Patent Publication Number: US-6222569-B1

Title: Laser thermal printer with dual direction imaging

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. 09/144,123, filed Aug. 31, 1998, by Roger Stanley Kerr and Robert W. Spurr titled “Linear Translation System Dithering For Improved Image Quality Of An Intended Image”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to image processors in general and in particular to a laser thermal printer having the capability of printing images in a forward direction and a reverse direction. 
     BACKGROUND OF THE INVENTION 
     Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that would be required to produce printing plates and set up a high-speed, high-volume, printing press to produce an example single of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy or meet the requirements of customers, resulting in a large loss of profits and ultimately higher cost to the final customer. 
     One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708 is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring colorant from a sheet of donor material to the thermal print media by applying a sufficient amount of thermal energy to the donor sheet material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a printhead, and vacuum imaging drum), and thermal print media and donor sheet material exit transports. 
     The operation of the image processing apparatus as described above comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of donor material (in roll form) is also metered out of the material supply assembly or carousel, measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum). 
     After the donor sheet material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the donor sheet material on the spinning vacuum imaging drum while it is rotated past the print head that will expose the thermal print media. The translation drive then traverses the print head and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media. 
     After the intended image has been written on the thermal print media, the donor sheet material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The donor sheet material is then transported out of the image processing apparatus by the donor sheet material exit transport. Additional donor sheet materials are sequentially superimposed with the thermal print media on the vacuum imaging drum, then imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport. 
     Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. In an image processing apparatus, as the imaging drum spins, the printhead moves along the vacuum imaging drum in a path that is parallel to the longitudinal axis of the vacuum imaging drum (referred to as the slow scan). The translation drive moves the printhead in the “slow scan” direction, from a home position (at the point where it begins writing the intended image using the data from the image processing apparatus) to the opposite end of the vacuum imaging drum. The combined movement of the printhead and the vacuum imaging drum rotation perpendicular to the motion of the printhead causes the resulting image to be written in a single, continuous helix about the vacuum imaging drum. However, with the present image processing apparatus, at the end of a writing cycle the printhead must be returned to the home position before writing the next color in order to assure, for example, color to color registration. Returning the printhead to the home position prior to unloading and loading of media and for the start of the next image scan adversely affects the throughput of the image processing apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide for an image processing apparatus that is capable of printing an image without a substantial loss of time or throughput when the printhead is returned to the home position at the end of a writing pass. 
     According to one embodiment of the present invention, an image processing apparatus for processing thermal print media comprises a vacuum imaging drum for holding thermal print media and colorant donor sheet material in registration; and a printhead, wherein a rotation of the vacuum imaging drum and lead screw can be reversed to allow the printhead to write in both a forward and a reverse linear direction. 
     According to another embodiment of the invention, the printhead is at an angle to a longitudinal axis of the vacuum imaging drum. In this embodiment, as the vacuum imaging drum is rotated in a reverse direction, channel delay signals are reversed when printing the intended image in a reversed direction. 
     The present invention permits the printhead to be positioned at the nearest home position at either end of the slow scan travel. This minimizes the time it takes to move the printhead to a home position to allow loading and unloading of the thermal print media and donor sheet material. In the case that the printhead is not required to be moved to a home position for loading and unloading of the thermal print media and donor sheet material, their would be no time required to move the printhead to a home position. 
     The present invention relates to an image processing apparatus which can write images in a forward direction and a reverse direction. The apparatus comprises a writing assembly; a translation assembly for moving the writing assembly; and a rotatable imaging member adapted to receive media thereon. The translation assembly moves the writing assembly in a forward linear direction and a reverse linear direction, such that a writing pass can be written on media on the imaging member in either of the forward linear direction or the reverse linear direction. 
     The present invention also relates to an image processing method which comprises the steps of loading media on a rotatable imaging member; and moving a writing assembly with respect to a surface of the imaging member in one of a first linear direction or a second linear direction which is opposite to the first linear direction, to provide for a writing pass on the media. 
     The present invention also relates to an image processing apparatus which comprises a writing assembly; a translation assembly for moving the writing assembly; a rotatable imaging member adapted to receive media thereon; and a control device operationally associated with the translation assembly and the imaging member to control a linear movement of the writing assembly and a rotation of the imaging member. The control device is adapted to cause a movement of the writing assembly in a forward linear direction and a rotation of the imaging member in a first direction, and being further adapted to cause a movement of the writing assembly in a reverse linear direction and a rotation of the imaging member in a second direction opposite to the first direction, such that at least one writing pass can be written on media on the imaging member in either of the forward linear direction or the reverse linear direction. 
     The present invention also relates to an image processing apparatus which comprises a writing assembly that is adapted to move in a forward linear direction and a reverse linear direction relative to a surface of a rotatable imaging drum, such that an image can be written on media on the imaging drum in either of the forward linear direction or the reverse linear direction based on a direction of rotation of the imaging drum and a linear direction of movement of the writing assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view in vertical cross section of an image processing apparatus according to the present invention; 
     FIG. 2 is a perspective view of a vacuum imaging drum, printhead and lead screw of the present invention; 
     FIG. 3 is a perspective view of a printing swath created by drum rotation and lead screw movement for printing an intended image in a forward direction; 
     FIG. 4 is a perspective view of a printing swath created by drum rotation and lead screw movement for printing an intended image in a reverse direction; 
     FIG. 5 shows a plan view of the imaging drum and the orientation of data in a forward direction according to the present invention; and 
     FIG. 6 shows a plan view of the imaging drum and the orientation of the data in a reverse direction according to the present invention; 
     FIG. 7 is a schematic illustration of a control system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 shows an image processing apparatus  10  according to the present invention. Image processing apparatus  10  includes an image processor housing  12  which provides a protective cover. A movable, hinged image processor door  14  is attached to the front portion of the image processor housing  12  permitting access to a lower sheet material tray  50   a  and an upper sheet material tray  50   b , that are positioned in an interior portion of the image processor housing  12  and support thermal print media  32 , thereon. Only one of the sheet material trays  50   a ,  50   b  will dispense thermal print media  32  out of its sheet material tray to create an intended image thereon; the alternate sheet material tray  50   a ,  50   b  either holds an alternative type of thermal print media  32  or functions as a back up sheet material tray. In this regard, the lower sheet material tray  50   a  includes a lower media lift cam  52   a  for lifting the lower sheet material tray  50   a  and ultimately the thermal print media  32 , upwardly toward a rotatable, lower media roller  54   a  and toward a second rotatable, upper media roller  54   b  which, when both are rotated, permits thermal print media  32  to be pulled upwardly towards a media guide  56 . Sheet material tray  50   b  includes an upper media lift cam  52   b  for lifting upper sheet material tray  50   b  and ultimately thermal print media  32  towards the upper media roller  54   b  which directs it towards media guide  56 . 
     The movable media guide  56  directs thermal print media  32  under a pair of media guide rollers  58  which engages thermal print media  32  for assisting upper media roller  54   b  in directing it onto a media staging tray  60 . Media guide  56  is attached and hinged to a lathe bed scanning frame  202  at one end, and is uninhibited at its other end for permitting multiple positioning of media guide  56 . Media guide  56  then rotates its uninhibited end downwardly, as illustrated in the position shown, and the direction of rotation of upper media roller  54   b  is reversed for moving thermal print media  32  resting on media staging tray  60  under the pair of media guide rollers  58 , upwardly through an entrance passageway  204  and around a rotatable vacuum imaging member such as a vacuum imaging drum  300 . 
     A roll  30  of donor material  34  is connected to a media carousel  100  in a lower portion of image processor housing  12 . Four rolls  30  are used, but only one is shown for clarity. Each roll  30  includes a donor material  34  of a different color, typically black, yellow, magenta and cyan. The colorant can be in the form of dyes, inks, pigments etc. These donor materials  34  are ultimately cut into donor sheet materials  36  and passed to vacuum imaging drum  300  for forming the medium from which colorants imbedded therein are passed to thermal print media  32  resting thereon. In this regard, a media drive mechanism  110  is attached to each roll  30  of donor material  34 , and includes three media drive rollers  112  through which donor material  34  of interest is metered upwardly into a media knife assembly  120 . After the donor material  34  reaches a predetermined position, media drive rollers  112  cease driving the donor material  34  and the two media knife blades  122  positioned at the bottom portion of the media knife assembly  120  cut the donor material  34  into donor sheet materials  36 . Lower media roller  54   a  and upper media roller  54   b  along with media guide  56  then pass the donor sheet material  36  onto media staging tray  60 , and ultimately to vacuum imaging drum  300  and in registration with thermal print media  32  using the same process as described above for passing thermal print media  32  onto vacuum imaging drum  300 . Donor sheet material  36  now rests atop the thermal print media  32  with a narrow gap between the two created by microbeads imbedded in the surface of the thermal print media  32 . 
     A laser assembly  400  includes a quantity of laser diodes  402  in its interior. Lasers  402  are connected via fiber optic cables  404  to a distribution block  406  and ultimately to a writing assembly which includes a printhead  500 . Printhead  500  directs thermal energy received from laser diodes  402  causing donor sheet material  36  to pass the desired color across the gap to the thermal print media  32 . As shown in FIG. 2, printhead  500  is attached to a lead screw  252  via a lead screw drive nut  254  and drive coupling (not shown) for permitting movement axially along a longitudinal axis of vacuum imaging drum  300  for transferring data to create an intended image onto thermal print media  32 . 
     For writing, vacuum imaging drum  300  rotates at a predetermined velocity for each color or material, and printhead  500  begins at one end of thermal print media  32  and traverses the entire length of thermal print media  32  for completing the transfer process for the particular donor sheet material  36  resting on the thermal print media  32 . After printhead  500  has completed the transfer process, for the particular donor sheet material  36  resting on thermal print media  32  the donor sheet material  36  is then removed from vacuum imaging drum  300  and transferred out the image processor housing  12  via a skive or ejection chute  16 . Donor sheet material  36  eventually comes to rest in a waste bin  18  for removal by the user. The above described process is then repeated for the other three or more rolls  30  of donor materials  34 . 
     After the required amount of color from all sheets of donor sheet materials  36  have been transferred and donor sheet materials  36  have been removed from the vacuum imaging drum  300 , thermal print media  32  is removed from vacuum imaging drum  300 . 
     FIG. 2 is a perspective view of a lathe bed scanning subsystem  200  of image processing apparatus  10 , including vacuum imaging drum  300 , printhead  500  and lead screw  252  assembled in a lathe bed scanning frame  202 . A translation assembly or system includes lead screw  252  and a drive motor  258  which drives lead screw  252 . Motor  258  can be a stepper motor or servo motor which operates in conjunction with lead screw  252 . However, the present invention is not limited to this arrangement. It is recognized that various translation systems such as a motor and belt arrangement where the motor can be a stepper or servo motor, or a linear motor assembly which can be a servo or stepper motor, can be utilized within the context of the present invention. Vacuum imaging drum  300  is mounted for rotation about an axis  301  and a motor  600  as shown in FIG. 7 rotates vacuum imaging drum  300 . Printhead  500  is movable with respect to vacuum imaging drum  300 , and is arranged to direct a beam of light to donor sheet material  36 . The beam of light from printhead  500  for each laser diode  402  (not shown in FIG. 2) is modulated individually by modulated electronic signals from image processing apparatus  10 , which are representative of the shape and color of the original image, so that the color on donor sheet material  36  is heated to cause volatilization only in those areas in which its presence is required on thermal print media  32  to reconstruct the shape and color of the original image. 
     Printhead  500  is mounted on a movable translation stage member  220  which, in turn, is supported for low friction slidable movement on translation bearing rods  206  and  208 . Translation bearing rods  206  and  208  are sufficiently rigid so that they do not sag or distort between their mounting points and are arranged as parallel as possible with axis  301  of vacuum imaging drum  300 , with the axis of printhead  500  being perpendicular to axis  301  of vacuum imaging drum  300 . The front translation bearing rod  208  locates translation stage member  220  in the vertical and the horizontal directions with respect to axis  301  of vacuum imaging drum  300 . The rear translation bearing rod  206  locates translation stage member  220  only with respect to rotation of translation stage member  220  about the front translation bearing rod  208 , so that there is no over-constraint condition of the translation stage member  220  which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the printhead  500  during the generation of an intended image. 
     During operation, motor  258  rotates lead screw  252  to cause a linear travel of printhead  500 . Printhead  500  travels in a path along imaging drum  300 , moved at a speed synchronous with drum rotation and proportional to the width of a writing swath  450 , as shown in FIGS. 3 and 4. The end and/or beginning of the path of travel of printhead  500  is represented by reference numerals  456 ,  458  which designate home positions for printhead  500 . The pattern that printhead  500  traces out along spinning vacuum imaging drum  300  is a helix. Writing swath  450  traced out on vacuum imaging drum  300  are shown separated for purposes of clarity, in actual operation, each writing swath  450  would be directly adjacent to the previous writing swath  450 , traced out on the surface of vacuum imaging drum  300 . Printhead  500  has a point at which it writes a first pixel  414 , as shown in FIG.  5  and FIG. 6, relative to the final image. First pixel  414  is a fixed distance from a drum index mark  454  of vacuum imaging drum  300  which can be preferably located within a non-writing area  455  of drum  300  (FIG. 7) or a writing area, depending on design considerations. This means that printhead  500  writes first pixel  414  at a fixed distance on the surface of vacuum imaging drum  300 , after vacuum imaging drum  300  has rotated past printhead  500 . 
     FIG. 7 schematically illustrates a control system in accordance with the present invention. As shown in FIG. 7, a control device  700  such as a processor (CPU) receives data with respect to a line or lines to be written. Control device  700  is operationally associated with motor  258  which drives lead screw  252 , as well as motor  600  which rotates drum  300 . 
     In one embodiment of the present invention, thermal print media  32  and a first donor sheet material  36  are loaded onto vacuum imaging drum  300 . Based on the data inputted to control device  700 , in this embodiment control device  700  controls motors  258  and  600  and thereby controls the drive of printhead  500  and the rotation of drum  300  as follows. With printhead  500  at home position  456 , vacuum imaging drum  300  is rotated in a forward writing direction and the translation system which includes motor  258  and lead screw  252  moves printhead  500  in the forward writing direction as shown in FIG.  5 . At the end of the first writing pass, motor  258  stops, donor sheet material  36  is replaced, vacuum imaging drum  300  is rotated in a reverse direction, and motor  258  and lead screw  252  are rotated in a reverse direction to move printhead  500  in the reverse direction, as shown in FIG. 6 for a subsequent writing pass. Motor  258  then stops and the process is repeated until the intended image is completed. 
     In another embodiment of the present invention, thermal print media  32  and a first donor sheet material  36  are loaded onto vacuum imaging drum  300 . Based on the data inputted to control device  700 , in this embodiment control device  700  controls motors  258  and  600  and thereby controls the drive of printhead  500  and the rotation of drum  300  as follows. With printhead  500  at home position  456 , vacuum imaging drum  300  is rotated in the forward writing direction and motor  258  and lead screw  252  move printhead  500  in the forward writing direction. At the end of the first writing pass, motor  258  and lead screw  252  move printhead  500  to second home position  458 , donor sheet material  36  is replaced, vacuum imaging drum  300  is rotated in a reverse direction, and the rotation of motor  258  and lead screw  252  are reversed to move printhead  500  in the reverse direction for a subsequent writing pass. Motor  258  then moves the printhead to home position  456  and the process is repeated until the intended image is completed. Sensors are positioned at each of the home positions  456 ,  458  to indicated the presence of printhead  500  and provide a signal indicative thereof to control device  700 . 
     In another embodiment of the present invention, thermal print media  32  and a first donor sheet material  36  are loaded onto vacuum imaging drum  300 . Based on data inputted to control device  700 , control device  700  controls motors  258  and  600  and thereby controls the drive of printhead  500  and the rotation of drum  300  as follows. With printhead  500  at home position  456 , vacuum imaging drum  300  is rotated in the forward writing direction and motor  258  and lead screw  252  move printhead  500  in the forward writing direction. At the end of the first writing pass, motor  258  and lead screw  252  move printhead  500  to home position  456  or second home position  458  whichever is the closest, donor sheet material  36  is replaced, vacuum imaging drum  300  is rotated in a forward or reverse direction, and motor  258  and lead screw  252  move printhead  500  in a forward or reverse direction for a subsequent writing pass. Motor  258  and lead screw  252  then move printhead  500  to second home position  456  or to second home position  458  whichever is the closest position and the process is repeated until the intended image is completed. The closest home position ( 456 ,  458 ) can be determined based on the count of motor  258  as the printhead is driven in a linear direction. That is, a value for this count is determined by control device  700  and used as a basis to determine the closest of the home positions ( 456 ,  458 ). 
     In another embodiment of the present invention, thermal print media  32  and a first donor sheet material  36  are loaded onto vacuum imaging drum  300 . Based on the data inputted to control device  700 , control device  700  controls motors  258  and  600  and thereby controls the drive of printhead  500  and the rotation of drum  300  as follows. With printhead  500  at home position  456 , vacuum imaging drum  300  is rotated in the forward writing direction and motor  258  and lead screw  252  move printhead  500  in the forward writing direction. At the end of the first writing pass, motor  258  and vacuum imaging drum  300  stop, vacuum imaging drum  300  is rotated in a reverse direction, motor  258  and lead screw  252  are reversed to move printhead  500  in a reverse direction to write a subsequent writing pass, motor  258  and lead screw  252  move printhead  500  to home position  456 , and the process is repeated until the intended image is completed. 
     In yet another embodiment of the invention printhead  500  is set at an angle in a known manner and channel delays are used to insure proper placement of pixels on a scan lie approximately parallel to the horizontal axis of vacuum imaging drum  300 . At a trailing end of each scan, printhead  500  stops writing by activating successively fewer pixels so that the net effect is a rectangular image area. Likewise, at a leading end printhead  500  starts writing by activating successively more pixels so that the net effect is a rectangular image area. Because vacuum imaging drum  300  is rotating, printhead  500  incorporates a set of channel delays so that the pixels line up correctly on the output image. In order to write to vacuum imaging drum  300  spinning in the reverse direction, these delays are reversed. Channel delay timing is executed by control device  700 . 
     In the embodiments of the present invention described above, during a reverse writing pass the image must be electronically inverted because the first pixel is now to last pixel and the last pixel is now the first pixel, and the top of page delay must be adjusted accordingly. In the embodiment where the printhead is at an angle, the above applies, and in addition the channel delays must be reversed. 
     The invention has been described with reference to the preferred embodiment thereof. However, it will be appreciated and understood that variations and modifications can be effected within the spirit and scope of the invention as described herein above and as defined in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, the invention is applicable to any drum. Also, the donor material may have dye, pigments, or other material which is transferred to the thermal print media. Thermal print media includes paper, films, plates, and other material capable of accepting or producing an image. Also, the printhead can be a laser thermal printhead, a resistive thermal printhead or an ink jet printhead.