JP2605914B2 - Focus control device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device for optically recording and reproducing information, and more particularly, to a focus control device for accurately irradiating a light beam on a recording surface.

[Conventional technology]

2. Description of the Related Art In an optical disk device that focuses laser light on a recording surface and irradiates information on the recording surface optically, it is required that the light beam be focused on an extremely small spot and accurately irradiated on the recording surface. For this reason, it is essential to perform focusing control that optically detects a focus shift (focus error) of the light spot with respect to the recording surface and moves the objective lens for beam focusing in a direction perpendicular to the surface so as to correct the shift.

[Problems to be solved by the invention]

However, in the conventional focusing control, there is no function of automatically adjusting the servo gain to an appropriate value by the device itself, and the gain is generally fixed to the value at the time of the initial adjustment. On the other hand, in the optical disk device, the focus error detection characteristics and the focus actuator driving characteristics vary depending on the optical head and the record carrier, and it is difficult to manually perform the gain adjustment even during the initial adjustment. Furthermore, changes in optical head characteristics over time, fluctuations in gain due to exchange of record carriers, etc. cannot be corrected,
There is a drawback that the servo gain may deviate from the optimum value, resulting in insufficient characteristics or unstable operation such as oscillation.

In order to solve these problems, it is only necessary that the device itself has a function that can automatically adjust the servo gain.
Conventionally, there was no appropriate method. As an example of a conventional gain adjustment method, a method of equipping an oscillator in a device, measuring a loop gain of a loop of a servo loop at a certain frequency, and adjusting the gain has also been proposed. It is necessary to separately set a special mode for performing adjustment from such measurement, and there is a disadvantage that it cannot be easily realized.

[Means for solving the problem]

The focus control device according to the present invention includes: a unit that detects a time from when a servo loop is turned on at the time of focus pull-in until the focus error signal crosses 0, a unit that detects a peak value of the overshoot after the 0 cross,
Means for changing the servo gain in accordance with the deviation of the time until the zero cross and the peak value of the overshoot from the respective prescribed values.

[Action]

The present invention monitors servo response characteristics at the time of focus pull-in, determines whether the gain is appropriate or not, and automatically sets the gain optimally.

The focus pull-in operation is always performed when the record carrier is exchanged, and can be executed without setting a special mode. In addition, when the gain is inappropriate, it is easy to repeat the pull-in operation again. Further, it is not necessary to provide an oscillator or the like, and according to the present invention, automatic gain adjustment of the focus servo system can be realized very easily.

〔Example〕

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

FIG. 1 is a block diagram showing one embodiment of the focus control device of the present invention.

The semiconductor laser 10 disposed in the optical head 2 is supplied with current from the laser drive circuit 30 to emit laser light.
The divergent laser light emitted from the semiconductor laser 10 is converted into parallel light by a collimator lens 11 and passes through a beam splitter 12 to enter an objective lens 13. The objective lens 13 focuses the light beam and forms a minute light spot on the surface of the record carrier 1. The objective lens 13 is attached to a focus actuator 14 and moves in a direction perpendicular to the surface of the record carrier 1, and is position-controlled so that a light spot is focused on a recording surface. It is well known that information is recorded or reproduced on the record carrier 1 by the light spot thus focused. The light reflected from the surface of the record carrier 1 passes through the objective lens 13 again and enters the beam splitter 12. The beam splitter 12 is constituted by a half mirror or a polarization beam splitter, and bends the optical path so that a part of the reflected light enters the focus detector 17. A part of the reflected light whose optical path is bent by the beam splitter 12 becomes a converging light beam by the converging lens 15. A knife edge 16 is disposed behind the focusing lens 15 to constitute a so-called knife edge type focus error detecting optical system. Note that this focus detection system is shown only as an example, and there is no difference in the basic function of the present invention between other detection systems. The luminous flux, which is almost half shielded by the knife edge 16, enters the focus detector 17. The focus detector 17 is a photodetector (photodetector) divided into two for the knife-edge type detection system, and an electric signal corresponding to a focus error is obtained by calculating a difference between outputs of the respective divided elements. can get. The differential amplifier 20 takes the difference between the outputs of the divided elements of the focus detector 17, amplifies the difference, and outputs a focus error signal 101. The focus error signal 101 is input to the focus servo circuit 21, where gain adjustment and phase compensation are performed and output as a focus servo signal 105. At the time of the focusing follow-up operation, that is, when the light spot is kept at the in-focus position with respect to the recording surface for recording / reproduction or track access, the analog switch 25 is set to the focus servo by the mode switching signal 109 from the focusing sequence control circuit 24. The signal 105 is passed and input to the power amplifier 31 as an actuator control signal 110. The power amplifier 31 uses this actuator control signal
A current corresponding to 110 is supplied to the focus actuator 14 to drive the position of the objective lens 13 so as to maintain an appropriate (focused) relationship with the record carrier 1. With such a configuration, the focusing following operation is realized. However, if the gain adjustment in the focus servo circuit 21 is not appropriate, insufficient focus tracking operation due to insufficient gain, that is, an increase in focus error occurs, or the tracking operation becomes unstable due to excessive gain, and further, oscillation of the servo loop may occur. There is a risk of causing it. In the focus control device of this embodiment, the servo gain set in the focus servo circuit 21 is automatically adjusted at the time of the focus pull-in operation.

Focus pull-in to shift from a state in which the objective lens 13 is not at the focal point position to the recording surface of the record carrier 1 to a focusing following operation in which the focused state is maintained, such as when the optical disc apparatus is started or after the record carrier 1 is replaced. The operation is performed. In this case, the focusing sequence control circuit 24
In response to the focus pull-in mode signal 108, the focus pull-in signal generation circuit 26 generates a ramp signal 106 for gradually raising the objective lens 13 from a state of being lowered (away from the recording surface) and approaching the record carrier 1. I do. At this time, the mode switching signal 109 indicates the pull-in mode, and the analog switch 25 passes the ramp signal 106 and is input to the power amplifier 31. This allows the objective lens
13 gradually approaches the record carrier 1. During this pull-in operation, the level detection circuit 22 monitors the focus error signal 101, and when the absolute value of the focus error becomes equal to or less than a predetermined value, that is, the objective lens 13 approaches a certain distance to the focal point position. When the focusing zone signal 11 indicates that focus servo is enabled
Outputs 1. Upon receiving the focusing zone signal 111, the focusing sequence control circuit 24 switches the mode from the focus pull-in mode to the focusing following operation. At this time, the focus pull-in mode signal 108
Is turned off to stop the generation of the ramp signal 106, the mode switching signal 109 indicates the focusing tracking mode, and the focus servo signal 105 is set to the analog switch 2
The analog switch 25 is switched so as to pass through 5. At the same time, the focus servo-on signal 107 is turned on to inform the focus gain adjustment circuit 23 that the servo loop has been closed from the state where the ramp signal for pulling in the focus has been generated. The focus gain adjustment circuit 23 operates the internal timer from this point to start measuring the time until the focus error signal 101 becomes 0 level (focus). The focus error signal 101 becomes level 0 when the objective lens 13 is further closer to the record carrier 1 by the operation of the servo loop and just at the in-focus position. The level detection circuit 22 detects the 0 level cross of the focus error signal 101 and transmits a focus 0 cross signal 102 to the focus gain adjustment circuit 23. The focus gain adjustment circuit 23 calculates the focus 0 cross signal 10
By the input of 2, the time measurement timer after closing the focus servo loop is stopped, and the time to zero cross is obtained. Even after the focus error signal 101 becomes 0, due to the transient response of the focus servo, the objective lens 13 slightly continues to approach the record carrier 1, so-called overshoot occurs. The level detection circuit 22 monitors the overshoot after the crossing of the focus error signal 0, detects the peak value of the overshoot amount, and uses the detected value as the overshoot peak signal 103 as the focus gain adjustment circuit 23.
Tell The focus gain adjustment circuit 23 compares the time from the focus servo ON to the focus error 0 cross obtained in the above-described time measurement and the overshoot peak value with a predetermined value, respectively, to determine the gain of the focus servo circuit 21. Is determined to be excessive or insufficient, and the gain adjustment signal 104 is used to reduce or increase the gain. That is, the position and speed of the objective lens 13 at the time of closing the focus servo at the time of focus pull-in are always substantially constant. Is too fast. Similarly, an excessive amount of overshoot also indicates that the gain of the servo loop is excessive. A long time until the focus 0 cross or an extremely small overshoot amount means that the gain is insufficient. Therefore, if the time until the focus 0 cross is short and the overshoot amount is large, it is determined that the gain of the focus servo circuit 21 is excessive, and
A gain adjustment signal 104 for lowering the gain of the focus servo circuit 21 is transmitted. Conversely, when the time to the zero cross is long and there is almost no overshoot amount, it is determined that the gain is insufficient, and a gain adjustment signal 104 for increasing the gain of the focus servo circuit 21 is transmitted.

Note that the level detection circuit 22 can be constituted by a plurality of comparators and a peak hold circuit, and the focus gain adjustment circuit 23 can be realized by a microprocessor having a timer. It can also be easily realized by an A / D converter for converting the focus error signal 101 into a digital value and a microprocessor.

By such an operation, the gain of the focus servo is automatically adjusted to an appropriate value. This adjustment operation can be repeated an arbitrary number of times as a focus re-pulling operation, and correction can be performed until the focus response characteristic, that is, the gain becomes an appropriate value.

FIGS. 2 (1), (2) and (3) are signal waveform diagrams showing the operation of the embodiment of FIG.

At the start of the focus pull-in operation, the mode switching signal 109 is in the pull-in mode (low level), and the ramp signal 106 is transmitted to the power amplifier 31. The ramp signal 106 has a large negative level at first, and lowers the objective lens 13 to a position sufficiently distant from the record carrier 1. Thereafter, as the level of the ramp signal 106 increases, the objective lens 13 gradually approaches the record carrier 1.
When the objective lens 13 approaches a certain distance from the record carrier 1, the level of the focus error signal 101 increases, and thereafter, as the focus lens position further approaches, the level decreases so that the focus error signal 101 approaches the 0 level. . The level of the focus error signal 101 is determined to a focus servo can becomes equal to or less than the value X _i to a predetermined mode switching signal 109 becomes a focusing follow mode (high level), the focus servo signal 105 to the power amplifier 31 Tell At the same time ramp signal 1
06 is returned to the 0 level. At this point, the servo tracking loop is closed, and the objective lens at this point
The 13 positions and speeds (that is, the value and the amount of change of the focus error signal 101) are the initial conditions of the servo response. These initial values the slope of the ramp signal 106 are the same, and for servo-on condition value X _i is the same, the same value for a number of times the pull-in operation. Therefore, the subsequent response of the objective lens 13, that is, the response of the focus error signal 101 reflects the characteristics of the focus servo loop. Focus error signal due to servo loop
101 approaches 0 level and crosses ₀ at time t0. Time T _R when closing the servo loop from the (t _S) until the 0 cross point (t ₀₎ is varied in accordance with the servo gain, T _R becomes shorter as the servo gain is large. Also, then the focus error signal 101 reaches overshot the negative peak values X _P. The magnitude X _P of the peak point is also changed by the servo gain, taking generally a large negative value as the gain is large. Thus, these by monitoring two values T _R and X _P of the servo gain suitable / unsuitable can be determined, can be corrected gain of the focus servo circuit 21 to a proper value. As a result, the gain of the focus servo loop is automatically set to an appropriate value, and a stable servo operation can be realized.

〔The invention's effect〕

As described above, the present invention monitors the timing up to the zero cross and the peak value of the overshoot at the time of the focus pull-in operation, compares them with the specified value assumed at the time of appropriate gain, and responds to the deviation. By correcting the servo gain, the focus servo gain can be automatically adjusted to an appropriate value without having a special oscillator or the like, and without having to set an operation mode for gain adjustment in particular. There is an effect that stable focus control can be performed even when the record carrier is replaced or the optical head changes over time.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a focus control device according to the present invention, and FIGS. 2 (1), (2) and (3) are waveform diagrams showing the operation of the embodiment of FIG. . 1 ... record carrier, 2 ... optical head, 10 ... semiconductor laser, 11 ... collimating lens, 12 ... beam splitter, 13 ... objective lens, 14 ... focus actuator, 15 ... focusing lens, 16 ... ... knife edge, 17 ... focus detector, 20 ... differential amplifier, 21 ... focus servo circuit, 22 ... level detection circuit, 23 ... focus gain adjustment circuit, 24 ... focusing sequence control circuit, 25 ... ... Analog switch, 26 ... Focus pull-in signal generation circuit, 30 ... Laser drive circuit, 31 ... Power amplifier, 101 ... Focus error signal, 102 ... Focus 0 cross signal, 103 ... Overshoot peak signal, 104 …… Gain adjustment signal, 105… Focus servo signal, 106 …… Lamp signal, 107 …… Focus servo on signal 108 …… Focus Pull mode signal 109 ...... mode switching signal, 110 ...... actuator control signals, 111 ...... focusing zone signal.

Claims (1)

(57) [Claims] 1. A focus control device for an optical disc device, wherein a focused laser beam is focused on a recording surface in order to record and reproduce information by irradiating a focused laser beam onto a record carrier. Means for detecting the time from when the servo loop is turned on to when the focus error signal crosses 0, means for detecting the peak value of the overshoot after the 0 cross, and the time to the 0 cross and the overshoot peak A focus control device for an optical disc device, comprising: means for changing a servo gain in accordance with a deviation of each value from a specified value.