Abstract:
A method and system for detecting the alignment of print heads relative to one another in an ink jet printer having a plurality of print heads arranged side-by-side in a first direction (Y) on a carriage, each print head having a conductive nozzle plate which defines an edge that extends in said first direction (Y), wherein a sensor is used for determining the position of a reference mark formed on the print head, the sensor being used to measure the capacitance between the nozzle plate, serving as the reference mark, and the sensor, said alignment detection including the steps of moving the carriage) relative to the sensor in said first direction (Y) into a first position (“b”) where said sensor) extends across the edge of one print head and a second position (“d”) where the sensor extends across the edge) of another print head, measuring the capacitances in said first and second positions, and comparing the measured capacitances, thereby determining the relative position of said print heads in a second direction (X) orthogonal to said first direction (Y).

Description:
[0001]     This application claims the priority benefit of European Patent Application No. 05109635.2 filed Oct. 17, 2005 which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a method of detecting the alignment of printheads relative to one another in an ink jet printer having a plurality of printheads arranged side-by-side in a first direction on a carriage, each print head having a conductive nozzle plate which defines an edge that extends in said first direction, wherein a sensor is used for determining the position of a reference mark formed on the print head. The present invention also relates to a printer for applying the present method.  
         [0003]     A printer, e.g., an ink jet printer, comprises one or more print heads each of which carries an array of print elements, e.g., nozzles, for printing pixels on a sheet of a print substrate that is moved along a predetermined path relative to the print head. Typically, the print heads are mounted on a carriage that is moved across the path of the substrate in a main scanning direction Y, whereas the substrate is advanced in a sub-scanning direction X. The print elements or nozzles typically form a linear array that extends in X-direction.  
         [0004]     In order to obtain a high quality of the printed image, the nozzles on the print heads must be positioned or positionable with high accuracy relative to the substrate. For example, in the typical set-up described above, the linear array of nozzles should form a specified angle with the main scanning direction Y, typically an angle of exactly 90°. Moreover, when a plurality of print heads are employed, for example, print heads for different colours, the print heads must be precisely aligned relative to one another in at least one direction, typically the sub-scanning direction X. In the other direction Y, it is at least required that the relative positions of the print heads and hence the nozzles are known with high accuracy, so that the individual print elements can be energised at correct timings in accordance with the image information.  
         [0005]     A conventional method of detecting the alignment of the print heads consist of printing a test pattern which may then be inspected visually.  
         [0006]     U.S. Pat. No. 6,371,591 B1 and U.S. Pat. No. 6,508,530 B1 disclose automatic detection methods of the type indicated above, utilising opto-electronic sensors.  
         [0007]     U.S. Pat. No. 4,941,405 discloses a capacitive measurement of the position of the carriage in the main scanning direction, for controlling the timings at which the print elements are filed.  
         [0008]     U.S. Pat. No. 6,498,685 discloses a capacitive system for detecting the two-dimensional alignment of a printhead (an optical exposure system) in a lithography system. Here, the individual printheads are aligned relative to the wafer which corresponds to the print substrate. The capacitive detection method relies upon a periodic pattern formed on the wafer.  
         [0009]     US-A-2003 0 0085 938 discloses a capacitive measurement system for measuring an alignment of two substrates which, together, form the printhead. This system is used only when the printhead is assembled.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an object of the present invention to provide a method and printing system which permits detecting the alignment of print heads with high accuracy and which requires only a simple and robust hardware.  
         [0011]     According to the present invention, a sensor is used to measure the capacitance between the nozzle plate, serving as the reference mark, and the sensor, and the alignment detection comprises the steps of moving the carriage relative to the sensor in a first direction into a first position where said sensor extends across said edge of one print head, and in a second position where said sensor extends across the edge of another print head; measuring the capacitances in said first and second positions, and comparing the measured capacitances, thereby determining the relative position of said print heads in a second direction orthogonal to said first direction.  
         [0012]     It has been found that a capacitive position measurement permits a remarkably high detection accuracy with measurement errors being only in the sub-micrometer range, as is required for modern high-resolution printers. The present invention provides the advantage that the reference mark on the print head, which is usually installed on a moving carriage, can simply be formed by the conductive nozzle plate which has very little risk of being damaged mechanically and can easily be machined with high precision so as to have the correct positional relationship relative to the print elements. The nozzle plate has a straight edge extending at right angles to the direction in which the position measurement is to be performed and has a well defined positional relationship to the print elements. The sensor may simply be formed by another conductive plate which forms a capacitor together with the nozzle plate on the print head. The capacitance of this capacitor is proportional to the amount of overlap between the two plates and thus depends upon the relative position of the sensor and the print head. The capacitance measurement can be performed with high accuracy, for example by means of an oscillator circuit, the frequency of which is determined by the capacitance, as is generally known in the art of capacitive sensors.  
         [0013]     The nozzle plate may have a rectangular cutout defining a straight edge, and the conductive plate of the sensor may have a rectangular shape and a size comparable to that of the cutout and may be arranged such that it overlaps with the cutout when the print head is in a predetermined position. Preferably, the sensor is stationary mounted in a predetermined position at a portion of the path of travel of the carriage where the carriage stops or moves with relatively low velocity, for example, in a cleaning section where the nozzle plates of the print heads are cleaned from time to time. Thus, sufficient time for performing a high precision capacitance measurements will be available.  
         [0014]     Some ink jet printers have two or more print heads for the same color, e.g., black, and these print heads are arranged such that the nozzles thereof are staggered or offset relative to one another in the sub-scanning direction X. Then, the reference mark preferably has two straight edges that are offset by the same amount as the nozzles and are moved past the sensor one after the other, so that both offset positions can be measured independently from one another. Thus, the offset between the two print heads can be checked in a simple and reliable way when both print heads are equipped with identical nozzle plates.  
         [0015]     When a conductive portion of the nozzle plate moves past the plate of the sensor, the capacitance will vary as a function of the position of the print head in the main scanning direction Y. The conductive plates may be arranged and configured such that the capacitance functions show at least one well defined peak, which permits detection of the position of the print head in the main scanning direction Y relative to the carriage, the position of which is monitored with an independent detection system. Such a detection system is required, anyway, for controlling the timings at which the print elements are energized. Thus, the invention provides a simple way of measuring the position of the print head in both the main scanning direction and the sub-scanning direction.  
         [0016]     The inclination of the row of print elements relative to the sub-scanning direction X may be also detected, for example, by employing two sets of reference marks and sensors near opposite ends of each print head.  
         [0017]     The reference marks and the sensor or sensors may permanently be integrated in the printer, whereas the electronic circuit needed for the capacitance measurements may either be integrated in the printer or may be formed by a separate unit that is electrically connected to the sensors and the conductive plates of the print heads only when the printer is being tested, e. g., in the manufacturing process or during maintenance or repair. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Preferred embodiments of the present invention will now be described in conjunction with the drawings, wherein:  
         [0019]      FIG. 1  is a schematic view of essential parts of a printer adapted to practice the method according to the invention;  
         [0020]      FIG. 2  is a capacitance position-diagram for the printer shown in  FIG. 1 ;  
         [0021]      FIG. 3  illustrates a printer according to a modified embodiment; and  
         [0022]      FIG. 4  is a capacitance position-diagram for the printer shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     As is shown in  FIG. 1 , an ink jet printer comprises a carriage  10  that is movable back and forth in a main scanning direction Y along a stationary frame  12 . In the example shown, the carriage  10  carries five print heads  14 ,  16 ,  18 ,  20  and  22 , among which the print heads  14  and  22  print with black ink, whereas the other print heads are provided for the colors, cyan, magenta and yellow. All five print heads have nozzle plates  24  which all have the same outer shape and each define a row of nozzles  26  serving as print elements. The rows of nozzles  26  extend in a sub-scanning direction X in which a sheet of a print substrate (not shown) is intermittently advanced relative to the frame  12  and the carriage. The print heads  14 ,  16 ,  18  and  20  are precisely aligned in the main scanning direction Y, so that their nozzles  26  have a well defined positional relationship in sub-scanning direction X, as is required for color printing.  
         [0024]     The print head  22  is slightly offset from the other print heads in the sub-scanning direction X, so that the nozzles  26  of the print heads  14  and  22  are staggered, which enables the printing of black images with higher resolution.  
         [0025]     The nozzle plates  24  are electrically conductive and serve to form reference marks for detecting or checking the alignment of the print heads. To this end, the nozzle plate  24  of each of the print heads  16 ,  18  and  20  has a rectangular cutout  28  defining an edge  30  that extends in the main scanning direction Y and is machined to have a well defined positional relationship to the nozzles  26  in sub-scanning direction X. The print heads  14  and  22  have similar cutouts  28  which, however have a stepped-shape so that they each define a first edge  32  and a second edge  34 . The offset between these edges corresponds exactly to the offset between the print heads  14  and  22  in X-direction.  
         [0026]     A sensor  36  is fixed relative to the frame  12  and has the form of an electrically conductive rectangular plate that is arranged in parallel with the nozzle plates  24  and forms a small clearance therewith when the carriage  10  and the print heads move past the sensor. Several possible positions of the sensor  36  relative to the nozzle plates  24  have been indicated in phantom lines in  FIG. 1 . The sensor  36  is arranged such that it extends over the edges  30 ,  32  and  34 , so that a part of the sensor plate may overlap with the cutouts  28 , whereas another part, which has been hatched in  FIG. 1 , overlaps with the nozzle plate  24  and forms a capacitance therewith. The capacitance of this capacitor is measured with a known measuring circuit  38  which, in the example shown, is adapted to be connected to the printer through a plug  40 , so that it will be electrically connected to the sensor  36  on the one hand and to each of the nozzle plates  24  on the other hand. Although this has not been shown in detail in the drawing, the circuit  38  is selectively connectable to each of the nozzle plates  24 . For example, the arrangement may be such that a specific nozzle plate  24  will be connected to the circuit  38  only in the condition where this nozzle plate is located in the vicinity of the sensor  36 .  
         [0027]     In practice, the frame  12  will be subdivided in an operating portion in which the carriage  10  moves back and forth when an image is being printed, and a cleaning station  42  which adjoins to one end of the operating portion and in which the sensor  36  is installed. Thus, the capacitance measurement can be performed when the carriage  10  stops in or is slowly moved through the cleaning station.  
         [0028]     As is generally known in the art of ink jet printers, a position measuring system (not shown) is provided for measuring and monitoring the position of the carriage  10  relative to the frame  12  in main scanning direction Y. The main function of this position measuring system is to determine the timing at which the nozzles  26  of the individual print heads have to be fired in order to form an image in accordance with the image information to be printed. In the present embodiment, the position measuring system also monitors the position of the carriage  10  when the same is in the cleaning station  42 , and the results of the capacitance measurements performed with the circuit  38  are recorded as a function of the position of the carriage and may be plotted or displayed in the form of a diagram as shown in  FIG. 2 .  
         [0029]     When the carriage  10  moves slowly to the left side in  FIG. 1 , the sensor  36  will first reach the position “a” relative to the carriage  10 , as has been indicated in  FIG. 1 . The same position has also been designated as “a” in the diagram in  FIG. 2 . In this position, the plate of the sensor  36  fully overlaps with a rim of the nozzle plate  24  surrounding the cutout  28 , so that the capacitance indicated by a curve  44  in  FIG. 2  reaches a maximum and will show a pronounced peak. Then, when the cutout  28  starts to overlap with the sensor  36 , the capacitance will decrease and will then remain on a stationary level while the edge  32  (position b) passes over the sensor. When the edge  34  is moved over the sensor, the capacitance will decrease again and will remain stationary for some time on a lower level, because the amount of overlap with the nozzle plate  24  has decreased (position c). When the carriage moves on, another peak in the capacitance curve will be found when the cutout  28  leaves the sensor, and the capacitance will drop to almost zero when the sensor is between the print heads  14  and  16 .  
         [0030]     A similar pattern is found when the print heads  16 ,  18  and  20  are moved past the sensor  36 . However, since the cutouts  28  of these print heads have only a single straight edge  30 , the capacitance will stay on a constant level while this edge (position d) is passed over the sensor. When the print heads  14  and  16  are aligned precisely, the edge  32  will be aligned exactly with the edge  30  of the print head  16 , and this can be checked by confirming that the capacitance curve  44  for the positions “b” and “d” is exactly on the same level “r”, as is shown in  FIG. 2 .  
         [0031]     When the print heads  18  and  20  are moved over the sensor  36 , the alignment of these print heads can be checked in the same way.  
         [0032]     When the print head  22  reaches the position “e” over the sensor  36 , the capacitance remains on a somewhat higher level, because the print head  22  is offset relative to the other print heads. However, when the position “f” is reached, this offset is compensated for by the offset between the edges  32  and  34  of the cutout  28  of the print head  22 , so that the capacitance curve  44  again reaches the level r, which confirms that the offset position of the print head  22  has been adjusted correctly.  
         [0033]     Thus, it can be confirmed in a simple measurement sequence that all the print heads are precisely adjusted in the sub-scanning direction X. Thanks to the stepped shape of the cutouts  28  of the nozzle plates of the print heads  14  and  22 , identical nozzle plates may be used for these print heads, and it does not matter when the mounting positions of the print heads  14  and  22  are interchanged.  
         [0034]     Of course, in a modified embodiment, the nozzle plates  24  of the print heads  16 ,  18  and  20  could have the same shape as those of the print heads  14  and  22 , i. e., with a stepped cutout, so that the nozzle plates could be manufactured more efficiently.  
         [0035]     A number of reference positions y 1 -y 10  have been shown along the Y-axis in  FIG. 2 . By checking whether the peaks of the capacitance curve  44  coincide with these reference positions, it can be confirmed that the print heads  14 - 22  are also adjusted correctly in the main scanning direction Y. If any deviations should be found, it is not necessary mechanically to correct the positions of the print heads, but it is sufficient to appropriately adapt the timings at which the nozzles  26  are fired. An accurate detection of the positions of the print heads in Y-direction is facilitated by the fact that the width of the plate-like sensor  36  is equal to the width of the rim of the nozzle plates  24  surrounding the cutouts  28 , so that sharp capacitance peaks are formed.  
         [0036]      FIGS. 3 and 4  illustrate a modified embodiment, in which the nozzle plates of each of the print heads  14 - 22  have cutouts  28  and  46  provided at both ends of the rows of nozzles. In addition to the sensor  36 , another similar sensor  48  is associated with the cutouts  46 .  
         [0037]     In the case of the print heads  14  and  22 , the cutouts  28  at one end of the nozzle row define straight edges  34 , but the height of these cutouts is larger than the height of the other cutouts  28  and  46  having the straight edges  30 ,  32 . The difference in height corresponds exactly to the amount of offset of the print head  22  relative to the other print heads. Thus, the edge  34  of print head  22  is aligned with the edges  30  of the cutouts  28  of the print heads  16 ,  18  and  20 , and for the cutouts  46  at the other end of the print heads, the edge  32  of the print head  14  is aligned with the edges  30  of the print heads  16 ,  18  and  20 .  
         [0038]      FIG. 4  shows, in continuous lines, a capacitance curve  50  that is recorded with the sensor  36 , and in broken, bold lines, another capacitance curve  52  that is recorded with the sensor  48 . In the position “g”, the capacitance curve  52  has the same level “r” as for the print heads  16 ,  18  and  20 , which shows that the print head  14  is correctly aligned with the print heads  16 - 20 . On the other hand, in the position “h” in  FIG. 4 , it is the capacitance curve  50  that has the level “r”, which shows that the print head  22  has the correct offset.  
         [0039]     Moreover, as has been shown exaggeratedly in  FIG. 3 , the print head  18  is somewhat tilted relative to the correct position. This type of misalignment is also indicated by the capacitance curves  50  and  51 , because the corresponding peaks of the capacitance curve  50  are shifted to the right, whereas the peaks of the capacitance curve  52  are shifted to the left, relative to their respective target positions. Thus, the embodiment according to  FIGS. 3 and 4  also permits the detection of a possible misalignment of the nozzle row of a print head relative to the sub-scanning direction.  
         [0040]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.