JPS5827889B2 - Loop gain adjustment circuit for servo circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a loop gain adjustment circuit for a servo circuit, and particularly to a loop gain adjustment circuit for a servo circuit, and in particular, by adjusting the change in gain of a tracking detection section of a video disc to a constant value, tracking can be achieved with a set loop gain. It is an object of the present invention to provide a loop gain adjustment circuit that can alleviate differences in reproduction states caused by servo circuits.

In devices that play back video discs and audio discs (hereinafter collectively referred to as discs) in which video and audio signals are recorded by forming spiral or concentric trunks using countless intermittent bits, a scanner is used. It is well known that various servo circuits such as a tracking servo circuit and, if necessary, a focus servo are provided in order to trace the information accurately to the information track.

75) The loop gain of this servo circuit varies depending on the combination of the disc and the playback device due to the fact that the bit shape of the disc is not standardized.

For example, in an optical reproducing device that detects and reproduces recorded information signals on a disc using changes in the light intensity of a light beam, the loop gain of the tracking servo circuit that responds to the eccentricity of the disc and controls the read spot on the bit is controlled. It consists of the circuit gain, the gain of servo elements such as galvanomirrors, and the tracking error detection gain depending on the disk bit shape.The gain value of the former two does not change due to the initial setting, but the error detection gain depends on the disk bit shape. Since the gain is formed by the mutual relationship between the bit shape and the reading spot, this gain value depends on the degree of standardization of the spot diameter formed by the lens used, the light intensity distribution, the focus shape of the disc, etc. It changes depending on the silk combination.

In fact, differences occur in the light intensity distribution of the beam nod due to differences in lens numerical apertures as well as differences in various aberration corrections.

Regarding discs, there are also differences in the process when force is applied vertically, for example, the depth of the bit is adjusted depending on the thickness of the photosensitive material, and the bit shape is determined by the intensity of the laser beam and the sensitivity of the photosensitive material. The diffraction effect differs between the two, causing a difference in detection gain.

Also, since photosensitivity depends on the intensity of the light and the irradiation time, the relative linear velocity of the recording light beam and the disk differs between the inner and outer circumferences of the disk, and therefore the ratio (irradiation time/area) differs, so the disk radius If the so-called radius compensator, which allows the beam power to change uniformly in response to the beam power, is not activated, uneven grooves will occur.

Furthermore, when the beam power of the reproducing device changes with temperature and power supply voltage, dirt also becomes a factor in the fluctuation of the detection gain.

The permissible one-shift tracking deviation of the read spot in the disk radial direction by the tracking servo circuit is defined as ±0.1 μm, which is an error in which crosstalk from trunks adjacent to the information trunk to be scanned is suppressed within the permissible value. Therefore, the necessary loop gain is determined from the variation values of eccentricity and trunk distortion during disk production.

The loop gain determined by this becomes a considerable value, and unless the characteristics of the servo element have perfect characteristics with respect to secondary resonance and unnecessary oscillation motion, a sufficient gain margin may not be obtained.

However, among the loop gains, the tracking error detection gain, which cannot be determined at the time of production, has considerable variation, and in the past, the above-mentioned slight gain margin could not cover it and oscillation could occur.

As a countermeasure against such instability, the present invention provides a gain adjustment unit between the tracking detection signal and the servo drive amplifier to adjust the loop gain, and enables stable servo operation with the loop gain initially determined by the circuit and servo element. An embodiment thereof will be described below with reference to the drawings.

FIG. 1 is a block system diagram of one embodiment of the circuit of the present invention, and FIG.
The figure shows an example of the relationship between bits on the disk surface and read spots.

In FIG. 1, Dl. D2 are detection photodiodes, respectively, KT and KT in FIG.

It receives the reflected light (or transmitted light) of the tracking beam indicated by T and /, photoelectrically converts it, and outputs a signal corresponding to its light intensity.

Here, to explain the beam irradiated onto the disk surface, the laser beam emitted from the laser tube is converted into 0th-order and ±1st-order diffracted light by a diffraction grating, and these three diffracted lights are separated by a magnifying lens. The three imaged spots are reduced by a reading lens and reimaged on the disk surface to form three spots as shown by T, . . . T'S in FIG.

The beam S, which has the strongest optical intensity due to the O-order diffracted light, moves on the trunk (the track pitch is indicated by P in Figure 2),
It reads the phase information formed by bits in the trunk direction and reproduces the image signal.

Also, spots l-T,,T,' formed by ±1st-order diffracted light
are placed before and after the spot S so as to scan the right and left sides of the track, respectively, and overlap the track by about 25 lines.

Typically, the spot diameter of,T,,T,′,S,is about 1 μm, and the trunk pinch is,1.6 μm.

Further, the interval between the three spots is about 15 μm.

The signal reading spot S reads the high frequency phase structure formed in the trunk scanning direction, while the tracking spots T, , T,' read changes in the phase grating in the radial direction orthogonal to the trunk scanning direction.

If the tracking spots T, , T, ' are between the focus and the bit, it is a complete violation and the reflection intensity is maximum.

Furthermore, when T,,T,' scans directly above the line of bits, the average intensity of the reflected light ideally decreases by half to 50%.

When T, , T, ' scans these intermediate parts, the average intensity of the reflected light changes between 100% and 50% depending on the position.
It shows a gentle change between %.

When the tracking spots T,,T,' sandwich the trunk from both sides as shown in the second figure, the phase of the change in light intensity with respect to the positional shift of T1°T,/ becomes an opposite phase.

Therefore, the differential output obtained by applying the output electric signals of the photodiodes D, D2 to the differential amplifiers shown at 1 in FIG. 1, respectively, is as shown in FIG. The tracking error voltage e has a different polarity depending on the deviation X.

The width of the deviation from the center of the trunk for each truck has the same shape, with opposite slopes between the tracks.

The tracking error detection gain is determined by the slope of the curve shown in the third diagram.

This value is determined by the width of the bit x, the shape of the focus, the light intensity distribution of the spot, etc.

As described above, this gain is likely to vary and may go up or down from the set value at the time of production.

Now, the output dragging error voltage of the differential amplifier 1 is amplified by the amplifier 3 via the variable gain section 2 (described later in FIG. 1), and then supplied to the drive amplifier 4, where the characteristics of the loop are stabilized. Phase adjustment is performed for this purpose.

This phase adjustment is performed using a leading phase circuit or a circuit consisting of a proportional amplifier and a differential amplifier, but for convenience of explanation,
The illustration of these components included in the block of the drive amplifier 4 is omitted.

The output of the drive amplifier 4 drives a galvanometer mirror (galvanometer with a built-in rotatable mirror) 5, which is an example of a servo element, to give an angle to the beam and change the positions of the three spots in the disk radial direction. S always tracks and scans the center of the trunk.

Here, the galvano mirror 5 is driven by current control based on the fact that the electromagnetic force is generated by a current, and the phase delay due to the inductance of the galvano mirror 5 is also compensated for at the same time.

The mirror/L/vano mirror 5 has a response characteristic of a secondary system having resonance due to mirror inertia and a holding spring, and the amplitude of the resonance is defined by a damping constant.

The one with good characteristics has a frequency characteristic of the rotation angle amplitude with respect to the input current of 12 dB/in the high frequency region.

. 1, but some structurally exhibit additional resonance.

This is especially true in cases where the mirror is supported on a cantilever, and less so on structures where the mirror is supported at both ends.

When it is supported at both ends, the rotation axis is fixed and it is forced to perform only rotational movement, whereas in the case of cantilever support, the rotational axis is fixed and the rotational axis is fixed due to the imbalance of the dynamic balance of the mirror. This is because a slight oscillating motion occurs, and a so-called secondary resonance occurs in a high frequency region.

Examining the open loop transfer characteristics of the tracking servo circuit shown in Figure 1, we find that as shown in Figure 4, the transfer characteristics of the galvanometer mirror 5 and the phase correction provided to improve the phase margin near where the loop gain crosses OdB. This is in addition to the increase in high-frequency gain caused by the circuit, which is OdB/ at low frequencies.

ct and 12 dB/ as the frequency increases.

. tl and 6dB/. . 1.Finally 12 dB/
.

. The slope of t is shown. In FIG. 4, the gain above OdB is the degree of improvement.

Normally, in terms of stability, the phase margin of a servo circuit is determined by how far the phase lag at the OdB intersection is from 1800, and how negative the gain is when the phase lag reaches 1800. Considering the gain margin, it is said that the values are preferably 400 to 600 degrees and 10 dB to 20 dB, respectively.

However, in this servo loop, it is difficult to create a configuration with such a margin unless servo elements and amplifiers with fairly good characteristics are used.

In particular, when secondary resonance is included, the loop gain impulse is limited to 1 in order to check the phase margin at its peak point.

Therefore, if sufficient phase margin and gain margin are provided, the necessary loop gain will not be obtained, and the tracking range will become narrow.

Therefore, it is often used with less gain margin.

If the loop gain changes in the detection system under such conditions, oscillation is likely to occur.

In the well-known tracking servo circuit as described above, the present invention provides a part (the first
In the figure, a variable gain section 2, a band amplifier 6, a rectifier circuit 7, and an oscillator 8) are installed to eliminate oscillations that may occur due to the combination of a disc and a reproducing device.

Therefore, a so-called automatic gain control (AGC) operation is performed to keep the tracking detection signal at a constant level.If the servo is not locked in, this detection signal will clearly reach its maximum level, but if the servo is not locked in, the maximum level will clearly appear. In this case, there is only a residual error component and there is no reference such as the level of the burst signal, synchronization signal, or biloft signal used in the field of normal signals.

Therefore, it is not possible to use a system in which the biloft signal is used for tracking control and AGC is applied to the level according to the linear velocity with respect to the disk.

Therefore, in order to find out the gain of the detection system, the circuit of the present invention uses a single reading beam wobbling method, which has been conventionally proposed as a tracking servo, in addition to the trunking method using three spots. The spot is configured to oscillate (meander) on the trunk at a certain frequency, and the gain is determined from the frequency component of this oscillation in the instantaneous tracking error signal.

That is, when locking in the servo circuit, an oscillation output of several Hz is applied from the oscillator 8 to the drive amplifier 4, and the galvanomirror 5 is displaced to produce beam spots T,, T,',
You can swing the S freely from side to side from the center of the track.

The value is 1.6 μm in track pitch and 1 in spot diameter.
Since it is on the order of μm, it is a minute range of about 0.1 μm or less.

As a result, a waviness detection signal having the same frequency as this oscillation frequency appears in the trunking error voltage.

Therefore, in this embodiment, the frequency component due to beam fluctuation is separated from the output signal of the amplifier 3 by the band amplifier 6 as a P wave,
The rectifier circuit 7 converts it into a DC component, detects the level, and feeds this back to the variable gain section 2 negatively.

If the detection gain increases due to the mutual relationship between the spot and the bit shape, the detection level of the beam swing frequency passing through the band amplifier 6 will increase, and the gain will be lowered in the gain variable section 2 to improve the overall tracking servo loop. Keep the payoff constant.

The bandpass amplifier 6 of this embodiment can be easily configured with an active filter using an operational amplifier, and the variable gain section 2 uses a junction field effect transistor to change its drain-source resistance by controlling its gate voltage. and the gain can be varied.

The configuration for knowing the normal gain value is as above,
The phenomenon of beam swing (meandering) also occurs in the phenomenon of servo loop oscillation.

The servo element that includes the above-mentioned secondary resonance has a high gain in this respect, so it is easy to oscillate, and in a normal servo element, oscillation may occur even if the gain configuration has a margin.

Although this is not originally a desirable phenomenon, since the disc playback signal is an FM wave, the level fluctuation due to beam fluctuation can be removed by a limiter, which may adversely affect the color banding of the color signal or interfere with the detection of the boost-out compensator. As long as it does not work, it is tolerable for the time being, and in this case, beam fluctuation occurs at the secondary resonance frequency even without the provision of an oscillator, and this can be used to adjust the gain.

By providing a configuration for monitoring the gain of the tracking detection section and controlling it to a constant value as described above, the gain of all loops becomes a constant value, thereby increasing the stability of the control system.

In addition, in a servo system where the gain margin is reduced due to secondary resonance, the tracking detection gain becomes higher than the set value, resulting in even sharper oscillation and producing a sharper AM component in the reproduced FM signal level. It is possible to suppress the effect of color banding on the color process system and the adverse effects of color banding on the color process system.

Incidentally, the beam may be oscillated for a certain period of time only at the start of playback of the disk each time the disk to be played back is changed, and thereafter the driving no-voking 1 novo circuit may be operated at the set value.

Further, the present invention is not limited to an optical reproducing device, but can also be applied to a device that uses a scanning stylus to reproduce recorded information signals from a disk in which a stylus guide groove is not formed.

As described above, the loop gain adjustment circuit of the servo circuit according to the present invention is based on the above-described structure of the scanner which relatively scans the recording medium on which the information signal is recorded by forming a trunk by changing the geometrical shape. An error signal corresponding to the positional deviation with respect to the track is generated, and this error signal is sent to the drive unit to displace the scanner in the trunk width direction so that the positional deviation is minimized by the lever drive unit.・A circuit for adjusting the loop gain of a tracking servo circuit that operates, the means for slightly oscillating the scanning element at a constant frequency in the width direction of the trunk, and the above-mentioned error signal containing a constant frequency component caused by this oscillation. a variable gain section that variably controls the level of the signal according to a gain signal and supplies it to the drive section;
The gain of the variable gain section is adjusted so that the frequency component in the output signal of the variable gain section is at a substantially constant level based on the frequency component generated by the waveforming filter circuit and the oscillation caused by the filter circuit. Since the loop gain of the servo circuit can be kept at a constant value because it is configured with a variable control means, the servo circuit can always operate stably even if the standardization of the recording medium is insufficient. Even if a servo element with imperfect performance such as next-order resonance is used, the servo element can be used while maintaining a constant value without increasing phenomena that are inconvenient to signal reproduction. The strictness of the specifications of the reading parts can be relaxed to some extent, and furthermore, the servo IL-!, which has no margin due to secondary resonance! In the J path, the loop gain becomes higher than the set value, resulting in thicker oscillations, producing AM components in the reproduced FM (g and bell), and the effect of the bulge-out compensator and color banding in the color process system. However, it has many advantages such as being able to suppress these adverse effects.

[Brief explanation of drawings]

FIG. 1 is a block system diagram of one embodiment of the circuit of the present invention, and FIG.
The figure shows an example of the relationship between the read spot and the bit on the disk "l1llj," and Figure 3 shows the relationship between the trunking error voltage and the spot when the male cross-resistance spot shown in Figure 2 is used. FIG. 4 is a diagram showing the open loop amplitude transfer percentage shown in FIG. 1. 1. Differential amplifier, 2. Gain variable section, 4.・Drive tank @ device, 5... Galvano mirror 6... Bandwidth amplifier, 7
... Rectifier circuit, 8... Oscillator.

Claims (1)

[Claims] 1 Generates an error signal corresponding to the positional deviation of the scanner relative to the trunk, which scans the recording medium in which the information signal is recorded forming a trunk due to a change in geometric shape, and A circuit that adjusts the loop gain of a tracking servo circuit that supplies a signal to a drive unit and operates to displace the scanner in the trunk width direction so that the positional deviation is minimized by the drive unit, ,
A means for slightly oscillating at a constant frequency in the width direction of the Jl-rank, and a variable device Th] for adjusting the level of the error signal containing a constant frequency component due to the oscillation according to the training signal.
a variable gain unit that supplies the drive unit; a filter circuit that converts the frequency component generated by the vibration into a ρ wave from the output signal of the variable gain unit; and a filter circuit that converts the frequency component generated by the vibration into one wave by the filter circuit; Based on this, the gain of the variable gain section is varied so that the frequency component in the output signal of the variable gain section is at a substantially constant level. 1. A loop gain adjustment circuit for a servo circuit, comprising means for adjusting the IJ quotient.