JP2548110B2 - Focus position control device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus position control device for an optical pickup in an apparatus for recording / reproducing information on / from an information recording medium (hereinafter referred to as a disk) with a light beam. Is.

2. Description of the Related Art In recent years, various methods have been devised and developed for such a focus position control device, and are actually used for video disk players, document files, and the like. An example of the above-described conventional focus position control device will be described below with reference to the drawings. FIG. 2 shows an example of a conventional focus position control device. Reference numeral 1 is a disk in which information pits having a length of about 1 to 4 μm are recorded as light or dark information on concentric or spiral tracks at a track pitch of 1.5 to 2.5 μm. Reference numeral 2 denotes a motor, which is rotationally controlled with high precision in order to extract information recorded from the disc 1 as a time-series signal, and is called a disc motor. Reference numeral 3 denotes an objective lens for narrowing down a laser beam on the recording surface of the disc 1 and reading out an information signal with high precision. Reference numeral 4 denotes a focusing actuator, which constantly irradiates the beam spot narrowed down by the objective lens 3 against surface wobbling of the recording surface caused by the warp of the disk 1 and the tilt at the time of mounting on the disk motor 2. As described above, the position of the objective lens 3 is constantly corrected by the focusing actuator 4. 5 is a λ / 4 plate,
The plane of polarization of the light flux that has passed through the λ / 4 plate 5 changes. Reference numeral 6 denotes a polarizing prism, which is used to separate the polarized light of the incident light and the reflected light on the disk by changing the polarization planes. Reference numeral 7 denotes a collimator lens for collimating the laser light radially emitted from the semiconductor laser 8. A circuit 9 controls the output power of the semiconductor laser 8. 10 is a collimator lens, 11 is a knife edge set at the optical axis center of the collimator lens 10, 12
Are two-division focusing detectors I and II. The light reflected from the disk surface is configured to be focused on the focusing detector 12 by the collimator lens 10, and when the disk 1 and the objective lens 3 are closer than the in-focus position by the knife edge 11, the detector I, When it is far, most of the reflected light is incident on the detector II. The above optical system is housed in the pickup 13 and is driven in the radial direction with respect to the disc 1. In addition to the above-mentioned means in the pickup 13,
Means for correcting track fluctuations due to track eccentricity are stored, but are omitted here. The output signal of each detector is converted into a differential signal by the differential amplifier 14, and the phase compensation circuit 16 for ensuring the stability of the loop, the loop switch 17, and the gain variation due to the reflectance of the disk are corrected. A focusing servo loop is configured so that the focusing actuator is driven via the gain control circuit 18 and the drive circuit 19 so that the objective lens faithfully follows the surface fluctuation of the disk. Reference numeral 15 is a summing amplifier of the output signals of the detectors I and II, which is used as a control signal of the gain control circuit 18 for keeping the loop gain constant. The differential signal A of the differential amplifier 14 has a waveform example as shown in FIG. 3 in the knife edge system as in this configuration example. This is known, and D _S shown in the figure depends on the optical element used, but
It is about 10 μm. This D _S is the detectable range for detecting the surface deviation of the disk 1 and the focus shift of the objective lens 3 due to the positional change of the components of the optical pickup 13, and D _{O in} the figure
Is the operating point of the focusing servo. Normally, the objective lens is placed far away from the recording surface of the disc before the focusing servo is retracted, and in order to retract the focusing laser from that state, the objective lens is forced onto the disc surface. Close to the above-mentioned D _S
The range needs to be detected. Therefore, as shown in FIG. 2, the pull-in start signal 22 is generated from the pull-in start circuit 21 by the pull-in start signal 22, the start signal is applied to the focusing actuator through the drive circuit 19, and the objective lens 3 is moved to the surface deflection of the disk. The disk recording surface is brought close to the detectable range. The pull-in detection circuit 20 detects that it is within the D _S range shown in FIG.
Turn 17 on to close the servo loop. At the same time, the pull-in start signal is turned off. FIG. 4 is a waveform example of each signal from the start of the pull-in activation to the closing of the servo loop. A to D in the figure are A to D shown in FIG.
D corresponds to, t _O is retracted starting start time, t ₁ denotes the time when the servo loop is closed.

Heretofore, an example of the focus position control device has been described. In this example, the knife edge method is adopted as the surface wobbling detection method, but as is well known, there are a critical angle method, an astigmatism method, etc. There is not much difference in the components.

Problems to be Solved by the Invention The above-mentioned configuration has the following problems.

Originally, the D _O point shown in FIG. 3 is the operating point of the focusing servo and should be the focus position, but each optical element 3, 5, 6, 7, 7, 10, in the pickup 13 shown in FIG. There is a problem that the D _O point does not always correspond to the in-focus position due to a slight positional deviation of the focusing detector 11, the focusing detector 12, and the semiconductor laser 8. Therefore, if the servo loop is closed with D _O as the operating point, the best focus servo will not be obtained, and defocusing will occur, and C / N and S / N will decrease when reproducing the information signal. In some cases, the information signal may not be normally read out.

In view of the above problems, the present invention sets the point at which the true focusing point is the operating point of the servo loop.

Means for Solving the Problems In order to solve the above problems, the focus position control device of the present invention is a focus position control loop for performing drive control in the optical axis direction of an objective lens for guiding light from a light source to a disc, A drive circuit provided in the focus position control loop that receives the signal from the pull-in start circuit to drive the lens to bring it into a focused state, and a pull-in circuit that detects that the lens is in the focused state and issues a signal. After receiving a signal from the detection circuit and the pull-in detection circuit, the focus position control loop operates and the offset signal for correcting the defocus of the lens after a lapse of a predetermined time from the in-focus state is obtained. And an offset adjusting means to be added to the focus position control loop.

Example Hereinafter, an example of the focus position control device of the present invention,
This will be described with reference to the drawings.

FIG. 1a shows a block diagram of the embodiment. In the figure, components having the same reference numerals as those in FIG. 2 are the same as those in FIG. 2 and perform the same functions. The components different from those shown in FIG. 2, that is, the summing amplifier 24, the DC voltage generating source 25, the switch 26 as the offset adjusting means, and the delay circuit 27 are additional elements according to the present invention. Signal waveform examples A to G in FIG.
Corresponds to A to G in FIG. Below, the first
The operation will be described with reference to FIGS. The constituent elements other than the additional elements according to the present invention are the same as those in the conventional example and will not be described. At t _O ′ in FIG. 1B, the pull-in activation start signal C is turned “ON”, the pull-in operation is performed, and the pull-in detection signal D is output at t ₁ ′. The pull-in detection signal D is delayed by a delay circuit for a desired time, and further, the waveform is blunted through an integrating circuit as shown at E in FIG.
Is gradually changing from “OFF” to “ON”. When the switch 26 is turned “ON”, the DC voltage F set by the volume 25 is added to the servo loop by the adding amplifier 24.
When the DC voltage F is added, the output G of the adding amplifier 24 is the first
As shown in FIG. B, the bias voltage of E'is applied. Here, E'is an operating point for bringing the disc recording surface and the objective lens to a true in-focus position, which reproduces the reference recording track in advance and changes the DC voltage F set by the volume 25. Then, the operating point at which the best condition is obtained is obtained from the C / N of the reproduced signal.

Now, for example, when detecting the pull-in of the bias voltage E '
When applied without delay to t ₁ ′, the DC bias voltage is applied at the same time as the servo loop is closed. Immediately, it becomes a step response, and it may not be a stable pull-in. Therefore, the delay circuit 27 plays a very important role in ensuring the stability of pulling in of the focusing servo. When a FET switch is used as the switch 26, the delay circuit 27 waits for the effect of delay even if the gate signal has a waveform as shown by E ″ in FIG. It is also possible to configure an integrating circuit with.

As described above, according to the present invention, after a desired delay time has passed from the time when the servo loop was closed, a preset DC voltage was added to the loop, so that an optical pickup caused by a temperature change or the like was generated. It is possible to correct the defocus of the objective lens caused by the positional fluctuation of the optical components inside.

Further, the focus position control device of the present invention, after the focusing state, that is, after the operating point of the focusing servo is located near the center of the detectable range of the position variation of the objective lens from the focused state, the offset Since the signal is applied to the drive circuit, even if the position of the objective lens fluctuates due to the application of the offset signal to the drive circuit and the position of the operating point of the focusing servo fluctuates, this operating point deviates from the above detection range. It is difficult to do so, and even when an optical head having a large defocus of the objective lens due to the position variation of the optical components constituting the optical pickup is used, this defocus amount is easy to be found and the zero point adjustment can be easily performed. Become.

[Brief description of drawings]

1a is a block diagram of an embodiment of a focus control device according to the present invention, FIG. 1b is a timing diagram of each output signal in the same embodiment, FIG. 2 is a block diagram of a conventional focus control device, FIG. 3 is a diagram showing a waveform example of a focus position error signal of a knife edge type focusing servo, and FIG. 4 is a timing diagram of each output signal in the conventional example. 1 ... Disc, 3 ... Objective lens, 4 ... Focusing actuator, 12 ... Focusing detector, 14 ... Differential amplifier, 16 ... Phase compensation circuit, 17 ... Loop switch, 19 ... Driving circuit, 20 …… Pull-in detection circuit, 27 …… Delay circuit, 26 …… Switch.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-121139 (JP, A) JP-A-59-19248 (JP, A) Practical application Sho-56-131543 (JP, U)

Claims (1)

(57) [Claims] 1. A focus position control loop for controlling the drive of an objective lens in the optical axis direction, which guides light from a light source to a disk, and receives the signal from a pull-in starting circuit to drive the lens to bring it into a focused state. A drive circuit provided in a focus position control loop, a pull-in detection circuit that detects that the lens is in the in-focus state and issues a signal, and a focus position control loop that receives a signal from the pull-in detection circuit And a focus position control device that applies an offset signal for correcting the defocus of the lens to the focus position control loop after a predetermined time has elapsed after the operation of the above-mentioned focusing state.