JPS5811693B2 - Focus servo method in optical reproducing equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus servo system in an optical reproducing device, in which the reading lens and the disc are optimized within a range where reflected light (or transmitted light) from an information recording medium (hereinafter referred to as a disc) exists. It detects when it approaches beyond the focus position and restricts it from approaching any further, and also adjusts the detector output to have a slight offset when the servo is retracted, allowing the servo loop to operate and read when there is no light. It is an object of the present invention to provide a focus servo system in an optical reproducing device that can prevent collision between a reading lens and a disk by configuring the lens to move away from the disk.

In an optical reproducing device that uses a light beam such as a laser beam to read and reproduce a disc on which information signals are recorded in a spiral or concentric manner by countless continuous pits,
It is well known that a focus servo is used to follow the surface runout of the disk and form a spot on the disk surface.

In this focus servo, a focus detection device using an auxiliary beam is conventionally known as an example of means for detecting the relative position between the reading lens and the disc information surface.

In this focus detection device, when a beam incident on the reading lens at a position far from the optical axis is reflected by the disk after passing through the reading lens, the position and angle at the time of reflection change depending on the position of the reflecting surface, and as a result, A position change is applied to the beam, and this is read out as a left/right displacement by two differentially coupled photodetection diodes.

Generally, the final control performance of focus servo is N, A,
±1.5 to 2.0/μm when using 0.40 reading lens
This degree is allowed as an error range, and it is necessary to detect the surface runout of the disk with an accuracy below this value.

Normally, the degree of amplification of the amount of displacement in the plane runout direction and the amount of lateral displacement of the differential detection position is about 10 times, and in order to use the main beam for signal reading and the auxiliary beam for focusing together, the aperture of the reading lens is , the size of the photodetection diode and the size of the light-receiving area), the effective detection range on the photodetection diode is several mm, and based on the amplification factor of 10,
Due to the displacement of the reading lens and the disk from the just focus position, the effective detection area is ± several hundred μm.

Therefore, if the disk position is within this range, a focus detection signal will be obtained, but if it is outside this range, the reflected beam will be located outside the photodetection diode, and the detection signal will be zero or a residual offset voltage. .

In addition, the focus detection signal is influenced by the size of the beam and the size of the detection diode as described later, and there is little linear slope area in the focus detection signal vs. distance characteristic between the disk and reading lens required for the servo loop. , which also includes a saturated region and a reverse polarity region.

When the sensitivity of focus detection is increased, the displacement of the reflected auxiliary beam becomes large and easily exceeds the range of the detection diode.

Therefore, as a method of expanding the detection range without reducing the detection sensitivity near the servo lock, a method of inserting a specially processed lens into the optical path of the reflected auxiliary beam was proposed in JP-A-51-43.
No. 948 ``Photo-electronic conversion corner point detection device''.

However, although this proposed device can expand the detection range to a certain extent, it has the drawback of increasing the number of optical elements and requiring special processing. It is questionable whether countermeasures can be taken without installing auxiliary equipment.

In other words, this proposed device can be configured to expand the detection range and not include the polarity inversion region, but when considering the insertion position of the special lens in practice, the auxiliary focus beam and the main signal reading beam coexist near the optical axis. Therefore, there are difficult problems such as the location is limited and the size is limited in order to affect only the auxiliary focus beam without affecting the signal reading main beam.

Generally, in order to widen the tracking range of the focus servo, increasing the current sensitivity of the unit or decreasing the spring constant to make it easier to obtain amplitude in the low frequency region (below the fundamental wave) will improve control performance. On the other hand, the reading lens easily moves toward the disk surface due to overshoot during retraction due to insufficient braking, fluctuations due to power line shock, and other C disturbances, creating the risk of collision.

As a measure to prevent this collision, Japanese Patent Application Laid-Open No. 50-110647
``Circuit Arrangement for Aligning the Image of an Information Structure with a Transducer'' and JP-A-51-89444 ``Information Reading Apparatus for a Record Carrier Having a Flat Reflective Surface'' have been proposed.

In the former case, if the lens is retracted when loading a disc and then extended to the focus position during playback, if the lens is extended too quickly, sufficient braking will not be applied, and if the working distance of the reading lens is particularly short, the lens may Since disk collisions occur, the voltage of the speed detection coil provided in the focus drive unit is used to prevent the ejecting speed from exceeding a certain value.

However, although this proposed circuit layout is effective for the purpose of preventing collisions mentioned above, it requires a speed detection coil,
This has the disadvantage of complicating the structure of the focus drive unit.

In addition, in the latter method, the focus lock-in process after loading the record is performed by a certain sequence of operations, and at the time of lock-in, the focus is passed through a pseudo pull-in point on the transparent surface of the transparent disc, and moreover, the focus lock-in process is performed at an excessively high speed. This is an information reading device that prevents lens collisions due to overshoot.

However, in this proposed device, the lock-in process is performed sequentially from an initial state, and the premise is only during the lock-in process, and does not refer to the prevention of collisions due to undesirable disturbances.

Concerning the collision between the record and the lens, countermeasures must be taken by assuming the worst-case conditions.

Conditions include the initial pull-in when playing after replacing and setting the disc, or when there is no light due to a failure of the laser source, when the reflected light becomes diffused light due to the proximity of the lens and the disc, and the auxiliary beam is not reflected. Measures must be taken under conditions such as the locking of the focus servo due to undesirable disturbances such as shocks caused by disconnection of the high power source by a plunger relay or the like.

The present invention detects that the reading lens and the disk have approached beyond the optimum focus position within the range where reflected light exists, and restricts the reading lens from approaching the disk any further, and also slightly changes the detector output when the servo is retracted. Adjust to have an offset,
When the beam disappears, the differential output detects the two focus points (
By configuring the system so that the servo loop operates and the reading lens moves away from the disk when the differential output is in the dark current region of the diode (auxiliary beam detection), all of the above-mentioned drawbacks have been eliminated. One embodiment will be described with reference to the drawings.

FIG. 1 shows an optical system and circuit system of an embodiment of the method of the present invention.

In the figure, a laser beam emitted from a laser source 1 is split into two beams by a beam splitter 2.

A reflective thin film 2a having a reflectance of about 10% is deposited on the surface of the beam splitter 2, and a part of the incident laser beam is reflected here, and the rest is transmitted.

The laser beam reflected by the reflective thin film 2a passes through a half mirror (or a polarizing prism) 3 as an auxiliary beam, and is directed toward the reading lens 4 parallel to the optical axis of the reading lens 4 and at a distance r from the optical axis. A point image is formed and reflected by the disk surface.

This auxiliary beam is diffused light and is reflected with a diameter sufficiently large compared to the size of the information track, so that the disk reflecting surface acts as a mere mirror.

On the other hand, the laser beam that has passed through the reflective thin film 2a is reflected by a total reflection mirror 5 formed on the back surface of the beam splitter 2, and is then divided into three diffracted lights (
The zero-order diffracted light is used as a signal reading beam, and the ±1st-order diffracted light is used as a tracking detection beam), and is further focused by a beam expanding lens 5, and then transmitted through a half mirror 3 to the aperture of a reading lens 4. The image is magnified and enters the reading lens 4.

These three diffracted beams are focused by the reading lens 4 as three minute spots on the information surface of the disc, and are reflected light whose light intensity is modulated in accordance with the information signal formed by the pits on the disc.

Since this signal reading beam is diffused light and its point light source is at a finite position, the focus position of the disk is set by the reading lens 4.
It is further away than the focal plane of

Therefore, the auxiliary beam, which is parallel light, is reflected as an expanded beam on the disk surface.

In FIG. 1, the reflected light reflected by the disk surface located at any of the positions -Xo, Xl, or X2 passes through the reading lens 4, is reflected by the half mirror 3, and is further transmitted to the focus detection diode D1. D2 and D3 signals are guided to photoelectric conversion sections such as a detection diode D4 and a tracking detection diode D5 t D6.

That is, among the reflected lights, the reflected light from the three spots (king beam) is magnified by the magnifying lens 6 to a detectable size, and then the reflected light from the signal reading spot is transmitted to the signal detection diode D4 and the two tracking beams. The light reflected by the detection spot is passed through a tracking detection diode D,
, D, respectively.

On the other hand, the auxiliary beam for focus detection is generated by two focus detection diodes D1. D2 receives the light, converts it into an electrical signal with a level corresponding to the light intensity, and then supplies it to the dynamic amplifier 7, which then sends the focus detection diode D1. The difference between the outputs of D2 is taken out as a focus detection voltage S1 and applied to the lens drive mechanism so that the height position of the reading lens 4 is always at a predetermined position.

Auxiliary beam focus detection diode D1. To explain the movement on D2, the parallel light auxiliary beam that enters the reading lens 4 at a distance r from the optical axis forms a point image at the focal position after passing through the lens.

As mentioned above, since the focus position of the disk is far from the focal plane of the reading lens 4, the auxiliary beam hits the disk in the form of a beam expanded from a point image, and the disk reflective surface is a conjugate image of the point image. Reflected light is formed as if the light were emitted.

Focus detection diode D1 t D2 of this reflected light
The position on the light receiving surface varies depending on the position of the disc.
The radius value when passing through the reading lens 4 is changed from a ratio to a dog when the lens is close to the reading lens 4 and when it is far from the focus point.

If the disc surface is close. The angle of the auxiliary beam reflected light becomes shallower, closer to the optical axis of the reading lens 4, and the focus detection diode D1. Of D2, light is received by Dl.

Conversely, if the distance is far, the angle becomes deep, and the reflected light is received by the focus detection diode D2.

Therefore, as the output of the differential amplifier 7, a focus signal of negative polarity is obtained when the disk surface is close, and a focus signal of positive polarity is obtained when the disk surface is far.

The auxiliary beam focus detection diode D1. The ratio of the displacement ΔZ on the crow is usually about 10 times.

In FIG. 1, the tracks on the disk surface are actually distorted and deviate from the ideal, but the tracking galvanometer mirror for making the reading spot follow the track is not shown in the figure. In order to prevent the effect of this mirror on the auxiliary beam from appearing as crosstalk in the changes in the beam, the auxiliary beam is incident on the rotation axis of the galvanomirror in the longitudinal direction of the focus detection diode (in the slit direction). ) where the movement of focus detection diode D1 is the least.
．． D2 is selectively arranged on the optical axis near the reflection conjugate point image.

At this position, the interval between the auxiliary beam reflected light a0 and the signal conjugate point image at the time of just focus is approximately equal to the radius r.

Also, since the conjugate point is a point image, the focus detection diode D1. D2 can be arranged with dimensions close to r.

Normally, r is 2 to 3 mm, so if we also take into account the magnification of 10, the disc will move ±200 to 200 mm from the focus position.
When the distance is within the range of 300 μm, the auxiliary beam reflected light is detected by the focus detection diode D1. When the light hits D2 and is displaced beyond that range, the reflected light is reflected by the detection diode D1.
．． It will be located outside D2.

When these relationships are shown in terms of the relationship between the focus detection voltage S1 and the relative distance d between the focus detection lens and the reading lens, a figure-eight curve is drawn as shown in FIG. 2A.

Here, the relative distance between the reading lens and the reading lens is usually indicated by the dimension measured from the main surface of the reading lens 4, but in reality it is better to measure it from the glass surface force at the tip of the reading lens 4. Since it is convenient to see the degree of risk of collision, this relative distance d is set as such here.

Further, the relative distance do is the working distance when the lens is just focused.

When loading a disc into the optical playback device, or when no action is applied to the reading lens 4 (when the current is zero), the height changes depending on the mutual relationship of the tapered holes in the clamping part of the disc. Normally, the reading lens 4 is initially located far away from the disk, and then approaches the disk during the transition period after the power is turned on and before the focus servo lock-in. The structure is considered.

When the reading lens 4 moves from such a far distance to the focus position, a voltage S1 as shown in FIG. 2A is obtained at the differential amplifier IC output. Since the disk is moving up and down due to surface runout, simply moving the height of the reading lens 4 closer to the disk surface without controlling the servo loop may cause a collision due to a large portion of the surface runout. Therefore, the reading lens 4 is normally brought close to the disk with the loop closed.

When the power is turned on, the focus detection diode D1.
Since the reflected auxiliary beam is not received by any of D2, the focus detection voltage S1, which was approximately zero, is reduced by the focus detection diode D2 first before the retracting operation of the focus servo loop as the reading lens 4 approaches the disk surface. Since the reflected auxiliary beam is only received by the auxiliary reflected beam (at this time, the disk surface is at the position indicated by
It begins to rise in the positive direction from the time when light is simultaneously received by Dl in addition to 2, and becomes approximately zero again when the same amount of light is received by both diodes D1 and D2.

As shown in FIG. 2A, the focus detection voltage S1 before the Forgus servo loop pull-in operation is in an inversion region with a slope opposite to that at the time of servo lock. An action occurs in which the material passes through rapidly.

The servo loop performs a negative feedback operation in the positive slope region beyond the reversal region, and a braking force is applied to the reading lens 4 to lock it in.

In this case, if the braking action is insufficient, an overshoot occurs, and sufficient braking is not possible within the normal slope region, and the vehicle enters a reversal region close to the disk, approaches the disk, and collides with the disk.

Further, normally, the reflected auxiliary beam, that is, the focus detection beam is connected to the focus detection diode D1. Although it is located on the slit of the junction of D2 and locked in, the voltage offset of the differential amplifier 7 during lock-in changes depending on the DC loop gain of the focus servo system.

The mechanical position adjustment of the slit is set so that the reproduction signal is at OV when the peak value is output at just focus, but this is not necessarily Ov.

The output of the differential amplifier T in the absence of light is connected to two diodes D1. This is the difference in output voltage due to the dark current of D2, and Dl
, D2 has an offset close to OV due to variations in dark current characteristics.

As mentioned above, one way to consider the problem of collision between the reading lens 4 and the disk under bad conditions is when the laser disappears due to a failure in the laser source 1, or when the tracking mirror is damaged due to a failure in the tracking servo system. There are cases where the beam rotates excessively and enters the aperture of the reading lens 4 from the tracking mirror, resulting in an extremely small amount of reflected light. This section explains countermeasures for (including

At this time of no light, the focus detection voltage is the focus detection diode D1. The output voltage is due to the dark current difference of D2.

The detection voltage value at this time is almost close to OV, but if the focus servo system has a DC gain of 60 dB or more, the difference between the focus detection voltage at servo lock-in and that at no light is important. It is.

The focus detection voltage during servo lock-in is determined by the focus detection diode D1. It is determined by the mutual positional relationship between D2 and the reflected auxiliary beam, and the loop gain.

That voltage is measured at distance d in FIG. 2A.

Ideally, it would be OV at Determine the polarity and use negative feedback to bring it within the residual error.

If the light becomes zero or extremely small in this servo lock-in state, the focus detection voltage becomes a constant value of the difference between the dark currents D1 and D2.

If this value is smaller than the voltage at lock-in, the polarity will be the same as the detected voltage when approaching the disk with the figure 8 curve shown in Figure 2A, and the servo loop will detect when the reading lens 4 approaches the disk. It detects this and moves it far away.

However, if the focus detection voltage during no light is higher than the detection voltage during servo lock-in, the servo loop senses that the reading lens 4 is far away and moves the reading lens 4 closer to the disk surface. This results in a collision.

Therefore, in the method of the present invention, one condition is that the focus detection system is set so that the detection voltage during no light is smaller than the detection voltage during loop pull-in.

Next, collision prevention measures when there is definitely reflected light will be explained.

If there is reflected light and the reading lens 4 approaches the disk further than the position indicated by When the lens 4 approaches the disk surface,
Beyond the optical axis above.

This is shown in Fig. 3, which is a cross-sectional view seen from the X-X direction in Fig. 1.
To explain with reference to the drawings, when the admonition lens 4 is far away, the reflected auxiliary beam is at the position indicated by al in Fig. 3, and as the reading lens 4 approaches the disk surface, it moves toward the middle pressure in the figure a.

The state exceeds the state of and becomes the state of a2. At this time, the diameter of the reflected auxiliary beam changes within the range shown by g and g', but this occurs because the incident auxiliary beam is parallel light, but since the total amount of light is received, the detection output does not change. There are no negative effects.

If the reading lens 4 approaches any further, the cross section of the reflected auxiliary beam that collides with the disk surface will move to the position indicated by a3 in FIG. 3.

Therefore, the present invention provides a focus detection diode indicated by D3 in FIGS. 1 and 3 to prevent collisions.

The output signal of the diode D3 is amplified and then supplied to the comparator 8, where it is detected whether it has reached a set level or higher and becomes "1".

The signal is converted to a "0" level signal.

Ideally, the detection output signal S2 obtained from the diode D3 at the beam position a3 beyond the focusable area should maintain the same output level during the period when the reading lens continuously contacts the disk from the original dangerous area. Yes, but
In reality, a mirror with a finite area is used in the optical path of the reflected light, so if the reflected light exceeds the range, it disappears.
It returns to zero level as shown by D3 in FIG. 2B.

Therefore, in order to maintain a constant level during the control period, the output of the comparator 8 is applied to a monostable multi-by-break (hereinafter referred to as M, M2) 9.

The outputs of M and M9 are supplied to the focus servo output level control circuit 11 to control the output of the drive amplifier 12.

The drive amplifier 12 is supplied with the output voltage of the servo amplifier 10 obtained from the output of the differential amplifier 7, and supplies the output obtained by power amplifying the input focus detection signal to the focus drive moving coil 13. Drive this.

Here, when it is detected that the reading lens 4 has entered a dangerous area, the driving amplifier 12 is configured to limit the driving voltage to that value or, conversely, to instantly apply a voltage that moves the reading lens 4 away from the disk. An output level control circuit 11 is inserted as a level control section between the input of the servo amplifier 10 and the output of the servo amplifier 10, and an output level control circuit 11 is inserted between the servo amplifier 10 and the drive amplifier 12 to facilitate level control. Make high impedance coupling.

Now, if we configure the servo loop so that the reading lens 4 approaches the disk when the output voltage of the drive amplifier 12 is positive, the output of the drive amplifier 12 will change when there is an output from the MM9 indicating that it has entered the dangerous area. The output level control circuit 11 is configured so that the voltage is negative.

Specifically, the output level control circuit 1 is controlled by the output voltage of M and M9.
1 is closed and a negative voltage is applied to the input of the drive amplifier 12 with low impedance.

At this point, the reading lens 4 is controlled away from the disk surface by the driving force moving in the opposite direction.

When the output voltages of M and M9 are restored, the output level control circuit 1
Gate 1 is opened, normal loop operation resumes, and the focus servo is locked in.

With this configuration, the reading lens 4 is always pulled back, and damage to the disk due to a collision phenomenon can be prevented.

Note that detecting that the reading lens 4 has come too close to the disk is not limited to the position of the focus detection diode shown as D3 in FIG. Detection diode D
It is also possible to use a focus detection diode D'3 t D//3 arranged at a position where it does not interfere with the detection of 4.

However, in this case, since the magnification by the magnifying lens 6 is added and the amount of movement of the reflected auxiliary beam increases, the focus detection diode D'3. D″3 needs to have a large light-receiving area.

Since the focus detection diode D'3 is positioned ahead of the focus detection diode D3 in the reflected auxiliary beam, its detection output signal becomes as shown by the broken line v3 in FIG. Detection is also possible at an early stage when the reading lens 4 and the disk are close to each other to some extent.

As described above, the focus servo system in the optical reproducing apparatus according to the present invention uses the focus detection voltage at the focus servo pull-in point during normal operation as the focus detection voltage between the reading lens and the information recording medium that the focus detection voltage indicates when there is no focus detection light. The focus detection system is configured to take a value indicating a distance slightly farther than the distance, and a detection unit is provided that receives and detects focus detection light and outputs a detection signal when the distance is less than or equal to a predetermined value. Since the focus drive voltage is controlled so that the reading lens is forcibly separated from the information recording medium by this detection signal, reading can be performed both when there is no focus detection light and when the focus detection light is present. It is possible to prevent the phenomenon of the lens colliding with the information recording medium, and it has a simpler structure than the conventional one, and even under any bad conditions, it is possible to prevent the collision between the reading lens and the information recording medium, and it is possible to prevent information recording due to collision. This makes it possible to avoid damage to the medium.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical system and circuit system of an embodiment of the method of the present invention, and FIGS. 2A and 2B are graphs of detection voltage characteristics with respect to the relative distance between the reading lens and the information recording medium in each part of FIG. 1, respectively. ,
FIG. 3 is a view taken along the line X--X in FIG. 1. 1...Laser line, 2...Beam splitter-13
...Half mirror 14...Reading lens, 7...Differential amplifier, 8...Comparator.

Claims (1)

[Claims] 1. When reading and reproducing a recorded information signal on an information recording medium with a light beam focused on the information recording medium by a reading lens, the surface of the information recording medium is adjusted such that the distance between the reading lens and the surface of the information recording medium is constant. In a focus servo system in an optical reproducing device that uses a focus drive voltage to displace the reading lens position in accordance with shake, the focus detection voltage at the focus servo pull-in point during normal operation is the same as the focus detection voltage when there is no focus detection light. The focus detection system is configured to take a value indicating a distance that is slightly further away than the distance indicated above, and the detection system detects that the distance has become less than or equal to a predetermined value by receiving focus detection light and outputting a detection signal. 1. A focus servo system for an optical reproducing apparatus, characterized in that the detection signal controls a focus drive voltage so that the reading lens is forcibly separated from the information recording medium.