Patent Application
20240384469
The present invention generally relates to quality control techniques for fabricating sheet materials and, more particularly, to methods facilitate correlating on-line scanning sensor readings to laboratory test results for sheet materials produced in a continuous process.
On-line measurements are used to detect properties of sheet materials during manufacture to enable prompt control of the sheetmaking processes and, thus, to assure sheet quality while reducing the quantity of substandard sheet material which is produced. One of the main complications in making on-line measurements during sheetmaking is that the physical properties of sheet materials usually vary in the machine direction as well as in the cross direction. (“Machine direction” refers to the direction of travel of the sheet material during manufacture, and the term “cross direction” refers to the direction across the surface of a sheet perpendicular to the machine direction.)
To detect variations in sheet materials, scanning sensors are employed that periodically traverse back and forth across a sheetmaking machine in the cross direction while detecting values of a selected sheet property such as basis weight or caliper along each scan. Normally, the sheet being produced is traversed from edge to edge during each scan. The time required for a typical scan is generally between a few seconds to tens of seconds depending on the cross-direction length which can be many meters. The rate at which measurement readings are provided by such scanners is usually adjustable; a typical rate is about one measurement reading every millisecond.
In practice, measurement information provided by scanning sensors is usually assembled after each scan to provide a “profile” of the detected sheet property in the cross direction. In other words, each profile is comprised of a succession of sheet measurements at adjacent locations in the cross direction. The purpose of the profiles is to allow cross-directional variations in sheet properties to be detected easily. Based upon the detected cross-directional variations in the detected sheet property, appropriate control adjustments may be made to the sheetmaking machine with the goal of reducing profile variations both in the cross direction and in the machine direction.
A scanning sensor that periodically traverses a sheet at generally constant speed cannot measure the selected sheet property at locations which are aligned exactly perpendicular to the longitudinal edges of the sheet. Because of the sheet velocity, scanning sensors actually travel diagonally across the sheet surface, with the result that consecutive scanning paths have a zig-zag pattern with respect to the direction perpendicular to the longitudinal edges of the sheet. In practice, it is typical to calculate an average of profile measurements over each scan.
In industrial papermaking machines, which are often referred as Fourdrinier machines, the scanning sensors are periodically tested but this testing process is time consuming.
The present invention is based, in part, on the recognition that correlating on-line scanning sensor readings to laboratory test results for the verification of accuracy and repeatability can be facilitated by changing the scanning parameters of the scanner so that it only scan a small portion of the sheet that is to be sampled. This feature makes the sensor's mini-scan readings more relevant and more likely to be a repeatable result. The entire dynamic correlation process achieves more reliable results in a shorter length of time.
In one aspect, the invention is directed to a method of operating a continuous sheet making system which includes a plurality of actuators that are positioned along a cross direction and a scanning sensor, which is positioned downstream of the plurality of actuators, for measuring and acquiring property data of the moving sheet of material, that includes:
In another aspect, the invention is directed to a method of correlating scanning sensor measurements to laboratory analyses that is employed in a system for manufacturing a continuous sheet product that moves in a machine direction (MD) which employs a scanning sensor that periodically traverse back and forth across the full width of the moving sheet during a production mode. The method includes:
In a further aspect, the invention is directed to a method of measuring process variables in a continuous sheet process which has a machine direction and a cross direction, which includes:
While the invention will be illustrated as being implemented in papermaking, it is understood that the invention is applicable in other continuous sheet making processes such as, for example, in the manufacturer of rubber sheets, plastic films, electrodes, metal foils, fabrics and the like.
The papermaking machine 2 includes a headbox 8, which distributes an aqueous pulp suspension uniformly across the machine onto a continuous screen or wire 30 that is moving in the machine direction (MD). Headbox 8 includes slice openings through which the pulp suspension is distributed onto screen or wire 30 which comprise a suitable structure such as a mesh for receiving a pulp suspension and allowing water or other materials to drain or leave the pulp suspension. The formation of the paper sheet 12 is influenced by a plurality of linear actuators 3 extending in the cross direction across the sheet 12 of paper being formed. Actuators 3 control the sheet's weight in the cross direction (CD). Sensors located downstream from the actuators measure the properties of the sheet. The feedstock is fed from the head box through a gap or elongated orifice 5 onto a wire section 30. The orifice or gap is a relatively narrow opening that extends across the width of the machine. Weight profile control in such an arrangement is achieved by locally adjusting the position of the slice lip across the machine with motorized linear actuators 3 to vary the dimensions of the gap or orifice immediately adjacent the actuator. As used herein, the “wet end” forming portion of sheetmaking system 10 comprises headbox 8 and wire 30 and those sections before the wire 30, and the “dry end” comprises the sections that are downstream from wire 30.
Sheet 12 then enters a press section 32, which includes multiple press rolls where sheet 12 travels through the openings (referred to as “nips”) between pairs of counter-rotating rolls in press section 32. In this way, the rolls in press section 32 compress the pulp material forming sheet 12. This may help to remove more water from the pulp material and to equalize the characteristics of the sheet 12 on both of its sides.
As sheet 12 travels over a series of heated rolls in dryer section 34, more water in sheet 12 is evaporated. A calendar 36 processes and finishes sheet 12, for example, by smoothing and imparting a final finish, thickness, gloss, or other characteristic to sheet 12. Other materials (such as starch or wax) can also be added to sheet 12 to obtain the desired finish. An array of induction heating actuators 24 applies heat along the CD to one or more of the rollers to control the roll diameters and thereby the size of the nips. Once processing by calendar 36 is complete, sheet 12 is collected onto reel 14.
Sheetmaking system 10 further includes an array of steam actuators 20 that controls the amount of hot steam that is projected along the CD. The hot steam increases the paper surface temperature and allows for easier cross direction removal of water from the paper sheet. Also, to reduce or prevent over drying of the paper sheet, paper material 14 is sprayed with water in the CD. Similarly, an array of rewet shower actuators 22 controls the amount of water that is applied along the CD.
In order to control the papermaking process, selected properties of sheet 12 are continuously measured and the papermaking machine 2 adjusted to ensure sheet quality. Typical physical characteristics of paper that are can be measured include, for example, thickness, basis weight, moisture content, chemical composition such as ash, surface roughness, gloss, caliper, color, and crepe pattern surface features. CD control may be achieved by measuring sheet properties using one or more scanners 26, 28 that are capable of scanning sheet 12 and measuring one or more characteristics of sheet 12. For example, scanner 28 could carry sensors for measuring the dry weight, moisture content, ash content, or any other or additional characteristics of sheet 12. Scanner 28 includes suitable structures for measuring or detecting one or more characteristics of sheet 12, such as a set or array of sensors. Scanner 28 is particularly suited for measuring the dry end dry weight and ash content of the paper product.
Measurements from scanners 26 and 28 are provided to control system 4 that adjusts various actuators or operations of papermaking machine 2 that affect machine direction and cross direction characteristics of sheet 12. A machine direction characteristic of sheet 12 generally refers to an average characteristic of sheet 12 that varies and is controlled in the machine direction. In this example, control system 4 is capable of controlling the dry weight of the paper sheet by adjusting the supply of pulp to the headbox 8. For example, control system 4 could provide information to a stock flow controller that regulates the flow of stock through valves and to headbox 8. Control system 4 includes any hardware, software, firmware, or combination thereof for controlling the operation of the sheetmaking machine 2 or other machine. Control system 4 can, for example, include a processor and memory storing instructions and data used, generated, and collected by the processor. CD scanner sheet property measurements are stored for future reference and/or accessed for real-time observation and analysis. Control systems for papermaking machines are described, for instance, in U.S. Pat. No. 9,309,625 to Backstrom and Forbes, U.S. Pat. No. 10,174,456 to Backstrom and Forbes and U.S. Pat. No. 10,358,771 to He et al., which are incorporated by reference.
Depending on the properties being monitored, the scanning sensors can be configured to operate in the transmissive mode or reflective mode. For instance, scanner head 54 can house a radiation source that directs a beam of radiation into a moving web or sheet 26 and scanner head 56 houses a radiation receiver that detects radiation that is transmitted through the material. As the dual scanner heads advances back and forth along the CD, the sensor measures one or more properties of the web or sheet 60. Sensors operating in the transmission mode are described, for instance, 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, US 2021/0382173 to Hughes et al. and US 2021/0262776 to Tixier and Hughes, which are incorporated herein by reference. Typically, in the normal production mode, the sampling rate of the scanning sensor ranges from one sample every 10 to 200 milliseconds.
In the reflective mode, the upper scanner head 54 can house both a radiation source and detector to measure one or more characteristics of the web or sheet 26. Sensors operating in the reflective mode are described, for instance, in in U.S. Pat. Nos. 9,182,360, 8,527,212, 7,298,492 and US2020/0096308 to Hughes et al, which are incorporated herein by reference.
Cameras can be secured to the upper or lower scanner head to capture surface images of the sheet 60. Cameras, for described, in U.S. Pat. No. 7,695,592 to Shakespeare and Kellomaki, which is incorporated herein by reference.
The moving scanner does not measure the selected sheet property at locations which are aligned exactly perpendicular to the longitudinal edges of moving sheet 60. Instead, because of the sheet velocity, the scanning device travel diagonally across the sheet surface, with the result that consecutive scanning paths have a zig-zag pattern with respect to the direction perpendicular to the longitudinal edges of the sheet.
In practice, measurement information provided by a scanning sensor is usually assembled after each scan to provide a “profile” of the detected sheet property in the cross direction. In other words, each profile is comprised of a succession of sheet measurements at adjacent locations in the cross direction. The characteristic measurements are averaged in finite sized bins of CD segments. The purpose of the profiles is to allow cross-directional variations in sheet properties to be detected easily. Maps displaying the scanner measurements are typically divided into points or bins across the width; for example, each bin can represent a distance of about 5 mm.
With the present invention, the mini-scan can be conducted anytime to correlate the on-line sensor readings to laboratory tests for verification of accuracy and repeatability. The procedure matches the paper that is being sampled in the laboratory with the corresponding readings from the sensor. In the normal scanning mode, the sensor traverses the entire sheet in x seconds (scan time). Since only a small portion of the sheet is tested, a great deal of time is spent reading paper that is not relevant to the test.
With the mini-scan, the scanning parameters are changed so that the scanner only scans the portion of the sheet that is to be sampled, thereby making all the sensor readings relevant and more likely to be a repeatable result. In practice, an operator reviews the cross directional profile and selects a CD region that exhibits relatively consistent readings. In
Mini-scans can be implemented when the take-up reel 14 (
Mini-scans can also be used to diagnose upstream actuators. For instance, as shown in
The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.
