CA1327239C - On-line paper sheet strength determination method and device - Google Patents

On-line paper sheet strength determination method and device

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Publication number
CA1327239C
CA1327239C CA000578616A CA578616A CA1327239C CA 1327239 C CA1327239 C CA 1327239C CA 000578616 A CA000578616 A CA 000578616A CA 578616 A CA578616 A CA 578616A CA 1327239 C CA1327239 C CA 1327239C
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Canada
Prior art keywords
sheet
paper
basis weight
signal
sensor
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CA000578616A
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French (fr)
Inventor
Leonard M. Anderson, Jr.
Lee Macarthur Chase
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Honeywell Measurex Corp
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Measurex Corp
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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/34Paper
    • G01N33/346Paper paper sheets
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84Systems specially adapted for particular applications
    • G01N21/86Investigating moving sheets
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84Systems specially adapted for particular applications
    • G01N21/86Investigating moving sheets
    • G01N2021/8609Optical head specially adapted
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84Systems specially adapted for particular applications
    • G01N21/86Investigating moving sheets
    • G01N2021/8663Paper, e.g. gloss, moisture content

Abstract

ON-LINE PAPER SHEET STRENGTH
DETERMINATION METHOD AND DEVICE

ABSTRACT

This invention relates to a device and method for determination of the strength of a sheet of paper, and more specifically, to a device and method to determine the strength based upon monitoring the variations in the intensity of a narrow beam of light transmitted through the sheet as the sheet moves perpendicularly through the beam. A process for determining the strength of a moving sheet of paper comprising: (a) measuring the average basis weight of the sheet; (b) measuring the local basis weight of the sheet; (c) measuring the transmittance of the sheet;
(d) measuring the line speed of the sheet; (e) measur-ing the floc size of the sheet; and (f) determining the strength of the sheet based upon the average basis weight, the local basis weight, the transmittance, the floc size and the line speed.

Description

3~723~ ~

.
oN-r~INE PAPER SHEET STRENGTH
DETERMINATION METHOD AND D~VICE
BACKGROUND OF THE INVENTION
This invention relates to a device and method for determination of the strength of a sheet of paper~ and more specifically, to a device and method to cletermine the ~trength based upon monltoring the variat:ions in the intensity of a narxow beam of light tran~;mitted through the sheet as the sheet moves perpendicularly through the beam.
Paper is produced frQm a ~uspension of flbers~ These fibers are usually made of cellulose, derived mainly from wood and rags. The evenness of the distribution of these fibers in a sheet of paper is of paramount importance to the optical and printing properties of the sheet. Thexefore, onç of the chief goals for a paper maker is to develop a paper making process and adjust the parameters o the process to achieve as even a "basis weight" or distribution of these fibers in the finished sheet material as possible.
In the paper making art, the term "basis weight" re~ers to the weight of the paper-forming fibers per unit area of the ~.
sheet surface.
Among the critical characteristics of paper and other ~heet materials which are important ~o.both manufacturers and users is strengthO Many different methods for measuring strength have been proposed in the past, but virtually all suffer from a great disadvantage, namely, the tests are destructive and cannot be used "on line." A number of standardized tests have been devised to provide a basls for ; ':
:

;
1~2~3~
specifications by which paper can be bought and sold, and these tests provide arbitrary but nevertheless useful strength indices for compari~g the strengths of various papers. Unfortunately, all are destructive, and none can be used "on line." The more common tests are a standardized tensile test, the so-called "STFI " compression test, and the "burst pressure" or "Mull~n" ~es~.
In the standard tensile test, a strip of paper is held between two clamps and loaded in tensioh at a predetermined ratc. The loading at failure is taken to be a measuxe of the tensile strength of the paper. There are a number of standardized procedures which have been adopted to perform this test, e.g., TAPPI Standard T4040s-76 and ASTM Standard D828.
The "STFI" compression test for heavy papers is a standardized test whose procedure has been established by the Swedish Technical Forest Institute, as spec~ied by the ident~fiers: Scan P46 Column 83~ In this test a strip of paper to be tested is held between a pair of clamps which are moved together at a fixed rate while the compressive force is monitored. "Rupturel' occurs when the compressive force passes a peak and begins to drop. The force at "rupture" is taken as the compressive strength of the paper~
Other standard specifications for this test are, eOg., TAPPI
78180s-76 and ASTM D1164.
The strengths of papers as measured by the foregoing tests typically have di~ferent values depending on whether .

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the test strip i5 cut in the machine d~rection or the cross direction.
A "Mullen" or burst pressure tes~ is conducted by clamping a sample of the paper between t:wo cir~ular clamping .
rings having a specified standard inside diameter, and building up pressure on one side of the paper until the paper bursts ~usin~ a rubber diaphragm and liquid pressure).
The pressure re~uired to burst the paper is known a~ ~he "burst pressure" and is the fi~ure often used to specify the required strength. Common burst pressure specifications are TAPPI 4030s-76 and ASTM D774.
Needless to say, none of these tests lend themselves to use in connectlon with the continuous measurement of paper strength~ Because of their widespread popularity, however, it is desirable that any method used to measure the strength of paper provides results which correlate with one of the recognized standard tests In addition to strenyth, another important paper parameter i~ the '~formation" of the sheet. There is, apparently, no standard definition of "formation." However, for the present purpose "formation" will be defined as ~he manner in which fibers forming a paper sheet are .
distributed, disposed and intermixed within the sheet. In all paper sheets, the sheet-forming fiber~ are, at least to a certain extent, unevenly distributed in bunchss ca}led "flocs." However, sheets of paper having generally evenly distributed, intertwined f~bers are ~aid to have good -3- :

132723~ - ~

formation. Conversely, when the fibers forming the sheet are unacceptably unevenly distributed in flocs, the paper sheet is grainy rather than uniform and is sald ts have p~or formation.
Some researchers have found a correlation between paper formation and strength. However, their research has been limited and primarily theoretical and hla~ not resulted in device which can be used in practical applications to measure strength of paper being produced on a paper machine.
Moreover, although a variety of devices have ~een tested for measuring paper formation, as will be discussed hereinafter, many are incapable of accurately measuring formation, and none are capable of measuring paper strength.
In one device for measuring formation, called a basis weight sensor (or microdensitometer), a beam of light is .. . ..
transmitted throu~h the sheet as the ~heet passe~
perpendicularly through the beam. The intensity of the beam is measured by a light detector after the beam is transmitted through the paper sheet~ This light detector is positioned on the opposite side of the sbeet from the light source. The light de~ector produces an electrical signal indicative of the intensity of the transmitted beamO As the .. . . ... . .
basis weight o the portion of sheet through which the light beam is passing increases, the intensity of the beam transmitted through the ~heet decreases. Thus, the electrical signal from the light detector is indicative of the basis weight of the sheet.

,.. ,. :: :-.. ;:: ::

~ 327239 As previously mentioned, the fibers forming every ~heet of paper tend to congregate ln flocs. I n any one sheet, these flocs will have a variety of sizes. Thus, as the paper moves perpendicularly through the l~ght beam, the electrical signal produced by the light detector will be modulated at a plurality of frequencies corresponding to the distribution of floc sizes and also to the speed with which the paper sheet moves through the light beam. As the sheet speed increases, the frequency with which the flocs modulate the electrical basis weight signal increases. Similarly, -:
smaller flocs modulate the signal at higher frequencies than larger flocs. The amplitude of these modulations corresponds to the local variations in basis weiyht or, what amounts to the same thlng, the local variations in the distribution of the fibers forming the flocs.
In one technique, the formation characterizing device displays the average peak-to-peak variation in the electrical signal produced by a basis weight sensor. The average peak-to-peak value o~ the electrical signal is said to indicate the magnitude of variations in the basis weight :.
of the sheet. However, this technique has not been applied to measure strength, and for the reasons discussed below, the technique may give a false indication of the sheet formation.
In many instances, the paper maker will want to make a sheet having as eVQn a fiber distribution as possible, i.e.
one havlng good fvrmation. To accomplish ~his r the paper 5- :

- 132723~ ;
maker will want to know, not only t:~le magnl~ude of the variations in basi~ weight, but also the si~e distrlbution of the flocs. The paper maker will also warlt to know the strength of the sheet. However, the prev~ously descr~bed techni~ue, which yields only ~he average peak to-pealc value of the basis weight signal, gives no indication of ~he size of the flocs creating ~hese variations in the basis weigh~
signal or the strength of the sheet.

SUMMARY OF THE INVENTION
. .
The present invention is dlrected to a method and device whiah pro~ide a set of electrical output siynals indicative of th~ strength of the shee~. The output signals may be converted i~to numeri~al values a~d dlsplayed to the paper mill operator~ The operator can then use these numerical values to monitor the strength of the manu~actuxed shee~ and ad~ust thP parameters of the paper ma~i~g process to achieve a paper sheet hav~ng the desired strength.
Alternatlvely, these electrlcal output s~gnals can be fed 1~27239 into a computer or other device whlch would then use these output signals to automati~ally ad~ust the paper makin~ ;
process to achieve paper havlng the desired strength.
The device of the present invention includes a basis weight sensor for accurately measuring local variations in the basis weight of a sheet of paper. ~e sensor includes a :~
light beam source, which is disposed on one side of the sheet, and a ~Ireceiver~ disposed on the other side of the sheet opposiny th~ light beam source. The receiver includes a light pipe, such as a narrow sapphire rod. On end of the rod abuts against the sheet on the opposite side of the sheet from the li~ht source. As the sheet passes through the sensor perpendicular to the light beam, the sheet is held against the end of the rod so that only light which passes through the sheet can enter the rod. This rod i~
directs at least a portion of the llght beam to a llght detectin~ device such as a photodiode. The photodiode then produces an electrical output proportional to the intensity o the light beam after the beam is transmitted through the sheet.
As the sheet of paper passes through the basis weight sensor, local variations in the basis weight of the sheet create variations in the intensity of the light beam transmitted through the sheet. The light detecting device in the receiver portisn of the sensor produces an ~lectrical signal proportional to the intensity of th~ transmitted beam ;.
and hence inversely proportional to the basis weight of the 132723~ ~
portion of the sheet through which the detected portion of the beam is passing. Because paper consists of flocs of a variety of sizes, the electrical signal Prom the sensor i5 modulated at number of fre~uencies as the paper sheet pas~es between the liyht source and receiver halves of the sensor.
These frequencieæ axe dependent upon both the speed with which the paper passes through the sens~r and the size of the various flocs forming the sheet. Hc)wever, the si~nal processing circuits vf the present invention account for changes in the speed with which the paper passes through the sensor. Thus, the output signals characterizing ormation are independent of the paper speed.
It should be understood that the basis weight sensor described above may not measure exactly the same "basis weight" as is measured by a conventional nuclear basi~
weight gauge. The basis weight sensor taught herein is based upon optical transmi~tance of the paper sheet which may vary with wood fiber type, paper additives or other factors, which do not affect the "basis weight" measured by a nuclear basis weight gauge. However, for the purposes of this application, the present gauge will be said to measure :
basis weight or local basis weight, whereas a conventional nuclear gauge will be said to measure~averaqe basis weight.
The signal processing circuitry of the present lnvention has a plurality of electrical channels. ~ach channel processes basis weight signals from the basis weight sensor corresponding to a diffexent predetermined minimum ._ 9 ~

. . , ; -: , ,.: . , . :

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13272~9 floc size, the floc sizes larger than that minlmum. This is accomplished by plac~ng a low pass Pilter at the lnput end of each channel. The signal from the basis welght sensor i~
fed into each of these low pass filter~. However, the low pass filter of each succeed~ng channel has a cutoff frequency lower than that of the low pass filter in the preceding channel. In addition, the cu~off frequency ~or each of these low pass filters is variable and is controlled to be proportional to the speed with which the paper passes through the sensor. Thus, the cutoff frequency for the low pass filter of each channel corresponds to flocs of a particular predetermined minimum size and continues to correspond to flocs of this predetermined minim~n size even when the speed with which the paper moves through the sensor is changed.
The output of each low pass filter is directed to a separate AC to DC converting circuit which converts the filtsred signal from the associated low pass filter to a DC
output proportional to the root mean-square (hereafter "RMS") value of the signal from the low pass filter. The ~:
output of each AC to DC converter therefore indicates th2 magnitude of the variations in the basis weight of the sheet created by flocs of a certain minimum size (i.e. the flocs modulating the basis weight signal at a frequency just below the cut-off fre~uency) and all flocs larger than that minimum size.

-10- ., ' ' :~

`- ~3~72~9 -Additionally, the signal from the low pa~s fllter of the flrst channel (the first channel low pass filter has the hi~hes~ cutoff frequenoy) can be directed to a peak de~ector circuit. This circuit can be made to indicate the maxlmum intensity of the basis welght signal over a predetermined length of paper which passes thrvugh the basis weight sen~or or the average of several siqnal peaks. A more in~ense transmitted light beam corresponds to a lower basis weight.
Therefore, when the peak detector is made to indicate the maximum intensity of the basis weight signal, the maynitude of the output of the peak detector circuit characterizes the strength of weakest point of the sheet. Alternatively, when the peak detector circuit is made to indicate the average of several signal peaks, then the output of this circuit characterizes an average of the strengths of several of the weakest points in the sheet.
Although the strength of the sheet a~ its weakest point may be usefui to the papermaker, of more importance may be the overall strength. To determine the overall strength additional information is necessary. In particular, the average basis weight of the sheet is determined with a conventional nuclear gauge, and the transmission of the sheet is determined with the local basis weight sensor and the line speed of the paper machine must be determined by conventional mean~. Once this data is available, it is utilized along with other data from the local basis weight sensor to de~ermine sheet strength.

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BRIEF DESCRIPTION OF THE D~AWINGS
Fig. 1 illustrates wavele~gth power spectra ~or several dif~erent g~ades of pape~.
Fig. 2 illustrates one embodiment of the basis welght sensor of the present invention.
Fig. 3 illustrates a chopper wheel for c,alibrating the device of the present inven~ion.
Fig. 4 illustrates a block diagram of one embodiment of the circuitry of the present invention used to process ~ignals from the basis weight sensor of ~ig. 2.
Fig. 5 illustrates a block dia~ram of one embodiment of a computer system to util~ze signals fro~ ~he circuitry o~
Fig. 4.
DETAILED DESCRIPTION OF THE PREFERRED_EMBODIMENT

In another tech~igue for characteriz$ng heet formation, a beta radiograph is made,of a sample sheet of paper. Light is then passed through or reflected o~f o~ the radio~raph. Variations in the i~tensit~ of 13 narrow beam of this light are converted into an electrical signal as the radiograph moves, at a uniform speed, perpendicularly with respect to the beam. A graphical display is produced of the amplitude of the modulations of this electrical signal as a function of the wavelengths comprising the slgnal. This display is called a wavelength power spectrum. Fig. 1 illustrates o~e such display ~or several ~rades of paper having good, intermediate and poor formation. This techn~ue has been discussed in great detail by Norman and Wahren in a number papers, including their symposium paper "Mass Distribution and Sheet Properties of Paper."
For some commercial paper manu~acturing situa~tions, the Norman and Wahren technique may be inappropriate to measure formation. ~ illu~tra~ed ln Fig. 1, at wavelengths below about one milllmeter, there is little differenoe between the lZ-, . .

1~272~

wavelength power spectra of a well-~ormed sheet and a poorly-formed sheet. However, fr.on.wavelengths of about one millimeter to th~rty two millimeters, signlflcant differences e~ist. Thus, the Norman and Wahren ~echniq~e produces more in~ormation than may be necessary for the paper maker to determlne formation o~ a sheet. Another possible d~sadvanta~e of this techni~u~ ~s that lt provldes so much information that its lnterpretat~on may be di~ficult for the non-expert. In many co~meraia~ manu~acturin~
situations, the paper maker may prefer a device and technique which provides him or her with only a few numbers, which together completely characterize the formation of the sheet, rather than an entire spectral display. Moreover, this techni~ue, like the previousl~ described technique for measuring the average peak-to-peak value of a basis weight signal, fails to provide the paper maker with an indicatlon of the strength of the sheet.

A. The Basis ~Weiqht Sensor Fig, 2 illustrates a pre~ently preferred embodiment of the basis weight sensor 10 of the present invention. This sensor 10 can be con~idered as consisti~g of ~wo halves, a "source" half 12 and a "receiver," halfl~l4.. The ~ource half 12, disposed on one slde of the sheet of paper 16, direct~ a beam of light through the sheet 16 whose ~orma~ion ls to be determined. The receiver half 14 is disposed ori the opposite side of the sheet 16 an~.~r:oduces an electrical signal proportional to the in~enslty of the ligh~ which is ~ransmitted through the shee~ 16. The source half 12 includes a light source 18, such as a high intenslty incandescent lamp 20, and a reflector 22 for direc~ing the i- ~12a - ~272~

beam of li~ht from the lamp 20 toward the sheet 16. As th~ -lîght travels toward the sheet 16, lt passes through a diffuser 24 which randomizes the direction of the pho~ons as the beam passes through it. It ls important to use a diffuse source of light. If a non-diffuse source of light is used, the receiver half 14 of the sensor 10 may measure variations in the intensity of the transmitted beam caused by variations in the reflectance of the sheet surface to light coming from one particular directlon, rather than variations in transmitted light intensity caused by local variations.in basis weight of the sheet 16.
The receiver half 14 of the sensor 10 includes a 1 mm.
diameter sapphire l~ght plpe 26 for directing a small spot of the diffuse light beam which is transmitted through the sheet 16 toward a lens system 28. This lens system 28 focuses the light from the light pipe 25 onto a light sensitive silicon photodiode 30. Th8 photodiode 30 produces an electrical output signal proportional to the intensity of the spot of transmitted light. Thus it can be seen that the sensor 10 measures the basis weight of a lmm diameter circle of paper.
It is important that the sheet 16 be held firmly against the end of the light pipe 36 as the sheet passes through the sensor 10 so that any light impinging upon the end of the li~ht pipe 36 must have traveled through the sheet 16. To accomplish this goal~ the source half 12 of the formation sensor 10 is formed with protrusions 32, --` 132723~

called "skid plates," on opposit~ sides of the light pipe 26. In addition, the end of the light pipe 36 ext~nd~
toward the sheet 16 and is protected by anothex skid plate 34 surrounding the light pipe 26 such that the paper sheet 16, travellin~ in the direction of the arrows 38 between the source an~ receivex halves of the sensor 10, is held by the skid plates 32, 34 against the end of the light pipe 36.
As the paper sheet 16 passes between and rubs against the skid plates 32, 34 and the end of the light pipe 36, the paper will tend to wear away the skid plates and the end of the light pipe 36. The skid plates 32, 34 are therefore constructed of an abras~on resistant material such as ~teel alloys and the light pipe 26 is made of sapphire or some other similarly transparent but abrasion resistant material.
B. The Signal Processin~ CircuitrY
As preYiously mentioned, the basis weight sensor 10 produces an electrical signal the magnitude of which is inversely proportional to the basis weight of the portion of the sheet 16 through which the detected spot of the light beam transmitted. The sheet 16 is formed from flocs so that the transmitted beam intensity, and hence the sensor signal, varies as the paper sheet 16 passes through ~he sensor 10.
The sensor signal is then amplified by amplifier 120 and the amplified sensor signal 121 is fed to the signal processing circuitry. The sensor signal 121 is indicative of the lo~al basis weight of that portion of the sheet contacting the end of the light pipe 36, i.e. a lmm diameter circle.
. .

13~7239 A presently preferred embodiment of the signal processing circuitry 50 is shvwn in block diagram form in Fi~. 4. This signal processing circuitry 50 compxises a .:
plurallty of low pass filters 52-62. Each filter 52 62 is associated with a particular electrical "channel." Each channel lncludes one of these low pass filters 52 62 and an RMS-AC to CD converter 78-88. The device of ~he present :
invention may have any number of channels (channel 4 is omitted to simpllfy the figure). In the embodiment of Fig.
4, the device has six channels. Each of the six low pass filters 52 62 receives two ~nput signals. The first input signal to each of the low pass filters 52-62 comes ~:rom the :.
previously described basis wei~ht sensor 10. This signal is .:
directed to the first input of each low pass filter 52-62.
The cutoff frequency for each low pass filter 52-62 is proportional to the frequency of a second input signal. The frequency of the second input signal is not the same of each low pass filter 52-62. Instead, the frequency of the second input to each low pass filter ~2-62 is one-half of the frequency of the signal fed to the second inpu~ of the low pass filter of the preceding channel. Thus, the cutoff frequency of the first channel low pass filter is highest and the cutoff frequency of the sixth channel low pass filter 62 is lower than the cutoff frequency of any of the ..
other low pass filters 52-62. In othex words, the first channel low pass filter 52 passes a signal from the basis weight sensor 10 the highest freguency component of which -15- `:

~32723~

corresponds to a certain minimum floc size. The sensor 10 cannot sense changes in basis weight that occur in less than mm since the lis~ht pipe 26 ~ Fig. 2 ) of the basis welght sensor 10 has a 1 mm. diameter. Thus, the highest frequency basis weight signal sent the low pass f:Llters corresponds to 1 mm. flocs~ Therefore, in the present embodiment, the frequency of the signal sent to the second input of the low pass filter 52 of channel 1 is adjusted so that ~his low pass filter 52 has a cutoff freq~ency corresponding to variations in the basis weight caused by 1 mm. flocs. The frequency of the signal sent to the second inputs of the low pass filters 54-62 of channels 2-6 is adjusted so that the cu~off frequencies of these low pass filters 54-62 correspond to floc slzes of 2mm, 4mm, 8mm, 16mm and 32mm, respectively. The frequency of the second input to each low pass filter 52-62 is also proportional to the speed with which the paper passes through the sensor 10. Thus, the cutoff frequency of each low pass filter 52-62 continues to correspond to a basis weight signal frequency characteristic of flocs of the above-mentioned sizes9 even when of the speed with which the paper sheet passes through the sensor 10 changes.
In the present preferred embodime~t, the second input signal to each low pass filter 52-62 is derived by first measuring the speed with which the paper sheet passes through the sensor 10. Devices which measure the speed of a paper sheet are well known in the axt. Many modern paper 13~72~ ~
mills are highly automated and include a computer which monitors and controls various ~arameters of the paper making process. Thus, in the present preferred embodiment, a digital paper speed-si~nal 61 from the mill's computer indicative of the paper speed is conveniently used to .
control the cutoff frequency o$ the low pass filter 52-62 of each channel. This digital speed si~nal 61 is directed to a digital to analog converter 64 whi~h receives the digital speed signal and outputs a voltage proportional to the paper speed. This voltage is then input to a voltage to frequency converter 66 (hereinafter "VFC"). The VFC 66 then outputs a signal having a frequency which is proportional to the output voltage of the digital to analog converter 64 and hepce to speed of the paper passing through the sensor lO.
Each channel, except the first channel, includes a frequency divider 68-76. The signal from the VFC 66 is fed directly into the second input of the first channel low pass ~ilter 52, and also into the requency divider 68 of the second channel. The freguency divider 68 of the second channel divides the fre~uency of the signal received from the VFC 66 and the resulting lower frequency divider 70 of the third channel. Thus, the second input to the low pass filter 52 of the first channel ls at frequency X. Frequeney X
corresponds to the speed with which the paper passes through the sensor lO. Since divide-by-two frequency dividers are used in the present preferred embodiment, the f~equency input to the low pass fllter 54 of the second channel is at '~ .

,,:.,,.,: : :,: .: :: ,. , ~327239 frequency X/2. The signal output by frequency divider 68 of the second channel is also fed the input of the frequency divider 70 of the third channel. ~ach succeedlng channel 4 and S also have frequency dlviders, for example frequency divider 76, which receive the si~nal from the frequency divider of the preceding channel and output a signal at one-half the frequency of the received slgnal. Thus, the freguency of the signal fed to the second input of the low .
pass filter 56 of the third chan~el is at frequency X/4, the frequency of the signal ed to the second input of the fourth channel low pass filter (not shown) is X/8, etc. In this way, the output of the low pass filter 52 of the first channel comprises fre~uencies corxesponding to floc sizes greater than or equal to a minimum size, i.e. lmm. The highest frequency passes through to the output of the low pass filter in each succeeding channel corresponds to floc sizes of increasingly larger minimum size, i.e. 2mm, 4mm, 8mm, 16mm and 32mm. The output of each ~hannel's low pass filter 5~-62 is then processed to indicate various formation parameters of the sheet being sensed for floc ~izes at and above the minlmum floc size for the particular channel.
To derive an output signal indicative of the magnitude of the varîations in the basis weight vf the sheet, the output of each low pass ilter 52-62 is directed to an associated AC to DC converter 78-88. Each AC to DC
converter 78-88 produces a DC voltage equivalent to the RMS
value of the AC signal output f rom the associated low pass ~7~3~

filter 52-62. The RMS Yalue of the DC voltage produced by each AC to DC converter 78-88 is proportional to the magnitude of variation in the basis weight of the sheet caused by flocs of a particular minimum size. Since the cutoff frequency of the low pass filters 52-62 in each succeeding channel is set to succeedingly lower frequencies, the magnitude of the RMS DC output voltage o~ each succeeding channel corre~ponds to the magnitude of variation in the basis weight of the sheet caused by succeedingly larger minimum floc sizes.
In certain sit~ations, the mill operator will want to know the magnitude of the basis weight variations in the sheet caused by flocs in a particular size range. The device of the present invention can provide this information by simply subtracting the RMS DC output of the AC to DC
converter of one channe} from the RMS DC output of the AC to DC converter of another channel. The difference between the value of these outputs corresponds to the magnitude of the basis weight vari~tions caused by flocs in the s$ze range between the cutoff frequencies of the low pass filters of the two channels.
A subtracting circui~ 122 may be provided to receive, at inputs 1 and 2, the outputs of any two selected AC to DC
converters. This subtracting circuit pxoduces an output voltage corresponding to the difference between the outputs of the two selected AC to DC converters. Alternatively, if the output of the various AC to DC converters are --19 - ,.

.

27~39 numerically displayed, then the paper mill operator can obtain the difference between any two such outputs by subtraction. For example, to determine the magnitude of the basis weight variations caused by flocs between 4mm and 8mm, the paper mill operator simply subtracts the value of the :
output of the fourth channel AC to DC converter from the value of the output of the third channel AC to DC converter.
Many standard "~MS" AC to DC converters actually measure the peak-to-peak voltage of the incoming signal and then provide an output DC signal which corresponds to the true RMS value of the input signal only if the input signal is sînusoidal. However, the basis weight signal waveshape is genexally not sinusoidal. It is, therefore, usually important that the AC to DC converters 78-88 of the present invention output a DC voltage corresponding to the true RMS
value of the basis weight signal, otherwise the output slgnal of these AC to DC converters 78-88 may provide an inaccurate measure of the basis weight variatlons.
The use of true RMS-AC to DC converts is partlcularly important when the ou~put of the converter of one channel is subtracted from the output of a converter of another channel to thereby determine the contribution,to the basis weight variations caused by flocs in a particular size range.
Flocs of different sizes may cause the same peak-to-peak changes in the basis weight signal, even though their contribution to the RMS value of the basis weight signal is different. Thus, subtracting an AC to DC conver~er output :. :.: . ... ::,: :.,. .: .

~:2723~
derived from a basis weight signal containing frequencies corresponding to 4 mm. minlmum floc sizes, should and would yield a signal indicative of the contribution to basis weight variation caused by flocs in the 4-8mm size range, if true RMS AC to DC converters are used. However, if the "RMS" signal is actually derived from ~ measurement of the peak-to-peak signal value, the flocs of different si~es are `.
causing the same peak-to-peak change in basl9 weight signal, then the difference between the outputs of the two AC to DC
converters would be zero. However, this would not be a correct indication of the contribution to basis weight .
variation caused by the flocs in the 4-8mm range. Thus, the use of standard peak-to-peak AC to DC converters may give false readings when used in the device of the present invention. :.
Another parameter, indicatlve of the strength of the weakest portion of the sheet, is obtained by feeding the output of the low pass filter 52 of the first channel to the input of a peak detecting circuit 90. Since, as previously mentioned, the magnitude of the intensity of the transmi~ted beam is inversely proportional to the basis wei~ht of the sheet~ the magnitude of the AC signal at the output of this low pass filter 52 will also be inversely proportional to the local basis weight of the portion of the sheet then being sensed by the sensor lO. The peak detecting circuit 90 may be designed to provlde a DC output proportional to the largest vol~age peak which passes through the first ~ 3 ~

channel low pass filter S~ in a predetermined tlme period or ~:
for a predetermLned length of sheet p~ssin~ through the sensor 10. The magnitude of this signal indicates the weakest point in the shPet, Alternatively, the peak detector circuit 10 may also be designed to produce an output proportional to the average of several signal peaks over a set period of time or length of sheet pas~ing through the sensor 10. In this latter case, the output o the peak detector circuit 90 would characterize an average weak spot in the sheet.
The signal processing circuits SO of the presentinvention may provide the paper manufacturer with yet another output signal indicative of another characteristic of the paper sheet -- the average flo~ size. To obtain this parameter, the output of the low pass filter 52 of the first channel Ls fed to a floc size measuring circuit 92. This circuit 92 counts the number of times, during a predetermined time interval, that the output signal from the low pass filter 52 of the first channel achieves a value corresponding to the average of the output signal~ (This can be called "crossings" i.e. the rate at whlch the signal from filter 52 "crosses" the average of~the signal.) The frequency of crossings divided by the speed of the paper through the sensor indicates the average slze of the flocs forming the sheet. A floc size measurin~ circuit, not shown, performs this division and outputs a signal corresponding to the average floc size. For example, if th~

:;. :.. , .. , :., ,.,.. . . ., .. , :

; ~ 1327?39 paper ~3heet i~ moving at 1000 m/min and th~ output ~r~m the low paæs filt~r o~ the fir~t ~hann~l achievod a value correspondir~ to the av~r~e of the out~u~ 3333 t~ne~ in one ~e~ d ~ime interval, 'chen the average ~lo~ ~ize o~ th~
shee~ i~ l9rnm ( lOOOm/min ' lmin/~O~ec 1o~/3333c:r '~arc~ings~flQa)~ Thus, b~f ~erl~in~ th~ lo~al ba~i~ we;L~h~
o~ ~he pal?e~ ~heet ~long ~ lin~ or aurv~ (hereirla~t~r collectively "li~e~' ) 210ng the sheet sur~ace, th~ d~vic~ and method of the present invention can provide th~3 p~p~r nnanu~acturer with an output signal indiaa~iv~ o~ ~he ~ o~
the ~lo~æ ~orming the ~heet.
C. U~;e ~nd Calibra~ion o~ the Ba~is W~i~h~ .Sex~p~
In a paper mill, paper is ty~iaally pxc:K~uced in she0ts about 25 ~ee~ wide. To characterize the ~ntire she~t, one baæis weight form~t~c~n sensor c rl b~ ved c~r "s~ ed" b~ok and forth in the "cross direotion" of the sheet ( ~ ¢~o s he wl~th o~ the she~t) A the ~heet moves along i~ ~h~:
"machine dir~ction" ~l,e, the lerlgthwlse ~irectio~).
A}ternatively, a plur~lity o~ ~en~ors ~a~ b~ ca~ ed back ::
and ~Eor~h in ~h~ croæs dire~ioI~ a~ross o~ly ~ ~ax~ o~ ~h~ ::
wid~h o~ ~he sheet. I~, ~or æxaml?le, 50 ba~s w~eight ~ensor~ are used o~ a ~5 fool~ wid~ ~he~ then ea~h ~en~or would be made to scan bac:lc and ~orth acros~ ? 6 incsh wide strip o~ the sheet. Typi~ally, paper mills produ~e su~h sheets at about more than 1~00 e~t p~r minut~ and the ~ac~k and forkh -~canrling ~peed o~ the ~ensor ir~ the ~re30nt :: `
~mbodiment may be set at ~0 iE~t ~er mi~ute. q!hu~, the .~,,. .. ;:, ;:. :,:,: ., . . : . ................ .
:,.'., '',.. '.. '.. ,,',. `' .,'.''' :

::::::: :.......... :.::. :: .
,.. ,. , . :,: . .. .
''., ': ''' :'''.''' .' ', ' :. :. . .: :.:, . :::: ; . . . .
:, :, ,. , ::,::::.: .
; .: .. .:,: . ::: ,:,, . ,:, :.:::,: : ~:::: ::.......................... . . . .
.:.:,.,:,,, :.;;::, . . .
. . : . :, ,: :: ,.;:,, . ::; ::.

~3~7~3-~

cutoff frequency of the low pass filters may be made proportional only to the speed with which the sheet moves in the machine direction without introducing substantial srror into the output readings. The additional contributlon to the speed with which the paper moves through the sensor, caused by the cross directional movement of the sensor, is minimal, and can usually be ignored.
For the receiver part of the basis weight censor 14 ~Fiy. 2) to operate properly~ the li~ht from the source ~ide of the sensor 12 must be aligned directly opposite the sheet from the receiver 14. However, the two halves of ~he basis weight sensor 10 cannot be directly connected together since the paper sheet 16 passes between the~e two halves. A
number of different mechanisms can be used to keep the two halves of the sensor 10 directly opposite to each other as they scan back and forth across the sheet 16. One such .
device, for example, consists of two tracks (not shown), one on each side o the sheet 16. The sour~e side of the ~ensor 12 rides on one of the tracks and the receiver side of the sensor 14 rides on the other track. A gear or pulley system moves the two halves of the sensor in unison and oppos~te each other back and forth across the width of the sheet 16.
In this way, the source 12 and receiver 14 halves remain directly opposite each other without the nec~ssity of penetrati~g the sheet with a connecting membex.
Calibration of the basis weight sensor 19 may be done "off sheet", i.e. without having a paper sheet between the ... ,.:. .,,,,.,;.,.,. :

132723~

two sensor halves. To callbrate the outputs of the low pass filters, a chopper wheel 100 ~Figs. 2-3) is po~itioned between the sensor's light source 18 and photodiode io. In the present embodiment, the chopper wheel 100 ~s positioned at the base of the light pipe 26. The chopper wheel 100 is made from a circular disc 102 of opaque material having a plurality of radlal slots 104 positioned around the wheel 100. The chopper wheel 100 is driven at a known rotational speed so that the photodiode 30 rece~ves pulse~ of light.
The pulsing rate is detexmined by the predetermined speed of rotation of the wheel 100. The paper speed signal can then be set so that the low pass filters o all chann~ls 52-62 (Fig. 4) will transmit signals to the associated RMS-AC to DC converters 78-88. Then, by inputting successively lower paper speeds, the low pass filter cutoff frequencies can be cali~rated. For ex~mple, if a chopper wheel 100 with four radial slots 104, like that of Fig. 3, is rotated at a speed of 142.5 rotations per second, the chopper wheel 100 will modulate the light impinging upon the light detector at 570 Hz. If the paper speed signal from the YFC 66 is faster than 1094 mJmin., then all channels will see the signal.
However, as the paper speed drops below 1094 m/min., only channels 1-5 will provide an output. Further decreases in the paper speed siynal will cause additional low pass filters to cutoff the signal from the basis weight sensor 10 . "

132723~
Any device which modulates ~he intensity of the light reaching the light detector can be used other than a ~hopper wheel 100. For example, a tuning fork, the arms of which oscillate into and out of the light beam at a known frequency, could be used in place of a cbopper wheel 100.
Additional information is gcnerated using the data produc~d with the basis weight sensor 10 in the "off sheet"
position and the chopper wheel 100 operati~g. In particular, sensor signal 121 from amplifier 120 is averaged ;~
over a predetermined time interval. This signal is divid~d by the value of this same signal 121 during the "offsheet"
condition of the sensor 10 and the resulting value, T, indicates the average transmittance of the paper. The .
average transmittance, T, is used as a corrector, as discussed below, for the effects of long term variations in paper color or transmittance, which may occur a~ the wood flber type, digester chemistry or additives are varied.
Differe~t types of paper will preferentlally ~bsorb or reflect ~ertain fre~uencies of light. Therefore, to optimize the sensitivity of th~ basis weight sensor to changes in basis weight, an optical band pass filter 100 (Fig. 2) may be placed in the path of the light beam. This band pass filter 110 will preferentia~ly pass light of certain frequencies to the photodiode 30.
To properly measure variations in the basis weight of the sheet, it is important that the amplified sensor signal 121 from amplifier 120 (Fig. 4~ be inversely proportional to :,: . . :,.: ,;.

~ 1327~9 ~

the basis weight of the sheet. To ensure that the amplitude of the sensor signal 121 fed to the low pass fllters 52-62 responds linearly to changes in basis weight, the amplifled sensor signal can be measured with and without a neutral density filter 130 (Fig. 2) placed in the path Oe the light beam. The neutral density filter 130 ettenuates the beam intensity by a known percentage. The ~nplitude of the amplified sensor signal 121 should be measured firs~ while the pivot 132 has pivoted the neutral density filter 130 out of the path of the beam. Then, the neutral densi~y filter 130 is pivoted, by the pivot 132~ into the path of the beam.
While the neutral density filter 130 is in the beam path, the amplified basis weight signal should again be measured.
Nonlinearities in the output of the sensor can then be compensated for by adjusting the amplifier (Fig. 4~ so that the change in the amplitude of the amplified basis weight signal caused by placing the neutral density filter 130 in :
the beam path linearly corresponds to the known change in the light beam intensity caused by po~itioning the neutral density filter 130 in the beam path.
D. Determination of Strenqth Fig. 5 illustrates the present computer system for .:
determining paper strength. The strength computer 130 can be a stand-alone computer or it can be a software program running on a computer which runs other programs as well.
The computer 130 receives: i) the digital paper speed si~nal 61; ii) the sensor signal 121 from the ampli~ier 120; .;

... ;:.; .: .,:.,. :.:;,................... ;.:. ., . .:

7 2 3 ~

iii) the output of channel 6 xom RMS AC to DC converter 88 iv) the signal from floc size circuit 92; and v) ~he average basis weight signal 134 from a nuclear basis weight gauge, not shown. The nuclear basls weight gauge is a conventional gauge e.g. acoordin~ to U.S. Patent 3,757,122.
Also, constants are entered into the computer 130. Based upon this data, the computer 130 determines the strength of the sheet as discussed below.
According to the present embodiment the computer utilizes the following algorithm to determine strength: ~-S = A ~ B*ABW + C*ABW ~ + D ~RM5 Where: S = Mullen strength RMS32 = Channel 6 output from ~MS ~C to DC converter 88 t>32mm) ABW = average basis weight signal 134 : T = transmittance SP = paper speed 61 ~`
A, B, C and D are constants CROSS = output of circuit 92 ~iOe. crossings) The transmittance, T, is determined aceording to the formula:
T = Aol Aos 2) Where: Aol = signal 121 while the sensor 10 is measuring the sheet of paper (on line) Aos = signal 121 while the sensor 10 is "off sheet"
during calibration.
: As an example of the application of our equation 1) in the determination of Mullen strength for 42 to 69 pound .

-- ~327239 liner board, e~uation 1) results in good correlation with laboratory measurements. In this example, we found that using RMS32, the output of channel 6 of circuit 50, achieved better results than usiny the o~tput of any of the other channels 1-S. However, i~ some cases the use of the output of another channel may lead to better results, and in some cases, it may be prefera~le to modify the circuit S0 to add a channel which measures only flo~s with a minimum size greater than 3 2mm and use the signal related to such larger '~.
floc size.
In this example, the units of measurement were as ~ollows: .
RMS32 is dimensionless ABW is in pounds/lOOOft2 T is dimensionless SP is in feet/minute In this example, our constants were determined by linear regression and were:
A = 99.4 B = 0.419 C = 0.113 = -177 In practice, the constants A, ~, C and D should be determined before the present system is used for actual measurement or control. This is done by operating the present device to measure a sheet and analyzing a portion of the same sheet in a laboratory using a conventional i 3 2 7 2 3 ~

laboratory method. Then the constants are calculated by linear regression of the laboratory data with respect to data from the present system~
It should be understood that equation 13 can be modified in a number of ways and still be used to determine strength. For example, as discussed above, S - Mullen strength; however, other strength parameters such as tensil strength, STFI, etc. can be determined by choosing the constants A, B, C and D (and in some cases other parameters) appropriatelyO '' It should also be understood that equation 1) is a member of a class of formulas which could be used to operate on local basis weight and floc size da~a to determine paper strength.
The expression C~OSS is inversely ~roportional to the T*SP
floc size. Larger floc sizes usually result in weaker paper. The expression, R~532 is proportional to the variability of local basis weight across the sheet. Large variability in local basis weight indicates local basis weight regions which are likely to be weak. (Thus, the coefficient D in eguation 1~ is negative.) Thus, we have found a method to determine the strength of a moving sheet of paper based upon the floc size and variability of the local basis weight of the sheet.
In some applications e~uation 1) can be modified to provide greater accuracy. For example, in some cases we can determine strength as follows:

: . . .

~32~23~
, . . .
S = A + B~ABW ~ %~CROSS ~ Y*~ 32 ThSP T

+ E La~Lc ~ F Lb~Ld 3) ~ )*~T+G) Where:

i ) the parameters in equation 3) are the same as those in eguation 1) with the same labels 5 ii) A, B, C, D, ~, F~ G, X and Y are constants;
iii) La, Lb, Lc and Ld are the output signals from khe segments of a seqmented load sensor such as that disclosed in Canadian Patent No.
1310752 issued on Novemher 24, 1992;

iv) z is the output of a web deflection sensor;
and v) T is a number representative of the tension of the web.

one pre~erred embodiment of the present invention has been described. Nevertheless, it will be understood that various modiflcations may be made to the system de~cribed herein without departing from the spirit and sco~e of the invention. Moreover, sheet materials other than paper may be passed through the sensor and characterized by the device of the present inventlon. Thus, the present lnventions is not limited to the pre~erred embodiments described here~.n, nor is it limited strictly to use with paper~

~3I- I ;

Claims (2)

1. A process for determining the strength of a moving sheet of paper comprising:
a) measuring the average basis weight of the sheet, b) measuring the local basis weight of the sheet;
c) measuring the transmittance of the sheet;
d) measuring the line speed of the sheet;
e) measuring the floc size of the sheet; and f) determining the strength of the sheet based upon the average basis weight, the local basis weight, the transmittance, the floc size and the line speed.
2. A process for determining the strength of a sheet of paper comprising:
a) measuring the variability of the local basis weight of the sheet;
b) measuring the floc size of the sheet, and c) determining the strength of the sheet based upon the variability of the local basis weight and the floc size of the sheet.
CA000578616A 1987-10-06 1988-09-27 On-line paper sheet strength determination method and device Expired - Lifetime CA1327239C (en)

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US105,635 1987-10-06

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JPH01132965A (en) 1989-05-25
US4936141A (en) 1990-06-26
DE3886336T2 (en) 1994-06-09
DE3886336D1 (en) 1994-01-27
FI95748C (en) 1996-03-11
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FI95748B (en) 1995-11-30
JP2578944B2 (en) 1997-02-05

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