US2763805A - Electromagnetic focus coil for cathode ray tube - Google Patents

Description

Sept. 18, 1956 s DE L 2,763,805

ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE Filed June 29, 1954 2 Sheets-Sheet 1 IN V EN TOR.

Sept. 18, 1956 s. 1.. BENDELL 2,763,805

ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE Filed June 29, 1954 2 Sheets-Sheet 2 JVE/YHL 01/7707 IN V EN TOR. Jan 5y Z. fli/yafl United States Patent O ELECTROMAGNETIC FOCUS COIL FOR CATHODE RAY TUBE Sidney L. Bendell, Haddon Heights, N. 3,, assignor to Radio Corporation of America, a corporation of Delaware Application June 29, 1954, Serial No. 440,039

Claims. (Cl. 313-84) The present invention relates to new and improved electromagnetic focus coils of the type employed in focussing an electron beam within a cathode ray tube such, for example, as a cathode ray television camera tube.

As is well known, conventional cathode ray tubes include a target of a type depending upon the function of the tube and an electron gun for forming an electron beam which is caused to impinge upon the target by virtue of a potential difference between the gun and the target. In the television field such devices are employed for a variety of uses, including the derivation of electrical signals representative of a subject and also as image reproducing devices which operate to form a light image reconstructed from such signals. It has been found necessary to provide some form of apparatus for focussing the electron beam in such tubes to provide a fine spo on the target, in order for acceptable image resolution to be afforded. In view of the necessity for focussing means, various proposals have been made in the prior art which fall primarily into two main classes, namely, electrostatic and magnetic arrangements. The latter class includes permanent magnet focussing devices and electromagnetic focussing devices both of which have in common the fact that they provide a magnetic field whose flux lines are substantially parallel to the nominal path of the beam. That is to say, the magnetic field is axial with respect to the tube.

Insofar as the electromagnetic focussing coil art is concerned, one problem has been that of forming a focus coil of the proper axial length and with the required number of conductor turns needed for focussing of the electrons in order to meet the requirement that the field be substantially uniform throughout its axial length. The problem in question has become even more aggravated in the case of multiple cathode ray tube arrangements wherein several cathode ray tubes are caused to scan a common object, as in a color television camera, for example.

It is, therefore, a primary object of the present invention to provide a novel electromagnetic focussing coil for use in conjunction with a cathode ray tube.

Among the various methods of winding focussing coils employed in the past, characterized in that a conductor is wound about a cylindrical coil form, are the so-called randon winding and layer winding methods. Random wound coils are normally made by hand and have the disadvantages of being costly to produce and of lacking uniformity with respect to other coils. Such a method of winding coils for focussing is, therefore, undesirable where it is necessary that a plurality of focussing coils be identical so that the cathode ray tubes with which they are associated may be operated in unison with respect to a common subject. Layer winding of focussing coils with layer-winding machines has also been found to be a source of difficulty, particularly where it is attempted to layer-wind a coil without the interposition of paper insulating sheets between successive layers. That is, the

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layers tend to become jumbled or non-uniform, so that the machine operator must thereafter exercise his judgment as to how best to compensate for the non-uniformity. It is readily apparent that such a method depends to a large extent upon the skill and judgment of the operator.

Hence it is a further object of the present invention to provide a novel electromagnetic focussing coil which is simple to form with readily available commercial winding apparatus and which does not give rise to situations requiring the exercise of judgment on the part of the person operating the winding apparatus.

In general, the present invention is characterized in that it provides an electromagnetic focussing coil comprising" a cylindrical coil form of suitable dimension for surrounding the neck of the cathode ray tube with which the coilis to be associated and a conductor wound about the form as a so-called universal winding. That is to say, the conductor is wound about the cylindrical form helically and with such a large pitch that successive convolutions of the conductor are spaced axially from each other, the conductor then being wound back over itself with the reverse pitch. The present invention affords the advantages that coils produced in accordance with its principles are self-supporting (i. e. do not require end supports), are extremely precise in their turns dis tribution and may be produced by mass production methods with extreme uniformity between coils.

Additional objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following detailed description of the accompanying drawings, in which:

Fig. l is an elevational view of an electromagnetic focus coil of the invention;

Fig. 2 illustrates diagrammatically, and in a simplified manner, apparatus by means of which the coil of Fig. 1

may be wound; and

Fig. 3 is a schematic diagram of a tricolor television camera arrangement in which coils of the present invention may be advantageously employed.

Referring to Fig. 1 of the drawing, it will be seen that the electromagnetic focus coil of the present invention comprises a cylindrical core or coil form 10, about which there is wound a wire conductor 12 in the form of a universal winding. That is to say, as may be seen from the drawing, the conductor is so wound about the form 10 as to have a substantially larger pitch than that which would be required for successive convolutions of the coil to be contiguous. In fact, the pitch is of such value that successive helical turns of the conductor are spaced axially from each other by an appreciable dis tance. While the term universal winding is well known to those engaged in the textile arts, wherein thread, twine and the like are wound in that manner, the method by which the universally wound focus coil of Fig. 1 is accomplished may be better understood by those skilled in the electronics art from the showing of Fig. 2.

In Fig. 2 there is illustrated, by way of a simplified isometric view a well-known arrangement for producing universal windings of cord or the like and which has been found quite effective in winding the cathode ray tube focus coils of the present invention. Specifically, the coil form 10 is supported on a shaft 14 driven through suitable gearing 16 and 18 by a motor shaft 20. The rotation of the coil form 10 is, by way of illustration, in the direction of the arrow 22. The higher conductor 12 is supplied from a reel 24 supported in suitable relationship with respect to the rotating coil form 10 and the wire feeding apparatus indicated in its entirety by reference numeral 26. In general, the conductor 12 is, after being connected at one end 12 to the coil form as through the frictional engagement by the sides of a slot 28 in the end support 30, is fed to the coil form 10 by means of a laterally reciprocating arm 32 which carries a V-grooved wire feeder roll 34 at its extremity adjacent the coil form 10. The arm 32 is caused to reciprocate in the directions indicated by the arrow 36 through a suitable mechanical arrangement synchronized with the form rotating mechanism as by means of the idler gear 38 enmeshed with the motor gear 18 and a cam gear 40. The cam gear 40 drives a shaft 42 through the agency of a gainer mechanism 44 which is in the nature of an escapement mechanism whose function will become more fully apparent hereinafter. Rigidly mounted on the shaft 42 is a nutating cam wheel 46 which is provided with a cam groove 48 in its periphery. The end of the wire-feeding arm 32 remote from the wheel 34 is provided with a low friction cam follower 59 which rides in the groove 48. Thus, rotation of the shaft 42 causes nutating movement of the cam wheel 43 in such manner as to cause reciprocation in the horizontal direction (arrows 36) of the wire feeding arm 32 back and forth per revolution of the cam. The arm 32 is illustrated diagrammatically as being supported and guided in its reciproeating movement by means of a slide member 52 which rides in a bearing slot 54 of a support member 56.

The gear ratio of the train employed is so chosen that the coil form executes a plurality of revolutions for each traverse of the wire feeding arm 32 between one of its extreme positions and its other extreme positions. That is to say, the shape of the cam 46 and its speed of rotation is such that, at the beginning of operation, the wire-feeding roller 34 is at the end of the coil form 10 adjacent the member 30 and, several revolutions of the coil form later (depending upon the gear ratio 16, 18, 38 and 44), the roller 34 has traversed the length of the coil form until it is at the end thereof remote from the arrangement 30. In Fig. 2, the apparatus is shown in its position after several revolutions of the form 10 in the direction of the arrow 22 when the arm 32 is approaching the extremity of its lateral excursion to the right. Such relative movement of the coil form 10 and the wire feeding :arm 32 causes the conductor 12 to be laid down helically, as shown, and with successive convolutions thereof spaced apart axially the distance d. In the terminology of the textile art in which apparatus of the type shown in Fig. 2 is commonly employed for winding thread or twine onto a bobbin, the distance d is termed the cross-over per turn. Depending, therefore, upon the value of cross-over per turn, the point at which the conductor 12 reaches the end of the coil form and begins its helical travel back toward the starting point will vary radially of the coil form. At the time that the wire-feeder arm reaches its extreme righthand position in Fig. 2, the cam groove 48 will cause the arm 32 abruptly to com mence lateral movement to the left, so that the conductor 12 will then be wound in the same manner, but with the reverse pitch, over the first group of turns.

The gainer mechanism 44, which forms a conventional adjunct to commercial thread winding machines such as those sold commercially under the style Universal Winding Machine by the Universal Winding Company of Cranston, Rhode Island, comprises means for providing a minute shift of the positioning of the cam shaft 42 to compensate for the diameter of the filament being wound, so that successive groups of convolutions lie adjacent each other rather than directly on top of each other. Since the specific apparatus for forming the focussing coils of Fig. 1 does not constitute a part of the present invention, additional detailed description is unnecessary here. It will be understood, however, that the gainer effectively adds an increment to the displacement of the cam after each complete rotation thereof, whereby to increase the wire pitch a slight amount.

In order that the above description of the typical universal winding means may be better correlated with a specific coil, it will be seen in Fig. 1 that the conductor is neatly and precisely arranged on the form 10 with successive cross-over points spaced from each other. The dimension cross-over per turn is indicated by the distance d in Fig. 1 and the coil in that figure is properly termed a 3-cross-over-per-turn universal winding.

One environment in which focussing coils in accordance with the present invention offer great utility is that illustrated diagrammatically in Fig. 3, wherein there is shown a conventional 3-Vidicon television color camera of conventional form. Each of the three Vidicons 6t), 62 and 64 is of the type described in detail in an article entitled The Vidicon photo conductive camera tube by Weimer, Forgue and Goodrich, Electronics, May 1950. The Vidicon, as illustrated in a greatly simplified manner in the drawing comprises an electron gun 64 located at one end of the evacuated cylindrical glass envelope 66, which produces a beam of electrons 68 for impingement upon a pho-toconductive target electrode 70 located at the other end of the envelope 66. The electron beam 68 is caused to scan a conventional, rectangular raster by means of suitable sawtooth currents of field and line frequency flowing through the associated vertical and horizontal deflection coils 72 and 74, respectively. In order that the electrons of the beam 68 may be focussed to a fine spot at the target 70, whereby acceptable resolution of the image may be realized, each of the Vidicons 6t), 62 and 64 is provided with an electromagnetic focus coil 76. Light from a subject in natural color (indicated by the X) is focussed via a suitable lens arrangement 78 onto a color selective light splitting arrangement 80 which may, for example, comprise the well-known di chroic reflectors 82 and 34 of the type described in detail in U. S. Patents 2,379,790 and 2,312,492, granted July 3, 1945, and December 10, 1946, respectively, to G. L. Dimmick. The optical arrangement of the dichroics 8t 82 and 84 is explained in U. S. Patent 2,672,072, granted March 16, 1954, to L. T. Sachtleben et al. for Color Television Optical System.

Briefly, the dichroic reflector 82 reflects red light from the subject to the Vidicon 62 and passes the blue and green light. The dichroic 84 reflects the blue light to the Vidicon 64 and permits the green light to pass to the Vidicon 60. As pointed out in the above-cited Sachtleben et al. patent, the lengths of the paths from the subject to the three cameras are equal to each other, in order that the size of the image focussed onto the tubes is the same for the three pickup devices. Since it is necessary, for proper registration of the three component color images, that the images be scanned in unison by the three camera tubes and with the same degree of preciseness, the universally wound focussing coils of the present invention are well adapted for such use. By way of example, the focus coils 76 are illustrated as being energized from the same source of focus voltage 86, in a wellknown manner. This common energization may be from a constant current source feeding the several focus coils in series, as is conventionally done.

While, as will be appreciated, the specific dimensions and specifications for electromagnetic focussing coils wound in accordance with the invention will vary, depending upon the use to which the coils are to be put, there follows by way of example a description of one focus coil, designed for use with a Vidicon television camera tube, which has been found to be extremely satisfactory in use:

Outer diameter of coil form=2.843 Outer diameter of focus coil=3.2l8" Axial length of focus coil=4 A cross-over per turn Throw of cam 46 (Fig. 2)=4" Ratio of gears 16 to 40:33/109 Number of turns=8500 Wire type: No. 33 single silk enamel wire Outer diameter of wire=0.0092

The specific Vidicon type with which the above-described coil was designed for use is the RCA 6326 Vidicon.

From the foregoing, those skilled in the art to which the present invention apertains will recognize that the novel universally wound electromagnetic focus coils of the invention are simple to produce with mass production methods, yet possess a degree of preciseness unequalled by coils constructed according to the usual prior art methods. Additionally, the fact that the coils of the invention may be wound to meet rigid specifications renders them extremely suitable for such environments in which several coils are required to have exactly the same characteristics and to produce identical focussing fields. The preciseness of the coils wound in accord ance with the present invention is, as stated, of particular importance by reason of the fact that variations in the axial magnetic focussing field which would be produced with non-uniformly wound coils are known to cause changes in the geometry of the scanned raster. Thus, where a plurality of rasters must be identical to each other, it is essential that the flux patterns produced by the focussing coils be identical, which requirement is readily met by the coils of the present invention.

While the invention has been described in accordance with a specific form in which the electromagnetic focus is in the form of a single universal winding, it should be borne in mind that, where desired as for purposes of facilitating manufacture or for electrical reasons, the focus coil may be made of a plurality of separate universally wound coils arranged coaxially alongside each other.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent:

1. An electromagnetic focus coil adapted to surround at least a portion of a cathode ray tube of the type having a target and means for directing a beam of electrons along a path toward such target, said coil being adapted to provide an axial magnetic field and comprising a coil form of substantially cylindrical shape and a conductor wound thereon as a universal winding of a plurality of crossovers per turn.

2. An electromagnetic focus coil adapted to surround a portion of a cathode ray tube, said coil comprising an elongated, cylindrical core and a conductor wound there- 6 on helically from one end thereof to the other and back, and of a pitch substantially greater than that necessary for successive helical turns to be contiguous, latter turns of said conductor being wound over prior turns and with reverse pitch.

3. An electromagnetic focus coil for a cathode ray tube, said coil comprising: an elongated cylindrical form and a conductor wound thereon helically from a first end thereof to the second end thereof and with such certain pitch that successive convolutions are spaced from each other axially of said form, said conductor being wound back over itself from said second end to said first end with a pitch substantially equal to such certain pitch but of the reverse direction.

4. An electromagnetic focus coil for use in conjunction with a cathode ray tube of the type having a generally cylindrical portion, a target, and means for directing a beam of electrons along a path within such generally cylindrical portion toward such target, said coil being adapted to surround at least a portion of such path and comprising a substantially cylindrical coil form and a conductor wound thereon as a universal winding of a plurality of layers and having a plurality of crossovers per turn.

5. An electromagnetic focus coil for use in conjunction with a cathode ray tube of the type having a generally cylindrical portion, a target, and means for directing a beam of electrons along a path within such generally cylindrical portion toward such target, said coil being adapted to surround at least a portion of such path and comprising a substantially cylindrical coil form and a conductor wound thereon as a universal winding of a plurality of layers and having a number of crossovers per turn greater than two.

References Cited in the file of this patent UNITED STATES PATENTS 2,071,516 Farnsworth Feb. 23, 1937 2,217,409 Hepp Oct. 8, 1940 2,243,893 Blurnlein June 3, 1941 2,296,355 Levin Sept. 22, 1942 2,298,673 Brown et a1. Oct. 13, 1942 2,420,156 Van Suchtelen May 6, 1947 2,550,592 Pearce Apr. 24, 1951

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