Tracking control apparatus and method

An apparatus for control of a beam of radiant energy, comprising:

• a detector of said beam having first and second outputs responsive to a position of said beam;
• a circuit coupled to said outputs of said detector for producing an error signal representing a displacement of said beam from a predetermined position, wherein said error signal has a periodic characteristic relative to said displacement;
• a servo responsive to said error signal for restoring said displaced beam to said predetermined position;
• a local feedback loop coupled to said outputs of said detector, said loop comprising:
• a first periodic function generator responsive to said error signal;
• and
• a second periodic function generator responsive to said error signal, said second periodic function generator having an output that differs from an output of said first periodic function generator by a phase angle;
• a first multiplier for multiplying said first output of said detector by said output of said first periodic function generator;
• and
• a second multiplier for multiplying said second output of said detector by said output of said second periodic function generator;
• wherein said outputs of said first and second multipliers are provided as inputs of said circuit.

A method for controlling a beam of radiant energy, comprising the steps of:

• generating first and second detection signals responsive to a position of said beam;
• producing an error signal representing a displacement of said beam from a predetermined position, wherein said error signal has a periodic characteristic relative to said displacement;
• restoring said displaced beam to said predetermined position in response to said error signal;
• and
• generating a first periodic signal responsive to said error signal;
• and
• generating a second periodic signal responsive to said error signal, said second periodic signal differing from said first periodic signal by a phase angle;
• wherein said step of producing an error signal is performed by the steps of:
• multiplying said first detection signal by said first periodic signal to yield a first product signal;
• multiplying said second detection signal by said second periodic signal to yield a second product signal;
• and
• determining a difference between said first product signal and said second product signal.

A tracking control apparatus for an optical disc system, comprising:

• a source for directing a beam of radiant energy toward a selected one of a plurality of information tracks of an optical disc;
• means for imparting relative rotational motion between said beam and said disc about an axis of rotation;
• a beam steering means for displacing the beam in a generally radial direction relative said disc to follow said selected information track;
• a detector responsive to radiant energy returning from said selected information track and having a first output signal and a second output signal, wherein said first output signal differs in phase from said second output signal according to a displacement of said beam from said selected track;
• a first multiplier electrically having a first input coupled to said first output signal of said detector;
• a second multiplier electrically having a first input coupled to said second output signal of said detector;
• a difference summing amplifier having a first input coupled to an output of said first multiplier and a second input coupled to an output of said second multiplier for generating an error signal;
• a first periodic function generator having an input coupled to an output of said difference summing amplifier and an output coupled to a second input of said first multiplier;
• a second periodic function generator having an input coupled to an output of said difference summing amplifier and an output coupled to a second input of said second multiplier;
• and
• a servo responsive to said output of said difference summing amplifier for operating said beam steering means.

A method for tracking control in an optical disc system, comprising the steps of:

• directing a beam of radiant energy toward a selected one of a plurality of information tracks of an optical disc;
• imparting relative rotational motion between said beam and said disc about an axis of rotation;
• displacing the beam in a generally radial direction relative said disc to follow said selected information track;
• detecting radiant energy returning from said selected information track and generating a first output signal and a second output signal, wherein said first output signal differs in phase from said second output signal according to a displacement of said beam from said selected track;
• multiplying said first output signal by a first periodic function of an error signal to yield a first product;
• multiplying said second output signal by a second periodic function of said error signal to yield a second product;
• subtracting said first product from said second product to produce said error signal;
• and
• steering said beam toward said selected information track in accordance with said error signal.