System and method for measuring the permeability of a material

A system for measuring the permeability of a material includes a light source for illuminating the material, and a stray light sensor for detecting stray light traveling through the material from the light source and outputting a stray light signal indicative of the stray light detected. The system further includes a direct light sensor for detecting direct light traveling through holes in the material from the light source and outputting a direct light signal indicative of the direct light detected. Finally, the system includes a digital processing device for receiving the stray light and direct light signals and calculating the permeability of the material.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to instruments for measuring the permeability of a material, and, more particularly to system and method for measuring the permeability of a material.

B. Description of the Related Art

Many products or materials are provided with holes or perforations. Such products and materials require their permeability to be measured. Examples of such products and materials needing permeability measurements include: wallpaper; filters used for air, chemicals, etc.; materials affording the appropriate degree of liquid (ink, varnish, sizing) absorption in printing; porous bags and materials used in food packaging and agricultural fumigation; insulating materials; paper; textiles; etc.

One particular material provided with such holes or perforations are the wrappers of filter cigarettes or similar rod-shaped tobacco products. The perforations allow cool atmospheric air to enter the column of tobacco smoke. Such wrappers are called tipping paper. Running webs of tipping paper making up rod-shaped tobacco products may be perforated mechanically, electrically, or optically. For example, British Patent No. 1,588,980 discloses a perforating unit that employs a set of needles or analogous mechanical perforating tools that puncture selected portions of the running web. U.S. Pat. No. 2,528,158 and British Patent No. 1,604,467 disclose electro-perforating tools that employ heat-generating electrodes that combust selected portions of the running web. An optical perforating tool, as disclosed in U.S. Pat. No. 4,265,254, uses coherent radiation from a laser to make perforations of a desired size and with a high degree of reproducibility.

Conventional filter-tipped tobacco products are perforated in the region of their filter plugs to insure that atmospheric air can enter the column of tobacco smoke irrespective of the length of combusted portion of the tobacco-containing section of the product. It is desirable to regulate the permeability of wrappers of all articles of a given tobacco product in such a way that the permeability is consistent or deviates only negligibly from a predetermined value.

It is known to control perforations of tipping paper in response to permeability measurements, as discussed in U.S. Pat. Nos. 4,569,359, 4,121,595, 4,648,412 and 5,092,350. Known permeability measuring devices include pneumatic systems for measuring the pressure drop through the tipping paper. However, such pneumatic systems are frequently inaccurate and difficult to implement in a high volume production line where the web can travel through the perforator at speeds of 5000 to 6000 feet per minute.

Pneumatic measurements are frequently made off-line on a sample basis. In some conventional production lines, quality monitoring and control are accomplished through a combination of sampling and perforator adjustments. Initial setup can be accomplished by iterative trial and error in which the focus and power settings of the laser perforator are adjusted. After making tentative settings, the line is run to generate samples. The resulting samples are then tested in a pneumatic pressure drop instrument gauge. Once the desired operating results are achieved, a manufacturing inspector periodically samples the perforated product, for example, a sample could be taken of five foot sections of paper from the end of every third bobbin (or of every bobbin) to check for correct pressure drop. The paper could also be inspected by visual monitoring by holding the paper up to light to check generally for hole position and size. However, since such measurements are neither continuous nor in real time, defective perforation, if detected at all, would be determined after a large quantity of tipping paper has been perforated.

Optical monitoring devices for tipping paper perforation lines are also known, as discussed in U.S. Pat. Nos. 4,569,359 and 5,341,824. A conventional optical system for monitoring a perforation line is illustrated inFIG. 1and described below. While such a system permits on-line monitoring of the process, in practice the output signal from this system has been found to correlate poorly with the pressure drops measured directly with pneumatic systems. Moreover, the system is affected by variations in the paper base sheet such as splices, extraneous holes, or thickness changes.

As shown inFIG. 1, the conventional optical monitoring system for monitoring perforations102in tipping paper100(traveling in direction101) includes a light or optical source or sources104that shines a large circular area of light106onto the tipping paper100. Typically, light source104is a halogen-based light source. Light108emanating through perforations102is received by a light or optical detector or detectors110, and used to monitor and/or control the quality of the perforations102in tipping paper100. The problem with such a conventional arrangement, as best shown inFIG. 3, is that the large circular area of light106has a diameter of about ten millimeters (mm) and illuminates an area having a number of perforations102. Thus, the fine scanning and resolution capabilities of the conventional optical monitoring system are poor, reducing the reliability and accuracy of such a system.

Thus, there is a need in the art to provide a system and method for measuring the permeability of a material such as tipping paper that overcomes the problems of the related art.

SUMMARY OF THE INVENTION

The present invention solves the problems of the related art by providing a system and method for measuring the permeability of a material such as tipping paper.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A system for measuring the permeability of a material in accordance with an aspect of the present invention is shown generally as reference numeral10A inFIG. 2Aand reference numeral10B inFIG. 2B.FIG. 2Ashows an arrangement where light sources12are provided above a tipping paper100, and light detectors18are provided below tipping paper100. Alternatively, as shown inFIG. 2B, light sources12may be provided below tipping paper100and light detectors18may be provided above tipping paper100. The alternative arrangement ofFIG. 2Badds a supplemental protection of light detectors18from the environmental light, which in most cases comes from the ceiling and can generate an error signal.

As used herein, the term “material” includes, but is not limited to, products or materials with holes or perforations that require their permeability to be measured. Examples of such products and materials needing permeability measurements include: wallpaper; filters used for air, chemicals, etc.; materials affording the appropriate degree of liquid (ink, varnish, sizing) absorption in printing; porous bags and materials used in food packaging and agricultural fumigation; insulating materials; paper; textiles; wrappers of filter cigarettes or similar rod-shaped tobacco products; etc.

A. System Overview

System10A or10B includes light-based permeability measuring instruments, such as, for example, a light or laser source or sources12and an optical or light sensor or sensors (detectors)18.FIGS. 2A and 2Bshow two light sources12and two light sensors18for use with tipping paper100, because tipping paper100typically includes two sets of rows of perforations102. However, system10A or10B is not limited to this number of light sources12and light sensors18, and may include more or less than two light sources12and two light sensors18, depending upon the application of system10A or10B. As shown inFIGS. 2A and 2B, light sources12produce narrow lines of light14that illuminate tipping paper100and, a portion of which, extends through and emanates from perforations102as light beams16which are eventually received by light sensors18. As discussed more fully below with reference toFIG. 12, light sensors18may convert the optical data received from light beams16into electrical data that may be used to determine the propriety of the quality of perforations102.

B. Types Of Light Sources

Preferably, light source12is a polarized light source (such as a laser) instead of the traditional non-polarized light source (usually a high-intensity halogen light) used in conventional optical monitoring systems, as shown inFIG. 1. With a polarized light source12, light traveling through perforations102, hereinafter referred to as “direct light”, remains polarized, while the light penetrating through the non-perforated areas of tipping paper100, hereinafter referred to as “stray light”, changes its polarization characteristics. This makes it possible to distinguish between direct light and stray light, as discussed more fully below with reference toFIGS. 6A,6B,7A, and7B.

Use of a laser for light source12provides a coherent, modulated or non-modulated light source with which to scan the material (e.g., tipping paper100). Coherent light properties, such as monochromaticity and low divergence, increase the performance of the optical configuration of system10A or10B. Other advantages of using a laser for light source12instead of a conventional halogen-based light source include: increased life (a laser has one order of magnitude more life than a halogen light); lower power requirements for the laser; smaller size of the laser; etc.

The wavelength of the laser used as light source12in system10A or10B may be in general in the red light spectrum (e.g., approximately 660 nanometers(nm)). However, a violet or ultra-violet laser light source may be used instead of, or preferably in combination with, the red laser light source. A light with a wavelength as low as 405 nm (violet light), or even as low as 350 nm (ultra-violet light), helps to reduce the stray light component, eliminating the differences between tipping papers having different colors (for example, tipping papers typically come in white, cork, and cork-on-white colors). However, currently, violet and ultra-violet light lasers are not the preferred choice for light source12because of their larger size and higher price than red light lasers, but as technology evolves violet and ultra-violet light lasers are expected to decrease in size and price.

The utility of using a violet or ultra-violet light laser as light source12is best seen inFIG. 8. As shown inFIG. 8, the paper absorption factor of tipping paper100is very small, but different for white, cork, and cork-on-white tipping paper. Therefore the stray light component will be different for different color tipping papers. However, decreasing the wavelength towards the ultra-violet, the paper absorption factor increases considerably so that around 350 nm the stray light component is expected to be negligible, leading to more accurate measurement resulting from a high signal-to-noise ratio. The use of violet or ultra-violet light for this purpose is not limited to use with lasers, but rather is applicable to any light source, including conventional halogen-based light sources.

FIG. 3shows the narrow line of light14produced by light source12, as compared to the large illumination area106produced by conventional light source104. The exemplary dimensions of the narrow line of light14, as shown inFIG. 3, are approximately 0.1 mm (or 100 microns) wide and approximately ten mm long. Although the dimensions of the narrow line of light14shown inFIG. 3are preferred for tipping paper100having a low permeability of 50 to 500 Coresta units (smaller holes) and having a high permeability of 500 to 2500 Coresta units (larger holes), the dimensions of narrow line of light14are in no way limited to these values. Rather, the dimensions of narrow line of light14may vary depending upon the application of system10A or10B. Narrow line of light14may be produced with special optics inserted in front the laser, rather than by limiting the light field with a physical aperture. As further shown inFIG. 3, the total illuminated area of narrow line of light14is approximately two orders of magnitude smaller than the illuminated area of the traditional light source104(as represented by circle106). This permits a very fine scanning of tipping paper100, which improves the resolution and quality of system10A or10B over the conventional light permeability measuring system.

As shown inFIGS. 4 and 5, the system10A or10B of the present invention may be used to detect skipped (or missing) perforations102down to the level of a single missing perforation102.FIG. 4shows narrow line of light14scanning a tipping paper100that is not missing any perforations102, whereasFIG. 5shows narrow line of light14scanning a tipping paper100that is missing one perforation102, wherein the missing perforation102is indicated by reference numeral112. The signal generated by system10A or10B when used to scan the tipping paper100shown inFIG. 5will be one half of the signal generated by system10A or10B when used to scan the tipping paper100shown inFIG. 4because the total area of the tipping paper allowing light to pass through (i.e., the perforations102) has been reduced in half. This approach is particularly efficient for tipping papers with one single row of perforations.

The direct digital pre-processing of optical signals allows inspection of very small portions of tipping paper100, hereinafter referred to as “segments” and “sub-segments”, at speeds up to 1500 meters per minute. The concept and capability of measuring defined length segments and sub-segments combined with fast processing of the data signals is instrumental for detecting skipped perforations (or missing holes) in tipping paper100.

D. Alternative Optical Arrangements

As shown inFIGS. 6A,6B,7A, and7B, system10A or10B of the present invention may have two different optical arrangements.FIGS. 6A and 6Bshow a first arrangement with an angled (or tilted) stray light sensor, andFIGS. 7A and 7Bshow a second arrangement with a polarized beam splitter and a straight stray light sensor. Each optical arrangement will be described in turn.

FIG. 6Ashows the path of direct light in the first optical arrangement, whereasFIG. 6Bshows the path of stray light in the first optical arrangement. As shown in these Figs., the first optical arrangement includes light source12that generates light through line forming optics20to create narrow line of light14. Line of light14illuminates tipping paper100, and direct light22travels through perforation102and enters light detector18through an aperture23. Light detector18further includes: a stray light sensor24for measuring stray light; an optical beam collimating lens26for focusing direct light22; a polarization filter28for filtering out stray light; a stray light filter30having an aperture31that further filters out stray light; and a direct light sensor32for sensing direct light22. Direct light22enters light detector18through aperture23, bypasses stray light sensor24due to aperture23, is focused by optical lens26, travels through polarization filter28and aperture31, and is sensed by direct light sensor32. Polarizing filter28filters out stray light, but allows direct light22to pass through, enhancing the separation between direct light22and the stray light by increasing the signal-to-noise ratio.

FIG. 6Bis identical toFIG. 6A, except thatFIG. 6Bshows the path of stray light34as it travels through tipping paper100. Although most of the stray light34fails to enter light detector18, some stray light34does enter light detector18through aperture23. It is not desirous to have stray light34enter direct light sensor32. As shown inFIG. 6B, the first optical arrangement prevents stray light34from being detected by direct light sensor32. Stray light34is prevented from being detected by direct light sensor32because first, the polarization filter28reduces those components of stray light34with different polarization than direct light22, and then aperture31reduces the components with the same polarization as direct light22. In addition, the different focusing distances for direct light22and stray light34prevents stray light34from being detected by direct light sensor32. Direct light22is generated at a distance g1from optical lens26, allowing the re-collimated direct light22to focus on direct light sensor32at a distance h1. At the same time, the stray light34is generated at the tipping paper100at a distance g2(which equals the focal distance f of optical lens26). This arrangement causes the re-collimated stray light34to focus beyond direct light sensor32, at a distance h2. Calculating mathematically using the following optical equations:

1f=1g1+1h1=1g2+1h2,
and solving for distance h2provides:

h2=g2*fg2-f.
Thus, as distance g2approaches the focal distance f, then distance h2approaches infinity. At the same time, aperture23and the angled position of stray light sensor24prevent direct light22from reaching stray light sensor24. The stray light signal generated by stray light sensor24may be used to identify changes in the transmissive property of tipping paper100that may be created by variations in tipping paper color intensity or thickness, so as to detect changes in the basis weight and allow these variations to be removed from the signal generated by direct light sensor32through software (see the calibration equation discussed below).

FIG. 7Ashows the path of direct light in the second optical arrangement, whereasFIG. 7Bshows the path of stray light in the second optical arrangement. As shown in these Figs., the second optical arrangement is identical to the first optical arrangement shown inFIGS. 6A and 6B, except the angled stray light sensor24is not angled in the second optical arrangement shown inFIGS. 7A and 7B. Rather, a polarized beam splitter36is provided and stray light sensor24is aligned with polarized beam splitter36. Such a configuration eliminates the need for precise angle mounting of stray light sensor24, improves the reproducibility of the optical arrangement, and improves the consistency of the sensor performance. Polarized beam splitter36directs most of the stray light34toward stray light sensor34, and the residual stray light34(having the same polarization as direct light22) is prevented from reaching direct light sensor32by optical lens26and aperture31. Another difference in the second optical arrangement is that polarization filter28is not used. Instead, a polarization filter38is provided between polarized beam splitter36and stray light sensor24to help remove residual, reflected components of direct light22from the stray light34entering stray light sensor24. Thus, the second optical arrangement separates the direct light from the stray light even more efficiently than the first optical arrangement.

Dithering of light source12may be used to minimize the effect of inherent differential non-linearity of the light intensity by averaging the intensity values across the narrow line of light14. The light intensity across the narrow line of light14usually has variations. Such variations are called “integral non-linearity” for the entire ten millimeter length of the narrow line of light14. Variations are called “differential non-linearity” for contiguous small segments of the ten millimeter length.

A typical cross profile of a laser light source intensity across the narrow line of light14is shown inFIG. 9, with an integral non-linearity of 9% and a differential non-linearity of 2%. If one considers only a six millimeter length of the line of light14(it is assumed that that a maximum of six rows of perforations102will encompass six millimeters), the differential non-linearity will be 2%. This means that the measuring error for tipping paper100having a single row of perforations could be as high as 2% if the position of the holes changes by 0.3 mm, as shown inFIG. 10. In order to reduce this error, laser light source12may be moved alternately left to right within ±1 mm from the center position, resulting in an average repeatability error of less than 0.5%, as shown inFIG. 11. The signal component resulting from the oscillating movement may be digitally filtered out. Such dithering may be accomplished in a number of ways, including mechanically with a mechanism using a servo motor, electrically with a piezoelectric crystal attached to light source12, etc. The dithering principle may be applied to any light source used for measuring tipping paper permeability, and may be extended to measuring other properties of different materials using light scanning. Dithering of light source12may be efficient for tipping paper winding systems with very stable lateral movement. For less stable systems in which the paper moves sideways randomly and continuously, the paper movement has the same effect as the light source dithering, so the light source12may remain in a fixed position without any dithering movement.

F. Calibration Of The System

System10A or10B of the present invention may be calibrated with the calibration targets (or standards) disclosed in co-pending U.S. patent application Serial No. 10/854,438, assigned to the assignee of the present invention, Philip Morris USA, Inc., the entire disclosure of which being incorporated by reference herein.

G. Signal Processing

FIG. 12is an electrical schematic showing the details of direct light sensor32and stray light sensor24, as shown inFIGS. 6A,6B,7A, and7B, and how they interact with a digital processing device such as a control board62. Control board62may be housed within light sensor18, but may also be external to light sensor18. In one aspect of the present invention, a smart digital light sensor is used for light sensor18for measuring light passing through perforations102of tipping paper100. Such a smart digital light sensor includes an integrated digitizer and digital signal pre-processing (“DSP”) for fast interpretation of signals generated by direct light sensor32and stray light sensor24. A smart digital light sensor does not need any physical adjustment related to brand changes or measuring range, whereas conventional analog sensors require several analog adjustments (e.g., potentiometers).

As shown inFIG. 12, the light from light source12is received by direct light sensor32and stray light sensor24and converted into an analog electrical signal with a photo sensor40. The analog electrical signal is then amplified with amplifiers42,44,46, and converted into a digital electrical signal with an analog-to-digital (“A/D”) converter or integrated digitizer48. One A/D converter48cooperates with a gain control50. The digital electrical signals are then provided to a digital pre-processor and control FPGA (field programmable gate array)52where they are pre-processed and output, via a serial input/output port60, to a computing device112for storage or further processing. Control board62further includes a power supply54(made up of three regulators/filters), an internal clock56, and an external clock58.

Computing device112represents a combination of hardware and software, and thus may comprise a conventionally programmed computer, a programmed logic controller (“PLC”), a microcontroller embedded with software, or any other intelligent system. Computing device112may be used in place or in conjunction with digital pre-processor and control FPGA52. Further, computing device112may not be used at all if digital pre-processor and control FPGA52includes at least a memory device.

Referring toFIG. 13, if computing device112is a conventionally programmed computer, then such a computer may include a bus200interconnecting a processor202, a read-only memory (ROM)204, a main memory206, a storage device208, an input device210, an output device212, and a communication interface214. Bus200is a network topology or circuit arrangement in which all devices are attached to a line directly and all signals pass through each of the devices. Each device has a unique identity and can recognize those signals intended for it. Processor202includes the logic circuitry that responds to and processes the basic instructions that the drive computer. ROM204includes a static memory that stores instructions and data used by processor202.

Computer storage is the holding of data in an electromagnetic form for access by a computer processor. Main memory206, which may be a RAM or another type of dynamic memory, makes up the primary storage of the computer. Secondary storage of the computer may comprise storage device208, such as hard disks, tapes, diskettes, Zip drives, RAID systems, holographic storage, optical storage, CD-ROMs, magnetic tapes, and other external devices and their corresponding drives. Main memory206and/or storage device208may store any of the data retrieved from any of the components of the present invention.

Input device210may include a keyboard, mouse, pointing device, sound device (e.g. a microphone, etc.), biometric device, or any other device providing input to the computer. Output device212may comprise a display, a printer, a sound device (e.g. a speaker, etc.), or other device providing output to the computer. Communication interface214may include network connections, modems, or other devices used for communications with other computer systems or devices.

Communication links216may be wired, wireless, optical or a similar connection mechanisms. “Wireless” refers to a communications, monitoring, or control system in which electromagnetic or acoustic waves carry a signal through atmospheric space rather than along a wire. In most wireless systems, radio-frequency (RF) or infrared (IR) waves are used. Some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing.

Computing device112consistent with the present invention may perform the tasks of receiving digital signals from control board62and storing the signals or producing an output that is the light permeability equivalent of the air permeability of tipping paper100from the signals generated by direct light sensor32and stray light sensor24, using the measuring algorithm discussed below. However, control board62may perform these tasks on its own as well. Computing device110may perform these tasks in response to a processor executing sequences of instructions contained in a computer-readable medium. A computer-readable medium may include one or more memory devices and/or carrier waves.

Execution of the sequences of instructions contained in a computer-readable medium causes the processor to perform the processes described below. Alternatively, hardwired circuitry may be used in place of or in combination with software instructions to implement processes consistent with the present invention. Thus, the present invention is not limited to any specific combination of hardware circuitry and software.

In order to calculate the equivalent air permeability of tipping paper100from the signals generated by direct light sensor32and stray light sensor24, the measuring algorithm uses specific parameters determined during system calibration. The calibration curve slope Cslopeand intercept Cint, as described in co-pending U.S. patent application Serial No. 10/854,438, are calculated during calibration. The algorithm used during calibration is tailored to the specific configuration of the sensor being calibrated. If the sensor configuration changes, then the algorithm will change as well. For example, a calibration equation which defines the correlation between light permeability and air permeability may be created by measuring two different, previously certified targets with an air-flow measuring instrument and a light measuring instrument. These measurements provide first and second air permeabilities AP1and AP2which correlate with first and second light permeabilities LP1and LP2. These values enable the calibration parameters of the calibration equation to be calculated, namely the slope Cslopeand the intercept Cintof the equation. The calibration equation will thus be AP=Cslope×LP+Cint, where:

The calibration equation defines the correlation between light permeability and air permeability, which can be considered linear for a limited range of permeability values. Once the slope Cslopeand intercept Cintare calculated, the light permeability of a material may be measured, and based upon the calibration equation the equivalent air permeability (AP) of the material may be calculated. Another parameter used in the calculation is called the paper factor (PF), which is the ratio between the signals generated by stray light sensor24and direct light sensor32as measured with non-perforated paper. The paper factor (PF) permits correction of the impact that the residual stray light on direct light sensor32, and helps determine inherent variations of the paper basis weight. The equations used to calculate the paper factor (PF) and permeability (P) are:

PF=ADdirectADstray,
and
P=∫{Cslope×[(ADdirect−Odirect)−PF×(ADstray−Ostray)]+Cint},
where Cslopeis the slope of the calibration curve, Cintis the intercept of the calibration curve, ADdirectrepresents the analog-to-digital (A/D) counts measured by direct light sensor32, Odirectis the offset of direct light sensor32, ADstrayrepresents the A/D counts measured by stray light sensor24, Ostrayis the offset of stray light sensor24, and PF is the paper factor. The offsets (Odirect, Ostray) represent residual currents of sensors24,32with light source18turned off.

H. Speed Independent Measurement

The permeability measurement by system10A or10B of the present invention is independent of the tipping paper velocity since the data is collected at sampling intervals determined by pulses generated with a shaft encoder (which is the external clock58shown inFIG. 12) installed on the rewinding drum of the tipping paper machine, which moves in synch with the tipping paper.

I. Automatic Correction Of Calibration Parameters

Accuracy of system10A or10B of the present invention may deteriorate over time due to aging of light source12, light sensor offset variations due to temperature changes, dust accumulation on the optical components, etc. In order to keep system10A or10B operating at maximum performance, a measurement of the light transmission through a very fine aperture (inserted in between the light source and light sensor, like a piece of paper, but in a very stable and mechanically repeatable position) may be used to compare the entire light transmission capability of the measuring head. A first measurement may be performed during system10A or10B installation, and then performed periodically (e.g., once per shift or before each bobbin run). A deviation larger than a predetermined amount would require application of a correction to the original values of either the slope Cslopeor the intercept Cintparameter of the calibration curve, which restores the original transmission characteristics of the measuring channel.

It will be apparent to those skilled in the art that various modifications and variations can be made in the calibration system and target of the present invention and in construction of the system and target without departing from the scope or spirit of the invention. Examples of such modifications have been previously provided.