Patent Publication Number: US-6211628-B1

Title: System for controlling the position of an electron beam in a cathode ray tube and method thereof

Description:
This Application claim benefit to Provisional Application 60/054,603 filed Aug. 2, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a system and method for detecting and controlling the position of an electron beam in a cathode ray tube using capacitive coupling. 
     BACKGROUND OF THE INVENTION 
     A conventional color television tube has a cathode ray tube, three electron guns (i.e., one gun for the red image, a second gun for the green image, and a third gun for the blue image) and a shadow mask or aperture grill which serves to block the three electron beams produced by the guns from hitting the wrong phosphors on an inner surface of a faceplate of the cathode ray tube. While the shadow mask acts as an effective block, it causes some difficulties. 
     For example, approximately eighty percent of the total electron beam current produced by a gun hits the shadow mask and is dissipated therein as heat. This heating causes the shadow mask to expand. The process is called doming and results in an upper limit on the tube&#39;s brightness because as higher electron beam currents are used to achieve greater brightness, more expansion occurs and causes the shadowmask to eventually lose its registration with the phosphors on the faceplate. 
     The shadow mask also limits the resolution of the display, which depends on the number and size of the holes in the mask. There are plainly only so many holes that one can put in the mask and still keep it stiff. Also, as the hole size decreases, less of the electron beam reaches the phosphor, thus lowering the brightness. 
     Without a shadow mask, the problems with doming and resolution are eliminated. However, without a shadow mask proper positioning of the electron beam becomes more crucial. To properly position the electron beam, it is necessary to be able to determine and adjust the position of the electron beam. 
     One technique for controlling the position of the electron beam involves detecting light on the outer surface of the faceplate of cathode ray tube when the electron beam strikes a phosphor. The detected light is then converted to a position signal indicating the position of the electron beam on the faceplate. The position signal is then compared against a desired location signal for the electron beam, an error signal is generated and the error signal is used to correct the positioning of the electron beam. One of the main problems with this technique is that it requires an expansive detection system outside of and separate from the cathode ray tube to control the position of the electron beam which renders it not commercially feasible. 
     Another technique for controlling the position of the electron beam involves generating an electronic current as the electron beam hits an electrode on the faceplate of the cathode ray tube and then coupling this current out from the cathode ray tube using transformers. One example of such a system is disclosed in U.S. Pat. No. 4,635,107 to Turner, which is herein incorporated by reference. 
     Although the technique using transformers works, it has problems. For example, the transformers used in this technique are expensive because they must be able to faithfully transform a few microamps of current into detectable levels of currents while successfully withstanding potential differences of twenty-five kilovolts or more. Additionally, the leads from the transformers must pass through the cathode ray tube to get to the current signal from the electrodes out. 
     SUMMARY OF THE INVENTION 
     A system and method for controlling an electron beam in accordance with the present invention includes an electron gun, a cathode ray tube with a faceplate, a deflection drive, a pair of positioning electrodes, an electron beam controller, and a pair of capacitors. The electron gun generates an electron beam in the cathode ray tube which is deflected in a desired direction towards and between at least one pair of positioning electrodes formed on the inner surface of the faceplate. Each of the positioning electrodes generates a position signal which is capacitively coupled by the capacitors to the electron beam controller. The electron beam controller adjusts the deflection of the electron beam in response to the position signals. The capacitors comprise a pair of first and second capacitor plates which are separated by the cathode ray tube. The first capacitor plates are disposed on opposing sides of an inner surface of the cathode ray tube adjacent to the faceplate. The second capacitor plates are disposed on the outer surface of the cathode ray tube, each of the second capacitor plates being disposed opposite one of the first capacitor plates. 
     The system and method in accordance with the present invention provides a number of advantages, including providing an inexpensive and effective control system for the position of an electron beam in a cathode ray tube. Instead of the prior system of using transformers which are expensive and require leads to pass through the cathode ray tube, the present invention uses a pair of capacitors formed on the funnel of the cathode ray tube at a minimal cost, i.e. only the cost of metallization to form the capacitor plates in and on the funnel of the cathode ray tube, and which can transfer the position signals generated by the electron beam in the cathode ray tube externally from the cathode ray tube without requiring leads to pass through the cathode ray tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial schematic and partial block diagram of a system for controlling an electron beam in a cathode ray tube in accordance with the present invention; 
     FIG. 2A is a perspective view of the cathode ray tube in accordance with the present invention; 
     FIG. 2B is a cross-sectional view of the cathode ray tube taken along lines  2 B— 2 B in FIG. 2A; 
     FIG. 3A is cross-sectional view of the cathode ray tube with a capacitor taken along lines  3 — 3  in FIG. 2B; 
     FIG. 3B is cross-sectional view of the cathode ray tube with another embodiment of the capacitor taken along lines  3 — 3  in FIG. 2B; 
     FIG. 4A is a diagram illustrating one embodiment of a pair of electrodes on an inner surface of a faceplate of the cathode ray tube; 
     FIG. 4B is a diagram illustrating another embodiment of a pair electrodes on an inner surface of the faceplate of the cathode ray tube; and 
     FIG. 4C is a diagram illustrating yet another embodiment of a pair electrodes on an inner surface of the faceplate of the cathode ray tube. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A system  10  for controlling an electron beam  12  in accordance with the present invention is illustrated in FIG.  1 . The system  10  includes an electron gun controller  14 , a cathode ray tube  16 , at least one capacitor  18 , optionally, a second capacitor  20 , and at least one pair of positioning electrodes  22  and  24 . The system and method provide a number of advantages including providing an effective and inexpensive system and method for detecting and controlling the position of an electron beam  12  in a cathode ray tube  16 . 
     It is to be noted that although FIGS. 1 and 2 show a pair of capacitors comprised of elements  18 ,  18 ( 2 ),  20 , and  20 ( 2 ), it is well within the scope of applicants&#39; invention to operate with only one capacitor when the electronics so permit. This will become evident in the discussion of the electrode pattern shown in FIG.  4 A. 
     Referring more specifically to FIGS. 1 and 2A,  2 B,  3 A, and  3 B, the system  10  includes the electron gun controller  14 , a deflection drive  26 , and an electron gun  28 . The electron gun controller  14  is coupled to the electron gun  28  in one end of the cathode ray tube  16 , to a second plate  18 ( 2 ) and  20 ( 2 ) of a pair of capacitors  18  and  20 , and to the deflection drive  26 . The electron gun controller  14  includes control circuitry used to control the position of the electron beam  12  and to correct for any errors in the electron beam&#39;s position based upon the electron beam&#39;s detected location. The electron gun controller  14  transmits control signals to the electron gun  28  and to the deflection drive  26  to, inter alia, control the intensity or brightness of and the position of the electron beam  12  generated by the electron gun  28 . Typically, the electron beam  12  has a current of only a few microamps and the high voltage applied between the cathode and anode in the cathode ray tube  16  is between about ten and thirty kilovolts. The general construction and operation of the electron gun controller  14 , the deflection drive  26 , and the electron gun  28  to generate and control the position of the electron beam  12  at horizontal and vertical locations are well known to those skilled in the art, such as those disclosed in U.S. Pat. No. 4,635,107 to Turner which has already been incorporated by reference, and thus will not be described in detail here. 
     Referring to FIGS. 1,  2 A, and  2 B, the system  10  includes the cathode ray tube  16 . The cathode ray tube  16  has a substantially, “funnel-like” shape or funnel  29  with a pair of opposing ends  30  and  32 , an inner surface  34 , and an outer surface  36 . The narrow end  30  of the cathode ray tube  16  houses the electron gun  28  which is coupled to the electron gun controller  14 . The electron gun  28  generates the electron beam  12  which is transmitted towards the other, wider end  32  of the funnel  29  of the cathode ray tube  16 . The deflection drive  26  is also located adjacent to the end  30  of the cathode ray tube  16  with the electron gun  28 . A faceplate  38  is secured to the other, wider end  32  of the cathode ray tube  16 . The faceplate  38  also has an inner surface  40  and an outer surface  42 . One or more phosphors, depending upon whether or not a monochrome or color screen is desired, are coated on the inner surface  40  of the faceplate  38  in a manner well known to those skilled in the art. A general discussion of the construction and operation of cathode ray tubes can be found in  The Cathode Ray Tube  by Peter A Keller, Palisades Press, New York, N.Y., 1991, which is herein incorporated by reference. 
     Referring to FIGS. 1,  2 A,  2 B,  3 A, and  3 B, the pair of capacitors  18  and  20  are located on opposite sides of the funnel  29  of the cathode ray tube  16  adjacent to the faceplate  38 . The capacitors  18  and  20  are shown separate from the cathode ray tube  16  in FIG. 1 simply for ease of illustration, but are actually formed on the cathode ray tube  16  as discussed below. A first capacitor plate  18 ( 1 ) and  20 ( 1 ) for each capacitor  18  and  20  is located inside the cathode ray tube  16  on the inner surface  34  of the funnel  29  of the cathode ray tube  16 . For ease of illustration, only a cross-sectional view of capacitor  20  is illustrated in FIGS. 3A and 3B, however capacitor  18  has an identical construction on the opposite side of funnel  29 . The first capacitor plates  18 ( 1 ) and  20 ( 1 ) are located on opposite sides of the cathode ray tube  16 . Second capacitor plates  18 ( 2 ) and  20 ( 2 ) are located on the outer surface  36  of the funnel  29 . Each of the second capacitor plates  18 ( 2 ) and  20 ( 2 ) is disposed substantially opposite one of the first capacitor plates  18 ( 1 ) and  20 ( 1 ). In this particular embodiment, the first and second capacitor plates  18 ( 1 ),  20 ( 1 ),  18 ( 2 ), and  20 ( 2 ) each have an area of about ten square inches each, although the size of the first and second capacitor plates  18 ( 1 ),  20 ( 1 ),  18 ( 2 ), and  20 ( 2 ) can vary as needed or desired. 
     The funnel  29  is typically formed of ceramic or glass and the funnel  29  acts as the dielectric between the first and second capacitor plates  18 ( 1 ),  20 ( 1 ),  18 ( 2 ), and  20 ( 2 ). In this particular embodiment, the funnel  29  is made of lead glass which has a dielectric constant of about eight to ten. 
     Each of the capacitors  18  and  20  is designed to withstand the high voltage differences typically found between the cathode and anode in the cathode ray tube  16 , i.e. in this particular embodiment a difference of about twenty-five Kilovolts. Typically, a capacitance of a few hundred picofarads is sufficient for each capacitor  18  and  20  to detect the position signal when the electron beam  12  strikes one of the positioning electrodes  22  and  24  coupled to the capacitor  18  and  20  by capacitive coupling of the electron beam  12 . 
     As shown in FIG. 3A, a coating  44  slurry of fine carbon in Na 2 SiO 3  paint and fire and is a conductive coating may coat the inner surface  34  of the funnel  29 , except for the regions where the first capacitor plate  20 ( 1 ) is located or an insulating layer  46  may be placed over the first capacitor plate  20 ( 1 ) inside the cathode ray tube  16  and then the coating  44  may cover the first capacitor plate  20 ( 1 ) as shown in FIG.  3 B. The coating may be of the conventional DAG used in CRT manufacture. It is generally a slurry of fine carbon particles in sodium silicate which may be painted on and fired. Although two embodiments are illustrated, other coating and insulating arrangements may also be used. 
     One of the features of the present invention is that the cost of the capacitors  18  and  20  is minimal, i.e. basically being just the cost of metallization to form the first and second capacitor plates  18 ( 1 ),  20 ( 1 ),  18 ( 2 ), and  20 ( 2 ) in and on the cathode ray tube  16  and the cost of lead attachments  48  and  50  coupling the first capacitor plates  18 ( 1 ) and  20 ( 1 ) each to one of each pair of positioning electrodes  22  and  24 , and is substantially less than the cost of the prior art technique using transformers. Additionally, by using capacitors  18  and  20 , rather than transformers, leads do not need to be passed through the cathode ray tube  16  to couple signals generated inside the cathode ray tube  16  externally. 
     Referring to FIGS. 4A,  4 B, and  4 C, three different embodiments for positioning electrodes  22  and  24  are illustrated. As shown in these figures, the inner surface  40  of the faceplate  38  includes at least one pair of positioning electrodes  22  and  24  which extend in a substantially horizontal direction across the inner surface  40  of the faceplate  38  and are separated be a gap or first distance  52 . In this particular embodiment, the gap or first distance  52  ranges between about 0.015 and 0.075. Phosphors (not shown) are coated on the inner surface  40  of the faceplate  38  between each pair of positioning electrodes  22  and  24 . By way of example, in a color screen or faceplate  38  the gap  52  between each pair of positioning electrodes  22  and  24 , a red substantially horizontal stripe of phosphor (not shown), a green substantially horizontal stripe of phosphor (not shown), and a blue substantially horizontal stripe of phosphor (not shown) are formed. One positioning electrode  22 ( 1 )- 22 ( 3 ) is coupled to one of the first capacitor plates  18 ( 1 ) and the other positioning electrode  24 ( 1 )- 24 ( 3 ) is coupled to the other first capacitor plate  20 ( 1 ) via leads  48  and  50 , respectively. Although only one pair of positioning electrodes  22  and  24  is shown in each example, the faceplate  38  can have more than one pair of positioning electrodes  22  and  24 . By way of example, a television screen may have about 480 pairs of positioning electrodes  22  and  24 . If multiple pairs of positioning electrodes  22  and  24  are used, one electrode  22  from each pair is coupled typically to a bus (not shown) which is coupled to one first capacitor plate  18 ( 1 ) and the other electrode  24  from each pair is also typically coupled to another bus (not shown) which is coupled to the other first capacitor plate  20 ( 1 ). 
     The positioning electrodes  22  and  24  may have a variety of different shapes. For example, as illustrated in FIGS. 4A,  4 B, and  4 C and also as discussed in copending patent application Serial No. 60/041,035, filed on Mar. 21, 1997 for a Mask-Free, Single Gun Color Television System, which is herein incorporated by reference, the edge  54  of each positioning electrode  22  and  24  facing its pair may have variety of shapes, such as a squared and stepped configuration, a sawtooth configuration, a substantially straight configuration, or a variety of other configurations as needed or desired. As discussed in greater detail below, if the electron beam  12  is not modulated then a patterned configuration, such as the squared and stepped configuration or the sawtooth configuration shown in FIGS. 4A and 4B, is formed in the edge  54  of each positioning electrode  22  and  24  facing the other to modulate the constant intensity electron beam  12 . If the electron beam  12  is modulated, then a substantially straight configuration may be used shown in FIG.  4 C. In this particular embodiment, the electron beam is modulated at a frequency of about ten MHz. 
     The operation of the system  10  and method for controlling the electron beam  12  will be discussed with reference to FIGS. 1-4. The electron gun controller  14  transmits control signals to the electron gun  28  in the cathode ray tube  16  to generate an electron beam  12 . The electron beam  12  is deflected in a desired direction by the deflection drive  26  in response to additional control signals from the electron gun controller  14 . Typically, the electron gun controller  14  in conjunction with the deflection drive  26  control the electron beam  12  to scan across the faceplate  38  in a pattern, such as a raster scan pattern or a serpentine pattern, as discussed in copending patent application Serial No. 60/041,035, filed on Mar. 21, 1997 for a Mask-Free, Single Gun Color Television System, which has already been incorporated by reference. To facilitate detection of the electron beam  12 , the electron beam  12  is modulated in intensity. Preferably, the electron beam  12  is modulated at a frequency of between about five to fifty MHz. Alternatively, the edge  54  of each positioning electrode  22  and  24  facing the gap  52  is patterned, such as the squared and stepped configuration or the sawtooth configuration illustrated in FIGS. 4A and 4B, so that the constant intensity electron beam  12  is converted to an AC signal at the capacitor  18  and  20 . 
     The electron beam  12  is directed to strike one of the stripes of phosphors between the positioning electrodes  22  and  24 . If the electron beam  12  misses the gap  52  and strikes one of a pair of the of positioning electrodes  22  and  24 , then the positioning electrode  22  and  24  which is struck converts the electron beam  12 , which is either modulated in intensity before striking the faceplate  38  or by the patterned positioning electrode  22  or  24 , into a first position signal. The first position signal is capacitively coupled via the capacitor  18  or  20  coupled to the positioning electrode  22  or  24  to the electron gun controller  14 . The first position signal is separated from the high DC voltage required to generate the electron beam  12  which typically ranges between ten and thirty kilovolts. The other of the pair of positioning electrodes  22  or  24 , will generate a second position signal that indicates that it was not struck by the electron beam  12 . The second position signal will be capacitively coupled via the other capacitor  18  or  20  to the electron gun controller  14 . 
     The electron gun controller  14  receives the first and second position signals and in response to these position signals, transmits control signals to the electron gun  28  and to the deflection drive  26  to adjust the position of the electron beam  12  to the gap between the positioning electrodes  22  and  24  to strike the phosphors. By way of example, if the electron beam  12  strikes positioning electrode  22 , then the first position signal will be high and the second position signal will be low. As a result, the electron gun controller  14  will transmit control signals so that the electron beam  12  is directed down towards the gap  52 . If the electron beam  12  strikes positioning electrode  24 , then the second position signal will be high and the first position signal will be low. As a result, the electron gun controller  14  will transmit control signals so that the electron beam  12  is directed up towards the gap  52 . If the electron beam  12  strikes the gap  52 , then the first and second position signals will be low. As a result, the electron gun controller  14  will not adjust the position of electron beam  12 . Accordingly, the present invention provides an inexpensive and effective method for controlling the position of the electron beam in a cathode ray tube  16  without a shadow mask. 
     Having thus described the basic concept of the invention, it will be readily apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These modifications, alterations and improvements are intended to be suggested hereby, and are within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims and equivalents thereto.