WO1997001793A1

WO1997001793A1 - Visual indicator of ground fault in grounding screen for cathode ray tube

Info

Publication number: WO1997001793A1
Application number: PCT/US1996/009820
Authority: WO
Inventors: Eugene Waite Chase; Jayantilal Shamjibhai Patel
Original assignee: Bell Communications Research, Inc.
Priority date: 1995-06-29
Filing date: 1996-06-11
Publication date: 1997-01-16
Also published as: TW298700B

Abstract

A passive fault indicator for the grounding link of a protective screen placed in front of a cathode ray tube (CRT) to prevent shock to the operator. The conductor of the protective screen is connected to ground through a grounding link. A real-time detector is always placed in the conductor to detect whether the grounding link and its connections have been maintained. Preferably, the detector includes a display that immediately notifies the operator of a ground interrupt. Preferably, also, the detector and display are passive. Such a passive fault indicator can be achieved with a liquid-crystal display (LCD) placed in parallel with a load resistor in the grounding link. The voltages and currents induced on the protective screen by the CRT are sufficient to power such a fault indicator. More preferably, the LCD displays a visual message when the grounding link has been interrupted. Such a message can be achieved by differential rubbing alignment of one of the alignment layers in a twisted nematic LCD.

Description

Visual Indicator of Ground Fault in Grounding Screen for Cathode Ray Tube

Field of the Invention This invention relates generally both to liquid crystals and to screens for cathode ray tubes and other high-potential displays. In particular, this invention relates to an indicator, preferably passive, of a grounding fault in a CRT filter screen.

Background Art

Cathode ray tubes (CRTs) are extensively used as display devices. For a long time, they have found a large market in television displays. Recently, however, they have become equally ubiquitous as computer displays or monitors, both in the office and in the home. Many corporate employees spend their entire workday in front of a CRT, often in customer telephone service.

Many computer operators complain about the glare from their CRT screens. As a result, anti-glare screens for CRTs have become commercially available.

CRT display screens have been recognized to develop relatively high potentials with respect to the earth ground, and measures have been developed to avoid harmful shocks from such high potentials, both to the human operator and to electrostatically sensitive electronic equipment, such as computers. Conceptually, the monitor is enclosed in a grounded Faraday cage. For the computer user, this reduces to providing a grounded transparent conductor Iayer in front of the CRT screen. The conductor Iayer may be a very thin metallic Iayer, a somewhat thicker Iayer of a nearly transparent conductor such as indium tin oxide, or a grid of nearly invisible wires. The conductor is then connected via a lead to a convenient grounding point, which may be associated with the computer terminal or may be separately available in the office. A grounding screen may be additionally adapted to reduce the optical glare from the monitor. Such grounding screens are widely

-1- available. One such protective screen, the Glare Sentry Plus, is available from INMAC, but many others provide substantially equivalent screening.

As illustrated in the pictorial view of FIG. 1, a screen assembly 10 including such a grounding screen 32 is placed directiy adjacent to a CRT display face at the front of a CRT display cabinet 12 and to protect a human operator 14 from the CRT. By various means, the screen 32 is both transparent or at least semi-transparent to the operator and is electrically grounded or otherwise connected to a fixed potential so as to ground any excess charge. Typically, the screen 32 is so closely adjacent to the CRT face 30 that it is capacitively coupled to the CRT display 30 so that its grounding prevents the CRT screen 30 from becoming charged.

However, the above operation requires that the grounding connection from the grounding screen 32, as well as other functional characteristics, not be degraded. For instance, if the ground connection 36 be somehow disconnected, although upon visual inspection the grounding screen 32 is well installed, it will not provide the desired level of protection. There is usually no apparent indication of a failed grounding screen 32. Incidents of failed grounding screens have been reported, and failure analysis revealed that a metallic U-spring connector to the glass was open circuited. Other possible failures are a poor ground, poor connection to that ground, and a cut or severely degraded grounding link. These types of problems have been discussed by Franey et al. In "Field Induced ESD from CRTs: Its Cause and Cure," 1994 EOS/ESD Symposium Proceedings, EOS-16, pp. 42-46.

It would, of course, be possible to periodically test continuity of the grounding link. However, such testing requires some level of technical competence and requires time. The typical equipment required for testing a grounding fault, though modest, involves active electronics and adherence to a fixed testing schedule and procedure.

-2- Summary of the Invention

Accordingly, an object of the invention is to assure grounding of a CRT filter screen.

A further object of the invention is to provide passive and automatic detection of a grounding fault.

The invention can be summarized as a voltage detector inserted into the grounding lead connecting the CRT filter screen to ground or other predetermined potential. The natural CRT scanning creates a significant potential on the filter screen. If the grounding link is somehow interrupted, the grounding side of the detector is left floating and the voltage across the detector rises above a predetermined maximum of the current falls below a predetermined minimum. Preferably, the ground fault detector is a passive liquid-crystal display which changes its visual appearance when the threshold is passed. Even more preferably, the passive liquid-crystal display provides a visual message of an ineffective grounding path.

Brief Description ofthe Drawings

FIG. 1 is a pictorial representation of a grounded CRT filter screen and its use.

FIG. 2 is a cross-sectional and schematic view of the ground fault indicator of the invention.

FIG. 3 is a perspective view of a computer terminal with attached grounding screen.

FIG. 4 is a cross-sectional and schematic view of an embodiment of the invention including a CRT grounding screen and associated visual indicator incorporating a liquid-crystal display.

FIG. 5 is an orthographic view of a reflective liquid-crystal display usable with the invention.

FIGS. 6 and 7 are schematic representations of two states of a visual fault indicator of the invention. FIG. 8 is a plan view of the two alignment layers of another embodiment of

SUBSIiTsiTE SHEET RULE2 the invention including an explicit fault message.

Detailed Description ofthe Preferred Embodiments

Figure 2 schematically illustrates a conventional cathode ray tube (CRT) 20 in which a cathode 22 is connected to a high-voltage DC source 23, of the order of 20kV, so as to thermionically emit electrons which are accelerated toward a grid 24 held at a much lower voltage. A magnetic yoke or electrostatic plates 26 direct the high energy electrons into an electron beam 28 toward the CRT display screen 30 and steers that beam 28 in two orthogonal directions peφendicular to the electron beam propagation direction. A typical raster scan operates at a 60Hz vertical frequency (the refresh rate for the screen 30) and a 35kHz horizontal scanning frequency. The high-energy electrons impinge on the CRT screen 30 which contains phosphors, possibly of different colors, that emit a two-dimensional display.

The CRT filter screen 32 is placed closely adjacent to the front of the CRT display face 30. In a later described embodiment, the filter screen 32 may alternatively be integrated into the CRT display 30. As well as reducing glare, the filter screen 32 has an electrical Iayer that is connected through a junction 34 to the grounding lead 36 ultimately connected to ground 38 or other predetermined potential. According to the invention, the grounding lead 36 includes an electrical power detector 40 that detects whether the power impressed upon the grounding lead 36 is ultimately being grounded or whether the grounding lead 36 has been somehow interrupted.

In view of high cathod voltages, it is not inconceivable that the electrical potential levels can reach high levels. Therefore, the electrical power meter 40 may be a voltmeter that determines whether the voltage is maintained below a predetermined and low maximum level. Alternatively, the electrical power meter 40 may measure the electrical current being grounded through the ground lead 36. If the grounding is interrupted, the grounding current falls below a predetermined minimum. The electrical power meter 40 may be a passive device without other electrical inputs or may be electrically powered by secondary sources, as is typical for detectors of low-level voltage.

-4-

SUBSTmiTE SHEET (KϋlE 26) The results of the measurement of the electrical power meter 40 are preferably displayed, as illustrated pictorially in FIG. 3, on a small display 50 integrated with the CRT filter screen assembly 10 affixed to the CRT cabinet 12 and including a CRT filter screen 32 positioned closely in front of the CRT display 30. As illustrated, the CRT filter screen 32 is grounded and the grounding must be maintained.

The configuration of the ground fault display 50 of FIG. 3 is preferably implemented without the input of additional electrical power. That is, the display 50 is preferably completely passive except for its proximity to the front of the CRT cabinet 12. We have observed that the electron beam 28 of FIG. 4 irradiating the CRT display screen 30 and ultimately inducing charge on the filter screen 32 is sufficient to power a ground failure display.

In particular, as illustrated schematically in FIG. 4, the grounding lead 36 is connected to ground 38 through a parallel circuit of a load resistor 60 and the liquid- crystal display 50 connected between junctions 64 and 66. We have observed voltages of the order of 1 volt induced by the CRT 20 across moderately sized load resistors. This voltage is sufficient to switch a typical liquid crystal display 50 from its passive to its active state, i.e., between its dark and light states. The liquid-crystal display itself consumes very little power and operates at the voltages we have observed on CRT protective screens. A typical liquid-crystal device, such as the twisted nematic structure commonly employed for watches and calculators, requires that the voltage exceed a certain threshold value, called the Freedericks voltage, a value that is typically around one or two volts. Such a display also should have a capacitance of less than 100pF. Other liquid-crystal non-threshold devices will work with both lower voltages and higher capacitances. The liquid-crystal display has the further advantage that it preferably operates at an AC biasing frequency of a few kilohertz, close to the CRT horizontal and vertical scan frequencies.

To demonstrate the available power, we inserted a resistor 60 into the grounding lead 36, and an oscilloscope measured the voltage trace across the resistor 60. Different amplitudes were observed for a DOS display of a few characters on a generally dark CRT screen and for a Windows^™ display which was denser and brighter. The Windows software is expected to induce more charge. For the Windows display, the resistor 60 was selected to be 1MΩ, and waveforms were observed having peak-to-peak amplitudes of about 1.2V with a period of about 16ms, corresponding to the 60Hz vertical scan, and of about 1.1 V with a period of 32μs, corresponding to a horizontal scan rate of about 31kHz. For the DOS display, the resistor 60 was selected to be 10MΩ, and waveforms were observed with amplitudes of about 1V at both these same periods.

A simple LCD 50 usable with the invention is illustrated in perspective and schematically in FIG. 5. It includes a nematic liquid crystal 72 filled into a gap between two orthogonally aligned alignment layers 74 and 76. The alignment layers 74 and 76 are coated with a homogeneous aligning material and are brushed or otherwise conditioned in respective perpendicular directions. Two semi-transparent electrodes 78 and 80, e.g., of indium tin oxide, are disposed outside of the alignment layers 74 and 76 so as to pass light 81 into the interior of the LCD cell 50. Polarizers 82 and 84 disposed outside of the electrodes 78 and 80 have polarization-passing directions in parallel to the adjacent alignment layers 74 and 76. A reflector 86 is disposed outside of the lower polarizer 84. Typically, two unillustrated substrates, such as glass plates, are disposed to have the liquid crystal 72 therebetween and are coated with the alignment layers 74 and 76 and electrodes 78. The polarizers 82 and 84 are disposed on the outsides of the glass plates with the reflector 86 affixed to the outside of the lower polarizer 84. A kilohertz AC voltage source 88 is connected between the two electrode layers 78 and 80 at terminals 64 and 66 respectively of the electrical power detector 40 of FIG. 4, and the AC voltage source 88 is modulated by a data line 89. When the LCD cell 50 is used as a ground fault detector, the modulated AC voltage source 88 is the parallel resistor 60. In the absence of an applied field from the voltage source 88, the homogenous nature of the alignment layers 74 and 76 prevails so that the liquid- crystal director is everywhere parallel to the plane of the cell 50 since the liquid- crystal molecules are aligned with the adjacent alignment layer 74 or 76, and the liquid-crystal director twists by 90° in the cell gap. As a result, light that is passed by the top polarizer 82 is waveguided and its polarization twisted by the liquid crystal 72

-6-

SUBS .EStiEET I.B so as to be passed by the bottom polarizer 84 whence the reflector 86 returns it in the reverse path. Unselected light is absorbed or scattered by the polarizers. Thereby, a bright image is produced. However, in the presence of a normal electric field applied by the voltage source 88, the liquid-crystal molecules of positive dielectric anisotropy line up with the electric field so as to destroy any waveguiding. Whatever light is passed by the first polarizer 82 is blocked by the second polarizer 84, and the image is dark.

In a typical twisted nematic liquid-crystal display, the director of the liquid crystal 72 is aligned in parallel to the adjacent respective alignment layer 74 or 76 but is mutually perpendicular at the two alignment layers 74 and 76. In the absence of an applied electric field, the director gently twists between the alignment layers 74 and 76 while maintaining its parallelism to the cell plane. If the twist has a sufficiently long period, as characterized by the Mauguin condition involving the cell gap and the difference in refractive indices in two orthogonal directions in the liquid crystal, the liquid crystal waveguides the incident light so as to rotate the light's polarization by 90° as it traverses the cell. However, if a voltage of the order of a volt is applied by the electrodes 78 and 80, the liquid-crystal director is aligned with the electric field across the bulk of the cell. As a result, the light is not waveguided and its polarization is not rotated. Thus, the display 50 of FIG. 5 is reflective and bright in its active state when a voltage is applied between the junctions 64 and 66 and is dark in its passive state when no voltage appears between the junctions 64 and 66.

However, in view of the fault detector's consumer application, it is desired that the occurrence of a ground fault be immediately evident without further consideration to the operator in front of the display 50. Whether the fault display 50 is dark or bright is not considered sufficient to notify the operator who is not necessarily looking for this difference or remembers which state indicates what. For example, as illustrated in FIG. 6, the fault display 50 may display a confirmatory message ("OK") when the grounding link 36 is operating correctly and that a different message, such as a character 56, e.g., "X", as illustrated in FIG. 7, is impressed over the "OK" message when the grounding link 36 is interrupted.

Such a display should have a message presented to the operator at least

-7-

Γ when the liquid-crystal display 50 is in its passive or unbiased state. This functionality requires that the LCD have portions that are distinguishable from neighboring portions and that the visual characteristics of the LCD change as a function of the applied voltage. One method of achieving a passive LCD message is illustrated schematically in FIG. 8 which shows a pair of alignment layers 90 and 92 that would be used in the twisted nematic cell 50 of FIG. 5. The background area 94 of the first alignment Iayer 90 and the entire second alignment Iayer 92 are brushed in the same direction so that they are aligned in parallel. However, in the areas 96 of the characters spelling out the desired message, e.g. "SCREEN FAILURE" or other graphical representation of a failure, the first alignment Iayer 90 is brushed in a peφendicular direction. Patel describes a method of differentially aligning an alignment layer in U.S. Patents 5,111,321 and 5,150,236, which additionally contain or reference descriptions of the desired liquid-crystal display and methods of making it. The resultant LCD causes the background area 94 to always be in one state, dark in the embodiment of FIG. 5, regardless of the voltage impressed across the electrical junctions 64 and 66 of the cell 50 because no deflective waveguiding and polarization twisting occurs in either state. However, the areas 96 of the failure message are dark when a sufficiently high voltage is impressed across the cell 50 but is bright when there is no appreciable voltage because the grounding path has been interrupted.

Example 1 A liquid-crystal display having the configuration of that of FIG. 8 was built and tested. However, rather than incorporating the reflective configuration of FIG. 5, it was operated in the back-lit or transmission mode; that is, it was illuminated from one side and observed from the other. Polarizers were placed on the input and output sides.

In one configuration, the polarizers were arranged perpendicularly, corresponding in general to the configuration of FIG. 5. The LCD displayed the failure message of FIG. 8 with bright characters on a dark background when the LCD was disconnected from the grounding path from an operating CRT so that no

-8-

SIIBSTTH SHEET RyLE26) voltage was applied across it. However, when the LCD was connected across a bias resistor to a properly connected grounding screen in front of an operating CRT, the LCD display was moderately uniform and fairly bright.

In the other configuration, the polarizers were arranged in parallel. In the absence of applied voltage, the LCD displayed the failure message of dark characters on a lighter background. When connected in a properly functioning grounding path, the LCD was uniformly dark.

Example 2 In a second example, the configuration of FIGS. 6 and 7 was built and tested. Two messages "OK" and an overlying "X" were patterned into one of the alignment layers and differentially rubbed. The interiors of the messages in the first alignment Iayer were rubbed in one direction, while the remainder of the first alignment Iayer and the entire second alignment Iayer were rubbed in the perpendicular direction. One of the indium tin oxide electrodes, preferably the one adjacent to the uniform alignment Iayer was patterned to form the "X" message in registry with the corresponding message in the alignment Iayer. As a result, in the absence of an applied electric field, i.e., the fault state, both messages were displayed. However, in the presence of an applied electric field, i.e. the non-fault state, only the message "OK" was displayed. The LCD so fabricated displayed both the first and second messages, that is, an "OK" in a box covered with an "X", when no power is applied to the LCD. However, when sufficient voltage was applied to the LCD, as in a properly connected grounding screen, only the "OK" and the surrounding box appeared. It is understood that, although a separate optional grounding screen is descπbed above, the invention is equally applicable to grounding screens that are integrally formed with the display screen, for example, by being deposited on the CRT face. It is also understood that the invention applies not only to the conventional cathode ray tube but also to field-emission flat-panel displays and other displays in which one or more cathodes or other electron sources emit high energy electrons toward the display screen.

The invention thus provides an easy and inexpensive way of assuring that a

-9-

SUBST1TUTE SHEET (RULE 26) CRT grounding screen is properly connected.

Claims

Claims What is claimed is:

1. A visual fault indicator connected in an electrical line between a screen adjacent to a CRT display face and a predetermined potential during a normal operation of said CRT display to indicate whether said screen is properly connected to said predetermined potential.

2. A visual fault indicator as recited in Claim 1 , wherein said predetermined potential is ground.

3. A visual fault indicator as recited in Claim 1 , wherein said screen acts as an electrostatic shield between said display face and an operator facing said display face.

4. A visual fault indicator as recited in Claim 1 , wherein said indicator receives no power in addition to power received from said screen and said CRT display face.

5. A visual fault indicator as recited in Claim 4, wherein said indicator comprises a liquid-crystal display connected in said electrical line.

6. A visual fault indicator as recited in Claim 5, wherein said liquid-crystal display displays a graphical message when said screen is not connected to said predetermined potential.

7. A liquid-crystal fault indicator for a grounding screen placed adjacent to a

CRT display screen and connectable to a predetermined potential through a grounding link, comprising a liquid-crystal display having two electrode layers sandwiching a liquid-crystal Iayer, one of said electrode layers being connected to said grounding screen through a first portion of said grounding link and a second of

-11-

T RULE 26 said electrode layers being connected to said predetermined potential through a second portion of said ground link.

8. A liquid-crystal fault indicator as recited in Claim 7, further comprising a load resistor placed in parallel to said liquid-crystal display to provide a switching voltage for said liquid-crystal display from power delivered to said screen from said CRT display face.

9. A liquid-crystal fault indicator as recited in Claim 8, wherein said liquid- crystal display includes a visually readable message when said grounding link is interrupted.

10. A liquid-crystal fault indicator as recited in Claim 8, wherein said liquid- crystal display includes at least one alignment Iayer having differentially aligned portions so as to create a graphic in only one of two states of said liquid-crystal display determined by a voltage applied across said liquid-crystal display.

11. A liquid-crystal fault indicator as recited in Claim 8, wherein said liquid- crystal display includes a nematic liquid crystal.

12. A liquid-crystal fault indicator as recited in Claim 7, wherein said liquid- crystal display comprises two alignment layers sandwiching said liquid-crystal Iayer and one of which is differentially aligned to form a first message.

13. A liquid-crystal fault indicator as recited in Claim 12, wherein at least one of said two alignment layers is patterned to form a second message.

14. A liquid-crystal fault indicator as recited in Claim 7, wherein at least one of said two alignment layers is patterned to form a message.

15. A liquid-crystal fault indicator as recited in Claim 7, wherein said

-12-

SUBSTTTUTE SHEET RULE 26 grounding screen is integrally formed in said display screen.

16. A liquid-crystal fault indicator as recited in Claim 7, wherein said liquid- crystal display is formed in a holder for said grounding screen.