IE970221A1 - Apparatus for Detecting Defects in a Fabric - Google Patents

Abstract

An apparatus for detecting defects in a moving fabric (12) includes a laser line generator (20) for generating a line scanning laser beam, and a detector (30) on the opposite side of the fabric for detecting the amount of light from the laser beam passing through the fabric. The detector (30) includes a bundle of optical fibres (31) whose one ends (32) receive the light from the laser beam and whose other ends (33) are optically coupled to a photodetector PD. An electrical circuit (40) provides an output signal when the amount of light passing through the fabric exceeds a threshold value. <Designate Fig. 1>

Description

The present invention relates to an apparatus for detecting defects in a fabric, such as a knitted cotton fabric.

During the manufacture of knitted cotton greige fabric, continuous and/or discrete defects may occur at irregular intervals in the fabric. These defects impact on the final manufactured product, for example a T-shirt, and results in the product being categorised as substandard. This has a significant impact on margin due to lower selling prices on irregular goods. In some cases, particularly with continuous type defects, the fabric does not even get assembled into a product, but is discarded as waste at an earlier stage in the manufacturing process.

To date detection of defects on knitted fabrics has not been possible using conventional reflective type scanners due to both fabric density and the non-homogenous surface characteristics inherent in the fabric itself.

It is an object of the present invention to provide an apparatus which allows fabric faults to be detected shortly after they have been generated, enabling a machine which is manufacturing the fabric to be stopped automatically.

According to the present invention there is provided an apparatus for detecting defects in a fabric, comprising means for generating a laser beam for scanning a moving fabric, means on the opposite side of the fabric for detecting the amount of light from the laser beam · ίχ··’ ^70221 - 2 passing through the fabric, and means for providing an output signal when the amount of light passing through the fabric exceeds a threshold value.

The invention further provides the combination of a knitting machine of the type which knits a hollow tube of knitted fabric and an apparatus for detecting defects as specified in the preceding paragraph, wherein one of the generating means and detecting means is located inside the tube and the other of the generating means and detecting means is located outside the tube.

An embodiment of the invention will now be described, by way of example, with reference to the· accompanying drawings, in which: Figure 1 is a schematic diagram of an apparatus according to the embodiment of the invention including a laser line generator and a sensor unit disposed on opposite sides of the fabric; Figure 2 is a schematic diagram of electronic circuitry associated with the sensor unit of Fig. 1; and Figure 3 is a schematic view of a knitting machine incorporating the apparatus of Figs. 1 and 2.

The apparatus comprises a laser line generator 20 and a fibre optic sensor unit 30 which are located on opposite sides of a knitted fabric 12. The fabric 12 is produced by a knitting machine 10, Fig. 3, to be described. The laser line generator 20 is a known type of device which emits a narrow laser beam which is repeatedly deflected at high frequency through an angle (fan angle) 22 to provide a repetitive linear scan of an object upon which the beam falls. In the present case the generator 20 is preferably a lmW Class II Visible line generating laser with a 40 degree fan angle.

The generator 20 is located at the focal point of a plano-convex lens 21, so that the angular deflection 22 of the laser beam on leaving the generator is converted to a scan path 23 in which the laser beam moves substantially parallel to itself. This ensures that the laser beam will scan along a line of substantially constant length on the fabric 12, irrespective of the distance between the laser 20 and the fabric.

The fibre optic sensor unit 30 includes a plurality of optical fibres 31; in the present embodiment there are thirty such fibres. At the end 34 of the unit 30 adjacent the fabric 12 the ends 32 of the optical fibres 31 are arranged in a line which is in register with the line traversed by the laser beam as it moves substantially parallel to itself over the scan path 23. Thus, in the absence of the fabric 12, the laser beam would scan along the ends 32 of the fibres 31.

The ends 33 of the fibres 31 remote from the fabric 12 are gathered together so that the light emitted from the fibres falls upon a medium area photodiode PD. As the laser beam passes through the fabric 12, the presence of a defect results in a variation in light intensity at the photodiode PD which is processed by electronic circuitry 40, Fig. 2, to be described.

In the present embodiment, the apparatus is used to detect defects in a tube of fabric produced by a conventional knitting machine 10, Fig. 3. Tin example of a suitable knitting machine is manufactured by Vanguard, USA, and full details will not be given here. Briefly, however, the knitting machine 10 includes a plurality of - 4 spools 13 which supply individual cotton threads 14 or other material to be knitted to needles (not shown) which form a hollow tube 15 of knitted fabric 12. The tube of fabric passes over a roller 16 which rotates around a vertical axis A. The roller 16 thus flattens the tube 15 of fabric into a double ply cloth 18 so that it can be gathered onto a pick up roller 17 which rotates with the roller 16. Rotating the rollers 16 and 17 also causes the flattened tube of fabric 12 to be drawn onto the roller 17. In order to accommodate the increasing diameter of the fabric roll 19 being gathered onto the roller 17, the roller 17 is allowed to move vertically as indicated by the arrows B. In order to support the roller 17, a further roller 17' which again rotates with the rollers 16 and 17, is disposed below the roller 17.

In the machine 10 the line generator 20 including the lens 21 is supported on a bracket 11 on the inside of the tube 15 of fabric 12, and the fibre optic sensor unit 30 including the fibres 31 and photodiode PD is supported on the outside of the tube 15. Thus the knitted fabric 12 is interposed between the generator 20 and unit 30 in the manner shown in Fig. 1. In order to ensure that the laser beam falls substantially perpendicularly on the fabric 12, a hoop 24 is disposed below the line generator 20, inside the tube of material 15, to retain the tube of fabric in a substantially cylindrical state between the generator 20 and unit 30. The hoop 24 is suspended on a number of spokes 25 projecting radially from the lower end of a support member 26 mounted axially within the fabric tube 15. The generator 20 is orientated such that the laser beam scans the fabric 12 in the vertical direction, i.e. parallel to the axis of the fabric tube 15, and the 32 of the optical fibres are likewise vertically ends I «— ~s i 1‘S' ! orientated.

It will be seen that because of the rotation of the rollers 16, 17 and 17', each point on the tube 15 of fabric 12 will follow a downward helical path passing between the line generator 20 and the sensor unit 30. Provided the distance travelled downwardly by the fabric 12 in one rotation of the roller 16 is less than the vertical distance scanned by the laser beam, then any defect, with only some exceptions explained below, can be detected by the sensor unit 30.

The ends of the rollers 16, 17 and 17' are mounted in respective arms 50 one on each side of the knitting machine. These arms 50 cover small portions of the tube of fabric as it rotates between the line generator 20 and the sensor unit 30. It will therefore be seen that defects in these portions will not be detected by the present apparatus.

Electronic signal processing circuitry 40, Figure 2, is coupled to the photodiode PD. The circuitry includes a unity gain high impedance amplifier and filter circuit incorporating an op-amp Al connected to the photodiode PD. The output of the op-amp Al is fed via a current limiting resistor R4 to a comparator circuit including a further op-amp A2. A defect threshold voltage is set at the negative (-) input to the op-amp A2 from the common leg of a potentiometer POT. An 8.2V zener diode ZD is connected in parallel with the potentiometer POT to set a constant reference which is divided by the potentiometer POT to set the threshold voltage. The present circuitry 40 only detects defects where the amount of light passing through the fabric exceeds a value corresponding to the threshold voltage, for example, holes in the fabric or runs in the fabric. - 6 In this case, the voltage at the positive (+) input to the comparator exceeds the threshold voltage and the output O/P of the op-amp A2 is driven high. The output signal O/P is coupled to relay circuitry (not shown) which is actuable to stop to knitting machine.

It will be seen that the processing circuitry 40 is not affected by the presence of the arms 50, as these arms occlude light emitted from the line generator 20 and thus keep the voltage at the positive (+) input of the op-amp A2 below the threshold voltage, and thus the output O/P is not asserted. Therefore, the arms 50 do not generate false defects signals, even though they may obscure defects from the sensor unit 30.

It will be seen that modifications of the above embodiment are possible. For example, the line generator 20 can be mounted outside the tube of material 15 with the sensor unit 30 located inside. Also, the processing circuitry 40 can be enhanced to include a microprocessor connected to the output O/P for identifying the type of defect in the material. Thus, the apparatus could be programmed to distinguish between a single hole and a continuous run, so that the knitting machine could be allowed to continue for the former and would be stopped for the latter.

It is also possible to divide up the ends 33 of the optical fibres 31 into a number of groups of fibres, each with a respective photodiode and comparator circuitry, rather than a single group. This would enhance the resolution of the sensor unit 30 and would enable the processing circuitry to distinsuish between holes of different sizes.

It will also be seen that the comparator circuit including the op-amp A2 could be adapted to become a window comparator circuit. This would enable a measured light signal to be compared both against an upper threshold and a lower threshold voltage. This would enable the invention to be used to detect defects such as pieces of dirt on the fabric.

Claims (5)

1. An apparatus for detecting defects in a fabric, comprising means for generating a laser beam for scanning a moving fabric, means on the opposite side of the fabric for detecting the amount of light from the laser beam passing through the fabric, and means for providing an output signal when the amount of light passing through the fabric exceeds a threshold value.

2. An apparatus as claimed in claim 1, wherein the detecting means comprises a plurality of optical fibres which at one end are arranged to receive light from the laser beam passing through the fabric'and which at the other end are optically coupled to at least one photodetector.

3. An apparatus as claimed in claim 2, wherein the laser beam performs a linear scan of the fabric, and wherein at the said one end the optical fibres are arranged in a line which is in register with the line traversed by the laser beam such that in the absence of the fabric the laser beam would scan along the ends of the fibres.

4. An apparatus as claimed in claim 3, wherein the generating means comprises a laser line generator which generates an angularly scanning laser beam, and a lens which converts the angular scanning of the laser beam to a scanning movement of the laser beam substantially parallel to itself.

5. The combination of a knitting machine of the type which knits a hollow tube of knitted fabric and an apparatus as claimed in any preceding claim, wherein one of the generating means and detecting means is located inside the tube and the other of the generating means and detecting means is located outside the tube.