JPH0614407B2 - Servo control circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a servo control circuit, and more particularly to a servo control circuit intended to perform stable servo control regardless of an operation mode.

The servo control circuit according to the present invention is applied to, for example, focus control and tracking control of an optical information recording / reproducing apparatus that records or reproduces information by using light.

[Prior Art] A case of focus control in an optical information recording / reproducing apparatus will be described below as an example.

FIG. 3 is a schematic configuration diagram of the focus control system. However,
Here, focus error detection by the beam eccentricity method is taken as an example, but of course, an astigmatism method or a focus error detection method using a knife edge may be used.

In the figure, a light beam from a light source 1 such as a laser passes through a beam splitter 2 and is condensed by an objective lens 3 to form an optical spot on a recording surface 4 of an information recording medium.
The reflected light from the light spot passes through the objective lens 3, is reflected by the beam splitter 2, and is condensed on the two-divided optical sensor 5. Here, since the incident optical axis from the light source 1 and the optical axis of the objective lens 3 are deviated, when the recording surface 4 is displaced from the in-focus position, the optical axis of the reflected light moves and the optical sensor is divided into two. The distribution of the amount of light incident on 5 changes.

Such a change in the light amount distribution is detected by the two-divided optical sensor 5, and the focus control circuit 6 controls the driver 7 so as to eliminate the focus error based on the two detection signals. The driver 7 drives the actuator 8 by this control signal, and the actuator 8 moves the objective lens 3 in the optical axis direction so that the light spot on the recording surface 4 is focused. Due to such an operation, even if the recording surface 4 causes surface deviation, the objective lens 3 moves following the surface deviation and tries to maintain the distance from the recording surface 4 within the focal depth of the light spot.

Actually, tracking control is performed in addition to such focus control, and information recording or reproduction is stably performed by these controls.

When recording information, the light beam from the light source 1 is modulated according to the information and its power is increased. That is, by irradiating the recording surface 4 with a modulated high power light spot, information is recorded as a row of pits on the recording surface 4. Although the pits differ depending on the type of the recording medium, if the recording medium is a read-only optical disk or the like, the minute holes or the like formed in the portion irradiated with the light spot will be the pits. In the case of a recordable / reproducible magneto-optical disk or the like, the temperature rises due to the light spot and the portion where the magnetization direction is reversed becomes a pit.

Also, when reproducing the information recorded in this way,
Information is reproduced by irradiating a light spot on the recording surface 4 with a light beam having a power low enough not to form pits and detecting a change in reflected light depending on the presence or absence of pits. For example, in the case of the above-described magneto-optical disk, the polarization plane of the reflected light rotates depending on the presence or absence of pits, and this change may be detected by an analyzer and an optical sensor.

Of course, the focus control is performed in parallel with such a recording or reproducing operation, and although not described further, tracking control is also performed so that the light spot scans a predetermined position on the writing surface 4. There is.

Next, a conventional servo control circuit in such an optical information recording / reproducing apparatus will be described.

FIG. 4 is a block diagram of a focus control system using a conventional servo control circuit. However, the focus error detection method is the third
The case of the beam decentering method shown in the figure will be described, but the present invention is not limited to this, and the same applies to the astigmatism method and the focus error detection method using a knife edge as described above.

In the figure, first, it is assumed that the objective lens 3 is at the in-focus position. Then, in this state, it is assumed that the recording surface 4 of the information recording medium causes surface wobbling and is displaced by the distance X, and the objective lens 3 is displaced by the distance x in order to follow the recording surface 4.

The control error ΔX = X−x at this time is detected as a change in the light amount distribution on the photosensor 5 divided into two, as described above. Therefore, with optics 9 and 10,
The control error ΔX is the light amount P ₁ of the two parts of the optical sensor 5 and
Converted to P ₂ . However, the optical systems 9 and 10 are not necessarily separated, and the light amounts P ₁ and P _{2 of the two} portions of the optical sensor 5 may be obtained as a result by some error detection method. In addition, the transfer function of the optical systems 9 and 10 is Go ₁
And Go ₂ are expressed as P ₁ = Go ₁ · ΔX and P ₂ = Go ₂ · ΔX.

The light amounts P ₁ and P ₂ are converted into electric signals by the detection circuits 11 and 12 each including the optical sensor 5 and an amplifier, amplified, and output as detection signals V ₁ and V ₂ , respectively. If the transfer functions of the detection circuits 11 and 12 are Gp ₁ and Gp ₂ , P ₁ =
Gp ₁ · P ₁ and P ₂ = GP ₂ · P ₂ .

The detection signals V ₁ and V ₂ are calculated by the subtractor 13 and the adder 14, and the difference signal Vd = 6V ₁ −V ₂ and the sum signal Vs = V ₁ + V ₂
Is obtained.

Subsequently, the difference signal Vd and the sum signal Vs are input to the divider 15,
The focus error signal Ve is output from the divider 15. here,
Ve = Vd / Vs = (V ₁ −V ₂ ) / (V ₁ + V ₂ ), and from this expression, even if the levels of the detection signals V ₁ and V ₂ change between recording and reproduction, The error signal Ve can maintain a constant level. Below, a subtractor 1 having such a function
3, the circuit configuration of the adder 14 and the divider 15 is called an auto gain control circuit.

The focus error signal Ve is input to the driver 19 (corresponding to the driver 7 in FIG. 3) via the phase compensation circuit 17 and the gain control circuit 18 after the voltage Vofs is added and the offset is adjusted by the adder 16. The driver 19 supplies the drive current Ie corresponding to the focus error signal Ve to the actuator 20.
(Corresponding to the actuator 8 in FIG. 3) to obtain the displacement x of the objective lens 3. Phase compensation circuit 17, gain control circuit 18, driver 19, actuator
When the transfer functions of 20 are Gph, Gg, Gdr, and Gac, the displacement of the objective lens 3 is expressed as x = Gph.Gg.Gdr.Gac. (Ve + Vofs).

In the same manner, the objective lens 3 is driven so that the control error ΔX becomes zero, and the recording surface 4 is controlled so that it is always in the in-focus position.

By the way, as described above, the power of the light beam from the light source 1 has different set values depending on the operation mode such as recording and reproduction. Thereby, the levels of the light amounts P ₁ and P ₂ incident on the optical sensor 5 are also detected signals V ₁ and V _2.
The level of will be different. However, even if the light amount changes depending on the operation mode as described above, the focus error signal Ve of a constant level can be obtained by the above-described auto gain control circuit. This will be described below with reference to FIG.

FIG. 5 shows the amount of positional deviation ± ΔX and the amount of light P ₁ on the optical sensor divided into two when an ideal light source and optical system are used.
3 is a graph showing the relationship between P and P ₂ . However, + ΔX
Is -ΔX when the objective lens 3 is far from the in-focus position.
Represents the case of approaching, and the suffixes w and r represent the time of recording and the time of reproducing, respectively.

As described above, the focus error signal Ve = Vd / Vs = (V ₁ −
V ₂ ) / (V ₁ + V ₂ ), but if the photoelectric conversion efficiency of each sensor portion and the amplification factor of each amplifier in the detection circuits 11 and 12 are the same, Ve = (P ₁ −P ₂ ) / ( It can be expressed as P ₁ + P ₂ ). Therefore, as shown in FIG. 5, in the ideal state in which the changes in the light amounts P ₁ and P ₂ incident on the respective sensor portions are symmetrical with respect to the positional deviation amount ΔX = 0, the focus error signal Vew during recording and Ver at the time of reproduction is equal to the following formula.

Vew = (Pw ₁ −Pw ₂ ) / (Pw ₁ + Pw ₂ ) = (Pr ₁ −Pr ₂ ) / (Pr ₁ + Pr ₂ ) = Ver Therefore, the position deviation amount ± ΔX regardless of the operation mode.
It is possible to obtain the focus error signal Ve corresponding to, and the servo system is maintained in a stable state.

[Problems to be Solved by the Invention] However, in reality, there are installation errors of optical systems such as an objective lens, a beam splitter, and an optical sensor. Therefore, each sensor part is centered on the positional deviation amount ΔX = 0. The changes in the incident light quantities P ₁ and P ₂ are not symmetrical. Such an error in the change of the light amounts P ₁ and P ₂ is particularly remarkable at the time of recording when the power of the light source is high. The state at this time is No. 6
Shown in the figure. FIG. 6 is a graph showing an example of the actual relationship between the positional deviation amount ± ΔX and the light amounts P ₁ and P ₂ on the photosensor divided into two.

As shown in the figure, in particular, changes in the light amount Pw ₁ and Pw ₂ during recording
Becomes non-symmetric, and when the positional deviation amount ΔX = 0, (Pw ₁ −P
w ₂ ) does not become zero. Therefore, in the conventional servo control circuit shown in FIG. 4, although the objective lens is held at the in-focus position, the focus error signal Vew = (Pw ₁
-Pw ₂ ) / (Pw ₁ + Pw ₂ ) drives the objective lens,
There is a problem that the recording operation is performed in the defocused state. When the light spot is defocused,
Due to the power distribution, pits may not be formed on the information recording medium, and the servo system may become unstable.

[Means for Solving Problems] In order to solve the above conventional problems, the servo control circuit according to the present invention controls a target value and a controlled object by using desired physical quantities having different set values depending on operation modes. A plurality of divided detection means for detecting an error from the quantity, an addition circuit and a subtraction circuit for calculating the sum and difference of two outputs of a desired combination of the plurality of divided detection means, and a difference output of the subtraction circuit And a division circuit for obtaining an error signal by dividing by the sum output of the addition circuit,
In the servo control circuit for operating the controlled object based on the error signal from the division circuit, switch means for switching at least the offset voltage of the subtraction circuit according to the operation mode is provided.

[Operation] By providing the switch means, it is possible to correct at least the difference output according to an operation mode such as recording or reproduction, and to obtain the target value (for example, the displacement distance X due to the surface deviation of the information recording medium). The above-mentioned error signal that corresponds accurately to the error with the controlled variable of the controlled object (for example, the moving distance x of the objective lens) is obtained, and accurate and stable control operation can be achieved.

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

FIG. 1 is a block diagram of a focus control system using an embodiment of a servo control circuit according to the present invention. However, the same parts as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted.

In this embodiment, the offset signal Vd
The difference signal Vd ′ added with f and the sum signal Vs ′ obtained by adding the offset voltage Vsf to the sum signal Vs of the adder 14 are input to the divider 15, and the divider 15 receives the corrected difference signal Vd ′. The focus error signal Ve is output using the sum signal Vs'.

The offset voltage Vdf becomes the voltage Vdfw at the time of recording and becomes the voltage Vdfr at the time of reproducing by the operation of the analog switch 21.
Further, the offset voltage Vsf becomes the voltage Vsfw at the time of recording and becomes the voltage Vsfr at the time of reproducing by the operation of the analog switch 22. With these correction voltages, for example, the difference signal caused by the deviation between the light amount changes Pw ₁ and Pw ₂ as shown in FIG.
By correcting the errors of Vd and the sum signal Vs, it is possible to obtain a focus error signal Ve 'equivalent to the error signal that would be obtained when the ideal light amount changes shown in FIG. Hereinafter, a specific circuit configuration will be described.

FIG. 2 is a specific circuit diagram of the automatic gain control circuit in this embodiment.

In the figure, each detection signal V ₁ from the detection circuits 11 and 12
And V ₂ are applied to the subtractor 13 via resistors R ₁ and R ₂ , respectively.
Operational amplifier (hereinafter referred to as operational amplifier 13).
Input to the non-inverting terminal and inverting terminal of. Furthermore, the offset voltage Vdf from the analog switch 21 is input to the non-inverting terminal of the operational amplifier 13 via the resistor R ₃ . Therefore, the difference signal Vd ′ = V ₁ −V ₂ + Vdf to which the offset voltage Vdf is added is output from the operational amplifier 13 to the divider 15.

The offset voltage Vdf is switched by the analog switch 21 to the voltage Vdfw during recording and to the voltage Vdfr during reproduction.
The voltage Vdfw and the voltage Vdfr are adjusted by the variable resistors VR1 and VR2 having the power supply voltage applied across them.

The detection signals V ₁ and V ₂ are input to the non-inverting terminal of an operational amplifier (hereinafter referred to as operational amplifier 14) that constitutes the adder 14 via resistors R ₄ and R ₅ , respectively. Further, the offset voltage Vsf from the analog switch 22 is input to the non-inverting terminal of the operational amplifier 14 via the resistor R ₇ . Therefore, the difference signal Vs ′ = V ₁ + V ₂ + Vsf to which the offset voltage Vsf is added is output from the operational amplifier 14 to the divider 15.

The offset voltage Vsf is switched by the analog switch 22 to the voltage Vsfw at the time of recording and to the voltage Vsfr at the time of reproducing.
The voltage Vsfw and the voltage Vsfr are adjusted by the variable resistors VR3 and VR4 having the power supply voltage applied across them.

Thus, during recording, offset adjustment is performed using the variable resistor VR1, and when the objective lens is at the in-focus position, the difference signal Vd ′ can be set to zero and the focus error signal Ve ′ can be zero-crossed. As a result, the recording operation can be performed in an accurately focused state. Also, during reproduction, the same adjustment can be performed using the variable resistor VR2.

Furthermore, by adjusting the sum signal Vs ′ by using the variable resistor VR3 during recording and the variable resistor VR4 during reproduction,
It is possible to match the sensitivities of the focus error signal Ve ′ during recording and during reproduction, and eliminate the conventional control instability due to the operation mode. Of course, the offset adjustment of the sum signal Vs ′ and the offset adjustment of the focus error signal Ve ′ can be omitted if there is some stability.
In this case, the focus error signal Ve 'can be sufficiently corrected only by adjusting the offset of the difference signal Vd'.

The switching operation of the analog switches 21 and 22 in this embodiment is performed by an operation mode switching signal from the control section of the information recording / reproducing apparatus.

Further, although the case of the focus control system has been described in the present embodiment, the present invention is not limited to this, of course. For example, it is obvious that the present invention as illustrated in FIGS. 1 and 2 can be applied to a tracking control system using a photo sensor divided into two.

Furthermore, it is apparent from the above detailed description that not only the optical information recording / reproducing apparatus but also a control system utilizing a physical quantity such as magnetism is within the scope of the present invention.

[Effects of the Invention] As described in detail above, the servo control circuit according to the present invention is provided with the switch means for switching at least the offset voltage of the subtraction circuit according to the operation mode, so that at least the differential output is corrected according to the operation mode. It is possible to obtain an error signal that accurately corresponds to the error, and it is possible to achieve an accurate and stable control operation. Further, by providing the second switch means for switching the offset voltage of the adder circuit and correcting the sum output, it becomes more stable regardless of the characteristics of the detection means used and the level change of the detection signal due to the operation mode. Control is achieved.

For example, when the present invention is applied to an optical information recording / reproducing apparatus, focus deviation and tracking deviation due to the switching of operation modes, which have been problems in the related art, are eliminated, and control operation is stabilized, so that reliability is improved. A high recording / reproducing operation can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram of a focus control system using an embodiment of a servo control circuit according to the present invention, FIG. 2 is a concrete circuit diagram of an automatic gain control circuit in this embodiment, and FIG. FIG. 4 is a schematic block diagram of a focus control system, FIG. 4 is a block diagram of a focus control system using a conventional servo control circuit, and FIG. 5 is a positional shift when an ideal light source and optical system are used. Amount ± ΔX and light amount P ₁ on the photo sensor divided into two
And P ₂ are graphs, and FIG. 6 is a graph showing an example of the actual relationship between the positional deviation amount ± ΔX and the light amounts P ₁ and P ₂ on the photosensor divided into two. 1 ... Light source 3 ... Objective lens 4 ... Information recording medium 5 ... Divided optical sensor 11, 12 ... Detection circuit 13 ... Subtractor (opamp) 14 ... Adder (opamp) 15 ... Dividers 21, 22 …… Analog switch

Claims (2)

[Claims] 1. A plurality of divided detecting means for detecting an error between a target value and a controlled variable of a controlled object by using desired physical quantities having different set values depending on operation modes, and a desired one of the plurality of divided detecting means. An addition circuit and a subtraction circuit for calculating the sum and difference of the two outputs of the combination, and a division circuit for dividing the difference output of the subtraction circuit by the sum output of the addition circuit to obtain an error signal, A servo control circuit for operating the controlled object based on an error signal from a division circuit, comprising a switch means for switching at least an offset voltage of the subtraction circuit according to the operation mode. 2. The servo control circuit according to claim 1, further comprising second switch means for switching the offset voltage of the adder circuit according to the operation mode.