JPH04351717A - Optical pickup focus servo mechanism - Google Patents

Abstract

PURPOSE:To prevent the state that servo cannot be applied for a long time by changing a threshold to be compared in accordance with the intensity of a read signal to more accurately discriminate whether the relative speed is within a leading-in permission speed. CONSTITUTION:The output of a photo diode 8a is subjected to current-voltage conversion and is inputted to a subtracting circuit 14 and an adding circuit 15a. The output of the adding circuit 15a is a signal RF of the read signal of a disk, and this signal has the high frequency component eliminated through a low pass filter 18a to obtain a signal RF0. This signal RFO and the threshold corresponding to the intensity of the signal outputted from a peak holding circuit 24a are compared by a comparator 20a, and a switching signal is outputted from an AND gate 22a to perform switching from a sweep signal to a servo signal if the period when the signal RF0 is larger than the threshold is longer than the time constant determined by a monostable multivibrator 21a. Thus, the occurrence of discrimination error due to variance of the signal intensity is prevented.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a focus servo mechanism for an optical pickup used in an optical disc device.
In particular, the present invention relates to a focus servo mechanism of an optical pickup in an optical disc device used in a state where it is affected by vibrations, such as a vehicle-mounted CD player.

0002

2. Description of the Related Art In an optical disc device, since minute information on the disc is read by an optical pickup, it is necessary to maintain the relative position of the optical disc and the optical pickup with high precision. Since it is not possible to maintain the relative position at the required accuracy with mechanical accuracy alone, the required accuracy is maintained using servo mechanisms such as a focus servo mechanism and a tracking servo mechanism.

A focus servo mechanism is a mechanism for focusing an optical pickup on the information recording surface of an optical disc. The relative position of the optical pickup to the optical disk surface must be maintained within ±1.0 μm of the focal point position, but it may fluctuate by a maximum of about ±0.5 mm due to warpage, unevenness, and surface runout with respect to the rotation axis of the optical disk surface. For,
A focus servo mechanism as shown in FIG. 4 maintains the focus within the required focus range. The lens 9c is held by an actuator coil 10c within a magnetic field formed by a magnet 11c and a magnetic circuit 12c, and is configured to move up and down with respect to the optical disk 7c by a current flowing through the actuator coil 10c.

The focal position is detected by a photodetector 8c. As this focus detection method, the astigmatism method, eccentric auxiliary beam method, knife edge method, etc. are known, but all of them use multiple photodetectors and The focus error detection circuit 3c determines the output difference between the two, and servo is applied according to this output to maintain the optical pickup at the focal position. However, no matter which method is used, the signal corresponding to the deviation from the focus can only be obtained within a range of about 10 μm from the focus position, and it is not possible to obtain the signal necessary to apply the servo outside this range. . As mentioned above, the relative position of the optical pickup with respect to the optical disk fluctuates by about ±0.5 mm at most, so it is first necessary to bring this relative position within a servo-enabled range. Therefore, the focus servo mechanism includes a sweep signal generation circuit 16c shown in FIG. 4 to perform a sweep operation to move the optical pickup up and down with respect to the optical disk, and to start the focus servo operation when the optical pickup reaches the focal position.
and an automatic focus detection circuit 2c. The sweep signal generation circuit 16c moves the optical pickup to the optical disc 7c.
This is a circuit that generates a signal that causes the sensor to move up and down at a constant speed. The automatic focus detection circuit 2c detects the moment when the optical pickup passes the focus position while moving up and down, and at this time outputs a switching signal to the switch 5c to convert the drive signal to the actuator from the sweep signal to the focus error detection circuit. Switch to servo signal from 3c. Since servo is thereby applied at the focal position, the relative position between the optical pickup and the optical disk 7c is maintained within a predetermined tolerance.

[0005] The optical pickup obtains an optical disc readout signal due to light reflected from the optical disc and a focus error signal corresponding to the deviation from the focal position, and the focus error signal is used as a signal for focus servo. These signals are also used to generate a switching signal from the sweep operation to the focus servo operation, and FIG. 5 shows the change in the vicinity of the focus. The read signal is called an RF signal, and since it contains a high frequency signal corresponding to the recording pits of the optical disc, it is passed through a low-pass filter and used as an RFO signal. Furthermore, the flip-flop in the figure needs to be reset before the sweep operation.

As described above, the relative position between the optical pickup and the optical disk varies due to warping of the optical disk and surface runout with respect to the rotation axis. These error factors are specified in the specifications, and the servo mechanism is designed so that the optical pickup can follow fluctuations due to errors within the specified range. The range that the servo mechanism can follow is determined by the acceleration above a certain frequency, and this is related to the driving force of the actuator coil 10c, the mass of the lens system, etc. The tracking ability of the servo mechanism determines the maximum passing speed at which the servo is applied when the optical pickup passes through the focal position by switching to the servo mechanism. This passing speed is called the maximum retractable speed of the servomechanism. If the optical pickup passes through the focal position at a speed exceeding this speed, the servo will not be activated even if the servo mechanism is switched to, and the optical pickup will deviate from the focal position. Therefore, the sweep speed is determined taking these conditions into consideration.

[0007] The actual sweep speed is the sweep speed plus the fluctuation of the optical disc, but as mentioned above, these are set below the maximum retractable speed, so this is almost no problem in stationary optical disc devices. will not occur. However, in recent years, optical disk devices such as CDs have come to be installed in automobiles. When used in an automobile, the entire optical disk device is constructed to be isolated from vibrations using anti-vibration rubber or the like, since the vibrations are large, but it is practically impossible to completely isolate vibrations. Therefore, during the sweeping operation, fluctuations due to these vibrations are further superimposed on the vertical movement for sweeping. This situation is shown in FIG. 6, in which fluctuations in the optical disc are omitted for the sake of simplification of explanation. If there is no influence of vibration, the sweep is performed along the broken line in the figure. The right part is an enlarged view of the part surrounded by the dashed-dotted line as it passes through the focal position, where P represents the sweep speed, Q represents the maximum retractable speed, and R represents the actual passing speed. ing. In this case, since the passing speed is higher than the maximum retractable speed, it is not possible to follow the focal position even if the focus servo is switched to.

[0008] Up until now, when the optical pickup passes through the focal position, the focus servo is always switched to due to the sweep operation, so if the passing speed is higher than the maximum retractable speed, the servo is not activated. This state continues for a while, but it detects from the signal that the servo has failed and starts sweeping again. This will be repeated until the servo is applied. Therefore, in some cases, it may take a long time until the servo is activated.

[0009] As a countermeasure for this, it is conceivable to improve the actuator etc. so that the maximum retractable speed is increased, but there is a problem that the device becomes large and expensive. It is also conceivable to take more strict anti-vibration measures, but this method also increases the size and cost of the device. Therefore, in order to reduce the time required for servo pull-in by eliminating failures in servo pull-in when switching from sweep operation to servo operation, the present applicant has filed patent application No. 2-21
No. 0534 and Japanese Patent Application No. 2-315804 propose an optical pickup focus servo mechanism that switches to servo operation only when the relative speed of the optical pickup passing near the focal point to the optical disk during a sweep operation is less than the maximum retractable speed. There is. FIG. 7 shows the basic configuration of such a mechanism, and compared to FIG. 4, an optical pickup speed detection means 1d is newly added.

In the above patent application, in order to determine the relative speed of the optical pickup with respect to the optical disk, R
The fact that changes in the FO signal and focus error signal correspond to relative speed is utilized. FIG. 8 is a diagram for explaining the determination method, in which the RFO signal and focus error signal are
A solid line indicates when the relative velocity is high, and a broken line indicates when the relative velocity is slow. (a) determines the period during which the RFO signal is greater than or equal to a certain threshold. (b) determines the time difference when the RFO signal reaches different threshold values, that is, the rising edge of the RFO signal. (c
) is the period during which the focus error signal is above a certain threshold,
(d) The rise of the focus error signal is determined. (e) determines the time difference between the RFO signal and the focus error signal when they exceed their respective thresholds. As is clear from the above explanation, in any of the methods (a) to (e), appropriate threshold values are determined for the RFO signal and the focus error signal, and then the It is determined whether or not retraction is possible based on whether the time required for retraction is greater than or equal to a predetermined value. By determining the relative speed in this way and then switching to servo mode, failures during retraction are reduced.

[0011]

[Problem to be Solved by the Invention] When determining the relative speed of the optical pickup with respect to the optical disk as described above, a predetermined threshold value is determined, and the time period during which the RFO signal and focus error signal are greater than or equal to the threshold value or when the threshold value is reached is determined. Time differences are detected, and these thresholds are fixed. Therefore R
If the intensity of the FO signal or focus error signal changes, an error will occur in the determination of relative velocity. If an error occurs in the judgment of the relative speed, the servo may not be activated because the switch has switched to servo operation even though the retractable speed is greater than the retractable speed, or the servo may not be activated because the retractable speed is determined to be greater than the retractable speed even though it is within the retractable speed. There may be cases where switching is not possible.

[0012] The RFO signal and the focus error signal are signals detected by the detector of the optical pickup of the light emitted from the optical pickup, reflected by the optical disk, and returned. Signal strength fluctuates due to changes in light intensity and light-receiving sensitivity. A semiconductor laser is used as a light source for an optical pickup, and its output deteriorates over time and changes due to temperature changes. There is also a similar change in the light receiving sensitivity of the optical pickup.

[0013] The RFO signal and focus error signal fluctuate due to the reasons described above, but if they fluctuate too much, the servo may not be applied forever. In other words, if the signal strength decreases significantly, most sweep operations will determine that the speed is below the retractable speed and will not switch to servo forever, and conversely, if the signal strength increases significantly, the retractable speed will be determined to be above the retractable speed. However, it is determined that retraction is possible and switches to servo, resulting in a situation where retraction to servo fails many times.

The present invention has been made in view of the above-mentioned problems, and is aimed at determining the relative speed of an optical pickup with respect to an optical disk by using at least one of an RFO signal and a focus error signal as a target signal during a sweep operation. The purpose of this invention is to prevent a situation in which a determination error occurs due to a change in the intensity of the focus servo and the focus servo is not applied forever.

[0015]

[Means for Solving the Problems] In order to solve the above problems, the optical pickup focus servo mechanism of the present invention has the following features:
Among the RFO signal and focus servo signal, the maximum intensity of a signal that is a target signal for determining the relative speed of the optical pickup is detected and fed back. That is, the optical pickup focus servo mechanism of the present invention performs a sweeping operation to move the optical pickup up and down with respect to the optical disk, and when the optical pickup enters a predetermined focal range, it maintains the optical pickup in a predetermined focal range with respect to the optical disk. An optical pickup focus servo mechanism configured to start a focus servo operation, the mechanism comprising:
In order to start the focus servo operation only when the relative speed between the optical pickup and the optical disk is such that it can be pulled into the focus servo operation, the optical disk read signal and the focus servo operation servo signal are subjected to low frequency pass processing. In an optical pickup focus servo mechanism, the optical pickup focus servo mechanism is provided with an optical pickup speed determination means that uses at least one of the focus error signals as a target signal and determines relative speed based on the time during which the target signal is equal to or higher than a predetermined threshold value or the time difference when the target signal reaches a predetermined threshold value. The threshold value optimization means is provided to change the threshold value according to the intensity of the threshold value.

[0016]

[Operation] The relative speed of the optical pickup with respect to the optical disk during the sweep operation is determined by using at least one of the RFO signal and the focus error signal, which are obtained by performing low frequency pass processing on the optical disk read signal, as the target signal, and this target signal is set to a predetermined threshold value. This is performed based on a time difference that is equal to or more than a predetermined threshold value or a time difference that is a predetermined threshold value. Therefore, if the intensity of the target signal changes, an error will occur in the determination, but since the threshold value is changed according to the maximum value of the target signal, the relative speed can be determined accurately.

In order to change the threshold value according to the maximum value of the target signal, it is necessary to perform a sweep operation once to detect the maximum value of the target signal.

[0018]

[Example] As mentioned above, the present applicant has filed Japanese Patent Application No. 2-2105
No. 34 and Japanese Patent Application No. 2-315804 propose various methods using RFO signals and focus error signals for determining the speed of an optical pickup with respect to an optical disk. RF shown
An embodiment of the present invention will be described by taking as an example a method using the O signal. However, the present invention is equally applicable to other relative speed determination methods.

A circuit of the first embodiment is shown in FIG. In the figure, 8a is a photodiode for light detection, and the outputs of these are converted into voltages by current-voltage conversion circuits 131a and 132a, and input to a subtraction circuit 14a and an addition circuit 15a. The output from the subtraction circuit 14a is a focus error signal, which passes through the compensation circuit 17a and becomes a servo signal. The output of the adder circuit 15a is an optical disc read signal, which is called an RF signal. The RF signal includes pit information of the optical disc, and this high frequency component is removed by a low pass filter 18a to produce an RFO signal. Here, the RFO signal is used to determine focus passage and relative velocity.

Reference numeral 24a denotes a peak hold circuit, which will be described later, but the voltage obtained by dividing the voltage outputted from this circuit by resistors Ra and Rb becomes the threshold value. The comparator 20a determines whether the RFO signal is greater than this threshold, and if the period during which the RFO signal is greater than the threshold is longer than the time constant determined by the monostable multivibrator 21a, a switching signal is output from the AND gate 22a. This switching signal causes the switch 5a to switch from the sweep signal to the servo signal.

FIG. 2 shows an example of a peak hold circuit. The circuit in Figure 2 is a modification of the well-known peak hold circuit, and since the voltage Va of the RFO signal is almost zero at the time of starting the sweep, the output voltage V
A voltage equal to Vr is outputted from b. By performing a sweep operation and once passing through the focal point, a high voltage is generated in the RFO signal, which charges the capacitor C.
Vb will be a voltage equal to the maximum value of the RFO signal. Then, this voltage is divided and becomes the threshold value, so the RFO in that state
A threshold value corresponding to the signal strength is output.

In the circuit of FIG. 2, Vr is set lower than the maximum value of the RFO signal, and once Va reaches a high value by passing through the focal point, this value is output to Vb, and then when Va decreases, it is maintained. The value is C. Rc
It gradually approaches Vr with Vr as a time constant. Therefore, if the time constant is large, the attenuation between sweep operations can be ignored, and the value corresponding to the maximum value of the RFO signal detected in the previous sweep operation becomes the threshold for the next sweep operation.

In the circuit shown in FIG. 2, Vr is set to be lower than the maximum value of the RFO signal, so the threshold value for the first sweep operation is small, and when a relative speed higher than the retractable speed is determined to be the retractable speed. When switching to servo operation, the servo does not work and it is necessary to perform the sweep operation again.However, from the second time onwards, an appropriate threshold value is set, so a good relative velocity judgment can be made. This prevents the servo from not working for long periods of time.

However, in the circuit shown in FIG. 2, when switching to servo operation during the first sweep operation, there is a risk that the servo will not be activated and a useless operation will be performed, so only the maximum value of the RFO signal is detected for the first time. , the switch to servo may be performed from the second time onward. In that case, switch 5a in FIG. 1 is not switched during the first sweep operation, but is switched after the second sweep operation, and Vr in FIG.
Just remove it and ground this terminal directly. In this way, even if the voltage Va of the RFO signal is much lower than Vr, it can be handled.

However, in the above case, the first sweep is always wasted, so it is not sufficient in terms of shortening the time until the servo is applied. Therefore, for the first time, the threshold value is determined based on a value that approximates the maximum value of the RFO signal that can occur, and from the second time onwards, the threshold value is determined based on the maximum value of the RFO signal obtained as a result of the sweep. This is the circuit shown in Figure 3. The switch 51d is set to V only during the first sweep.
r side, and otherwise connected to the output of the sample and hold circuit.

As mentioned above, in the circuit of FIG. 3, Vr is set to a high value, but if the relative speed is slow enough, it is determined that retraction is still possible, so in that case, the servo operation is switched to Servo can be applied. Furthermore, in general, large fluctuations in signal strength, which are the problem of the present invention, occur in most cases when the signal strength decreases due to dirt on the disk or the like. Therefore, it is possible to set Vr in Fig. 3 to correspond to the normal signal output, perform the normal judgment operation from the first time, and correct the threshold value based on the actual RFO signal strength from the second time onwards. good.

[0027] The sweep operation is performed when the servo is applied for the first time and when the servo is no longer applied due to an impact or the like during servo operation.If the servo is disengaged during servo operation, the previous RFO is It is sufficient to use a threshold value corresponding to the maximum value of the signal. An example of determining the relative speed based on the period during which the RFO signal is greater than or equal to the threshold value has been described above, but as mentioned above, other determination methods such as determining the relative speed using the focus error signal may also be used based on the actual signal strength. It is desirable to change the threshold value.

[0028]

Effects of the Invention According to the present invention, in the optical pickup focus servo mechanism that switches to servo operation after determining whether the relative speed of the optical pickup to the optical disk during the sweep operation is within the retractable speed, the relative speed is within the retractable speed. This makes it possible to more accurately determine whether the servo is working properly, and prevents situations where the servo is not activated for a long time.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of the peak hold circuit in FIG. 1;

FIG. 3 is a diagram showing another example of the peak hold circuit in FIG. 1;

FIG. 4 is a diagram showing a conventional focus servo mechanism.

FIG. 5 is a diagram showing a signal from an optical pickup when passing near the focal point during a sweep operation.

FIG. 6 is an explanatory diagram of speed when vibration is superimposed during sweeping.

FIG. 7 is a diagram showing the configuration of an optical pickup focus servo mechanism that controls switching to servo operation according to relative speed during sweep operation.

FIG. 8 is a diagram showing a plurality of methods for determining the relative speed of an optical pickup with respect to an optical disk.

[Explanation of symbols]

8a...Photodetector 14a...Subtraction circuit 15a...Addition circuit 16a...Sweep circuit 17a...Compensation circuit 18a...Low pass filter 20a...Comparator 21a...Monostable multivibrator 24a...Peak hold circuit

Claims (1)

[Claims] 1. Performs a sweep operation to move the optical pickup up and down with respect to the optical disk, and when the optical pickup enters a predetermined focal range, performs a focus servo operation to maintain the optical pickup in a predetermined focal range with respect to the optical disc. an optical pickup focus servo mechanism configured to start the focus servo operation only when the relative speed of the optical pickup and the optical disk is a speed that can be pulled into the focus servo operation during the sweep operation; In order to start the process, at least one of the optical disk read signal that has undergone low frequency pass processing and the focus error signal that is the servo signal for the focus servo operation is used as a target signal, and the target signal is at least a predetermined threshold or a predetermined time. The optical pickup focus servo mechanism includes an optical pickup speed determining means for determining the relative speed based on a time difference that becomes a threshold value, characterized in that the optical pickup focus servo mechanism includes a threshold value optimization means for changing the threshold value in accordance with the intensity of the target signal. Optical pickup focus servo mechanism.