Automatic inter-frame alignment monitoring for sheet manufacturing applications

Abstract

Contour alignment with a cross-correlation is employed to determine alignment between two measured contours obtained at different locations along the sheet of material. Aligning the two contours in sheet manufacturing is crucial so that the coating weight on the sheet can be calculated. The coating weight profile is calculated by subtracting the weight of the pre-coating sheet from the total weight of the coated sheet measured after coating. When the profile is not correctly aligned for this subtraction, the coating weight profile will be inaccurate, and corrective control actions for significant deviations in the coating weight may be applied to incorrect portions of the sheet and may lead to poor quality. When new measurement data becomes available, contour misalignment is monitored by repeatedly performing cross-correlation analysis. The offset is tracked and displayed over time. An automatic alarm is triggered when the offset deviates from zero and / or the value changes (within a particular threshold).

Description

[0001] Citation of Related Applications

[0002] This application claims priority under 35 U.S.C. §119(e) to co-pending application No. 63 / 522,252, filed on June 21, 2023, which is incorporated herein by reference. Technical Field

[0003] The present invention relates generally to quality control techniques for making sheet materials, and more particularly to an automatic monitoring system that determines the level of in-line measured profile misalignment between scanners employed in a continuous sheet production process. Background Art

[0004] In-line measurements are used to detect the properties of sheet materials during manufacturing to enable rapid control of the sheeting process and thus ensure sheet quality while reducing the amount of reject sheet material produced. One of the major complications in making in-line measurements during sheeting is that the physical properties of sheet materials typically vary in the machine direction as well as the cross direction. ("Machine direction" refers to the direction in which the sheet material travels during manufacturing, and the term "cross direction" refers to the direction across the surface of the sheet perpendicular to the machine direction).

[0005] To detect changes in the sheet material, a scanning sensor is employed which periodically traverses back and forth across the sheeting machine in a cross direction while detecting the value of a selected sheet property (such as basis weight or card thickness) along each scan. In practice, the measurement information provided by the scanning sensor is typically aggregated after each scan to provide a "profile" of the detected sheet property in the cross direction. In other words, each profile consists of a series of sheet measurements at adjacent positions in the cross direction. The purpose of the profile is to allow easy detection of cross-directional changes in sheet properties. The property measurements are averaged over a finite size grid of cross-directional segments. Maps showing scanner measurements are typically divided into points or grids spanning the width; for example, each grid may represent a distance of approximately 5 mm.

[0006] In papermaking and lithium-ion battery electrode manufacturing, multiple scanners located at different positions along the machine direction are often used. Each scanner generates a machine-crossing measurement profile on the production line. The measurement profile from each scanner can include hundreds of measurements covering discrete areas of the sheet from one side to the other. For measurement and quality control purposes, the profile alignment must be maintained for all scanners.

[0007] Scanner misalignment also impacts the tracking of anomalies from the beginning of the production line through to the end of the line. If the sheet being manufactured is coated, the misaligned profile can lead to inaccurate coating weight estimates 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 performed when necessary, but this can require a high level of domain knowledge and is typically only performed when a problem is discovered. In addition, poor sheet quality can persist or go undetected because manufacturers are unaware that misalignment is the root cause. The industry needs an automated alignment monitoring system that can immediately notify manufacturers when misalignment occurs. Summary of the invention

[0008] The present invention is based in part on the recognition that a given grid at one scanner device should correspond to the same grid at all other scanner devices; both representing the same discrete portion of the sheet. Due to variations in process operation, the alignment of the profiles may vary over time, such that the profiles from one scanner device may be better aligned if the alignment is offset by a certain number of grids or corresponding physical distance. Scanner misalignment can lead to problems such as poor cascade control, as downstream control loops adjust the wrong portion of the target profile of upstream control loops.

[0009] By 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. When the offset deviates from zero and / or changes in value (within a certain threshold), an automatic alert to plant personnel is triggered.

[0010] In one aspect, the present invention is directed to a method of operating a sheeting system that produces a continuous sheet moving in a machine direction (MD), the sheeting system comprising one or more actuator beams, wherein each beam comprises a plurality of actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuator beams at a first position along the MD, and a second sensor positioned downstream of the first sensor at a second position along the MD, the method comprising:

[0011] (a) measuring sheet properties of a continuous sheet with a first sensor and generating a first sheet property profile, and recording the first sheet property profile in a database of historical first profiles (such as a cloud);

[0012] (b) measuring sheet properties of the continuous sheet with a second sensor and generating a second sheet property profile;

[0013] (c) comparing the second sheet material characteristic profile to one or more first profiles from the database to determine alignment information; and

[0014] (d) Repeating steps (a), (b) and (c). The sensor is preferably a scanning sensor and the technique employs cross-correlation analysis.

[0015] In another aspect, the invention relates to a system for monitoring the formation of a sheet of traveling material moving in a machine direction (MD), the system comprising:

[0016] a first device configured to measure a first profile of a sheet of material at a first location;

[0017] a second device configured to measure a second profile of the sheet of material at a second location, the second location being downstream from the first location; and

[0018] Means for comparing the first contour to the second contour to determine alignment between the first contour and the second contour.

[0019] In another aspect, the present invention is directed to a method of operating a sheeting system that produces a continuous sheet moving in a machine direction (MD), the sheeting system comprising one or more actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuators at a first position along the MD, and a second sensor positioned downstream of the first scanning sensor at a second position along the MD, the method comprising:

[0020] (a) measuring sheet properties of a continuous sheet with a first sensor and generating a first sheet property profile;

[0021] (b) measuring sheet properties of the continuous sheet with a second sensor and generating a second sheet property profile; and

[0022] (c) comparing the first sheet material characteristic profile to the second sheet material characteristic profile to determine alignment information.

[0023] In one application of the present invention, profile data from a sheet manufacturing production line is sent to the cloud, where it is stored in a cloud server database for analysis. The data includes profile measurements of many different variables and profile data from at least two scanners. The data monitoring service in the cloud server will regularly collect recent 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 beneficial for comparison because dry weight is very stable along the machine. Analysis can be based on individual profiles; however, averaging multiple profiles can improve subsequent analysis by reducing the effects of random noise. Additional processing will be performed, such as automatically selecting portions of the profile away from the edge so that changes in values ​​near the edge of the profile will not have a negative impact on the analysis. Other processing steps may be included. Once the profile or average profile is collected and processed, alignment analysis is performed. In a preferred embodiment of the present invention, cross-correlation is used, in which cross-correlation is calculated for many different offset values ​​between the profiles; the offset where the maximum correlation occurs is the position where the profile is best aligned.

[0024] There may be other techniques for determining alignment, such as identifying the location of the edges of the profile and using the edge locations to determine alignment. Another technique is to use the sum of the absolute errors between the profiles at different offsets. Once the alignment offset is determined, it can be used in a variety of ways. First, the offset value over time can be displayed to allow a maintenance technician to see if the alignment is correct and stable. Second, a threshold can be determined for how much misalignment is acceptable and at what point a misalignment should trigger an alarm. The threshold can be based on the number of grids or the physical distance in the customer unit. If a pair of scanners is detected to be misaligned, the operator is notified through the operator display, and the status should optionally be sent via email or text message. The alarm will include the amount (degree) of misalignment and the actions required to correct the alignment and avoid losses due to poor quality.

[0025] The automatic inter-frame alignment technology is particularly suitable for sheet manufacturing production such as lithium-ion battery electrodes, paper, plastic, metal foil, coated metal substrates or fabrics. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A filmmaking system having two scanning sensors arranged along the machine direction of a moving sheet is shown;

[0027] Figure 2 is a flow chart of the contour alignment monitoring technique;

[0028] Figure 3A and Figure 3B are the dry weight profiles from two scanners that are not perfectly aligned;

[0029] Figure 4Ashows the calculated weight alignment offset between the size press scanner and the reel scanner over a 24 hour period; and

[0030] Figure 4B The calculated weight alignment offset between the wet stock scanner and the reel scanner over a 24 hour period is shown. DETAILED DESCRIPTION

[0031] The present invention relates to a method of determining the alignment between two or more measured profiles taken at different locations along a continuous sheet of material. A preferred profile alignment monitoring technique employs 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 a coating on a sheet (such as an electrode for a lithium-ion battery) can be correctly calculated. The coating weight profile is calculated by subtracting the weight of the sheet before coating from the total weight of the coated sheet measured after coating. If the profiles are not correctly aligned for this subtraction, the coating weight profile will be inaccurate, and corrective control actions for significant deviations in coating weight may be applied to the wrong portion of the sheet; this results in poor quality.

[0032] Figure 1 A pair of scanning sensors 100 and 104 are shown, which are positioned along the machine direction (MD) of the filmmaking process. The scanning sensor 100 includes a scanner head 102, which is supported on a frame including two crossbeams and moves back and forth repeatedly in a cross direction (CD). Similarly, the scanner sensor 104 includes a scanner head 106, which is supported on a frame and moves back and forth repeatedly in the cross direction. The distance along the MD between the two scanning sensors is known, and the speed of the moving sheet 110 is known. The first scanning sensor 100 measures the property of the moving sheet 110 at a first position, and the second scanning sensor 104 measures the same property of the moving sheet 110 downstream at a second position. A controller 108 including a processor and a memory collects data from the two sensors. The controller can be coupled to a cloud storage device via a network (e.g., the Internet).

[0033] Due to the sheet speed, each scanning sensor travels diagonally across the sheet surface, with the result that the continuous scan path has a zigzag pattern in a direction perpendicular to the longitudinal edge of the sheet. In practice, the measurement information provided by the scanning sensors is typically aggregated after each scan to provide a "profile" of the detected sheet property in the cross direction. In other words, each profile consists of a series of sheet measurements at adjacent positions in the cross direction. The property measurements are averaged over a grid of limited size over the CD segment. The purpose of the profile is to allow easy detection of cross-directional variations in sheet properties. Maps showing scanner measurements are typically divided into points or grids across the width; for example, each grid may represent a distance of approximately 5 mm.

[0034] In the case of a sheeting process producing a continuous sheet of paper product, a paper material having a specific width is produced from a pulp suspension stock comprising an aqueous mixture of wood fibers and other materials. The stock undergoes various unit operations performed by actuators, which are monitored and controlled by a quality control system. The actuators include, for example, a slicer positioned along the width of the headbox (the slicer discharges the stock onto a wire screen at the wet end of the process), a sprayer and a heater positioned downstream. Papermaking is described in U.S. Pat. Nos. 9,309,625 to Backstrom and Forbes and 8,021,517 to Hughes and Tixier, which are incorporated herein by reference.

[0035] In order 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 wrinkle pattern surface features. CD control can be achieved by measuring sheet properties using one or more scanners capable of scanning the sheet. For example, each scanner can carry multiple sensors for measuring dry weight, moisture content, ash content, or any other or additional characteristics.

[0036] like Figure 1As shown, scanning sensor 104 is located downstream of scanning sensor 100. In one configuration, the two scanning sensors measure properties of the paper before and after a particular operation, such as heating, calendering, or coating. For example, the paper may be coated with a chemical coating or a plastic film. As an illustration, scanning sensor 102 measures properties of the paper, such as its weight, and produces a first profile 112 having three peaks of weight located at distances a1, b1, and c1, respectively, from the edge of the paper. Thereafter, after the coating is applied to the paper to form a coated sheet of paper, scanning sensor 104 measures the basis weight of the coated sheet of paper and produces a second profile 114 having three peaks of basis weight located at distances a2, b2, and c2, respectively, from the edge of the paper. The present invention continuously monitors the alignment of the two profiles.

[0037] It is important that the two profiles are aligned in continuous sheet manufacturing so that the weight of the coating on the sheet can be calculated correctly. The coating weight profile is calculated by subtracting the weight of the sheet before coating from the total weight of the coated sheet measured after coating. If the profiles are not aligned correctly for 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 and result in poor quality.

[0038] By the present invention, the alignment analysis is repeatedly performed as new measurement data becomes available, allowing tracking of changes in the offset over time. The results can be displayed for monitoring; automatic alerts can be sent when the offset deviates from zero and / or changes in value (within a certain threshold).

[0039] Figure 2 is a flow chart describing a method for determining alignment between two measured profiles taken at different locations along a sheet of material. In initial steps 202, 204, and 206, cross-correlation values ​​are calculated for the new profile measurements. The lag with the highest cross-correlation is selected as the offset to be used for profile alignment. Next in step 208, the latest offset is compared to the historical offset to determine whether the offset violates acceptable limits. If not, the process is repeated for another profile after an update interval 216. However, if a violation is detected, then in steps 210 and 212, an alert can be sent to factory personnel and the technician takes corrective action, such as correcting the scanner / measurement settings to align the profiles with 0 offset. Thereafter, monitoring is resumed in step 214.

[0040] 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 measures the same width, which generally extends beyond both edges of the sheet. Due to the movement of the sheet as it travels along the process, material located at a particular location on the first scanning sensor may not be located at the same location on the second scanning sensor. It is important to determine the positional differences between the locations. For these measurements, it is assumed that the sheet properties are measured at N locations across the length of the sheet, and to simplify the explanation, it is also assumed that the same number of measurements are made at the two locations. It should be understood that the technique can be easily extended to handle situations where the number of measurements between the measurement locations is different.

[0041] The cross-correlation function gives a measure of how much two signals move together as a function of the displacement of one signal relative to the other. The cross-correlation will be used to determine how best to align the two measured profiles.

[0042] The cross-covariance function of two profiles y1 and y2 is:

[0043]

[0044] The cross-correlation function is the cross-covariance normalized by the zero-lag autocovariance

[0045]

[0046] For the above equation, y1 and y2 are measured profiles of paper (or other types of sheet material) properties, such as dry weight. R{y1,y2} is the cross-covariance function between y1 and y2, indicating how much the values ​​of the two profiles move together across the width of the paper machine (or other sheet material manufacturing 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 cells in the profile). r{y1,y2} is the cross-correlation, which is the cross-covariance normalized by the zero-lagged autocovariance of the two variables y1 and y2.

[0047] When the profiles vary exactly together, the cross-correlation gives a maximum value of 1. The cross-covariance function of the two profiles is calculated to find the lag at which the cross-correlation is highest, as the shift required for optimal alignment of the profiles. The value m at which r is highest is the shift required for optimal alignment of the profiles.

[0048] In many cases, the edge of the sheet is not located at the same location on every scanner. Also, there may be unexpected and variable behavior of sheet properties near the edge. For these reasons, it is best to avoid using edge portions of the contour when determining how to best align the contour. The edge portion of the sheet is not included in the cross-correlation calculation. For a given system, there may be user-entered values ​​(start position and end position) to determine which portions of the contour are used for the alignment calculation. For illustration purposes, a safe initial / default value of starting at 20% of the sheet width and ending at 80% of the sheet width will be used unless the user overrides these values.

[0049] Figure 3A and Figure 3B are examples of two measurement profiles obtained in a paper machine. The dry weight of the paper product is measured before the size press, which is located halfway through the papermaking process. The size press applies a solution of starch material to the surface of the dry paper. The dry weight of the finished paper is then measured at the winding reel where the continuous paper is wound. The weight is measured with a beta gauge scanning sensor operating in transmission mode.

[0050] Figure 3A Shown is a measurement profile of the sheet weight (sheet weight at a location across the width of the paper machine) measured by a size press scanner (in the middle of the paper machine). Figure 3B The weight of the profiles of the paper weight (the weight of the paper at a location across the width of the paper machine) as measured by a reel scanner (towards the end of the paper machine) is shown. If the paper does not shrink or drift relative to the cross-machine direction as it moves along the paper machine, the edges of the profiles and the peaks and valleys of the profiles should be in the same position on the x-axis of each graph. The scanner alignment analysis invention determines if the profiles are indeed positioned in the same position across the x-axis or how much one profile has shifted in the cross-machine direction.

[0051] like Figure 3A As shown in FIG, the measured profile along the width of the paper fluctuates above and below the target profile. Figure 3B In Figure 1, the measured profile of the coated paper also fluctuates around the target profile of the coated paper. The two measured profiles have the same basic shape, but the peaks in the profiles are not precisely aligned at the same location (as seen in 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). In addition, the two profiles do not start and end at the same location, and the two profiles behave very differently near the edges. In this case, the algorithm focuses on the inner portion of the profile (the default inner 60%, in this case from 46 inches (117 cm) to 184 inches (467 cm)).

[0052] Figure 4Aand Figure 4B Alignment analysis between pairs of profiles over a 24-hour period is presented. Figure 4A The profile measured at the size press scanner is compared to the profile measured at the reel scanner. Plotted is the profile shift calculated by the method described above. Figure 4B The positioning of the profile measured at the wet stock scanner is compared with the positioning of the profile measured at the reel scanner. The wet stock scanner is located after the wet end of the paper machine where the pulp slurry, which is a mixture of fibers and water, is dewatered. The two graphs (curve Figure 4A and curve Figure 4B ) shows that the profiles were initially well aligned with a near zero offset for approximately the first 10 hours of the time period shown. However, in the second half of the time period, the sheet appears to have shifted in the shirt position between scanners as the graph shows a continued offset between profiles of 2.78 inches. The vertical dashed line in the middle of the graph indicates where a grade change occurred (a paper machine operation switched to making a different size of paper). The dotted boxes indicate where sheet breaks occurred (and where no data is available).

[0053] With the present invention, multiple profiles at each location can be averaged over a period of time and then a cross-correlation calculation can be performed to determine the offset between the profiles. By repeating this process at fixed time intervals, a trend of the offset over time can be derived. For best product quality, the offset should be kept close to zero. By observing the trend over time, a threshold for deviation from zero can be determined. Then, an alarm can be generated when the offset exceeds the threshold over a period of time.

[0054] While the present invention has been described in the context of a papermaking process, it will be appreciated that the present invention may be applied to other sheet making processes including, for example, plastics, fabrics, metal foils and coated metal foils, such as electrodes for lithium ion batteries.

[0055] Instead of using scanning sensors, one or more fixed sensors may be used. For example, multiple sensors may be strategically positioned along the width of the sheet 110 ( Figure 1 ). Each static sensor provides only a fixed point measurement. Multiple fixed sensors are required to cover the entire sheet in the cross direction.

[0056] Depending on the properties being monitored, the scanning sensor may be configured to operate in transmission mode or in reflection mode. For example, the upper scanner head may house a radiation source that directs a radiation beam into a moving web or sheet, and the lower scanner head houses a radiation receiver that detects radiation transmitted through the material. As the dual scanner heads advance back and forth along the CD, the sensor measures one or more properties of the web or sheet. For example, sensors operating in transmission mode are described 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, US2021 / 0382173 to Hughes et al., and US 2021 / 0262776 to Tixier and Hughes, which are incorporated herein by reference. Typically, in normal production mode, the sampling rate of the scanning sensor is in the range of one sample per 10 milliseconds to 200 milliseconds.

[0057] In the reflection mode, the upper scanner head can house both a radiation source and a detector to measure one or more characteristics of the web or sheet. Sensors operating in the reflection mode are described, for example, in U.S. Pat. No. 9,182,360, U.S. Pat. No. 8,527,212, U.S. Pat. No. 7,298,492, and US2020 / 0096308 to Hughes et al., which are incorporated herein by reference.

[0058] The camera may be affixed to the upper scanner head or the lower scanner head to capture an image of the surface of the sheet 60. The camera system is described in US Patent No. 7,695,592 to Shakespeare and Kellomaki, which is incorporated herein by reference.

[0059] The foregoing describes the principles, preferred embodiments and modes of operation of the present invention. However, the present invention should not be construed as being limited to the specific embodiments discussed. Therefore, the above embodiments should be considered as exemplary rather than restrictive, and it should be understood that variations may be made in these embodiments by those skilled in the art without departing from the scope of the present invention as defined by the following claims.

Claims

1. A method of operating a sheeting system that produces a continuous sheet moving in a machine direction (MD), the sheeting system comprising one or more actuator beams, wherein each beam comprises a plurality of actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuator beams at a first position along the MD, and a second sensor positioned downstream of the first sensor at a second position along the MD, the method comprising: (a) measuring the sheet properties of the continuous sheet with the first sensor and generating 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 with the second sensor and generating a second sheet property profile; (c) comparing the second sheet material characteristic profile to one or more first profiles from the database to determine alignment information; as well as (d) Repeat steps (a), (b) and (c). 2 . The method of claim 1 , wherein the first sensor comprises a first scanning sensor and the second sensor is a second scanning sensor. The method of claim 1 , wherein alignment information including offset data is collected over time and the offset data is displayed. The method of claim 1 , wherein alignment information including offset data is collected and an automatic alarm is triggered when the offset exceeds a threshold.

5. The method of claim 1, wherein the sheet property is the weight of the continuous sheet.

6. The method of claim 1, wherein the continuous sheet comprises paper, plastic, metal foil, a coated metal substrate, or fabric.

7. A system for monitoring the formation of a sheet of traveling material moving in a machine direction (MD), the system comprising: a first device configured to measure a first profile of the sheet of material at a first location; a second device configured to measure a second profile of the sheet of material at a second location, the second location being downstream from the first location; and Means for comparing the first contour to the second contour to determine alignment between the first contour and the second contour.

8. The system of claim 7, wherein the means for comparing the first profile to the second profile uses cross-correlation.

9. The system of claim 7, further comprising means for determining an offset value between the first contour and the second contour.

10. A method of operating a sheeting system that produces a continuous sheet moving in a machine direction (MD), the sheeting system comprising one or more actuators arranged along a cross direction, a first sensor positioned downstream of the one or more actuators at a first position along the MD, and a second sensor positioned downstream of the first scanning sensor at a second position along the MD, the method comprising: (a) measuring sheet properties of the continuous sheet with the first sensor and generating a first sheet property profile; (b) measuring sheet properties of the continuous sheet with the second sensor and generating a second sheet property profile; as well as (c) comparing the first sheet material characteristic profile to the second sheet material characteristic profile to determine alignment information.

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