WO2000040951A1

WO2000040951A1 - Method and apparatus for detecting a non-uniform coating on a core member

Info

Publication number: WO2000040951A1
Application number: PCT/GB2000/000021
Authority: WO
Inventors: David Winterbottom
Original assignee: Samfit Uk Limited
Priority date: 1999-01-06
Filing date: 2000-01-05
Publication date: 2000-07-13
Also published as: GB9900225D0; AU1883900A; GB2345539A

Abstract

A coated wire (1) is illuminated at different locations by optic fibres (14). Scattered light received by collecting chambers (4) is sensed by photo detectors (11) preferably connected in a differential arrangement to a computer (18). A difference signal is generated when a shade variation occurs in the colour of the coating, due to lack of concentricity between the wire and the coating.

Description

METHOD AND APPARATUS FOR DETECTING A NON-UNIFORM COATING ON A CORE MEMBER

This invention relates to a method and apparatus for detecting a non-uniform coating on a non-transparent core member (such as wire).

Wire is often coated with plastic material such as Nylon, (Registered Trade Mark) in order to improve its appearance and/or to provide a protective layer. Such coatings are often very thin, for example , of the order of 0.002-0.010 inches (0.05 - 0.25mm) .

As the coating is thin, the material used for coating is often heavily loaded with (for example) a titanium pigment, to give a dense opaque white colour and therefore a pleasing appearance. In a typical coating process, a non-transparent substrate, such as wire, is drawn through a die and it is then coated with molten plastics material (Nylon) during the wire drawing process. Whilst the wire drawing machine can be accurately set up to co-operate with the coating device, i.e. so that the wire core is always concentric with the coating, changes can take place with the passage of time which cause this concentricity to be lost. The central axis of the wire then becomes eccentric to the coating. This in turn means that the coating is non-uniform, i.e. it is thin along one side of the wire compared to the other. Where it is thin, it appears to change shade or colour. Where white is used as a pigment, a thin coating appears as a grey streak, because the wire, which is not transparent, can then be seen through the thin coating, which is partly transparent or translucent. This is aesthetically unacceptable, especially where the coated wire is used in making foundation garment accessories, such as hooks and eyes, wire supports, etc. If the coating does not appear of a uniform white colour on these accessories, they can cause the garment to be rejected, by a potential customer, as being of inferior quality. It is therefore very important to maintain a coating of uniform thickness to avoid this problem.

As the wire used in making (e.g.) hooks and eyes is of a narrow gauge, and as several thousand metres will be consumed in the mass-production of wire products, it is not an easy task to inspect the coated wire before it is used in wire bending and forming machines. Hence it may not be until after the hooks and eyes have been manufactured that a defective coating is seen. These products then need to be rejected, which adds to the expense of manufacture. Whilst wire can be inspected by eye, this is a very time-consuming and laborious process and it also slows down the rate of manufacture.

The problem facing the invention is therefore to provide a method of detecting a non- uniform coating on a non-transparent core which does not have the above mentioned disadvantages. The inspection method should also preferably facilitate mass production, as in the case of pre-inspecting coated wire before it is fed to a wire bending machine. It should also preferably facilitate control in the manufacture of coated wire so that non-uniform coating can be substantially avoided or reduced to a minimum, whereby the wire manufacture can offer a better quality product to the wire user.

This problem is solved by the invention in a method of detecting a non-uniform coating on a non- transparent core member, the method including: (a) illuminating the coated non-transparent core member with incident light at different circumferential locations, and (b) detecting a variation in light scattered at different circumferential locations by the coating, due to a variation in the thickness of the coating on the core member, and

(c) detecting a difference, or differences between detected amounts of the scattered light.

The invention advantageously detects scattered light (instead of reflected light) in order to detect a shade variation in the coating. For example, the core member can be illuminated with light which is directed at an angle of about 45° to a normal to the core member axis and at grazing incidence to the coating. The reflected component (at an angle of 45)° on the other side of a normal is ignored. However, a detecting device, such as a photo diode, located (for example) on the axis of the normal, is used to detect some of the light scattered by the coating. Instead of directing light at an incident angle of 45°, it could be directed at other angles (including along a normal) and the scattered light detectors can be arranged at complimentary or other angles, as may be found best.

As the coated core member is illuminated with light which is incident at different circumferential locations, the light scattered at one of the locations differs from that scattered at another, so that the difference can be detected. This detected difference (or differences) can be measured to provide a reading, or used to provide a warning, or used to control a process for manufacturing or inspecting the coated core member.

The method could be carried out with light incident at two circumferentially opposite locations, but it is preferably carried out with illumination at three or more equally spaced locations. In particular, the light is preferably incident at four equally circumferentially spaced locations, because respective detectors at these locations can then be connected in opposite pairs in order to make differential measurements of the relative amounts of scattered or reflected light, (rather than making absolute measurements). Four circumferentially spaced locations also enables the detectors to see more easily "around" the cylindrical exterior of the coated core member wire. Whilst absolute measurements of the amount of scattered light can be made, these would require compensation for any variation in the level of illumination. By using the differential arrangement, even if the level of illumination changes, there will still be a similar difference. The differential arrangement can also improve the sensitivity of detection, and it provides inherent compensation against drift.

The invention also provides apparatus for detecting a non-uniform coating on a non- transparent core member, the apparatus including:

(a) means for illuminating the coated non-transparent core member with incident light at different circumferential locations,

(b) means for detecting a variation in the amount of light scattered at different circumferential locations by the coating, due to a variation in the thickness of the coating on the core member, and

(c) means for measuring a difference or differences between detected amounts of the scattered light.

The means for illuminating the coated core member preferably includes a source of light and a plurality of optic fibres which direct narrow beams, at grazing angles of incidence, to respective circumferential portions of the coated wire. However, other means may be employed to illuminate the coated core member, at respective circumferential locations, with uniform light. The advantage of using optic fibres is that a single source of light can be used, so that any variation in the intensity of the source is equally seen at different circumferential locations around the coated core member.

Preferably, the detecting device is mounted in a scattered light collecting chamber, the chamber extending closely towards the wire coating. In a preferred arrangement, the coated core member passes through a mounting which incorporates a plurality of radially extending scattered light collecting chambers, in each of which a respective detecting device is arranged to provide an output signal which varies with the amount of scattered light received. Preferably, the radial chambers are circumferentially spaced by equal amounts, (e.g. 90°) and the optical axis through each chamber extends at right angles from the direction in which the coated core is drawn (either during manufacture, or inspection). The arrangement assists in avoiding picking up external stray light, by enabling close running tolerances to be maintained between the coated core member, the incident light and light detecting arrangements.

Whilst the invention is particularly useful in inspecting wire having a white coating (e.g. filled with titanium oxide), it may be used for inspecting wire having coatings of other colours. In the case of a white coating, a white source of light can be used to illuminate the coating and we have found that this works well when the white light has a bias at the red end of the spectrum. However, the source of light and the use of filters can be determined in accordance with the specific application, since there may be advantages in using light in a particular bandwidth in order to provide a required effect. When assessing the shade of a coating subjectively, the illumination and detection system can be designed to simulate the response of the human eye. In a preferred embodiment of the invention, where light is incident at four equally spaced locations and respective detecting devices are used for collecting the reflected or scattered light, detectors on opposite sides of the core are connected such that the difference between their outputs may be measured. For example, with the photodiodes arranged at circumferential intervals of 90° (with respect to the axis of the core), the two pairs of photodiodes are connected so that two differential signals are measured. If the core member is off centre towards one photodiode the pair containing this photodiode will give a difference signal whilst the other pair will give no difference. Alternatively, if the core member is off centre towards one of the other pair, this pair will give a difference signal. If the core member is off centre between two adjacent photodiodes, both pairs will give a smaller difference signal. Both these signals are measured by the computer and are summed by vector addition to give a single shade variation measurement corresponding to the off centredness, independent of the direction. However, the direction in which the core memeber is off centre may also be determined from the vector sum of the two differential signals.

There can be an advantage in providing both differential and absolute measurements of shade variation. For example, when the pigment in the coating starts to become exhausted, it will become uniformly less dense, whereby differential measurement would be unaffected. An absolute measurement of shade variation can be used to monitor this change in pigment. Absolute measurements can also be useful when setting up or calibrating the apparatus, e.g. to counteract any stray light effects, cross talk.

The apparatus can also include means for indicating where, along the length of the wire, there is a shade variation. This can include means for measuring the the length of wire as it passes through a detection head. The measuring means could simply include some form of mechanical arrangement, such as a pulley and a rotational counter which is driven by passage and contact with the coated wire. Such means enables the coated wire to be passed through the detecting head and then taken up on a drum whilst a record of its shade variation with length is stored in some form of register (e.g. in a computer). When the wire is used for mass production, the section of the wire in which there is an unacceptable shade variation, can then be caused to bypass the metal forming machine, at high speed, in order to reject the faulty section before the following section is used for making products.

Preferably, a non-contact length measuring device is used, so as not to damage the coating. This could include a doppler device which is sensitive to wire speed.

In a wire manufacturing process, means may be included for causing relative movement between the axis of wire being drawn and the axis of a coating device. In this case, any differential shade variation is used to provide a drive signal for causing a shift in relative movement to compensate for any detected eccentricity.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:

Fig. 1 is a side view, partly in cross-section, of a detecting head sub-assembly with four light scattering collecting chambers,

Fig. 2 is a plan view of a wire inspection station (partly in section) in which the detecting head sub-assembly has been fitted,

Fig. 3 is a side elevation, partly in cross-section, through diametrically opposite light collecting chambers showing an optic fibre light outputs, and

Fig. 4 is a circuit diagram showing how the photo detectors are connected in an electrical detecting arrangement.

Referring to the drawings, Fig. 2 shows a wire inspection station W in which a detecting head sub-assembly D, sandwiched between two plates P, is used to inspect coated wire 1. The coated wire 1 is fed along the central axis of a through-hole 2, of square cross-section, which passes through each of the plates P. The wire W is supported by spaced V-pulleys V (e.g. Hepco journals). The Hepco journals or V- 00/40951

7 pulleys help to maintain an accurate and close running clearance between the light output ends of the optic fibres 14 and the wire 1. The wire is fed from a supply drum (not shown) onto a take-up reel (not shown).

Fig 1 shows the detecting head sub-assembly in more detail where four detectors are mounted symmetrically and co-axially with the central axis of through-hole 2. The detectors include four diametrically opposed, scattered-light, collecting chambers 4a- 4d. Each chamber 4 consists of a conical portion 5 and a rectangular portion 6. The conical portion 5 has a light inlet port 7 closely adjacent the through hole 2a and an outlet port 8 located so as to direct light into a lens arrangement 9 mounted in an end wall 10 of the rectangular portion 6. The conical portion 5 collects scattered light (as explained below) whereby the lens arrangement 9 focuses the scattered light onto a photo diode situated in an opposite end wall 12 of the rectangular portion 6. Each light collecting chamber 4 is of similar construction and the four conical portions 5 are arranged diametrically opposite one another at 90° intervals around the through hole 2.

Fig. 3 shows two of the scattered light collecting chambers (4a and 4c) in sectioned elevation. Each chamber 4a-4d is associated with an optic fibre (only fibres 14a and 14c are shown in Fig.3), which is gripped in a supporting member 15. Each optic fibre is positioned so as to direct light onto a portion of the coated wire 1 opposite the light input port 7 of its respective scattered light collecting chamber 4. As will be appreciated from the optical arrangement shown in the drawing, reflected light (where the angle of incidence and of reflection are the same) is ignored, and only scattered light passes into the conical portion 5 of the light collecting chamber 4. The amount of light scattered will depend on the coating thickness. When the coating becomes non-uniform, it becomes thinner on one side and this will affect its light scattering property. This can be termed a "shade variation".

Referring to Fig. 4, this schematically illustrates the four diametrically opposed photo detectors 11a- l id which are each connected to a respective amplifier 16a-16d in a first stage of amplification. The wire and its coating (1) is shown in section in this 00/40951

8

Figure. In the first amplification stage, the amplifier gains and offsets are set, for example, as 0V for black (i.e. no light) and IN for a maximum white wire shade (whereby the sum, or average, of the four measurements from the photo detectors 11, in this stage, can be used to indicate overall colour).

A second stage of amplification 17a, 17b is in a differential measuring arrangement 18. Difference signals are amplified by about x20 and the arrangement is such that when the difference measurement produces 0V, there is no variation in shade and the wire is substantially centralised in the coating. However, an output of IV would represent a 5% variation in shade. As the direction of the shade variation can be determined, with respect to the location of the photo detector, the circumferential position in which the wire is offset from the coating can also be determined. More particularly, where light is incident at four equally spaced locations and respective detecting devices are used for collecting the reflected or scattered light, detectors on opposite sides of the wire 1 are connected such that the difference between their outputs is measured. For example, with the photodiodes arranged at circumferential intervals of 90° (with respect to the axis of the core), the two pairs of photodiodes are connected so that two differential signals are measured. If the wire 1 is off centre towards one photodiode the pair containing this photodiode will give a difference signal whilst the other pair will give no difference. Alternatively, if the core is off centre towards one of the other pair, this pair will give a difference signal. If the core is off centre between two adjacent photodiodes, both pairs will give a smaller difference signal. Both these signals are measured by a computer 19 and are summed by vector addition to give a single shade variation measurement corresponding to the amount of off-centre, independent of the direction. However, the direction in which the core is off centre may also be determined from the vector sum of the two differential signals.

The computer 19 is programmed to measure voltages at 6 inputs when pulsed by an encoder (not shown) for every 5mm of wire, with a wire speed of 400m/min. Five measurements are taken and then averaged. The two orthogonal difference measurements are also combined to give one shade variation value. Running averages 0/40951

9 of 256 and 32768 measurements are calculated to give averages over 1.28 and 163m. (These values are by way of example only).

The output could control warning lights, a correction system, machine shutdown, defect marking system etc. (not shown).

A suitable computer included a 233MHz pentium processor with an A/D capture card. One computer per line would be required. However, data collection could be networked onto a central data collection computer. Alternatively, special hardware can be designed and used for control purposes.

Claims

00/4095110CLAIMS

1. A method of detecting a non-uniform coating on a non-transparent core member, the method including: (a) illuminating the coated non-transparent core member with incident light at different circumferential locations, and

(b) detecting a variation in light scattered at different circumferential locations by the coating, due to a variation in the thickness of the coating on the core member, and (c) detecting a difference, or differences between detected amounts of the scattered light.

2. A method according to claim 1 , wherein the method is carried out with light incident at two circumferentially opposite locations, but preferably at three or more equally spaced locations.

3. A method according to claim 1 , wherein the method is carried out with light incident at four equally circumferentially spaced locations, and the variations in the amounts of scattered or reflected light are detected in opposite pairs in order to make differential measurements.

4. A method according to any preceding claim, wherein the core member has a white coating and wherein a white source of light is used to illuminate the coating, the white light having a bias at the red end of the spectrum.

5. A method according to any of claims 1-4, wherein a source of light and filters are selected in accordance with the colour of the coating, whereby light in a particular bandwidth, and/or with a particular bias in the spectrum, is used to provide a required effect.

6. A method according to any of the preceding claims wherein the coated core member is inspected along its length and a record of amount of variation in the scattered light is kept with respect to its position along the length of the coated core member, the record being used later to reject a length portion of the coated core member with an unacceptable variation in the thickness of its coating, said portion being rejected when the coated core member is used in mass-production.

7. A method according to any of the preceding claims where the core member is a wire and where the method is used in a wire manufacturing process, means being included for causing relative movement between the axis of wire being drawn and the axis of a coating device, any differential variation being used to provide a drive signal for causing a shift in relative movement to compensate for any detected eccentricity.

8. Apparatus for detecting a non-uniform coating on a non-transparent core member, the apparatus including: (a) means for illuminating the coated non-transparent core member with incident light at different circumferential locations,

(b) means for detecting a variation in light scattered at different circumferential locations by the coating, due to a variation in the thickness of the coating on the core member, and (c) means for measuring a difference or differences between detected amounts of the scattered light.

9. Apparatus according to claim 8, wherein the means for illuminating the coated core member includes a source of light and a plurality of optic fibres which direct narrow beams, at grazing angles of incidence, to respective circumferential portions of the coated wire.

10. Apparatus according to claim 9, wherein only scattered light is collected, light being directed at an angle of about 45 ° to a normal to the wire axis and at grazing incidence to the coating, the reflected component (at an angle of 45°) on the other side of a normal being ignored; a detecting device being located on the axis of the normal to detect light scattered by the coating.

11. Apparatus according to claim 10, wherein the detecting device is mounted in a scattered light collecting chamber.

12. Apparatus according to any of claims 8-11 , wherein the coated core member passes through a mounting which incorporates a plurality of radially extending scattered light collecting chambers, in each of which a respective detecting device is arranged to provide an output signal which varies with the amount of scattered light received.

13. Apparatus according to claim 12, wherein the radial chambers are circumferentially spaced by equal amounts, and the optical axis through each chamber extends at right angles from a direction in which the coated core is drawn.

14. Apparatus according to any of claims 8-13, wherein close running tolerances are maintained between the coated core and the incident light.

15. Apparatus according to any of claims 9-15, wherein light is incident at four equally spaced locations and respective detecting devices are used for collecting the reflected or scattered light, detectors on opposite sides of the core are connected such that the difference between their outputs may be measured.

16. Apparatus according to claim 15, wherein detecting devices are arranged at circumferential intervals of 90° (with respect to the axis of the core), two pairs of the latter devices being connected so that two differential signals can be generated, whereby:

(a) if the coating on the core is off centre towards one detecting device, the pair containing this device will give a difference signal whilst the other pair will give no difference signal;

(b) if the coating is off centre towards one of the other pair, this pair will give a difference signal;

(c) if the coating is off centre between two adjacent detecting devices, both pairs will give a difference signal; both difference signals being summed by vector addition to give a single shade variation measurement corresponding to the amount that the coating is off centre, independent of direction.

17. Apparatus according to any of claims 8-11, wherein detecting devices are arranged at circumferential intervals and the direction in which the coating is off centre is determined by means of comparing the detected signals.

18. Apparatus according to any of claims 8-17, wherein absolute measurements of shade variation are made to determine when the pigment in the coating starts to become exhausted.

19. Apparatus according to any of claims 8-18, including means for indicating where, along the length of the wire, there is a variation in coating thickness.

20. Apparatus according to any of claims 8-19, including means for measuring the length of the core as it advances through the apparatus.

21. Apparatus according to claim 19, wherein a non-contact length measuring device is used, so as not to damage the coating.

22. Apparatus according to any of claims 8-21 , including means for recording said variation with length.

23. Apparatus according to any of claims 8-22, including means for bypassing a machine in order to reject a faulty section of the core before a following section is used for making products.