JPH0621048Y2 - Focus control device for optical pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a focus control device for an optical pickup.

[Conventional technology]

A focus control device for an objective lens of an optical pickup that records or reproduces information on an optical disc, while irradiating a light beam on the optical disc with the focus control loop opened, brings the objective lens closer to the disc by a sawtooth wave and reflects the reflected light. The focus error signal is obtained by changing the shape of the object, the objective lens reaches the just focus point, and the zero-cross point at which the focus error signal becomes zero is detected.The focus servo loop is closed at this position, and the objective lens moves to this just focus point. Control to be stable.

In order to detect this zero-cross point, as is well known, a portion where the focus error signal is equal to or higher than a certain slice voltage is sliced to form a square-wave-shaped zero-cross detection signal, and the trailing edge of the zero-cross detection signal is used as the zero-cross point.

[Problems to be solved by the device]

However, especially when the focus servo loop is open, noise is included in the focus error signal due to the influence of lens vibration, etc., and chattering occurs at the leading edge of the zero-cross detection signal. Incorrect, sometimes focus search could not be performed.

The present invention has been made for the purpose of solving such problems.

[Means for Solving the Problems]

The optical disc device according to the present invention detects the just focus point of the objective lens by comparing the focus error signal with a reference voltage, and closes the focus servo loop of the objective lens. It is characterized in that a DC voltage is added to the focus error signal or the reference voltage so that the focus error signal does not coincide with the reference voltage before reaching the focus point.

[Action]

Therefore, before the objective lens reaches the just focus point, the focus error signal and the slice voltage do not cross each other, and the malfunction due to chattering can be prevented.

〔Example〕

 FIG. 1 is a block diagram showing an embodiment of the present invention.

In the figure, the focus error signal a from the focus error signal detection circuit 1 and one contact of the switch 9 are
It is connected to one fixed contact of the switch 3 via summing resistors 7 and 8, respectively, and is applied to the input terminal of the comparator 6. The output g of the triangular wave generator 2 is applied to the other fixed contact of the switch 3. The movable contact of the switch 3 is connected to the focus control coil 5 via the amplifier 4.

The output of the comparator 6 is connected to the clock input terminal of the D flip-flop circuit 12 via the inverting circuit 14.
The reflected light detection signal (HF signal) b obtained by detecting the reflected light from the optical disk is applied to the input terminal 11 a of the comparator 11. The comparator 11 outputs a focus point approach signal c that becomes high level when the objective lens comes near the focus point and the output of the reflected light detection signal b becomes a predetermined level or more. The focus point approach signal c
Is applied to the D input terminal of the D flip-flop circuit 12. The focus servo switch signal e obtained at the D output terminal of the D flip-flop circuit 12 is applied to the one-shot multi circuit 13 and controls the switch 3. A reference power supply 10 is connected to a movable contact of the switch 9, and a motor start signal f obtained at an output terminal of the one-shot multi-circuit 13 rotates a spindle motor (not shown) and controls the switch 9.

The operation of the above configuration will be described in detail below with reference to the waveform chart of FIG. The waveform diagrams shown by a to g in the figure show the waveforms appearing in the portions denoted by the same reference numerals in FIG.

The focus error signal a is obtained by a well-known astigmatism method or the like, and as the objective lens is brought closer to the optical disc, the focus error signal a as shown in FIG. 2A is obtained.

Since the objective lens is initially far away from the optical disc, the reflected light detection signal b is extremely small, and therefore the focus point approach signal c is at a low level. Further, the focus error signal a is at zero level, but since the switch 9 is closed, a constant voltage by the DC power supply 10 is applied through the resistor 8, and the focus error signal a ₁ applied to the comparator 6 is the offset voltage a. Only _{3 is} raised,
It becomes higher than the slice voltage a ₂ . Therefore, the zero-cross detection signal d is at high level and the output of the inverting circuit 14 is at low level.

Next, the reflected light detection signal b increases as the objective lens approaches the disc, and the focus completion signal c becomes high level when the reflected light detection signal b becomes equal to or higher than the predetermined level b ₁ determined by the comparator 11. Further, the focus error signal a ₁ also gradually increases, decreases toward the just focus point, passes through the zero cross point a ₄ , and further passes through the slice voltage a ₂ (ground level in this case). Becomes a low level, so that the output of the inverting circuit 8 becomes a high level.

In this way, with the focus completion signal c added to the D input terminal of the D flip-flop circuit 12 at a high level,
When the output of the inverting circuit 8 applied to the clock terminal becomes high level, the focus servo switch signal e which is the output of the D flip-flop circuit 12 becomes high level, the switch 3 is switched, and the focus servo loop is closed. Further, the one-shot multi 13 operates at the same time, and the motor start signal f, which is normally at a high level, has a predetermined time t
It goes low later. When this signal f becomes low level,
The spindle motor of the turntable (not shown) rotates,
As the signal reproducing operation from the optical disk is performed, the contact of the switch 9 is switched, and the offset voltage a ₃ becomes zero. Therefore, the focus error signals a and a ₁
, And correct focus control that is not affected by the offset voltage a ₃ is performed.

Note that the focus zero-cross point detection signal d causes chattering as shown in the figure after the zero-cross point is detected as described above, but the focus-point approach signal c applied to the D terminal of the D flip-flop 12 remains at a high level. The focus servo switch signal e does not change. Although by adding a constant voltage to the focus error signal a to obtain the offset voltage a ₃ in the above embodiment,
It is a matter of course that the present invention is not limited to this, and the reference voltage of the comparator 6, for example, may be a voltage corresponding to the offset voltage a ₃ .

[Effect of device]

As described above, according to the present invention, since chattering does not occur before the objective lens reaches the just focus point, malfunctions of focus search can be extremely reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a waveform diagram showing its operation. 1 ... Focus error signal detection circuit 3, 9 ... Switch 6, 11 ... Comparator 12 ... D flip-flop

Claims (1)

[Scope of utility model registration request] 1. A focus error signal detecting section for detecting a focus error signal at the time of a focus search for driving an objective lens from an distant point to a near point on an optical disk to fetch a focus, and a reference voltage generating section for generating a reference voltage. A DC voltage adding unit that adds a predetermined DC voltage to the focus error signal or the reference voltage, and a comparison unit that compares the focus error signal and the reference voltage,
In the focus control device of the optical pickup for detecting the coincidence of the comparison and closing the focus servo loop of the objective lens to complete the focus search, the direct current voltage is the coincidence of the comparison, the objective lens is the just focus of the objective lens. A focus control device for an optical pickup, wherein the addition is performed so that the optical disc is closer to the optical disc than the point, and the addition is canceled after a predetermined time has passed after the completion of the focus search.