Automatic frame-to-frame alignment monitoring for sheet manufacturing applications

Abstract

To provide an automated monitoring system for determining a level of on-line measurement profile mis-alignment employed in a continuous sheet production process.SOLUTION: Aligning two profiles in sheet production is important to be able to calculate a coating weight on a sheet. A coating weight profile is calculated by subtracting a weight of the sheet before coating from a total measured weight of a coated sheet after coating. When the profile is not properly aligned for this subtraction, the coating weight profile becomes inaccurate and a corrective control action for an apparent deviation in the coating weight may be applied to a wrong part of the sheet, resulting in reduced quality. A profile mis-alignment is monitored by repeatedly performing cross-correlation analysis when a new measurement data can be used.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority under 35 U.S.C. §119(e) to co-pending application Ser. No. 63 / 522,252, filed June 21, 2023, which is incorporated herein by reference.

[0002] FIELD OF THE INVENTION The present invention relates generally to quality control techniques for producing sheet materials, and more particularly to an automated monitoring system for determining the level of on-line measurement profile misalignment between scanners employed in a continuous sheet production process. [Background technology]

[0003] Online measurements are used to detect sheet material properties during production to enable accelerated control of the sheet making process, thus ensuring sheet quality while reducing the amount of substandard sheet material produced. One of the main problems with performing online measurements during sheet making is that the physical properties of the sheet material typically vary in the machine direction and the cross direction. (The term "machine direction" refers to the direction of movement of the sheet material during production, and the term "cross direction" refers to the direction across the surface of the sheet perpendicular to the machine direction.)

[0004] To detect variations in sheet material, a scanning sensor is used that periodically traverses back and forth across the sheet-making machine in the transverse direction, detecting the value of a selected sheet property, such as basis weight or caliper, along each scan. In practice, the measurement information provided by the scanning sensor is typically assembled after each scan to provide a "profile" of the detected sheet property in the transverse direction. In other words, each profile consists of a series of sheet measurements at adjacent locations in the transverse direction. The purpose of the profile is to facilitate detection of transverse variations in the sheet property. The property measurements are averaged over finite-sized bins of the transverse segment. Maps displaying the scanner measurements are typically divided into points or bins across the width. For example, each bin can represent a distance of approximately 5 mm.

[0005] In papermaking and lithium-ion battery electrode manufacturing, multiple scanners positioned at different positions along the machine direction are often used. Each scanner generates a cross-machine measurement profile on the production line. The measurement profile from each scanner can include hundreds of measurements covering discrete areas from one side of the sheet to the other. For measurement and quality control purposes, it is necessary to keep the profiles aligned for all scanners.

[0006] Scanner misalignment also impacts the tracking of anomalies from the beginning of the production line to the end of the line. If the sheets being produced are coated, a misaligned profile can result in an inaccurate coating weight estimate when the uncoated sheet weight profile is subtracted from the coated weight profile. Currently, sheet manufacturers must manually check scanner alignment by collecting and analyzing appropriate data. Time-consuming corrective actions are implemented when necessary, but this can require a high level of domain knowledge and is often only undertaken once a problem is discovered. Furthermore, manufacturers may be unaware that misalignment is the root cause, allowing poor sheet quality to persist or go undetected. The industry needs an automated alignment monitoring system that can immediately notify manufacturers when misalignment occurs. Summary of the Invention

[0007] The present invention is based in part on the recognition that a specified bin in one scanner device corresponds to the same bin in all other scanner devices, and both represent the same distinct portions of the sheet. Due to changes in process operation, profile alignment can change over time, and therefore profiles from one scanner device can be better aligned if the alignment is shifted by a specific number of bins or a corresponding physical distance. Scanner misalignment can cause problems such as poor cascade control, as downstream control loops operate to adjust incorrect portions of the target profile relative to upstream control loops.

[0008] In the present invention, profile misalignment can be monitored by repeatedly performing cross-correlation analysis as new measurement data becomes available. Changes in offset can be tracked over time and the data can be displayed for monitoring. Automatic alerts to plant personnel are triggered when the offset deviates from zero and / or changes in value (within certain thresholds).

[0009] In one aspect, the present invention provides a method of operating a sheet manufacturing system producing a continuous sheet moving in a machine direction (MD), the sheet manufacturing system including one or more actuator beams, each beam including a plurality of actuators arranged along a cross direction, a first sensor disposed downstream of the one or more actuator beams at a first location along the MD, and a second sensor disposed downstream of the first sensor at a second location along the MD, the method comprising: (a) measuring sheet properties of a continuous sheet using a first sensor to generate a first sheet property profile, and recording the first sheet property profile in a database (e.g., in the cloud) of historical first profiles; (b) measuring sheet properties of the continuous sheet using a second sensor to generate a second sheet property profile; (c) comparing the second sheet characteristic profile with one or more first profiles from the database to determine alignment information; (d) repeating steps (a), (b) and (c).The sensor is preferably a scanning sensor and the technique uses cross-correlation analysis.

[0010] In another aspect, the present invention is directed to a system for monitoring the formation of a moving sheet of material moving in a machine direction (MD), the system comprising: a first device configured to measure a first profile of the sheet at a first location; a second device configured to measure a second profile of the sheet at a second location downstream from the first location; and means for comparing the first profile to the second profile to determine an alignment between the first profile and the second profile.

[0011] In a further aspect, the present invention provides a method of operating a sheet manufacturing system producing a continuous sheet moving in a machine direction (MD), the sheet manufacturing system including one or more actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuators at a first location along the MD, and a second sensor positioned downstream of the first scanning sensor at a second location along the MD, the method comprising: (a) measuring a sheet property of a continuous sheet using a first sensor to generate a first sheet property profile; (b) measuring sheet properties of the continuous sheet using a second sensor to generate a second sheet property profile; (c) comparing the first sheet characteristic profile to the second sheet characteristic profile to determine alignment information.

[0012] In one application of the present invention, profile data from a sheet manufacturing production line is transmitted to the cloud, where it is stored in a cloud server database for analysis. The data includes profile measurements for many different variables and profile data from at least two scanners. A data monitoring service in the cloud server periodically collects the latest data from common measurements between a pair of scanners at appropriate intervals, such as every hour. For example, dry weight profiles from two different scanners may be advantageous for comparison, since dry weight is very stable along the machine. Analysis can be based on individual profiles. However, averaging several profiles may improve subsequent analysis by reducing the influence of random noise. Additional processing is performed, such as automatically selecting portions of the profile away from the edges so that fluctuations in values ​​near the profile edges do not adversely affect the analysis. Other processing steps may also be included. Once the profiles or average profiles are collected and processed, an alignment analysis is performed. In a preferred embodiment of the present invention, cross-correlation is calculated for many different offset values ​​between the profiles. The offset at which the maximum correlation occurs is where the profiles best align.

[0013] There may be other techniques for determining alignment, such as identifying the location of the profile edge and using the edge location to determine alignment. Another technique is to use the sum of the absolute errors between profiles at different offsets. Once the alignment offset is determined, it can be used in a number of ways. First, the offset value over time can be displayed to allow a service technician to check whether the alignment is correct and stable. Second, a threshold can be determined for how much misalignment is acceptable and at what point the misalignment should trigger a warning. The threshold can be in terms of the number of bins or the physical distance of customer units. If misalignment of a pair of scanners is detected, the operator should be notified via the operator display, and optionally, the status should be sent via email or text message. The warning includes the amount of misalignment and the action required to correct the alignment and avoid losses due to quality defects.

[0014] The automatic frame-to-frame alignment technique is particularly suitable for sheet manufacturing production such as electrodes for lithium-ion batteries, paper, plastic, metal foil, coated metal substrates, or fabrics. [Brief explanation of the drawings]

[0015] [Figure 1] 1 shows a sheet making system having two scanning sensors aligned along the machine direction of a moving sheet. [Figure 2] 1 is a flowchart of a profile alignment monitoring technique. [Figure 3A] Dry weight profiles from two scanners that are not perfectly aligned. [Figure 3B] Dry weight profiles from two scanners that are not perfectly aligned. [Figure 4A] Figure 1 shows the calculated weight alignment offset over 24 hours between the size press scanner and the reel scanner. [Figure 4B] Figure 1 shows the calculated weight alignment offset between the wetstock scanner and the reel scanner over a 24-hour period. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention relates to a method for determining the alignment between two or more measurement profiles taken at different locations along a continuous sheet of material. A preferred profile alignment monitoring technique uses cross-correlation. For example, in one application, it is important to know how to align two profiles in sheet manufacturing so that the weight of the coating on the sheet, such as an electrode for a lithium-ion battery, can be accurately calculated. The coating weight profile is calculated by subtracting the weight of the sheet before coating from the measured total weight of the coated sheet after coating. If the profiles are not properly aligned relative to this subtraction, the coating weight profile will be inaccurate, and corrective control actions for apparent deviations in coating weight may be applied to the wrong part of the sheet, resulting in poor quality.

[0017] FIG. 1 shows a pair of scanning sensors 100 and 104 positioned along the machine direction (MD) of a sheet-making process. Scanning sensor 100 includes a scanner head 102 supported on a frame including two cross beams and moving back and forth repeatedly in the cross direction (CD). Similarly, scanner sensor 104 includes a scanner head 106 supported on a frame and moving back and forth repeatedly in the CD. The distance along the MD between the two scanning sensors is known, and the velocity of a moving sheet 110 is known. The first scanning sensor 100 measures a characteristic of the moving sheet 110 at a first location, and the second scanning sensor 104 measures the same characteristic of the moving sheet 110 at a second downstream location. A controller 108, including a processor and memory, collects data from both sensors. The controller may be coupled to a cloud storage device via a network, such as the Internet.

[0018] Due to the sheet velocity, each scanning sensor moves diagonally across the sheet surface, resulting in successive scan paths with a zigzag pattern relative to the direction perpendicular to the sheet's longitudinal edge. In practice, the measurement information provided by the scanning sensors is typically assembled after each scan to provide a "profile" of detected sheet properties in the CD. In other words, each profile consists of a series of sheet measurements at adjacent locations in the lateral direction. The property measurements are averaged in finite-sized bins of the CD segment. The purpose of the profile is to facilitate the detection of lateral variations in sheet properties. Maps displaying scanner measurements are typically divided into points or bins across their width. For example, each bin can represent a distance of approximately 5 mm.

[0019] When a sheet-making process produces a continuous sheet of paper product, paper material having a specific width is produced from a pulp suspension feedstock containing an aqueous mixture of wood fibers and other materials. The feedstock undergoes various unit operations performed by actuators monitored and controlled by a quality control system. The actuators include, for example, slices arranged along the width of a headbox that discharges the feedstock onto a wire mesh at the wet end of the process, downstream sprayers, and heaters. Papermaking is described in U.S. Patent No. 9,309,625 to Backstrom and Forbes and U.S. Patent No. 8,021,517 to Hughes and Tixier, both of which are incorporated herein by reference.

[0020] To control the papermaking process, selected properties of the sheet are continuously measured, and the papermaking machine is adjusted to ensure sheet quality. Typical physical properties of paper that can be measured include, for example, thickness, basis weight, moisture content, chemical composition such as ash content, surface roughness, gloss, caliper, color, and crepe pattern surface characteristics. CD control may be achieved by measuring sheet properties using one or more scanners capable of scanning the sheet. For example, each scanner can carry a sensor to measure dry weight, moisture content, ash content, or any other or additional property.

[0021] As shown in FIG. 1 , scanning sensor 104 is positioned downstream of scanning sensor 100. In one configuration, two scanning sensors measure paper properties before and after a particular operation, such as heating, calendaring, or coating. For example, the paper may be coated with a chemical coating or a plastic film. Illustratively, scanning sensor 102 measures a paper property, such as its weight, and generates a first profile 112 having three weight peaks located at distances a1, b1, and c1 from the edge of the paper. Then, after a coating is applied to the paper to form a coated paper sheet, scanning sensor 104 measures the basis weight of the coated paper sheet and generates a second profile 114 having three basis weight peaks located at distances a2, b2, and c2 from the edge of the paper. The present invention continuously monitors the alignment of the two profiles.

[0022] It is important to align the two profiles in continuous sheet production so that the weight of the coating on the sheet can be accurately calculated. The coating weight profile is calculated by subtracting the weight of the sheet before coating from the measured total weight of the coated sheet after coating. If the profiles are not properly aligned for this subtraction, the coating weight profile will be inaccurate and corrective control actions for obvious deviations in coating weight may be applied to the wrong part of the sheet, resulting in reduced quality.

[0023] In the present invention, alignment analysis is performed repeatedly as new measurement data becomes available to allow tracking of offset changes over time. Results may be displayed for monitoring. Automatic alerts may be sent when the offset deviates from zero and / or when the value changes (within certain thresholds).

[0024] FIG. 2 is a flowchart illustrating an approach for determining alignment between two measured profiles taken at different locations along a sheet of material. In initial steps 202, 204, and 206, a cross-correlation value is calculated for a new profile measurement. The lag with the highest cross-correlation is selected as the offset used for profile alignment. Next, in step 208, the most recent offset is compared to historical offsets to determine whether the offset violates tolerance limits. If no violations exist, the process is repeated for another profile following the update interval 216. However, if a violation is detected, steps 210 and 212 can alert plant personnel, who can take corrective action, such as correcting the scanner / measurement setup to align the profiles with zero offset. Monitoring then resumes in step 214.

[0025] As an illustration of the process of the present invention, consider a profile y1 measured by a first scanning sensor and a profile y2 measured by a second scanning sensor located downstream of the first scanning sensor. Each scanning sensor typically measures the same width, extending beyond both edges of the sheet. Due to sheet motion as the sheet moves along the process, material located at one position on the first scanning sensor may not be located at the same position on the second scanning sensor. It is important to determine the positional differences between the sites. For these measurements, it is assumed that sheet properties are measured at N positions along the length of the sheet, and for simplicity, it is assumed that the same number of measurements are made at both sites. It is understood that this technique can be easily extended to accommodate cases where the number of measurements varies between measurement sites.

[0026] The cross-correlation function gives a measure of how well two signals move together as a function of the displacement of one relative to the other. Cross-correlation is used to determine how best to match two measurement profiles.

[0027] The cross-covariance function of two profiles y1 and y2 is:

[0028]

number

[0029] The cross-correlation function is the cross-covariance normalized by the autocovariance at 0 lag.

[0030]

number

[0031] In the above equation, y1 and y2 are measured profiles of paper (or other type of sheet) properties, such as dry weight. R{y1,y2} is the cross-covariance function between y1 and y2, indicating how closely the values ​​in the two profiles move together across the width of the paper machine (or other sheet-making equipment). m is the position offset between the two profiles. N is the number of data points used in the analysis (in this case, the number of bins in the profiles). r{y1,y2} is the cross-correlation, which is the cross-covariance normalized by the zero-lag autocovariance of the two variables y1 and y2.

[0032] The cross-correlation has a maximum value of 1 when the profiles co-vary exactly. The cross-covariance function of the two profiles is calculated to determine the lag at which the cross-correlation is highest, which is the offset needed to best match the profiles. The value m at which r is highest is the offset needed to best match the profiles.

[0033] In many cases, the edge of the sheet is not located in the same location on each scanner. Also, there may be unexpected and variable behavior of the sheet properties near the edge. For these reasons, it is better not to use the edge portion of the profile when determining how to best align the profiles. The edge portion of the sheet is not included in the cross-correlation calculation. For a given system, there may be user input values ​​(start and end positions) to determine which portion of the profile to use in the alignment calculation. For illustration purposes, safe initial / default values ​​of starting at 20% of the sheet width and ending at 80% of the sheet width are used unless the user overrides these values.

[0034] Figures 3A and 3B are examples of two measured profiles taken on a papermaking machine. The dry weight of the paper product was measured before the size press, which is located midway through the papermaking process. The size press spreads a solution of starch material onto the surface of the dry paper. The dry weight of the finished paper was then measured at the take-up reel where the continuous paper was wound. The weight was measured using a Beta Gauge scanning sensor operating in transmission mode.

[0035] Figure 3A shows a measured profile of paper weight (weight of paper at a location across the width of the paper machine) measured by a size press scanner (midway along the paper machine). Figure 3B shows a profile of paper weight (weight of paper at a location across the width of the paper machine) measured by a reel scanner (towards the end of the paper machine). If the paper does not shrink or wobble in the cross-machine direction as it moves along the paper machine, the profile edges and the profile peaks and valleys should be at the same location on the x-axis of each plot. Scanner alignment analysis determines whether the profiles are actually located at the same location across the x-axis or how much one profile is shifted in the cross-machine direction.

[0036] As shown in Figure 3A, the measured profile along the width of the paper fluctuated above and below the target profile. Similarly, in Figure 3B, the measured profile of the coated paper also fluctuated around the target profile of the coated paper. Both measured profiles have the same basic shape, but the peaks in the profiles do not match at exactly the same location (as seen by the peak at 142.05 inches (360.81 cm) in the size press measurement and the same peak at 141.42 inches (359.21 cm) in the reel measurement). Furthermore, the two profiles do not start and end at the same location, and the behavior of the two profiles near the edges is quite different. In this case, the algorithm focuses on the inner portion of the profile (by default, in this case, the inner 60% from 46 inches (117 cm) to 184 inches (467 cm)).

[0037] Figures 4A and 4B show an alignment analysis between pairs of profiles over a 24-hour period. Figure 4A compares the profile measured with a size press scanner to that measured with a reel scanner. Plotted is the profile offset calculated using the method described above. Figure 4B compares the profile positioning measured with a wetstock scanner to that measured with a reel scanner. The wetstock scanner is located after the wet end of the papermaking machine, where the pulp slurry, a mixture of fiber and water, is dewatered. Both plots (Figures 4A and 4B) initially show that the profiles were well aligned, with an offset near zero, for the first approximately 10 hours of the time period shown. However, later in the time period, the sheet appears to be misaligned between the scanners, as the plots show a persistent offset between the profiles of 2.78 inches. The central dashed vertical line across the plot indicates where a grade change occurred (the paper machine operation switched to producing paper with a different specification). The dotted box indicates where there was a sheet break (data was not available).

[0038] In the present invention, several profiles at each site can be averaged over a period of time, and then a cross-correlation calculation is performed to determine the offset between the profiles. By repeating this process at regular time intervals, a trend in the offset over time can be developed. For best product quality, the offset should remain near zero. By observing the trend over time, a threshold for deviation from zero can be determined. An alert can then be generated when the offset exceeds the threshold over a period of time.

[0039] Although the invention has been described in a papermaking process, it will be appreciated that the invention may also be applied to other sheet manufacturing processes, including, for example, plastics, textiles, metal foils, and coated metal foils such as electrodes for lithium ion batteries.

[0040] Instead of using a scanning sensor, one or more stationary sensors can be used. For example, multiple sensors can be strategically placed along the width of the sheet 110 (FIG. 1). Each static sensor provides only a fixed-point measurement. To cover the entire sheet in the CD direction, multiple stationary sensors are required.

[0041] Depending on the characteristic being monitored, the scanning sensor can be configured to operate in a transmission mode or a reflection mode. For example, an upper scanner head may house a radiation source that directs a beam of radiation into a moving web or sheet, and a lower scanner head houses a radiation receiver that detects the radiation transmitted through the material. As the dual scanner heads advance back and forth along the CD, the sensors measure one or more characteristics of the web or sheet. Sensors operating in a transmission mode are described, for example, in U.S. Pat. No. 9,182,360 to Tixier and Hughes, U.S. Pat. No. 8,527,212 to Hughes and Tixier, U.S. Pat. No. 7,298,492 to Tixier, U.S. Patent Application Publication No. 2021 / 0382173 to Hughes et al., and U.S. Patent Application Publication No. 2021 / 0262776 to Tixier and Hughes, which are incorporated herein by reference. Typically, in normal production mode, the sampling rate of the scanning sensor ranges from one sample every 10 to 200 milliseconds.

[0042] In reflective mode, the upper scanner head may house both a radiation source and a detector to measure one or more properties of the web or sheet. Sensors operating in reflective mode are described, for example, in U.S. Patent Nos. 9,182,360, 8,527,212, 7,298,492, and U.S. Patent Application Publication No. 2020 / 0096308, which are incorporated herein by reference.

[0043] A camera may be fixed to the upper or lower scanner head to capture an image of the surface of sheet 60. A camera system is described in U.S. Patent No. 7,695,592 to Shakespeare and Kellomaki, which is incorporated herein by reference.

[0044] The foregoing has described the principles, preferred embodiments, and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Accordingly, the above-described embodiments are to be considered illustrative rather than restrictive, and it will be understood that modifications can be made therein by those skilled in the art without departing from the scope of the invention as defined by the following claims.

Claims

1. 1. A method of operating a sheet manufacturing system producing a continuous sheet moving in a machine direction (MD), the sheet manufacturing system including one or more actuator beams, each beam including a plurality of actuators arranged along a cross direction, a first sensor disposed downstream of the one or more actuator beams at a first location along the MD, and a second sensor disposed downstream of the first sensor at a second location along the MD, the method comprising: (a) measuring sheet properties of the continuous sheet using the first sensor to generate a first sheet property profile and recording the first sheet property profile in a database of historical first profiles; (b) measuring sheet properties of the continuous sheet using the second sensor to generate a second sheet property profile; (c) comparing the second sheet characteristic profile with one or more first profiles from the database to determine one or more offset values ​​between the first and second sheet characteristic profiles; (d) comparing the one or more offset values ​​to a previous offset value to determine whether the one or more offset values ​​exceed a threshold, the previous offset value being determined based on a historical first profile; (e) sending an alert to a user based on the one or more offset values ​​exceeding the threshold; (f) repeating steps (a), (b), (c), (d) and (e).

2. 1. A system for monitoring the formation of a moving sheet of material moving in a machine direction (MD), comprising: a first device configured to measure a first profile of the sheet at a first location; a second device configured to measure a second profile of the sheet at a second location downstream from the first location; means for comparing the first profile to the second profile to determine one or more offset values ​​between the first profile and the second profile; means for comparing the one or more offset values ​​to a previous offset value to determine whether the one or more offset values ​​exceed a threshold, the previous offset value being determined based on a historical first profile; and means for sending an alert to a user based on the one or more offset values ​​exceeding the threshold; Including, the system.

3. The system of claim 2 , wherein the means for comparing the first profile to the second profile uses cross-correlation.

4. 1. A method of operating a sheet manufacturing system that produces a continuous sheet moving in a machine direction (MD), the sheet manufacturing system including one or more actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuators at a first location along the MD, and a second sensor positioned downstream of the first sensor at a second location along the MD, the method comprising: (a) measuring sheet properties of the continuous sheet using the first sensor to generate a first sheet property profile; (b) measuring sheet properties of the continuous sheet using the second sensor to generate a second sheet property profile; (c) comparing the first sheet characteristic profile to the second sheet characteristic profile to determine one or more offset values ​​between the first and second sheet characteristic profiles; comparing the one or more offset values ​​to a previous offset value to determine whether the one or more offset values ​​exceed a threshold, the previous offset value being determined based on a historical first profile; sending an alert to a user based on the one or more offset values ​​exceeding the threshold; A method comprising:

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