JPH0223927B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus servo pull-in device in an optical information reading device.

A conventional device of this type is shown in FIG. In FIG. 1, 1 is a focus error detection means for detecting the convergence state of the pick-up spot light, that is, the amount and direction of deviation from the normal distance between the objective lens and the information recording surface, and outputting a focus error signal a; 3 is a target value adjustment volume for adjusting the focus error signal a to be zero at the normal distance; 3 is a gain adjustment volume for correcting variations in the loop gain so that constant servo characteristics are always obtained; 4
is a loop switch for opening and closing the servo loop; 5 is a phase compensation circuit for compensating the phase of the focus error signal a; 6 is a comparator for comparing the focus error signal a with a reference voltage Vref, which is a reference for detecting a focused point; In the initial state, 7 turns off (open) the loop switch 4 and generates an activation signal to start the focus actuator 10, and further turns on (closed) the loop switch 4 in response to the output of the comparator 6. A pull-in control circuit controls to start the focus servo; 8 is an adder circuit which receives the focus error signal a and the activation signal as two inputs; 9 is a focus actuator 10 in response to the output of the adder circuit 8; This is the drive circuit that drives the.

The focus error signal a output from the focus error detection means 1 has a waveform as shown in FIG. 2 in the case of the well-known astigmatism method, and a waveform as shown in FIG. 3 in the case of the well-known critical angle method. becomes. In either case, the focus error is theoretically zero at the in-focus point, and converges to zero at locations significantly away from the in-focus point. Note that in the critical angle method, there are points other than the focused point that cross the zero level. In the case of the astigmatism method, the reference voltage Vref is as shown in Figure 2.
In the case of the critical angle method, the settings are as shown in FIG. The focus error signal a is then directly compared with the reference voltage Vref in the comparator 6.

Before starting the information reading operation, the objective lens is generally placed farthest from the disk recording surface, and in this initial state, when the focus servo loop is turned on (closed) and the servo is applied, Since the spot light is not converged on the recording surface and is in a so-called out-of-focus state, it is not possible to obtain a focus error signal based on the light that has passed through the recording surface, and therefore servo operation is impossible. Therefore, in the initial state, the focus servo loop is kept off (open), and a start signal is generated from the retraction control circuit 7 to activate the focus actuator 1.
0, and after moving the objective lens to the vicinity of the normal distance, turning on the loop performs a so-called servo operation pull-in that operates the focus servo.

In the case of the astigmatism method, servo pull-in is performed as follows. That is, in the initial state, the pull-in control circuit 7 controls the loop switch 4 to turn off (open), and therefore the output of the phase compensation circuit 5, that is, the input of the adder circuit 8, is zero volt. Next, the retraction control circuit 7 gradually moves the objective lens from a place far away from the disk and sends a waveform activation signal that acts to cross the in-focus point to an addition circuit (8).
and is applied to the focus actuator 10 through the drive circuit 9. Therefore, the focus error signal a changes on the curve shown in FIG. 2 from point b to point c, and crosses the reference voltage Vref just past the in-focus point. Therefore, the output level of the comparator 6 is inverted, and in response, the retraction control circuit 7 turns on (closes) the loop switch 4.
At the same time, the start signal outputted to the adder circuit 8 is set to zero, and the pull-in is completed.

Regarding the drawing in the case of the critical angle method, as disclosed in Utility Model Application No. 1976-1976 by the applicant of the present application, it is carried out as follows. That is, the pull-in control circuit 7 controls the loop switch 4 to be turned off in the initial state, so that the output of the phase compensation circuit 5, that is, the input of the adder circuit 8, is zero volt. Next, the retraction control circuit 7 moves the objective lens from a place extremely far away from the disk to a place extremely close to the disk, and then sends a waveform starting signal to the adder circuit 8 and the drive so that the objective lens moves away from the disk again. It is applied to the focus actuator 10 through the circuit 9. Therefore, the focus error signal a gradually changes from point d to point e on the curve shown in FIG. 3, and then from point e to point d.
Then, the pull-in control circuit 7 responds to the inversion of the output level of the comparator 6 only when the point e changes to the point d, turns on the loop switch 4 only near the true focal point, not the point f, and adds The activation signal output to the circuit 8 is set to zero, and the pull-in is completed. It has been explained that in both the astigmatic method and the critical angle method, the starting signal output to the adder circuit 8 is set to zero when the loop switch 4 is turned on. It does not necessarily have to be set to zero as long as it is a value that can be considered as a disturbance that does not have any influence.

Here, in both the astigmatism method and the critical angle method, the focus error signal a is determined by the theoretical value shown in Figures 2 and 3 only after precisely adjusting the precisely finished optical components. However, there are limits to the accuracy of parts and adjustment, and if you try to increase the accuracy unnecessarily, you will need very expensive optical parts and increase the number of adjustment steps, making it difficult for consumer use. This becomes a major obstacle in manufacturing inexpensive equipment.

Variations are particularly likely to occur in target value deviation and detection gain. Since the shapes near the in-focus point are almost the same in both FIGS. 2 and 3, enlarged views of the vicinity are shown in FIG. 4. What is target value deviation?
The focus error signal a becomes zero at a certain distance away from the true in-focus point created by the optical system, and does not become zero at the true in-focus point, and l in Fig. 4 is the distance of the target value deviation. be. Note that the focus error signal a does not necessarily become zero on the side farther from the true in-focus point as shown in FIG. Furthermore, the variation in detection gain is the variation in the amount of displacement of the focus error signal a for a unit distance shift near the in-focus point.
This refers to the variation in the slope of the straight line in FIG. 4 (which can be regarded as almost a straight line near the in-focus point).

Since there is no need to move the objective lens when it is in true focus, no drive signal should be applied to the focus actuator 10. That is, at the true focal point, the focus error signal a, which is the output of the focus error detection means 1, must be zero. Assume now that the absolute value of the target value deviation generated by the focus error detection means 1 is at most l, and the detection gain is at most x (in the case of X in Figure 4) and at minimum y (in the case of X in Figure 4). In the case of Y), it is assumed that it is within the range of

In order to make the focus error signal a zero at the true in-focus point, an offset is added electrically after converting the light amount into an electrical signal in the focus error detection means 1, but the amount of offset is equal to the target value. It is adjusted by adjustment volume 2. Thus, for X in FIG. 4, the added offset is p=x×l, so that X becomes X'. Further, in the case of Y, q=y×l, and as a result, Y becomes Y'. The focus error signal a corrected in this way is shown in FIGS. 5 and 6 for the astigmatism method and the critical angle method, respectively. In each figure, the solid line shows the state before correction, and the dashed-dotted line shows the state after correction.

In the astigmatism method, if the focus error signal a at the true in-focus point before correction is negative as shown in Fig. 5, a positive offset is added.
The maximum value of the offset is p=x×l. Therefore, the potential at a point significantly far from the focal point is at most +
(x×l). If the reference voltage Vref is smaller than this value, the focus error signal a will cross the reference voltage Vref before the true in-focus point in the process of pushing out the objective lens during retraction, so the vicinity of the in-focus point will be accurately detected. Can not do it. Therefore, the reference voltage Vref needs to be at a higher potential than (x×l).

In the critical angle method, as shown in Figure 6, if the focus error signal a at the true focus point is positive before correction, the potential at a point significantly closer to the focus point after correction is -(x×l) at its maximum. becomes. Therefore, in order to prevent the focus error signal a from crossing the reference voltage Vref before the true in-focus point while the objective lens is brought close to and then moved away from the disk recording surface, it is necessary to increase the reference voltage Vref. Vref needs to be a negative potential whose absolute value is larger than (x×l).

Note that the focus error signal a is the reference voltage
It is very difficult to judge whether or not the point is truly in focus depending on which direction it crosses Vref. This is because the disk reflective surface (recording surface) is not necessarily stationary when viewed from the pickup, but is moving at a fairly high speed due to external vibrations, warping, etc., so the focus error signal a is the standard. This is because if discrimination is attempted based on the direction across the voltage Vref, the moving speed of the objective lens during retraction must be greater than that speed, which will impede stable retraction. Therefore, in both the astigmatism method and the critical angle method, the absolute value of the reference voltage Vref needs to be larger than x×l.

In this way, it is necessary to determine the reference voltage Vref based on the case where the detection sensitivity varies the most, as shown by X' in Figure 4.
Conversely, in the case of Y' where the sensitivity varies to a minimum, the absolute value of Vref, ie, x×l, corresponds to a focus error signal at a point separated by (x/y)×l from the true focus point.

Here, when calculating the value based on a concrete numerical example, if the maximum value of the target value deviation l is 2 μm and the difference between the maximum detection sensitivity x and the minimum detection sensitivity y is 4 times, then the maximum sensitivity In this case, a point that is at least 2 μm away from the in-focus position will be detected, but in the case of minimum sensitivity, a point that is at least 8 μm away will be detected. In this way, 8μ from the true focus position
In the area where the distance is m, the focus error signal a is no longer a straight line and the slope is generally smaller, so in the above calculation, assuming a straight line, the focus error signal a is 8 μm.
However, in reality, the points are 10 to 20 μm apart. Therefore, if the loop is closed by detecting a point that is far away from the true focus position, retraction will not necessarily be performed stably, resulting in a decrease in reliability.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a focus servo pull-in device for an optical information reading device that improves the reliability of focus servo pull-in by stably detecting the vicinity of a focused point.

The focus servo pull-in device in the optical information reading device according to the present invention sets the signal level of the focus error signal that has passed through the gain adjustment means provided in the focus servo loop and adjusts the gain of this loop to a predetermined reference level. The configuration is such that the servo loop is controlled on and off based on the comparison output.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 7 is a block diagram showing one embodiment of the present invention. In one embodiment, the comparison input of the comparator 6 is not the focus error signal a which is the output of the focus error detection means 1, but the focus error signal a' which has passed through the gain adjustment volume 3. The rest of the structure is exactly the same as the conventional example shown in FIG. 1, and the same parts are designated by the same reference numerals.

Next, the operation of the present invention will be explained. The gain adjustment volume 3 is for correcting variations in the open-loop gain to always obtain constant servo characteristics. Variations in the open-loop gain include variations in circuit gain caused by variations in resistance values, variations in the sensitivity of the focus actuator 10, etc., but the most dominant one is the sensitivity of the focus error detection means 1. There is variation. Now, if we ignore other variations, adjusting the open-loop gain is nothing but adjusting the sensitivity variations of the focus error detection means 1. Therefore, the focus error signal a' after the gain adjustment is the gain adjustment volume 3 when the detection sensitivity of the focus error detection means 1 is maximum, that is, in the case of X' in FIG.
If the detection sensitivity is Y', which has the minimum detection sensitivity, it is not attenuated and remains Y'. Therefore, the maximum value (x×l) of the focus error voltage after correction at a point significantly away from the in-focus point in FIGS. 5 and 6 also undergoes attenuation and becomes (y×l), which is originally (y× 1) remains as is. Therefore, the magnitude of the reference voltage Vref is (y×
l), and it is possible to always detect a point that is a certain distance away from the true focused point (2 μm in the specific value given in the explanation of the conventional example) regardless of variations in focus error detection sensitivity. can.

In the above embodiments, only the astigmatism method and the critical angle method have been described, but the present invention is not limited thereto, and even with other detection methods, target value deviation and detection gain This is effective when the variation is so large that it cannot be ignored. Also, gain adjustment is not limited to the method of attenuating by volume; for example, a method of adjusting the gain of an amplifier can be considered, but even in such a method, the focus error signal after passing through the gain adjustment circuit is used as the reference. Needless to say, it is sufficient to compare it with the voltage Vref.

As described above, according to the present invention, the focus error signal after passing through the open-loop gain adjustment means is compared with the reference voltage, so regardless of the variation in the detection gain of the focus error detection means. It is possible to always detect a point that is a certain distance away from the true focal point, significantly improving the reliability of focus servo pull-in.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional example, Fig. 2 is a waveform diagram of a focus error signal by the astigmatism method,
Fig. 3 is a waveform diagram of the focus error signal obtained by the critical angle method, Fig. 4 is an enlarged view of the vicinity of the in-focus point in Figs. FIG. 6 is a waveform diagram of the focus error signal after correction, FIG. 6 is a waveform diagram of the focus error signal before and after target value deviation correction using the critical angle method, and FIG. 7 is a block diagram showing an embodiment of the present invention. It is. Explanation of symbols of main parts, 1...Focus error detection means, 2...Target value adjustment volume, 3...
...gain adjustment volume, 4 ... loop switch, 6 ... comparator, 7 ... pull-in control circuit,
10...Focus actuator.

Claims (1)

[Claims] 1. A gain adjustment means provided in a focus servo loop to adjust the gain of this loop, and a comparator that compares the signal level of the focus error signal passed through the gain adjustment means with a predetermined reference level, 1. A focus servo pull-in device for an optical information reading device, characterized in that the servo loop is controlled on/off based on the output of the servo loop.