Abstract:
An apparatus positions an ink jet printhead relative to a print medium onto which the printhead jets ink. A conduit is attached to the printhead and has an opening disposed in opposition to the print medium. An air-moving device moves air through the conduit. A pressure-sensing device senses pressure within the conduit. A gap-adjusting assembly moves the printhead and/or the print medium dependent upon the pressure sensed by the pressure-sensing device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an ink jet printer, and, more particularly, to setting a gap between a printhead and a print medium, i.e., a printhead gap, in an ink jet printer.  
           [0003]    2. Description of the Related Art  
           [0004]    It is generally known that improvements in ink jet printing can be achieved if the printhead can be positioned closer to the paper and if this gap distance between the printhead and the paper can be controlled within desired limits. An ink jet printhead moves or scans across the width of a sheet of paper, depositing ink drops onto the paper as the printhead scans. The printhead is generally slidingly coupled to a guide rod or shaft, which is oriented parallel to the direction of movement of the printhead. The printhead is coupled tightly enough to the guide rod that the straightness of the guide rod largely determines the straightness of the path of the printhead. Thus, the straightness of the guide rod partly determines the variations in the gap between the printhead and the paper, hereinafter referred to as the “printhead gap”, as the printhead scans. The straightness or flatness of the platen that supports the paper in opposition to the printhead also partly determines the variations in the printhead gap.  
           [0005]    It is known to very precisely manufacture the guide rod with a high degree of straightness in order to limit variations in the printhead gap. It is also known to manufacture the platen very precisely with a high degree of straightness or flatness. Further, it is known to support the platen with one or more very precisely manufactured shafts having a high degree of straightness in order to provide the paper-engaging surface of the platen with a desired degree of straightness or flatness. A problem is that such precisely manufactured guide rods, platens and shafts are very expensive.  
           [0006]    What is needed in the art is a method of dynamically adjusting the printhead gap while printing, thereby eliminating the need for expensive, precisely manufactured guide rods, platens and shafts.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method and apparatus for sensing the size of a printhead gap and dynamically adjusting the printhead gap to a desired size.  
           [0008]    The invention comprises, in one form thereof, a method of operating an ink jet printer. A printhead is positioned to jet ink onto a print medium. A conduit is attached to the printhead. The conduit has an opening opposing the print medium. Air is caused to flow through the conduit. A pressure within the conduit is measured. A printhead gap between the printhead and the print medium is adjusted dependent upon the measuring step.  
           [0009]    The invention comprises, in another form thereof, an apparatus that positions an ink jet printhead relative to a print medium onto which the printhead jets ink. A conduit is attached to the printhead and has an opening disposed in opposition to the print medium. An air-moving device moves air through the conduit. A pressure-sensing device senses pressure within the conduit. A gap-adjusting assembly moves the printhead and/or the print medium dependent upon the pressure sensed by the pressure-sensing device.  
           [0010]    The invention comprises, in yet another form thereof, an ink jet printer for printing on a print medium. A conduit is attached to an ink jet printhead. The conduit has an opening disposed in opposition to the print medium. An air-moving device moves air through the conduit. A pressure-sensing device senses pressure within the conduit. A gap-adjusting assembly moves the printhead and/or the print medium dependent upon the pressure sensed by the pressure-sensing device.  
           [0011]    An advantage of the present invention is that expensive, high precision guide rods, platens and shafts are not needed.  
           [0012]    Another advantage is that the apparatus of the present invention does not touch or contact the print medium.  
           [0013]    Yet another advantage is that the printhead can be positioned closer to the platen and dynamic gap adjustments can be made to accommodate print mediums of different thicknesses, such as envelopes.  
           [0014]    A further advantage is that air flowing through the nozzle and onto the paper facilitates drying of the ink.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    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 an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0016]    [0016]FIG. 1 is a schematic view of one embodiment of a printer of the present invention;  
         [0017]    [0017]FIG. 2 is an enlarged, schematic view of the nozzle, paper and platen of FIG. 1; and  
         [0018]    [0018]FIG. 3 is a flow chart of one embodiment of the method of the present invention. 
     
    
       [0019]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring now to the drawings, and particularly to FIG. 1, there is shown one embodiment of a printer  10  of the present invention, including a printhead  12 , a guide rod  14 , a platen  16 , a gap-sensing assembly  18 , a fixed structure  20 , and a gap-adjusting assembly  22 .  
         [0021]    Ink jet printhead  12  is slidingly coupled to guide rod  14 . The axis  14   a  of guide rod  14  defines a bi-directional scanning path for printhead  12 . Guide rod  14  is oriented parallel to a width of platen  16 .  
         [0022]    Gap-sensing assembly  18  includes an air pressure source, or air-moving device,  24 . Air-moving device  24  can be, for example, a small cooling fan which supplies positive air pressure to an upper section  26  of a first flexible conduit or tube  28  through an opening  29 . Tube  28  is attached to printhead  12 . The air pressure in upper section  26  is approximately the static pressure of fan  24  and can be approximately 0.1 inches of water. A very small fraction of the air from fan  24  is supplied to a nozzle  30  in a lower section  32  of first tube  28 . Nozzle  30  tapers to an opening at an end  34 .  
         [0023]    The opening at end  34  of nozzle  30  is disposed closely adjacent and in opposition to a sheet of print medium, such as paper  36  supported on platen  16 . The opening is also oriented parallel to paper  36 . End  34  is disposed approximately 0.7 millimeter from paper  36  and is also disposed closely adjacent to printhead  12 . End  34  as well as the remainder of first tube  28  has a circular cross section. A width or diameter  38  (FIG. 2) of end  34  is approximately four times greater than a maximum height of a gap  40  between nozzle  30  and paper  36 . That is, gap  40  is less than 25% of width  38 .  
         [0024]    A cylindrical fluid resistor  42  is disposed in first tube  28  and separates, interconnects and defines upper section  26  and lower section  32 . Fluid resistor  42  has a channel  44  fluidly connecting upper section  26  and lower section  32 . Channel  44  has a circular cross section and a diameter  46  approximately equal to diameter  38  of nozzle end  34 . Fluid resistor  42  constricts the flow of air from upper section  26  to lower section  32 , thereby causing the air pressure in upper section  26  to be greater than that in lower section  32 .  
         [0025]    The resistance of fluid resistor  42 , i.e., the size of diameter  46 , is chosen to maximize the change in backpressure within lower section  32  for a given change in gap  40 . Thus, the sensitivity of gap-sensing assembly  18  is maximized.  
         [0026]    Lower section  32  includes an opening  49  in fluid communication with and fluidly connected to a first end  48  of a second conduit in the form of a flexible tube  50 . A second end  52  of tube  50  is fluidly connected to a cavity  54  of a housing  56  of a pressure-sensing device. The pressure-sensing device is attached to tube  50 . An extremely flexible, circular diaphragm  58  forms a side of housing  56  opposite from end  52  of tube  50 .  
         [0027]    A compression spring  60  is attached to fixed structure  20  and engages diaphragm  58 . Spring  60  biases diaphragm  58  in the direction indicated by arrow  62 .  
         [0028]    A grounded electrical contact  64  is attached to diaphragm  58 . Contact  64  is a part of a potentiometer  66  which, in turn, is part of a voltage divider  68 . Voltage divider  68  also includes a resistor  70 .  
         [0029]    A voltmeter  72  is connected across resistor  70 . As the last sequential element of gap-sensing assembly  18 , an output of voltmeter  72  is connected to gap-adjusting assembly  22 .  
         [0030]    Gap-adjusting assembly  22  includes electrical controller  74 , voltage sources  76 ,  84 , motors  78 ,  86 , actuators  82 ,  90  and guide rod holders  92 ,  94 . An output of voltmeter  72  is connected to an input of electrical controller  74 , which can include, for example, a microprocessor and associated memory that executes control instructions. A first output of controller  74  is connected to an input of a first voltage source  76 . An output of voltage source  76  is connected to a first motor  78 . An output shaft  80  of motor  78  is mechanically coupled to a first actuator  82  which is attached to platen  16 .  
         [0031]    A second output of controller  74  is connected to an input of a second voltage source  84 . An output of voltage source  84  is connected to a second motor  86 . An output shaft  88  of motor  86  is mechanically coupled to a second actuator  90  which is attached to a movable guide rod holder  92 . Another movable guide rod holder  94  is attached to guide rod holder  92  such that holder  94  follows the movement of holder  92 . Holders  92  and  94  are attached to respective opposite ends of guide rod  14 . Actuators  82  and  90  are the last sequential elements of gap-adjusting assembly  18 .  
         [0032]    Referring now to FIG. 3, during use, at step S 100 , printhead  12  is positioned to jet ink onto paper  36 . Since nozzle  30  is attached to printhead  12 , nozzle  30  moves with printhead  12  both laterally, i.e., in the directions indicated by double arrow  96 , and into and out of the page of FIGS. 1 and 2.  
         [0033]    At step S 102 , air is caused to flow through upper section  26 . More particularly, fan  24  blows air into opening  29 , which is in direct communication with fan  24  to receive air therefrom. Air flows through nozzle  30  and then radially outward in the area between end  34  of nozzle  30  and paper  36 . The height of gap  40  between end  34  of nozzle  30  and paper  36  determines the rate of airflow through first tube  28 . The airflow through nozzle  30  may assist in drying the ink that has been immediately previously jetted onto paper  36  by printhead  12 , regardless of the direction of scan of printhead  12 .  
         [0034]    At step S 104 , the backpressure within nozzle  30  is determined. As gap  40  between nozzle  30  and paper  36  changes due to, for example, variations in paper thickness, straightness of guide rod  14 , straightness of platen  16 , etc., the flow area for the air between end  34  of nozzle  30  and paper  36  also changes. Because of this change in the flow area, the mass flow rate of air through nozzle  30  and the backpressure within nozzle  30  changes. The backpressure acts on diaphragm  58 . When cavity  54  is at atmospheric pressure, diaphragm  58  is planar and is disposed in zero position  98 . When the backpressure acts upon diaphragm  58 , diaphragm  58  is forced outward against the action of spring  60 , as shown in FIG. 1. The smaller gap  40  is, the greater is the backpressure, and the greater is the outward deflection of diaphragm  58 . Thus, the axial position of diaphragm  58  is a function of the gap  40  between the nozzle opening and paper  36 . The volume of cavity  54  and the fluid resistance of second tube  50  can control the time response of the movement of diaphragm  58 .  
         [0035]    At step S 106 , the backpressure is associated with a height of gap  40 . The axial position of diaphragm  58  is measured by the combination of voltage divider  68  and voltmeter  72 . As diaphragm  58  moves in the direction opposite to direction  62 , contact  64  also moves in the same direction, thereby reducing the resistance of potentiometer  66 . As the resistance of potentiometer  66  decreases, the voltage across resistor  70 , as measured by voltmeter  72 , increases. Thus, the backpressure with tube  28  is measured, and the output of voltmeter  72  is indicative of the size of gap  40 . This actual size of gap  40  is compared with a desired, or target, size of gap  40 .  
         [0036]    At step S 108 , if the actual size of gap  40  is not equal to a desired size of gap  40 , then gap  40  is adjusted based on the measured backpressure. Controller  74  receives the voltage output of voltmeter  72  and activates voltage source  76  and/or voltage source  84  accordingly. Upon activation, voltage source  76  powers motor  78 , which drives actuator  82  to raise or lower platen  16  in one of the directions of double arrow  100 . By moving platen  16  up or down, gap  40  is decreased or increased, respectively. Likewise, upon activation, voltage source  84  powers motor  86 , which drives actuator  90  to raise or lower guide rod holders  92 ,  94  in one of the directions of double arrow  100 . By moving guide rod holders  92 ,  94  up or down, and hence guide rod  14  and printhead  12  up or down, gap  40  is increased or decreased, respectively. Thus, gap  40  is adjusted to the desired size.  
         [0037]    Both platen  16  and guide rod  14  can be moved to adjust gap  40 . It is also possible to move only platen  16  or to move only guide rod  14  to adjust gap  40 . Both adjustments can be made with the same gap-adjusting assembly  22 .  
         [0038]    By the method described above, using gap-sensing assembly  18  and gap-adjusting assembly  22 , feedback control is used to maintain a constant, desired gap  40  as printhead  12  scans in directions  96 . Since nozzle  30  is attached to printhead  12 , and the relationship between gap  40  and the gap between printhead  12  and paper  36  is known, a constant, desired gap between printhead  12  and paper  36  is also maintained by feedback control as printhead  12  scans in directions  96 . The steps of causing air to flow within tube  28 , measuring pressure within tube  28 , and adjusting the gap between printhead  12  and paper  36  are preformed continuously during the scanning of printhead  12 .  
         [0039]    It is alternatively possible to detect and/or measure the axial position of diaphragm  58  by use of several different devices, including contact points, capacitance-measuring devices, electro-optics, etc. These sensors/detectors can be used with elements of common feedback-control to control an electrical/mechanical device such as a motor to dynamically change the paper-print head gap to be within the desired range. Various feedback and control techniques can be used, such as on-off, proportional, microprocessor controlled, etc.  
         [0040]    It is also possible for an inexpensive pressure transducer to replace the flexible diaphragm. The diaphragm or control switch (pressure sensors) can be located physically close to the print head, and can also move with the print head. The diaphragm or pressure sensor could be an integral part of the nozzle assembly. For instance, the diaphragm or pressure sensor could be an integral part of first tube  28 .  
         [0041]    The magnitude of the fluid resistance of fluid resistor  42  can be adjusted to obtain optimum performance. Although fluid resistor  42  is shown in FIG. 1 as being a discrete element, it is also possible for the fluid resistor to be formed integrally with first tube  28 . For example, the diameter and length of upper section  26  could be chosen to provide a desired level of fluid resistance.  
         [0042]    Channel  44  of fluid resistor  42  has been shown herein as having a diameter approximately equal to diameter  38  of nozzle end  34 . However, it is also possible for the diameter of the channel of the fluid resistor to be different than diameter  38  of nozzle end  34 .  
         [0043]    An external spring  60  is shown in FIG. 1. However, it is also possible for the flexibility of the diaphragm to serve as the spring-like element.  
         [0044]    The gap between printhead  12  and paper  36  is shown in FIG. 1 to be equal to gap  40  between nozzle end  34  and paper  36 . However, it is also possible for the gap between printhead  12  and paper  36  to be greater than or less than gap  40 .  
         [0045]    Platen  16  is shown as being a planar object. However, it is also possible for the platen to be a roller having an axis parallel to the scanning direction of the printhead.  
         [0046]    The present invention has been described as keeping the gap between printhead  12  and paper  36  constant as printhead scans in directions  84 . However, it is to be understood that the present invention can also be used to keep the gap between printhead  12  and paper  36  constant as printhead  12  moves in any other direction.  
         [0047]    The present invention has been described as blowing air downwardly through tube  28  and measuring a positive pressure in housing  56 . However, it is also possible for fan  24  to suck air upwardly through tube  28  and measure a negative pressure in housing  56 .  
         [0048]    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.