GB2068113A - Detection of faults in fabrics - Google Patents

Abstract

Faults can be detected in a fabric, such as a tufted fabric, by scanning an illuminated surface of the fabric with a light sensitive device (4) which produces a train of electrical pulses corresponding for example to backstitches (7) projecting at the scanned surface. The pulses are monitored for irregularities and, for example, an alarm signal is produced in response to the detection of a missing pulse corresponding to a missing backstitch. In order to avoid production of false fault signals caused by variations in the background level of illumination, appropriate compensatory measures are taken during monitoring of the pulses. For example, filter circuitry may be used, or, alternatively, the pulses may be compared with slightly delayed versions thereof to facilitate accurate identification of the occurrence of a pulse. <IMAGE>

Description

SPECIFICATION Detection of faults in fabrics This invention relates to the detection of faults in surfaces and is particularly although not exclusively concerned with the detection of faults in tufted fabr ics.

As described in British Patent Applications Nos.

20953/78 and 7917452 (2027191), detection of faults in a tufted fabric can be effected by scanning the backstitch side of the fabric with a light sensitive device which receives light reflected from ortrans mitted through the fabric such that a missing or mal formed tuft or backstitch gives rise to a deviation in the output of the light sensitive device which can be utilised to produce a fault signal. With this arrange ment, however, fluctuations tend to occur in the intensity of light reaching the light sensitive device, due for example to variations in level of illumination of the fabric or in the spacing of the fabric surface from the device, and problems may arise with regard to distinguishing between deviations in the output of the light sensitive device due to such fluctuations and deviations due to fabric fault conditions.Problems may also arise with regard to the production of clearly distinguished pulses having regard to variations in light transmitting or reflecting properties of fabrics and also due to inevitable slight, permissible localised variations in backstitch structure and the like.

An object of the present invention is to overcome or at least appreciably reduce these problems.

According to one aspect of the invention therefore there is provided a method of detecting faults in a regular configuration on a surface in which said surface is scanned with an electrical light sensitive device which receives light from such surface and which produces a train of pulses corresponding to said configuration superimposed on a base signal determined by the general level of illumination to which the device is exposed, and the train of pulses is monitored to detect a deviation from a norm representative of a fault, characterised in that compensation is made for changes in said base signal during said monitoring of said train of pulses.

The invention also provides apparatus for use in detecting faults in a regular configuration on a surface comprising an electrical radiation sensitive device arranged for scanning said surface to receive radiation therefrom and adapted to produce a train of pulses corresponding to the physical configuration of said surface superimposed on a base signal determined by the general level of said radiation to which the device is exposed, and electrical monitoring means connected to said device to receive said train of pulses therefrom and adapted to produce an output in response to detection of a deviation of said pulses from a norm representative of a fault, characterised in that said monitoring means includes compensation means adapted to compensate for changes in said base signal.

With the method and apparatus of the invention it will be appreciated that it is possible to distinguish between deviations in the output of the radiation sensitive device due to a fault and deviations due to other factors such as fluctuations in the ambient level of radiation or spacing of the device from the surface.

Whilst the invention may be used in any suitable context it is visualised that it will find particular application in the detection of faults in a fabric having a ribbed surface especially a tufted fabric.

Most preferably, the radiation sensitive device is an electrical light sensitive device and may take any suitable form but preferably comprises a phototransistor, photoelectric cell orthe like which is positioned behind a lens or the like so that the light sensitive device can receive light from a small region only of the surface of the fabric. In the case where a ribbed surface is scanned, the arrangement is preferably such that light is received from a narrow elongate region extending longitudinally of the ribs.

Preferably also, the fabric surface is illuminated with a light source and the light sensitive device receives light which is reflected from the fabric surface. The light source may be arranged to direct light at an angle onto the fabric surface and the light sensitive device may be arranged to receive light reflected at least substantially perpendicularly from the surface, whereby projecting structures on the yarn surface cast long shadows and there is a sharp contrast between the level of illumination at the projecting structures and the level of illumination between such structures.

Reference is made herein to "light"forthe sake of convenience, but it is to be understood that this term is not intended to be restricted to radiation within the visible spectrum but is also intended to encompass similar non-visible radiation such as infra-red radiation.

In orderto monitor the train of pulses whilst allowing for fluctuations in the base signal, the output of the light sensitive device may be compared with a slightly delayed version of such output. In this way, maxima and minima of the pulses can be identified as constituting positions at which a rising signal is coincident with a falling signal for the two outputs.

After identification of the maxima and minima of the pulses a missing or abnormal pulse can be readily detected for example by using the maxima and minima to trigger a square wave generator, whereby a train of square waves is produced the marklspace ratio of which will vary if a fault condition arises. A variation in mark/space ratio may be determined by using leading edges of the square waves to trigger and re-set a ramp generator, an alarm signal being produced if the output of the ramp generator rises above a reference level as would be the case for example if there is a missing pulse. Alternatively, the pulses may be separated from the fluctuating base signal by means of a filter which removes relatively low frequency fluctuations whilst passing the relatively high frequency pulses. The pulses can then be used to trigger the square wave generator as described above.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Fig. is a diagrammatic sectional view (on the line I-I of Fig. 2) of a scanning device positioned above a fabric surface; Fig. 2 is a sectional view on the line ll-ll of Fig. 1; Fig. 3 is a sectional view on the line III-lil of Fig. 2; Fig. 4 is a circuit diagram of a system for monitoring the output of the scanning device of Fig. 1 in accordance with one embodiment of the invention; Figs. 5 to 10 show wave forms at various stages of the system of Fig. 4;; Fig. 11 shows a block circuit diagram of an alternative embodiment of the invention; Fig. 12 shows a wave form for the arrangement of Fig. 11; and Fig. 13 shows an enlarged diagrammatic section of the scanned fabric surface.

With reference to Fig. 1, the backstitch side of a tufted fabric 1 such as carpeting is scanned on a tufting machine immediatelyafterthetufting needles. The fabric surface 2 is scanned with a scanning device containing a light source 3 and a light sensitive device 4 in a common sealed housing 8. A plurality of scanning devices are mounted at equally spaced intervals on a bar (not shown) which extends across the fabric (transversely to the direction of fabric advancement) and is reciprocated longitudinally so that each scanning device moves backwards and forwards scanning a respective strip of the fabric 1. Reference is madeto co-pending Applications Nos. 20953178 and 7917452 (2027191) for a more detailed description of a suitable arrangement for effecting such movement of the scanning devices.

The common housing 8 of each scanning device is a generally rectangular box structure which is arranged at an angle to and spaced above the surface 2 as shown in Fig. 2. Within the housing 8, the light source 3 is mounted at the end of a tube 11 which is provided with a lens 5, and a prism 6 is arranged in a bottom wall of the housing so that a beam of light is directed through the prism 6 at an angle of about 30 to 450 to the fabric surface 2 and at an angle to the parallel ribs 7 (which are defined on the surface 2 by the backstitches) as determined by the inclination of the housing 8.With this arrange ment, the backstitches 7 cast long shadows (as shown in Fig. 13) and there is therefore a strong contrast between the illumination of the backstitches 7 and the illumination of adjacent portions of the fabric surface 2 between the backstitches.

The device 4 comprises a phototransistor 9 which is mounted at the end of a tube 10. In front of the phototransistor 9 in the tube 10 there is a screen 12 with a very narrow slit 13 therein (say 0.005 ins.

0.127 mm) and a lens 13a.The phototransistor9 receives light via the slit 13, the lens 13a and a prism 13b in the bottom wall of the housing 8, such prism 1 3b being arranged to transmit only light received perpendicular to the fabric surface 2. With this arrangement, at any instant, the image of a very nar row strip of the fabric (extending longitudinally relative to the ribs 7) is focussed on the phototransistor.

Due to the use of this focussing arrangement in conjunction with the inclination of the beam from the light source 3 it is possible to achieve an output from each device 4 in the form of a train of pulses which pulses are sharply defined even in the case where the fabric is a very fine gauge tufted fabric.

The pulses are produced as the device 4 scans the back stitches 7 and, as can be seen from Fig. 5, the pulses are generally of sine wave form and are superimposed on a base signal in the form of a wave of much lowerfrequencyand much greater amplitude. The fluctuating base signal is produced by fluctuations in the general level of illumination reaching the device 4 due for example to variations in intensity of ambient light and spacing of the fabric from the device.

The output of each device 4 is fed to an electronic monitoring system as shown in Fig. 4.

The pulse train is amplified with amplifier 14to give a wave form as shown in Fig. 6 and this is then passed through a filter stage 15 in which the relatively low frequency background fluctuations are removed (by blocking) whilst the relatively high frequency pulses are passed through. The output of stage 15 is therefore a level train of pulses as shown in Fig.7.

The maxima and minima of the pulses are used, as shown in Fig. 8 to trigger switching of a square wave generator 16 so that the generator switches on at each minimum and off at each maximum.

The leading edges of the square wave pulses are used to trigger and re-set a ramp generator 17 as shown in Fig. 9.

In the event that there is no fault condition, square wave pulses of fixed mark/space ratio will be produced and these in turn will produce a train of equal triangular pulses. In a comparison stage 18 the amplitude of the triangular pulses is compared with a d.c.

reference level 19 as shown in Fig. 9. The reference level is set in relation to the pre-adjusted rate of rise of the ramp generator so that in the no4ault condition the ramp generator is re-set before the triangular pulses produced thereby can rise to the d.c. reference level. In the event that a fault condition arises, due for example, to a missing backstitch, the marklspace ratio of the square wave pulses may change and the output of the ramp generator may then rise above the d.c. reference level.

This increase over the reference level is utilised (Fig. 10) to trigger output stages 20, 21, 22, 23 which actuate a visual indicator 24 and produce fault signals at 25 for processing as desired. The indicator 24 may be located on top of the housing 8 to facilitate location of the fault. It is also possible to actuate a control device to stop or modify operation of the tufting machine andior a position identifying device which identifies the location of the fault for example in terms of a numerical identification of the pertain ing tufting needle.

Fig. 4 also shows control circuitry 26 which pro duces a warning output at 26a if the system fails. The circuitry 26 compares the amplitude of the triangular waves (produced by circuit 17) with an upper refer ence level (higherthan level 19) which is reached if the circuit 17 is never re-set by a square wave pulse (as would happen for example if the source 3 failed).

With the alternative embodiment of Fig. 11, the output of each device 4 is fed to an electronic monitoring system comprising an amplifier27 and a delay circuit 28. The delay circuit 28 generates a wave form which is a slightly delayed version of the wave form produced by the device 4. The two wave forms i.e. the original wave form 29 and the delayed version 30 of this, are compared, as shown in Fig. 12 in a comparison device 31. As can be seen, immediately following the maximum of each pulse of the original wave form, a falling original signal coincides with a rising delayed signal, and immediately following the minimum of each pulse of the original wave form a rising original signal coincides with a falling delayed signal.Thus, the maxima and minima of the pulses can be readily identified (by detecting coincidence of opposite signals) irrespective of any fl uctuation occurring in the base signal.

The identified maxima and minima are used, as with the embodiment of Fig. 4 to trigger switching of a square wave generator 32 which controls a ramp generator 33 the output of such generator being fed to a comparison stage 34 and output stages 35. The stages 32-35 correspond to the stages 16-18, 20-23, of Fig. 4.

With the embodiments described above, the opti cal system of the scanning device uses a focussing arrangement which receives light perpendicuierly frnm a long, narrow strip of fabric parallel to the backstitches 7 and this has an averaging effect in the sense that a backstitch will be identified even if there is some slight deviation from linearity or discontinuity at some position along its length. Also, the optical system receives light perpendicularly whilst the light source directs light at a steep angle onto the fabric surface whereby there is a strong contrast effect and sharp signals can therefore be produced irrespective of factors influencing reflectivity such as colour and the like. Thus, sharply defined pulses can be reliably produced.These sharply defined pulses are then accurately and reliably monitored using the electronic monitoring system which compensates for background changes and converts the pulses to triangular pulses of rising amplitude which are well suited to the identification of a missing or deformed pulse corresponding to a backstitch fault.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only. Thus, for example in a modification of the arrangements of Fig. 4 and 11, the trailing edges (rather than the leading edges) of the square wave pulses may be used to trigger the ramp generator, the leading edges being used to re-set the generator.

Also, whilst reference has been made to the scanning of fabrics having ribbed surfaces it is to be understood that the invention may be applicable to the scanning of other surfaces having ribbed or other regular raised conformations.

Claims (15)

1. A method of detecting faults in a regular configuration on a surface in which said surface is scanned with an electrical radiation sensitive device which receives radiation from such surface and which produces a train of pulses corresponding to said configuration superimposed on a base signal determined by the general level of said radiation to which the device is exposed, and the train of pulses is monitored to detect a deviation from a norm representative of a fault, characterised in that compensation is made for changes in said base signal during said monitoring of said train of pulses.

2. Amethod according to claim 1, characterised in that said radiation sensitive device is a light sensitive device arranged to receive reflected light radiation at least substantially perpendicularly from said surface and said surface is illuminated by light from a source at an angle to the perpendicular.

3. A method according to claim 1 or 2, wherein said configuration comprises regular parallel elongate structures, characterised in that said radiation sensitive device is a light sensitive device arranged to receive light radiation from said surface focussed via a narrow slit extending parallel to said structures.

4. A method according to any one of claims 1 to 3, characterised in thatthe merkispace ratio od suc cessive pulses is monitored to detect said deviation.

5. A method accorelling to claim 1, wherein said surface Leing scanned is the iLiced backstitch side of a tuJr-5-ed fabric and seid pusses correspond to the Ibac:siioschss upetendino on said side, characterissd in that said radiation sensitive device is a light sensi tivs device arranged to receive reflected light radia tlon s7 art least suhstentiell' perpendicularly iFrorr; said SUriaGe and said surface is illuminated by light from a source at an angle to the perpendicular; said radiation sensitive device is arranged to receive light radiation from said surface focussed via a narrow slit eending parallel to said becketitches; and the rnark/space ratio of successive pulses is monitored to detect said deviation.

6. Apparatus for use in detecting faults in a regu Bar configuration on a surface comprising an electrical radiation sensitive device arranged for scanning said surface to receive radiation therefrom and adapted to produce a train of pulses corresponding to the physical configuration of said surface superimposed on a base signal determined by the general level of said radiation to which the device is exposed, and electrical monitoring means connected to said device to receive said train of pulses therefrom and adapted to produce an output in response to detection of a deviation of said pulses from a norm representative of a fault, characterised in that said monitoring means includes compensation means adapted to compensate for changes in said base signal.

7. Apparatus according to claim 6, characterised in that said compensation means comprises filter means adapted to remove low frequency signal fluctuations whilst passing said relatively high frequency pulse train.

8. Apparatus according to claim 6, characterised in that said compensation means comprises means adapted to produce a delayed version of said pulse train and means adapted to compare the original pulse train and the delayed version thereof to produce outputs at correspondence of rising and falling signals which identify maxima and minima of the original pulses.

9. Apparatus according to any one of claims 6 to 8, wherein missing or abnormal pulses are to be detected, characterised in that said monitoring means includes means responsive to the marklspace ratio of said pulses.

10. Apparatus according to claim 9, characterised in that said pulses are converted to square wave pulses before monitoring of the markispace ratio thereof.

11. Apparatus according to claim 9 or 10, characterised in that said means responsive to the mark/space ratio comprises a ramp generator adapted to produce triangular pulses respectively of amplitudes determined by the duration of each said square wave pulse, and means adapted to produce an alarm output if said amplitude exceeds a predetermined reference level.

12. Apparatus according to any one of claims 6 to 11, wherein said radiation sensitive device is sensitive to light radiation characterised in that said device is positioned alongside a light source, said light source being arranged to direct light at an angle to said surface to be scanned and said device being arranged to receive light perpendicularly from said surface.

13. Apparatus according to any one of claims 6 to 12, wherein said radiation sensitive device is a light sensitive device characterised in that a focussing arrangement comprising a screen with a narrow slit is positioned in front of said device.

14. Amethod according to claim 1, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. Apparatus according to claim 6, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.