A SHEET BREAK DETECTOR APPARATUS

Abstract: A sheet break detector apparatus (10) is disclosed for detecting a sheet break in a dryer section (12) of a paper machine. The apparatus (10) includes a vacuum transducer (14) which is sensitive to the vacuum level within a vacuum roll (16) of a dryer section (12). The transducer (14) detects only a sudden change in vacuum level within the vacuum roll (16) resulting from a sheet break rather than a relatively gradual change in vacuum level resulting from an operator augmented vacuum level change. A guide device is connected to the transducer (14) and responsive to the transducer (14) for redirecting the sheet to a broke chute (22) upstream relative to the transducer (14) when the transducer (14) detects the sudden change in vacuum level. The arrangement is such that excessive wind-up of the broken sheet (W) around a dryer (24) of the dryer section (12) is inhibited.

Description: PATENT APPLICATION TITLE: A SHEET BREAK DETECTOR APPARATUS Background Of The Invention Field Of The Invention The present invention relates to a sheet break detector apparatus for detecting a sheet break in a dryer section of a paper machine. More particularly, the present invention relates to a sheet break detector for particular application to a dryer section having no open draws therein. Information Disclosure Statement Particularly in the Bel-Champ™ type dryer section, it is important that in the event of a sheet break, such breakage be detected in order to avoid excessive wind-up of the sheet around a drying cylinder. Bel-Champ is a common law trademark of Beloit Corporation. The Bel-Champ™ single tier drying section has provided a means for drying alternate sides of a web of paper without subjecting the paper to the long open draws typically incorporated into a conventional two-tier, double felted dryer section. While the aforementioned Bel-Champ™ type dryer section has greatly lessened the occurrence of sheet breaks during movement of the web through the dryer section, such breakages do still occur occasionally. When a break in a Bel-Champ™ dryer section occurs, the excellent runnabiiity of the Bel-Champ™ also contributes to the increased likelihood that the web will wrap one of the dryers. The geometry of the Bel-Champ™ dryer section makes it difficult to remove the wrapped paper from the dryer if the wound layers around the drying cylinder become excessively thick. A reliable sheet break detector would sense the absence of a web and would initiate a dropping of the sheet upstream relative to the dryer section, thereby preventing excessive wrapping of a dryer cylinder. However, generally the Bel-Champ™ concept does not include any open draws. Therefore, the employment of a conventional sheet break detector is not possible. A prior art break detection method includes an optical sensor to determine if a web is present. A light emitter is located on one side of the web and a receiver on the other. When a web is running between the emitter and receiver, the passage of light is obstructed, indicating the presence of the web. When a sheet breaks, the light passes from the emitter to the receiver, indicating the absence of a web. In order to utilize the aforementioned arrangement, it is essential that an open draw exist in the dryer section. Accordingly, in the Bel-Champ™ type arrangement, a modified optical break detector has in the past been employed. More particularly, an optical sensor includes an emitter and a receiver. However, the emitter and receiver are located on the same side of the web. Such configuration utilizes reflected light rather than transmitted light in order to detect the presence of a web. More specifically, the sensor is of a type that can detect a color difference. Therefore, in order for the aforementioned method to be effective, a colored felt must be used. The sensor senses the color difference between a felt and a sheet of paper so that in the event of a sheet break, such color difference is sensed by the sensor. Nevertheless, optical sensors are sensitive to dust and the build-up of dirt on the light sensitive lenses thereof. Such build-up of dust and dirt restricts the passage of light, thus indicating the presence of a web even when the web is absent due to a web breakage. When such a false indication occurs, there can be a significant amount of paper wrapped up on a dryer before a sheet break is discovered. Cleaning the wrapped paper from the dryer cylinder leads to excessive downtime of the dryer section. In an effort to obtain a more reliable sheet break detection method, a vacuum transducer, according to the present invention, has been utilized. Vacuum sensors are not as sensitive to dust and dirt as optical transducer type counterparts. Conceptually, by sensing .the vacuum level in the Bel-Champ™ vacuum roll, it is possible to determine if a web is present. Under normal running conditions, the vacuum would be high. However, during a sheet break, the vacuum would drop to a much lower level due to the absence of the web. The problem with vacuum detection is that it becomes difficult to determine at what level the sheet break detector should trip. Machine operators during routine operation are constantly adjusting vacuum levels in the Bel-Champ™ vacuum rolls. Accordingly, if the operator raises the vacuum level above the trip point setting, a sheet break will not be detected. However, if the operator drops the vacuum below the trip point setting, a false break indication will be signalled. The present invention provides a vacuum transducer for detecting a sheet break which is insensitive to operational changes in the normal adjustments in vacuum levels of the vacuum rolls, while reliably detecting a sheet break. Therefore, it is a primary objective of the present invention to provide a vacuum transducer for detecting a sheet break in a dryer section which is sensitive to a sudden change in vacuum within the vacuum roll, rather than a relatively gradual change in vacuum level caused by routine operational adjustments. Another object of the present invention is the provision of a vacuum sheet break detecting apparatus which overcomes the aforementioned problems of the prior art arrangements and which makes a considerable contribution to the art of detecting a sheet break in a dryer section. Summary Of The Invention The present invention relates to a sheet break detector apparatus and method for detecting a sheet break in a dryer section of a paper machine. The apparatus includes a vacuum transducer which is sensitive to the vacuum level within a vacuum roll of the dryer section. The transducer detects only a sudden change in the vacuum level within the vacuum roll resulting from a sheet break, rather than a relatively gradual change in the vacuum level resulting from an operator augmented vacuum level change. The apparatus also includes means connected to the transducer and responsive to the transducer for redirecting the sheet to a broke chute or pit upstream relative to the transducer when the transducer detects the sudden change in vacuum level. The arrangement is such that excessive wind-up of the broken sheet around a dryer of the dryer section is inhibited. In a more specific embodiment of the present invention, the transducer is a piezoelectric pressure transducer which senses the vacuum level within a center shaft of the vacuum roll. In another embodiment of the present invention, the transducer senses the vacuum level within a duct which connects the vacuum roll to a source of partial vacuum. The transducer is such that during normal operation of the dryer section, when the unbroken sheet extends through the dryer section, a first output from the transducer is zero, and when a sheet break occurs, an output signal is generated which is proportional to the change in vacuum level within the vacuum roll as a result of the sheet break. Many modifications and variations of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter. However, such modifications and variations fail within the spirit and scope of the present invention as defined by the appended claims. Brief Description Of The Drawings Figure 1 is a side-elevational view of a portion of a Bel-Champ™ type dryer section showing the location of the transducer; Figure 2 is a similar view to that shown in Figure 1 , but shows how the web is directed to a broke chute or pit in the event of a downstream web breakage; Figure 3 is a schematic representation of a sheet break detector apparatus including a vacuum transducer according to the present invention; Figure 4 is a schematic representation of the sheet detector apparatus according to a further embodiment of the present invention in which the transducer detects the vacuum level within a duct; Figure 5 is a graph showing the transducer response to a steady state condition; Figure 6 is a graph showing the transducer response to a sheet break; and Figure 7 is a graph showing the transducer response to an operator augmented damper adjustment. Similar reference characters refer to similar parts throughout the various views of the drawings. Detailed Description Of The Drawings Figure 1 is a side-elevational view of a portion of a Bel-Champ™ type dryer section including a sheet break detector apparatus, generally designated 10 according to the present invention, for detecting a sheet break in the dryer section, generally designated 12. The apparatus 10 includes a vacuum transducer 14 which is sensitive to the vacuum level within a vacuum roll 16 of the dryer section 12. The transducer 14 detects only a sudden change in the vacuum within the vacuum roll 16 resulting from a sheet break rather than a relatively gradual change in vacuum resulting from an operator augmented vacuum level change. Figure 2 is a view similar to that shown in Figure 1 , but showing the operation of the apparatus 10 in the event of a web breakage. More particularly, means 18 are connected to the transducer 14 and responsive to the transducer 14 for redirecting the sheet W to a broke pit 22 disposed upstream relative to the transducer 14 when the transducer 14 detects a sudden change in vacuum level. The' arrangement is such that excessive wind-up of the broken web W around a dryer 24 of the dryer section 12 is inhibited. As shown in Figure 2, the broken web W is shown as wrapping around dryer cylinder 24 and the trailing end of the downstream broken web passing the vacuum roll 16 in order to extend around drying cylinder 26. In normal operation, a high vacuum is applied within vacuum roll 16, and a breakage of the web will cause a sudden decrease in the vacuum level within vacuum roll 16. Such sudden decrease in vacuum level is sensed by the transducer 14 which is electrically or otherwise connected to the means 18 disposed upstream relative to the group of drying cylinders 24,26. Typically, the means 18 may include a transverse web cutting device for cutting the web as it extends around a vacuum transfer roll 28. Additionally, the means 18 may include an arrangement for cutting off the vacuum to the vacuum roll 28 such that the moving web will fall to the broke pit 22 rather than extending around the vacuum roll 28 for transfer into the downstream dryer group, which includes drying cylinders 24 and 26 and vacuum roll 16. By the aforementioned arrangement, only a short length of the web from the vacuum roll 28 to the drying cylinder 24 will become wrapped around the drying cylinder 24. However, the present invention is directed particularly to the transducer 14, which senses the sudden change in vacuum level, and it will be understood by those skilled in the art that such transducer may be used for triggering a visual or audible alarm so that an operator may manually direct the web to the broke pit. Additionally, the aforementioned transducer 14 may generate a signal for cutting the web upstream of the entire dryer group or, for in any other manner, interrupting the movement of the web along the dryer section. In a preferred embodiment of the present invention, the transducer 14 is a piezoelectric pressure transducer. Figure 3 is a schematic representation showing the transducer 14 for sensing the vacuum level existing within a center shaft 30 of the vacuum roll 16. Figure 4 is a similar view to that shown in Figure 3, but shows an alternative embodiment of the present invention in which a transducer 14A senses the vacuum level within a duct 32 which connects the vacuum roll 16A to a source of partial vacuum 34. As shown in Figure 3, the transducer 14 is arranged in such a manner that during normal operation of the dryer section 12, when the unbroken web W extends through the dryer section 12, the output from the transducer is zero. When a sheet break occurs, the change in vacuum is sensed by the transducer and an output signal is generated which is proportional to the change in vacuum level within the vacuum roll 16 as a result of the sheet break. Figure 5 is a graph 36 showing the response of the piezoelectric transducer 14 to a steady state condition with the graph 36 representing the pressure against a number of time constants. Figure 6 shows four graphs 38,40,42 and 44 indicating the transducer output for various time constants in response to a sheet break. Figure 7 shows graphs 46,48 and 50 indicating the response of the transducer 14 to routine damper adjustments of the vacuum roll for various time constants. More specifically, Figure 7 shows the graphs for the pressure relative to the time of adjustment. More particularly, the output from the transducer 14 decays to zero when a new steady state condition exists, as shown particularly in the graph 44 shown in Figure 6, following the sheet break. The difference in the output signal between a fast and a relatively slow change makes it possible to detect the difference between a sheet break and damper movement. More specifically, such output signal generated as a result of a sheet break occurs within a relatively small time constant. In a preferred embodiment of the present invention, the time constant is within the range 1 millisecond to 3 seconds. After the break occurs, vacuum decreases to a new steady state condition. The output from the transducer 14 will then decay to zero following the function: Output = A # e ( - t/T) Where: A = Maximum output following step change e = base of natural logarithm t = time T = time constant The time constant is defined as the period required for the output to reach e ' \ or .368, of its original value. Figure 5 shows the graph 36 of this curve. The objective is to configure the transducer system such that it has a time constant on the order of milliseconds. In such configuration, the vacuum change occurring during a sheet break happens fast enough to be fully sensed by the transducer, as shown in the graph depicted in Figure 6. The much slower change in vacuum that occurs when an operator adjusts a vacuum damper, however, is competing with the constantly decaying output of the transducer. Given a sufficiently short time constant, the sensor will exhibit only a slight ripple in its output, as shown in Figure 7. The difference in output between a fast and a relatively slow change makes it possible to discern between sheet breaks and damper movement within the vacuum roll. A comparison of Figures 6 and 7 shows that under the conditions plotted, a time constant of .05 seconds is suitable. By adjusting the peak voltage setting in the detection circuit based on the minimum steady state vacuum level with the sheet on, vacuum levels can be manually changed by the operators without requiring a corresponding change in the detection level or without a false indication of a sheet break. During transient operations, such as sheet threading, sensing of the transducer break detector must be disabled, the same as with other more conventional detectors. The means, generally designated 18, may include a cross-machine web cutter such that the web is directed to the broke chute 22. Alternatively, the means may include an audible or visible alarm for directing an operator to the presence of a sheet break. What is disclosed is a method and apparatus for using a piezoelectric vacuum transducer to sense sheet breaks in a Bel-Champ™ type dryer section. The arrangement is more durable than the prior art optical detectors currently used and does not require an open draw to operate. Additionally, the break detector, according to the present invention, can be set up to operate without false break signals due to operator adjustment of vacuum levels. The present invention provides a reliable means for detecting a sheet break that is particularly advantageous for detecting a sheet break within a dryer section having no open draws.

Claims: What is Claimed is: 1 . A sheet break detector apparatus (10) for detecting a sheet break in a dryer section (12) of a paper machine, said apparatus comprising: a vacuum transducer (14) for sensing the vacuum level within a vacuum roll (16) of the dryer section (12), said transducer (14) detecting only a sudden change in said vacuum level within said vacuum roll (16) resulting from a sheet break rather than a relatively gradual change in vacuum level resulting from an operator augmented vacuum level change; and means (18) connected to said transducer (14) and responsive to said transducer (14) for redirecting the sheet to a broke chute (22) upstream relative to said transducer (14) when said transducer (14) detects said sudden change in vacuum level, the arrangement being such that excessive wind-up of the broken sheet (W) around a dryer (24) of the drying section (12) is inhibited. 2. A sheet break detector apparatus (10) as set forth in claim 1 , wherein said transducer (14) is a piezoelectric pressure transducer. 3. A sheet break detector apparatus (10) as set forth in claim 2, wherein said transducer (14) senses the vacuum level within a center shaft (30) of said vacuum roll (16). 4. A sheet break detector apparatus (10) as set forth in claim 2, wherein said transducer (14A) senses the vacuum level within a duct (32) connecting said vacuum roll (16A) to a source of partial vacuum (34). 5. A sheet break detector apparatus (10) as set forth in claim 2, wherein said transducer (14) is adjusted such that during normal operation of the dryer section (12) when the unbroken sheet (W) extends through the dryer section (12), an output from said transducer (14) is zero, and when a sheet break occurs, a further output signal is generated which is proportional to the change in vacuum level within the vacuum roll (16) as a result of the sheet break. 6. A sheet break detector apparatus (10) as set forth in claim 5, wherein said further output signal decays to zero when a new steady state condition exists following said sheet break, said further output signal being generated only when said change in vacuum level occurs within a relatively small time constant. 7. A sheet break detector apparatus (10) as set forth in claim 6, wherein said time constant is within the range 1 millisecond to 3 seconds. 8. A sheet break detector apparatus (10) as set forth in claim 1 , wherein said means (18) includes: a cross-machine web cutter for cutting the web; a broke chute (22) disposed beneath said cutter for collecting the cut web. 9. A sheet break detector apparatus (10) for detecting a sheet break in a single tier drying section (12) of a paper machine, said apparatus comprising: a vacuum transducer (14) for sensing the vacuum level within a vacuum roll (16) of the dryer section (12), said transducer (14) detecting only a sudden change in said vacuum level within said vacuum roll (16) resulting from a sheet break rather than a relatively gradual change in volume level resulting from an operator augmented vacuum level change; and means (18) connected to said transducer (14) and responsive to said transducer (14) for generating a signal indicating that a sheet break has occurred when said transducer (14) detects said sudden change in vacuum level, the arrangement being such that excessive wind-up of the broken sheet (W) around a dryer (24) of the single tier drying section (12) is inhibited.

Inventors: PULKOWSKI JEFFREY H, PULKOWSKI, JEFFREY, H.
Applicants: BELOIT TECHNOLOGIES INC, BELOIT TECHNOLOGIES, INC.
Application year: 1993
Main classification: D21F7
IPC codes: D21F7