Abstract:
A quality control method for two-dimensional matrix codes on metallic workpieces, the codes being in the form of stamped marking dots is disclosed. The stamping process for the marking dots is carried out by a marking tool ( 17 ) with the aid of predetermined digital positional data. The corresponding image data is then recorded for analysis by an image processing device ( 22 ), exclusively at the locations that have been predetermined by the positional data, or additional image data that has been previously generated is also used for the analysis, to establish whether a correct marking dot with the required quality characteristics is present.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention refers to a method for quality control for two-dimensional matrix codes on metallic workpieces, which are present in the form of embossed marking dots, with an image processing device. 
   2. The Related Art 
   The marking dots of such a two-dimensional matrix code are normally embossed by means of a hard-metal needle of a marking tool. This is done on one hand very fast and on the other hand in a very narrow arrangement, wherein such matrix areas can be very small and possess a length and width of only a few millimeters. For being able to read back the information of the matrix code without any errors, the precision in placing the marking dots is of high importance, with the exact shape, size, position and depth of the marking dots being important quality features. It is therefore very critical to check, during or after the production of such a matrix code, whether the information can be read back without any errors, i.e. whether the marking dots are present in the correct place in the necessary quality. 
   Known methods for the quality control of two-dimensional matrix codes use a “finder function” like the one which is used at reading for the position determination of the code array. The square matrix codes have two adjacent outlines with marking dots in narrow sequence. These two outlines are searched in the image data collected by a camera as an image processing device for determining the position of the matrix code in this manner. The two other outlines have marking dots whose distance corresponds, respectively, to the distance of the grid lines of a grid possessing the marking dots as grid points. Thus, these grid points are determined from the image data and, subsequently, the calculated points are checked as to whether a marking dot of the desired quality is positioned there. Due to image capture, image processing and subsequent calculation, multiple error factors are included in quality control which render the entire process very imprecise, in particular in case of very small distances between the dots. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is provision of a method for fast, easy and precise quality control for two-dimensional matrix codes. 
   This objective is accomplished, according to the invention, by the embossing process for the marking dots taking place based on preset digital position data, using a marking tool, and by the subsequent capturing of the corresponding image data by means of the image processing device, only at the sites precisely predetermined by the position data, for checking whether at each site there is a correct marking dot with the desired quality characteristics. 
   The advantages of the method according to the invention lie in particular in the fact that the known finder function can be omitted and is replaced by a much more precise method for finding the marking dots and checking the same. According to the invention, the position data for the marking tool, which are preset anyway, are supplied to the image processing device so that it possesses the exact coordinates of the marking dots even without image capture. The quality check then needs only to be performed at these coordinate points. All errors of the known methods, which are related to determination of the coordinate positions, are therefore almost completely eliminated so that the quality check of the marking dots can be performed with much higher precision. 
   For checking the desired quality characteristics of the respective marking dots, one or more of the following parameters are detected and compared with default data: area, depth, length, width, area midpoint or centroid, ellipticity. This check takes place by means of the image information recorded at the coordinate sites. During this process, deviations from the default data can be documented or even evaluation criteria given. 
   To compensate for any still existing small position offset, the average of the positional deviations of all marking dots, respectively, is subtracted as an offset value. This is because such a position offset does not contain any quality deviation of the code to be tested, but only the imprecision of the positional relation used by the image processing device or camera on the one hand and the marking tool on the other hand. 
   An advantageous measure for achieving high-grade quality control consists in the fact that before a matrix code is applied, a reference image of the corresponding surface portion of the workpiece is recorded and stored and/or evaluated. If the surface portion is recognized as being too bad, e.g. having a surface texture which is too rough or severe defects, such as blow holes or mechanical damages, a shift to a different, better surface portion takes place which is, of course, also checked first. In this manner, a marking which can not be used can be avoided in the first place. 
   Another advantage of recording a reference image of the material surface before applying a matrix code consists in the fact that a correlation between the reference image and the corresponding image data after application of the matrix code is performed to eliminate interfering surface texture data. By such a correlation of the reference image with the image to be analyzed, differences between the actual physical-geometrical properties of a marking dot and its optical representation by the camera in the form of pixel patterns are reduced and omitted from the evaluation of the coding to be evaluated. These differences can also be due to illumination, optics, CCD sensor and signal filtering. 
   Another advantageous method for improving quality control consists in recording the image data with different focus adjustments, in particular in recording them at varying distances between camera and workpiece surface and correlating them. Interferences of various kinds can be filtered more easily in this manner. 
   Another advantageous measure for improving the quality control consists in using an individual marking dot, previously recorded in a real manner as an image, electronically in an “artificial” image which is generated by superimposing this “reference image” of a marking dot at all preset target positions of marking dots of the code image to be measured currently. This “reference image” of a marking dot may have been generated and be used in respect to various characteristics with different qualities. As an alternative, this “reference image” may have been generated by statistical averaging of many marking dots really generated. 
   Advantageously, the marking tool and the image processing device can be moved in mechanical coupling with each other. In this manner, a tolerance-free alternative positioning of the image processing device or camera, respectively, on the one hand and of the marking tool on the other hand above the marking position of the workpiece is possible by moving both with the same three-axis carriage system. Tolerance-free positioning, also in the z axis, allows an imaging which is true to scale and, therefore, a size measurement which can be calibrated as well. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a perspective view of a marking tool attached to an adjustable support and firmly connected to a camera, used in practice of a preferred embodiment of the present invention; and 
       FIG. 2  is a flow chart of a preferred embodiment of the method of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a supporting table  10  for taking up metallic workpieces  11  to be provided with a two-dimensional matrix code, as including a column  12  arranged on which is mounted a holder  13  which can be vertically moved and positioned by means of a motor drive  14 . For guidance on the column  12  during movement in the vertical direction z, guide slots  15  are used. 
   On the holder  13 , a carriage arrangement  16  is arranged through which a marking tool  17  can be driven and positioned in the two horizontal axes (x axis and y axis). The carriage arrangement  16  consists of an x-carriage  18  for the direction x and a y-carriage  19  for the direction y. Each of the carriages  18  and  19  is equipped with motor positioning drives, with only the positioning drive  20  for the x-carriage  18  being recognizable in the perspective drawing. 
   By means of a spacing element  21 , the marking tool  17  is firmly connected to an image processing device devised as a camera  22  which can be implemented e.g. as a CCD camera. Around the lens  23  of the camera  22 , an illumination device  24  is arranged for illuminating the matrix area as well as possible. Naturally, the illumination device  24  can also be arranged in a different position on the camera  22  and/or the marking tool  17 . 
   The marking tool  17  possesses a striking tool devised e.g. as a hard-metal needle  25  which, after suitable positioning, executes striking movements against the workpiece  11  for creating the marking dots executed as striking recesses. A multiplicity of such marking dots then forms a two-dimensional matrix code  26 , with the presence or lack of the marking dots at the respective grid points representing the binary encoded information. During this process, the motor drive  14  for the holder  13  and the carriage arrangement  16  are used for positioning the marking tool  17  above the site to be encoded on the metallic workpiece  11 . 
   Before the marking process starts, the camera  22  is positioned above the site to be encoded and captures a reference image signal of the area to be encoded. Based on this reference image signal, an evaluation of the surface texture can take place, e.g. gray-scale value distribution and variance. By means of a threshold evaluation, it can be tested whether the site intended for marking is at all suitable for this purpose or whether e.g. the surface roughness is too great or severe defects, such as blow holes etc., are present, or whether this area shows substantial mechanical damages making it unsuitable for code marking. If the evaluation indicates that the envisaged site is not suitable for marking for the reasons stated above, a better marking site more free from interference is searched by moving the carriage arrangement  16 . The found site is then taken into account or accepted, respectively, in the marking control with its offset values. 
   Subsequently, the marking tool  17  is shifted by means of the x-carriage  18  over a distance corresponding precisely to the distance between the lens  23  of the camera  22  and the hard-metal needle  25 . In this manner, the marking tool is positioned exactly where the camera  22  was positioned before. Now, by a sequence of striking movements of the hard-metal needle  25  and movements of the carriage arrangement  16 , the matrix code  26  is generated. The positions of the individual matrix points are preset by means of stored digital position data. 
   Subsequently, the camera  22  is driven back into its original position by movement of the x-carriage  18 , i.e. in a position above the now existing matrix code  26 . The camera now records a test image signal. By correlation with the reference image signal which has already been recorded and stored, e.g. the surface texture of the metallic workpiece  11 , which interferes with the quality check for the marking dots, can be masked out. This means de facto a masking of any interference signals due to the surface texture which would falsify the image point information to be evaluated for the marking dots. Other disturbing influences for the quality check can be caused by the illumination, the optics, the camera or signal filtering. These influences can be eliminated by such a correlation as well. 
   Now the actual quality check of the marking dots at the sites indicated by the stored coordinates takes place. These coordinates are basically the same as those controlling the marking process of the marking tool. The quality check can be performed in different ways and with different amounts of effort. Quality criteria are e.g. the area, the length, the width, the ellipticity, the depth, the area midpoint and centroid of the marking dots. At the position data, the corresponding image data of the marking dots are recorded and compared with stored default values. A quality check is performed according to detected deviations. This can be reproduced in detail or, in the simplest case, exceeding of maximum permissible deviations leads to optical and/or acoustic alarms. 
   For compensation of any existing position offset, i.e. a homogeneous displacement of all x and y values by a certain amount, the average of all positional deviations is determined as an offset value and a corresponding correction is performed. Such an offset does not represent any quality deviation of the marking dots to be checked, but only an imprecision of the employed positional relation of camera  22  and marking tool  17 , whatever may have caused this imprecision. 
   As a variation of the presented embodiment, the linear movement described can also be replaced by a swinging movement for the reciprocal positioning of the marking tool  17  and the camera  22 . For instance, a swivel axis could be arranged on center between the marking tool  17  and the camera  22  so that the change in position can be performed respectively by a 180° swinging movement. In a simpler embodiment, the movement of the camera  22  can be uncoupled from the marking tool  17  as well and, for instance, the camera can be firmly marked or have an independent drive. Since this makes the recording position of the camera deviate from that of the marking tool, this must be taken into account during conversion of the position data for the marking dots. Also the tolerance-free positioning possible during synchronous movement and representation to scale must accordingly be compensated for electronically as well. 
   In the described embodiment, image processing for quality control is combined with the production machine, i.e. with the marking tool. The purpose is mainly to guarantee the quality of the generated matrix codes such that they can be read perfectly by reading devices during later use. In principle, the method according to the invention can also be carried out independently of a marking tool. 
   In a simpler embodiment, the recording of a reference image signal and the comparison with the test image signal can also be omitted, in particular if workpieces, which have none or only a minimum surface texture, are to be provided with a matrix code  26 .