Abstract:
A method for monitoring the quality of the primer layer applied to the body of a motor vehicle prior to painting (enamelling) envisages provision of at least one manipulator robot carrying a monitoring head. The monitoring head includes a light source constituted by an array of LED sources and a videocamera that are held in a position fixed with respect to one another while they are displaced with respect to the surface to be monitored following the profile of said surface. The signals at output from the videocamera are processed by dividing the area monitored into an array of sub-areas and executing the same processing procedure simultaneously on all the sub-areas.

Description:
This application claims priority to EP 11192716.6 filed 9 Dec. 2011, the entire contents of each of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for monitoring the quality of the primer layer applied to a motor-vehicle body prior to painting (enamelling). In particular, the invention regards a method of the type comprising: provision of a light source for illuminating the surface to be monitored and of at least one videocamera for inspecting the illuminated surface; and processing of the signals at output from the videocamera to obtain information on the quality of the primer layer. 
     A method of the type specified above is, for example, known from the document No. KR 20090053079. 
     The primer layer applied on a motor-vehicle body can present different types of defects, which, after application of the enamel, can give rise to different effects. A first type of defects gives rise to a variation of the colour of the painted body, which can assume different appearances: non-uniform brightness, halos, stains or shadows, shiny parts, regions with shortage of paint. Other types of defects give rise to a non-uniform surface (orange-peel effect, stripes) or to accumulation of paint (running of paint). 
     The main class of defects of the primer layer is, however, constituted by defects of a localized type (“point defects”) that take different forms (impurities, blistering, craters, degassing, flaking, peeling, pin-holing, streaking, scratches, porosities, blurring, bubbles). The method according to the invention is principally directed at monitoring defects of the latter type. 
     OBJECT OF THE INVENTION 
     The object of the invention is to provide a method that can be implemented with relatively simple and low-cost means, that will guarantee a high precision in the operation of monitoring, and that will enable the desired result to be obtained with a very low processing time. 
     SUMMARY OF THE INVENTION 
     With a view to achieving said object, the invention regards a method presenting the characteristics that have been referred to at the start and further characterized: 
     in that a manipulator robot is provided, which carries a monitoring head including both the aforesaid light source and the aforesaid videocamera, with said light source and said videocamera held in a fixed position with respect to one another; 
     in that said robot is controlled for moving the light source and the videocamera with respect to the surface monitored in a main direction of movement and according to a path parallel to the surface to be monitored, keeping the light source and the videocamera each at a constant distance from said surface in such a way that said path of movement follows a profile corresponding to the profile of said surface; 
     in that said light source is constituted by an array of LED sources designed to emit a beam of light on an area of the surface to be monitored so as to obtain a uniform intensity of illumination on the aforesaid surface; 
     in that said videocamera is positioned and oriented with respect to said light source in such a way as to collect by reflection the image of the illuminated area of the surface monitored; 
     in that the aforesaid processing step comprises:
         dividing the area monitored into an array of sub-areas; and   executing the same processing procedure simultaneously on all the sub-areas,       

     and in that the processing procedure executed for each sub-area comprises:
         identifying regions of each sub-area with a luminosity lower than a threshold level as potential defects of the primer layer;   rejecting the potential defects that have dimensions smaller than a minimum threshold area or larger than a maximum threshold area,   rejecting the potential defects that are distant from the edges of the sub-area by a length shorter than a threshold length; and   identifying the potential defects that are not rejected following upon the aforesaid operations as confirmed defects, and classifying each confirmed defect as slight defect, medium defect, or serious defect, according to the area of the defect.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the annexed drawings, which are provided purely by way of non-limiting example and in which: 
         FIG. 1  is a schematic perspective view of a station for monitoring the quality of the primer layer applied on motor-vehicle bodies, on a production line; 
         FIG. 2  is a schematic perspective view, which shows the essential components of the system used for implementation of the method according to the invention; 
         FIG. 3  is a schematic front view of the array of LED sources constituting the light source used in the method according to the invention; 
         FIGS. 4 and 5  are block diagrams that illustrate the method according to the invention; 
         FIG. 6  illustrates further block diagrams that show the details of the method according to a preferred embodiment of the invention; and 
         FIGS. 7A ,  7 B and  8 A,  8 B are schematic representations of defects analysed with the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , the reference number  1  designates as a whole a production line, illustrated only schematically in the drawing, along which there advance motor-vehicle bodies  2  on which a primer layer has been applied, in preparation for the painting operation. The reference number  3  designates a station for monitoring the quality of the primer layer, comprising a pair of multi-axis manipulator robots  4 , of any known type, prearranged at the two sides of the line  1  for analysing simultaneously the left-hand half and the right-hand half of the surface of each body  2 . Each robot  4  has its distal wrist that carries a monitoring head  5  for execution of the method according to the invention. 
     As illustrated schematically in  FIG. 2 , the monitoring head  5  comprises a light source  6  and a videocamera  7 , which are supported in a pre-set fixed position with respect to one another by a supporting frame  8  having an attachment  9  of any known type (not illustrated) for fast connection to the wrist of the respective robot  4 . 
     According to the invention, the electronic control unit of each robot  4  feeds the monitoring head  5  in the direction of advance A of the bodies  2  along the line  1 , with a speed of advance higher than the speed of advance of the bodies  2 , so as to create a relative movement between the monitoring head  5  and each body  2 , sufficient for carrying out scanning of the entire surface of the body, preferably without the need to stop the body at the monitoring station  3 . In the course of advance in the direction A, the monitoring head  5  is moved also in a direction orthogonal to the direction A so that it will follow a path T corresponding to the profile of the surface S to be monitored in such a way that both the light source  6  and the videocamera  7  remain constantly at the same distance from the surface S. 
     With reference to  FIG. 3 , the light source  6  comprises an orderly array of LED sources  60  that gives rise to a collimated beam of light  61  that illuminates an area I of the surface S with a light intensity substantially uniform in all the points of said area I. 
     The videocamera  7  is a CCD videocamera of any known conventional and standard type for detection of the image of the illuminated area I reflected by the surface S. For this purpose, the orientation of the source  6  and of the videocamera  7  is of course such as to guarantee the maximum optical efficiency, with the source  6  the optical axis of which is inclined by an angle preferably of between 20° and 30° with respect to the surface to be monitored S and the videocamera  7  is also inclined so as to be specular by a corresponding angle with respect to the surface itself. 
     The signals at output from the videocamera  7  are sent to an electronic processing unit E. 
     With reference to  FIG. 7 , in order to reduce as much as possible the processing time, the method according to the invention envisages that the area I (x, y) illuminated is divided into an array of sub-areas I 1 , I 2 , . . . I N . The example illustrated regards the case N=4, i.e., the division of the entire area I into four quadrants I 1 , I 2 , I 3 , I 4 . The images of the four sub-areas are processed simultaneously by the unit E in order to be able to identify simultaneously the defects of each of said sub-areas (see penultimate row of the block diagram of  FIG. 4 ), after which the results obtained are united to obtain information on the defects of the entire area I. Said procedure is also clearly indicated in the block diagram of  FIG. 5 . 
       FIG. 6  shows block diagrams that illustrate a preferred embodiment of the processing method that is performed for each of the sub-images I 1 , I 2 , . . . I N  associated to the sub-areas into which the area I is divided. 
     The top part of  FIG. 6  shows the step in which the regions of each sub-area with luminosity lower than a threshold level are identified as potential defects of the primer layer. For this purpose, a filter F, which is a kernel filter h (m 1 , m 2 ), is used 
               y   ⁡     [       n   1     ,     n   2       ]       =       ∑       m   1     =   0         N   1     -   1       ⁢       ∑       m   2     =   0         N   2     -   1       ⁢       h   ⁡     [       m   1     ,     m   2       ]       ⁢     x   ⁡     [         n   1     -     m   1       ,       n   2     -     m   2         ]                   
where the kernel h (m 1 , m 2 ) used is a disk of radius  5 :
 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
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                 0 
                 0 
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     With reference once again to  FIG. 6 , the output of the filter (y(n 1 ,n 2 )), presents the defects of the primer highlighted with low values (close to 0) and the background light with high values (close to 255). All the values of y(n 1 ,n 2 ) lower than the threshold “Threshold Surface” are brought to the value 0 so that, after the AND operation (z(n 1 ,n 2 )) represented in  FIG. 6 , we have either zero values (corresponding to probable defects) or the values of the original image x(n 1 ,n 2 ). 
     Once again with reference to the top part of  FIG. 6 , after the AND an “Amplification of Defects (Dilate)” is executed that enables amplification of the regions of the array z(n 1 ,n 2 ) with zero values (probable defects) so that a probable defect is dilated.  FIGS. 7A and 7B  show a potential defect before and after the “Dilate” operation, respectively. 
     There is then executed an operation of “binarization” with two thresholds (“Threshold Low” and “Threshold High”) that sends all the bright (illuminated) regions to the zero value and the probable defects to one. 
     The operation “seek objects” detects all the regions with value of unity (probable defects) and determines the area and the position of each object present in the binarized array. 
     With reference to the bottom part of  FIG. 6 , for each object found two checks are carried out to determine whether the object found (probable defect) is a real defect or not. 
     The first check verifies that the area of the object found is comprised between the thresholds “Area Min” and “Area Max” (objects that are too small or too big are rejected). 
     The second check extracts a square (“Particular”) around the probable defect found for verifying that the particular extracted is isolated at the centre of the square, i.e., that it is at a distance from the edges of the sub-area monitored by a length greater than a threshold value.  FIG. 8A  shows the example of a defect that satisfies said condition, whilst  FIG. 8B  shows a potential defect that is to be rejected as real defect. 
     The potential defects that are not rejected following upon the aforesaid operations are considered confirmed defects. 
     For the confirmed defects the method according to the invention envisages a classification as slight, medium, or serious defects, according to their extension. 
     Of course, without prejudice to the principle of the invention, the details of construction and the embodiments may vary widely with respect to what has been described and illustrated herein purely by way of example, without thereby departing from the scope of the present invention, as defined in the annexed claims.