Abstract:
A printer assembly with lateral and longitudinal self-alignment. The printer assembly has a frame, a door which swings open, a motor-driven platen disposed on the door, a print head movably disposed on the frame so as to engage the platen, and a spring coupled to the print head so as to produce both backward force to urge the print head against the platen when the door is closed and torque to rotate the print head upwardly when the door is opened. The frame is provided with a semi-cylindrical paper well and a generally triangular shape for ease of access to the paper well.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to printers. More specifically, it relates to printer assemblies that employ linear arrays of symbol generation elements by which the recording medium is passed, particularly thermal printers having heating elements arranged in a linear array on a print head which creates characters or other symbols on thermal printer paper as it passes by the print head.  
           [0002]    Thermal printers are well known and widely used for many applications, particularly where high printing speed or small physical space are required. For example, a thermal printer may be particularly suitable for incorporation in a portable medical patient monitoring instrument, where the principle function of the instrument is to measure accurately, display and record physiological waveforms and parameters, as well as trend information about the patient&#39;s vital signs. Thermal printers lend themselves to high printing speeds which are needed to print waveforms, such as an electrocardiogram, in real time so as to be immediately available to clinicians.  
           [0003]    At the same time, image quality is always a concern with a printer to ensure that the information recorded thereby can easily and accurately be read. In essentially any printer where, as in a thermal printer, a two dimensional image is created by moving the printing paper between a platen and a linear array of printing elements disposed on an elongate print head forced against the paper, it is important that the force applied by the print head to the paper be substantially uniform throughout the length of the paper. Meeting this requirement can be a challenge because of the need also to provide a mechanism for loading paper into the printer so that it passes between the print head and the platen.  
           [0004]    A thermal printer assembly that is particularly effective in providing such uniform force while permitting paper to be loaded therein is disclosed in Mahoney U.S. Pat. No. Re. 35,026 entitled SELF-ALIGNING THERMAL PRINT HEAD AND PAPER LOADING MECHANISM, hereby incorporated by reference in its entirety. The printer assembly of this Mahoney patent employs a lateral self-alignment mechanism which urges the print head against the platen to provide uniform lateral distribution force. However, this assembly does have some important limitations. First, it employs a sliding door for opening the printer to load paper, which requires space behind the printer to open the door. Second, it requires the use of flat, folded printer paper which takes more space and often is not as easy to use as a cylindrical roll of printer paper. Third, the paper can be difficult to install because it must be placed into a deep paper well, partially under the sliding cover. Fourth, because the motor for driving the platen is mounted on the frame of the printer and the platen itself is mounted on the cover, the cover must be carefully aligned with the frame so that a gear on the frame will engage a gear on the cover to operate the platen.  
           [0005]    Another approach is to use a cylindrical roll of paper and a cover that is hinged so as to swing, rather than slide, open. A printer assembly which uses this approach is shown, for example, by Gustavsson et al. U.S. Pat. No. 5,791,796, entitled THERMAL PRINTER WITH SPRING-BIASED DRIVE ROLLER/PLATEN. In addition, the Gustavsson et al. printer employs a platen drive motor mounted on the door, which avoids the problem of aligning the door with the frame so as to ensure that gears properly mesh. However, the Gustavsson et al. device does not provide for lateral self-alignment of the print head with the platen so as to apply uniform force against the platen.  
           [0006]    Although it would be desirable to employ a cylindrical roll of paper and a door that is hinged so as to swing, rather than slide, open, the application of uniform force is important to achieve good print quality. The problem is that a lateral self-alignment mechanism of the type shown in Mahoney, which has a sliding door, will not readily work with a swinging door of the type shown in Gustavsson et al., at least without undue complexity, because the platen and the print head will interfere with one another when the door is opened and closed.  
           [0007]    Accordingly, there is a need for a printer assembly that provides the advantages of lateral self-alignment of a print head with a platen and the advantages of a door that swings open and closed so as to provide better utilization of space, ease of use and high print quality.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention meets the aforementioned need by providing a printer assembly with both lateral and longitudinal self-alignment. The printer assembly has a frame, a top door hinged at the back of the frame to swing open, a motor-driven platen disposed at the front, underside of the door, a print head disposed at the front of the frame, and a spring coupled to the print head so as to produce both backward force to urge the print head against the platen when the door is closed and torque to rotate the print head upwardly when the door is opened. The print head is disposed on a mounting assembly which has a centrally located tab that provides a fulcrum and engages a spring. The spring provides both backward force and torque to rotate the print head upwardly when the door is open.  
           [0009]    The platen is driven by a motor which is also mounted on the door and connected by a gear train to the platen so as to transmit rotational motion to the platen. The frame is provided with a semi-cylindrical well for receiving a cylindrical roll of printer paper, and has a generally triangular shape for easy access to the well when the door is open.  
           [0010]    Therefore, it is a principal object of the present invention to provide a novel and improved printer assembly with both lateral and longitudinal self-alignment. 
       
    
    
       [0011]    The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a side elevation of a preferred embodiment of a printer assembly according to the present invention, with the door thereof shown in an open position and in partial cross-section.  
         [0013]    [0013]FIG. 2 is a side elevation of a the preferred embodiment of the printer assembly according to the present invention, with the door thereof shown in a closed position and in partial cross-section.  
         [0014]    [0014]FIG. 3 is a top view of a the preferred embodiment of the printer assembly according to the present invention.  
         [0015]    [0015]FIG. 4 is a front elevation of a preferred embodiment of the printer assembly according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring first to FIGS. 1 and 2, the printer assembly generally comprises a frame  10 , a print head  12 , a hinged door  14 , and a motor-driven platen  16  mounted on the door  14  for engaging the print head  12  with paper  18  disposed therebetween when the door  14  is closed. For purposes of reference, the printer assembly and frame have a front  20 , a back  22 , a top  24 , a bottom  26 , and two sides  28  and  30  (see FIGS. 3 and 4), respectively. The dimension from side-to-side of the printer assembly is referred to herein as the lateral dimension, and the dimension from top-to-bottom is referred to herein as the longitudinal dimension.  
         [0017]    The door  14  is attached to the frame at or proximate the back thereof by two hinges  32   a  and  32   b , as shown in FIG. 3. The type of hinge mechanism is not critical to the invention, and other hinge mechanisms not inconsistent with the object of the invention may be used without departing from the principles of the invention. The hinges enable the door  14  to be pivoted to an open position, as shown in FIG. 1, or to a closed position, as shown in FIGS.  2 - 4 . The platen  16  is rotatably mounted at or proximate the front, underside of the door  14  and driven by a motor  34 , also mounted on the door. The motor transmits rotational motion to the platen through a gear train  36  mounted on the door. The operation of the motor and gear train to drive a platen in a printer of this type is commonly understood in the art.  
         [0018]    The door  14  is provided with a pair of latches  60  to hold it closed. The latches are interconnected by a shaft  62  to a push button  64  which, when actuated, rotates the shaft and releases the latches so the door can open.  
         [0019]    The frame  10  includes a semi-cylindrical well  38  for receiving a roll of printer paper  40 . The preferred embodiment of the invention contemplates use of a thermal print head and heat-sensitive printer paper to create an image. Such paper is commonly available in cylindrical rolls as shown in FIGS. 1 and 2. However, where uniformly distributed force applied by the print head to the platen is important, other types of print heads and print paper might be used without departing from the principles of this invention. Preferably, the frame is generally triangular shaped, as viewed from the side, so as to facilitate easy access to the paper well.  
         [0020]    The print head  12  is pivotally mounted to the frame at the front  20  thereof so that, when the door  14  is opened, the print head will rotate upwardly along the longitudinal dimension to disengage with the platen  16  and, when the door  14  is closed, the print head will rotate downwardly along the longitudinal dimension as the platen engages the print head. These two positions can be particularly seen in FIGS. 1 and 2. The print head also is mounted so as to float in the lateral dimension when the door  14  is closed and the platen  16  engages the print head. Thus, the printer assembly provides both longitudinal and latitudinal self-alignment when the door is closed with paper  18  disposed between the platen and the print head.  
         [0021]    The print head  12  is supported by a mounting assembly  42 . The mounting assembly includes a frontwardly-directed tab  44  which is disposed substantially centrally of the mounting assembly in the lateral dimension. The tab fits through a slot  46  in the front of the frame. The tab includes a pair of wings  48   a  and  48   b  extending laterally thereof but set back from the front surface  50  of the mounting assembly  42  so as to be disposed on the front side of the front  20  of the frame. The tab  44  also includes a slot  52  for receiving a spring  54 . The spring  54  urges the tab toward the back of the frame, the back edges  56   a  and  56   b  of the wings  48   a  and  48   b , respectively, limiting the backward distance that the mounting assembly  42  can travel. Preferably, the spring  54  is an elongate wire spring having a circular cross-section and is retained at its ends  58   a  and  58   b  to the frame  10 . This permits the tab  44  to rotate about the spring  54  in the slot  52 . The ends  58   a  and  58   b  of the spring are attached to the frame  10  backwardly from the slot  52  so that the spring is under tension and thereby applies force against the inner wall of the slot  52 . This both urges the print head  12  backwardly toward the platen  16 , while allowing the print head to pivot laterally about the point of contact of the spring  54  and slot  52  so as to provide lateral self alignment, and produces rotational torque to provide longitudinal self alignment, as described hereafter.  
         [0022]    The slot  52  is offset downwardly in the longitudinal dimension from the wings  56   a  and  56   b . When the door  14  is closed, the platen  16  pushes frontwardly on the print head so that the back edges  56   a  and  56   b  of the wings  48   a  and  48   b , respectively, are pushed slightly away from the front surface of the frame  10 . Thus, the print head mounting assembly  42 , and print head, float in the lateral dimension. When the latches  60  are released so that the door  14  can open, the mounting assembly  42  and print head will rotate in the longitudinal dimension around the spring  54  in the slot  52 . This permits the platen  16  to disengage the print head; indeed, as the door  14  is opened and the platen begins to swing upwardly and backwardly, the tabs  56   a  and  56   b  will be forced against the front of the frame  20  and act as a fulcrum so that the spring in the offset slot  52  provides rotational torque to force the platen upwardly, thereby opening the door. Once the door is open, the spring  54  keeps the print head in an upwardly rotated position so as to receive the platen when the door is closed.  
         [0023]    It is to be appreciated that, while a particular preferred type of spring and tab assembly have been shown, other spring and tab assemblies might be employed to provide both backward force and rotational torque such that the print head self-adjusts laterally and longitudinally, without departing from the principles of the invention.  
         [0024]    The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention of the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.