GB2270227A

GB2270227A - Linescan visual monitoring system

Info

Publication number: GB2270227A
Application number: GB9218400A
Authority: GB
Inventors: Martin Coulthard
Original assignee: Surface Inspection Ltd
Current assignee: Surface Inspection Ltd
Priority date: 1992-08-28
Filing date: 1992-08-28
Publication date: 1994-03-02
Also published as: GB9218400D0

Abstract

A visual monitoring system uses a linescan camera 10 to view a product 11 on the production line. The camera repeatedly scans and produces lines of image data which are collected in a frame store. The frame store generates a video signal that is displayed as a rectangular image on a video monitor 17, 15. The operator of the system views the image on the monitor in order to inspect the product, and can manipulate the image displayed by means of a control panel 18. <IMAGE>

Description

LINESCAN CAMERA VISUAL MONITORING SYSTEM TECHNICAL FIELD This invention relates to a linescan visual monitoring system.

BACKGROUND Direct visual inspection on the production line of moving products such as metal strip for surface and other defects can be problematic due to the speed of the product and the hostile environment. Defects are often difficult to see by eye under normal lighting conditions. If such defects are not found early in the production process then large quantities of defective material may be produced.

In many cases it is preferable to inspect the product indirectly, that is to say by producing an image of the product on a video monitor for a human inspector to look at.

In this document the term 'visual monitoring system' is used to describe such a system which acquires and displays images of a product and acts as an aid for a human inspector. This is done to distinguish this class of system from an 'automatic inspection system' which carries out the inspection task without human involvement.

Visual monitoring systems have been built using matrix cameras - ie. cameras which produce a rectangular image, such as standard vidicon or CCD (Charge Coupled Device) cameras.

A disadvantage of this approach is that the camera views a rectangular area of strip, so that a relatively large amount of space is required on the production line. Also, it is difficult to illuminate this area of strip in such a way that surface defects are visible and that an image of uniform brightness is produced.

More sophisticated scanning devices, such as linescan cameras and flying-spot laser scanners, have been used in automatic inspection systems, but not in visual monitoring systems.

ESSENTIAL TECHNICAL FEATURES According to the present invention there is provided a linescan visual monitoring system comprising an illumination system, a linescan camera, a frame store, a video monitor and means for manipulating the image displayed on the monitor.

Thus, the present invention is directed to a linescan visual monitoring system, especially but not exclusively for monitoring strip products, in which relative movement occurs between the product being monitored and the system, comprising illumination means to illuminate a transverse line on the product being monitored, a linescan camera positioned to receive light from the illumination means which has been reflected by that product to produce signals representative of an image of the line being monitored, a frame store connected to receive signals from the linescan camera representative of the images of successive such lines and to store those signals, a video monitor connected to the store to display images represented by the stored signals, and adjustment means of the system arranged to adjust the relative manner in which different points on the product being monitored are mapped onto the display of the video monitor.

Advantageously, the illumination means comprise an array of spot lamps having respective axes of illumination which converge substantially to a single point at or substantially at the camera.

Preferably, the said array is a linear array.

In one preferred embodiment of the present invention the adjustment means enable the area image displayed by the video monitor to be compressed so that an image representative of a larger length of the product may appear at a given instant on the display of the video monitor, than would appear without such compression.

Advantageously, the system further comprises display control means of the system connected to enable successive images to be displayed on the video monitor representative of successive areas of the product being monitored, and variable timing delay means of the display control means, which are variable to alter the real time delay between the monitoring of the said successive areas whereby the relative spacing between successive equi-spaced repeated defects of the product being monitored can be ascertained from the real time delay that results in an apparently stationary image of the defect as successive images are viewed on the video monitor.

The present invention extends to an array of spot lamps for use in a linescan visual monitoring system, the array being arranged so that their axes of illumination substantially pass through points spaced apart along a common line, and so that these axes converge substantially to a single point.

EXAMPLE A specific embodiment of the invention, configured as a visual monitoring system for metal strip, will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a diagram showing the general arrangement of the linescan visual monitoring system; Figure 2 is a block diagram of the frame store of the system shown in Figure 1; Figure 3 is a diagram showing the layout of the controls on the control panel of the system shown in Figure 1; Figure 4 is a diagram illustrating in greater detail the illumination system of the system shown in Figure 1; and Figure 5 is a further diagram illustrating the image compression feature of the system shown in Figure 1.

Figures 6, 7 and 8 are further diagrams illustrating the view selection features of the system shown in Figure 1.

Figure 9 is a further diagram illustrating how the camera data is handled for the image compression feature of the system shown in Figure 1.

A linescan camera 10 is mounted on the production line so that it views a line 11 across the strip 12 at a position where the strip is in contact with a roller 13. This line is lit by an illumination system 14. The camera is connected electrically to a frame store 20 located in a control cabinet 15. Also mounted in the control cabinet are a video monitor 17 and a control panel 18, both of which are connected electrically to the frame store. An additional electrical input to the system is a strip speed signal 16.

The system described operates as follows. The camera, having one thousand seven hundred and twenty-eight pixels, repeatedly scans the viewed line, typically at a rate of two thousand scans per second, as the strip moves past and sends image data to the frame store. The scanning speed is controlled by a strip speed signal so that a scan is made each time the strip moves a given distance.

Multiple image lines are collected in the frame store, which can store up to two hundred and fifty-six lines of up to one thousand and twenty-four pixels each and are displayed as a two-dimensional image on the video monitor, to be viewed by the system's operator.

The manner in which the incoming lines of image data 21 are stored in the frame store memory 23 is controlled by the write control circuits 22 of the frame store, which in turn are controlled by the signals 26 received from the control panel switches. A line of data arriving at the frame store, for instance, can be stored in its entirety, part of it can be stored, or none of it can be stored, dependent on the signals from the control panel.

Similarly the size and position of the area of the frame store memory that is read out to produce the video signal 25 to be displayed on the video monitor is controlled by the read control circuits 24, which are again controlled by the signals from the control panel.

The operator of the system can therefore manipulate the image that is displayed by means of the control panel, which consists of an array of push-buttons and rotary switches.

He can, for instance, freeze the image on the video monitor by pressing the button 30 on the control panel labelled 'Freeze'. This button sends a signal to the write control circuits of the frame store causing no new lines of data to be stored, and to the read control circuits so that the same area of frame store is repeatedly displayed. The effect is reversed by pressing the 'Live' button 31, which restores the live image on the display.

Alternatively he can zoom in to see an area of interest magnified by pressing one of the 'View' buttons 32, such as the button 33 which causes the left part of the strip to be displayed at times two magnification. In this case the first one thousand and twenty-four pixels from the image data 60 are stored in the frame store memory 61 and the first seven hundred and thirty-six of these 62 are then displayed on the monitor.

Greater magnification is possible by pressing one of the 'View' buttons 32, such as the button 34 which causes the far left part of the strip to be displayed at times four magnification. In this case the first one thousand and twenty-four pixels from the camera data 70 are stored in the frame store memory 71 and the first three hundred and sixtyeight of these are then each displayed twice in succession 72 on the monitor.

To resume the normal display the Full View button 35 is pressed which causes every second pixel from the camera data 80 to be stored in the frame store memory 81, i.e. eight hundred and sixty-four pixels for a typical camera having one thousand seven hundred and twenty-eight pixels, and of these the central seven hundred and thirty-six are then displayed 82 on the monitor.

Illumination System The illumination system 14 is effective on certain strip products such as cold-rolled steel, and uses multiple spot lamps. The lamps are arranged in a row and are angled so that the axes of light output of all of the lamps converge to a single point.

The row of lamps is positioned so that the light from the spot lamps is directed onto the line 11 across the strip that is viewed by the linescan camera 10. It is convenient, though not essential, that the row of lamps be mounted so that the line passing through all the lamps is parallel to the strip surface.

The camera is positioned with its axis of view at an equal angle to the normal to the strip as the axis of the lights, and so that it is at the point of the convergence of the light from the lamps (after the reflection of the light in the surface of the strip). The camera therefore views the light from the lamps that is specularly or directly reflected by the strip. This lighting/viewing arrangement is generally known as 'bright field' illumination.

In an alternative configuration of the same illumination system the camera can be positioned away from the direction of the directly reflected light, but still viewing the illuminated line across the strip, so that it views light from the lamps that is diffusely reflected or scattered by the strip. This is known as 'dark field' illumination.

An advantage of using this lighting arrangement to provide either bright or dark field illumination is that it is efficient in its use of light; that is to say a relatively high proportion of the light output of the lamps is collected by the camera. Secondly, the directional characteristics of the light from the spot lamps allow the camera to acquire images in which topographic defects such as dents in the surface of the strip are visible with strong contrast.

Image Compression One of the image manipulation functions that can be carried out by the system is "image compression", i.e. the display of an image in which a large length of strip is shown. The operator can increase the compression factor using the control panel button 36 labelled 'Compress'.

A compressed display is achieved by sub-sampling the lines of camera data 90 received by the frame store, so that, for instance, only one in every four scan lines from the camera is stored in the frame store memory 91.

The button 37 labelled 'Stretch' reverses the action and reduces the sub-sampling factor.

The compress function is particularly useful on strip products where repeating defects 50, such as roll marks, occur on the strip 12. If the circumference of the roller which caused the roll mark is, say, two metres then the roll mark will be produced every two metres of strip (assuming that no elongation of the strip has occurred since contact with the roller). In a standard view the image 51 on the monitor would show, say, a two metres long length of strip 52, so only one defect would be visible. If however the system is set to compress the image by a factor of four then the image 53 shows eight metres of strip 54, so that four defects are visible. This makes diagnosis of the defect considerably easier for the operator of the system.

Defect Interval Measurement An additional function of the system is defect interval measurement. This allows the operator to measure the distance between adjacent repeating defects such as roll marks in order to determine the cause of the defect.

When this function is activated successive static images of the product under inspection are displayed on the monitor.

The repeat rate of the display of these images is adjusted by the operator, using a potentiometer 38 on the control panel, until the defect appears to be stationary (because successive defects appear in the same position in consecutive images).

The variation of the repeat rate can be achieved in a number of ways that will be apparent to those familiar with the art, including varying the integration time or scan rate of the camera, and altering the ways that image data is written to and read from the frame store memory.

The distance between the defects is then calculated by the system from the strip speed (known from the strip speed signal) and the time interval between the images by the following equation Defect Interval = Strip Speed x Time Interval Between Images The defect interval is displayed on a digital read-out.

Claims

1. A linescan visual monitoring system comprising an illumination system, a linescan camera, a frame store, a video monitor and means for manipulating the image displayed on the video monitor.

2. A linescan visual monitoring system, especially but not exclusively for monitoring strip products, in which relative movement occurs between the product being monitored and the system, comprising illumination means to illuminate a transverse line on the product being monitored, a linescan camera positioned to receive light from the illumination means which has been reflected by that product to produce signals representative of an image of the line being monitored, a frame store connected to receive signals from the linescan camera representative of the images of successive such lines and to store those signals, a video monitor connected to the store to display images represented by the stored signals, and adjustment means of the system arranged to adjust the relative manner in which different points on the product being monitored are mapped onto the display of the video monitor.

3. A linescan visual monitoring system according to claim 2, in which the illumination means comprise an array of spot lamps having respective axes of illumination which converge substantially to a single point at or substantially at the camera.

4. A linescan visual monitoring system according to claim 3, in which the said array is a linear array.

5. A linescan visual monitoring system according to any one of claims 2 to 4, in which the adjustment means enable the area image displayed by the video monitor to be compressed so that an image representative of a larger length of the product may appear at a given instant on the display of the video monitor, than would appear without such compression.

6. A linescan visual monitoring system according to any one of claims 2 to 5, further comprising display control means of the system connected to enable successive images to be displayed on the video monitor representative of successive areas of the product being monitored, and variable timing delay means of the display control means, which are variable to alter the real time delay between the monitoring of the said successive areas whereby the relative spacing between successive equi-spaced repeated defects of the product being monitored can be ascertained from the real time delay that results in an apparently stationary image of the defect as successive images are viewed on the video monitor.

7. A linescan visual monitoring system substantially as described herein with reference to the accompanying drawings.

8. For use on a linescan visual monitoring system, an array of spot lamps arranged so that their axes of illumination substantially pass through points spaced apart along a common line, and so that these axes converge substantially to a single point.