System and process for measuring the strength of sheet material

Abstract: The invention relates to a system for measuring the strength of a sheet which is being produced wherein the sheet is traveling between a first roll and a second roll. This system comprises: a) load measuring means to measure the load of the sheet at a plurality of points between the first and second rolls and to generate signals corresponding to the measured load; and, b) computing means to receive the signals from said load measuring means and to determine the strength of the sheet at at least one point relative to the strength at at least one other point.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention concerns a system for measuring the strength of a sheet such as paper while the sheet is being produced. State of the Art In the manufacture of sheet material such as paper it is often important to know the strength of the sheet. For example, strength of paper can be measure in various ways including burst, tensile and pierce strengths. Normally, these strength parameters are determined by various destructive tests in a laboratory. However, on-line determination of paper strength during the manufacture of paper can aid in producing paper which can meet specified strength requirements. U.S. Patent 4,291,577 teaches a system for measuring the strength of paper as it is being produced. The patent provides for measurement of the velocity of ultrasonic waves through the moving paper sheet. Based upon the velocity, the strength of the paper is determined. The patent teaches a device having two wheels which are spaced apart from one another and which roll along the moving paper web. The first wheel contains a transducer in the form of a rectangular button mounted on the periphery of the wheel so that as the wheel rotates the button periodically contacts the paper. Each revolution of the wheel, when the transducer contacts the paper, it receives an electrical signal from a signal generator and imparts a mechanical signal to the paper. The second wheel contains a receiving transducer substantially the same as the transmitting transducer, which also is mounted on the periphery of the wheel and occupies a small percentage of the total circumference of the wheel. The receiving transducer contacts the paper once each revolution of the wheel and receives the signal from the transmitter by picking up the ultrasound signal from the paper and converting it to an electrical signal. The system also includes a position detector to monitor the rotational position of the first wheel and to trigger the firing of the ultrasound pulse by the transmitter when the wheel is in a predetermined position. The rotation of the receiving wheel is coordinated with the transmitting wheel so that the receiving transducer is in contact with the paper at the appropriate time to receive the transmitted signal. The signal from the receiving transducer is transmitted to a metering and recording apparatus which measures the velocity of the ultrasonic waves. The system described in the patent has a number of disadvantages. Since the transmitting transducer and the receiving transducer each contact the paper during only a small percentage of the total rotation of the wheels, the rotation of the wheels must be carefully synchronized and the transmission of the pulse must be carefully timed so that the pulse is received by the transmitter when it is in contact with the paper. Furthermore, since the transmitter and the receiver are only in contact with the paper for a small percentage of the total rotation of the wheels, a substantial portion of the paper is not subject to velocity measurement. Another system is disclosed in the document US-A-3 933 035. The system disclosed in this patent comprises two driving means which respectively drive a strip at two different speeds. Tension measuring devices are respectively disposed in front of the driving means and they deliver signals which are proportional to the tensions measured. A comparator receives these signals and determines their difference which is proportional to the elastic coefficient of the strip. The main disadvantage of this system is that it cannot indicate the strength of the strip at different localized areas of this strip. OBJECTS OF THE INVENTION An object of the present invention is to provide a system and process for continuously measuring the strength of a moving sheet. A further object is to provide such a system which utilizes the tension and elastic modulus of the sheet and also the velocity of the sheet to determine strength. To achieve these objects, the present invention provides a system for non-destructively determining the strength of a sheet of material being drawn under tension to travel between a first roll and a second roll, the sheet of material having a width, the system comprising: a) mounting means located to support a load sensor means for a movement across the width of the sheet while said sheet travels between the first and second rolls; b) a load sensor attached to the mounting means for contact with the sheet, said load sensor including means for controllably deflecting said sheet across a localized area between the first and second rolls, said load sensor sensing the force exerted by the sheet against the deflecting means at a plurality of localized areas between the first and second rolls and to generate signals corresponding to said force; and c) computing means for receiving and processing the signals from the load sensor to provide an indication of the strength of the sheet at one localized area relative to the strength at least at one other localized area. The present invention further provides a process for non-destructively determining the strength of a sheet which is being drawn under tension between a first roll and a second roll comprising the steps of: a) detecting a deflecting load exerted on the sheet at a plurality of localized areas between the first and second rolls; and, b) computing the relative strength of the sheet at least at one of said localized areas compared to the strength at least at one other localized areas based upon detected deflecting loads while the sheet is moving between the first and second rolls. Further objects and advantages of the present invention can be ascertained by reference to the specification and drawings herein which are offered by way of example and not in limitation of the invention which is defined by the claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of the present embodiment. Figure 2 is an illustration of part of the present embodiment. Figure 3 is a top view of the part shown in Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As an aid in understanding the invention, the paper making process is briefly described with reference to Figure 1. A more elaborate description is provided is U.S. Patent 3,757,122 issued September 4, 1973 and entitled "Basis Weight Gauging Apparatus, System and Method Using A Digital Count". With reference to Figure 1, raw stock is supplied to paper machine 2 through a stock valve 3. White water from collection reservoir 4 is drained by a line 4a, mixed with raw stock and supplied through stream valve 5 and fan pump 6 to head box 7. The pulp and white water mixture jets from head box 7 through slice 8 on top of and parallel with wire 9. Water passes through wire 9 and is collected by reservoir 4. However, the majority of fiber is left behind to form a wet sheet of paper 10. After leaving wire 9, sheet 10 is passed through press 11 consisting of a plurality of rollers 12 which removes much of the water from sheet 10. Thereafter the paper sheet passes into a dryer section 14 consisting of a plurality of rollers 15 through which steam is supplied by a steam control valve 16. The steam heats the rollers and consequently evaporates much of the water in the paper sheet so that the paper emerging from dryer section 14 has the desired moisture content. The paper passes from dryer section 14 to a calendar stack 17. The calendar stack 17 includes a plurality of calendar rollers 18, 20, 22, 24, 26 and 28. The bottom most roller, 28, is larger than the other rollers and is driven by a motor 30 which is powered by electricity from line 32. Electricity from line 32 is supplied to the motor 30 through an ammeter 34 which measures the current passing through line 32. A tachometer 35 is coupled to the motor 30 to measure the speed of the motor. After leaving the calendar rolls 26 and 28, the sheet passes through scanner 36 and is wound onto reel 40. The reel 40 is driven to rotate by a pope roll 42 which in turn is rotated by motor 44. The motor 44 receives electric current through line 46 and ammeter 48. The scanner 36 is conventional and will therefore not be discussed in detail herein. Basically, the scanner 36 includes two heads, one disposed above and the other below the sheet of paper. The scanner 36 also includes a drive system to cause the scanners to reciprocate back and forth across the moving sheet of paper. Further details of a scanner can be ascertained from U.S. Patent 3,621,259 titled "Sheet-Gauging Apparatus". Turning to Figures 2 and 3, there is shown part of an upper gauging head 50 and part of a lower gauging head 52. Both heads 50 and 52 are coupled to the scanner 36 so that they can be moved back and forth across the moving sheet of paper. A support member 54 is coupled to the upper gauging head 50 by pivot 55. The support member includes two brackets 56 mounted at its lower end, and a U-shaped bracket 58 is coupled to the brackets 56 by pivot 57. Thus it can be seen that the U-shaped bracket 58 can rotate horizontally about pivot 55 and vertically about pivot 57. A wheel 60 is mounted in the U-shaped bracket 57 by an axle 62 on bearings, not shown. Thus, the wheel 60 is free to rotate as indicated by the arrow and also to move upward and downward as the bracket 58 rotates about the pivot 57. The support member 54 includes an indentation 66 adjacent the upper part of U-shaped bracket 58. A load cell 70 is coupled to the support member 54 in the indentation 66 and also to the bracket 58. The load cell 70 is a conventional device to measure the compressive force on the load cell and generate a signal corresponding to the compressive force. Thus it can be seen that the load cell 70 measures the upward force on the wheel 60. The load cell 70 is coupled by line 71 to transmit signals to a computing system 72 mounted In the upper gauging head 50. A tachometer 74 is coupled to the bracket 58 and to the axle 62 to measure the rate of rotation of the wheel and generate a signal corresponding thereto. The signals from the tachometer 74 are conveyed to the computing system 72 by a line 76. A support 80 is coupled to the lower gauging head 52, below the sheet 10, and a sheet support member 82 is coupled to the support 80. The sheet support member 82 is substantially toroidal in shape and has a smooth, rounded upper surface. In operation, the wheel 60 is mounted above the sheet of paper 10, and the sheet support member 82 is mounted directly below the wheel, and on the opposite side of the sheet of paper. The wheel 60 and sheet support member 82 are positioned so that the sheet 10 is in contact with the upper surface of the support member 82, and the wheel 60 pushes the paper in the center of the support member 82 downwardly a predetermined distance which in practice can be about 0.2 - 0.5 centimeters, and preferably about 0.4 centimeters, below the upper edge of the support member 82 for a support member 82 about 12,7 cm (5 inches) in diameter. Thus increases and decreases in the tension and elastic modulus (i.e. the ratio of stress to strain in the paper) of the sheet 10 impart variable forces on the load cell 70. Higher sheet tension or higher elastic modulus results in higher force on the load cell 70. Also, the wheel 60 is caused to rotate as the sheet 10 moves. The computer 72 receives signals from the load cell 70 corresponding to the tension and elastic modulus in the paper and signals from the tachometer 74 corresponding to the velocity of the paper. The computing system 72 utilizes the data to determine the strength of the sheet of paper as will be discussed in more detail hereinafter. As the heads 50 and 52 scan across the sheet, the support member 54 and wheel 60 pivot slightly about pivot 55 so that the wheel 60 rolls smoothly on the sheet. The computing system 72 receives signals from the ammeter 34 which measures the load on the motor 30 driving calendar roller 28. The load on motor 30 is representative of the average tension on the sheet of paper between rollers 15 and the first calendar stack roller 18. The computing system 72 also receives signals from the ammeter 48 which indicate the load on the motor 44 which in turn is representative of the tension on the sheet 10 between the calendar roller 28 and the pope roll 42. The computing system 72 also receives signals from tachometer 35 measuring the speed of motor 30, which is representative of the speed of the sheet of paper 10 as it passes over roller 28. The computing system 72 utilizes the parameters discussed above to determine the strength of the paper according to the following equation: Where: S = Strength C and K = Constants L = Load at the particular point in the paper measured by the load cell 70 as the system scans across the sheet of paper T = Average tension of the sheet of paper between roller 15 and roller 18 V2 = Velocity of the sheet measured by the tachometer 74 V1 = Velocity of the sheet as determined from the tachometer 35 In some cases we have found that the calculation of strength can be simplified. In particular, if one desires to know the relative strength of the sheet at one point with respect to the strength at another point, without knowing the absolute value of the strength, it is unnecessary to measure the average tension T or the velocities V1 or V2. Rather, the only parameter necessary is the load, L measured by the load cell 70. We have found that the relative strength of the paper can be determined simply from the relative loads, L, of the paper as measured by our system at various points on the sheet, according to the following equation: Where: S1 and S2 are strengths at two points L1 and L2 are loads at the same two points K is a constant In some cases we have found that better results can be obtained by determining the strength based upon both the load L and the velocity V2 according to the following equation: S = LV2 A + B Where: S = Strength L = Load V2 = Velocity A and B = Constants In some cases we have found that better results can be determined by the utilization of the density, ρ, of the sheet as well as the load L and velocity V2. In this case the appropriate equation is: S = ρ LV2 A2 + B2 Where: The parameters are the same as above, and A2 and B2 are constants. The density ρ in this case can be determined utilizing the conventionally measured parameters of basis weight and caliper wherein density equals basis weight divided by caliper.

Claims: A system for non-destructively determining the strength of a sheet of material (10) being drawn under tension to travel between a first roll and a second roll (28,42), the sheet of material having a width, the system comprising: a) mounting means (50,52) located to support a load sensor for a movement across the width of the sheet while said sheet travels between the first and second rolls; b) a load sensor (54-58,60,62,70,80,82) attached to the mounting means for contact with the sheet, said load sensor including means for controllably deflecting said sheet across a localized area between the first and second rolls, said load sensor sensing the force exerted by the sheet against the deflecting means at a plurality of localized areas between the first and second rolls and to generate signals corresponding to said force; and c) computing means (72) for receiving and processing the signals from the load sensor to provide an indication of the strength of the sheet at one localized area relative to the strength at least at one other localized area, A system for non-destructively determining the strength of a sheet of material (10) traveling under tension between a first roll and a second roll (28,42), said sheet of material having a width, the system comprising: a) mounting means (50,52) located to support a load sensor for movement across the width of the sheet of material while said sheet travels between said first and second rolls; b) a load sensor (54-58,60,62,70,80,82) attached to the mounting means for contact with the sheet, said load sensor including means for controllably deflecting said sheet across a localized area between the first and second rolls, said load sensor including means for detecting the force required to accomplish the deflection and for generating an output signal indicative of said force; c) velocity detecting means (74) for detecting the velocity of the sheet within said localized area and to generate signals indicative of said velocity; and d) computing means (72) for receiving and processing the output signals from the load sensor and the velocity detecting means to provide an output indicative of the strength of the sheet. A system as defined in claim 2, wherein the sheet deflecting means includes a wheel (60) which contacts and deflects the sheet, and said velocity detecting means provides output signals indicating the rate of rotation of said wheel and thereby the velocity of the sheet (10) within said localized area. A system as defined in claim 3, further including density sensing means to sense the density of the sheet (10) and to provide signals corresponding to the sensed density to the computing means (72), the computing means providing an indication of the strength of the sheet based upon signals provided by the load sensor, the density sensing means, and the velocity detecting means. A system as defined in claim 3, including a second velocity detecting means (35) for providing output signals indicative of the velocity of the sheet (10) at a location between said first and second rolls other than within said localized area, the system further including tension sensing means (30,34) mounted to sense the average tension on the sheet and to provide output signals indicative of said average tension, the computing means (72) providing an indication of the strength of the sheet based upon signals provided by the load sensor, the first and second velocity detecting means and the tension sensing means. A process for non-destructively determining the strength of a sheet (10) which is being drawn under tension between a first roll and a second roll (28,42) comprising the steps of: a) detecting a deflecting load exerted on the sheet at a plurality of localized areas between the first and second rolls; and, b) computing the relative strength of the sheet at least at one of said localized areas compared to the strength at least at one other localized areas based upon detected deflecting loads while the sheet is moving between the first and second rolls. A process according to claim 6, in which the computing step comprises finding values in accordance with the following equation: where: S1 and S2 = strengths at two localized areas; L1 and L2 = loads at the same two localized areas; and K = a constant. A process for non-destructively determining the strength of a sheet (10) of material traveling under tension between a first roll and a second roll (28,42), the process comprising the steps of: a) controllably deflecting the sheet across a localized area between the first and second rolls; b) detecting the forces exerted by the sheet when the sheet is deflected at a plurality of localized areas between the first and second rolls; c) detecting the velocity of the moving sheet at the localized areas; and d) computing the strength of the sheet at least at one of said localized areas based upon the velocity of the sheet at the localized areas and the deflection forces at said localized areas. A process according to claim 8, wherein the computing step comprises finding values in accordance with the following equation: S = LV2 A + B where: S = strength; A and B = constants; L = the deflection force; and V2 = detected velocity of the sheet at the localized areas. A process according to claim 8, further including the step of: detecting the density of the traveling sheet (10), and wherein the computation of the strength of the sheet is further based on the detected density. A process according to claim 10, in which the computing step comprises finding values in accordance with the following equation: S = ρ LV2A2 + B2 where: S = strength; L = load; V2 = velocity of the sheet at the localized areas; A2 and B2 = constants; and ρ = density. A process according to claim 8, further including the steps of: detecting the velocity of the sheet (10) at a point between said first and second rolls (28,42) distant from the localized areas; and detecting the average tension of the traveling sheet, and wherein the computation of the strength of the sheet is further based on said detected distant velocity and said detected average tension. A process according to claim 12, wherein the computing step comprises finding values in accordance with the following equation: where: S = strength; C and K = constants; L = detected load; T = average tension; V2 = velocity of the sheet at said localized areas; and V1 = velocity distant from the localized areas.

Inventors: CHASE LEE MAC ARTHUR, GOSS JOHN DOUGLAS, HOUGHTON PAUL, NORTON MICHAEL KENT, CHASE, LEE MAC ARTHUR, GOSS, JOHN DOUGLAS, HOUGHTON, PAUL, NORTON, MICHAEL KENT
Applicants: MEASUREX CORP, MEASUREX CORPORATION
Application year: 1986
Main classification: G01N33
IPC codes: G01N3, G01N33