Magnetic field compensator for a CRT

Abstract: In a television receiver, a magnetic field compensator of a horizontal component (H h ) of the earth's magnetic field includes a fluxgate magnetometer (61) having a pair of sensing windings (100 U ,100 V ) for generating first (S U ) and second (S V ) sensing signals representative of corresponding components (H U ,H V ) of the horizontal component (H h ) of the earth's magnetic field. The sensing signals (S U ,S V ) generate current (i₁₁,i₁₂) in a pair of substantially planar coils (11,12) disposed in a non-parallel vertical configuration on the rear periphery of a cathode ray tube (10). The currents in the coils produce a magnetic field that compensates or neutralizes the effect of the horizontal component (H h ) of the earth's magnetic field on the electron beam in the cathode ray tube (10). Whenever the position of the receiver is changed, rotationally or geographically, the current in the coils (11,12) is automatically adjusted to provide automatic neutralization.

Description: The invention relates to an automatic cancellation of external magnetic fields and, in particular, to apparatus for compensating for an external magnetic field affecting the operation of a cathode ray tube (CRT) display. The earth's magnetic field in the northern hemisphere is characterized by magnetic lines of force dipping downward and toward what is commonly referred to as the earth's north magnetic pole, but which by electromagnetic theory is actually a south magnetic pole. At a point of one of the lines of force a vector H may be drawn to represent the direction and intensity of the earth's field. The vector H that is substantially uniform at any geographical location may be separated into vertical and horizontal components, H y and H h , respectively. For the continental United States the horizontal intensity H h in the direction of the magnetic North varies from about .27 gauss in the southern parts to about .16 gauss in the northern parts. Color purity in a color television receiver tends to be adversely affected by the earth's magnetic field. An example of such a color television receiver is the shadow mask receiver. In such a color television receiver, the CRT has three electron guns and an apertured mask through which the electron beams pass to impinge upon the screen, which has phosphor stripes thereon emissive of light of different colors. For proper color rendition, it is desirable that the electron beams be properly directed and properly convergent so that at each scanning instant when an elemental area of the color image is being reproduced, the electron beams will pass through the correct aperture of the mask and impinge on the proper phosphor stripe of the screen. A CRT having a large screen, such as a 32-inch (81 cm) CRT, may be more sensitive to the earth's magnetic field than a CRT having a small screen. Greater sensitivity may exist for large screen CRT than for a smaller screen CRT because a given bending of the electron beam path caused by the earth's magnetic field results in a larger change in landing position on the face of the larger screen CRT, due to the longer distance from the yoke to the screen. A larger change in landing position requires a tighter tolerance for color purity. Such sensitivity to the earth's magnetic field may occur even when a magnetic shield is used, since ideal magnetic shielding is not feasible. In a given geographical location, the effect of the vertical component H y on the electron beams that affects the color purity is substantially the same at any directional angle δ of the CRT. Directional angle δ is defined as the angle between an electron beam axis Z of the CRT and the direction of the magnetic North. Therefore, automatic neutralization of the vertical component H y as a function of the directional angle δ may not have a significant effect on color purity. On the other hand, after the CRT is rotated in the horizontal plane by, for example, a viewer, a situation encountered in normal usage of television receiver, directional angle δ changes. It is well known that a change of angle δ may tend to degrade or reduce the tolerance of the color purity in the CRT. It may be desirable to neutralize the horizontal component H h of the earth's magnetic field when directional angle δ changes. An apparatus for neutralizing a first horizontal magnetic field component according to the preamble of claim 1 is disclosed in US-A-3,757,154. In Patent Abstract of Japan, vol. 11, no. 154, & JP-A-61289788 an apparatus for neutralizing the horizontnal component for the Earth's magnetic field which uses three magnetic field neutralizing coils is described. The invention is as set out in claim 1. The subclaims relate to preferred embodiments. In a magnetic field compensator of a television apparatus embodying an aspect of the invention, a fluxgate magnetometer is used for generating first and second magnetic field sensing signals representative of corresponding magnetic field components of horizontal component H h of the earth's magnetic field. The sensing signals are applied to a pair of substantially planar coils disposed in a nonparallel, side-by-side vertical direction arrangement fitted on the periphery of the CRT. The sensing signals are used for generating corresponding currents in the coils that compensate or neutralize the effect of the horizontal component H h of the earth's magnetic field on the electron beam inside the CRT. Whenever the position of the receiver is changed, rotationally or geographically, the currents in the coils are automatically adjusted to compensate for the effect of the horizontal component H h of the earth's magnetic field. Thus, neutralization is obtained by directly interacting and sensing the horizontal component H h of the earth's magnetic field by means of the transducing magnetometer. The magnetometer generates the sense signals in a manner that is independently derived from the generation of the electron beam in CRT 10. The sense signals are independent of and unaffected by the beam landing position on the CRT screen. In contrast, in one prior art compensator, an optical output produced on the CRT screen by the electron beam is utilized for sensing the effect of the earth's magnetic field on the electron beam and for generating a neutralizing magnetic field in accordance with the direct effect of the earth's magnetic field on the electron beam landing position. Such prior art arrangement, disadvantageously, requires a complicated optical interface with the CRT screen for monitoring the electron beam landing position on the CRT screen, and a complicated signal processing arrangement that may be costly. In accordance with another aspect of the invention, an apparatus for neutralizing a first magnetic field that undesirably affects electron beam landing in a video apparatus includes an arrangement responsive to the first magnetic field for generating a first sense signal that is indicative of a magnitude of the first magnetic field. A first current that is determined in accordance with the first magnetic field is generated. A first magnetic field neutralizing coil coupled to conduct the first current produces the neutralizing second magnetic field. The second magnetic field is of a magnitude that is in accordance with the magnitude of the first magnetic field. The second magnetic field automatically changes to maintain neutralization of the first magnetic field when a change in position of the video apparatus occurs. In the Drawing: FIGURE 1 illustrates an arrangement, embodying an aspect of the invention, including a magnetometer and neutralizing coils, for neutralizing the effect of the horizontal component of the earth's magnetic field on an electron beam of a CRT, shown in FIGURES 2, 3 and 4a; FIGURE 2 is a back view of the CRT that is to be magnetically neutralized; FIGURE 3 is a sectional view of the CRT of FIGURE 2; FIGURE 4a illustrates a perspective view of the CRT of FIGURE 2 fitted with a pair of magnetic field neutralizing coils; FIGURE 4b illustrates the magnetometer of FIGURE 1; FIGURE 4c illustrates a vectorial diagram representing horizontal magnetic field components useful for explaining magnetic field neutralization by the coils; FIGURE 5 is a vectorial diagram representing the lines of force of the earth's magnetic field relative to an electron beam path of a CRT; FIGURE 6a illustrates a fluxgate magnetometer of the arrangement of FIGURE 1; and FIGURE 6b is a vectorial diagram useful for the explanation of the operation of the magnetometer. FIGURE 1 illustrates a fluxgate magnetometer 61 and a current driver arrangement, embodying an aspect of the invention, of, for example, a television receiver, for generating currents i 11 and i 12 that are applied to magnetic field neutralizing coils 11 and 12, respectively. As described later on, coils 11 and 12 are fitted near CRT 10 of FIGURE 5 of the television receiver. Fluxgate magnetometer 61 of FIGURE 1 includes a drive winding 100 and a pair of sensing windings 100 U and 100 V , having axes U and V, respectively, that are, for example, perpendicular. The windings are constructed in the manner shown in FIGURE 6a. Similar symbols and numerals in FIGURES 1, 5 and 6a and 6b illustrate similar items or functions. Coils 11 and 12 of FIGURE 1 produce magnetic fields that are applied to the electron beam path of CRT 10 of FIGURE 5 for neutralizing the horizontal component of the earth's field, as described later on. A saturable toroidal core 150 of magnetometer 61 of FIGURE 6a, on which windings 100 V and 100 U are wound, is alternately saturated and unsaturated. This is accomplished by winding core 150 with toroidal drive winding 100, which is excited by an approximately square wave current i in of FIGURE 1. Square wave current i in , alternating from a positive value to a zero value at a frequency of, for example, 50 kH z , is applied to input terminals 100a and 100b of drive winding 100. During transition times of current i in , toroidal core 150 of FIGURE 6a is unsaturated. Between transition times when current i in is flowing, the core is saturated. Toroidal core 150 and toroidal drive winding 100, confine the magnetic field produced by drive winding 100 to circular core 150 itself, so that, by itself, the circular magnetic field, that is produced as a result of current i in , induces a voltage of only a negligible amplitude into sensing windings 100 V and 100 U . The geometric symmetry of the toroidal core permits the use of more than one sensing winding on a single saturating core. The successful use of more than one sensing winding on a single core is contingent upon the selection of a core exhibiting uniform magnetic properties and cross-sectional area around its circumference. When core 150 of FIGURE 6a becomes unsaturated, during transitions of current i in of FIGURE 1, the permeability of core 150 of FIGURE 6a becomes high. Therefore, the core material acts as a low impedance path to lines of magnetic flux in the surrounding environment, formed by horizontal component H h of the earth's magnetic field. When the core material is saturated, and its permeability is low, it no longer presents a low impedance path for external lines of flux, and they return to their original paths. Each time the core passes from the saturated state to the unsaturated state, the surrounding flux lines are drawn into it, cutting the winding turns of sensing windings 100 U and 100 V surrounding the core and inducing a voltage pulse S U in sensing winding 100 U , and a voltage pulse S V in sense winding 100 V . Each time the core passes from the unsaturated to the saturated state, the flux lines caused by horizontal component H h of the earth's magnetic field are driven out of the core, generating a pulse of equal amplitude, but of opposite polarity. Pulses S V of FIGURE 1 are applied to an input terminal of an amplifier U1A through a resistor R6. A half-wave, phase-sensitive demodulator of pulses S V is implemented with an analog switch arrangement IC1, operating synchronously with current i in for shorting out the signal at the input terminal of amplifier U1A that is produced by sensing winding 100 V during the occurrence of alternate pulses of pulses S V . Such alternate pulses occur when core 150 of FIGURE 6a goes out of saturation. The other alternate pulses of pulses S V of FIGURE 1, occurring when core 150 of FIGURE 6a goes into saturation, are applied to the input terminal of amplifier U1A of FIGURE 1 to form a demodulated portion of signal S V . The demodulated portion of signal S V is voltage amplified in amplifier U1A low pass filtered by a capacitor C6 and applied via a variable voltage divider that includes a resistor R15 to an input terminal of a voltage amplifier U1B having a power stage 60 that includes transistors Q1 and Q2. In accordance with an aspect of the invention, an output voltage V out of output stage 60, that is indicative of that portion of horizontal component H h of the earth's magnetic field of FIGURE 6b, which cuts across sensing winding 100 V , is applied via a resistor R V to neutralizing coil 11 of FIGURE 1. Variable resistor R15 is used for adjusting the level of current i 11 . In a similar manner, current i 12 , that is adjustable by adjusting a resistor R24, is generated from pulses S U and flows in coil 12. As 40 explained later on, neutralizing coils 11 and 12 produce corresponding magnetic fields that automatically neutralize the horizontal component H h of the earth's magnetic field at any directional angle δ between the electron beam axis Z of CRT 10 of FIGURE 5 and the magnetic north. Assume that pulses S V and S U of FIGURE 1, that are used for generating currents i 11 and i 12 , respectively, are unaffected by the magnetic fields that are produced by coils 11 and 12 of FIGURE 1. Such assumption is justified when the fluxgate magnetometer is located inside the television receiver cabinet, not shown in the FIGURES, but sufficiently far from coils 11 and 12. As shown in the vectorial diagram of FIGURE 6b, horizontal component H h has a direction that forms corresponding angles with axes U and V, respectively. Consequently, horizontal component H h may be vectorially separated into a first component H U having a direction that is coaxial with axis U and a second component H V having a direction that is coaxial with axis V and that is perpendicular to axis U. At any directional angle δ of CRT 10 of FIGURE 5, the amplitude of pulses S V , for example, produced between terminals 100e and 100d of sensing winding 100 V of FIGURE 1 is linearly proportional to the intensity of the surrounding magnetic field component H V which lies parallel to the axis V of sensing winding 100 V of FIGURE 6a. The amplitude of pulses S V of FIGURE 1 is unaffected by component H U of horizontal component H h of FIGURE 6b that is perpendicular to axis V. The polarity of pulses S V of FIGURE 1 depends upon the vectorial direction of component H V with respect to sensing winding 100 V of FIGURE 6a that, in turn, is determined by directional angle δ of FIGURE 5. The polarity and amplitude relationship between pulses S U of sensing winding 100 U and field component H U is analogous to that between pulses S V and field component H V . The ratio between the amplitude of pulses S V and the magnitude of the corresponding component H V is unaffected by the directional angle δ of CRT 10 of FIGURE 5. Similarly, the ratio between the magnitude of pulses S U of FIGURE 1 and that of component H U of FIGURE 6b is unaffected by angle δ. In this way, pulses S V and S U are representative of components H V and H U , respectively. Because current i 11 of FIGURE 1 is linearly proportional to pulses S V , it is also linearly proportional to component H V of FIGURE 6b and is unaffected by component H U of the horizontal component H h of the earth's magentic field. Similarly, current i 12 in coil 12 of FIGURE 1 is linearly proportional to component H U of FIGURE 6b but is unaffected by component H V of the horizontal component H h of the earth's magnetic field. Each of FIGURES 2, 3 and 4a illustrates neutralizing coils 11 and 12 of FIGURE 1, mentioned before, that are fitted on CRT 10. Coils 11 and 12 of FIGURE 1 are used for producing a resultant neutralizing magnetic field that neutralizes the effect of horizontal component H h of the earth's magnetic field on the electron beam in CRT 10. Similar symbols and numerals in FIGURES 1-6 indicate similar items or functions. Neutralizing coils 11 and 12 of FIGURE 3 are fitted along the outer periphery of CRT 10 to spatially surround a magnetic shield 16 of FIGURE 3 received within a conical section 22 of CRT 10. Inner vertical portions 11b and 12b of neutralizing coils 11 and 12, respectively, partly surround a neck section 17 of CRT 10 and intersect each other at vertically disposed two opposite points across neck section 17. Inner portions 11b and 12b are disposed between a rear end portion 16b of magnetic shield 16 and a forward end portion 27a of a deflection yoke arrangement 27. Outer portions 11a and 12a of coils 11 and 12, respectively, are positioned near a shadow mask 16a, which is also part of magnetic shield 16, in parallel relationship with inner portions 11b and 12b, respectively. The number of turns in each coil may be such so as to provide the required number of ampere-turns, such as, three, for example. It should be understood that deflection yoke arrangement 27 is not shown in FIGURE 4a so as to provide a better view of coils 11 and 12. When CRT 10 of FIGURE 3 is placed relative to a horizontal plane in a position similar to its position in normal usage, portions 11a and 11b of coil 11 define a vertical plane having an axis V' of coil 11 that is perpendicular to such horizontal plane. The spatial orientation between coil 11 and winding 100 V of fluxgate magnetometer 61, shown in FIGURE 4b, is arranged such that the axis V of winding 100 V is close to being in parallel with the axis V' of coil 11 as shown in FIGURE 3. In a like manner, coil 12 defines a corresponding vertical plane having an axis U' of FIGURE 2 that is close to being in parallel with axis U of winding 100 U . Magnetometer 61 of FIGURE 4b is secured in a cabinet of the television receiver, not shown in the Figures, behind the back portion of CRT 10 of, for example, FIGURE 4a. The orientation of axes U and V of magnetometer 61 relative to CRT 10 is shown in FIGURE 3. Axes U and V are disposed approximately symmetrically relative to axis Z of CRT 10. Current i 11 , flowing in the winding turns of coil 11 of FIGURES 3 or 4a, produces a linearly proportional neutralizing magnetic field shown in the vectorial diagram H 11 of FIGURE 4c outside magnetic shield 16 of CRT 10 of FIGURE 3 in a volume that includes magnetic shield 16 and the electron beam path of CRT 10. Assume that neutralizing magnetic field H 11 , shown by the vectorial diagram of FIGURE 4c, is uniform, perpendicular to axis U and coaxial with axis V of winding 100 V . Similarly, assume that current i 12 , flowing in coil 12, produces a neutralizing magnetic field H 12 of FIGURE 4c that is uniform, perpendicular to axis V and coaxial with axis U. It should be understood that the above mentioned assumptions represent only an approximation that is used for simplifying the explanation. The magnitude and polarity of current i 11 of FIGURE 1 may be manually adjusted by varying resistor R15 so as to produce magnetic field H 11 of FIGURE 4c at the opposite direction and at the same magnitude of component H V . Thus, magnetic field H 11 neutralizes component H V or reduces the effect of component H V of horizontal component H h of the earth's magnetic field on the electron beam of CRT 10. Such adjustment of current i 11 of FIGURE 1 is made in a given geographical location such as, for example, at the factory and at a given direction angle δ of CRT 10 of FIGURE 5. Similarly, current i 12 of FIGURE 1 is manually adjusted so as to produce magnetic field H 12 that neutralizes component H U . By neutralizing components H V and H U , horizontal component H h of the earth's magnetic field is neutralized. Current i 11 that produces magnetic field H 11 of FIGURES 4c is linearly proportional to component H V , as described before. Magnetic field H 11 is linearly proportional to component H V , is in a direction that opposes that of component H V and is unaffected by component H U . The above mentioned relationship between magnetic field H 11 and component H V is maintained at any directional angle δ of CRT 10 of FIGURE 5. A similar, corresponding relationship between magnetic field H 12 of FIGURE 4c and component H U is also maintained at any angle δ. Consequently, in accordance with a feature of the invention, once currents i 11 and i 12 of FIGURE 1 are adjusted in the manner described before, neutralization of horizontal component H h of the earth's magnetic field is maintained automatically, that is without manual or operator intervention, at any other directional angle δ of CRT 10 and at any other geographical location. It should be understood that even though the assumptions made before represent only an approximation, a sufficient cancellation or neutralization of the horizontal component H h of the earth's magnetic field is obtained at any angle δ of CRT 10 of FIGURE 5.

Claims: An apparatus for neutralizing an external horizontal first magnetic field to prevent said first magnetic field from undesirably affecting electron beam landing in a cathode ray tube of a video apparatus, comprising: first and second sensing windings (100 U ,100 V ) responsive to said first magnetic field for generating first and second sense signals (S U ,S V ) indicative of a magnitude and direction of said first magnetic field, said sense signals varying when said first magnetic field varies; characterized by first and second generating means (U2A,U2B; U1A,U1B) responsive to said sense signals (S U ,S V ) for generating first and second currents (i 12 ,i 11 ) that are determined and continuously vary during operation of said apparatus in accordance with said first magnetic field; and first and second magnetic field neutralizing coils (12, 11) coupled to said first and second current generating means (U2A,U2B; U1A,U1B) for producing a neutralizing second magnetic field in said video apparatus at a magnitude that continuously varies during operation of said apparatus in accordance with variation of said magnitude and direction of said first magnetic field, said first and second magnetic field neutralizing coils being symmetrically disposed, with respect to said cathode ray tube, in substantially perpendicular vertical planes, when said cathode ray tube is placed in a normal usage position, in proximate relation to a flared envelope portion of said cathode ray tube, such that said second magnetic field automatically neutralizes said first magnetic field when a change in a position of said cathode ray tube (10) occurs during operation of said apparatus. An apparatus according to Claim 1 characterized in that said first sensing winding (100 U ) comprises a sense winding of a fluxgate magnetometer (61). An apparatus according to Claim 1 characterized in that a second magnetic field neutralizing coil (11) produces a neutralizing third magnetic field (H 11 ) such that said second (H 12 ) and third (H 11 ) magnetic fields neutralize the effect of corresponding field components (H U ,H V ) of the horizontal component (H h ) of the earth's magnetic field on an electron beam in said cathode ray tube (10). An apparatus according to Claim 1 characterized in that said second magnetic field neutralizing coil (11) having an axis that is disposed in a nonparallel manner with an axis of said first magnetic field neutralizing coil (12) and a second sensing winding (100 V ) having an axis that is disposed in a nonparallel manner relative to an axis of said first sensing winding (100 U ) such that said axes of said neutralizing coils (12,11) are oriented in a predetermined manner relative to said axes of said sensing windings (100 U ,100 V ) for producing a second sense signal (S V ) in said second sensing winding (100 V ), wherein said second sense signal (S V ) is applied to said second coil (11) for producing in accordance therewith a neutralizing third magnetic field (H 11 ) in said second neutralizing coil (11). An apparatus according to Claim 6 characterized in that said first sensing winding (100 U ) has an axis that is disposed approximately in the same direction as an axis of said first neutralizing coil (12) and wherein an axis of said second sensing winding (100 V ) is disposed approximately in the same direction as an axis of said second neutralizing coil (11). An apparatus according to Claim 6 characterized in that the axes of said sensing windings (100 U ,100 V ) are substantially perpendicular. An apparatus according to Claim 1 characterized in that second magnetic field neutralizing coil (11), said first (12) and second (11) substantially planar neutralizing coils are disposed in corresponding vertical planes, when said cathode ray tube is placed in a normal usage position, and are arranged symmetrically in proximate relation to a flared envelope portion (22) of a cathode ray tube (10) so as to produce, in response to currents (i 12 ,i 11 ) flowing in said coils (12,11), corresponding magnetic fields (H 12 ,H 11 ) in counteracting relation to corresponding magnetic field components (H U ,H V ) of the horizontal component of the earth's magnetic field (H h ) and means responsive to said sense signals (S U ,S V ) for supplying said currents (i 12 ,i 11 ) to said coils (12,11) such that said magnetic fields (H 12 ,H 11 ) produced by said coils (12,11) substantially compensate for said magnetic field components (H U ,H V ) of the horizontal component (H h ) of the earth's magnetic field.

Inventors: HOOVER ALAN ANDERSON, MCGLASHAN KENNETH WOERNER, HOOVER, ALAN ANDERSON, MCGLASHAN, KENNETH WOERNER
Applicants: THOMSON CONSUMER ELECTRONICS, THOMSON CONSUMER ELECTRONICS, INC.
Application year: 1989
Main classification: H04N9
IPC codes: H04N9