JPH03104030A - Servo circuit for optical disk device - Google Patents

Abstract

PURPOSE:To automate switching to a suitable rage and to widen the dynamic range of a servo circuit by comparing a sum signal with a threshold level and switching the gain of an I/V converter when the sum signal exceeds the level. CONSTITUTION:Currents from photodiodes 11 and 12 are inputted to a subtracting circuit 15 through I/V converters 13 and 14 and difference, which is obtained by the circuit 15, between the outputs of the converters 13 and 14 is inputted to a division circuit 19 through a low-pass filter 17. Then, the sum of outputs obtained by an adder circuit 16 is also inputted to the circuit 19 through a low-pass filter 18. A logic generation part 20 is provided for range switch and a signal A is compared with the threshold level caused by a threshold voltage generating circuit 24. When the signal A exceeds the level, an output B of a comparator 21 is made low and the analog switches of the converters 13 and 14 are turned ON to lower the gains of the converter.

Description

Detailed Description of the Invention Industrial Application Fields The present invention provides automatic gain control a (A
GC) Regarding servo circuits.

BACKGROUND ART In an optical disc device, an optical disc is rotated and is irradiated with a laser beam emitted from an optical head. When writing a signal, the optical power is increased to thermally make holes (pits) in the recording film or magnetize the recording film, and when reading signals, the optical power is lowered and the reflected light from the recording surface is detected. in this case,
The laser beam is narrowed down by an objective lens, and focus servo is applied so that a predetermined minute spot hits the recording film surface. A tracking servo is also applied so that the laser beam follows the track of the optical disc. Note that the optical head is provided with an actuator, and the actuator displaces the objective lens, thereby making it possible to focus the laser beam or follow the track.

In this case, since the loop gain varies depending on the reflectance of the disk, the power of the laser diode that generates the laser beam, etc., AGC is often applied. AGC usually consists of an analog divider and the like.

Since the AGC has a wide dynamic range, the reflectance is specified and the range is switched by software, or the range is switched in read/write mode.

Problems to be Solved by the Invention> However, when specifying the reflectance, there is a possibility of making a mistake in the case of an unknown disc, and when changing the range in READ/WRITE mode, the power during WRITE is 3 mW (duty - It is also necessary to cover a range 40 times greater, such as from 10%) to 12mW (100% duty), and the problem is that the range to be covered is still wide.

The present invention has been made in view of the above points, and its purpose is to provide a servo circuit for an optical disk device that can widen the dynamic range of the AGC servo of an optical disk and automatically select an appropriate range. It's about doing.

Means for Solving the Problems> To achieve such objects, the present invention has an actuator that displaces an objective lens, writes and reads signals using a laser beam, and generates a servo error signal. an optical head that can perform two I/V converters that convert into voltage signals, a subtraction circuit that generates a voltage proportional to the difference in the amount of light entering the photodiodes, and an addition circuit that generates a voltage proportional to the sum of the amounts of light that enters the photodiodes; a low-pass filter that removes unnecessary frequency components from the output of the subtraction circuit; a low-pass filter that extracts only the fundamental wave component of the output of the addition circuit; and a low-pass filter that extracts only the fundamental wave component of the output of the addition circuit; a divider circuit that divides by the output of the adder circuit and outputs a value proportional to the divided value; and a gain of the 1/V converter when the output of the adder circuit that has passed through the low-pass filter exceeds a certain threshold level. a servo circuit that generates an actuator drive signal based on the output of a divider circuit of the preamplifier, and a power amplifier that supplies current to the actuator in accordance with the output of the servo circuit. It is characterized by the following:

Function> An I/V converter converts the current output of the two photodiodes into a voltage signal, a subtraction circuit and an addition circuit calculate the difference and sum of the two signals from the photodiodes, and a division circuit converts the difference signal. Divide by the sum signal.

This signal drives the actuator.

In this case, the sum signal is compared with a certain threshold level, and when the threshold level is exceeded, the gain of the I/■ converter is switched.

As a result, an appropriate range is automatically selected, and the dynamic range of the AGC servo can be widened.

Example The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a structural diagram showing one embodiment of a servo circuit of an optical disc device according to the present invention. In the figure, 1 is an optical disk, and 2 is an optical head having an actuator for displacing an objective lens. The optical head 2 is capable of writing and reading signals using a laser beam, and is also capable of generating a servo error signal. 3 is a preamplifier that extracts a servo error signal as an electrical level; 4 is a servo circuit that generates an actuator drive signal from the servo error signal; and 5 is a power amplifier that supplies current to the actuator.

FIG. 2 is a block diagram showing details of the preamplifier 3. 11
and 12 are photodiodes provided in the optical head for generating servo error signals; 13 and 14 are current/voltage converters (hereinafter referred to as 1/V converters) having the same configuration; 15;
16 is a subtraction circuit that calculates the difference between the outputs of the two I/V converters 13.14, 16 is an addition circuit that calculates the sum of the outputs of the two I/V converters 13.14, and 17 and 18 are both low-pass filters. A dividing circuit 19 divides the output of the subtracting circuit 15 that has passed through the low-pass filter 17 by the output of the adding circuit 16 that has passed through the low-pass filter 18. 20 is a logic generation section.

FIG. 3 is a configuration diagram showing details of the logic generation section 20.
1 is a converter, 22.23 is a hysteresis resistor,
24 is a threshold voltage generation circuit.

FIG. 4 is a structural diagram showing details of the I/V converter 13 (14), where 31 is an operational amplifier and 34 is an I/V conversion resistor connected to the feedback path of the operational amplifier 31. Further return. A series circuit of an analog switch 32 and a resistor 33 is connected in parallel to a resistor 34 in the feedback path, and the resistance value of the entire feedback path changes by turning on and off the analog switch 32.

The operation in such a structure will be explained next.

Note that FIG. 5 is a diagram showing waveforms at each part. In optical disks, the servo error signal is usually obtained as the difference in light falling on a photodiode. A current proportional to the light entering the photodiode 11.12 is connected to an I/V converter 13.1, respectively.
4, is converted to a voltage level, and is output. The subtraction circuit 15 obtains the difference between the outputs of the 1/V converters 13 and 14, and the resulting signal is subjected to a low-pass filter 17 in which unnecessary frequency components are removed (only the fundamental wave component is extracted), and the signal is input to the division circuit 19.

On the other hand, in the adder circuit 16, the I/V converter 13. The sum of the 14 outputs is obtained, and the signal passes through a low-pass filter 18 and is input to one divider circuit. In one divider circuit, the output of the low-pass filter 17 is passed through the low-pass filter 18.
Divide by the output of , then multiply it by a constant to get the result.

The difference in the amount of light entering the photodiodes 11 and 12 has a property that it is proportional to the displacement from the control target position, and is also proportional to the reflectance of the disk and the amount of light emitted from the head. On the other hand, the sum of the amounts of light entering the photodiodes 11 and 12 has the property of being proportional to the reflectance of the disk and the amount of light emitted from the optical head. Therefore, the calculation result of one division circuit is an output proportional only to the displacement from the control target position.

However, the range of the product of the output power of the optical disc device and the reflectance of the disc can extend over 100 times, and it is difficult to cover this range with a single divider circuit. It is necessary to switch.

The logic generation section 20 is a circuit for range switching, and compares the signal A (output of the low-pass filter 18) with the threshold level generated by the threshold voltage generation circuit 24. If the signal A exceeds the threshold level, the comparator 21 outputs the signal A (the output of the low-pass filter 18). Output B goes LOW, which causes the analog switch 32 in the I/V converter 13.14 to
is turned on and works to reduce the gain of the I/V converter.

Through such an operation, an appropriate range selection is automatically performed.

Effects of the Invention> As explained in detail above, according to the present invention, since range switching is performed using a signal proportional to the product of reflectance and output power, an appropriate range is automatically selected, and two Since the lower limit of the range in the first stage can be set close to the upper limit of the range in the first stage, a wide dynamic range can be covered.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the servo circuit of an optical disc device according to the present invention, FIG. 2 is a configuration diagram showing details of the preamplifier 3, and FIG. 3 is a detailed diagram of the logic generator 20. FIG. 4 is a diagram showing the details of the I/V converter 13 (14), and FIG. 5 is a diagram showing the waveforms of each part. 1... Optical disk 2... Optical head 3...
Preamplifier 4... Servo circuit 5... Power amplifier 11.12... Photodiode 13.14... I/V converter 15... Subtraction circuit 16... Addition circuit 17.1
8...Low pass filter 19...Divider circuit 20...Logic generating section Fig. 1 Fig. 2

Claims (1)

[Scope of Claims] An optical head that has an actuator that displaces an objective lens, that can write and read signals using a laser beam, and generate a servo error signal, and two photos provided in the optical head. An amplifier that receives a current signal from a diode and obtains a signal proportional to displacement from a control target position, and includes two I/V converters that convert the current outputs of two photodiodes into voltage signals, and a photodiode. A subtraction circuit that generates a voltage proportional to the difference in the amount of light entering the photodiode, an addition circuit that generates a voltage proportional to the sum of the amount of light entering the photodiode, and a low-pass filter that removes unnecessary frequency components from the output of the subtraction circuit. , a low-pass filter that extracts only the fundamental wave component of the output of the adder circuit, and the output of the subtracter circuit that has passed through the low-pass filter is divided by the output of the adder circuit that has passed through the low-pass filter, and a value proportional to the divided value is calculated. a preamplifier comprising a divider circuit for outputting an output, and means for switching the gain of the I/V converter when the output of the adder circuit that has passed through the low-pass filter exceeds a certain threshold level; A servo circuit for an optical disk device, comprising: a servo circuit that generates an actuator drive signal based on the output of a divider circuit; and a power amplifier that supplies current to the actuator in accordance with the output of the servo circuit.