JPS5919246A - Focus servo pull-in device of optical information reader - Google Patents

Abstract

PURPOSE:To always detect a point distant away from a real focused point despite the variance of the detecting gain of a focus error detecting means, by comparing the focus error signal obtained after it passed through a 1-circulation loop gain control means with the reference voltage. CONSTITUTION:The comparison input of a comparator 6 is used as a focus error signal (a') which passed through a gain control volume 3 and not a focus error signal (a), i.e., the output of a focus error detecting means 1. The volume 3 compensates the variance of a 1-circulation loop gain to obtain fixed servo characteristics at all times. The variance of the 1-circulation loop gain contains the most dominant sensitivity variance of the means 1 in addition to the variance of the circuit gain due to the variance of resistance value, etc. and the sensitivity variance of a focus actuator 10, etc. Thus adjustment of the 1-circulation loop gain is the same as the adjustment of the sensitivity variance of the means 1 if other variance are ignored.

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 detects the convergence state of the pickup spot light, that is, the amount and direction of deviation from the normal distance between the objective lens and the information recording surface, and the focus error signal (
a); 2 is a target value adjustment knob IJ for adjusting the focus error signal (a) to be zero at the normal distance; A gain adjustment button IJ, - 4 for correcting the variation in the loop gain so that the
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 (opens) the loop switch 4 and generates a start signal to start the focus actuator 10, and then turns on (closes) the loop switch 4 in response to the output of the comparator 6 to start the focus. a pull-in control circuit that controls the servo to start; 8 an adder circuit that receives the focus error signal (a) and the activation signal as two inputs; 9 drives a focus actuator 10 in accordance with the output of the adder circuit 8; This is a drive circuit that

The focus error signal (a) output from the focus error detection means 1 is the second one in the case of the known astigmatism method.
The waveform is as shown in the figure, and in the case of the well-known critical angle method, the waveform is as shown in FIG. In either case, the focus error is theoretically zero at the in-focus point, and converges to zero at a significant distance 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 dimension critical angle method, the settings are as shown in Figure 3.

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, the spot Since the light is not converged on the recording surface and is in a so-called out-of-focus state, it is possible to obtain a focus error signal based on the light that has passed through the recording surface, so servo operation is impossible. Therefore, in the initial state, the focus servo loop is turned off (
The retraction control circuit 7 generates a start signal to drive the focus actuator 1°, and after moving the objective lens to the vicinity of the normal distance, the focus servo is activated by turning on the loop. It is configured to perform a so-called servo operation pull-in operation.

In the case of the astigmatism method, servo pull-in is performed as follows. That is, the pull-in control circuit 7 controls the loop switch 4 to be turned off (opened) in the initial state, so that the output of the phase compensation circuit 5, that is, the input of the addition circuit 8, is zero volt. Next, the retraction control circuit 7 gradually approaches the objective lens from a place far away from the disk, and sends a waveform start signal that acts across the focused point to the focus actuator 10 through an adder circuit 8 and a drive circuit 9. Apply. Therefore, the focus error signal (a
) changes from point (b) to point (c) on the curve shown in FIG. 2, so it 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 pull-in control circuit 7 turns on (closes) the loop switch 4 and sets the activation signal outputted to the adder circuit 8 to zero, thereby completing the pull-in.

Regarding the drawing in the case of the critical angle method, as disclosed in U.S. Pat. That is, the pull-in control circuit 7 controls the loop switch 4 to be turned off in the initial state, so the output of the phase compensation circuit 5, that is, the input to the adder circuit 80 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 transmits a waveform starting signal to the adder circuit 8 and the drive circuit so that the objective lens moves away from the disk again. 9 to the focus actuator 10. Therefore, the focus error signal (a) once moves on the curve shown in Fig. 3 (d
) to point (e), and then from point (e) to point (d). And retraction control circuit 7
responds to the reversal of the output level of the comparator 6 only when changing from point (e) to point (d), and turns on the loop switch 4 only at J near the true focal point, not at point (f). , the activation signal output to the adder 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 natural limits to component accuracy and adjustment accuracy, and if you try to increase the accuracy unnecessarily, you will need very expensive optical components 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. Target value deviation means that the focus error signal (a) becomes zero at a certain distance away from the true focal point created by the optical system, but does not become zero at the true focal point. is the distance of target value deviation.

Note that the focus error signal (a) becomes zero at the fourth
As shown in the figure, it is not necessarily the side farther from the true focal point. ! ,
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, and the straight line in Figure 4 (which can be regarded as almost a straight line near the in-focus point) ) is the variation in the slope of

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) output from the focus error detection means 1 must be zero. Assuming that the absolute value of the target value deviation caused by the focus error detection means l is at most ℃, the maximum detection gain is X (in the case of X in Figure 4), and the minimum is 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 electrical offset is added after converting the light intensity into an electrical signal in the focus error detection means 1, but the amount of offset is equal to the target value. Adjustment volume 2
adjusted by. Therefore, in the case of X in Figure 4,
The offset to be added is p = x x B,
As a result, X becomes X'.

Further, in the case of Y, q=yXn, and as a result, Y becomes Y'. The focus error signal (a
) are 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 focus point before correction is negative as shown in Figure 5, a positive offset is added, but the maximum value of the offset is p = x×fl. Therefore, the potential at a point significantly far from the focal point is at most ±(x x n ). If the reference voltage Vref is smaller than this value, the focus error signal (a) will cross the reference voltage Vref long 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 is (zX
It is necessary to have a higher potential than n).

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 -(zX
R). Therefore, while the objective lens is brought close to the disk recording surface and then moved away from it, the focus error signal (a) reaches the reference voltage Vre long before the true in-focus point.
In order not to cross f, the reference voltage Vref
must be a negative potential whose absolute value is greater than (+, X°C).

Note that the focus error signal (a) is the reference voltage Vref.
It is very difficult to judge whether or not the point is truly in focus depending on which direction the point is crossed. 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 at the reference voltage. This is because if discrimination is attempted based on the direction across Vref, the moving speed of the objective lens during retraction must be made faster 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 zxA.

In this way, it is necessary to determine the reference voltage Vref based on the case where the detection sensitivity varies to the maximum as shown by X' in FIG. On the other hand, Y' where the sensitivity varies to the minimum
In the case of (z/y)xx from the true focus point
This corresponds to the focus error signal at a point that is separated by .

Let's find the value based on a specific numerical example:
If the maximum value of the target value deviation °C is 2 μm and the difference between the maximum detection sensitivity However, in the case of the minimum sensitivity, points separated by at least 8 Iim will be detected. In this way, the focus error signal (
Since a) is no longer a straight line and the slope is generally smaller, in the calculations in section 2, it is assumed to be a straight line and is 81 Jm.
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, the 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 compares 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 with a predetermined reference level. The configuration is such that the servo loose is controlled on and off based on this 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 this 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 (3) which has passed through the gain adjustment buttons IJ and -3. The rest of the structure is exactly the same as the conventional example shown in FIG. 1, and the same parts are indicated by the same reference numerals.

Next, the operation of the present invention will be explained. The gain adjustment volume 3 is used to correct variations in the open-loop gain and always obtain constant servo characteristics. - Variations in loop gain include variations in circuit gain caused by variations in resistance, etc., and variations in focus actuator 10.
However, the most dominant one is the sensitivity variation of the focus error detection means 1. Therefore, if we ignore other variations, adjusting the -circuit loop gain is nothing but adjusting the sensitivity variations of the focus error detection means 1. Therefore, the focus error signal (a') after gain adjustment is attenuated by the gain adjustment water IJ, -m 3 when the detection sensitivity of the focus error detection means 1 is maximum, that is, in the case of X' in FIG. When Y' corresponds to Y' and the detection sensitivity is minimum, it is not attenuated and remains the same as Y'. Therefore, the maximum value (zxn) of the focus error voltage after correction at a point significantly away from the in-focus point in FIGS. It is. Therefore, the reference voltage V
ref only needs to have a size larger than (yxl2), and regardless of variations in focus error detection sensitivity, it is always a point that is a certain distance away from the true focus point (2 in the numerical value specifically given in the explanation of the conventional example). 〃m) can be detected.

In the above embodiments, only the astigmatism method and the critical angle method have been described, but the present invention is not limited thereto. This is effective when the variation is so large that it cannot be ignored. The precise gain adjustment is not limited to the method of attenuating by volume IJ, -.For example, a method of adjusting the gain of the amplifier can also be considered, but even in such a method, the focus error signal after passing through the gain adjustment circuit. It goes without saying that it is only necessary to compare the voltage Vref with the reference voltage Vref.

As mentioned above, according to this investigation, - loop gain control!
Since the focus error signal after passing through the focus error detection means is compared with the reference voltage, it is possible to always detect a point a certain distance away from the true focus point, regardless of variations in the detection gain of the focus error detection means, and the focus The reliability of servo pull-in can be significantly improved.

[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 a focus error signal by the critical angle method, and Fig. 4 is a waveform diagram of a focus error signal by the astigmatism method.
Figure 5 is a waveform diagram of the focus error signal before and after target value deviation correction using the astigmatism method, and Figure 6 is a target value deviation correction using the critical angle method. Waveform diagrams of focus error signals before and after correction, and FIG. 7 is a block diagram showing an embodiment of the present invention. Explanation of symbols of main parts 1...Focus error detection means 2...Target value adjustment hole 43...Gain adjustment volume 4...
Loop switch 6...Comparator 7/Retraction control circuit 10...Focus Actuake Applicant Pioneer Co., Ltd. Agent Patent Attorney Motohiko Fujimura

Claims (1)

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