JPS5919247A - Focus controller of optical pickup - Google Patents

Abstract

PURPOSE:To detect accurately the coincidence between a focusing point and a disk surface to the surface deflection of a disk and the variance of a focus controller, by moving largely an objective lens forward and backward by means of a focus searching circuit. CONSTITUTION:A sensor 1 has large outputs S1 and S2 and small outputs S3 and S4 in case a focusing point exists at the front of a disk. Then these outputs S1-S4 are approximately equal to each other when the focusing point is coincident with the disk. A sensor input circuit 2 obtains (S1+S2)-(S3+S4) and then delivers a signal F showing a shift by means of a differential amplifier. The signal F is applied with the phase compensation corresponding to the mechanical characteristics of an optical pickup by a phase compensation circuit 3 and making the servo control with high sensitivity possible. The circuit 3 consists of an operational amplifier, etc. and applied with the feedback whose time constant is matched for the mechanical characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to a focus control device for an optical big amplifier for extracting signals from a disk in a DA-D (digital audio disk) system.

(b) Background of the Technology In a DAD system, an optical pickup using laser light is used to read bits recorded on a disk. The optical pickup focuses laser light onto the disk surface and detects changes in the reflected light using a sensor such as a photodiode to obtain a signal. The laser beam is focused by an objective lens, but its depth of focus is about ±2 μm, and if it is not focused on the disk surface, it will be difficult to read the pits. Therefore, due to variations in the placement of the disk or surface runout of the disk, it is necessary to control the laser beam so that the focused point is always on the disk surface.

A critical angle method is a method for detecting a focus shift.

In this method, a critical angle prism is used, and if the focal point is in front of the disk, the reflected light becomes focused light, so the reflection on one side of the critical angle prism is weak, and if the focal point is behind the disk, The reflected light becomes scattered light, and the reflection from the other critical angle prism becomes weak. Therefore, if the divided sensor is installed behind the critical angle prism, a difference in output will occur in the sensor due to K, and this signal will be used as a defocus signal to drive the coil that moves the objective lens and focus the sensor. Adjust.

However, the characteristics of the distance between the objective lens and the disk and the sensor output difference are as shown in FIG. Therefore, the range that can be used as a control signal is +80 .mu.m and -40 .mu.m from the focal point, as shown in FIG. 1, and the servo control state can only be entered within this range. When the disc is installed, it is fine if the disc surface variation is always within the range of 120 μm, but in reality, the standard allows the disc surface runout to be ±0.3 yrp, and it is also necessary to reduce the disc surface variation during installation. If this is taken into account, the variation will be about ±0.6''. Therefore, it is difficult to enter a servo control state based on the sensor output difference, and it is necessary to make the sensor output difference available in some way.

(C) Disclosure of the Invention The present invention has been made in view of the above-mentioned points, and includes a servo gain adjustment circuit that determines the gain in the servo control state, and an objective lens that moves back and forth largely and at low speed to find a focused point. A focus search circuit is provided, and a monitor circuit detects from the signal read from the disk that the focus point has been reached.
When the monitor circuit detects that the focus point has been reached, the output of the monitor circuit stops the operation of the focus search circuit and activates the servo gain adjustment circuit, so that the optical pinoqua can enter the servo control state based on the sensor output difference. The object of the present invention is to provide a control device for the servo mode, and to smoothly and reliably transition from the state of searching for a focused point to the servo state.

2) Description of the embodiment FIG. 2 is a block diagram showing an embodiment of the present invention.
) is a sensor arranged in four parts behind the critical angle P1 nosm, (2) is each output S5, S of sensor (1),
. (4) is a servo gain adjustment circuit which inputs the phase compensated signal F and determines the gain of the servo control state; (5) is a coil (5) for moving the objective lens of the optical pickup; 6), and the above constitutes a servo control system. In addition, (7) is the sensor (each output Sr of 11)
, St, S9, and S4, a signal reading circuit reads the presence or absence of pits recorded on the disc and creates a data signal called an HF signal.(8) detects whether the disc is in the focused point using HI3. Monitor circuit, (9
) is a focus search circuit which oscillates a low frequency signal, for example, an IHz signal, and applies it to the drive circuit (5) to cause the objective lens of the optical pickup to swing back and forth significantly to find a focused point.

When the focused point is in front of the disk, the outputs S and S2 of the sensor (1) are small, and the outputs S and S4 are small, and when the focused point is behind the disk, the output S is small. , and S2 are small, the outputs S3 and S4 become dog, and when the in-focus point and the disk coincide, the outputs S7, S
2, S8, and S4 are approximately equal, and the output at this time is 81
The output change in ~S4 corresponds to a bit change. Therefore, by finding (SI+82)-(S3+84), a signal indicating the deviation between the in-focus point and the disk can be obtained. Sensor input circuit (2) C (S, + S, ) - (
S3 + 34) and outputs a signal F indicating the deviation, and a differential amplifier is used. As mentioned above, the signal F has the characteristics shown in Figure 1, and can be used for servo control at +80μ from the focused point.
m, in the range of -40 μm. The signal F is passed through the phase compensation circuit (
3), additive compensation commensurate with the mechanical characteristics of the optical pickup is performed, making highly sensitive servo control possible. The phase compensation circuit (3) is composed of an operational amplifier, etc.
Feedback is performed with a time constant that matches the mechanical characteristics.

On the other hand, the signal readout circuit (7) to which the outputs S and ~S4 of the sensor (1) are applied is S+ +St +83-1-
84 extracts a signal that changes depending on the presence or absence of bits recorded on the disk and outputs it as an HF signal. The HF signal is a data signal containing digital information, which is applied to a signal processing circuit, undergoes predetermined processing to obtain a quantized digital signal, and is then converted into an analog signal. The HF signal is also applied to a monitor circuit (8), and the monitor circuit (8) determines whether the focused point is on the disk surface by detecting the presence or absence of the HF signal. That is, the output S of sensor (1) is +82-1-8
．． -1-84 changes depending on the bit when the disk surface is within the range of focal depth ±2 μm, and only in this case the HF signal is output. The output CNT of the monitor circuit (8) is applied to a servo gain adjustment circuit (4) and a focus surge circuit (9) to switch and control their operations. That is, if the focused point is not on the disk surface and no HF signal is output, the servo gain adjustment circuit (4) is turned off, the focus search circuit (9) is operated, and the disk is brought to the focused point. The sides match and IN
When the F signal is output, the focus search circuit (9) is turned off and the servo gain adjustment circuit (4) is operated.

The focus search circuit (9) is a CR oscillator, and in the operating state, a signal with a frequency of IHz is sent to the drive circuit (5).
to be applied. When the coil (6) is driven by the IITz signal, the objective lens of the optical big amplifier is ±055mm.
move back and forth with an amplitude of Disc surface runout is 8Hz~
As mentioned above, the surface runout is within ±0.6 mm (including variations in disc mounting), so a movement of the objective lens of ±0.55 mm will definitely bring the in-focus point to the disk surface. In addition, the basic dimensions of the objective lens and the disk are set to 1.8 mm, but even if surface runout and focus search overlap, the minimum gap is (1.65 mm). Therefore, the objective lens and the disk do not come into contact with each other.Then, when the in-focus point matches the disk surface by focus search, the bit is read out and an HF signal is output.The period during which the HF signal is output is It lasts only about 5 mS, and if the servo control system is not operated during this period, it will immediately go out of focus.When the monitor circuit (8) detects the i(F signal, its output CNT is output from the search circuit (9). ), and the servo gain adjustment circuit (4) is activated.

The servo gain adjustment circuit (4) turns on and off the servo control system and sets the gain.

As shown in Fig. 1, the signal F used for servo control has an extremely narrow range of use, and there is also the mechanical inertia of the objective lens, so when turning on the servo control system,
If you suddenly increase the gain, noise will occur. Therefore, it is necessary to gradually increase the gain, but even if the focus search detects that the in-focus point matches the disk surface, if the servo control is too weak, the in-focus point will shift immediately.

Therefore, when the servo gain adjustment circuit (4) is turned on by the signal CNT, it suddenly turns on with a low sensitivity gain that allows servo control within the range of 120/zm shown in Figure 1, and then turns on. , slowly increases to the required gain. As a result of the experiment, the initial gain is 25 d
B to about 30 dB, it is pulled into a range of ±30 μm, and it takes about 0.7 seconds to set a high sensitivity gain of 50 d, 13, and it is pulled into a depth of focus of ±2 μm.

As mentioned above, the focus search circuit (9) searches for the focused point, instantaneously switches to a low-sensitivity servo control system, and gradually increases its gain, making it easy and reliable to control the focus of the optical pink amplifier. It is possible.

Figure 3 shows the drive circuit (5), monitor circuit (8), focus search circuit (9), and servo gain adjustment circuit (4).
) is a circuit diagram.

The drive circuit (5) includes an operational amplifier GO) and an operational amplifier OQ.
When the output of the focus search circuit (9) is applied to the ten terminals of the operational amplifier QO) via the resistor az, the coil (6) is driven by the current according to the output. Furthermore, when the output of the servo gain adjustment circuit (4) is applied to one terminal, the coil (6) is current-driven in accordance with the output.

The reason why the coil (6) is driven by current is that the coil (6) is driven by a voltage.
) is driven, the phase of the current is delayed due to induction.

The monitor circuit (8) includes a diode α, a capacitor α4, and a resistor 09 for rectifying and smoothing the applied HF signal, and an operational amplifier (16) to which the terminal voltage of the capacitor (14) is applied to its child terminal. It consists of an NPN type transistor OQ to which the output of the operational amplifier ae is applied to the base via a resistor α force, a capacitor a1 and a resistor (2) connected to the emitter of the transistor 0, and the emitter potential is set to the focus search (9 ) and output as a signal CNT that controls the servo gain adjustment circuit (4). A feedback resistor (21) is connected to one terminal of the operational amplifier 0e, and a reference voltage is applied through the resistor (dualization).
Since the terminal voltage of is lower than the reference voltage, the output of the operational amplifier θe is -12V, and the transistor 0
The output is turned off, and the signal CNT has a potential of -12V. When the focused point and the disk surface match and the HF signal is applied, the terminal voltage of the capacitor (14) increases and becomes higher than the reference voltage. As a result, the output of the operational amplifier (16) becomes 12V, so the transistor 79720 is turned on and the signal CNT becomes 12V potential. Since the capacitor 01 is at the focused point, it will respond to temporary interruptions in the HF signal.
It holds the potential of the signal CNT.

The focus search circuit (9) is an RC oscillator that uses an operational amplifier (c) and charges and discharges a resistor C (5) and a capacitor (5), and the oscillation frequency is set to approximately IHz. The terminal voltage waveform of the capacitor (27) is a triangular wave of l11z, and is applied as an output to the drive circuit (5). In addition, an N-channel junction FET (G) is connected between the connection point of the feedback resistor cs+ (2'l) connected to the output of the operational amplifier I2 (A) and the child terminal, and the ground.
A diode c31) is provided between the gate and the signal CNT. When the signal CNT is at a potential of 112V,
FET (7) is off and oscillation occurs, but when the focused point is detected and the signal CNT becomes +12V, FET (7) is turned off.
) is turned on, and the connection point of the feedback resistor (281(a)) is grounded, so oscillation stops.

The servo gain adjustment circuit (4) receives the phase compensated signal F.
a resistor 02, a variable resistor (to), and an N-channel junction type FETC 34) connected in series between the drive circuit (5) and the drive circuit (5);
N-channel junction FET connected in parallel with resistance θ
(c), the diode 06)07) connected to the gate of each FET C34J (c), the resistor 2 and capacitor 0 that create a predetermined time constant, and the diode connected in parallel to the resistor (to) (consists of 41, diode □□□
A signal CNT is applied to the connection point between the phase and the resistor (9). Resistor (32 is the low sensitivity of the servo control system, 25 dB to 30 d
B is set, and the variable resistor (2) is used to set a high sensitivity of 50 dB.

When the signal CNT is 112V, the forward direction is t.
-tclc37) (4*d, J: li, each F E T
C34)C35)+l-7, and the signal F is the drive circuit (
5) is not applied, and the servo control system is not working. When in focus and signal CNT reaches +12V, F
Only the ET (34) is turned on, and the servo control system operates with a gain determined by the resistor Q21 and the variable resistor (T). At this time, capacitor C31 and resistor (2)
Due to the time constant of
is fully turned on. Therefore, the servo control system initially has 25
Operates from dB to 30 dB, gradually increasing gain,
It operates with a high sensitivity of 50 dB after about 7 seconds.

In this way, the output signal CNT of the monitor circuit (8) is
2■, that is, when the focal point and the disc surface do not match, the FET (To)/, 34JO→ are turned off, and the focus search circuit (9) operates, and the focus search circuit (9) is activated. Matching of the focus is detected and the output signal CNT becomes +
When it comes to 12V, F E T C30) (34) C35
) turns on, the focus search circuit (9)
The operation of the servo gain adjustment circuit (4) stops and the servo gain adjustment circuit (4) starts operating.

(e) Effects According to the present invention, by moving the objective lens back and forth by a large distance using the focus search circuit, the coincidence between the focused point and the disk surface can be reliably detected despite disk surface runout and device variations. HF
By detecting coincidence with the in-focus point from the signal, switching to the servo control system, and gradually shifting from a low-sensitivity gain to a high-sensitivity gain, it is possible to smoothly enter the servo control state. Therefore, an optical pickup control device that does not malfunction can be obtained, and the circuit configuration is also simple.
This contributes to lowering the price of DAD systems.

[Brief explanation of the drawing]

Figure 1 is an output characteristic diagram of the optical pickup sensor, Figure 2 is
The figure is a block diagram showing an embodiment of the present invention, and FIG. 3 is a circuit diagram showing the main part of the block diagram shown in FIG. 2. Description of main drawings (1)...Sensor, (2)...Sensor input circuit, (3)...Phase compensation circuit, (4)...Servo gain adjustment circuit, (5)... Drive circuit, (6)...Coil, (7)...Signal readout circuit, (8)...Monitor circuit, (9)...Focus search circuit. Figure 1 °i. Tadasu, color 2nd map ward Ofu Castle〉 N＞

Claims (1)

[Claims] 1. A focus control device for an optical pickup used for reading out signals from a disk, which includes a sensor input circuit for detecting the front and back of the focal point from the difference in output between the divided sensors of the optical pickup, and a mechanical system of the optical pickup. a phase compensation circuit that compensates for phase lag; a drive circuit that drives a coil for moving the focal point of the optical pickup back and forth; and a servo gain adjustment circuit to which the output of the phase compensation circuit is applied and determines the gain of the drive circuit. and a focus search circuit that greatly moves the focus of the optical pickup back and forth in order to search for a focused point, and a monitor circuit that inputs a signal read from the disk and detects that it is at the focused point,
A focus control device for an optical pickup, characterized in that the servo gain adjustment circuit and the focus search circuit are switched and controlled by the output of the monitor circuit, thereby changing from a focused point search state to a focused point servo control state.