A METHOD AND SYSTEM FOR MONITORING OR SCANNING AN OBJECT, MATERIAL OR THE LIKE

Abstract: In a method for monitoring or scanning an object, material or the like with the aid of a camera (2) movable in relation to the object or material, data recorded by the camera is stored sequentially as pixel values. A reference value is determined from one or more selected, stored pixels. The difference between the value of the actual pixel and the reference value is determined, after which changes in the object or material are detected from said difference. A system for such monitoring or scanning comprises a camera and a data-storage unit for sequential storage of recorded data as pixel values. A reference-producing unit (10) is arranged to produce a reference value from the value or values of one or more selected, stored pixels, and a comparison unit (6, 8) is arranged to form the difference between the value of the actual pixel and the reference value.

Description: A METHOD AND SYSTEM FOR MONITORING OR SCANNING AN OBJECT, MATERIAL OR THE LIKE The present invention relates to a method for monitoring or scanning an object, material or the like with the aid of a camera which is movable in relation to the object or material. The invention also relates to such a system with a data-storage unit for sequential storage of recorded data as pixel values. The present invention is primarily intended for various types of quality control, e.g. in the production of steel tubes, rods, insulating tape for electric cables and other electrical applications, paper webs in the manufacture of paper, etc. According to known technology pixel values, often measured using a so-called line camera, are compared with a fixed reference level, usually representing a faultless state or a state without foreign substances in the object or material in question, see DE 4 200 801 A1 , for instance. The object of the present invention is to provide a new technology which will give increased sensitivity for detecting small changes in the object being monitored or scanned, and which will also enable selection of the sensitivity of the monitoring or scanning being undertaken. This object is achieved with a method as claimed in claim 1 and a system as claimed in claim 7. In accordance with the present invention, thus, the object or material is scanned using a line camera and recorded data is sequentially stored as pixel values, i.e. light values, to serve as references in subsequent scans. By comparing subsequent pixel values with stored references it is thus possible to determine whether any pixel has changed value, i.e. to detect a change in the light flow, which may indicate a change in the object being scanned. According to the present invention, therefore, the pixel value in question is not compared with a fixed basic level as is the case in conventional technology, but with a floating reference value representing the state of the object as monitored or scanned a specific time earlier. The sensitivity for detecting small changes in the scanned object is thus increased, and by then selecting as reference a value a longer or shorter time prior to the actual pixel, the detecting sensitivity can be regulated. When changes in the object or material being scanned occur slowly, a reference further back in time may need to 2 — be chosen than if the change is rapid. It can thus be determined that the detected change occurred during a certain period, i.e. a kind of time-derivative determination is performed. A change detected in the object or material may constitute some form of inhomo- geneity, foreign substances in the object or material, etc. According to an advantageous embodiment of the invention the reference value is formed continuously as the mean value of a predetermined number of pixels im- mediately before the actual pixel being currently scanned. The momentary pixel value is thus compared with a reference value derived from earlier recent pixel values, making this embodiment suitable for detecting rapid changes in the object being monitored or scanned. According to other advantageous embodiments of the invention, wherein the camera is a line camera for scanning the object or material line by line, scanned lines are stored line by line and the value of the pixel in a selected, stored line that corresponds to the actual pixel in the line being scanned, is chosen as reference value, or the mean value of the pixels in a predetermined number of selected lines of said stored lines that correspond to the actual pixel in the line being scanned, is chosen as reference value. These stored reference lines constitute a kind of historical record containing information on how the appearance of the scanned object has varied backwards in time. In these embodiments of the invention, thus, changes in the scanned object or material are determined over a time which can be selected by the choice of reference line. By choosing a "recently" stored reference line immediately prior to the new, incoming line, the change over a short interval of time is measured, which is advantageous if rapid changes shall be detected. To detect slower changes, the line being scanned is compared with a reference line stored further back in time, i.e. a comparison in time or a time derivative determination is performed over shorter or longer times depending on the requirements of the measurement in progress. Selecting the time over which the change shall be detected means that the detection sensitivity can also be adjusted to current needs. The time displacement between the stored lines in turn corresponds to a certain displacement in space of the movable object. A typical distance between consecu- tive lines may be 10-30 μm. According to still other embodiments of the invention a change in the object or material is indicated if the difference between the actual pixel value and the reference value exceeds a predetermined threshold value, or alternatively, if the difference between the actual pixel value and the reference value exceeds a predetermined threshold value in a predetermined number of consecutive pixels. Actual comparison of pixel values, i.e. light values, in a new, in-coming line being scanned and the selected reference line takes place pixel by pixel and when a change in one or more pixels is detected a triggsignal is suitably generated for possible further investigation of the relevant area of the object or material being scanned. According to the invention, therefore, the current "appearance" of the scanned object is compared with its previous appearance in order to detect possible changes. Both positive and negative changes can be studied and changes in the base level thus have no effect. According to an advantageous embodiment of the system in accordance with the invention the comparison unit, which is arranged to form the difference between the value of the actual pixel and the reference value, comprises two subtracting units in order to determine the sign of the difference between the actual pixel value and the reference value. In this way it is possible not only to detect the occurrence of a change in the scanned object, but also to obtain information as to the nature of the change. To explain the invention further, embodiments of the system according to the invention, chosen by way of example, will now be described in more detail with reference to the accompanying drawings in which Figure 1 illustrates one embodiment of the system according to the invention in flowchart form and Figures 2-4 illustrate the function of three variants of the system in accordance with the invention, with the aid of schematically indicated scanned and stored pixel values In the embodiment shown in Figure 1 , the object or material in question is scanned by a line camera 2 at regular time intervals along scanning lines, and the corre- sponding light values are stored sequentially, pixel by pixel in a data-storage unit 4. Each line of the camera comprises pixels, i.e. light-sensitive elements, in a number of typically between 1024 and 8192. The pixels are read continuously with high frequency and the pixel values for up to 128 scanning lines are stored in the datastorage unit 4. The stored values serve as references to be used in subsequent comparisons to determine whether any pixel has changed value, i.e. to detect a change in the light value. As mentioned, up to 128 lines can be stored in the datastorage unit 4 and when a new line is stored, the oldest stored line is deleted, so that the stored lines are continuously updated. The above indicated number of 128 for the number of lines stored in the datastorage unit 4 is of course only an example and can be varied. In practice the camera is suitably stationary while the scanned object is moved past the camera during scanning. The in-coming, newly scanned line is supplied to subtracting units 6, 8, to which a selected reference line is also supplied from the data-storage unit 4. A selector unit 10 is provided to select a specific reference line among the lines stored in the datastorage unit 4. In the subtracting unit 6 the chosen reference line is subtracted from the scanned line that has arrived most recently, and in the subtracting unit 8 the most recent line is subtracted from the selected reference line. The subtraction is performed pixel by pixel. The use of two parallel subtracting units 6, 8 allows determination of the direction a change is taking, i.e. if the change is positive or negative. If the subtraction performed produces a positive value, the subtracting unit in ques- tion delivers a corresponding output signal for comparison with a predetermined trigg level in a subsequent comparator 9, 12. This trigg level is adjustable. The subtracting unit producing a negative result delivers no output signal, i.e. either the subtracting unit 6 or the subtracting unit 8 delivers an output signal dependent on whether the change is positive or negative, and if this output signal exceeds the predetermined trigg level stored in the associated comparator 9, 12, the output step 14 of the system will deliver an output signal indicating that a change in the scanned object or material has been detected. This output signal can then be used to initiate possible further investigation, using standard methods, of the relevant area of the object or material being scanned. Naturally positive or negative trigg levels may be chosen for the above embodiment. The camera may consist of a commercially available standard camera and data- storage unit, selector, subtracting units, comparators and output step can be realized by means of an image processing circuit board. Thus, in the embodiment above the pixel value in question is compared with the corresponding pixel value of a reference line stored previously in the data-storage unit 4. This line may be chosen from among the 128 stored lines. The principle of how this embodiment functions is illustrated in Figure 2. The pixel value in question in the relevant line is 125, indicated in suitable units, and the stored line 10 has been chosen as reference line. The corresponding pixel value of this reference line is 119 and the difference between the reference value and the pixel value in question is thus -6 which, in this example, is sufficient to generate an output signal from the associated comparator. Figure 3 shows an alternative embodiment in which the reference value is determined by forming an average value in the actual scanning line from a predetermined number of pixels immediately prior to the actual pixel. In Figure 3 the reference value is obtained from the mean value of the four pixel values immediately prior to the actual pixel. The mean value in the example shown is 106 and the actual pixel value of the actual line is 25. The trigger level set in the associated comparator is -45. In this case, thus, the output step 14 will deliver an output signal too. In this embodiment, thus, the actual pixel is compared with the immediately pre- ceding pixels in the same scanning line, which makes the system particularly sensitive to rapid changes. Figure 4 illustrates yet another alternative embodiment in which the actual pixel value is compared with a reference value consisting of the mean value of corre- sponding pixels in a predetermined number of lines chosen from amongst the previous lines stored. In the example shown in Figure 4 the mean value is formed from the preceding lines 1-10, i.e. the reference values consists of the mean value of ten previous pixel values at the same position in the lines. The mean value of the pixels in question is 65. The trigg value in the associated comparator for deliv- ering an output signal is -45 in the example. To realise the embodiments in Figures 3 and 4 the system must include a mean value forming means, while the function is otherwise substantially the same as that described in connection with Figure 1. The functions of the system according to the invention illustrated with reference to Figures 2-4 can be included in one and the same system, in which case the function most suitable for the application in question can be chosen. Combinations of the methods described can also be used for detecting an area of interest in the 6 Z_ object being scanned, whereafter this area may possibly undergo further investigation. When an area of interest has thus be detected in the object being scanned, by means of the invention as described above, this area is normally further analysed. Additional measurements and evaluations can be performed using so-called DSPs (Digital Signal Processors), for instance. In an additional step, the last measures such as classification of various types of foreign substances detected, are performed using a PC, for instance.

Claims: C L A I M S 1. A method for monitoring or scanning an object, material or the like with the aid of a camera which is movable in relation to the object or material, characterized in that data recorded by the camera are stored sequentially as pixel values, in that a reference value is determined from one or more selected, stored pixels, in that the difference between the value of the actual pixel and the reference value is determined, after which changes in the object or material are detected from said difference. 2. A method as claimed in claim 1 , characterized in that the reference value is formed continuously as the mean value of a predetermined number of pixels immediately before the actual pixel being currently scanned. 3. A method as claimed in claim 1 , wherein the camera is a line camera for scanning the object or material line by line, characterized in that scanned lines are stored line by line and in that the value of the pixel in a selected, stored line that corresponds to the actual pixel in the line being scanned, is chosen as reference value. 4. A method as claimed in claim 1 , wherein the camera is a line camera for scanning the object or material line by line, characterized in that scanned lines are stored line by line and in that the mean value of the pixels in a predetermined number of selected lines of said stored lines that correspond to the actual pixel in the line being scanned, is chosen as reference value. 5. A method as claimed in any of the preceding claims, characterized in that a change in the object or material is indicated if the difference between the actual pixel value and the reference value exceeds a predetermined threshold value. 6. A method as claimed in claim 5, characterized in that a change in the object or material is indicated if the difference between the actual pixel value and the reference value exceeds a predetermined threshold value in a predetermined number of consecutive pixels. 7. A system for monitoring or scanning an object, material or the like comprising a camera arranged to scan the object or material and a data-storage unit for 8 - sequential storage of recorded data as pixel values, characterized in that a reference producing unit is arranged to produce a reference value from the value or values of one or more selected, stored pixels, and in that a comparison unit is arranged to form the difference between the value of the actual pixel and the refer- ence value. 8. A system as claimed claim 7, characterized in that the camera is a line camera which is movable in relation to the object or material, and designed for 1024 - 8192 pixels per line. 9. A system as claimed in claim 8, characterized in that the data storage unit is designed for storing scanned lines, line by line. 10. A system as claimed in claim 9, characterized in that the data storage unit is designed for storing the pixel values from up to 128 lines. 11. A system as claimed in claim 9 or claim 10, characterized in that the data storage unit is adapted such that when a new line is entered into a full data storage unit, the oldest stored line is deleted. 12. A system as claimed in any of claims 7-11, characterized in that the reference-producing unit is arranged to produce a reference value from the mean value of the values of a predetermined number of pixels immediately before the actual pixel being currently scanned. 13. A system as claimed in any of claims 9-11 , characterized in that the reference-producing unit is arranged to choose the value of the pixel in a selected, stored line that corresponds to the actual pixel in the line being scanned, as reference value. 14. A system as claimed in any of claims 9-11 , characterized in that the reference-producing unit is arranged to produce the mean value of the pixels in a predetermined number of selected lines of said stored lines that correspond to the actual pixel in the line being scanned, as reference value. 15. A system as claimed in any of claims 7-14, characterized in that a control unit is arranged to control the camera to execute scanning at regular intervals. 9 — 16. A system as claimed in any of claims 7-15, characterized in that the comparison unit comprises two subtracting units in order to determine the sign of the difference between the actual pixel value and the reference value. 17. A system as claimed in any of claims 7-16, characterized in that an output unit is arranged, as indication of a change in the object or material, to deliver an output signal if the absolute value of the difference between actual pixel value and the reference value exceeds a predetermined limit value. 18. A system as claimed in claim 16 or claim 17, characterized in that the output unit comprises a comparator connected to each one of the subtraction units for comparison of signals representing the difference between actual pixel value and reference value, from the subtracting units with predetermined threshold values to deliver an output signal from one comparator when said difference signal ex- ceeds the threshold value, indicating a change in the object or material.

Inventors: AASEMYR GOERAN, AASEMYR, GOERAN
Applicants: AASEMYR GOERAN, PHOTONIC SYSTEMS AB, AASEMYR, GOERAN
Application year: 1999
Main classification: G01N21
IPC codes: G01B11, G01N21, G06T1, G06T7