JPS63241732A - Focus servo device for optical disk device - Google Patents

Abstract

PURPOSE:To eliminate the asymmetric property of a servo gain due to an operating direction, by providing a gain correcting means, responding to positive and negative asymmetric properties in the sensitivity characteristic of a focus error detecting means, and changing the amplifying gain of a focus error signal. CONSTITUTION:A gain correction circuit 15 amplifies the focus error signal FE4 passing through a phase compensation circuit 10, and supplies its output FE5 to a driving circuit 11. At the circuit 15, the gain is changed corresponding to the polarity of the signal FE4. And it is possible to adjust at least either a gain of positive input or a gain of negative input. In such constitution, by adjusting the gains of operational amplifiers 3 and 4 so as to be different from each other, the zero point of an error is aligned to that of a signal. Thereby, it is possible to obtain different inclination of sensitivity characteristic of the signal FE on a positive side and a negative side. The gain of the circuit 15 is adjusted corresponding to the asymmetric property. Thus, adjustment is performed so as to eliminate the positive and the negative asymmetric properties as a whole of servo loop. In such a way, it is possible to obtain the optimum servo gain in spite of the displacing direction of an objective lens.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an optical disc device for recording and reproducing information on an optical disc, in which a laser beam from an optical head is accurately applied to an information recording layer of an optical disc. This invention relates to an improvement of a focus servo device.

FIG. 5 shows the basic structure of a focus adjustment mechanism incorporated in a conventional optical head. The laser beam is reflected by an objective lens 14 and focused onto the optical disk 13. The objective lens 14 and its lens barrel 21 are attached to the base of the optical head so as to be vertically movable, and the lens barrel 21 is vertically displaced by a drive coil 12 and an electromagnetic actuator using a magnet or a spring. The amount of displacement is controlled by the current supplied to the coil 12. The focus servo system, which will be explained next, uses the objective lens 14.
The distance between the optical disc 13 and the optical disc 13 is kept constant, and the optical disc 1
This is a control system that adjusts the current of the coil 12 so that the beam 20 is always correctly focused on the information recording layer No. 3.

FIG. 4 shows a general configuration of a conventional focus servo device.

This device includes a pair of photodetectors 1 and 2 provided on the optical head to receive the reflected light of the laser beam irradiated from the optical head to the optical disk, and amplifies the outputs of the photodetectors 1 and 2. Preamps 3 and 4 and Preamps 3 and 4
A differential amplifier 5 obtains a focus error signal FEI by adding the differential output of the preamplifiers 3 and 4, and a sum amplifier 6 obtains a received light level signal FSI by adding the outputs of the preamplifiers 3 and 4. Error signal FBI and received light level signal FS
Gain switching circuits 7 and 8 (outputs are respectively FE2 and FS2) that switch the amplification gain of I, and a focus error signal FE2 based on the received light level signal FS2.
An AGC (automatic gain control) circuit 9 obtains a focus error signal FE3 that compensates for changes in the reflectance of the optical disk by changing the amplification gain of the focus servo, and stabilizes the servo by increasing the phase margin at the gain intersection of the focus servo. A phase compensation circuit 10 for increasing the servo gain at low frequencies, and the coil 1 for focus adjustment by current amplifying the focus error signal FE4 that has passed through this circuit 10.
The drive circuit 11 supplies excitation current to the drive circuit 2.

Since the power of the laser beam irradiated to the optical disc is switched between recording and reproducing information, the intensity of light received by the photodetector 1.2 naturally changes as well. Gain switching circuits 7 and 8 compensate for this difference, and the gain is switched between recording and reproduction. Note that in a playback-only device, this circuit 7
．． There is no 8.

If the laser beam is accurately focused on the information recording layer of the optical disc 13, the focus error signal FE
The level of (FEI, FE2.FE3.FE4) is zero, and if the information recording layer is out of focus before or after the information recording layer, the level of the focus error signal FE increases to be positive or negative depending on the direction and amount of shift.

The current supplied to the coil 12 is adjusted according to the polarity and level of the focus error signal FE, the position of the objective lens 14 is changed, and the distance between the objective lens 14 and the optical disk 13 is kept constant. In this way, the focus error signal FE
The servo loop functions in such a way that the optical disc 130 surface runout is minimized, and the laser beam application can always be imprinted on the information recording layer.

The characteristics of the focus error signal FE will be described in detail. 6th
In the graph shown, the objective lens 14 and the optical disc 1
The appropriate value of the distance from
level.

Characteristic ■ of prisoners in Figure 6 is an ideal specification.

In characteristic (2), the error signal FE becomes exactly zero at the point where the actual distance error between the lens and the disk is zero, and the change in the error signal FE with respect to positive and negative distance errors exhibits symmetrical characteristics.

It is extremely difficult to realize such ideal characteristics (1) on an actual device. The mounting of photodetectors 1 and 2 depends on the accuracy of the position, the mounting of the optical system for detecting focus errors related to photodetectors 1 and 2 depends on the accuracy of the position, and the relative positional relationship of these, etc. On the device, the characteristics of the focus error signal are usually as shown in Figure 6.
As shown in FIG.

In characteristic (2) with an offset, the error signal FE does not become zero at the point where the interval error is zero, so correct focus servo cannot be performed as is. For each assembled device, it is not impossible to repeatedly fine-tune the positions of the photodetector 1 and the optical system and measure the characteristics, and finally achieve the ideal characteristics (2). However, in an actual mass production system, this is impossible in terms of time and cost. To this end, the above-mentioned offset is corrected by circuit countermeasures that can be implemented easily.

As means for correcting the above-mentioned offset using a circuit, the first method is as follows.
Another possibility is to externally apply a correction voltage corresponding to the offset amount to the servo loop.

However, since the specific amount of offset varies depending on the amount of light reflected from the optical disc, it cannot be corrected by applying a constant voltage, and it is difficult to always accurately correct the varying amount of offset due to the circuit. also becomes very complicated.

Therefore, correction using this method is not actually adopted.

The countermeasure actually taken is to individually adjust the amplification gains of preamplifiers 3 and 4 shown in FIG. 4 to change the characteristics of focus error signal FE and eliminate the offset. Specifically, for the characteristic ■ with offset in Figure 6 (4), preamplifiers 3 and 4
The gains are adjusted unequally so that the error signal FE also becomes zero at the point where the interval error is zero, as shown in characteristic (3) in FIG. 6 (03).

In this method, it is only necessary to make the gains of the preamplifiers 3 and 4 adjustable, and there is no need to add a special correction circuit. Further, adjustment work is also easy.

Therefore, this method is generally adopted.

Problems to be Solved by the Invention As mentioned above, if the offset of the focus error signal FE is eliminated by making the gains of the preamplifiers 3 and 4 different, the positive side of the error signal FE will naturally appear as shown in the characteristic (■) in FIG. The rate of change is different on the negative side, resulting in asymmetrical characteristics. This means that the detection sensitivity is different depending on whether the lens/disk distance is too large or too small, and furthermore, the servo gain is different for the operation of moving the objective lens 14 closer to the optical disk 13 and the operation of moving it away from it. . Although this is not a fatal flaw in the focus servo, it is one of the obstacles to realizing a high-performance servo.

This invention was made in view of the above-mentioned conventional problems, and its purpose is to eliminate the asymmetry of servo gain depending on the direction of operation, and to realize a focus servo with high precision and high performance. be.

Means for Solving the Problems Therefore, in the present invention, in response to the positive/negative asymmetry of the sensitivity characteristics of the focus error detection means, the lens/disk distance is driven in the direction of increasing it and when it is driven in the direction of decreasing it. A gain correction means is provided to vary the amplification gain of the focus error signal.

The characteristics of the action focus error detection means are, for example, the characteristics shown in FIG.
As shown in the figure, the slope of the error signal FE is large in the positive region (the lens/disc distance is too large) and is large in the negative region (the distance is too large).
Suppose that it is small. If the gain correction means according to the present invention were not provided, the servo gain during driving that increases an excessively small interval would be smaller than in the opposite case. Therefore, in the present invention, the gain correction means is used to make the amplification gain during driving in the direction of increasing the distance larger than in the opposite case. This eliminates the asymmetry in the positive and negative gains of the servo loop as a whole.

Embodiment FIG. 1 shows the overall structure of a focus servo device according to one embodiment of the present invention. In FIG. 1, photodetectors 1 and 2, preamplifiers 3 and 4, differential amplifier 5, sum amplifier 6, gain switching circuits 7 and 8, AGC circuit 9, phase compensation circuit 10, drive circuit 11, focus adjustment coil 1
Since the configuration and operation of 2 are the same as those of the conventional example shown in FIG. 4, the previous explanation will be used for these.

This embodiment differs from FIG. 4 in that a gain correction circuit 15, which is the aforementioned gain correction means, is inserted between the phase compensation circuit 10 and the drive circuit 11.

The gain correction circuit 15 is a circuit that amplifies the focus error signal FE4 and supplies its output FE5 to the drive circuit 11, and is characterized in that its gain differs depending on the polarity of the input signal FE4.

At least one of the positive input gain and the negative input gain can be adjusted as appropriate.

Two specific examples of the gain correction circuit 15 are shown in FIGS. 2 and 3.

In the example shown in FIG. 2, an operational amplifier 16 that amplifies only the positive input through the diode D1 and an operational amplifier 17 that amplifies only the negative input through the diode D2 are provided in parallel, and the gain of the operational amplifier 17 is variable. It can be adjusted by resistor VRI.

The example shown in FIG. 3 consists of one operational amplifier 18, and its feedback path is provided with a series circuit of a diode D3 and a variable resistor VR2 through which current flows only when there is a negative input. The gain of the negative input can be adjusted with a variable resistor VR2.

In the above configuration, the optical head was assembled as described above, and the sensitivity characteristics of the focus error signal FE were actually measured.
Since an offset like the characteristic ■ in Figure 6 is normal,
Adjust the gains of operational amplifiers 3 and 4 to be different,
Match the error zero point and the signal zero point as shown in characteristic ■.

Then, the slope of the sensitivity characteristic of the error signal FE becomes different between the positive side and the negative side.

The gain of the gain correction circuit 15 is adjusted in accordance with this asymmetry. For example, the negative side (the interval is too small) as in characteristic ■
If the sensitivity is small, the gain of the negative input (drive command in the direction of increasing the interval) of the gain correction circuit 15 is increased appropriately. In this way, the function of the gain correction circuit 15 may be included in the drive circuit 11, or between the AGC circuit 9 and the phase compensation circuit 10. There is no problem even if it is set to .

Effects of the Invention As described in detail above, according to the present invention, the positive and negative asymmetry of the sensitivity characteristics caused by the gain adjustment of the preamplifier of the focus error detection section can be corrected by adding a very simple gain correction circuit and adjusting the gain. This can be resolved by making adjustments, and a stable and optimal servo gain can always be obtained regardless of the direction of displacement of the objective lens.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a focus servo device according to an embodiment of the present invention, FIGS. 2 and 3 are circuit diagrams showing two specific examples of a gain correction circuit in the same device, and FIG. 4 is a block diagram of a conventional device. 5 is a schematic diagram of the focus adjustment mechanism, and FIG. 6 is a characteristic diagram of the focus error signal. 1.2...Photodetector, 3,4...Preamplifier, 5.
... Differential amplifier, 6 ... Sum amplifier, 7, 8 ... Gain switching circuit, 9 ... AGC circuit, 10 ... Phase compensation circuit, 11 ... Drive circuit, 12 ... Focus Adjustment coil, 13... Optical disk, 14... Objective lens, 15... Gain correction circuit

Claims (1)

[Claims] a focus error detection means having asymmetrical detection sensitivity characteristics depending on whether the distance error between the objective lens of the optical head and the optical disk is positive or negative; and focus adjustment for adjusting the distance between the objective lens and the optical disk by displacing the objective lens. means, a servo amplification means for driving the focus adjustment means based on a focus error signal from the detection means to minimize the interval error, and a servo amplification means included in the servo amplification means to accommodate the asymmetrical characteristic of the detection means. A focus servo device for an optical disk device, further comprising gain correction means for varying the amplification gain of the focus error signal when driving in a direction to increase the lens/disc distance and when driving in a direction to decrease the lens/disk distance.