Bucking field system and method for mitigating the effects of an external magnetic field on a cathode ray tube display

A system for mitigating the effects of an external magnetic field on a cathode ray tube (CRT), comprising:

• a magnetic shield having an open end for housing said CRT;
• a metal face plate that is formed along the circumference of said shield's open end;
• a coil that is disposed on said face plate and around said shield's open end for providing a bucking magnetic field in response to a drive signal to oppose said external magnetic field;
• a magnetic field sensor that is disposed inward from said coil for sensing a differential field between said external and said bucking magnetic fields;
• a transverse magnetic field sensor that is positioned outside said shield for sensing a transverse component of said external field;
• and
• a circuit for adjusting said drive signal in response to said sensed differential field such that said bucking field opposes said external field to reduce the magnitude of said differential field, and for producing a degaussing trigger signal when said transverse magnetic field component passes through a predetermined value.

A system for mitigating the effects of an external magnetic field on a color cathode ray tube (CRT), comprising:

• a five sided rectangular shaped magnetic shield having an open end for housing said CRT;
• an annular metal face plate that is attached to said shield at its open end, said face plate being substantially perpendicular to the sides of said shield and extending inward therefrom;
• a coil that is disposed circumferentially on the outside of said face plate, said coil responding to a drive signal to produce a bucking magnetic field in opposition to said external magnetic field;
• a pair of linear Hall effect sensors that are disposed substantially perpendicular to said face plate and positioned opposite each other inside said coil for sensing differential fields between said external and said bucking magnetic fields;
• and
• a circuit for adjusting said drive signal in response to an average of said differential fields such that said bucking field opposes said external field to reduce the magnitude of said average.

A system for mitigating the effects of an external magnetic field on an image display, comprising:

• a CRT having a phosphor face plate, said CRT writing images onto the face plate which in turn displays the images;
• a magnetic shield having an open end for housing said CRT and said shadow mask;
• a metal face plate is attached to said shield at its open end such that it is substantially parallel to said face plate;
• a coil that is circumferentially disposed on said face plate for providing a bucking magnetic field to oppose said external magnetic field in response to a drive signal;
• a first magnetic field sensor that is disposed inward from said coil and substantially perpendicular to said face plate for sensing a differential field between said external and said bucking magnetic fields;
• and
• a circuit for adjusting said drive signal in response to said sensed differential field to reduce the magnitude of said differential field.

A system for mitigating the effects of an external magnetic field on a cathode ray tube (CRT), comprising:

• a magnetic shield having an open end for housing said CRT;
• a coil that is disposed circumferentially around said shield's open end, said coil responding to a drive signal to produce a bucking magnetic field in opposition to said external magnetic field, said bucking field having a non-uniform profile;
• a first magnetic field sensor that is disposed inward from said coil for sensing a differential field between said external and said bucking magnetic fields;
• a field concentrator for modifying the external field's profile to approximately match the bucking field's non-uniform profile;
• and
• a circuit for adjusting said drive signal in response to said sensed differential field to reduce the magnitude of said differential field.

A method for mitigating the effects of an external magnetic field on a cathode ray tube (CRT), comprising:

• generating a bucking field having a known profile to oppose the external magnetic field;
• modifying the external magnetic field's profile to approximate the bucking field's profile;
• sensing a differential field between the external and bucking magnetic fields;
• and
• adjusting the bucking field in response to the sensed differential field to reduce the magnitude of the differential field.