Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-06-09
2001-11-06
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
Reexamination Certificate
active
06314069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a control arrangements for optical disc drives. More particularly this invention relates to an improved servo control which extends the operating range of a closed loop mode of operation of a focus or tracking servo to a plurality of positions on a disc.
2. Description of the Related Art
In optical disc drives wherein information is stored in a plurality of spiral or concentric information tracks, a lock of a recording or reproducing beam on an information track of interest is commonly maintained by a tracking servo, for example the servo disclosed in Ceshkovsky et al., U.S. Pat. No. 4,332,022. The tracking servo responds to minimize an error signal V
p
derived from the intensity of a reflected light beam returning from the optical disk medium and is given by the equation:
V
p
=
A
sin
(
2
π
x
p
)
(
1
)
where
A is a constant;
x is beam displacement from the track center; and
p is the track pitch.
Terashi, U.S. Pat. No. 5,177,725, discloses a servo apparatus for expanding the pull-in range using a velocity detector for detecting the velocity of a driven element.
Kobayashi et al, U.S. Pat. No. 4,853,918, proposes an arrangement where signals from tracking pits offset from each other about the center of a track are provided to sample-and-hold circuits, and are compared with one another to yield a sawtooth signal tracking signal having discontinuities midway between tracks.
In Burroughs, U.S. Pat. No. 4,779,251 an arrangement is disclosed wherein a circuit generates a ramp waveform that is used to introduce a controlled offset into a tracking servo. The servo error signal, which is derived from pre-formatted fine tracking features, is inverted in phase when the read beam is moving between tracks. The ramp waveform is adjusted according to memorized tracking error information from previous microjumps between tracks.
In focus control applications, the focus servo operates in the negative feedback region of the focus error “S curve”. Conventionally a special sequence is required, using open loop operation, to place the servo into a negative region which is surrounded by positive feedback regions. If focus is lost for any reason, the entire acquisition sequence must be repeated. This is very time consuming.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to extend the operating range of a focus servo operating in a closed loop mode.
It is another object of the invention to improve the focus servo's performance and allow automatic focus reacquisition in response to disc defects, noise, shock and vibration.
These and other objects of the present invention are attained in an optical disc drive by providing an optical pickup having a plurality of outputs for producing an error signal that supplies a servo loop. The error signal also supplies a local feedback loop, which includes a plurality of sine function generators to modify the outputs of the optical pickup, such that the focus error signal, plotted against the position of the read beam, is transformed from a sinusoidal waveform into a substantially linear ramp. The local feedback loop is independent of the main tracking servo loop, although it may be designed to share some components.
In addition to the sine function generators, the local feedback loop comprises two multipliers, a difference summing amplifier, a local loop gain element, a phase compensator, and a summing circuit to add a phase shift value to one of the two sine function generator inputs.
The invention provides an apparatus for controlling a focused beam of radiant energy, which includes an optical element for directing the focused beam, an actuator operative on the optical element for displacing the focal point of the focused beam in a direction of a predetermined position, and a detector receiving light from the beam. The detector is responsive to a displacement of the focal point of the beam from the predetermined position. A circuit coupled to the outputs of the detector produces an error signal representing the displacement of the focal point of the beam from the predetermined position. A servo is coupled to the actuator and to the error signal, and the actuator is responsive to the servo to displace the focal point of the beam onto the predetermined position.
According to the invention a local feedback loop circuit is coupled to the outputs of the detector. The loop includes a first periodic function generator responsive to the error signal, and a second periodic function generator responsive to the error signal. The second periodic function generator has an output that differs from an output of the first periodic function generator by a phase angle. The loop includes a first multiplier for multiplying the first output of the detector by the output of the first periodic function generator, and a second multiplier for multiplying the second output of the detector by the output of the second periodic function generator, wherein the outputs of the first and second multipliers are provided as inputs of the circuit.
According to an aspect of the invention the periodic characteristic is substantially sinusoidal, and the first periodic function generator and the second periodic function generator are sine generators.
According to another aspect of the invention the first and second outputs of the detector have an approximate mutual quadrature relationship with respect to the displacement of the beam and the phase angle is approximately ninety degrees. The phase angle may range from approximately 60 degrees to approximately 120 degrees.
The invention provides a method for focusing a beam of radiant energy, which is performed by generating first and second detection signals responsive to a position of a focus of the beam, and producing an error signal representing a displacement of the focus from a predetermined position, wherein the error signal has a periodic characteristic relative to the displacement. The method includes restoring the displaced focus to the predetermined position in response to the error signal by generating a first periodic signal responsive to the error signal, and generating a second periodic signal responsive to the error signal, wherein the second periodic signal differs from the first periodic signal by a phase angle. The error signal is produced by multiplying the first detection signal by the first periodic signal to yield a first product signal, multiplying the second detection signal by the second periodic signal to yield a second product signal, and determining a difference between the first product signal and the second product signal. Preferably the first and second detection signals, and the first and second periodic signals are substantially sinusoidal.
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Discovision Associates
Masaki Keiji
Tran Thang V.
Wong Steve A.
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