"Fibre Assessment Apparatus and Method"

Field of the Invention

The present invention relates to an apparatus and method for the assessment of the quality and form of fibres. The apparatus and method are particularly relevant to the assessment of monofilament fibres, such as yarns, wire, and optical fibres.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Furthermore, throughout the specification the term fibre is used to denote any fibre, filament, yarn, thread, optical fibre, wire or other threadlike structure.

Background Art

The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention.

It is often necessary to assess fibres for quality and form, for example to measure the diameter of fibres for a variety of reasons. The diameter of textile fibres such as wool can be used to determine the quality and type of the fibres, and the measurement of fibre diameters and in particular the detection of sudden changes in diameter are indicators of fibre damage. Accurate diameter measurement will reduce wastage of expensive material in the case of wires. Online measurement of thin places in extruded fibres allows extrusion to be stopped before kilometres of fibre may be wasted. Optical fibre performance can be affected by the mean diameter, variation in diameter and ovality of the fibre, as well as impurities and imperfections within the fibre itself.

There are fibre diameter measuring apparatus that use laser technology. Laser systems are slow which means that they can miss small defects on fast production lines, are difficult to calibrate due to laser drift, can only measure one fibre at a time, cannot take images of the fibre and are sensitive to dust. An instrument is available from a company called Sensoptic™ SA that uses a collimated LED and a photodetector which produces relative fibre diameters to detect regions of thickness and thinness. It is also affected by dust build up and so cannot distinguish between a gradual build up of dust and a gradual increase in fibre diameter. It can only measure one fibre at a time and is sensitive to vibration and requires the fibre to be well guided through the measurement zone.

The present invention seeks to provide a fibre assessment apparatus and method that alleviates some or all of these problems to at least some extent.

Disclosure of the Invention

In accordance with a first aspect of the present invention, there is provided a fibre assessment apparatus comprising a first light source, a first image sensing means and a first focusing means for focusing an image of a fibre arranged between the first light source and the first focusing means on the first image sensing means, the first light source, first focusing means and first image sensing means all being arranged on a first measuring axis substantially perpendicular to the longitudinal axis of the fibre, and the apparatus further comprising processing means coupled to the first image sensing means and operable to digitize the sensed image and to store the digitized image as an array of pixels, each pixel having a value indicative of the amount of light falling on the first image sensing means at that location, to scan the array of pixels and to determine pixel values for each pixel, whereby a first edge of the digitized image is determined by a significant drop in pixel value, and a second edge of the digitized image is determined by a significant rise in pixel value, and to determine the diameter of the fibre from the detected first and second edges of the stored digitized image.

Preferably, the processing means is operable to determine the diameter of the fibre by summing the value of each pixel stored across the stored digitized image,

subtracting this sum from the average light value at the fibre edges multiplied by the number of pixels of the stored digitized image, and dividing this value by the average light value.

Alternatively, the processing means is operable to determine the diameter of the fibre by determining the pixel value at the outside edges of the stored digitized image as a percentage of the pixel values outside the fibre, carrying out a linear interpolation at each edge to provide a sub-pixel location of the edge, and determining the difference between the two sub-pixel edges to generate the diameter of the fibre.

Preferably, the light source is an ultra bright light emitting diode.

Preferably, the image sensing means is a Charge Coupled Device ("CCD") or Complementary Metal Oxide Semiconductor ("CMOS") sensor array. The sensor array may be a single row or a two-dimensional array.

Preferably, the apparatus is arranged to measure a plurality of fibres, and the light source comprises a plurality of individual light sources, with the apparatus having a diffusing means provided adjacent the plurality of individual light sources for diffusing the light incident on the plurality of fibres.

Preferably, each individual light source in the plurality of individual light sources is independently adjustable for uniformity of emitted light.

Preferably, the diffusing means comprises a holographic diffuser.

Preferably, the apparatus comprises a second light source, a second image sensing means and a second focusing means for focusing the image of a fibre arranged between the second light source and the second focusing means on the second image sensing means, the second light source, second focusing means and second image sensing means being arranged on the same second measuring axis which is substantially perpendicular to the first measuring axis along which the first light source, the first focusing means and the first image sensing means are located.

Preferably, the apparatus comprises polarizing filters provided along both the first and second measuring axes.

Preferably, the apparatus comprises additional measuring axes, with each additional measuring axis having arranged thereon a corresponding additional light source, additional image sensing means and additional focusing means for focusing the image of a fibre arranged between the additional light source and the additional focusing means on the additional image sensing means.

Preferably, the fibre is translatable along its axis and the light source is arranged to strobe to thereby generate a still image of the translating fibre.

Preferably, the image sensing means includes shutters switchable between open and closed positions, whereby the shutter of one image sensing means is open only when the corresponding light source is flashing.

Preferably, the apparatus is provided with additional light sources offset from the first measuring axis, wherein the additional light sources are arranged to strobe.

In accordance with a second aspect of the present invention, there is provided a method of fibre assessment, the method comprising: arranging a fibre to be assessed between a first light source and a first focusing means, the fibre being arranged such that its longitudinal axis is substantially perpendicular to a first measuring axis along which the first light source and the first focusing means are arranged; focusing an image of the fibre onto a first image sensing means also arranged on the first measuring axis; digitizing the sensed image; storing the digitized image as an array of pixels, each pixel having a value indicative of the amount of light falling on the first image sensing means at that location; scanning the array of pixels to determine pixel values for each pixel; determining a first edge of the digitized image from a significant drop in pixel value; determining a second edge of the digitized image from a significant rise in pixel value; and determining the diameter of the fibre from the detected first and second edges of the stored digitized image.

Preferably, the diameter of the fibre is determined by summing the value of each pixel stored across the stored digitized image, subtracting this sum from the

average light value at the fibre edges multiplied by the number of pixels of the stored digitized image, and dividing this value by the average light value.

Alternatively, the diameter of the fibre is determined by determining the pixel value at the outside edges of the stored digitized image as a percentage of the pixel values outside the fibre, carrying out a linear interpolation at each edge to provide a sub-pixel location of the edge, and determining the difference between the two sub-pixel edges to generate the diameter of the fibre.

Preferably, the light source is independently adjustable for uniformity of emitted light.

Preferably, the method further comprises providing a second measuring axis that is substantially perpendicular to the first measuring axis and arranging a second light source, a second image sensing means and a second focusing means along the second measuring axis and arranging the fibre between the second light source and the second focusing means, wherein the method further comprises: focusing a second image of the second fibre onto the second image sensing means; digitizing the sensed second image of the second fibre; storing the digitized second image of the fibre as an array of pixels, each pixel having a value indicative of the amount of light falling on the second image sensing means at that location; scanning the array of pixels to determine pixel values for each pixel; determining a first edge of the digitized image of the fibre from a significant drop in pixel value; determining a second edge of the digitized second image of the fibre from a significant rise in pixel value; and; determining a second value of the diameter of the second fibre from the detected first and second edges of the stored digitized second image.

Preferably, the method further comprises providing additional measuring axes, with each additional measuring axis having arranged thereon a corresponding additional light source, additional image sensing means and additional focusing means for focusing the image of a fibre arranged between the additional light source and the additional focusing means on the additional image sensing means.

Preferably, the method comprises translating the fibre along its axis and strobing the light source to thereby generate a still image of the translating fibre.

Preferably, the image sensing means comprises shutters switchable between open and closed positions, wherein the method comprises opening the shutter of one image sensing means only when the corresponding light source is flashing.

Preferably, the method further comprises providing additional light sources offset from the first measuring axis, wherein the additional light sources are arranged to strobe.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic representation of a side view of a first embodiment of a fibre assessment apparatus in accordance with an aspect of the present invention;

Figure 2 is a plan view of the apparatus of Figure 1 in the direction of arrow Il in Figure 1 ;

Figure 3 is a schematic block diagram representing the electronic components of the apparatus of Figure 1 ;

Figure 4 is a schematic representation of a second embodiment of a fibre assessment apparatus in accordance with an aspect of the present invention using dual axis measurement;

Figure 5 is a schematic representation of a third embodiment of a fibre assessment apparatus in accordance with an aspect of the present invention for multiple fibre assessment; and

Figure 6 is a schematic diagram illustrating the timing of LED flashes in the apparatus of Figure 5.

Best Mode(s) for Carrying Out the Invention

Figure 1 shows - schematically - the arrangement for a first embodiment of a

! fibre assessment apparatus 1 in accordance with the present invention. The fibre assessment apparatus 1 comprises light sources in the form of one or more light emitting diodes ("LED's") 2, focussing means in the form of a focussing lens 3, and image sensing means in the form of an image sensor 4. A fibre 5 whose dimensions are being measured is located between the LED 2 and the lens 3. The fibre 5 is arranged to continuously move in the direction of its length. The apparatus 1 can be used to measure fibres as they are extruded, in which case they are moving as part of the extrusion process. If the fibre is being measured later, then the assessment apparatus 1 can be provided with fibre handling means which will simultaneously move and rotate the fibre using stepper motors (not shown). Any suitable means of movement and/or rotation can be used to handle the fibre. A consequence of its fast movement is that the fibre 5 will vibrate. The LED 2 is therefore arranged to strobe in order to 'freeze' the image of the fibre 5 as will be further discussed.

Preferably, the LED 2 is an ultra bright LED. The brighter the LED, then the shorter the strobe time required to 'freeze' the image of a vibrating fibre - as will be discussed in further detail below.

The LED 2 can be collimated or diffuse. Collimated LED's provide a greater depth of field, whereas diffuse LED's are better used when more than one fibre is being imaged. An embodiment for imaging more than one fibre is discussed below. As mentioned above, the LED 2 is arranged to operate in a strobe fashion using a short high current pulse in order to 'freeze' the image of the vibrating fibre.

The image sensor 4 is a linear sensor, i.e. a single line of photo-sensors using Charge Coupled Device ("CCD") or Complementary Metal Oxide Semiconductor ("CMOS") technology. In alternative embodiments of the invention, alternative

sensing means may be used such as an area sensor or sensors using alternative detection technologies. The use and operation of CCD and CMOS technologies are well known and need not be described in any further detail herein except as is relevant to the present invention. High precision, fast linear CCD sensors are costly, so, in this embodiment, the image sensor 4 is a low cost CCD sensor which has most of its optical sensing area covered by an opaque tape so that only a small portion of the array is used. This allows a typical fax machine 2000 pixel sensor to become a 100-200 pixel high speed sensor. This can be achieved because the line scan rate is inversely proportional to the number of pixels used on the array, so a speed increase of more than 10 times can be obtained by obscuring some of the pixels in the array.

The LED 2, lens 3, and image sensor 4 are all arranged on substantially the same measuring optical axis 6 - as shown in Figure 1. The fibre 5 to be measured is then placed substantially at right angles to the optical axis 6 between the LED 2 and the lens 3. The actual separation distances between the various components will depend upon the fibre size range. Typically, however, the distance between the fibre 5 and the lens 3 is in the range 15mm to 40 mm for a fibre diameter range of 20μm to 1 mm.

To reduce the effect of light transmission through transparent fibres, the LED 2 can have a narrow slit placed in front of it at right angles to the fibre direction, and the fibre 5 rotated so that light through the fibre 5 is reduced.

In use, light from the LED 2 passes by the fibre 5 and is focused by the lens 3 onto the image sensor 4 to provide an image which can be detected by the image sensor 4.

The output from the image sensor 4 is output to an amplifier 7 and digitized using a digitizer 8. The output from the digitizer 8 is input into a memory array 9 provided in processing means in the form of a computer 10 or embedded microprocessor - see Figure 3 - to provide a captured line of pixels, each pixel having a pixel value which is indicative of the amount of light that fell on the

corresponding portion of the image sensor 4. The higher the pixel value, the more light that was incident on that portion.

The computer 10 is operable to execute application software stored in memory thereof, such as a fibre assessment program that is operable to enable the computer 10 to perform various functions, described in further detail below.

The computer 10 is operable to analyse the image stored in the memory array 9 to determine the diameter, the focus and the opacity of the fibre 5 as will be described in more detail below. The amplifier 7, digitizer 8, computer 10 and memory array 9 can be of any suitable, known, type.

The computer 10 can detect diameter defects based on preset thresholds entered by a user, and an alarm can be triggered and/or a picture of the defect captured from the digitized and stored image. Results of the measurement are collated into histograms and these are transferred at regular intervals to a display (not shown) via any suitable serial, wireless or wired network link. This will be described in further detail below.

As mentioned above, a high pixel value in the memory array 9 indicates more light is falling on that pixel.

The captured line of pixels is scanned until a falling edge, i.e. a dramatic drop in the pixel value, is detected - indicating the start of a fibre 5.

This start position is saved in memory of the computer 10 and the scan continues until a rising edge is detected, which indicates the end of the fibre 5. This finish position is also saved in memory in the computer 10.

The diameter of the fibre 5 is measured by one of two ways depending on the diameter of the fibre 5 and its transparency:

a) Summation of the shadow: the value of each pixel across the fibre image is summed; this sum is subtracted from the average light value at the fibre edges multiplied by the number of pixels. This is then normalized by

dividing by the average light value to remove the influence of slight variations in light intensity.

b) Threshold width: a threshold is calculated as a percentage of the light value at each edge of the fibre, and a linear interpolation is performed at each edge to provide a sub-pixel location of the edge. A threshold value of 50% would indicate that the pixel value at the edge is half, i.e. 50%, of that beyond the edge of the fibre 5. The difference between the 2 sub- pixel edges gives the sub-pixel width of the fibre image. The ideal threshold is that which gives the least variation when the fibre 5 is slightly out of focus. Typically the threshold is close to or slightly above 50%.

The computer 10 is operable to display on its display (not shown) a calibration menu which allows a user to input variables such as fibre type, the distance of diameter width per pixel (in μm per pixel), and the number of measurements per second to be taken, via a keyboard, mouse, or other suitable user interface (not shown). Typically, 5μm to 100μm of diameter width per pixel of the captured image is used. This will depend upon the width of the fibre - the greater the diameter; the more μm per pixel is required.

After the user has selected the appropriate values for the parameters, then the computer 10 is operable to select the more appropriate of the two algorithms discussed above. If the fibre type is entered as transparent, then the threshold width algorithm would be used, and if the fibre type is selected as opaque, then the summation algorithm would usually be used.

The focus of the fibre is measured by the difference between the widths of the fibre image at two thresholds (a first or high and a second or low threshold), as measured using the sub pixel algorithm described above. Typically, the high threshold is near 90% and the low threshold is near 50%. To save calculation time, the low threshold is usually the same as the diameter measurement threshold. Bad focus values depend upon the fibre diameter and the required accuracy. For less accurate measurement, higher focus values (i.e. greater measured differences at the two thresholds) can be tolerated. Typically, a suitable focus value would be less than 10um. The thresholds are normally set

during manufacture and stored in memory of the computer 10 in a set up file. The allowed focus value can be set using the calibration menu.

A higher focus difference indicates the fibre 5 is less focused. The software will reject a fibre 5 that is too far out of focus and alert the user to reposition the fibre 5 or realign the apparatus 1.

A major advantage of using the focused image approach over the collimated approach of the laser or simple LED system is that dust buildup on the face of the optics has little effect on the measurement. Furthermore, dust particles floating in the air can be rejected since they appear out of focus. As dust level increases, or as the LED 2 wears out, the flash length of the LED strobe is increased by the computer 10 or microprocessor to compensate. When the dust level is detected to be too high, the user is alerted to clean the optical surfaces with, for example, an air blast or brush. The dust level is detected by a drop in the light level on either or both sides of the fibre. This relative drop can be set in the calibration menu.

Further embodiments of the invention will now be described. Corresponding numerals are used to denote like elements of the first and further embodiments.

In a second embodiment of the invention, the apparatus 1 can be arranged to measure the diameters of more than one fibre 5 simultaneously. This is illustrated in Figure 4. In this embodiment, there are provided two or more image sensors 4, two or more lenses 3 and LED's 2 which are arranged in a linear array 12 to allow very wide scans. A semi-opaque diffuser 11 can create a smooth, diffuse light source from the multiplicity of LED's 2. An ideal type of diffuser is a holographic diffuser. Each LED 2 or group of LED's 2 needs to be separately strobed to allow for differences in LED 2 intensity and for reduction in light captured from LED's 2 towards the ends of the array 12. Each LED 2 is also separately adjustable to improve light uniformity. This embodiment can measure hundreds of fibres 5 in one plane up to several metres wide.

In yet another embodiment of the invention, the apparatus 1 can be arranged to measure the fibre diameter from one, two or more axes. This is particularly useful for measuring the ovality of a fibre 5. Some fibres - such as optical fibres - need

to be very circular. Differences in the diameter measurements in orthogonal direction indicate a fibre that is non-circular. The fibre can then be rejected, for example.

In the third embodiment shown in Figure 5, the apparatus 1 is arranged to measure along two orthogonal axes. To measure along two axes 13, 14, two LED's 2a, 2b are used with two lenses 3a, 3b and three image sensors 4a, 4b. As mentioned above, these are placed perpendicular to each other, and focused at the fibre position. To prevent light from one LED entering the wrong image sensor, images can be strobed at different times. This can be achieved by flashing one LED during the time that the electronic shutter of the image sensor used to capture the image of the fibre 5 created by the other LED is closed - and vice versa. This is illustrated in Figure 6, which shows the timing of the image sensor synchronizing pulse, the image sensor shutter opening time and LED flash time for the apparatus 1 as illustrated in Figure 5. Figure 6 shows that the sensor synchronizing pulse and the image sensor shutter time is set so that flash time for the first LED 2a falls within the shutter open period for image sensor 4a and within the shutter closed period for image sensor 4b. Conversely, the flash time for the second LED 2b falls within the shutter open period for image sensor 4b and within the shutter closed period for image sensor 4a. In this way, the flash for LED 2a is seen only by image sensor 4a and not by image sensor 4b, and vice versa.

Alternatively, the beams can be separated along the length of the fibre or polarized light can be used to separate the two beams by providing a polarizing filter 15, 16 on both LED's 2a, 2b and both image sensors 4a, 4b, polarizing direction rotated at 90 degrees to the other LED/image sensor.

In another embodiment, more than two axes can be used to more accurately determine the degree of non-circularity.

The assessment apparatus 1 can also include a correction matrix stored in the setup file stored in memory of the computer 10. The correction matrix is used to correct the diameter measurements to allow for optical and light source distortions. The correction matrix is generated during an automated calibration

process after manufacture. The correction matrix can be used for any of the embodiments described above.

In the calibration process, the apparatus 1 measures several fibres of known diameter. These fibres are moved across the measurement region and measured at regular intervals to create an array of correction factors. This array is saved in memory of the computer 10 in the setup file, and when a fibre is measured its position is also measured and this position is used as the index to the correction array, allowing the measured diameter to be corrected. This correction eliminates most of the optical distortion, allowing smaller and much cheaper optics to be used. This array may be 1 or 2 dimensional depending on whether 1 or 2 axes of measurement are available.

A fibre guide may be additionally provided to reduce the vibration of the fibre 5 and to keep the fibre 5 within the measuring area.

In a further embodiment, additional off-axis light sources can be provided. In this embodiment, these off-axis light sources are also light emitting diodes. The additional light sources are alternately strobed at a different time to the on-axis

LEDs provided on the measurement axis. The off-axis light sources allow assessment of light through non-opaque fibres. Light from the off-axis light emitting diodes is received by the image sensor 4 and is processed as described before. In this case, values for light pixels are determined and summed. This summing is carried out at regular intervals and where variations in this summation value from previous summation values are determined, then this is usually an indication of non-circular fibres or the presence of defects within the fibre. This allows for the rapid detection of impurities within fibres, such as bubbles or particles that would degrade the performance of, for example, optical fibres.

It should be appreciated by the person skilled in the art that the invention is not limited to the embodiments described.

Further modifications are possible within the scope of the present invention. For example, the apparatus 1 can be linked to a network central server and results

transmitted via the Internet, either to a remote location or by wireless to a handheld web enabled device to allow an engineer to see the measurements at the point where the fibre 5 is being measured.

A two-dimensional area scan sensor and two dimensional LED array may be provided where it is required to measure grids or meshes of fibres. In this case, the fibres may be at several orientations and the fibre diameter and the area of the holes can be measured.

It should be further appreciated by the person skilled in the art that variations and combinations of features described above, not being alternatives or substitutes, can be combined to form yet further embodiments falling within the intended scope of the invention.