper machine there is a running web, i. e. one single layer of paper. The background of the paper is visible. Thus, the on-line measurement has to com¬pensate for the opacity of the paper. This is performed by two measurements in sequence, one with a white background, one with a black background.
. • Fluorescence transforms UV light into visible light. Thus the fluorescence effect changes the appearance of the paper e. g. in daylight. Fluorescence has to be measured separately when optical brighteners are added to the paper.
This explains why brightness and opacity measurements are usually part of a color measurement.
Gloss is a measure of the reflectance of the paper at a given angle. The measure¬ment is calibrated:
. • The appearance of black velvet corresponds to the measurement value, i. e. a gloss value of zero.
. • The appearance of black glass corresponds to the gloss measurement value of 100 %.
9.2 Quality Control System (QCS)
Other available sensors include the measurement of formation, roughness, and porosity.
Further sensors are still in the process of development and cannot be treated as standard sensors. These include sensors for fiber orientation, strength properties, etc.
However, there still remain many important paper properties which are not yet measurable on-line, and where on-line measurements will probably not be availa¬ble in the near future. Most important to note in this category is printability. A printability sensor, e. g., would be able to predict “missing dots” in a real printing test.
Some of those properties will be measurable using so called “soft sensors”. For example a soft sensor for porosity would “measure” porosity based on a calibration of about 100 process data against corresponding laboratory measurements of porosity.
22.214.171.124 Machine Direction Control
Machine direction control uses MD measurement values to control the down¬stream quality of the paper. Advanced MD controls are based on physical process models of the papermaking process. This means, mathematical models describe the physical process and are able to predict the effect of control actions on the process and on paper quality. Such model-based controls are especially essential to achieve
. • Start up control: to control the paper quality as fast as possible during start up of the machine. This is a topic for machines which restart quite often during the day, as for example off-machine coaters.
. • Coordinated speed change control: to keep quality constant even during speed changes. This requires a coordinated change of at least stock, ash, drying ca¬pacity and speed. For example: If the stock valve is turned it takes a while until the consistency change arrives at the headbox. This is due to the transport delay time between the stock valve and the headbox, and also to the mixing time in the chests. The increase in machine speed should start when the consistency at the headbox changes (Fig. 9.3). Control of ash content and the settings of the dryer section have to be done in a similar way.
. • Production maximization control: to increase the machine speed when the proc¬ess is not at the limit, for instance when drying capacity is still left, and machine speed is below a user defined maximum speed. In fact this involves a series of coordinated speed changes.
. • Grade change control: to keep the time for a grade change as short as possi¬ble.
126.96.36.199.1 Basis Weight MD Control
Basis weight controllers adjust the stock valve, and adjust the amount of blended stock to achieve a constant basis weight at the reel. Basis weight is the total weight per square meter. Thus it includes not only fibers but also water and fillers. To avoid too much coupling with other controls means mostly “basis weight oven dry”, which consists only of fiber and fillers, is controlled. Advanced basis weight controls are in fact “dry fiber weight controls”, which leave fillers and moisture to other control loops which run almost independently.
188.8.131.52.2 Moisture MD Control
Moisture controllers usually adjust a heating device, such as
. • steam pressure in drying cylinders
. • infrared radiation devices
. • valves to control the air flow in contactless air impingement drying hoods.
In some cases moisture at the reel is controlled by adjusting rewetting devices having a series of spray nozzles across the web.
184.108.40.206.3 Filler MD Control
The filler control adjusts an ash valve, to meter i. e. to add fillers within the ap¬proach flow of the paper machine. Filler control is needed because of filler varia¬tions in the blended stock, especially if DIP is used.
220.127.116.11.4 Wet End Control
Wet end control is not a classical MD control but it belongs nowadays to the category of MD quality controls as it stabilizes the process in the MD direction.
In the past quality was mainly assured by quality tests after the manufacturing process. i. e. quality controls looked only at the outcoming paper at the end of the machine.
9.2 Quality Control System (QCS)
Nowadays the process itself is the target of improvement, assuming that a good and stable process ensures good quality production by itself. So intermediate con¬trollers ensure process stability and the traditional quality controllers achieve their target more easily.
Wet end control comprises the following parts:
. • Retention control: In reality this is a white water consistency control. The amount of retention aid is controlled to ensure that the white water consistency stays constant. This means that the percentage of fibers, fines and fillers remain¬ing on the wire after the filtration process in the former stays constant. Reten¬tion control is linked to filler control, as adding fillers means that more retention aid is needed to keep retention in the forming process constant.
. • Charge control: Fixatives are added to the stock to keep the charge constant. This is important because charged particles influence the effectiveness of retention aids. Therefore retention control and charge control are frequently used simulta¬neously (Fig. 9.4).
. • Gas control: Defoamers are added to keep the air content in the stock constant at a low level. Using gas control reduces the amount of defoamer needed. Air in
the stock has many side effects, e. g. it alters the calibration of consistency meters.
. • Consistency sensor calibration: Measurement of fiber or filler consistency is one of the most demanding process measurements. Accuracy has to be very high for control purpose but consistency meters are sensitive to a lot of process parame¬ters like gas content, flow rates, etc. Therefore wet end control comprises a means to improve the calibration of consistency meters, by taking other process parameters into account. Thus the advanced consistency sensor calibration uses a soft sensor calibration technique.
. • Basis weight prediction: When a paper machine starts up, it takes a while until the paper is fed through the machine and finally passes the scanning measure¬ment device. After starting the scanner it takes another 20 to 30 s to get the first measurements. To shorten the time until on-grade production is reached, basic controls like basis weight have to start earlier.
Therefore basis weight is pre¬dicted based on wet end measurements. MD control can adjust basis weight even before the scanner starts measuring. This prediction is again realized using soft sensor technology.
18.104.22.168 Cross Direction Control
Cross directional control uses measured CD profiles to adjust actuators which are spaced across the machine width. The goal of CD controls is to ensure even quality in the CD. The challenges of CD control are:
. • The effect of an actuator adjustment can also be seen in adjacent zones. From a control point of view the zones are coupled with their neighboring zones. A spatial decoupling controller is required for good CD control.
. • Sometimes more than one actuator controls the same quality profile. For exam¬ple both a steam box and a moisturizer control the CD moisture profile at the reel. The two actuators have to be coordinated in their actions.
. • Sometimes one actuator has an effect on more than one paper quality. For exam¬ple a headbox actuator has effect on the basis weight profile and, as a side effect, also on the moisture profile.
22.214.171.124.1 CD Basis Weight Control
Actuators at the headbox are used to control the CD basis weight profile. Two principles are widely used:
. • Slice lip actuators: Linear displacement actuators with a spacing of between 75 mm and 150 mm deflect the slice lip of the headbox nozzle.
. • Dilution actuators: Motorized valves feed dilution water into the stock distrib¬utor, which is located upstream of the headbox nozzle. Typical spacing between actuators is 35 to 100 mm.
The advantage of dilution actuators is, that the headbox nozzle is not changed mechanically. Local changes in the headbox nozzle result in a nonuniform jet
9.2 Quality Control System (QCS)
velocity and jet direction. This leads to CD components in the jet flow, which widens up the response of an actuator movement and reduces the capability to control narrow streaks.
The uneven jet velocity also has negative side effects on other paper properties like the fiber orientation profile.
126.96.36.199.2 CD Moisture Control
For CD moisture control different actuators can be used:
. • Steam box actuators: A steambox in the press section is used to increase the temperature of the web and hence decrease the viscosity of the water in the web. Thus zonal heating results in a zonal effect of improved press dewatering. The dryness of the sheet entering the drying section should be as uniform as possible or the profile should have a certain shape. Deviations in CD dryness profiles before the dryer section result e. g. in uneven shrinkage, curl, overdried edges, edge cracks, paper web breaks or poor moisture profile at the reel. To ensure uniform dryness after the press, moisture profile measurement after the press is required to control the steam box CD actuator settings.
For good CD profile control capability steamboxes can have an actuator spacing down to 75 mm and quite small overlapping responses of neighboring actuators in the paper. However, as steam boxes are often used to increase production, the mean value of all CD actuators should be as high as possible. The remaining potential to change the actuators is sometimes no longer sufficient for good high resolution CD control performance. In these cases the steam box may control only long wave profile deviations, whereas the correction of short wave deviations is left to the moisturizers.
Thus, for steamboxes an actuator spacing of 150 mm or larger is in many cases completely sufficient, especially if moisturizers are also availa¬ble in the machine.
. • Moisturizers: Moisturizers with spray nozzles are used to rewet the paper to adjust the moisture CD profile. The spray nozzles can be controlled individually by a CD control computer. Rewetting for profiling purposes is usually kept as low as possible. Rewetting is in fact a waste of drying energy, if it is not needed for other reasons, like curl control. Moisturizers usually have an actuator spac¬ing of about 100 mm. A typical design is where each actuator consists of four pairs of valves and nozzles. Thus the amount of water added to a CD location is defined by which valves are opened. Thus, for the amount of water added in a given control zone, 24 = 64 different combinations can be chosen. To avoid streaks, the nozzles have a defined spray angle and a defined distance to the sheet. Spray angle and distance are chosen to ensure an overlapping spray pattern. Thus, each nozzle sprays 50 % of the water into neighboring control zones. This avoids moisture streaks in the rewetting process.
. • Air water moisturizers: Air water moisturizers are used to control droplet size and droplet speed over a wide operating range. Droplets are much finer and faster than with pure hydraulic moisturizers. This allows one to apply more water.
Air water moisturizers usually have only one nozzle per CD control zone, which is controlled continuously. This avoids the limitation to the 64 control steps of the moisturizer and gives better controllability. Additionally the CD spacing of the nozzles is much less. In some applications more than 300 nozzles are con¬trolled individually, having a CD distance of 25 mm. Air water moisturizers are used
. • in sensitive applications, where too large droplets have a negative impact on paper quality, e. g. on printability.
.• in applications, where paper is for quality reasons overdryed in the drying sec¬tion, and then rewetted e. g.
.– to control curl
.– to reduce mottling
.– to reduce two-sidedness of roughness and gloss
.– to achieve the best possible moisture CD profiles, down to a 26 deviation of
.0.1 % relative moisture.
. • Infrared heating: Zonal infrared heating corrects moisture profile deviations within the drying section and has the additional advantage of improving the drying capacity of the machine, instead of reducing it as do moisturizers. Infra¬red heaters use either gas or electric energy. Both are more expensive than dry¬ing by steam which limits the number of applications in industry. The zone width of the infrared heating system is usually around 150 mm, but half this size is possible.
188.8.131.52.3 CD Caliper Control
For CD caliper control in on-line calenders, off-line calenders or supercalenders their nip pressure is zonal controlled by local change of the roll shell diameter or shape.
• Zone-controlled calender rolls: The roll shell is internally supported by an oil cushion or a series of oil hydraulic pistons which can be controlled individually. The number of CD control zones per roll varies, usually between 8 and 60, depending on the construction. Limiting factors are for example the number of pipes inside the roll, which are needed to allow individual oil pressures in the various zones. Additionally counter zones are frequently used. They are on the opposite side of the nip. The purpose is to get more degrees of freedom in the zonal nip adjustment. The mechanics of roll shell together with nip pressure zones and counter zones are quite complex. For example, if only an edge zone is loaded, the whole shell is affected. Therefore the control is done in two steps:
1. 1. The CD control algorithm calculates the required line load profile in the nip.
2. 2. A simulation model of the roll is used to calculate the required zonal pres¬sures to achieve the line load profile. This model of the roll is generated by the machine builder and also takes safety limits into account, to avoid roll damage by profile control actions.
9.2 Quality Control System (QCS)
.• Zonal heating of rolls using eddy current: The principle is similar to eddy cur¬rent breaks. A magnetic field generates current in the moving metal roll. The current heats the roll. With increased temperature the roll diameter increases as well. The advantages of using eddy current are:
.– high energy efficiency
.– heating may have a positive side effect, if higher gloss is required at that position
.– zone spacing can be smaller than with calender rolls and is typically around
.100 to 120 mm.
The downsides are:
.– A zone cannot actively decrease the roll diameter. This limits the ability to react on small streaks in the profile.
.– The heat flow in the roll does not allow short wave profile corrections. For example, if every second zone is heated, heat also flows to the nonheated zone in between from both sides. In narrow spaced applications its temperature tends to be like that of its neighbours.
.• Zonal heating/cooling of rolls using hot/cold air: This kind of equipment is usually applied at heated rolls, to control the surface temperature profile of the roll. The advantages are:
.– Less energy consumption and less investment than eddy current actuators.
.– Compared to the eddy current solution, short wave profile corrections are much easier to achieve, as between two heated zones the middle zone can be actively cooled. Only cold air can actively decrease roll diameter i. e. between two heated zones.
.– Smaller zone spacing can be achieved due to the active cooling principle. Usual applications have 75 mm zonal spacing, 38 mm spacing is possible if the demands on caliper profile are very high.
.– If the actuator is used together with a zone controlled calender roll, it is bene¬ficial if the smallest possible zone width is used. In this case CD control activities are split: Long wave corrections are done with the calender roll, whereas the hot/cold air device takes care of the short wave deviations, which cannot be handled by the calender roll itself.
184.108.40.206.4 CD Gloss Control
CD gloss control uses actuators in on-line calenders, off-line calenders or superca¬lenders. Gloss is affected by heat, line load and surface moisture in the nip. As the CD line load and CD roll temperature also affect caliper, CD gloss control usually uses zone controllable steam boxes to change surface moisture. As steam affects the paper very locally, controllability of gloss profiles is very good. Zone spacing is not less than 150 mm, due to space restrictions in the calender, and each zone in the steambox requires its own steam chamber with a steam valve.