JPH031751B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tracking servo signal generation circuit in an information reading device.

For example, there is a tracking servo control device for an optical information reading device having a configuration as shown in FIG. That is, the irradiated light beam from the laser light source 1 passes through the lens 2 , the beam splitter 3 , the 1/4 wavelength plate 4 and the objective lens 5 and is incident on the recording surface of the recording disk 6 . The laser beam is converged by the objective lens 5 and becomes a pickup light spot as a minute information detection point on the recording surface. The reflected light (or transmitted light) by this disk 6 is separated by a beam splitter 3 and irradiated onto each light receiving surface of a pair of photoelectric conversion elements 9a and 9b. The outputs of both photoelectric conversion elements 9a, 9b are outputted by amplifiers 10a, 1
0b and LPF (low pass filter) 11a, 11
The signals are applied to the differential amplifier 12 through the respective channels b. This difference output is passed through the equalizer 13 to the drive amplifier 14.
The signal is input to the drive coil 8 and serves as a drive signal for the drive coil 8 for moving the objective lens 5 in a direction perpendicular to the track.

As shown in FIG. 2, a pair of photoelectric conversion elements 9a and 9b are installed so that their light-receiving surfaces are divided by a single dividing line 9c.
are parallel to the recording track tangential direction (indicated by arrow Y), and these elements 9a and 9b are provided so as to be symmetrical with respect to the optical axis 7' of the reflected light of the light spot. Note that 7 is an optical axis of incident light, and 15 is a spindle motor for rotating the disk.

With this configuration, when the center of the pickup light spot deviates from the center of the recording track in a direction perpendicular to the track, the photodetectors 9a and 9b shift according to this deviation.
The intensity distribution of the incident light becomes asymmetrical, and a difference occurs between the outputs of both detectors. Therefore, by obtaining the difference between the reduction components of these two detectors using the differential amplifier 12, a so-called tracking error signal is obtained, and this error signal is used to move the objective lens 5 in the direction perpendicular to the track (disk radius). direction), the pick-up light spot is biased accordingly, making it possible to always perform accurate tracking operations.

However, in such an apparatus, due to the inclination of the recording surface of the disk 6, the movement of the objective lens 5 in the radial direction of the disk, etc., the detectors 9a and 9 of the reflected light 16 from the disk as shown by the dotted line in FIG.
This has the drawback that the center on point b deviates from the dividing line 9c, causing serious target deviation in the tracking servo control system.

SUMMARY OF THE INVENTION An object of the present invention is to provide a tracking servo signal generation circuit that can eliminate the adverse effects of disturbance factors on tracking error signals and perform accurate tracking control.

The tracking servo signal generating circuit according to the present invention includes a set of detection means provided to detect the amount of deviation of the information detection point in the direction orthogonal to the track, and to generate a difference between the detection outputs corresponding to the amount of deviation. , means for respectively extracting the upper envelope component and the lower envelope component of each detection output of the set of detection means, and means for obtaining a difference signal between these upper envelope components and a difference signal between the lower envelope components. , and means for generating a tracking servo signal by subtracting these difference signals at a desired ratio.

The present invention will be explained below using the drawings.

FIG. 3 is a block diagram of an embodiment of the present invention, in which parts equivalent to those in FIG. 1 are designated by the same reference numerals. In this example, the outputs A ₁ of amplifiers 10a and 10b,
_A2 are upper envelope extraction circuits 17a and 17a, respectively.
17b and lower envelope extraction circuit 18a, 1
8b, and the outputs Ue 1 and Ue ₂ of the upper envelope extraction circuits 17a and _17b are led to the differential amplifier 19, and the difference output F is applied to one side of the differential amplifier 22 via the variable resistor 21. is applied to the input of In addition, the lower envelope extraction circuit 18a,
The outputs Le ₁ and Le ₂ of 18b are led to a differential amplifier 20, and the difference output F is applied to the other input of the differential amplifier 22. The output of the differential amplifier 22 becomes a tracking servo signal I, which is input to the drive amplifier 14 via the equalizer 13.

FIG. 4a shows an example of a circuit for extracting the lower envelope, in which the input signal is applied to the cathode of diode _D1 and output from its anode. Resistor R ₁ and capacitor C ₁ between anode and power supply +Vcc
are connected in parallel.

FIG. 4b shows an example of a circuit for extracting the upper envelope, in which the input signal is applied to the anode of diode _Q2 and output from its cathode. The cathode is grounded by a resistor R ₂ and a capacitor C ₂ is connected in parallel with the resistor R ₂ . Figure 5 a-i
are the operation waveforms of each part of the circuit blocks in FIGS. 1 and 3, and these waveforms show that due to disturbance, the center of the reflected light flux from the disk is at the dividing line 9c between the photodetectors 9a and 9b.
When the tracking servo loop is open and the optical spot is moving diagonally across the recording track on the disk surface, in the state shown by the dotted line in Figure 2, the tracking servo loop is open. It shows the time change of each waveform. In the figure, to indicates the time when the light spot coincides with the center of one track, and t- ₁ and t+ ₁ indicate the times when the light spot coincides with the center of the track and both adjacent tracks on the inner and outer peripheries, respectively. The vertical axis is the signal level.

Figures 5a and 5b show a set of photoelectric conversion elements 9a and 9b.
The amplified signals A ₁ and A ₂ of the respective detection outputs are shown respectively, and the output A ₁ of the detector 9a having a large irradiation area is
The amplitude of the RF component and the DC component (average value component) are both large compared to the output A ₂ of the detector 9b, which is smaller, and they are in a substantially proportional relationship.

It should be noted that the phase relationship between the envelope components of both signals A ₁ and A ₂ has a phase difference from each other from the track center as shown in Figures a and b. This phase difference is caused by the fact that the intensity distribution of the light reflected from the disk becomes asymmetrical with respect to the optical axis due to the deviation between the light spot and the track center.

FIG. 5c shows only the reduced components of the signals A ₁ and A ₂ extracted, and shows the waveforms of the signals B ₁ and B ₂ in FIG. 1. Figure d is a waveform showing the difference between the two signals B ₁ and B ₂ , which is the tracking error signal in the conventional example of Figure 1, and as seen in the figure, the DC
Since the offset exists, the tracking servo will not be able to lock. This DC offset represents the influence of disturbance, and the signal of this embodiment shown below cancels this.
Signals A ₁ and A ₂ are input to the envelope extraction circuit shown in FIG. The values of capacitors C ₁ and C ₂ in FIG. 4 are set to hold the maximum or minimum values of the signals A ₁ and A ₂ , so that each envelope component is extracted. Figure 5e shows the signals Ue ₁ , which are obtained by extracting only the upper envelope components of the signals A _{1 ,} A ₂ .
Figure g shows the waveforms of signals Le ₁ and Le ₂ from which only _the lower envelope components are extracted. Ue ₁ , Ue ₂ , Le ₁ , and Le ₂ can be approximated by the following formula.

Ue ₁ =K ₁・{Yo+y(ωt+δ)} ……(1) Ue ₂ =K ₂・{Yo+y(ωt−δ)} ……(2) Le ₁ =K ₁・{Xo+x(ωt+δ′)}… …(3) Le ₂ = K ₂・{Xo+x(ωt−δ′)} …(4) Here, K ₁ and K ₂ are proportionality constants that change due to disturbances,
Yo, Xo are DC components, y(t), X(t) are AC components, ω is the angular frequency when one track interval scanning time is one period, and δ, δ' are the phase differences mentioned above. .

_{FIG . _ _ _ _ _ _} −δ) ...(5). Figure 5h shows the waveform of the _difference component H between _the signals Le ₁ and Le ₂ , H=Le ₁ −Le ₂ =(K ₁ −K ₂ )・δ) ...(6). As can be seen by comparing the waveforms f and h in Figure 5, the DC offsets at time _tp are in phase, so if you mix both signals F and H at an appropriate ratio and take the difference component, Offset components can be removed.

In this case, the DC offset component of signal F is larger, so we can create a signal by multiplying signal F by an appropriate attenuation constant α using variable resistor 21, and obtain the difference from signal H using differential amplifier 22. good. That is, the signal that has passed through the variable resistor 21 is α・F=α・(K ₁ −K ₂ )・Yo+α・K ₁・y (ωt+δ)−K ₂・y (ωt−δ)} ...(7) Therefore, from equations (6) and (7), choose the value of α that satisfies the following equation: (K ₁ − _{K 2} )・Xo=α・(K ₁ − K ₂ )・Yo... According to
A difference signal I=H-α·F with the DC offset removed can be obtained as shown in Figure i.

I and α are expressed as follows.

I=K ₁ {X(ωt+δ)−α・y (ωt+δ)}−K ₂・{x(ωt−δ) −α・y(ωt−δ)} ……(9) α=Xo/Yo …… (10) Note that the AC components of signals F and H are also in phase, and by taking the difference, the AC component of signal I will also be reduced, but in reality, the upper envelope components of signals A ₁ and A ₂ The rate at which is modulated by the recording track is sufficiently small compared to the rate at which the lower envelope component is modulated, and the reduction in the alternating current component of the difference signal I can be ignored.

As is clear from equation (9), the signal for the drive coil 8 for optical spot position deviation, that is, the tracking servo signal H, has the DC offset due to disturbance removed and the target deviation reduced, as shown in Figure 5i. It can be seen that a good tracking servo signal is obtained.

FIG. 6 shows the amplifiers 10a and 10 in FIG.
This shows an example in which the components from b to the differential amplifier 22 are implemented using another circuit configuration. In FIG. 6, the amplifier 10a is the same as in FIG.
b′ is an inverting amplifier, and its output signal is −
It has become A ₂ . The lower envelope extraction circuit for signal _-A2 consists of a diode _D3 connected with the opposite polarity to that in Figure 4a, and a resistor biased to the power supply -Vcc.
It consists of _R3 and capacitor _C3 , and its output signal is _-Le2 . Also, the upper envelope extraction circuit for signal -A ₂ is constructed using a diode D ₄ connected with the opposite polarity and a resistor R ₄ biased to OV.
and a capacitor _C4 , and its output signal is _-Ue2 . The signal _A1 is the same as the above example. Signals Le ₁ and −Le ₂ are resistors R ₇ ,
Served as the inverting input of the differential amplifier 23 via _R8 ,
The differential amplifier 23 adds the signal -(Le ₁
−Le ₂ ). Then, it is applied to the inverting input of the differential amplifier 25 via the variable resistor 24. On the other hand, the signals Ue ₁ and -Ue ₂ are applied to the same inverting input of the differential amplifier 25 via resistors R ₅ and R ₆ , and the signals -
(Le ₁ −Le ₂ ) is added. The values of the resistors _R5 to _R9 are all Rx ( _R9 is the feedback resistance of the amplifier 23), the value of the variable resistor 24 is R _Y , and the differential amplifier 25
If the value of the feedback resistor is Rf, then the differential amplifier 25
When the value of the feedback resistor 27 is Rf, the output signal J of the differential amplifier 25 is J=Rf/Ry・(Le ₁ −Le ₂ ) −Rf/Rx(Ue ₁ −Ue ₂ ) ……(11) = (Rf/R _Y )・H−(Rf/Rx)・F. Since the signal I from which the DC offset has been removed is expressed as I=H−αF, the formula
Coefficients Rf/R _Y of H and F in (11): Rf/Rx is 1:
If the ratio is set to α, the DC offset can be removed from the signal J. In other words, the value of the variable resistor
R _Y may be set to satisfy the following equation.

Rf/R _Y :Rf/Rx=1:α That is, R _Y =α·R _X ...(12) However, from equation (10), α=Xo/Yo.

In the embodiment of FIG. 6, by using an inverting amplifier as the amplifier 10b', the number of subtraction circuits can be reduced, the cost can be further reduced, and the same effect as in FIG. 3 can be obtained. Of course, various other circuit configurations are possible. Furthermore, so far we have described the case where a disturbance, such as an offset caused by movement of the objective lens in a direction perpendicular to the track, is completely removed, but below we will explain a case where the offset is actively utilized.

Consider a case where the objective lens is supported by a spring or the like in the tracking direction. At this time, when the objective lens is displaced to track the recording track, due to the restoring force due to the stiffness component of the support spring,
Steady-state deviation occurs. This steady deviation, or offset, is proportional to the displacement of the lens, and has a proportionality constant β determined by the stiffness of the support spring and the tracking servo loop gain. On the other hand, an offset proportional to the displacement of the lens occurs in the signal I (=H-αF), and the amount of offset can be freely determined by changing the attenuation constant α. In other words, the proportionality constant γ that relates the displacement of the lens to the amount of offset can be freely determined. Therefore, the damping constant α is
By setting the formula γ=-β (13) to be satisfied, the steady-state deviation caused by the stiffness of the support spring can be canceled out. The circuit configuration of FIG. 3 or FIG. 6 can be realized by adjusting the variable resistor 21 or 24, respectively.

In this way, it is possible to cancel the steady-state deviation component that occurs in the tracking servo loop due to the stiffness of the lens support system, so that accurate tracking servo is always possible even when the amount of eccentricity of the recording track is large.

According to the present invention, the difference in the upper envelope component and the difference in the lower envelope component of each detection output of a set of detection means each contain components due to disturbance in the same phase. It is possible to prevent the tracking servo from malfunctioning due to disturbances simply by performing electrical processing, and there is no need to use any additional optical components. Furthermore, the circuit configuration is simple and disturbances can be removed with high accuracy.

In the above description, the objective lens is moved as the light spot deflecting means, but other means such as a tracking mirror may also be used. In addition, the present invention is not limited to an optical information reading device, but can be performed with any other type of information reading device as long as it generates equivalent tracking error signal information, and the recording medium does not need to be in the form of a disk.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional tracking servo device, Figure 2 is a diagram showing the relationship between a photoelectric conversion element and a light spurt, Figure 3 is a block diagram of an embodiment of the present invention, and Figure 4 is a diagram showing the relationship between a photoelectric conversion element and a light beam. 5 is an operational waveform diagram of each part of the block in FIG. 3, and FIG. 6 is a circuit diagram showing another modification of the circuit block in FIG. 3. Explanation of symbols of main parts, 1... Laser light source, 5
...Objective lens, 6...Disk, 9...Photodetector, 12, 19, 20, 22...Differential amplifier, 1
7, 18...Envelope extraction circuit.

Claims (1)

[Claims] 1. A servo signal generating device for a tracking servo control device, which is configured to bias an information detection point of a pickup in a direction substantially orthogonal to the recording track so that the information detection point of the pickup accurately tracks the recording track, a set of detection means provided to detect the amount of deviation of the information detection point in the direction perpendicular to the track, so that a difference occurs between the detection outputs corresponding to the amount of deviation; and each of the one set of detection means. means for extracting an upper envelope component and a lower envelope component of the detection output, means for obtaining a difference signal between these upper envelope components and a difference signal between lower envelope components, and subtracting these difference signals at a desired ratio. and means for generating a tracking servo signal.