Claims
- 1. An apparatus for controlling a magnetic field strength of bias coil at a focal point of a lens during recording on a medium, said apparatus comprising:
- a first controller for determining a distance between said focal point and a reference point on said medium, said first controller outputting said distance;
- a source generating a stray magnetic field having a stray field strength at said focal point;
- a second controller responsive to said distance for outputting a digital signal, said digital signal being a function of said distance and said stray field strength;
- a digital-to-analog converter responsive to said digital signal for providing a driver voltage;
- a current driver responsive to said driver voltage for providing a bias current to said bias coil wherein said magnetic field strength is controlled at a predetermined value;
- a current sensor for measuring said bias current; and
- a feedback device responsive to said current sensor for providing to said current driver a feedback voltage wherein said feedback voltage is a function of said bias current and a desired minimum bias current.
- 2. The apparatus according to claim 1 wherein said medium is in the form of an optical disk and said reference point is the center of the disk.
- 3. The apparatus according to claim 1 wherein said stray field strength is a function of said distance.
- 4. The apparatus according to claim 1 wherein said medium rotates about said reference point and said stray field strength is a function of the rotational velocity.
- 5. A method for controlling a magnetic field strength of a bias coil at a focal point of a lens during recording on a medium, said method comprising the steps of:
- determining a distance between said focal point and a reference point on said medium;
- generating a stray magnetic field having a stray field strength at said focal point;
- calculating a digital signal, said digital signal being a function of said distance and said stray field strength;
- converting said digital signal to a driver voltage;
- applying said driver voltage to a current driver for providing a bias current to said bias coil wherein said magnetic field strength is controlled at a predetermined value;
- measuring said bias current; and
- processing said measurement for providing to said current driver a feedback voltage wherein said feedback voltage is a function of said bias current and a desired minimum bias current.
- 6. The method according to claim 5 wherein said medium is in the form of an optical disk and said reference point is the center of the disk.
- 7. The method according to claim 5 wherein said stray field strength is a function of said distance.
- 8. The method according to claim 6 further including the step of: rotating said medium about said reference point, wherein said stray field strength is a function of the rotational velocity.
Parent Case Info
This is a division of application Ser. No. 08/418,965, filed Apr. 7, 1995, now U.S. Pat. No. 5,666,333.
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