Patent Publication Number: US-2007108883-A1

Title: Unified magnetic shielding of tensioned mask/frame assembly and internal magnetic shield

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/574,887, entitled “CRT Having a Unified Magnetic Shielding of Tensioned Mask/Frame Assembly and Internal Magnetic Shield” and filed May 27, 2004, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION  
      This invention generally relates to cathode ray tubes (CRTs) and, more particularly, to a shielding arrangement for a tensioned mask/frame assembly and an internal magnetic shield (IMS).  
     BACKGROUND OF THE INVENTION  
      A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask having strands or a membrane with slitted apertures with or without tie bars, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel.  
      A tension mask comprises a set of strands that are tensioned onto a mask frame to reduce their propensity to vibrate at large amplitudes under external excitation. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT.  
      Another source of electron beam misregister and beam motion is residual magnetism within the CRT. To remove this residual magnetism, a degaussing process is performed. One of the controlling parameters for optimizing magnetic performance of a tube is degauss recovery. Good degauss recovery manifests itself in low beam motion with the tube located in the external earth magnetic field and in good register of the electron beam with the phosphor element on the screen, after the tube has undergone a degaussing process to set up balancing fields in the IMS, mask, and frame components inside the CRT. With the introduction of true flat CRT&#39;s that use tension masks, including focus tension masks, optimization of magnetic shielding by degaussing has become more difficult.  
      During tube degaussing, existing IMS&#39;s must achieve effective magnetic field coupling with the mask through an intervening frame. In tension mask CRT designs, the mask is attached to a rigid frame. In order to maintain tension in the tension mask, the frame has to have high yield stress, which is usually accompanied by poor magnetic properties, i.e., high coercive force and low permeability. This makes degaussing the frame difficult, provides poor flux coupling during the degaussing process, and leaves very high residual magnetic fields inside the CRT. These residual magnetic fields cause the CRT to have very high electron beam misregister, poor purity and poor picture quality.  
      It is desirable to develop an improved mask frame assembly that allows tension masks to be uniformly degaussed.  
     SUMMARY OF THE INVENTION  
      The present invention therefore provides a cathode ray tube (CRT), comprising: a tensioned mask frame for supporting a tension mask inside the CRT at a cantilevered edge thereof, a tension mask mounted on the tension mask frame at the cantilevered edge; and an internal magnetic shield mounted on the tension mask frame. At least one of the tension mask and the internal magnetic shield have an extension extending along the tensioned mask frame to a point proximate or contacting the other of the tension mask and the internal magnetic shield to provide magnetic coupling between the tension mask and the internal magnetic shield independent of the tensioned mask frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described by way of example with reference to the accompanying figures of which:  
       FIG. 1  is a sectional plan view of a typical cathode ray tube;  
       FIG. 2  is a front view of a tension mask/frame assembly from the cathode ray tube of  FIG. 1 , showing a partial cut-away of the tension mask;  
       FIG. 3  is a perspective sectional view of an existing tension mask/frame assembly and internal magnetic shield arrangement;  
       FIG. 4  is a sectional side view of a tension mask/frame assembly and internal magnetic shield arrangement according to an exemplary embodiment of the present invention;  
       FIG. 5  is a sectional side view of a tension mask/frame assembly and internal magnetic shield arrangement according to another exemplary embodiment of the present invention;  
       FIG. 6  is a sectional perspective side view of a tension mask/frame assembly and internal magnetic shield arrangement according to yet another exemplary embodiment of the present invention;  
       FIG. 7  is a sectional perspective side view of a tension mask/frame assembly and internal magnetic shield arrangement according to yet another exemplary embodiment of the present invention;  
       FIG. 8  is a sectional side view of a tension mask/frame assembly and internal magnetic shield arrangement according to yet another exemplary embodiment of the present invention;  
       FIG. 9  is a sectional side view of a tension mask/frame assembly and internal magnetic shield arrangement according to yet another exemplary embodiment of the present invention; and  
       FIG. 10  is a sectional side view of a tension mask/frame assembly and internal magnetic shield arrangement according to yet another exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows a cathode ray tube (CRT)  1  having a glass envelope  2  comprising a rectangular faceplate panel  3  and a tubular neck  4  connected by a funnel  5 . The funnel  5  has an internal conductive coating (not shown) that extends from an anode button  6  toward the panel  3  and to the neck  4 . The panel  3  comprises a substantially cylindrical or a rectangular viewing faceplate  8  and a peripheral flange or sidewall  9 , which is sealed to the funnel  5  by a glass frit  7 . A three-color phosphor screen  12  is carried by the inner surface of the faceplate  3 . The screen  12  is a line screen with the phosphor lines arranged in triads, each of the triads including a phosphor line of each of the three colors. A color selection tension mask assembly  10  is removably mounted in predetermined spaced relation to the screen  12 . An electron gun  13 , shown schematically by dashed lines in  FIG. 1 , is centrally mounted within the neck  4  to generate and direct three inline electron beams, a center beam and two side or outer beams, along convergent paths through the tension mask assembly  10  to the screen  12 .  
      The tube  1  is designed to be used with an external magnetic deflection yoke  14  shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke  14  subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen  12 .  
      The tension mask assembly  10 , as shown in  FIG. 2 , has a metal frame  20  that includes two long sides  22  and  24 , and two short sides  26  and  28 . The two long sides  22 ,  24  of the frame are parallel to a central major axis, X, of the tube; and the two short sides  26 ,  28  parallel a central minor axis, Y, of the tube. Although the tension mask assembly  10  is shown here diagrammatically as a sheet for simplicity, it includes an apertured shadow mask  30  that contains a plurality of metal strips (not shown) having a multiplicity of elongated slits (not shown) therebetween that parallel the minor axis of the shadow mask  30 . The long sides  22 ,  24  have a cantilever edge  25  extending toward the screen  12 .  
      In an existing arrangement of a tension mask assembly  10  and an internal magnetic shield (IMS)  50 , as shown in  FIG. 3 , the tension mask  30  is attached to the cantilever edge  25  of the long sides  22 ,  24  of the tensioned mask frame. The attachment may be performed, for example, by welding. The IMS  50  is attached to the long sides of the frame  20  at a location removed from the tension mask  30 . In the embodiment illustrated in  FIG. 3 , the long sides  22 ,  24  of the frame  20  comprise L-shaped bars or angles formed by two legs at a right angle to each other with the cantilever edge  25  on the end of one leg and the IMS  50  attached to the other leg. Thus, the existing shielding arrangement provides magnetic flux coupling through the tensioned mask frame  20 .  
      In this arrangement, the IMS  50 , tension mask  30  and frame  20  are made from low carbon steel or iron-nickel alloys. Magnetic shielding and degaussing ability of the tension mask  30 , tensioned mask frame  20 , and IMS  50  system are improved if each of the components has high anhysteretic permeability and low coercivity. However, the tensioned mask frame  20  must have high yield stress to provide the rigidity necessary for proper function. This high yield stress is usually accompanied by poor magnetic properties, e.g., high coercivity and low permeability. Even if the coercivity of the tension mask  30  and the IMS  50  are low, indicating good magnetic properties, the overall performance of the tube is deteriorated if the coercivity of the tensioned mask frame  20  is high, indicating poor magnetic properties. Having high magnetic reluctance, the tensioned mask frame  20  increases the reluctance of the IMS/frame/mask assembly. Additionally, a residual magnetic field is retained after degaussing at the interface of the tension mask  30  and the tensioned mask frame  20  that is difficult to remove and leads to beam misregister. Conventional degaussing is performed using a special degaussing coil placed close to the IMS  50 , and will degauss the IMS  50  adequately. Conventional degaussing, however, will do very little to remove residual magnetic fields from the high coercivity tensioned mask frame  20  and the tension mask  30  behind it. The tensioned mask frame  20  causes the earth magnetic field to be distorted and concentrated at particular points, which can magnetize the tension mask  30  and IMS  50  when the tube is degaussed. In addition, a residual magnetic field exists due to the high coercivity tensioned mask frame  20  that is difficult to remove and leads to beam misregister.  
      In an exemplary embodiment of the present invention, as shown in  FIG. 4 , the tension mask  30  is attached to the long sides  22 ,  24  of the frame  20  at the cantilever edges  25 . The IMS  50  has extensions  55 ,  56  formed on the end of the IMS  50  at a right angle to one another corresponding to the surfaces of the long sides  22 ,  24  of the tensioned mask frame  20 . When the IMS  50  is attached to the tensioned mask frame  20 , the extension  55  extends along the tensioned mask frame  20  to a location proximate the cantilevered edge  25 , where the tension mask  30  is attached to the tensioned mask frame  20 . The extension  55  may, but does not have to contact the tension mask  30 . Optionally, the IMS  50  of this embodiment may be attached to the tensioned mask frame, only at the extension  56  for ease of access during assembly. It should be noted that the extension  55  need not touch the IMS  50  to provide magnetic coupling. Thus, the tension mask  30  and the IMS  50  may be magnetically coupled through a small gap with minimal magnetic flux leakage. Optionally, the IMS  50  of this embodiment may be attached to the tensioned mask frame, only at the extension  56  for ease of access during assembly.  
      In an alternative exemplary embodiment of the present invention, as shown in  FIG. 5 , one or more joining members  60  are attached to the tension mask  30  at the cantilevered edge  25  of the tensioned mask frame  20  and are attached to the IMS  50  at a location removed from the cantilevered edge  25 . The joining members  60  may be formed of a material having a high magnetic permeability, such as steel. The joining members  60  may be very thin to minimize the risk of contact with the walls of the tube or other structures within the tube. Also, the ends of the joining members  60  may be flat or bent on either or both sides to aid contact with the tension mask  30 . The joining members  60  may be attached to the tensioned mask frame  20 , but such attachment is not required.  
      In another alternate embodiment of the present invention, as shown in  FIG. 6 , a flexible mesh  70  comprising a ferromagnetic material extends between the tension mask  30  and the IMS  50 . The mesh  70  may extend under the tension mask  30  and the IMS  50  as shown in  FIG. 6 . The IMS  70  may extend around the corner of the angled long side  22 ,  24  of the tensioned mask frame, as shown in  FIG. 6 . Alternatively, the mesh  70  may extend around the corner of the angled long side  22 ,  24 , allowing additional clearance. The mesh  70  may be attached to the tension mask  30  and IMS  50  using attachment means, such as welding, and may be welded to the tensioned mask frame  20  together with the tension mask  30  and IMS  50 .  
      In another alternate embodiment of the present invention, as shown in  FIG. 7 , one or more tabs  80  are formed on the end of the IMS  50 , such that they extend toward the tension mask  30  when the IMS  50  is mounted on the tensioned mask frame  20 . The tabs  80  may vary in size and spacing to provide adequate magnetic coupling between the IMS  50  and the tension mask  30 . The tabs  80  may extend under the tension mask  30  at cantilevered edge  25  or may be attached to the tensioned mask frame  20  proximate cantilevered edge  25 . While tabs  80  are shown extending from the IMS  50 , tabs may alternatively be formed that extend from the tension mask  30  and extend toward the IMS  50 .  
      In another alternative embodiment of the present invention, as shown in  FIG. 8 , a coating  90  is applied to the tensioned mask frame  20  between the attachment locations for the tension mask  30  and the IMS  50 . The coating comprises a material with a high magnetic permeability, providing magnetic coupling of the tension mask  30  to the IMS  50 . The coating  90  may extend onto the cantilevered edge  25  of the tensioned mask frame  20  and to a location on the tensioned mask frame  20  removed from the cantilevered edge  25 , such that it contacts the tension mask  30  and the IMS  50  when they are attached to the tensioned mask frame  20 . Alternatively, the coating  90  may extend proximate the tension mask  30  and IMS  50  but not be in contact with either or both.  
      In yet another alternative exemplary embodiment of the present invention, as shown in  FIG. 9 , the tension mask has extensions  35  which extend beyond the cantilevered edges  25 . These extensions  35 , which are formed as part of the tension mask  30  are then positioned extending along the tensioned mask frame  20  to a position proximate or in contact with the IMS  50 , removed from the cantilevered edges  25 , where the extensions  35  are attached to the tensioned mask frame  20 . The IMS  50  and extensions  35  may be attached to the tensioned mask frame  20  using common attachment means, which may be, for example, spot welds.  
      In yet another alternative embodiment, shown in  FIG. 10  an extension  100  is attached to the long sides  22 ,  24  of the frame  20 , extending from the IMS  50  along the inside surface of the frame toward the mask  30 . The extension  100  comprises a material having high magnetic permeability, as compared to the frame  20 . The extension  100  may be a separate part attached by press fitting, for example onto the edge opposite the cantilevered edge  25 . The extension  100  can be inserted from the interior of the tension mask frame  20  providing enhanced shielding of the frame  20 . It should be noted that the extension  100  may contact the mask  30 , but physical contact is not required, as long as the extension  100  extends to a point proximate the mask  30 .  
      The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.