JPH06325394A - Tracking servo circuit for optical disk device - Google Patents

Abstract

PURPOSE:To provide a tracking servo circuit for an optical disk device capable of automatically adjusting offset while securing traceability. CONSTITUTION:This circuit is constituted so that the difference between respective light receiving outputs from a photodetector 2 is corrected by selectively switching one side gain of first and second current/voltage conversion amplifiers 4, 5 by using a switching transistor 10, and the offset voltage of a gain adjusting amplifier 21 is corrected by feeding back and applying a DC bias voltage for adjusting offset to the gain adjusting amplifier 21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking servo circuit of an optical disk device for tracing a light beam from an optical pickup by following a signal track of the disk.

[0002]

2. Description of the Related Art As is well known, in an optical disk apparatus for reproducing or recording on a disk with a light beam using an optical pickup, as is well known, the light beam emitted from the optical pickup is traced by following a signal track of the disk. In order to perform tracking control, the error amount between the trace position of the light beam and the signal track is detected, and the tracking that drives the objective lens of the optical pickup in the radial direction of the disc according to the tracking error signal indicating the error amount. A servo circuit is provided. In the tracking servo circuit, it is necessary to adjust the tracking error signal so that the trace position of the light beam becomes the center of the signal track of the disk when the tracking error signal becomes zero.

At present, as a tracking servo circuit,
The 3-beam method is predominant. As is well known, the three-beam method detects a tracking error by detecting a difference in light receiving output obtained from each light receiving area of a photodetector in which reflected light of two sub beams formed before and after the main beam is received. It seeks a signal. Therefore, in the three-beam method, each received light output is converted from a current signal to a voltage signal by a current / voltage conversion amplifier, and then each received light output of the voltage signal is subtracted by a subtraction circuit to obtain a tracking error signal. However, in the case of the three-beam method, the gain of one of the current / voltage conversion amplifiers is adjusted to correct the difference between the light receiving outputs, thereby adjusting the trace position of the main beam and the tracking error signal level. There is a need.

Further, the tracking servo circuit is provided with a gain adjusting amplifier which amplifies the tracking error signal to a predetermined level in order to adjust the tracking servo gain. The tracking error signal output from the gain adjusting amplifier is It is necessary to correct the offset voltage.

That is, in the tracking servo circuit, as described above, the drive voltage for driving the tracking actuator of the optical pickup is centered with respect to the reference voltage by correcting the difference between the respective light receiving outputs and the offset voltage of the gain adjusting amplifier. Need to be adjusted.

[0006]

By the way, as disclosed in, for example, Japanese Patent Laid-Open No. 4-85728, automation of offset adjustment of a drive voltage for driving a tracking actuator of an optical pickup has been achieved.

By performing such an offset adjustment, the drive voltage of the tracking actuator can be adjusted to the center with respect to the reference voltage when the variations in the received light outputs are small. However, when the variations in the received light outputs are large, the offset can be obtained. There is a problem that the traceability due to tracking control deteriorates if the voltage exceeds the adjustable range or if a too large bias voltage is applied for offset adjustment. Therefore, even if automation of offset adjustment is achieved, the variation of each received light output must be manually adjusted in the manufacturing process, and it is necessary to adjust the drive voltage of the tracking actuator to the center with respect to the reference voltage. Automation was not achieved.

[0008]

The present invention has been made in view of the above points, and converts two predetermined types of light reception signals from the photodetector used for tracking control into voltage signals. First and second current / voltage conversion amplifiers,
A subtraction circuit that subtracts respective output signals from the first and second current-voltage conversion amplifiers to generate a tracking error signal indicating an error amount between the trace position of the light beam and a signal track; and the first and second subtraction circuits. A resistor connected to switch one of the gains of the second current-voltage conversion amplifier,
A switching element for selectively using the resistance, and detecting whether the average level of the tracking error signal from the subtraction circuit is the positive polarity side or the negative polarity side, and controls the switching element according to the detection condition. A control means, a gain adjustment amplifier that amplifies the tracking error signal from the subtraction circuit and adjusts the tracking servo gain, and a DC bias voltage according to the DC component of the tracking error signal obtained through the gain adjustment amplifier. And a bias voltage generating circuit for

[0009]

According to the present invention, depending on whether the average level of the tracking error signal is the positive polarity side or the negative polarity side, the first and second currents
After selectively switching one of the gains of the voltage conversion amplifier, the DC bias voltage corresponding to the DC component of the tracking error signal obtained via the gain adjustment amplifier is fed back to the gain adjustment amplifier for offset adjustment. The drive voltage of the tracking actuator is adjusted to the center with respect to the reference voltage. In particular, when the average level of the tracking error signal is on the negative side due to the difference in the received light output, it is more advantageous for the passage of the black dots, and in consideration of the situation that the traceability is good, When the average level is on the positive side, the tracking error signal is shifted to the negative side.

[0010]

1 is a circuit block diagram showing an embodiment of the present invention. In the figure, reference numeral 1 denotes an optical pickup for reading a signal from a disc (not shown) by using a light beam, which is configured so that tracking control can be performed by the three-beam method. The optical pickup 1 is provided with a photodetector 2 for receiving the reflected light of the light beam reflected by the disc, and a tracking actuator 3 for displacing the trace position of the light beam in the radial direction of the disc. Has been.

Numerals 4 and 5 are _first and _second , respectively, for converting respective light receiving outputs S ₁ and S ₂ obtained from two predetermined light receiving elements used for tracking control of the photodetector ₂ from current signals to voltage signals. It is a second current / voltage conversion amplifier (first and second I-V amplifiers). The first IV amplifier 4
Is connected to the negative feedback path with a capacitor 6, resistors 7, 8 and 9, a grounded emitter switching transistor 10 and a gain switching resistor 11 connected to the collector of the switching transistor 10. On the other hand, the second I-V amplifier 5 includes a capacitor 12, resistors 13 and 14 in the negative feedback path.
And 15 are connected.

Reference numeral 16 denotes the first and second I-V amplifiers 4 and 5.
Optical pickup 1 by subtracting each output signal from
The subtraction amplifiers 17 and 18 for generating a tracking error signal indicating the error amount between the trace position of the light beam from the optical disc and the signal track of the disk, respectively.
A resistor and a capacitor that form a low-pass filter for extracting the DC component that is the average level of the tracking error signal from the device, 19 is a first comparison circuit that compares the terminal voltage of the capacitor 18 with a reference voltage of the ground potential, and 20 is Depending on the comparison output from the first comparison circuit 19, whether the average level of the tracking error signal is the positive polarity side or the negative polarity side is detected, and the negative feedback path of the first IV amplifier 4 in the negative feedback path is detected according to the detection condition. It is a polarity detection circuit that controls the conduction state of the switching transistor 10.

Reference numeral 21 is a gain adjustment amplifier for amplifying the tracking error signal from the subtraction amplifier 16 to adjust the tracking servo gain, and 22 is the gain adjustment amplifier 21.
A phase compensation circuit for compensating the tracking error signal via the phase compensation circuit 3 according to the tracking actuator 3, and a drive 23 for driving the tracking actuator 3 according to the phase compensation processed tracking error signal obtained via the phase compensation circuit 22. It is an amplifier.

Reference numeral 24 is a loop switch transistor for opening and closing the tracking servo loop, and 25 is a tracking servo control circuit for controlling ON / OFF of the loop switch transistor 24.

Reference numerals 26 and 27 respectively denote a resistor and a capacitor forming a low pass filter for extracting the DC component of the tracking error signal through the gain adjusting amplifier 21, and 28 denotes a terminal voltage of the capacitor 27 as a ground potential reference voltage. A second comparison circuit for comparison, 29 generates a PWM signal (pulse width modulation signal) whose duty cycle is increased or decreased according to the comparison output from the second comparison circuit 28. P
The WM signal generating circuit 30 and 31 are resistors and capacitors which form a low-pass filter for smoothing the PWM signal generated from the PWM signal generating circuit 29 into a DC voltage.

Next, the operation of FIG. 1 will be described.

In the initial state when the disc is mounted, the loop switch transistor 24 is turned on by the operation of the tracking servo control circuit 25, and the tracking servo loop is opened. Therefore, each of the light receiving outputs S ₁ and S ₂ obtained from the two light receiving elements for tracking control of the photodetector ₂ has a signal level when the sub beam for tracking control crosses the signal track as the disk rotates. Shows a waveform pattern in which fluctuates. The received light output S ₁ is applied to the non-inverting input terminal of the subtraction amplifier 16 via the first I-V amplifier 4, while the received light output S ₂ is inverted via the _{second I} -V amplifier 5 of the subtraction amplifier 16. It is applied to the input terminal. The subtraction amplifier 16
Generates a tracking error signal according to the difference between the received light outputs S ₁ and S ₂ , and the tracking error signal is supplied to the gain adjusting amplifier 21 and the DC component is extracted via the resistor 17 and the capacitor 18. Are supplied to the first comparison circuit 19.

The first comparison circuit 19 compares the DC component of the extracted tracking error signal with the reference voltage of the ground potential, and when the DC component is higher than the reference voltage, that is, the polarity detection circuit, the polarity detection circuit. 20 generates an ON signal for turning on the switching transistor 10,
On the other hand, when the DC component is lower than the reference voltage, that is, when the DC component is on the negative polarity side, the polarity detection circuit 20 generates an OFF signal for turning off the switching transistor 10.

Here, the first I-V amplifier 4 operates when the switching transistor 10 is off.
Only the resistor 9 is connected to the connection point of the resistors 7 and 8. However, when the switching transistor 10 is turned on, the resistor 11 is connected in parallel to the resistor 9 and the negative feedback amount. Is increased, the gain is increased.

Therefore, the switching transistor 1
When 0 is turned on, the voltage level applied to the non-inverting input terminal of the subtraction amplifier 16 is higher than that when the switching transistor 10 is turned off. Therefore, the first and second I-V amplifiers 4 are The average level of the tracking error signal output from the subtraction amplifier 16 is shifted to the negative polarity side due to the balance with the phases 5 and 5.

The first comparison circuit 19 is so constructed as to take in the DC component of the extracted tracking error signal only for a predetermined period before the reproduction of the disc is started, and once the comparison is made, the reproduction of the disc is carried out. The same output is continuously generated from the polarity detection circuit 20 until the above is completed.

On the other hand, it is supplied to the gain adjusting amplifier 21,
The tracking error signal obtained via the gain adjusting amplifier 21 has a DC component extracted via a resistor 26 and a capacitor 27, and is supplied to a second comparison circuit 28. The second comparison circuit 28 compares the DC component of the extracted tracking error signal with the reference voltage of the ground potential, and when the DC component is higher than the reference voltage, the duty of the PWM signal generated from the PWM signal generation circuit 29. The PW generated from the PWM signal generating circuit 29 when the DC component is lower than the reference voltage so that the setting of the cycle is increased from the reference duty cycle at the initial setting.
The duty cycle of the PWM signal is changed so that the setting of the duty cycle of the M signal is reduced from the reference duty cycle. Further, the PWM signal generated from the PWM signal generating circuit 29 is smoothed by the resistor 30 and the capacitor 31 and negatively fed back to the gain adjusting amplifier 21, so that the DC of the tracking error signal output from the gain adjusting amplifier 21 is obtained. The ingredients are being reduced.
Then, the second comparison circuit 28 outputs the PWM signal generation circuit 29 until the DC component reaches the same ground potential as the reference voltage.
Since the duty cycle of the PWM signal generated from is gradually changed, an appropriate DC bias voltage will be negatively fed back to the gain adjustment amplifier 21, and the DC of the tracking error signal output from the gain adjustment amplifier 21 will eventually be fed back. The component, that is, the offset voltage is corrected.

When the DC component of the tracking error signal from the gain adjusting amplifier 21 becomes equal to the reference voltage in the comparison by the second comparing circuit 28, the PWM signal generated by the PWM signal generating circuit 29 is fixed to the duty cycle. , Then P for that fixed duty cycle
Continue to generate WM signal.

Therefore, the PWM signal generation circuit 29
From this, a DC signal of the tracking error signal output from the gain adjustment amplifier 21, that is, a PWM signal with a duty cycle changed to correct the offset voltage is generated, and an appropriate bias voltage for the gain adjustment amplifier 21 is generated. Is applied, the offset voltage of the gain adjusting amplifier 21 is corrected.

When the correction of the difference between the respective light receiving outputs S ₁ and S ₂ of the photodetector ₂ and the correction of the offset voltage of the gain adjusting amplifier 21 are completed by the above-described operation, the tracking servo control circuit 25 operates to make a loop switch. The transistor 24 is turned off and the tracking servo is closed.

In the closed state of this tracking servo, starting from the photodetector 2 of the optical pickup 1, the first and second IV amplifiers 4 and 5, the subtracting amplifier 16, the gain adjusting amplifier 21, the phase compensating circuit 22, And drive amplifier 2
Since a closed loop whose end point is the tracking actuator 3 of the optical pickup 1 is formed via 3, the tracking actuator 3 is supplied with a drive current according to the tracking error signal for which gain adjustment and phase compensation have been performed. Like Then, by switching the gain of the first I-V amplifier 4, the difference between the light receiving outputs S ₁ and S ₂ input to the subtraction amplifier 16 is corrected, and the offset voltage of the gain adjusting amplifier 21 is absorbed.
Therefore, after ensuring the traceability by tracking control, the tracking error signal input to the phase compensation circuit 22 becomes zero when the main beam used for reading the signal on the disk is tracing the signal track correctly. The tracking actuator is driven according to the correctly adjusted tracking error signal.

[0027]

As described above, according to the present invention, the first
By selectively switching one of the gains of the second and current / voltage conversion amplifiers by using a switching transistor, the difference between the respective light receiving outputs from the photodetector is corrected, and the gain adjustment amplifier is used for offset adjustment. Since the DC bias voltage is fed back and applied to correct the offset voltage of the gain adjustment amplifier, the drive signal for driving the tracking actuator can be automatically adjusted without degrading the traceability. A tracking servo circuit of the device can be provided. In particular, according to the present invention, one of the gains of the first and second current / voltage conversion amplifiers is selected so as to shift the average level of the tracking error signal to the negative polarity side in order to correct the difference between the light receiving outputs. Since it is switched to each other, it is advantageous for the passage of the black dots and is advantageous in terms of the tracing ability.

[Brief description of drawings]

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

[Explanation of symbols]

 1 Optical Pickup 2 Optical Detector 3 Tracking Actuator 4, 5 Current / Voltage Conversion Amplifier 10 Switching Transistor 11 Gain Switching Resistor 16 Subtraction Amplifier 19 First Comparison Circuit 20 Polarity Detection Circuit 21 Gain Adjustment Amplifier 28 Second Comparison Circuit 29 PWM Signal Generator circuit

Claims (1)

[Claims] 1. A tracking servo circuit of an optical disk device for performing tracking control in which a light beam from an optical pickup is made to follow a signal track of a disk and traced, is obtained from a photodetector that receives reflected light from the disk, A first one that converts two predetermined types of received light signals used for tracking control into voltage signals
And a second current-voltage conversion amplifier, and subtracting the respective output signals from the first and second current-voltage conversion amplifier,
A subtraction circuit for generating a tracking error signal indicating an error amount between the trace position of the light beam and a signal track,
A resistor connected to switch one of the gains of the first and second current / voltage conversion amplifiers, a switching element for selectively using the resistor, and an average level of the tracking error signal from the subtraction circuit are A control unit that detects whether the polarity is the positive polarity side or the negative polarity side, and controls the switching element according to the detection status; and a gain adjustment amplifier that amplifies the tracking error signal from the subtraction circuit and adjusts the tracking servo gain. A bias voltage generating circuit for generating a DC bias voltage according to the DC component of the tracking error signal obtained via the gain adjusting amplifier, wherein the average level of the tracking error signal from the subtraction circuit is positive or negative. The first according to the sex side
Also, one of the gains of the second current / voltage conversion amplifier is selectively switched, and a DC bias voltage corresponding to the DC component of the tracking error signal obtained via the gain adjustment amplifier is fed back to the gain adjustment amplifier to provide an offset. A tracking servo circuit for an optical disk device, which is adapted to be adjusted.