GB2507069A

GB2507069A - Monitoring the quality of an electrostatic coating by measuring light reflected from a spray

Info

Publication number: GB2507069A
Application number: GB201218662A
Authority: GB
Inventors: John Cox; Stephen Brand
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2012-10-17
Filing date: 2012-10-17
Publication date: 2014-04-23
Also published as: GB201218662D0

Abstract

An optical monitoring system for monitoring quality of an electrostatically applied coating, such as oil or molten wax, applied to a product is disclosed. It comprises a light source 6; an optical detector (2, Fig. 3) mounted on a linear scanning mechanism 7 to detect light signals reflected from a spray 22 of the coating during application; and an actuator to pass the product 30 along its transport path through the scanning mechanism. The detector may detect spray from one or more lines 22 comprising a sub-section of all spray forming lines, preferably in a detection zone after lines form, but before they disperse. The detector may comprise a light guide; the light guide or light source may comprise a gas purge tube. A method comprises positioning the light source, illuminating a line of coating spray, detecting light reflected off the spray, and moving the detector to repeat the process. The received light may be compared with an expected profile, an alarm being activated if the comparison exceeds a threshold. The system may comprise a display, which preferably outputs a graphical representation of the comparison. A magnified image may be displayed if the comparison exceeds a threshold.

Description

OPTICAL MONITORING SYSTEM

This invention relates to an optical monitoring system, in particular for monitoring quality of electrostatically applied coatings, such as coatings applied by an electrostatic oiler to a strip or plate in a rolling mill.

Electrostatic oilers, for example blade applicator types, are prone to clogging which causes thy stripes and uneven coating. This can be hard to spot, even if the strip is kept under constant surveillance by mill staff. There may be other causes for dry stripes, but whatever the cause, it is important that dry stripes are spotted as soon as possible, or else the product being coatcd will need to be scrapped, or resprayed, or may be rejcctcd, or become corroded, or stick in presses during later processing.

Conventionally, the quality of a coating applied by an electrostatic oiler has been inspected by eye or by means ofoffline sampling, but this is a slow and labour intensive process, which may result in several hundrcd meters of dry stripe developing before it is detected.

DE19S4725 describes illuminating a wedge shaped spray chamber from one end with a laser, the laser being positioned perpendicular to a direction of travel of a strip being coated and positioning a camera at a sham angle to the spray surface to detect inhomogeneities in the distribution of spray particles. However, the camera must be positioned to view the spray along its length, which could mean imaging between 30cm and 2030cm, causing difficulties with focusing and resolution, particularly at the far end of the spray. Furthermore, a powerfid laser is requircd to illuminate along the full extcnt of the spray chamber, which is expensive and potentially hazardous. An additional disadvantage of this is that the scattered light illuminates stray oil particles along the camera's line of sight, thus occluding the spray even more. The wedge shape of the oil spray and the sharp angle from which it is viewed means there is a substantial parallax error, which may hide even wide dry stripes JP10253537 describes illuminating a metal sfrip coated with oil with a single sensor to determine oil thickness on the strip directly beneath the sensor. The sensor measures fluorescence in response to irradiation, but the use of a single sensor does not improve resolution across the full width of the strip. In this design the sensor is outside the spray enclosure, so difficult to retrofit.

In accordance with a first aspect of the present invention an optical monitoring system for monitoring quality of an electrostatically applied coating applied to a product comprises a light source; an optical detector mounted on a linear scanning mechanism to detect light signals reflected from a spray of the coating during application; and an actuator to pass the product along a transport path of product through the scanning mechanism; wherein the scanning mechanism is installed across the width of the transport path and perpendicular to a direction of travel of the product on the transport path.

A scanning detector allows for a a fixed single point focus and a narrow field of view, so grcater accuracy, than a fixed dctcctor system and enables indirect non-contact scanning.

In one embodiment, the spray coating is derived from lines formed on the blade and the detector detects spray from a line or lines forming a sub-section of all spray forming lines.

Reducing the number of lines from which reflected light is detected improves accuracy.

Alternatively, the detector detects in the detection zone after lines form, but before they disperse.

Detecting lines of coating, rather than dispersed spray, gives a better result.

Preferably, the light source is mounted on the same linear scanning mechanism as the optical sensor.

This allows the light source and sensor to be moved together.

Preferably, the detector further comprises a light guide.

This restricts the width of field of the reflected light received by the detector, improving resolution.

Preferably, at least one of the light guide and the light source further comprise a gas purge tube.

This helps to keep coating away from the light source or keep the sensor in the detector clean.

Preferably, the detector comprises an optical sensor.

Preferably, the detector further comprises a lens and a filter.

Preferably, the detector further comprises a processor to process signals output from the sensor.

Prcfcrably, the systcrn further comprises a display.

The coating may be any suitable liquid, but preferably the coating comprises one of oil or molten wax.

Tn accordancc with a second aspcct of thc prcscnt invention, a rncthod of monitoring an electrostatically applied coating comprises positioning a light source; illuminating a line of coating in a detection zone with the light source; positioning a detector; receiving at the detector light from the light source reflected off the line of coating; moving thc dctector; and repeating thc illumination and detection steps.

Prcfcrably, thc light source and dctcctor arc movcd togcthcr.

Preferably, the method further comprises comparing the received light with an expected light profile.

Preferably, thc method furthcr comprises deriving a graphical representation of the result of the comparison and outputting the graphical representation.

The processing of the image also mensurates the image, the results of which may be compared with set parameters, such as gap, or cusp density variation.

Preferably, the graphical representation is output to a display, or to a store.

In one embodiment, if the result of the comparison breaches a threshold value an alarm is activated.

Alternatively, if the result of the comparison breaches a threshold value, a magnified image is displayed.

By adjusting thc scanning to zoom in if a problem is detected, the operator can analyse a smaller area in more detail, if required.

An example ofan optical monitoring system for an electrostatically applied coating and a method of opcration will now be described with reference to thc accompanying drawing in which: Figure 1 illustrates how cusps of coating material form on an electrostatic oiler blade; Figure 2 illustrates an example of an optical monitoring system according to the present invention; Figure 3 illustrates operation of the system of Fig.2 in more detail; Figure 4 illustrates an electrostatically charged coating forming cusps; and, Figure 5 illustrates illumination of the coating of Fig.4, using the system and method of the present invention.

As described above, there are various methods of detecting spray or applied coating thickness using lasers and cameras. However, needing to covcr the full width of a strip or plate with one array, as in DE1947258, can give poor resuhs at the extremities of the image. In the present invention, a light source and optical detector, such as a point sensor are used, which traverse back and forth on a linear scanning mechanism and detect reflected light from a limited part of the width of the spray or coating. As the focus is on a single point at a fixed distance, the field of view is narrow and the results are much more accurate. The narrow field of vision and shallow depth of field together improve the accuracy of the results. The invention assesses the quality of the spray or coating as it is applied and if any region gives a result outside an acceptable tolerance, then action can be taken swiftly before too much of the strip or plate has been treated. This is useful for both maintenance of the coating system, such as an electrostatic blade oiler and quality control of a coated product. The invention can be retrofitted to many different types of coating assemblies or oilers to improve oiling quality.

In the description, the term cusp refers to the shape of liquid coating being formed into a discrete flow as a result of being electrically charged, more particularly the arch shape formed bctween two lines of liquid coating as they part on the blade and the term detection zone refers to an area in which the cusps become thin jets of liquid coating before they start spreading out to become a fine mist. This area is typically at a distance of about 3mm to 10mm from the blade.

Fig. 1 shows formation of cusps 10 of coating on a pieal electrostatic coating applicator blade 11 and also shows the detection zone 20 in whichjets 22 from the cusps are monitored. As coating emerges from the blade, it forms thejets, or lines 22 of coating, generally parallel and evenly spaced and separated from one another, although the lines begin to disperse into a mist 23 as they are drawn toward the strip being coated. By monitoring the formation of these lines 22 of coating and by detecting in the region of each line as it is formed, it is possible to determine where dry stripes or uneven coating arc occurring. Fig.2 illustrates an example of an optical monitoring system according to the present invention. The lines 22 can be seen at regular inteivals across the full width of the detection zone. A section through the lines in Fig.2 is within the detection zone illustrated in Fig.1. An actuator (not shown) causes a plate or strip product 30 to pass through the electrostatic coating applicator in a direction of travel indicated by the arrow 14. As the product passes beneath these detection zones, S it receives a coating from the blade II. When operating correctly, the lines 22 of coating supplied from the blade form an even coating across the width of the strip or plate.

A detection module 1 mounted on a linear drive 7 is arranged so that it can be scanned back and forth in the direction of the arrows 8, 9 across the width of the blade 11 and strip. Control of the linear drive may simply move the detection module back and forth with a start and end sensor, integral with the linear drive system?, to signal when to capture the data. In this example, thc detection module I comprises a sensor 2, lens 3 and filter 4, although a simple embodiment with only a sensor is equally possible. The sensor monitors a very small area of spray at any one time. This enables a particular section of the spray, even down to a single line 22 of spray within the detection zone 20, to be analysed. This in turn gives much better resolution, using less expensive sensors and safer light sources, than prior art devices. As illustrated in Fig.3, a light source 6, shown in this example mounted on the same linear drive 7, is scanned with the detection module 1 across the width of the blade 11. The light source may be of any suitable type, for example a laser, or a pulsed infrared diode. A further feature is the use of a light guide 13, typically a tube mounted in the detection module. This restricts the field of view, so that only one line 22 is viewed at a time to improve accuracy and in combination with a small air supply inlet 5 that keeps the pressure in the tube higher than the external pressure, helps to prevent coating from entering the detection module. Similarly, a protective tube around the light source and a purge tube to keep pressure in the protective tube higher than the external pressure may also be provided, in order to keep coating away from the light source.

Operation of the system is illustrated in more detail in Fig.3. hi an elecfrostatic coating applicator, applying an electrostatic charge to the blade applicatorwhieh has coating material flowing through it produces cusps 10 of spray along the blade edge 11, as illustrated in Fig.1. The number of lines 22 formed and their distance apart are dependent upon the high voltage charge applied, the flow of material and the characteristics of the material (such as resistance, viscosity and surface tension). Each line needs to be individLially monitored to ensure an even spray with no dry lines. This needs to be done remotely, i.e. a non-contact method because the quality of the spray is only evident at a determined distance from the exit, so preferably, the detection zone 20 is not at the point on the blade where the lines start to form, but a little way down the line 22 itself, preferably about 10mm from the exit of the blade. Tf there is a blockage, there will be limited or no line, just dnps in that detection zone.

The light source 6 is directed at a single line 22b and illuminates tlus to make the spray visible to the detection system. The light source 6 is aimed at the lines 22 to coincide with the detection zone of the line 22, just below the exit of the blade, but at a different angle to the detection module 1. Typically, the light source is adjustable to allow the sensor to be set up for different distances. Preferably, the angle of the light source is as perpendicular as possible to the blade, without illuminating the background of the sensor view. The light source is chosen to match the detection device and optical filters may be incorporated to match both the light source and the detection device.

A previous line 22a and a next line 22c in the detection zone 20 are shown adjacent to the line 22b. The sensor field of view 12 is preferably restricted by means of the light guide 13 to only receive light reflected from a small part of the line 22b in the detection zone. The angle of the light source may be adjusted to aecuiately illuminate a specific cusp and line, although it is acceptable to illuminate adjacent cusps and lines, as they are outside the sensor's field ofview. If desired, the light source may be moved on the linear drive, independently of the detection module, with the angle of the light source altered as necessary to illuminate a each cusp. However, this is more complex than moving the light source and detection module together.

Having received reflected light from line 22b, the linear drive moves the combination of light source and detection module into position for line 22c and that is then illuminated by the light source 6. This continues across the width of the blade until the outermost detection zone is reached and then the linear drive 7 reverses in direction and the lines in the detection zone are scanned back to the other edge of the blade. This alternating scan is preferably continuous during oiling.

Figs. 4 and 5 illustrate the illumination and scanning. Fig.4 shows the lines of coating in the detection zones 10 being formed on the blade 11 and Fig.5 shows a series of detection zones that have been illuminated by the light source. In this example, a long exposure was used and the light source was moved by hand, so only one line is illuminated at a time.

The linear drive is preferably positioned at the entry or exit wall of the electrostatic spray applicator, one for each spray, with the detection module mounted on the linear drive. The linear drive sets the detection module in position for pixel capture. The speed of the scan is a trade off between reliability, i.e. linear drive lifetime, required resolution and the required error detection speed. By controlling the linear drive, it is possible to zoom in and analyse a small section of the blade output, if required. A linear drive 7 and detection module I including a sensor assembly is a better shape than prior art devices, such as in DE1984725S. The sensor 2 is close to the detection zone 20 that it is imaging, so the scattering effect of stray droplets is reduced and the need for a wide depth of field is removed. However, with its narrow focus and field of view, the detection module maybe able to process images up to a metre away from the coating, although more typically processing is not more than 0.6m away. The resolution is determined by the sensitivity of the sensor 2 and the scan speed, but is well in excess of what is required to see just one missing section of spray, or unevenly spaced sections. This provides useful data on the condition of the oiler and allows remedial action, manual or closed loop, to be taken, adding extra functionality which is not available in conventional dry stripe detection methods.

Filtered, focused, reflected light received at the sensor may be converted into electrical signals and output to a processor 15 for flarther analysis, or to a display 16 for display to the operators, for example using waterfall displays, automatic detection of errors and automatic purging. The detection module may capture greyscale data for each point along the blade and either carry out local pre-processing, or forward the raw data to a remote processor. Data rates are preferably low, to simplify cost and electronics. Each scan may be analysed to identify anomalies in the line pattern. This enables operators to see changes in the detected spray pattern which might give rise to concerns about quality, or the interpretation may be automated and a comparison with an expected spray pattern made by a comparator and some type of alarm or other action initiated by the processor 15 when the result of the comparison falls outside an acceptable range, such as sending a signal to a controller 17 to modify or stop the coating application process. For example, problems with the supply of coating to the blade 11 in the electrostatic coating applicator may be resolved by purging, running the metering pump at maximum revolLitions, for a short time or by removing or reducing thc high voltage for a short time. Early notification of a blocked scction of blade is therefore usethl for maintenance, quality control and production efficiency.

The total number of cusps 10 and the lines 22 formed on either side of a cusp is not predetermined and varies according to conditions. The processing of the image gives a cusp (and hence line) density, as a guide to oiler quality. During processing irregularities are sought, such as different cusp densities across the spray including gaps with total absence of cusps and associated lines. By identifying the position of each individual cusp, it is possible to detcrmine cusp density which indicates the evenness of thc coating, as well as detecting the absence of cusps which identifies dry stripc.

Calculation of the cusp density gives the operator the ability to identify light and heavy stripes.

Thc optical monitoring system of the present invcntion is unaffcctcd by thc oil and high voltages present and does not affect the oiling process itself The sensor may comprise a photoelectric device and filter mounted at the end of a short plastic tube and may inctude a simple lens. The optical oiler quality monitor may be mounted as a free standing device, or inside the oiler enclosure. Thc invention limits thc area which needs to be imaged at any one time by scanning an optical detector along a detection zone in which cusps are at an optimal shape for detection.

Claims

CLAIMS1. An optical monitoring system for monitoring quality of an electrostatically applied coating applied to a product, the system comprising a light source; an optical detector mountcd on a lincar scanning mechanism to dctcct light signals reflected from a spray of the coating dunng application; and an actuator to pass the product along a transport path of product through the scanning mechanism; wherein the scanning mechanism is installed across the width of the transport path and perpendicular to a direction of travel of the product on thc transport path.
2. A system according to claim 1, wherein the spray coating is derived from lines formed on the blade and the detector detects spray from a line or lines forming a sub-scction of all spray forming lincs.
3. A system according to claim 1, wherein the detector detects in the detection zone after lines form, but before they disperse.
4. A system according to any preceding claim, wherein the light source is mounted on the same linear scanning mechanism as the optical sensor.
5. A systcm according to any prcccding claim, wherein the detector further compriscs a light guide.
6. A system according to any preceding claim, wherein at least one of the light guidc and the light source furthcr comprisc a gas purge tube.
7. A system according to any preceding claim, wherein the detector comprises an optical sensor.
2. A system according to claim 7, wherein the detector further comprises a lens and a filter.
9. A system according to claim 7 or claim 8, wherein the detector further comprises a processor to process signals output from the sensor.
10. A system according to any preceding claim, wherein the system further comprises a display.
11. A system according to any preceding claim, wherein the coating comprises one of oil or molten wax.
12. A mcthod of monitoring an clcctrostatically applicd coating, thc rncthod comprising, positioning a light source; illuminating a line of coating in a detection zone with the light source; positioning a detector; receiving at the detector light from the light source rcflectcd off the linc of coating; moving thc dctcctor; and rcpcating thc illumination and detection steps.
13. A method according to claim 12, wherein the light source and detector are moved together.
14. A method according to claim 12 or claim 13, wherein the method further comprises comparing the reccived light with an expected light profile.
15. A method according to claim 14, whcrcin thc method further comprises deriving a graphical representation of the resuh of the comparison and outputting the graphical representation.
16. A method according to claim 15, wherein the graphical representation is output to a display, or to a store.
17. A method according to any of claims 14 to 16, wherein if the result of the comparison breaches a threshold value an alarm is activated.
18. A method according to any of claims 14 to 16, wherein if the result of the comparison breaches a threshold value, a magnified image is displayed.