FR2651921A1 - ELECTRONIC BEAM DEVIATOR WITH MAGNETIC FIELD OF CORRECTION AND AUXILIARY MAGNETIC SHIELD INCORPORATED. - Google Patents

Abstract

The invention relates to an improved electron beam deflector with incorporated permanent image correcting magnets characterized in that said magnets and the magnetic field generated by these magnets are surrounded by an auxiliary magnetic shield integral with said deflector and positioned with a high geometric precision with respect to said magnets. This auxiliary shielding makes the image correction efficiency insensitive to the shape and positioning geometry of the metal elements in the vicinity of the deflector during its assembly and operation. Application to image tubes and more particularly to cathode ray tubes.

Description

The present invention relates to electronic image tubes, and more

  precisely beam deflectors

  associated with these tubes.

  Electronic image tubes concerned by the invention are electron beam tubes in which the deflection of the beam is used, either to draw the image on a display screen, in the case of cathode ray tubes (CRTs), or for image capture in the case of

camera tubes.

  Remember that a TRC consists of a vacuum chamber in which there is an electron gun to form an electron beam, and a display screen which emits light when struck by the beam. Means for modulating and deflecting said beam make it possible to draw the image; these means can be inside or

the outside of the vacuum chamber.

  The electron beam can be modulated or deflected by electromagnetic fields; said deflection and modulation means are, in general, devices for creating

  electromagnetic fields appropriate to obtain the desired image.

  If the modulation and the electrostatic deflection are often obtained using electrodes carried at high voltages and placed inside the tube, the electromagnetic deflection is generally obtained using magnets,

  electromagnets, or coils mounted outside the tube.

  The geometric accuracy of the image depends on the modulation and deviation accuracy obtained by the means employed, and can be degraded by spurious electromagnetic fields from the tube environment when installed in equipment. For this reason, CRTs intended to operate in complex environments are generally surrounded by an electromagnetic shielding device designed to prevent the penetration of surrounding electromagnetic fields into the interior of the tube and

  thus avoid disturbances of the image.

  This electromagnetic shielding device can be an envelope of good electrically conductive material and high magnetic permeability. Such electromagnetic shielding means are commonly used for onboard TRCs, for example, on board aircraft, tanks, ships or helicopters. In some advanced TRCs known from the prior art, corrections to the geometry of the image are applied using electromagnetic fields provided by permanent magnets. The settings of these correction fields are made at the factory, and consist in positioning the magnets

  permanent to obtain the desired image geometry.

  These settings, to be effective in the operating conditions of the TRC, must be performed under similar conditions of electromagnetic environment, namely: the TRC must be surrounded by its shielding

  electromagnetic during these correction field settings.

  This because, as we know, the magnetic fields are disturbed by the metal masses that surround them. Thus the image geometry correction obtained depends on the relative position of the magnets and the shield; this positioning becomes critical for the quality of the image; This has several drawbacks for the industrial manufacture of shielded CRT assemblies, among which the drawbacks indicated below. The manufacture of TRCs rather the glassware, and the shielding rather the sheet metal, often the latter is entrusted to a subcontractor by the tube manufacturer. To make correction adjustments, the manufacturer of the CRTs must have the shields and therefore must store them. If a different shield in geometry is used, the correction field of the magnets must be modified. A deflector provided for a shield can not be used with another shield of different shape. This implies that the manufacturer must make different shields for each type of TRC, so often for a large number of different TRCs. The different diverters will also be of the same number as the number of different CRTs and different shields. If the deflector and the shield are not perfectly aligned, the action of "shunt" of the electromagnetic field of the magnets by the metal of the shielding is asymmetrical, which

  introduces a distortion of the image.

  The object of the present invention is to overcome these drawbacks by proposing a deflector with field correction by permanent magnets which is not sensitive to the shape or the relative geometry of mounting the electromagnetic shielding. The device according to the invention avoids either making long adjustments by the addition of additional magnets, or individualized settings depending on the shielding. The device according to the invention makes it possible to use practically any geometry and form of shielding and thus makes it possible to reduce the number of deviators considerably.

  different used by a TRC manufacturer.

  To achieve these aims, the invention proposes an electronic beam deflector with correction magnetic field supplied by permanent magnets surrounded by an auxiliary electromagnetic shield integral with said deflector, said shielding assembled on the periphery of said magnets and mounted with

  a high geometric precision with respect to these.

  The presence of this auxiliary shield around the correction magnets according to the invention has the effect of isolating the fields generated by these magnets from any disturbance external to the deflector, and in particular the influence of the main electromagnetic shielding of the TRC, due to the geometry of his

shape or its positioning.

  A very sensitive saving of assembly time o the number of spare parts (deviators, auxiliary magnets, ...) to be used is thus obtained. Other characteristic objects and results of

  the invention will emerge from the following description given to

  As a non-limitative example and illustrated by the appended figures which represent: FIG. 1a, a sectional view of the main shield assembly of the beam deflection CRT (with the permanent image correction magnets incorporated), and TRC vacuum chamber in symmetrical arrangement according to the known art, with a schematic view of the resulting correction, in FIG. 1b; FIG. 2a, a sectional view of the main shielding, deflection assembly (with permanent magnets); incorporated correction), and the vacuum chamber of the TRC according to the known art in slightly asymmetrical arrangement, with a schematic view of the resulting (defective) correction, in Figure 2b, - Figure 3, a schematic longitudinal sectional view of the main shielding assembly of the CRT, a deflector according to the invention with its incorporated permanent correction magnets and its auxiliary electromagnetic shielding according to the invention,

arranged around a bare CRT.

  In the different figures, the same references designate the same elements, but, for the sake of clarity, the sides

  and proportions of the various elements are not respected.

  FIG. 1 shows a diagrammatic view, in section, of a main shielding assembly (4) of the CRT, beam deflector (2) with these incorporated permanent image correction magnets (3), and the enclosure (1). vacuum TRC according to the known symmetrical and therefore concentric arrangement. This represents the ideal configuration because it is the simplest to give a satisfactory correction, as shown in FIG. In FIG. 1a, the main screen (4) of the TRC, of circular section, is positioned concentrically around the deflector (2), which is also of circular section. The deflector (2) comprises permanent image correction magnets (3) arranged symmetrically about the same geometric center as the shield (4) and the deflector (2). Inside, the deflector (2) and concentric

  with this one is the vacuum chamber (1).

  In FIG. 1b, the dotted line (5) shows the image of the uncorrected CRT and in solid line (6) the image corrected by

  the symmetrical system shown in section in Figure la.

  The uncorrected image is not rectangular as it should be, but on the contrary it looks like a "cushion": slightly narrowed in the middle of its four sides and pointed at the four corners. The corrected image (6) using magnets (3) correction perfectly positioned relative to the shield (4)

  is square as shown by the solid line (6).

  Figure 2 is identical to Figure i with one difference: the geometry is not perfectly symmetrical. In FIG. 2a it can be seen that the main shield (4) of the TRC is not concentric with the deflector assembly (2), permanent magnets (3) for correction, and the vacuum enclosure (1), therefore the right magnet is closer to the shield than the magnet of

  left. This could be the result of a manufacturing defect.

  In FIG. 2b, the uncorrected image (5) and the desired corrected image (6) as shown in FIG. 1b are dashed and the mis-corrected image resulting from the asymmetry of the assembly as shown in Figure 2a. In the specific example shown in Figure 2, the symmetry is conserved around the horizontal median plane (which contains the TRC axis); and it can be seen that the resulting image correction remains satisfactory for the horizontal lines of the image which are straight. On the other hand, the vertical lines are in curves by the dissymmetry right / left of the

correction geometry.

  The dissymmetry of the example of Figure 2 is among the simplest that can be imagined; the dissymmetry that could be introduced by the hazards of large-scale industrial manufacturing would be likely to prove much more complicated. It is easy to see how critical assembly becomes critical for ensuring

rectilinear geometry of the image.

  The object of the invention is to make this part of the assembly less critical and thus to ensure better

  image correction more easily in industrial production.

  FIG. 3 shows in section an example of the system according to the invention, where the same elements arranged in a geometry similar to that of FIG. 1, with the exception of an auxiliary shield, made of a material of high permeability are seen; magnetic (mu-metal, ferrite ...) (8) fixed around the deflector (2) and more particularly around its permanent magnets incorporated (3) image correctors, and positioned with a

  high accuracy compared to these.

  The assembly accuracy of the set TRC (1), deflector (2), shielding (4) becomes less critical for image correction when the assembly precision of the deflector assembly (2), permanent magnets ( 3), auxiliary shield (8) is satisfactory because the magnetic field lines due to these permanent magnets (3) are trapped inside the auxiliary shield (8) and therefore remain insensitive to what is

  goes outside of it.

  In particular, not only the position of the main shield (4) becomes non-critical, but also its shape which has no influence on the isolated magnetic correction fields

  inside their auxiliary shield (8).

  This results in a very sensible economic time of assembly, and an economy of the number of types of deviators to

use can also be obtained.

R E V E N D I CATI Y N S

  1. Diverter (2) of electron beam with correction magnetic field provided by permanent magnets (3), characterized in that said deflector comprises a magnetic shield (8) said "auxiliary" integral deviator said, said magnetic shield being assembled on the periphery of said permanent magnets and positioned with great accuracy

  geometric in relation to these.

  2. Electronic tube (1) comprising an electron beam deflector according to claim 1 and a shielding

around the deflector.

  Electronic tube according to claim 2,

  characterized in that it constitutes a cathode ray tube.

  4. An electron tube according to claim 2,

  characterized in that it constitutes a camera tube.