Patent Publication Number: US-2007110491-A1

Title: Method and apparatus adjusting the position of a printhead

Description:
The present invention relates to a method of printing on an image receiving medium, such as a tape, and to a thermal printer and printhead arrangement to be used in such a method. In particular, the present invention relates to adjusting the position of a printhead.  
      Thermal printers are known. The printhead for such printing apparatus generally comprises a plurality of printing elements which are selectively activated, that is heated. An image is generated in one of two ways. An intermediate ink ribbon can be provided so that when the activated printing elements of the printhead heat up the ink from the parts of the ink ribbon in contact with the heated printing elements is transferred to the image receiving tape. Alternatively, the heated printing elements may directly contact a thermally sensitive image receiving tape which causes an image to be formed thereon by thermal activation.  
      In order to print an image the image receiving tape is passed through a print zone defined by the printhead and a platen which supports the image receiving medium during printing. The gap between the printhead and the platen defining the print zone allows the image receiving tape to pass through the print zone, whilst allowing the printhead to contact the image receiving tape in order to print an image on the tape.  
      Image receiving tapes are made of various types of material, having various thicknesses that are best suited to their application. Image receiving tape of different types of material can require the printhead to exert different pressures on the tape in order to print an image on the tape. Furthermore, it is necessary for the printing device to allow image receiving tapes of different thicknesses to pass through the print zone whilst allowing the printhead to contact the image receiving tape.  
      EP 1066975 (Brady) discloses an arrangement whereby a pivotally mounted printhead exerts pressure on the platen to provide a variable platen pressure when the print head thermally transfers ink from an ink ribbon to a labelling media. However, when used with labelling media of various thicknesses, a pivotally mounted printhead will contact the labelling media at different angles, depending on the thickness of the media, causing a misalignment between the printing area of the printhead and the labelling media.  
      It is therefore an object of the present invention to provide a printer for printing on image receiving mediums of different materials and of different thicknesses, and to overcome the problems presented by the prior art.  
      According to a first aspect of the present invention there is provided a printhead assembly comprising: a printhead arranged to print on an image receiving means; a platen; a fixed support; a first frame slideably connected to said fixed support, one of said printhead and said platen being mounted on said first frame; and driving means for driving said first frame relative to said fixed support to cause the one of said printhead and platen to move in a linear direction toward the other.  
      According to a second aspect of the present invention there is provided a printhead assembly comprising: a printhead arranged to print on an image receiving means; a platen; a fixed support; a first frame slideably connected to said fixed support, one of said printhead and said platen being mounted on said first frame; and driving means for driving said first frame relative to said fixed support in accordance with information stored with said image receiving means, to cause the one of said printhead and platen to move in a linear direction toward the other.  
      According to a third aspect of the present invention there is provided a printer comprising: inputting means for inputting data; a printhead arranged to print on an image receiving means; a platen; a fixed support; a first frame slideably connected to said fixed support, one of said printhead and platen being mounted on said first frame; and driving means for driving said first frame relative to said fixed support to cause the one of said printhead to move in a linear direction toward the other.  
      According to a fourth aspect of the present invention there is provided a method of controlling a printhead assembly comprising: a printhead arranged to print on an image receiving means; a platen; a fixed support; and a first frame slideably connected to said fixed support, one of said printhead and said platen being mounted on said first frame; wherein said method comprises the steps of driving said first frame relative to said fixed support to a predetermined position.  
      According to a fifth aspect of the present invention there is provided a method of printing with a printer comprising: inputting means for inputting data; a printhead arranged to print on an image receiving means; a platen; a fixed support; a first frame slideably connected to said fixed support, one of said printhead and platen being mounted on said first frame; wherein said method comprises the steps of driving said first frame relative to said fixed support to a predetermined position; and  
      printing on said image receiving means in accordance with said inputted data. 
    
    
      For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:— 
       FIG. 1  is a schematic cross sectional view of a tape printing device embodying the present invention;  
       FIG. 2  is a schematic cross sectional view of a tape printing device in accordance with an alternative embodiment of the present invention.  
       FIG. 3  is a diagram of an adjustable printhead arrangement in accordance with an embodiment of the present invention.  
       FIG. 4  is a diagram of an adjustable printhead arrangement in accordance with an embodiment of the present invention.  
       FIG. 5  is a diagram of an adjustable printhead arrangement in accordance with an embodiment of the present invention.  
       FIG. 6  is a diagram of an adjustable printhead arrangement in accordance with an embodiment of the present invention.  
       FIG. 7  is a diagrammatic sketch showing control circuitry for the printing device of  FIG. 1  according to an embodiment of the present invention.  
       FIG. 8  is a diagram of an adjustable printhead arrangement in accordance with an alternative embodiment present invention.  
       FIG. 9  is a diagram of an adjustable printhead arrangement in accordance with an alternative embodiment of the present invention.  
       FIG. 10  is a diagram of a look up table in accordance with an embodiment of the present invention. 
    
    
       FIG. 1  shows in plan view a tape printing device  61  embodying the present invention which has a cassette  60  arranged therein. Typically this tape printing device  61  is a hand-held or small desktop device. The cassette  60  is located in a cassette bay  62  and contains a supply spool  64  of image receiving tape  63 . The cassette bay  62  also accommodates a thermal print head  9  and a platen  10  which cooperate to define a print zone  65 . The cassette  60  also has an ink ribbon supply spool  48  and an ink ribbon take up spool  50 . An ink ribbon  12  is guided from the ink ribbon supply spool  48  through the print zone  65  and taken up on the ink ribbon take up spool  50 . The image receiving tape  63  passes in overlap with the ink ribbon  12  through the print zone  65  with its image receiving layer in contact with the ink ribbon  12 . The print head  9  is connected to an adjustable printhead arrangement  20 , in accordance with an embodiment of the present invention, which will be discussed in more detail hereinafter. The adjustable printhead arrangement  20  is movable so that it can be brought into contact with the platen  10  for printing and moved away from the platen  10  to enable the cassette  60  to be removed and replaced. In the operative position, the platen  10  is rotated to cause the image receiving tape  63  to be driven past the print head  9  and the print head is controlled to print an image on the image receiving tape  63  by thermal transfer of ink from the ink ribbon  12 . The printhead  9  comprises a thermal print head having an array of printing elements, each of which can be thermally activated in accordance with the desired image to be printed.  
      The platen  10  is driven by a DC motor or stepper motor  67  (see  FIG. 7 ) so that it rotates to drive the image receiving tape  63  through the print zone  65  of the tape printing device  61  during printing. In this way, an image is printed on the tape and fed out from the print zone  65  to an outlet  66  of the tape printing device  61 .  
      The image is printed by the print head  9  on the image receiving tape  63  on a column by column basis with the columns being adjacent one another in the direction of movement of the tape  63 . Pixels are selectively activated in each column to construct an image in a manner well known in the art. The motor  67  may be provided with a shaft encoder for monitoring the speed of rotation of the motor. Sequential printing of the columns of pixels by the printhead  9  is controlled in dependence on the monitored speed of rotation of the motor  67 . The control of the speed of the motor  67  is achieved by the microprocessor chip  100  (see  FIG. 7 ) to generate data strobe signals each of which causes a column of pixel data to be printed by the print head  9 .  
      The tape printing device  61  may include at cutting location  68  a cutting mechanism  69  which carries a blade  70 . The blade  70  cuts the image receiving tape  63  then enters a slot  71  located in the cassette  60 .  
       FIG. 2  shows an alternative embodiment of the present invention.  FIG. 2  shows a printer having two cassettes arranged in a cassette receiving bay  62 ′ of a printing device. The upper cassette  72  contains a supply of the image receiving tape  63 ′ which passes through a print zone  65 ′ of the printer to an outlet  66 ′ of the printer. The image receiving tape  63 ′ comprises an upper layer for receiving a printed image on one of its surfaces. In one embodiment, image receiving tape  63 ′ has on its other surface an adhesive layer to which it is secured to a releasable backing layer. In such an embodiment a portion of the tape may be applied to a surface as an adhesive label. Alternatively the image receiving tape  63 ′ may not have an adhesive layer and portions of the tape may be applied to articles using plastic tags or the like. The cassette  72  has a recess  80  for accommodating a platen  10 ′ of the printer. The platen  10 ′ is mounted for rotation.  
      The lower cassette  74  contains an ink ribbon  12  which extends from a supply spool  76  to a take up spool  78  within the cassette  74 . The thermal transfer ribbon  76  extends through the print zone  65 ′ in overlap with the image receiving tape  63 ′. The cassette  74  has a recess  71  for receiving the adjustable printhead arrangement  20  of the printer. The print head  9  is movable between an operative position, in which it bears against the platen and holds the ink ribbon  12  and the image receiving tape  63 ′ in overlap between the print head  9  and the platen  10  and an inoperative position in which it is moved away from the platen to release the ink ribbon  12  and the image receiving tape  63 ′. In the operative position the platen  10  is rotated under the action of a motor  67 , shown in  FIG. 7 , in a manner as described in relation to  FIG. 1 . The print head is controlled to print an image onto the image receiving tape by thermal transfer of ink from the ribbon.  
      The ink ribbon  12  can be omitted in certain embodiments where the image receiving tape is of a thermally sensitive material. In this case, the image is printed by the thermal printhead directly onto the thermally sensitive image receiving tape.  
      The adjustable printhead arrangement  20 , in accordance with a first embodiment of the invention will now be described in relation to  FIG. 3 .  FIG. 3  shows an exploded view of the adjustable printhead arrangement  20 . The adjustable printhead arrangement comprises a frame  1  comprising two parallel supports  240  and  241  interconnected by an interconnecting section  22 . The internal walls of supports  240  and  241  of frame  1  have opposing recesses  242  and  243  in which two slotted support members  41  and  42  are formed. Support member  42  is shown as a dotted line visible through support  241 . Support member  241  further comprises a motor housing  29  having an opening on the external wall of the support member  241 , said housing extending toward the opposite support member  240 .  
      Interconnecting section  22  is located between supports  240  and  241 . Interconnecting section  22  comprises two cylindrical tubes  340  and  341  through which two springs  45  and  46  are located.  
      The adjustable printhead arrangement further comprises a printhead support  8  for supporting the printhead holder  19  and printhead  9 . The printhead support  8  comprises two parallel projections  260  and  261  provided at each end of a plate  35 . Plate  35  comprises two holes  320  and  321  through which the end of springs  45  and  46  are inserted. Printhead holder  19  is connected to the underside of plate  35  such that the ends of springs  45  and  46  are in contact with the printhead holder  19 . Printhead  9  is glued to the underside of printhead holder  19 . The two parallel projections  260  and  261  of printhead support  8  are slideably connected with slotted support members  41  and  42  of frame  1  such that plate  35  of the printhead support printhead holder  19  and printhead  9  may be moved toward and away from interconnecting section  22  of the frame  1 .  
      The adjustable printhead arrangement  20  further comprises print head actuation frame  7  comprising a plate  80  and two parallel receiving portions  91  and  92  disposed at either end of said plate  80  for receiving said two parallel projections  260  and  261  of the print head support. Two threaded spindles  48  and  49  extend through two holes in the top plate  80  of print head actuation frame  7 . Two nuts threaded onto each spindle connect each spindle to the top plate  80  such that, for each spindle, one nut is arranged above the top plate  80  and one nut is arranged below the top plate. This arrangement allows the printhead actuation frame to move along the axis of the spindle, by turning the spindle relative to the nut. Printhead actuation frame  7  is arranged to fit between the supports of frame  1  such that the parallel receiving portions  91  and  92  of printhead actuation frame  7  are slideably mounted within the two slotted support members  41  and  42  of frame  1 .  
      A printhead cover  3  is fixedly mounted on top of slotted support members  41  and  42  of frame  1  by adjustable pins  2 . The upper ends of spindles  48  and  49  project through two holes  280  and  281  in printhead cover  3 . Two circular toothed gears  51  and  52  are connected to the projecting ends of spindles  48  and  49  respectively, such that gears  51  and  52  are disposed over printhead cover  3 . In a preferred embodiment of the present invention toothed gears  51  and  52  are each 40 teeth gears. Drive gear  6  is arranged to rotate on spindle  90  which is positioned on the printhead cover  3  between gears  51  and  52  such that drive gear  6  interconnects with both gears  51  and  52 .  
      A motor  13  is arranged in the motor housing of support member  241 . Motor  13  is arranged to drive a worm gear  14  which interconnects with drive gear  6 .  
      Printhead actuation frame  7  further comprises two hollow cylindrical members  30  and  31  which extend below the top plate  80  of the printhead actuation frame  7 . The two cylindrical members  30  and  31  are arranged to receive the two springs  45  and  46  which extend from the printhead holder  19 , through holes  320  and  321  of plate  35  and through cylindrical tubes  340  and  341  of interconnecting section  22 .  
      The operation of the adjustable printhead arrangement in accordance with a preferred embodiment of the invention will now be discussed in relation to  FIG. 4 . When motor  13  is driven in a counter clockwise direction denoted by arrow A, the worm gear  14  rotates drive gear  6  such that gears  51  and  52  rotate in a clockwise direction. Rotating gears  51  and  52  in a clockwise direction rotates spindles  48  and  49  in a clockwise direction causes the nuts attached to the printhead actuation frame to move downwards along the axis of the spindles.  
      This causes the parallel receiving members  91  and  92  of printhead actuation frame  7  to move along the slotted support members  41  and  42  of interconnecting section  22 . Printhead actuation frame  7 , the printhead support frame  8 , the printhead holder  19  and the printhead  9  are therefore slideably driven downwards relative to frame  1 , toward platen  10 . The motor  13  is provided with a shaft encoder for monitoring the number of rotations of the motor. The control of the number of rotations of the motor  13  is achieved by the microprocessor chip  100  (see  FIG. 7 ) to drive the printhead actuation frame along a predetermined distance.  
      Reference will now be made to  FIG. 5 .  FIG. 5  shows the printhead  9  in contact with the image receiving tape  63  on the platen roller  10 .  
      The distance between the printhead  9  and the printhead cover  3  is denoted by distance Y. Distance Y increases as the print head is moved towards the platen. The maximum value of distance Y between the print head cover  3  and the printhead  9  will be when the printhead  9  contacts the image receiving tape and will therefore depend on the thickness of the image receiving tape.  
      Since the printhead cover is fixedly connected to the slotted support members  41  and  42 , the distance between the printhead cover  3  and the platen  10  is fixed. Therefore, the maximum value of distance Y will not exceed the fixed distance between the printhead cover  3  and the platen  10 . For image receiving tapes of negligible thickness the maximum value of Y will be equal to the fixed distance between the printhead cover  3  and the platen  10 .  
      At the point when the print head  9  contacts the image receiving tape, springs  45  and  46  are under a minimum amount of compression. Accordingly, the distance between printhead  9  and plate  80  of printhead actuation frame  7 , denoted by distance X, will be at a maximum. The distance travelled by the printhead actuation frame is denoted by reference Z.  
      As the motor continues to drive the printhead actuation frame  7  toward the platen  10 , the springs  45  and  46  are placed under increasing compression between the printhead, which is prevented by the platen from moving, and plate  80  of the printhead actuation frame  7 , which continues to move toward the platen. As the distance X between the printhead and plate  80  decreases, the pressure applied to the image receiving tape by the printhead increases. The pressure applied to the image receiving tape can therefore be said to be a function of X. Or, 
 
 P=f ( −X ) 
 
 where P is the pressure applied to the image receiving tape by the printhead. 
 
      Since: 
 
 Y=Z+X  
 
therefore, 
 
 P=f ( Z−Y ) 
 
      Since Y is dependent only on the thickness of the image receiving tape, for each printing operation on a particular thickness of tape, Y is fixed and the pressure applied to the image receiving tape varies as a function of Z. Therefore, the pressure applied to the image receiving tape can be directly calculated for known values of the position of the printhead actuation frame and the thickness of the image receiving tape.  
       FIG. 6  shows a diagram of the adjustable printhead arrangement with the printhead actuation frame  7  wound to the end of spindles  48  and  49 . Plate  35  of the printhead support frame  8  abuts the parallel receiving members  91  and  92  of the printhead actuation frame. In this position the distance travelled by the printhead actuation frame, and thus the value of Z, is a maximum. The springs  45  and  46  cannot be compressed any further and thus the printhead applies a maximum pressure to the image receiving tape.  
      In an alternative embodiment of the present invention the printhead actuation frame is fixed relative to the printhead support frame and the springs  48  and  49  are omitted. In this embodiment the motor  13  drives the printhead toward and away from the platen, to operative and inoperative positions respectively. Thus in this embodiment the distance X between the printhead  9  and plate  80  of the printhead actuation frame is fixed.  
      Reference is now made to  FIG. 8 .  FIG. 8  shows an adjustable printhead arrangement  20 ′ in accordance with a further embodiment of the present invention. Components of the arrangement that are the same as those described in relation to  FIG. 3  are referred to with like reference numerals. In this arrangement the printhead support frame  8  and the printhead actuation frame  7  shown in  FIG. 3  have been replaced with a fixed printhead actuation and support frame  333 . Since this is a single frame, compression springs  45  and  46 , cylindrical members  30  and  31  and interconnection section  22  are omitted.  
      In this embodiment of the invention the platen  10  is compressibly supported relative to the printing device. Platen  10  is supported on a platen support frame  338 . Platen support frame  338  and printhead actuation frame  333  are slideably connected within the two slotted support members  41  and  42 . Platen support frame  338  is slideably mounted on platen base plate  336 , on projections  350  and  351 . Springs  450  and  460  are attached between the platen base plate  336  and the underside of a centre section of platen support frame  338 .  
      Reference is now made to  FIG. 9 . The print head is moved toward the platen by the rotation of motor  13 . The distance travelled by printhead actuation frame  333  is denoted by distance Z′. Distance Z′ increases as the print head is moved towards the platen. The springs are placed under compression by the movement of the printhead actuation frame when the printhead contacts the image receiving tape. The position of the printhead actuation frame when the printhead contacts the image receiving tape will depend on the thickness of the image receiving tape. The position of the platen when the springs are uncompressed will be referred to as the resting position of the platen.  
      At the point when the print head contacts the image receiving tape on the platen, springs  450  and  460  are under a minimum amount of compression. Accordingly, the distance between platen and the platen base plate  336 , denoted by distance B, will be at a maximum.  
      As the motor continues to drive the printhead support frame  333  towards the platen  10 , the springs  450  and  460  are placed under increasing compression between the movable platen and the base plate  336 . As the distance B between the platen  10  and the base plate  336  decreases, the pressure applied to the image receiving tape by the printhead increases. The pressure applied to the image receiving tape can therefore be said to be a function of B. Or, 
 
 P=f ( −B ) 
 
 where P is the pressure applied to the image receiving tape by the printhead. 
 
      After the point where the printhead contacts the image receiving tape, any further increase in distance Z′ will be equal to the decrease in distance B. Therefore when Z′ is greater than the value of Z at the point when the printhead contacts the image receiving tape, then, 
 
 P=f ( Z′ ) 
 
      The value of distance Z′, at the point when the movement of the printhead causes the springs to be compressed, depends on the thickness of the image receiving medium. Therefore, the pressure applied to the image receiving medium and can be directly calculated for known values of the position of the printhead actuation frame (Z′) and the thickness of the image receiving tape.  
      It can therefore be seen that in the above embodiments, pressure can be directly calculated from the distance travelled by the printhead actuation frame and the thickness of the image receiving tape. Since the distance travelled by the printhead actuation frame is controlled by the rotation of the motor  13 , it is therefore possible to control the pressure applied to the image receiving tape by controlling the number of rotations of the motor  13 .  
      In an embodiment of the present invention information may be stored with the image receiving tape to indicate the required pressure to print an image on the image receiving tape. The information stored with the image receiving tape could be detectable with a detecting device located in the printing device. For example the information may be stored on an RFID device located on a cassette containing the image receiving tape and a transponder could be located in the printing device to detect the information stored on the RFID tag. In alternative embodiments of the invention, the information may be stored in a barcode on the cassette and read by a barcode reader installed in the printing device or on an electrically readable ROM chip located on the cassette.  
      In one embodiment of the invention the information stored with the image receiving tape may specify the pressure required to print on the image receiving tape. Additionally, or alternatively, information relating to the thickness of the tape could be provided. This information may either be stored with the image receiving tape, or input by the user. In a preferred embodiment of the invention the printing device may be arrange to print on image receiving tapes that range between 0.13 mm to 1 mm in thickness. Information relating to the specified pressure, or tape thickness, could then be used to control the action of the adjustable printhead described herein after.  
      In an embodiment of the invention information stored on the printing device is used to directly control the position of the adjustable printhead.  
      The control of the adjustable printhead will now be described with relation to  FIG. 7 .  FIG. 7  shows the basic circuitry for controlling the printing device  1 . There is a microprocessor  100  chip having a read only memory (ROM)  102 , a microprocessor  101  and random access memory capacity (RAM)  104 . The microprocessor chip  100  outputs data to drive a display via a display driver chip  109  to display a label to be printed (or part thereof) and/or a message for the user. The display driver alternatively may form part of the microprocessor chip. The microprocessor receives an input from keyboard  108 . Additionally, the microprocessor chip  100  also outputs data to drive the printing elements of the print head  9  to form a label. The microprocessor chip  100  also controls the DC motor  67  driving the platen  10  and the motor  13  for driving the adjustable print head arrangement. In an alternative embodiment of the present invention a separate microprocessor chip may control the motor  13 . The microprocessor may also control the cutting mechanism  28  to allow lengths of tape to be cut off.  
      As previously described, the motor  13  is provided with a shaft encoder for monitoring the number of rotations of the motor such that the distance the printhead actuation frame is driven can be controlled by the microprocessor  100 .  
      According to one embodiment of the present invention a look up table stored in the ROM may be used to determine the number of rotations and thus the distance the printhead actuation frame is driven. As shown in  FIG. 10 , the look up table stores a list of motor rotations in column  510  that correspond to a detectable parameter of the image receiving tape stored in column  520 .  
      In one embodiment of the invention the detectable parameter may be the tape thickness. The number of motor rotations required to drive the printhead actuation means to provide a predetermined pressure on a particular tape thickness can be stored in correspondence with the tape thickness in the look up table. Separate look up tables may be stored in the ROM for each different type of material to be used with the printing device that requires a different printing pressure. The thickness of the tape and the material of the tape may be input by the user or may be sensed using a sensing arrangement described earlier.  
      In a preferred embodiment of the invention the detectable parameter may be an arbitrary value stored on the image receiving tape. The value specifies the number of rotations of the motor that are necessary for printing on the image receiving tape contained within the cassette at the required pressure. Since in this case the position of the printhead actuation frame for printing on the material has been precalculated by the manufacture, to take into account the type of material and the thickness of the image receiving tape, the printing device does not require any further information to print on the tape. In one embodiment of the invention the value may specify the number of rotations of the motor by means of a look up table as described in relation to  FIG. 10 . In an alternative embodiment of the invention the value may be used in an algorithm stored in the ROM to calculate the number of rotations of the motor.  
      In a further embodiment of the present invention the position of the printhead actuation frame may be calculated using an algorithm stored in the ROM of the microprocessor. As previously stated, pressure can be directly calculated from the position of print actuation frame and the thickness of the image receiving tape. Since different pressures are required for different image receiving tapes, different algorithms may be stored in the ROM, to be used when different image receiving tapes are installed in the printer.  
      In a preferred embodiment of the present invention the printer device is provided with a printhead actuation frame home switch which causes the printhead actuation frame to return to a reference position. The home switch may be operated to allow the user to remove the image receiving tape. The home switch may also be operated in the event of a power failure, such that the position of the printhead actuation frame can be relocated at the reference position.  
      The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any of the present claims. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.