JP2595559B2 - Focus servo circuit - Google Patents

Description

The present invention relates to a focus servo circuit used in an optical disk device.

[Conventional technology]

FIG. 5 is a block diagram showing a conventional focus servo circuit.

The error signal calculation circuit 3 calculates a disc-lens variation ΔX (hereinafter referred to as a disc-lens variation ΔX) used for accurately converging the light beam on the recording material thin film with the sensitivity Sd.・ Output ΔX. The analog filter 14 has a gain Gf, receives Sd · ΔX, and outputs a filter output Gf · Sd · ΔX. The control circuit 16 has a sensitivity Sa, enter the filter output Gf · Sd · ΔX, and outputs the optical system following amount X _2. Subtracter 2 subtracts the optical system following amount X ₂ from the disc variation X ₁ input from the input terminal 1,
The result is output as a disc-lens variation ΔX.
Here an optical system following amount X ₂ is represented by Formula (1).

X ₂ = Sa · Gf · Sd · ΔX (1) Further, the subtractor 2 satisfies the expression (2).

X ₁ −X ₂ = ΔX ‥‥ (2) Equation (3) holds from equations (1) and (2).

Since Equation (3) is Sa ・ Gf ・ Sd≫1, Equation (4) holds.

Therefore, the variation of the optical disc (the amount of runout) is
0 μm, this is the variation ΔX of the relative distance = ± 0.5 μm
, It is necessary to set the loop gain (Sa, Gf, Sd) to 1000 or more. However, the sensitivities Sd and Sd sharply decrease (-12 dB / _DCt ) with respect to an input fluctuation _equal to or higher than the cutoff frequency determined by the mechanism, and the phase is delayed. Loop gain (S
If a · Gf · Sd) is 1 or more, stable operation cannot be expected. Therefore, the analog filter 14 performs phase compensation.

FIG. 6 is a circuit diagram showing the analog filter 14 of FIG. 5 in detail, and FIG. 7 is a characteristic diagram showing a frequency characteristic of the analog filter 14.

Operational amplifier _4-9, a non-inverting input terminal receives a reference voltage through an input terminal 4 _2, an output terminal connected to the output terminal 4 _3. Input terminals 4 ₁ connected in series resistor 4 ₄ and via a capacitor 4 ₇ is connected to the inverting input of the operational amplifier 4 _9, is also connected to the inverting input terminal via the resistor 4 ₅ .
Respectively resistor 4 ₆ and the capacitor 4 ₈ is connected to the output terminal of the operational amplifier 4 ₉ and an inverting input terminal. The resistances of the resistors 4 ₄ , 4 ₅ , and 4 ₆ are 470 KΩ, 10 KΩ, and 150 KΩ, respectively, and the capacitors 4 ₇ ,
0.33μF 4 ₈ volume respectively, when 10 pF, the frequency 22KH
Below z, the gain varies from about 25 dB to 49 dB, and the phase varies from about 0 degrees to +60 degrees.

[Problems to be solved by the invention]

Conventional focus servo circuit described above is using the analog filter 14 must perform phase compensation analog filter 14 for stability, because it requires a capacitor 4 _7, 4 ₈ large capacity However, there is a disadvantage that it is not suitable for integration into a circuit, and makes it difficult to reduce the cost, reduce the size, and make no adjustment.

[Means for solving the problem]

The present invention relates to a focus servo circuit that moves an objective lens of an optical disk device to accurately converge a light beam on a rotating optical disk, and inputs a disk fluctuation amount that is a signal indicating a deviation of the optical disk from a reference position. A calculation unit for calculating a disc-lens variation which is a difference from an optical system following amount which is a control signal for moving the objective lens; and an error signal corresponding to the disc-lens variation calculated by the calculation unit. , An A / D converter for A / D converting the error signal, a digital filter for filtering the output of the A / D converter, and a PWM converter for pulse width modulation of the output of the digital filter A focus servo circuit comprising: an amplifier that amplifies the output of the PWM converter; and a control circuit that outputs the optical system following amount based on the output of the amplifier.

[Action]

As described above, since a digital filter is used in place of a conventional analog filter, a large-capacity capacitor is not required, and integration into an integrated circuit is easy.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a focus servo circuit according to the present invention, FIG. 2 is a block diagram showing the control circuit 6 of FIG. 1 more specifically, and FIG. FIG. 4 is a block diagram showing the digital filter 5 shown in detail, and FIG. 4 is a characteristic diagram showing a frequency characteristic of the digital filter 5.

Subtractor 2 calculates the disc variation input from input terminal 1
By subtracting the optical system following amount X ₂ of X _1, and outputs the variation ΔX between disk lens. The error signal calculation circuit 3 receives the disc-lens variation amount ΔX, amplifies it, and outputs an error output. The analog / digital converter 4 (hereinafter referred to as A / D converter 4) receives the error output and performs A / D conversion. The digital filter 5 receives the output of the A / D converter 4 and
A predetermined phase compensation is performed. The control circuit 6 receives the output of the digital filter 5 which is phase-compensated, the PWM converter ₆₁ for performing a pulse width modulation, a driver 6 ₂ for amplifying an output of the PWM converter _61, driven by a driver 6 _{and second} output And
And controlling the optical system including a lens so as to accurately converge the light beam onto the recording material thin film, which is an actuator 6 ₃ for outputting an optical system following amount X _2.

The digital filter 5 of this embodiment is a digital version of the analog filter 14 shown in FIG. 5, and a calculation formula for setting the circuit of the digital filter 5 will be described.

The S function of the analog filter 14 is given by equation (1).

Here, a ₁ , a ₂ , b ₁ , and b ₂ are constants determined by the resistance and the capacitance element.

Frequency characteristic of the sensitivity of the actuator 6 ₃ controlled 0
Since the sampling frequency is a sufficiently high frequency of 44.1 KHz while the frequency of dB is 2 to 3 KHz, the equation (1) can be Z-transformed by the difference method. Equation (1)
Is replaced by a first-order S function as shown in equation (2) because the frequency used is low.

When this is Z-transformed, Equation (3) is obtained.

Here, A, B, and G are constants.

The digital filter 5 based on the equation (3) will be described with reference to FIGS.

The unit delay element 32 is connected to the A / D converter 4 via the input terminal 31.
And outputs a unit delay Z- ¹ . The multiplier 33 is
The output of the unit delay element 32 is multiplied by a constant A. The adder 34
The output of the A / D converter 4 and the output of the multiplier 33 are added. The multiplier 35 multiplies the output of the adder 34 by a constant G. Subtractor 36
Subtracts the output of the multiplier 38 from the output of the multiplier 35 and outputs the result to the output terminal 39. The unit delay element 37 receives the output of the subtractor 36 and performs a unit delay Z- ¹ . The multiplier 38 multiplies the output of the unit delay element 37 by a constant B.

The values of a ₁ and b _{1 in} the equation (2) are -3 × 10 ³ and −6.45 × 1 respectively.
0 ⁴ and when, A of formula (3), B, G respectively -0.9347, -0.
4062,4. With this setting, the frequency characteristic of the digital filter 5 changes in the range of about -7 dB to 14.5 dB and the phase changes in the range of 0 to +60 degrees at a frequency of 22 KHz or less as shown in FIG. Therefore, it is clear that the frequency characteristic is almost similar to that of the conventional example shown in FIG.

〔The invention's effect〕

As described above, according to the present invention, a signal having a large capacitance value is not required by processing a signal which has been conventionally processed by an analog filter in a focus servo circuit by a digital filter. The circuit can be easily formed, and as a result, there is an effect that the price of the optical disc device can be reduced, the size can be reduced, no adjustment can be made, and the quality can be improved. The change of the focus servo circuit accompanying the change of the mechanism including the system can be easily performed, and there is also an effect that the flexibility in designing and manufacturing the optical disk device is enhanced.

[Brief description of the drawings]

1 is a block diagram showing an embodiment of a focus servo circuit according to the present invention, FIG. 2 is a block diagram specifically showing a control circuit 6 in FIG. 1, and FIG. 3 is a digital filter 5 in FIG. FIG. 4 is a characteristic diagram showing a frequency characteristic of the digital filter 5, FIG. 5 is a block diagram showing a conventional example, and FIG. 6 is a circuit diagram showing an analog filter 14 of the conventional example in FIG. FIG. 7 is a characteristic diagram showing the frequency characteristic of the analog filter 14. FIG. 1 ... input terminal, 2 ... subtractor, 3 ... error signal operation circuit, 4 ... A / D converter, 5 ... digital filter, 6 ... control circuit, 6 ₁ ... PWM converter, 6 ₂ …… Driver, 6 ₃ …… Actuator.

Claims (1)

(57) [Claims] 1. A focus servo circuit for moving an objective lens of an optical disk device so as to accurately converge a light beam on a rotating optical disk, wherein the focus servo circuit detects a disk fluctuation amount which is a signal indicating a deviation of the optical disk from a reference position. A calculation unit for calculating a disc-lens variation, which is a difference from an optical system following amount, which is a control signal for moving the objective lens, and a disc-lens variation calculated by the calculation unit. An error signal calculation unit that outputs an error signal, an A / D converter that A / D converts the error signal, a digital filter that filters the output of the A / D converter, and a PWM that pulse-modulates the output of the digital filter A focus carbo circuit comprising: a converter; an amplifier for amplifying an output of a PWM converter; and a control circuit for outputting the optical system following amount based on the output of the amplifier.