Abstract:
An inkjet printhead position detection system includes a light emitting device emitting at least one beam of light having a first edge and a second edge nonparallel to the first edge. An inkjet printhead includes a light detecting device detecting the at least one beam of light as the inkjet printhead scans across the light. A time period between when the light detecting device crosses the first edge of the at least one light beam and when the light detecting device crosses the second edge of the at least one light beam is dependent upon a position of the inkjet printhead.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for detecting the position of a component in an inkjet printer, and, more particularly, to a method and apparatus for detecting the position of a printhead in an inkjet printer. 
     2. Description of the Related Art 
     The alignment of inkjet printheads in an inkjet printer is usually achieved by controlling the tolerances during the manufacture of the printhead and printer. As printheads increase in size and customer quality expectations increase, aligning inkjet printheads solely by controlling tolerances becomes increasingly expensive. 
     A normal ink jet printer uses a scanning carriage to move the printhead back and forth across the print medium. After each printing pass, the medium is moved forward and stopped for the next print pass. To achieve full color output, three or four primary color printheads are used, with a full range of color achieved by mixing the primary colors in various proportions. When a single, integrated printhead contains all of the primary colors (such as cyan, magenta and yellow), their relative alignment on the final output is very well controlled, since their relative placement is controlled by existing photolithography processes and numerically controlled laser devices. However, as the need for higher throughput increases, the number of nozzles increases, and the possibility of integrating the various colors in a monolithic device becomes unfeasible. The drops of ink from each primary color are then generated from separate devices, and their relative placement is controlled by automatic placement equipment that has much looser tolerances. Even when the separate devices are placed on a single printhead unit, the achievement of tolerances approaching a pixel between colors is very difficult. If each color is placed on a separate printhead, and they are joined on a carriage assembly, the relative placement accuracy is even worse. 
     In existing printers, test pages are sometimes generated which can be used to adjust the relative placement of each color. This solution becomes less effective in a business inkjet environment, when less user intervention is desired, and consistent operation is expected when various supplies, such as a printhead, are replaced. 
     What is needed in the art is a method of automatically detecting the positions of printheads in both a scan direction and a paper feed direction, as well as the skew angle of each printhead relative to the paper feed direction. Printed information can then be automatically corrected for position and skew as shown in related application Ser. No. 09/827,805 filed on Apr. 6, 2001. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sensor arrangement in an inkjet printer that produces direct feedback to correct for misalignment of printheads. 
     The invention comprises, in one form thereof, an inkjet printhead position detection system including a light emitting device emitting at least one beam of light having a first edge and a second edge nonparallel to the first edge. An inkjet printhead includes a light detecting device detecting the at least one beam of light as the inkjet printhead scans across the light. A time period between when the light detecting device crosses the first edge of the at least one light beam and when the light detecting device crosses the second edge of the at least one light beam is dependent upon a position of the inkjet printhead. 
     The invention comprises, in one form thereof, a method of detecting a position of an inkjet printhead, including emitting at least one light beam having a first edge and a second edge, the second edge being nonparallel to the first edge. A light detecting device is provided on the inkjet printhead. The inkjet printhead is scanned across the light. The first edge of the light beam is detected with the light detecting device during the scanning step. The second edge of the light beam is detected with the light detecting device during the scanning step. The position of the inkjet printhead is calculated dependent upon a time period between the detecting steps. 
     An advantage of the present invention is that the positions of printheads can be sensed in both a scan direction and a paper feed direction. 
     Another advantage is that the detection of printhead position can be performed automatically, i.e., without the need for human intervention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic, front view of one embodiment of a printhead position detection system of the present invention; 
     FIG. 2 is a schematic, side view of the optical encoder, optical encoder sensor and rear wall of the carriage of FIG. 1; 
     FIG. 3 is an enlarged, fragmentary, schematic side view of the printhead position detection system of FIG. 1; 
     FIG. 4 is an enlarged, top view of the apertures of the light source assembly of FIG. 3; 
     FIG. 5 is an enlarged, top view of the apertures of the light source assembly and the position sensor of FIG. 3; 
     FIG. 6 is an enlarged, bottom view of another embodiment of a position sensor assembly of a printhead position detection system of the present invention; 
     FIG. 7 is an enlarged, top view of the apertures of the light source assembly of FIG.  3  and the position sensor assembly of FIG. 6; 
     FIG. 8 is an enlarged, top view of another embodiment of an aperture of the light source assembly and the position sensor of FIG. 3; 
     FIG. 9 is an enlarged, top view of another embodiment of the apertures of the light source assembly of FIG.  3  and yet another embodiment of a position sensor assembly; 
     FIG. 10 is an enlarged, top view of yet another embodiment of the apertures of the light source assembly of FIG.  3  and the position sensor assembly of FIG. 9; 
     FIG. 11 is an enlarged, top view of a further embodiment of the apertures of the light source assembly of FIG.  3  and the position sensor assembly of FIG. 9; 
     FIG. 12 is a fragmentary, schematic side view of another embodiment of the printhead position detection system of the present invention; 
     FIG. 13 is an enlarged, top view of one embodiment of the position sensor assembly of FIG.  12  and illumination patterns on the bottom of the heater chip when the light source assembly of FIG. 12 is at a relatively low elevation; and 
     FIG. 14 is an enlarged, top view of the position sensor assembly of FIG.  13  and illumination patterns on the bottom of the heater chip when the light source assembly of FIG. 12 is at a relatively high elevation. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is shown one embodiment of a printhead position detection system  10  of the present invention. Detection system  10  includes a fixed position light source assembly  12 , a carriage  14  carrying four printheads  16 ,  18 ,  20 ,  22 , a drive belt  24 , an optical encoder  26 , a guide rod  28  and a microcontroller  29 . 
     Carriage  14  is driven along guide rod  28  by drive belt  24 . The lateral position of carriage  14  along guide rod  28  is derived from optical encoder strip  26  and an optical encoder sensor  30  (FIG. 2) which is attached to a wall  32  of carriage  14 . Guide rod  28  is oriented parallel to a scan direction of carriage  14 , which is indicated by double arrow  33  in FIG.  1 . 
     Each of printheads  16 ,  18 ,  20  and  22  includes a respective light sensing position sensor  34 ,  36 ,  38 ,  40 . The outputs of position sensors  34 ,  36 ,  38 ,  40  are used by microcontroller  29  to determine the relative location of each printhead  16 ,  18 ,  20 ,  22  in two directions, i.e., the carriage scan direction  33  and a paper feed direction indicated by double arrow  42  in FIG.  2 . Scan direction  33  is perpendicular to paper feed direction  42 . 
     The structure of each of printheads  16 ,  18 ,  20 ,  22  and their associated position sensors  34 ,  36 ,  38 ,  40  is substantially identical. Thus, the structure of only printhead  16  and its position sensor  34  will be described in further detail herein. FIG. 3 is a more detailed view of printhead  16  as it is positioned above fixed position light source assembly  12 . Printhead  16  includes a body  44 , a heater chip  46  having position sensor  34  embedded therein, and a nozzle plate  48 . 
     Light source assembly  12  includes a light source  50  disposed in a housing  52  having an aperture plane  54 . Apertures  56  and  58  (FIG. 4) in aperture plane  54  are disposed above light source  50 , which may include a lens, and are used to create a well-defined light image with distinct edges on the bottom of nozzle plate  48 . Apertures  56  and  58  produce respective light beams having edges corresponding to the four sides of each of apertures  56 ,  58 . Apertures  56  and  58  can be created in aperture plane  54  by use of a photomask (not shown), similar to that used in photolithography. Other methods could be used, such as cutting or etching though a metal mask. 
     Apertures  56 ,  58  are in the form of two strips or slits with a relative angle A 1 . The direction of movement M of carriage  14  is perpendicular to a line B 1 , which bisects angle A 1 . A slit length V determines the detection range in paper feed direction  42  of position sensor  34 . 
     FIG. 5 shows position sensor  34  and apertures  56 ,  58  together. Sensor  34  follows path M across apertures  56 ,  58 , generating output transitions at times T 1 , T 2 , T 3  and T 4  when sensor  34  crosses an edge of one of the light beams. A width W 3  of sensor  34  is much less than a slit width W 1  of aperture  56 , so the sensor output signal changes relatively quickly between its minimum and maximum at each of times T 1 , T 2 , T 3  and T 4 . The lateral position of printhead  16  in scan direction  33  is determined from the average of times T 1  and T 4 . Times T 2  and T 3  could be included in the calculation for more accuracy or noise suppression. The position of printhead  16  in paper feed direction  42  is determined from the difference between times T 1  and T 4 . Again times T 2  and T 3  could be included in the calculation. R 1  and R 2  show the detection range in paper feed direction  42 . Reversing carriage direction and collecting four more transition times may further improve accuracy, especially if the sensor has any hysteresis. Gap G 1  is set greater than W 3  if times T 2  and T 3  are used. The sensor sensitivity in paper feed direction  42  (value of T 4  minus T 1 ) is controlled by angle A 1 . Small angles are to be avoided due to poor sensitivity, while angles above 120° cause slower signal transitions at times T 1  through T 4 , and thus reduce accuracy. Preferred angles A 1  are between 45° and 90°. 
     The intensity of light source  50  and the area of position sensor  34  must be great enough to achieve an adequate signal for amplification and comparison. Else, expensive electronics may be required to manipulate the signal at increased system cost. However, simply increasing the area of position sensor  34  causes slower signal transitions at times T 1  through T 4 , and can also reduce the position accuracy. 
     In another embodiment, a position sensor assembly  60  (FIG. 6) overcomes the problems discussed above. Sensor assembly  60  includes two elongate, nonparallel sensors  62  and  64 . Sensors  62  and  64  are shaped similarly to apertures  56 ,  58 , except that sensor assembly  60  is rotated 180° relative to apertures  56 ,  58 . Sensor  62  is the leading sensor, as it will generate an output first. A line B 2  bisects an angle A 2  defined between sensors  62 ,  64 . Sensors  62 ,  64  are arranged such that direction of carriage movement M is perpendicular to line B 2 . Angle A 2  is set equal to angle A 1  between apertures  56 ,  58  for best position sensing operation. 
     Both sensor assembly  60  and apertures  56 ,  58  are shown in FIG.  7 . As sensors  62 ,  64  travel along path M, the two sensors generate several output signals. Of most importance are the times when sensor  62  crosses aperture  56  (S 1 T 1  and S 1 T 2 ) and the times when sensor  64  crosses aperture  58  (S 2 T 3  and S 2 T 4 ). As in the previous embodiment, the lateral position of printhead  16  in scan direction  33  is calculated from the average of times S 1 T 1  and S 2 T 4 . S 1 T 2  and S 2 T 3  could be included in the calculation if needed. The position of printhead  16  in paper feed direction  42  is calculated from the difference between S 1 T 1  and S 2 T 4 . 
     R 3  and R 4  show the detection range in paper feed direction  42 . Gap G 1  is set greater than W 2  (FIG. 6) if times S 1 T 2  and S 2 T 3  are used. In addition, gap G 2  must be set greater than W 1  so that the output of sensor  64  does not interfere with the output of sensor  62 . In order to minimize overall sensor size, and thus minimize system cost, it is possible to use only times S 1 T 1  and S 2 T 4  in the calculations. Sensors  62 ,  64  may be electrically connected with little change in the overall operation. The same observations regarding angle selection apply as with a single position sensing element. 
     Additional information about each printhead can be gained by putting two sensors or sensor pairs on each printhead. The sensors can be placed along path M so that they can share the fixed light source, or an additional light source could be added to the system. Because each sensor determines the location in paper feed direction  42  and the location in scan direction  33  of the printhead, the two sensors can be used to find printhead rotation information, which is significant in determining print quality. Any rotation error causes image misplacement at each swath boundary. The location and rotation information could be used by the printer firmware to correct the image before printing, or it could be used for manual adjustment after each printhead is installed. 
     Any configuration in which the sensor crosses two aperture edges that are not parallel could be used to determine printhead locations in both paper feed direction  42  and scan direction  33 . The arrangement of the present invention provides the optimum sensor sensitivity, range, length and area. 
     With the system of the present invention, the positions of each of printheads  16 ,  18 ,  20  and  22  can be determined. Based on the relative positions of printheads  16 ,  18 ,  20  and  22 , the print data for each color plane can be adjusted so that final color images are aligned properly on the print medium. 
     In yet another embodiment (FIG.  8 ), the aperture plane includes a single rectangular aperture  66  which is aligned parallel to paper feed direction  42 . Thus, sensor  34  detects the position of the printhead only in scan direction  33 . Sensor  34  could be extended in paper feed direction  42  in order to increase sensitivity without losing resolution. 
     In a further embodiment (FIG.  9 ), two position sensors  34  and light sources  66  are displaced in the paper feed direction  42 . The two position sensors  34  are part of the same heater chip  46 . A difference in the times at which sensors  34  sense the light beam from apertures  66  can be used to calculate the skew of the printhead. Print data to the printhead can then be adjusted accordingly in order to correct for the effects of the skew. Thus, the visible misalignment between the swaths of a skewed printhead can be eliminated. 
     In a still further embodiment (FIG.  10 ), a first pair of apertures  56 ,  58  is displaced in paper feed direction  42  from a second pair of apertures  56 ,  58 . A difference in the times at which sensors  34  sense the light beam from apertures  56  or from apertures  58  can be used to calculate the skew of the printhead. The times at which a sensor  34  detects light from an associated pair of apertures  56 ,  58  can be used to calculate the position of the printhead in both scan direction  33  and paper feed direction  42 . 
     In another embodiment (FIG.  11 ), V-shaped apertures  56 ,  58  are used in conjunction with aperture  66 . The light from aperture  66  is sensed by a first position sensor  34 , while the light from apertures  56 ,  58  is sensed by a second sensor  34 , with the second sensor  34  being displaced from the first sensor  34  in paper feed direction  42 . Thus, the signal from the second sensor  34  is used to calculate the position of the printhead in directions  33  and  42 , while the signals from both the first and second sensors  34  is used to calculate the skew of the printhead. 
     In the embodiments shown above, a light source and sensor arrangement are used to determine the position of a printhead in two horizontal directions, i.e., scan direction  33  and paper feed direction  42 . Print defects can also result from variations in printhead elevation due to the change in flight time from the printhead to the print medium. In another embodiment (FIG.  12 ), position sensors  34  are used to determine the elevation of printhead  16  above a light source assembly  68 , and thus the elevation of printhead  16  above the print medium. 
     Light source assembly  68  includes two light sources  70  disposed in a housing  72  having an aperture plane  74 . In aperture plane  74 , a first pair of apertures  56 ,  58  is displaced in paper feed direction  42  from a second pair of apertures  56 ,  58 , as shown in FIG.  10 . Light paths  76 , which each define the substantially planar edges of the beams of light, each form an angle A 3  with an elevation direction  78 . Because the two light sources are not directly below position sensors  34 , but are offset by angle A 3 , the illumination pattern on the bottom of heater chip  46  changes with the elevation above light source assembly  68 . FIG. 13 shows the positions of representative illumination patterns  56 A,  56 B,  58 A,  58 B on the bottom of heater chip  46  when printhead  16  is at a relatively low elevation. In contrast, FIG. 14 shows the positions of representative illuminations patterns  56 A,  56 B,  58 A,  58 B on the bottom of heater chip  46  when printhead  16  is at a relatively high elevation. It can be seen that illumination patterns  56 A,  58 A are slightly closer to illumination patterns  56 B,  58 B when printhead  16  is at a relatively low elevation, as shown in FIG.  13 . 
     At the printhead elevation of FIG. 13, sensor data is produced from patterns  56 A,  58 A at times S 10 T 1 , S 10 T 2 , S 10 T 3  and S 10 T 4 . Further, sensor data is produced from patterns  56 B,  58 B at times S 11 T 1 , S 11 T 2 , S 11 T 3  and S 11 T 4 . The calculation of printhead position in scan direction  33  and paper feed direction  42  is substantially the same as in previous embodiments. However, the calculation of printhead position in paper feed direction  42  must take into account that the distance between the outermost edges of the illumination patterns, as indicated by times S 10 T 1  and S 10 T 4 , for example, vary not only with printhead position in paper feed direction  42 , but also with the elevation of printhead  16 . For example, the lower the elevation of printhead  16 , the longer the difference value V 1  between times S 10 T 1  and S 10 T 4 , and the shorter the difference value V 2  between times S 11 T 1  and S 11 T 4 . The average of the difference values [(V 1 +V 2 )/2] is constant and does not vary with the elevation of printhead  16 . Thus, the position of printhead  16  in paper feed direction  42  is calculated by averaging the difference V 1  between times S 10 T 1  and S 10 T 4  and the difference V 2  between times S 11 T 1  and S 11 T 4 . 
     The elevation of printhead  16  above light sources  70  varies linearly with the difference between the two time periods (V 2 −V 1 ). The rate of the linear change is dependent upon offset angle A 3 . 
     The timing difference (V 2 −V 1 ) is negative when printhead  16  is at a relatively low elevation, as in FIG.  13 . In the case where printhead  16  is at a relatively high elevation (FIG.  14 ), the difference between the two time periods (V 2 −V 1 ) is positive. 
     Because the drop velocity is much greater than the carriage velocity, for example, more than ten times as great, the image placement error is less sensitive to elevation than it is to errors in the other two dimensions. For that reason, a shallow angle, such as between 2° and 20° is preferred. 
     Although two light sources  70  are shown herein, it is also possible to use a single light source that is centrally located between the two sensors  34 . In this case, the light source assembly would need to be larger to achieve a shallow angle A 3 . A single light source could also be used with various reflective surfaces to achieve the same effect. Light pipes may also be used to direct the light. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.