JPH0553015B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a servo circuit for an optical head in an optical disk device.

2. Description of the Related Art Optical disk devices that record/reproduce signals optically or magneto-optically using an optical disk and an optical head are becoming popular.

Such optical disk devices require follow-up control (“tracking”) to always keep the optical head in a predetermined position with respect to the optical disk, and
Tracking within a vertical plane is called tracking servo/focusing servo, respectively.

It is desirable to increase the servo gain to a considerable degree in order to improve tracking responsiveness and accuracy, but if the servo gain is increased too much, high frequency components may increase in the servo band due to scratches on the optical disk. , the servo system becomes unstable and jumps between tracks (sound jumps) are likely to occur.

Therefore, conventionally, the servo gain is set to an intermediate value within a range that is compatible with the disc specifications, taking into account the density of scratches expected on the optical disc.

The problem that the invention seeks to solve.

Recently, as optical disk devices have become smaller and more economical, they have become popular for in-vehicle applications such as car stereos, but such in-vehicle optical disk devices are subjected to a considerable amount of vibration as the vehicle runs. Therefore, a larger servo gain than before is required to reduce tracking errors caused by vibration.

However, simply increasing the servo gain causes the problem that sound skips are more likely to occur due to scratches on the optical disk, as described above.

Further, regarding the servo system, there is a problem that the servo system oscillates when the amount of phase rotation at the gain intersection of the open loop approaches 180°.

Means for Solving the Constituent Problems of the Invention The servo circuit of an optical head according to the present invention for solving the problems of the prior art described above is based on the signal level within the vibration band included in the tracking error signal of the optical disk device. means for detecting the magnitude of vibration applied to the optical head; means for detecting flaws on the optical head based on the amplitude of the reproduced signal; The apparatus includes gain control generating means for generating a gain control signal, and a variable gain amplifier circuit for increasing or decreasing the amplification gain for the tracking error signal in accordance with the increase or decrease in the generated gain control signal.

This variable gain amplifier circuit has a low-frequency band that maintains a nearly constant gain regardless of increases or decreases in frequency, and a low-frequency band that maintains a nearly constant gain regardless of increases or decreases in frequency.
Variable gain amplification characteristics include an intermediate frequency band that increases at a rate of 20 dB/dec, and a high frequency band that maintains a nearly constant gain regardless of increase or decrease in frequency, with the boundaries of each frequency band shifting toward the lower frequency side as the gain decreases. Means for realizing the multiplication based on the multiplication of the gain control signal and the tracking error signal is provided.

Hereinafter, the operation of the present invention will be explained in detail together with examples.

Embodiment FIG. 1 is a functional block diagram showing the configuration of the main parts of a servo circuit of an optical head according to an embodiment of the present invention.

The main parts of this servo circuit include a variable gain amplifier circuit 1, a gain control signal generation circuit 2, a drive circuit 3, and an actuator 4.

The input terminal 5 is supplied with a tracking error signal output from a known appropriate tracking error detection circuit. This tracking error signal is amplified by a variable gain amplifier circuit 1, and is supplied via a drive circuit 3 to an actuator 4 for driving the optical head.

The gain control signal generation circuit 2 includes a vibration detection system 10 that detects a frequency component caused by vibration from a reproduction signal supplied to an input terminal 5, and a vibration detection system 10 that detects a frequency component caused by vibration from a reproduction signal supplied to an input terminal 6. It is equipped with a flaw detection system 20 that detects flaws on the optical disk based on its size.

The vibration detection system 10 is composed of a vibration band BPF 11, a limiter 12, an absolute value circuit 13, an offset voltage addition circuit 14, and a peak hold circuit 15 connected in series. Since the vibration component included in the tracking error signal as the vehicle travels appears in a low frequency region from several Hz to about 200 Hz, a vibration band BPF 11 is installed that passes only this frequency band.

The flaw detection system 20 includes a peak hold circuit 21/bottom hold circuit 22 that holds the maximum value/minimum value of the eye pattern of the reproduced signal, and detects the size of the dip appearing in the eye pattern from the difference between the maximum value and the minimum value. a subtraction circuit 23 that calculates the magnitude of the dip (the magnitude of the influence of scratches on the optical disk), a limiter 24 that limits the amplitude of the dip, a peak hold circuit 25 that holds the amplitude limit value, and an offset voltage for the holding signal. The offset voltage adding circuit 26 adds the offset voltage, the amplitude inverting circuit 27 inverts the amplitude of the signal to which the offset voltage is added, and diodes 28 and 29.

A servo error signal is input to the input terminal 5 and distributed to the vibration detector 10 of the variable gain amplifier circuit 1 and the gain control signal generation circuit 2. The servo error signal input to the vibration detector 10 has a vibration band BPF11.
The signal in the band generated by the vibration is extracted.

The extracted signal indicating the magnitude of vibration is subjected to amplitude limitation by a limiter 12 to avoid runaway due to rapid movement of the objective lens of the optical head.
The signal output from the limiter 12 is transmitted to the absolute value circuit 1
3 into a non-polar signal indicating only the magnitude of vibration, and an adder 14 adds an offset value for optimizing the servo gain in a non-vibration state.

Since vibrations applied to the device have an effect over several milliseconds to several seconds, a peak hold circuit 15 is installed to prevent the servo gain from decreasing over such a period of time.

If there are no scratches on the optical disc, as the vibration increases
Vy increases, and as the vibration decreases, Vy decreases and approaches the offset value. The gain of the variable gain amplifier circuit 1 is increased or decreased in accordance with the increase or decrease in Vy, and is configured to perform optimal phase compensation in accordance with the gain. Therefore, a highly stable control system is constructed in which optimal phase compensation is always performed even if the gain changes depending on the magnitude of vibration.

Furthermore, if there are scratches on the optical disk, it is necessary to reduce the servo gain. Therefore, when a scratch on the optical disk is detected by the scratch detection system 20, Vy is lowered. That is, a reproduced signal (RF signal) from the optical disk inputted from the input terminal 6 is supplied to a peak hold circuit 21 and a bottom hold circuit 22 for extracting the envelope of this reproduced signal.

If there is a scratch on the optical disc, the amount of reflected light decreases and the amplitude of the reproduced signal decreases, so the difference between the outputs of the peak hold circuit 21 and the bottom hold circuit 22, and therefore the amplitude of the output of the subtracter 23 decreases.
However, since the amount of reflected light varies depending on the manufacturing conditions of the optical disc, the amplitude is limited by the limiter 24 at the subsequent stage in order to determine that there is no scratch when the amplitude of the reproduced signal is greater than a predetermined value.

Furthermore, after the temporal change is made gentle by the peak hold circuit 25 at the subsequent stage, an offset value is added by the adder 26 to set the minimum gain when a scratch occurs on the optical disc. The amplitude of the added output from the adder 26 is inverted by an inverting circuit 27 so that the voltage decreases when a scratch on the optical disc is detected.

The vibration detection system 10 and the flaw detection system 20 are coupled through a diode 29. Therefore, when there is no scratch on the optical disc, the output voltage of the scratch detection system 20 becomes higher and the diode 29 turns off, and the gain control voltage Vy is determined by the output voltage of the vibration detection system 10. be done.

On the other hand, when there is a scratch on the optical disc, the output voltage on the scratch detection system 20 side becomes lower, so the diode 29 is turned on, and the gain is controlled by the voltage on the scratch detection system 20 side. Voltage Vy is determined. Therefore, as the vibration level increases, the gain control voltage Vy increases, and if there is a scratch on the optical disk, the gain control voltage Vy decreases.

Since the amplification gain of the variable gain amplifier circuit 1 is controlled by the gain control voltage Vy, the servo gain increases as the vibration level increases, and when scratches appear on the optical disk, the servo gain decreases, ensuring that the servo is always properly controlled. Gain is set.

Noise components included in the reproduced signal due to scratches on the optical disk appear in a high frequency region of about 500 Hz to several KHz, and the noise power is detected as the size of the dip on the eye pattern.

When the output of the peak hold circuit 15 of the vibration component detection system increases as the vibration component in the reproduced signal increases, the gain control voltage Vy supplied to the variable gain amplifier circuit 1 increases, increasing the gain of this amplifier circuit. .

On the other hand, when the output of the amplitude inversion circuit 27 of the high-frequency component detection system decreases due to the increase in the number of optical disks, the gain control signal Vy supplied from the gain control signal generation circuit 2 to the variable gain amplifier circuit 1 decreases, and the amplification Decrease the gain of the circuit.

Actuator 4 in the servo circuit shown in Figure 1
The gain-frequency response characteristic can be approximated by a curve in which the gain is constant up to a certain frequency and decreases at a slope of 40 dB/dec at higher frequencies, as shown by the solid line in FIG. Further, it is assumed that the gain-frequency characteristic of the drive circuit 3 is flat over the entire servo band.

Under this premise, as shown in FIG. 3, the variable gain amplifier circuit is constructed such that the open loop gain of the servo system including the drive circuit 3 and actuator 4 increases as the gain control voltage Vy increases. 1 gain-frequency characteristics are varied. In this open loop gain, in order to avoid instability of the servo system due to phase rotation on the high frequency side, the open loop gain is -
Frequency where the slope changes from 40dB/dec to -20dB/dec and the gain becomes 0dB (zero crossing point)
is pushed toward the high range side.

In order to realize the open loop gain of the servo system as shown in FIG. 3 under the gain-frequency characteristic of the actuator as shown in FIG. 2, the gain-frequency characteristic of the variable gain amplifier circuit 1 must be It should be something like the one shown in the figure. In other words, the gain in the low range is flat and increases as the gain control signal Vy increases, and in the high range it increases.
The gain increases with a slope of 20 dB/dec, and the frequency at which this increase starts is proportional to the gain control signal Vy (draws a locus with a slope of 40 dB/dec).

However, if the gain of the variable gain amplifier circuit 1 continues to increase at 20 dB/dec on the high frequency side as shown in FIG. 4, the output amplitude will reach an excessive value exceeding the power supply voltage. Therefore, as shown in FIG. 5, it is desirable to limit the gain of the variable gain amplifier circuit 1 on the high frequency side.

In the end, the frequency characteristics of the open loop gain of the servo system, which is a combination of the gain-frequency characteristics of the actuator shown in FIG. 2 and the gain-frequency characteristics of the variable gain amplifier circuit 1 shown in FIG. 5, are shown in FIG. 6A. The phase characteristics are as shown in FIG.

A configuration example of the variable gain amplifier circuit 1 suitable for realizing characteristics close to the gain-frequency characteristics shown in FIG. 5 is shown in the seventh example.
As shown in the figure.

This variable gain amplifier circuit 1 includes a partial circuit 31 that reduces the gain on the high frequency side at a slope of approximately -20 dB/dec.
It is configured as a cascade-connected circuit with a partial circuit 32 that increases the gain on the high frequency side at a slope of approximately 20 dB/dec.

The partial circuit 31 includes a multiplier 31, an input resistor R, and a feedback capacitor C, and its voltage gain Ga is Ga=(Vy/10)/[1+jωCR(1-Vy/10)]
... is given by (1). However, Vy is a normalized voltage normalized to 1 volt.

The high-frequency side of this gain Ga is shown by the dotted line in Figure 8, but as shown by the solid line in the same figure, it decreases at a slope of -20 dB/dec on the high-frequency side, and the decrease starts. It can be approximated by a group of curves that lie on a thin line with a frequency slope of 20 dB/dec.

Note that it is possible to realize a gain that exactly matches the solid line in FIG. There is a problem with N.

The partial circuit 32 includes a multiplier 32a, an arithmetic circuit 32
b, resistors Rs, Rm, Rf and capacitors Cs, Cf, whose voltage gain Gb is Gb=-(RfVy/10Rm)×[1+jωCs(10Rm/Vy
+1)]/[1+jωCsRs] ...(2) is given. However, the resistance Rm is a normalized resistance value normalized in KΩ.

If 10Rm/Vy is sufficiently larger than 1,
Equation (2) is approximated by the following equation.

Gb=-(RfVy/10Rm)×[1+jωCs(10Rm/
Vy)]/[1+jωCsRs]...(2) As shown in Figure 9, this gain Gb is a flat gain in the low range that is proportional to Vy in dB,
It starts to increase at 20 dB/dec from the frequency of Vy/(20πCsRm), becomes flat at the frequency of 1/(2πCsRs), and starts to decrease at −20 dB/dec at the frequency of 1/(2πCfRf).

Therefore, the total gain G of the variable gain amplifier circuit having the configuration shown in FIG. 7 becomes that shown in FIG. 10 by combining the gain Ga shown in FIG. 8 and the gain Gb shown in FIG. 9.

A more specific example of the variable gain amplifier circuit 1 shown in FIG. 7 is shown in FIG.

In order to compensate for the narrow dynamic range and non-linearity of the analog multipliers 31a and 32b, the coefficient multiplier 3
1b and 32c are added, and DC servo circuits 31c and 32 are added to reduce the influence of offset voltage.
d has been added. Furthermore, a low frequency compensation circuit 30 for adjusting the gain-frequency characteristics in the low frequency range.
has been added.

The gain-frequency characteristic of the variable gain amplifier circuit 1 shown in FIG. 11 is as shown in FIG. 12.

Fig. 13 shows the simulation results of the gain-frequency characteristics of Fig. 12, and Fig. 14 shows the simulation results of the frequency characteristics of the open loop gain of the servo circuit in this case, and shows the expected results. The same open-loop gain is achieved.

Above, we have exemplified a configuration in which the multiplication of the gain control signal and the tracking error signal is realized by a combination of an analog multiplier and an adder. It is also possible to have a configuration that realizes this.

Effects of the Invention As explained in detail above, the servo circuit for an optical disk according to the present invention includes a means for detecting the magnitude of vibration applied to an optical disk device and a scratch on the optical disk, and a means for detecting the magnitude of vibration applied to an optical disk device and a scratch on the optical disk, and a means for detecting a servo gain as the vibration component increases. Since the configuration includes an amplifier circuit that adaptively controls the servo gain by increasing the servo gain and decreasing the servo gain as the number of scratches increases, it is possible to simultaneously prevent interference due to vibrations and sound skipping due to scratches.

Further, according to the present invention, the variable gain amplification circuit
Based on the multiplication of the gain control signal Vy and the tracking error signal, there is a low frequency band where the gain is almost flat and a low frequency band where the gain is almost flat.
The structure realizes a variable gain amplification characteristic in which the intermediate frequency band increases at 20 dB/dec and the high frequency band where the gain is almost flat, and the boundary between each frequency band shifts toward the lower frequency side as the gain decreases. The inconvenience of oscillation when the amount of phase rotation at the intersection approaches 180° can be effectively prevented, and the effect is that highly stable servo characteristics can be realized.

[Brief explanation of the drawing]

FIG. 1 is a functional block diagram showing the configuration of the main parts of a servo circuit according to an embodiment of the present invention, and FIGS.
The figure is a gain-frequency characteristic diagram for explaining the principle of the main part of the servo circuit in FIG. 1, FIG. 7 is a circuit diagram showing an example of the configuration of the variable gain amplifier circuit 1 in FIG. 1, and FIGS. Figure 10 shows the gain for the circuit of Figure 7.
A frequency characteristic diagram, FIG. 11 is a circuit diagram showing an example of a more specific configuration of the variable gain amplifier circuit 1 of FIG. 1, and FIG. 12 is a gain-frequency characteristic diagram of the variable gain amplifier circuit 1 of FIG. Figure 13 is a characteristic diagram showing the simulation results of the gain/phase-frequency characteristics of the variable gain amplifier circuit in Figure 11, and Figure 14 is the open loop gain/phase of a servo circuit using the variable gain amplifier circuit in Figure 13. - It is a characteristic diagram showing simulation results of frequency characteristics. DESCRIPTION OF SYMBOLS 1... Variable gain amplification circuit, 2... Gain control signal generation circuit, 3... Drive circuit, 4... Actuator, 10... Vibration component detection system, 20... Flaw detection system, 31... High frequency side Partial circuit for decreasing gain, 31a... Multiplier, 32... Partial circuit for increasing high frequency side gain, 32a... Multiplier, 32b
...Operation amplifier.

Claims (1)

[Scope of Claims] 1. In an optical head servo circuit that controls the position of the optical head with respect to the optical disk based on a tracking error signal, the optical head servo circuit: means for detecting the magnitude of vibrations applied to the optical disk device from the optical head; means for detecting flaws on the optical head based on the amplitude of the reproduced signal; gain control signal generating means for generating a gain control signal that decreases as the gain control signal increases; and a variable gain amplification circuit that increases or decreases the amplification gain for the tracking error signal in accordance with the increase or decrease in the generated gain control signal; Gain amplifier circuits have a low frequency band in which the gain remains almost constant regardless of increase or decrease in frequency, and a low frequency band in which the gain remains almost constant as the frequency increases.
Variable gain amplification characteristics include an intermediate frequency band that increases at a rate of 20 dB/dec, and a high frequency band that maintains a nearly constant gain regardless of increase or decrease in frequency, with the boundaries of each frequency band shifting toward the lower frequency side as the gain decreases. A servo circuit for an optical head, comprising means for realizing the multiplication based on the multiplication of the gain control signal and the tracking error signal.