EP0418126A1 - Deflection yoke with corrective magnetic field and with an auxilary magnetic shield - Google Patents

Abstract

The subject of the invention is an improved electron beam deflector (2) with incorporated permanent magnets (3) for image correction, characterised in that the said magnets (3) and the magnetic field generated by these magnets (3) are surrounded by an auxiliary magnetic shield (4) integral with the said deflector (2) and positioned with high geometric precision with respect to the said magnets (3). This auxiliary shield (4) renders the effectiveness of image correction insensitive to the form and to the geometry of positioning of the metal elements (8) in the vicinity of the deflector (2) during its setting-up and operating. Application to image tubes and more particularly to cathode ray tubes. <IMAGE>

Description

The present invention relates to electronic picture tubes, and more specifically to electron beam deflectors associated with these tubes.

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

Recall that a TRC consists of a vacuum chamber in which is an electron gun to form an electron beam, and a display screen which emits light when struck by the beam. Modulation and deflection means of said beam make it possible to draw the image; these means can be found inside or outside the vacuum enclosure.

The electron beam can be modulated or deflected by electromagnetic fields; said deflection and modulation means are therefore, in general, devices for creating suitable electromagnetic fields to obtain the desired image.

If the modulation and the electrostatic deflection are often obtained using electrodes brought to high voltages and placed inside the tube, the electromagnetic deflection is generally obtained using magnets, electromagnets , or coils mounted outside the tube.

The geometric precision of the image depends on the modulation and deviation precision obtained by the means used, and can be degraded by parasitic electromagnetic fields coming from the environment of the tube when it is installed in equipment. For this reason, TRCs intended to operate in complex environments are generally surrounded by an electromagnetic shielding device designed to prevent the penetration of the surrounding electromagnetic fields inside the tube and thus avoid disturbances of the image.

This electromagnetic shielding device can be a casing made of a material which is a good conductor of electricity and of high magnetic permeability. Such electromagnetic shielding means are commonly used for on-board TRCs, for example, on board airplanes, tanks, ships or helicopters.

In certain improved TRCs known from the prior art, corrections to the geometry of the image are applied using electromagnetic fields supplied by permanent magnets. The adjustments of these correction fields are carried out at the factory, and consist in positioning the permanent magnets so as to obtain the desired image geometry. These adjustments, to be effective in the operating conditions of the TRC, must be carried out under similar conditions of electromagnetic environment, namely: the TRC must be surrounded by its electromagnetic shielding during these adjustments of correction fields. This is because, as we know, magnetic fields are disturbed by the metallic masses which surround them. So the correction of the image geometry obtained depends on the relative position of the magnets and the shielding; this positioning becomes critical for the quality of the image.

This has several drawbacks for the industrial manufacture of armored TRC assemblies, among which the drawbacks indicated below. The manufacture of the TRC rather relating to the glassware, and that of the shielding rather relating to the sheet metal work, often this last is entrusted to a subcontractor by the manufacturer of the tubes. To make the correction adjustments, the manufacturer of the TRCs must have the shields and so must store them. If a different geometry shield is used, the magnet correction field must be modified. A diverter intended for a shield cannot be used with another shield of different shape. This implies that the manufacturer must manufacture different shields for each type of TRC, therefore often for a large number of different TRCs. The different deflectors will also be the same number as the number of different TRCs 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 shield is asymmetrical, which introduces a distortion of the image.

The object of the present invention is to remedy these drawbacks by proposing a deflector with field correction by permanent magnets which is not sensitive either to the shape or to the relative mounting geometry of the electromagnetic shielding.

The device according to the invention makes it possible to avoid either making long adjustments by the addition of additional magnets, or individual adjustments depending on the shields. The device according to the invention makes it possible to use practically any geometry and form of shielding and therefore makes it possible to considerably reduce the number of different deflectors used by a manufacturer of TRC.

To achieve these goals, the invention provides an electron beam deflector with a magnetic correction field provided by permanent magnets surrounded by an auxiliary electromagnetic shield integral with said deflector, this said shield assembled on the periphery of said magnets and mounted with high geometric precision. compared to these.

The presence of this auxiliary shielding 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 from the influence of the shielding. main electromagnetic of the TRC, due to the geometry of its shape or its positioning.

A very significant saving in assembly times where the number of spare parts (deflectors, auxiliary magnets, ...) to be used is thus obtained.

• - la figure 1a, une vue en section de l'ensemble blindage principal du TRC déviateur de faisceau (avec les aimants permanents de correction d'image incorporés), et l'enceinte à vide du TRC en disposition symétrique selon l'art connu, avec une vue schématique de la correction résultante, en figure 1b,
• - la figure 2a, une vue en section de l'ensemble blindage principal, déviateur (avec les aimants permanents de correction incorporés), et l'enceinte à vide du TRC selon l'art connu en disposition légèrement dissymétrique, avec une vue schématique de la correction (défectueuse) résultante, en figure 2b,
• - la figure 3, une vue schématique en coupe longitudinale de l'ensemble blindage principal du TRC, déviateur selon l'invention avec ses aimants permanents de correction incorporés et son blindage électromagnétique auxiliaire selon l'invention, disposés autour d'un TRC nu.

Other characteristic objects and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• FIG. 1a, a sectional view of the main shielding assembly of the beam deflecting TRC (with the permanent magnets for image correction incorporated), and the vacuum enclosure of the TRC in symmetrical arrangement according to known art, with a schematic view of the resulting correction, in FIG. 1b,
• - Figure 2a, a sectional view of the main shield assembly, deflector (with permanent correction magnets incorporated), 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 view in longitudinal section of the main shield assembly of the TRC, deflector according to the invention with its permanent correction magnets incorporated and its auxiliary electromagnetic shielding according to the invention, arranged around a bare TRC.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the sides and proportions of the various elements are not respected.

FIG. 1a shows a schematic view, in section, of a main shielding assembly (4) of the TRC, beam deflector (2) with these permanent magnets (3) of incorporated image correction, and the enclosure (1) vacuum of the TRC according to the known art symmetrical and therefore concentric arrangement. This represents the ideal configuration because it is the simplest to give a satisfactory correction, as shown in Figure 1b.

The figure shows the main shield (4) of the TRC, of circular section, is positioned concentrically around the deflector (2) this being also of circular section. The diverter (2) comprises permanent magnets (3) for image correction arranged symmetrically around the same geometric center as the shield (4) and the diverter (2). Inside the deflector (2) and concentric with it is the vacuum enclosure (1).

In FIG. 1b we see in dotted line (5) the image of the uncorrected TRC and in solid line (6) the image corrected by the symmetrical system shown in section in FIG. 1a. The uncorrected image is not rectangular as it should be, but on the contrary it has the appearance of a "cushion": slightly narrowed in the middle of its four sides and pointed at the four corners. The corrected image (6) using the magnets (3) of correction perfectly positioned with respect to the shielding (4) is well square as shown by the solid line (6).

Figure 2 is identical to Figure 1 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 correction magnets (3), and the vacuum enclosure (1), therefore the right magnet is closer to the shield than the left magnet. This could be the result of a manufacturing defect.

In FIG. 2b, we see in dotted lines the uncorrected image (5) and the desired corrected image (6) as shown in FIG. 1b and in solid line (7) the poorly 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 preserved around the horizontal median plane (which contains the axis of the TRC); and it can be seen that the resulting image correction remains satisfactory for the horizontal lines of the image which are well rectilinear. On the other hand, the vertical lines are curved by the right / left asymmetry of the correction geometry.

The asymmetry of the example in Figure 2 is among the simpler than one can imagine; the asymmetry that could be introduced by the vagaries of large-scale industrial manufacturing would most likely prove to be much more complicated. It is easy to see how critical the assembly precision becomes to ensure the rectilinear geometry of the image.

The invention aims to make this part of the assembly less critical and therefore to ensure better image correction more easily in industrial manufacturing.

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

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

In particular, not only the position of the main shield (4) becomes non-critical, but also its shape which has no influence on the magnetic correction fields isolated inside their auxiliary shield (8).

This results in a very appreciable economy in assembly times, and an economy in the number of types of deflectors to be used can also be obtained.

Claims (4)

1. deflector (2) of electron beam with magnetic correction field provided by permanent magnets (3), characterized in that said deflector comprises a magnetic shield (8) called "auxiliary" integral to said deflector, said magnetic shield being assembled around the periphery of said permanent magnets and positioned with great geometric precision with respect thereto. 2. An electron tube (1) comprising an electron beam deflector according to claim 1 and a main shield around the deflector. 3. Cathode ray tube (1) according to claim 2. 4. Image sensor tube according to claim 2.

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