Abstract:
An image forming apparatus includes a fiducial reference point sensor that individually monitors the position of first and second fiducial reference points on a moving print device that are in spaced relation to one another.

Description:
FIELD OF THE INVENTION  
         [0001]    The present inventions are related to an image forming apparatus and, more specifically, to an image forming apparatus having a position monitor.  
         BACKGROUND  
         [0002]    Image forming apparatus are used to form text and graphic images on a variety of print media including, but not limited to, paper, card stock, mylar and transparency stock. Certain image forming apparatus include a print device that consists of a scanning carriage and one or more printing elements. During an image forming operation, the scanning carriage will traverse back and forth over the surface of the print media along the scan axis. As the scanning carriage traverses back and forth, a controller causes the printing element(s) to print at positions intended to result in portions of the desired image. The print media is periodically advanced along the media axis, which is transverse to that of the movement scanning carriage, so that the image may be completed.  
           [0003]    One example of an image forming apparatus with this type of print device is an ink jet printer. Here, one or more ink jet pens are carried by the scanning carriage. The pens often include a printhead with a plurality of ink ejecting nozzles arranged in a two-dimensional array of rows and columns that print individual ink spots (or “drops”) as the carriage scans across the media. A 600 dpi (dots-per-inch) printhead with a ½ inch swath will, for example, typically have two columns with 150 nozzles in each column. Ink drops are fired through the nozzles by an ink ejection mechanism, such as a piezo-electric or thermal ejection mechanism, to create the desired dot pattern (or “image”).  
           [0004]    The ability to accurately track the position of the printing elements as the scanning carriage moves along the scan axis is typically important, regardless of the type of printing element that is carried by the carriage, because position data is used to more accurately control the printing process and reduce dot placement and other printing errors. A linear encoder strip and sensor arrangement are frequently used for this purpose. The encoder strip, which includes a series of graduations, is mounted in parallel with the scan axis and the sensor, such as a light source and detector, is carried by the carriage in close proximity to the encoder strip. Position information from the encoder strip and sensor arrangement is used to control actuation of the printing element and, in the case of an ink jet printer pen, the firing of individual nozzles on the pens. Position information may also be used to control carriage movement.  
           [0005]    The accuracy of a conventional encoder strip and senor arrangement decreases as the distance between the sensor and the printing element increases because the relative positions of the printing elements and sensor do not remain constant during a printing operation. This is due to the fact that there is typically some “slop” in the bearings that support the scanning carriage and some flexure of the carriage as it moves along the scan axis. In a multi-printing element image forming apparatus, such as an ink jet printer with a plurality of pens, the distance between some of the printing elements and the sensor can be relatively large, which adversely effects the positional accuracy of those printing elements by increasing the likelihood of dot placement errors. The same problems may be encountered when relatively tall printing elements (i.e. elongated in the media axis) that print relatively tall swaths are used. Here, the distance between the sensor and certain portions of the relatively tall printing element may be large enough to result in erroneous position data for those portions and dot placement or, possibly, other printing errors.  
         SUMMARY  
         [0006]    An image forming apparatus includes a fiducial reference point sensor that individually monitors the position of first and second fiducial reference points on a moving print device that are in spaced relation to one another. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Detailed description of preferred embodiments of the inventions will be made with reference to the accompanying drawings.  
         [0008]    [0008]FIG. 1 is a perspective view of an image forming apparatus in accordance with a preferred embodiment of a present invention.  
         [0009]    [0009]FIG. 2 is a schematic block diagram of the image forming apparatus illustrated in FIG. 1.  
         [0010]    [0010]FIG. 3 is a perspective view of a print device in accordance with one embodiment of a present invention.  
         [0011]    [0011]FIG. 4 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
         [0012]    [0012]FIG. 5 is a perspective view of a print device in accordance with one embodiment of a present invention.  
         [0013]    [0013]FIG. 6 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
         [0014]    [0014]FIG. 7 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
         [0015]    [0015]FIG. 8 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
         [0016]    [0016]FIG. 9 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
         [0017]    [0017]FIG. 10 is a schematic block diagram of a print device and sensor system in accordance with a preferred embodiment of a present invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]    The following is a detailed description of the best presently known modes of carrying out the inventions. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the inventions. Additionally, it is noted that detailed discussions of various internal operating components of image forming apparatus which are not pertinent to the present inventions, such as specific details of the image processing system, print control system, and interaction with a host computer, have been omitted for the sake of simplicity.  
         [0019]    Although the present inventions are not limited to any particular image forming apparatus, the exemplary embodiments are described in the context of large format ink jet printers. The inventors herein have determined that one example of a conventional large format printer which could be reconfigured in such a manner that it would embody, incorporate or perform the present inventions is one of the Hewlett Packard DesignJet 2500 Series printers. Impact printers are another example of image forming apparatus to which the present inventions may be applied.  
         [0020]    As illustrated for example in FIGS. 1 and 2, an image forming apparatus  100  in accordance with one embodiment of a present invention includes a housing  102  and a movable print device  104 . The position of the print device  104  is monitored by a sensor system  106  which preferably includes a device having indicia that can be sensed, such as an encoder strip  108  with visible graduations, and at least two sensors  110   a  and  110   b . The sensor system  106  is discussed in greater detail below. The exemplary housing  102  is provided with end portions  112  and  114 , a window  116 , a cover  118  that covers a print media roll (not shown), a receiving bin  120  and a shelf  122 . The housing end portion  112  preferably encloses a scanning motor  124  that drives print device  104  back and forth over the print media  126  and a plurality of pen refill stations (not shown). The print media  126  is pulled though a slot  128  and carried by a roller  130  that is driven by a motor  132  in conventional fashion. The motor  132  and a printing element cleaning station (not shown) are located within the housing end portion  114 . A control panel  134 , including a display  136  and control buttons  138 , is preferably supported on the exterior of the housing end portion  114 .  
         [0021]    The print device  104 , sensor system  106 , motors  124  and  132 , and control panel  134  are connected to a printer controller  140  in conventional fashion in the exemplary embodiment. Suitable printer controllers include, for example, microprocessor based controllers. A clock  141  provides time information to the controller  140  which, when combined with position information from the sensor system  106 , may be used to calculate the velocity and acceleration of the print device  104 , which may in turn be used by the controller as it controls the operation of the print device. Generally speaking, the printer controller  140  receives image data from, for example, an application program, position data from the sensor system  106  and time information from the clock  141  as it controls the operation of the print device  104  and motors  124  and  132  to produce an image that corresponds to the image data. Additional aspects of the operation of the exemplary printer controller  140  are discussed in greater detail below.  
         [0022]    Referring to FIG. 3, the print device  104  in the exemplary image forming apparatus  100  includes a plurality of printing elements. Preferably, the print device  104  is provided with a plurality of ink jet pens  142  (sometimes referred to as “printhead cartridges,” “pen cartridges” and “print cartridges”) that are carried by a scanning carriage  144  in a formation referred to herein as a “bank.” The pens  142  may, for example, be of the readily removable type that include a self-contained ink reservoir, the type that carry a small amount of ink and are refilled by tubes that connect the pens to a remote ink reservoir (in what is sometimes referred to as an “off-axis” system), or the type that are periodically moved to the remote ink reservoirs where they are filled (in what is sometimes referred to as a “take a gulp” system). A suitable pen for use in the exemplary embodiment is the Hewlett Packard Model No. C1806A pen for large format printers such as the aforementioned Hewlett Packard DesignJet 2500 Series printers. Such pens include nozzle plates  143  (FIG. 5) with two columns of  124  nozzles (248 total nozzles).  
         [0023]    Although the number of pens  142 , the number of pen banks, and the arrangement of the pens within the bank(s) may vary to suit particular applications, the exemplary embodiment illustrated in FIGS.  1 - 4  includes eight pens in a single bank. The number of pens  142  in a single bank can, however, vary from one to twelve, or even more if applications so require. The banks may be arranged such that each pen is aligned with the other pens (as shown), or such that one or more of the pens in the bank is offset (or “staggered”) in the media axis from one or more of the other pens. Additionally, the pens  142  may be arranged such that the nozzle columns are either parallel to the media scan axis or diagonal to the media scan axis.  
         [0024]    The exemplary scanning carriage  144 , which reciprocatingly slides (or scans) on slide bearings back and forth along slider rods  146   a  and  146   b  (FIG. 3) to define the carriage scan axis, consists primarily of a main body  148  having a plurality of pen slots  149  that respectively receive the pens  142 . A pivotable latch  150  may be used to hold the pens  142  in place. A rear tray  152  carries electronic devices such as a pen interface printed circuit board. The electronic devices may also be mounted vertically or in other orientations. The scanning motor  124  is connected to the scanning carriage  144  in the exemplary embodiment by a drive belt  154  in conventional fashion. Other mechanisms for driving a scanning carriage, such as a motor and cable arrangement or linear motor, may be used if desired.  
         [0025]    As noted above, and as illustrated for example in FIGS.  2 - 4 , the exemplary image forming apparatus  100  includes a sensor system  106  that consists of a transparent linear encoder strip  108  and a pair of sensors  110   a  and  110   b . More specifically, the graduations are sensed as the scanning carriage  144  moves to determine the position of the scanning carriage on the scan axis. A suitable sensor is a conventional light source and light sensor arrangement where light from the source is directed through the encoder strip and sensed by the sensor on the other side of the encoder strip. The position data, based on the number of graduations sensed as the scanning carriage  144  moves away from its home location, is used to determine the pen nozzle firing times (i.e. the times at which the nozzles eject ink) during each pass of the scanning carriage  144  over the print media  126 . Preferably, the sensors  110   a  and  110   b  are located at the longitudinal ends of the scanning carriage  144  within respective sensor housings  156  (only one visible) and as close to the adjacent pens  142  as practicable. In one embodiment, the data from sensor  110   a  is used to control the nozzle firing times of the four closest pens  142 , i.e. those identified with an “A” in FIG. 4, while the data from sensor  110   b  is used to control the nozzle firing times of the other four pens, i.e. those identified with a “B.” Position data from either one of the sensors  110   a  and  110   b  may be used in conventional fashion, with time information from the clock  141 , for carriage motion control purposes.  
         [0026]    In an alternate embodiment, data from the sensors  110   a  and  110   b  is combined and the controller  140  interpolates (and extrapolates, if necessary) positional data for locations between (or beyond) the sensors. Positional data for the location of each pen  142  is interpolated and used to individually control the firing the pens.  
         [0027]    Depending on the configuration of the scanning carriage employed and other manufacturing constraints, the sensors  110   a  and  110   b  may be relocated in order to further reduce the distance between the sensors and the associated pens  142  or other printing elements. For example, the sensors  110   a  and  110   b  may be moved to the dash line positions shown in FIG. 4. Additionally, the number of sensors  110   a  and/or  110   b  may also vary depending on the configuration of the associated scanning carriage, the size, number and type of pens (or other printing elements), and the desired level of printing accuracy as measured by, for example, dot placement error. Each pen could even have its own corresponding sensor if an application so required or, as described below with reference to FIG. 8, a single pen could have more than one sensor associated therewith.  
         [0028]    The present inventions are not limited to exemplary image forming apparatus illustrated in FIGS.  1 - 4 . Turning to FIGS. 5 and 6, a print device  158  in accordance with another preferred embodiment includes two banks of pen slots with nozzle plate openings that allow the nozzle plates  143  to face the print media. The print device  158  may be reciprocatingly driven back and forth over print media by a motor and belt arrangement in the manner described above. The pens  142  are supported on a scanning carriage  160  that, in the exemplary embodiment, includes a main body  162  with two banks of six pen slots and a pair of slide bearings  164   a  and  164   b  that allow the carriage to slide along a pair of rails (not shown). Two pen interface printed circuit boards  166   a  and  166   b,  i.e. one for each pen bank, are also provided.  
         [0029]    With respect to carriage and, therefore, pen position sensing, the scanning carriage  160  in the exemplary embodiment illustrated in FIGS. 5 and 6 is preferably employed in image forming apparatus including sensor systems having at least two encoder strips  108   a  and  108   b  and at least two sensors  110   a  and  110   b . To that end, the encoder strips  108   a  and  108   b  pass through a pair of sensor housings  168   a  and  168   b  that are positioned adjacent to the pen banks. The data from sensor  110   a  is used to control the nozzle firing times of the pens  142  identified with an “A” in FIG. 6 and the data from sensor  110   b  is used to control the nozzle firing times of the pens identified with a “B.” 
         [0030]    The sensors  110   a  and  110   b  are preferably positioned at the midpoint of each bank of pens  142  in order to minimize the distance between the sensors and the farthest pens therefrom. Alternatively, as illustrated for example in FIG. 7, a print device  158 ′ that is otherwise identical to print device  158  is provided with four sensors  110   a ,  110   b ,  110   c  and  110   d  in order to further increase dot placement accuracy. The data from sensor  110   a  is used to control the nozzle firing times of the pens  142  identified with an “A,” the data from sensor  110   b  is used to control the nozzle firing times of the pens identified with a “B,” the data from sensor  110   c  is used to control the nozzle firing times of the pens  142  identified with an “C,” and the data from sensor  110   d  is used to control the nozzle firing times of the pens identified with a “D.” Another alternative, if possible given the scanning carriage configuration and manufacturing constraints, is to position the sensors  110   a ,  110   b ,  110   c  and  110   d  in the positions shown in dash lines in FIG. 7.  
         [0031]    The present inventions are also applicable to image forming apparatus in which print devices capable of printing relatively tall swaths are employed. As illustrated for example in FIG. 8, an exemplary print device  170  may include one or more pens  172  or other printing elements on a carriage  174 . The pens  172  are relatively tall and print a relatively tall swath (i.e. typically greater than one inch). In order to decrease the distance between the sensor system and the individual nozzles of the relatively tall pens  172 , the exemplary print device  170  includes a sensor system consisting of at least two encoder strips  108   a  and  108   b  and at least two sensors  110   a  and  110   b . The encoder strips  108   a  and  108   b  pass through a pair of sensor housings similar to those discussed above with reference to FIG. 5 and are positioned adjacent to the mid-line of the pen bank. Here, however, the sensors  110   a  and  110   b  are associated with particular nozzles, as opposed to particular pens. More specifically, data from sensor  110   a  is used to control the firing times of the nozzles in the portions of the pens  172  identified with an “A” and data from sensor  110   b  is used to control the firing times of the nozzles in the portions of the pens identified with a “B.” 
         [0032]    In other implementations of the present inventions, the positions of two or more locations on a movable print device may be monitored using devices other than encoder-based sensor systems. Here, one or more sensor devices are provided within the image forming apparatus and one or more fiducial reference points on the print device facilitate the sensing of position at two different locations on the print device. The fiducial reference points may be additional devices (i.e. “cooperative elements”) mounted on the print device or readily identifiable portions of the print device itself such as shiny brackets.  
         [0033]    As illustrated for example in FIG. 9, an exemplary print device  176  may include one or more pens  142  or other printing elements on a carriage  178 . Movement of the print device  176  is sensed by a laser interferometer system. Here, the laser interferometer system includes a pair of light source and sensor devices  180   a  and  180   b  that are mounted within the associated printing apparatus, preferably at one end of the scan axis, and a pair of reflectors  182   a  and  182   b , preferably mirrors, that are carried in spaced relation on the carriage  178  and act as the fiducial reference points. The reflectors  182   a  and  182   b  may be located on the top, bottom or sides or the carriage  178 . Light beams, including all suitable electromagnetic energy both in and out of the visible spectrum, emitted by the source and sensor devices  180   a  and  180   b  are reflected by the reflectors  182   a  and  182   b  back to the source and sensor devices in the manner illustrated in FIG. 9 to individually determine how far the reflectors have moved from their respective original home locations. Data from sensor  180   a  is used to control the nozzle firing times of the pens  142  identified with an “A” and data from sensor  180   b  is used to control the nozzle firing times of the pens identified with a “B.” 
         [0034]    Additional source and sensor devices and reflectors may be provided as applications require. Moreover, the individual source and sensor devices  180   a  and  180   b  may be incorporated into a single device capable of providing and sensing more than one light beam and the individual spaced reflectors  182   a  and  182   b  may be incorporated into a single component capable of reflecting light from two different locations on the print device.  
         [0035]    The laser interferometer sensor system described above with reference to FIG. 9 may be incorporated into any of the print devices disclosed herein in place of, or in combination with, other sensor systems. For example, the print device  184  illustrated in FIG. 10 includes a carriage  186  that supports two banks of six pen  142 . Here too, a pair of light source and sensor devices  180   a  and  180   b  are mounted within the associated printing apparatus and a pair of reflectors  182   a  and  182   b  are carried in spaced relation on the carriage  186 . Data from sensor  180   a  is used to control the nozzle firing times of the pens  142  identified with an “A” and data from sensor  180   b  is used to control the nozzle firing times of the pens identified with a “B.” 
         [0036]    The present apparatus and methods provide a number of advantages over conventional apparatus and methods. For example, obtaining position data at more than one location on a movable print device reduces the distance between respective portions of the print device and the associated sensor, thereby increasing the accuracy of the print device and reducing the likelihood of dot placement or other errors. Obtaining position data at more than one location on a movable print device also allows print devices that are manufactured with lower tolerances, lower cost materials and/or simplified manufacturing processes to achieve the same dot placement accuracy as those manufactured with tighter tolerances, higher cost materials and/or more complicated manufacturing processes. Additionally, in the event that an individual position sensing subsystem fails, position data from one or more other position sensing subsystems can be used to continue operation, albeit at a reduced level of performance.  
         [0037]    Although the present inventions have been described in terms of the preferred embodiments above, numerous modifications and/or additions to the above-described preferred embodiments would be readily apparent to one skilled in the art.  
         [0038]    By way of example, but not limitation, relatively tall swaths may be formed using a print device that aligns two or more pens or other printing elements end to end instead of the relatively tall pen described above with reference to FIG. 8. The present inventions are also susceptible to use with a wide variety of sensors in addition to those described above and are not limited to encoder-based and laser interferometer systems. Other suitable sensor systems include photo-reflective encoder strip systems, magnetic encoder strip systems, triangulation sensor systems, magnetostrictive sensor systems, ultrasonic sensor systems, cable extension transducer systems, linear variable differential transformer systems, and digital camera systems. Additionally, sensors and/or fiducial reference points may be carried by some or all of the pens themselves, instead of being carried by the carriage.  
         [0039]    It is intended that the scope of the present inventions extend to all such modifications and/or additions.