Patent Publication Number: US-7914098-B2

Title: Device for patterning workpieces

Description:
TECHNICAL FIELD 
     The invention relates to a device for patterning workpieces, which preferably consist at least partially of wood, wood materials or the like, according to the preamble of claim  1 . 
     PRIOR ART 
     A device of the type mentioned at the outset is known, for example, from DE 100 31 030 B4 and has sensors for roughly detecting the contour and thickness of the workpieces to be imprinted which are attached to a conveyor device or to a portal. Nevertheless, it has been found that the workpieces imprinted using a device of this type often have a distorted, smudged printed image or else a printed image displaying defects and colour variations. 
     Furthermore, European patent application EP 05 009 326.9, which was filed by the Applicant and has not yet been published, also relates to a device according to the preamble of claim  1 . 
     PRESENTATION OF THE INVENTION 
     The object of the present invention is therefore to provide a generic device for patterning workpieces that allows an improved printed image quality. 
     According to the invention, this object is achieved by a device according to claim  1  and a method according to claim  7 . Particularly advantageous developments of the invention are specified in the dependent claims. 
     The underlying idea of the invention is that of basing the printing process, by targeted preprocessing and/or postprocessing, on completely new information. For this purpose, provision is made for the device according to the invention also to have at least one image detection sensor. The provision of at least one image detection sensor opens up entirely new possibilities for controlling and optimising the printing process. It is thus possible, for example, to determine using the image detection sensor the precise position of the workpiece relative to the printing means even before the printing process (“preprocessing”). This allows distortion and smudging of the printed image, which are caused by inadequate relative positioning, to be avoided or minimised. An “overspray”, i.e. printing being carried out beyond a free edge of the workpiece and ink mist being deposited on an adjacent surface of the workpiece, can also be effectively prevented. 
     However, even after a printing process (or before a subsequent printing process), the image detection sensor provided in accordance with the invention can be effectively used to analyse the workpiece surface. For example, the image sensor can be used to carry out an analysis of the workpiece surface with regard to possible defects, colour variations or the like in order to check and, for example, to maintain, to clean, to adjust, etc. the printing means on this basis. 
     These and further possible applications of the image detection sensor allow the quality of the printed image to be greatly increased over the prior art. The term “image detection sensor” refers in this case at least to any means which operates on an optical basis and supplies information about the image formed (which may or may not be visible to the human eye), so use may be made of a broad range of means including, for example, spectrophotometers with or without (RGB) filters. 
     In order to be able efficiently to implement the above-described and further possible applications of the image detection sensor to improve the quality of the printed image, a development of the invention provides for the device further to have a control means which is connected to the at least one image detection sensor and the ink-jet printing means. 
     Within the scope of the present invention, the image detection sensor can have a broad range of configurations. However, with regard to rapid and precise detection of the respective workpiece surface, a development of the invention provides for at least one image detection sensor to have a camera and/or a colour measuring device. A CCD camera has proven especially advantageous in this regard, as the digital data obtained can be forwarded particularly simply and rapidly to a control means or the like. 
     Within the scope of the present invention, the control means can also have a broad range of configurations. Nor is the possibility ruled out that the control means requires in some cases the involvement of an operator. However, with regard to the desired improved quality of the printed image, a development of the invention provides for the control means to be configured to analyse image data obtained by the image detection sensor, preferably at least with regard to the colour spectrum and/or defects and/or geometry and/or colour space (for example, RGB colour space) of each workpiece. 
     Specifically if the at least one image detection sensor is used to detect the colour spectrum, but also to detect defects, it is important that a reproducible printed image is present at the moment of detection. Against this background, a development of the invention provides for the device further to have a drying means, in particular a UV drying means. This allows a predetermined drying state of the applied ink to be produced before the respective image detection is carried out. 
     The above-described advantages of the device according to the invention can be achieved particularly advantageously by a method for patterning workpieces according to claim  7 . It is in this case particularly preferred that the actual image data obtained through the image detection is compared with desired image data, in particular with regard to the colour spectrum and/or defects and/or geometry and/or colour space (for example, RGB colour space). This desired/actual data comparison can advantageously be utilised as a basis for improving the print quality, for example through the measures described hereinbefore. 
     A development of the invention provides in this case for the control signals issued by the control means to the printing means to be corrected on the basis of the desired/actual data comparison in order to minimise deviation between the desired/actual data. The control means thus ensures, on the basis of the image detection performed by the image detection sensor, that the printed image which is actually produced approximates the desired printed image as closely as possible, i.e. optimum printed image quality is achieved. 
     In order to achieve this, the control means can carry out a broad range of measures. However, it has proven particularly effective that the control means corrects, in accordance with a development of the invention, the control signals issued to the printing means in such a way that the intensity and/or the course of the control pulses to at least one nozzle of the printing means are altered. Control pulses can also be dispensed with altogether, i.e. the control signals issued by the control means to the printing means can be corrected in such a way that one or more nozzles are switched on or off. 
     However, alternatively or additionally, the control means can also ascertain or decide that the determined deviation between the desired/actual data is caused by soiling of the print head, in particular the nozzles. In this case, a development of the method according to the invention provides for cleaning of the printing means to be carried out on the basis of the desired/actual data comparison in the event of a predetermined deviation between the desired/actual data. The control means can also decide, for example if corrective measures are unsuccessful, that basic maintenance of the device or even an emergency stop of the device is required. 
     It should also be noted that the image detection sensor can operate continuously or discontinuously and that it can operate during normal operation of the device or else during check operation of the device in which, for example, predetermined test patterns are printed and detected by the image detection sensor. Furthermore, the image detection sensor can be positioned at any desired point of the device, wherein it has proven advantageous for at least one image detection sensor to be provided on the printing means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a device for patterning workpieces according to an embodiment of the invention; 
         FIG. 2  is a schematic partial plan view of the device shown in  FIG. 1 ; 
         FIG. 3  is a schematic, partially cut-away front view of the device shown in  FIG. 1 ; 
         FIG. 4  shows further details of the printing means of the device shown in  FIG. 1 ; 
         FIG. 5  shows further details of the printing means of the device shown in  FIG. 1 ; and 
         FIG. 6  illustrates the operation of the device shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic perspective view of a device  1  for patterning workpieces  2  as a preferred embodiment of the present invention. The device is used for patterning and, if appropriate, machining workpieces  2  which, in the present embodiment, consist at least partially of wood, wood materials, plastics materials or the like, such as are frequently used in the field of furniture and kitchen design. 
     The device  1  comprises an ink-jet printing means  10  which, in the present embodiment, operates in accordance with the drop-on-demand principle. As may be seen most clearly in  FIG. 4 , the ink-jet printing means  10  comprises a plurality of nozzles  12  from which drops of ink can be expelled and which, in the present embodiment, are disposed in a plurality of rows, each row being provided for expelling a predetermined colour, for example the colours cyan, magenta, yellow and black. 
     The printing means  10  is in the present embodiment provided on or inserted into a spindle unit  6 , as may be seen most clearly in  FIG. 3 . The spindle unit is preferably a spindle unit which is also suitable for the insertable and exchangeable receiving of machining tools or machining installations and which, for this purpose, has a tool receptacle  6 ′ and an interface  6 ″, wherein the interface can, for example, be configured to transfer data, power, drive, fluid, etc. 
     In order to allow the printing means (printing unit)  10  to be inserted into the spindle unit, the printing unit  10  is provided in the present embodiment with a connecting piece  18  which can be inserted into the tool receptacle  6 ′ of the spindle unit (cf.  FIG. 3 ). Furthermore, the printing unit  10  has transfer means  16  (cf.  FIG. 3 ) which are able to communicate with the interface  6 ″ of the spindle unit. This allows, for example, data, power, drive, fluids, etc. and, in particular, also ink to be transferred. In addition, the printing unit  10  can have an ink reservoir and/or a wireless data transfer means, although this is not shown in the figures. 
     The spindle unit  6  is provided so as to be movable in the y direction on a portal  4  which can itself, in turn, be configured so as to be movable in the x direction. There are in this case provided in the present embodiment two portals  5 , each of which can carry one or more spindle units  6  which can optionally be disposed on opposing sides of the respective portal  4 . It should be noted in this regard that the portals  4  can also optionally be configured as jibs. 
     The spindle units  6  can be automatically or manually fitted, via tool magazines  32  respectively provided at the portals  4 , with machining tools and/or machining installations  30  and also one or more printing units  10  ( FIG. 3 ). In other words, the printing units  10  are configured in such a way that they can also be deposited in the magazines  32 . 
     Within the scope of the present embodiment, use may be made of a broad range of machining tools and/or machining installations  30  such as, for example, cutting tools (drills, milling cutters, etc.), edge-banding installations, extruding installations, coating installations, laminating installations, cleaning installations, degreasing installations, installations for improving the adhesive and wetting properties of the surfaces to be imprinted, and installations for reducing the electrostatic charging of the surfaces to be imprinted. Obviously, these tools and installations can also be stand-alone means (means independent of a spindle unit). 
     In the present embodiment, there extends below the portals  4  a workpiece table  20  for carrying the respective workpieces  2  to be patterned, which table is movable in the x direction shown in  FIG. 1 . The workpiece table  20  can have a broad range of configurations and, for example, also be formed by a circulating conveyor belt or the like. On account of its movability, the workpiece table  20  forms at the same time a workpiece carrier means and a portion of the conveyor device according to the present invention. 
       FIG. 2  is a detailed plan view of the disposal of a plate-like workpiece  2  on the workpiece table  20 . In the present embodiment, the workpiece table  20  has extensible stop pins  22  against which the workpiece  2  can be placed for rough positioning. Also disposed on the workpiece table  20  is a plurality of distance sensors  52  which are part of a rough detection means  50 . The distance sensors shown in  FIG. 2  are configured to detect the distance between the sensors and a lateral surface (narrow surface) of each workpiece  2 . In the present embodiment, the sensors  52  are in this regard rotatable about an axis extending orthogonally to the surface of the workpiece table  20  and are optionally movable parallel to the surface. The rough detection means  50  is thus used for roughly detecting the geometry and positioning of each workpiece  2 . 
     Further details of the workpiece table  20  are shown in  FIG. 3  which is a partially cut-away front view of the device shown in  FIG. 1 . It may be seen from  FIG. 3  that the respective workpiece  2  may be fixed on the workpiece table  20 , for example via vacuum suction means  24 . It is also possible to integrate appropriate suction means or suction openings into the workpiece table or a workpiece belt. 
     The device  1  according to the invention further comprises a detection means  40  for detecting the relative position of the ink-jet printing means  10  and the respective surface to be patterned of a workpiece  2 . In the present embodiment, the detection means  40  has a plurality of types of sensors  42 ,  46  which can be seen most clearly in  FIGS. 3 ,  4  and  5 . In the present embodiment, the detection means  40  comprises first of all three distance sensors  42  which are disposed on the printing means  10  adjacently to the nozzles  12  and measure in a direction substantially parallel to the direction in which ink is expelled from the nozzles  12  ( FIG. 4 ). On the one hand, these distance sensors can be used to determine the absolute distance between the printing means  10  and the workpiece  2 ; however, in addition, the precise contour of each workpiece  2  can also be inferred from the distance data obtained. 
     As may be seen most clearly in  FIGS. 3 and 5 , further distance sensors  42  are disposed on the printing means  10 , in each case via an element  44  which, in the present embodiment, is able to pivot. The pivotable element  44  allows each sensor to be brought into an extended position which can be seen most clearly in  FIG. 5 . In this position, the sensors  42 , shown in  FIG. 5 , measure in a direction substantially orthogonal to the direction in which ink is expelled from the nozzles  12 . This allows the thickness or height of each region to be imprinted to be detected and an overspray to be avoided. 
     In addition, in the present embodiment, there are disposed on the printing means  10 , adjacently to the ink expelling nozzles  12 , two image detection sensors  46  which also measure in a direction substantially parallel to the direction in which ink is expelled from the nozzles  12 . However, it should be noted in this regard that, alternatively or in addition to these image detection sensors, one or more image detection sensor(s) can also be—stationarily or movably—disposed at another suitable point of the device. 
     The image detection sensors  46  may, for example, be a CCD camera or the like which can produce a complete image of a region of the respective workpiece  2  that is to be imprinted or has already been imprinted. 
     Although not shown in the figures, all of the sensors, on the one hand, and the printing means and preferably also the remaining operating components of the device  1 , on the other hand, are connected to a control means which evaluates the respective data collected by the sensors and on this basis controls the operation of the device, in particular of the printing means. The control means is in this case configured to analyse the image data obtained by the image detection sensors, at least with regard to the colour spectrum, defects and geometry of the imprinted or unimprinted workpiece surface. 
     Also provided ( FIG. 3 ) on the printing means  10  are drying units  14 , for example UV driers, which are used promptly to dry the ink applied by the printing means in order to prevent possible distortion or smudging of the printed image. Although not shown in the figures, the drying units  10  can also advantageously be positioned between the nozzles  12  and the image detection sensors  46 , so ink applied to the respective workpiece  2  can be dried at least in part by the drying means  14  before image detection is performed by an image detection sensor  46 . 
     The device according to the invention may in this regard be operated as follows. First of all, a workpiece  2  is roughly positioned on the workpiece table  20  via the stop pins  22  and fixed via the vacuum suction means  24 . Subsequently, the positioning and/or contour of the workpiece  2  on the workpiece table  20  are detected by the sensors  52  and this data is forwarded to the control means. 
     The workpiece table  20  is then moved in the x direction, so the workpiece  2  can be machined or refined by tools, installations or printing units inserted into the spindle units  6 . In this regard, the printing means is, for example, operated as follows. 
     Based on the data from the sensors  52 , the printing means  10  is moved with the corresponding spindle  6  along the portal  4  to the workpiece  2  to be imprinted. In this regard, the sensors  42 ,  46  continuously perform a measuring operation, thus allowing the presence and, if appropriate, the distance of each workpiece and, in addition (by way of the leading image detection sensor  46 ), further information about the workpiece  2  to be obtained. 
     Based on this data, the control means issues print signals to the respective nozzles  12  (or the associated piezoelectric actuators or thermocouples), so the workpiece  2  is imprinted. Individual nozzles or groups of nozzles can in this regard be switched on or off as a function of the detection data of the sensors  42 ,  46  in order to compensate for dimensional, positional or other tolerances or deviations of the workpiece  2 . Alternatively or additionally, it is also possible, within the scope of the invention, for individual nozzles or a plurality of nozzles of the printing means  10  to be produced via piezo adjustment means or the like, in order to adapt the position or direction of expulsion thereof to the workpiece  2 . 
     When imprinting a large lateral surface of a workpiece  2 , there operate, in addition to the image detection sensor  46 , primarily the sensors  42  which are disposed next to the nozzles  12  and can be seen most clearly in  FIG. 4 . In order to imprint a narrow surface of the workpiece  2 , use is alternatively or additionally made of the sensors  42  which are extensible via pivotable elements  44  in order to detect the height of the narrow surface and thus to prevent an overspray. 
     Once a surface portion has been imprinted, it can optionally be dried by the drying units  14 , if necessary simultaneously to the printing process. 
     Furthermore, not only the leading image detection sensor  46  (the image detection sensor  46  located in front during the printing process in the direction of movement of the printing unit  10 ) but also the trailing image detection sensor carries out image detection during the printing process. This actual image data is forwarded to the control means and compared with actual image data (i.e. with the image data on which the printing process is based) with regard to the colour spectrum and possible defects of the printed image applied. If deviations are ascertained in this actual/desired data comparison, the control means can take various measures. In particular, the control means can correct the control signals (control pulses) issued to the printing means on the basis of the deviation, for example with regard to the waveform and/or amplitude of the control signals. Alternatively or additionally, the control means can cause the printing process to be interrupted at an appropriate moment and automatic or manual cleaning of the printing means to be carried out. In the event of large deviations between the desired/actual data, the printing means can also completely stop the operation of the device or issue error messages stating, for example, that the printing means requires maintenance, other printing inks have to be used, etc. 
       FIG. 6  illustrates schematically the paths of movement of the printing means  10  and/or the workpiece  2 . The left-hand drawing in  FIG. 6  shows an operation in what is known as transverse printing in which the printing means  10  moves back and forth in the y direction, together with the spindle unit  6 , along the portal  4 , and the workpiece table  20  further clocks the workpiece  2  in the x direction. 
     Alternatively, it is also possible to use the printing model which is shown on the right-hand side in  FIG. 6  and is referred to as longitudinal printing. In this model, the printing means  10  is itself substantially stationary during the printing process, and the workpiece  2  is moved back and forth in the x direction with the workpiece table  20 . The printing means  10  has therefore merely to be further clocked in the y direction once the printing of a web is completed. In addition, within the scope of the present invention, combinations of both operations are also possible, and webs disposed, for example, obliquely or the like can be printed.