JPH0612594B2 - Servo device for information detection point position control - Google Patents

Description

The present invention relates to tracking servo and tangential servo devices for controlling the position of an information detection point in a recorded information reading device.

In the recorded information reading device, a so-called tracking servo system is provided for always accurately tracking the information detection point on the recording track, and a so-called tangential for correcting the time base fluctuation of the reproduction signal. Servo system is provided. FIG. 1 is a schematic block diagram of a tracking servo device in an optical VDP (video disc player).
The two spot lights 2 to 4 are emitted with the positional relationship shown. That is, when the information detecting spot light 2 is on the track 1, the other two spot lights 3 and 4 are located on both side edges of the track 1. Therefore, when the detection spot light 2 is deviated in the track orthogonal direction (radial direction of the video disk), the light amount difference between the two spot lights 3 and 4 corresponds to the deviation direction and the size thereof. When the signal outputs of 6 and 6 are amplified by the differential amplifier 7, a tracking servo error signal is obtained.
This is applied to the drive amplifier 10 via the equalizer 8 and the loop switch 9. The output of the drive amplifier 10 causes a current to flow in the drive coil 12 that controls the rotation of the tracking mirror 11. The rotation of the mirror 11 is controlled according to the current supplied to the coil 12, and the spot light 2 for information detection is displaced in the radial direction of the track 1. Incidentally, 13 is a light receiving element for receiving the reflected light amount of the spot light 2, and the output of this element 13 becomes a reproduction RF signal. Further, a switch controller 14 is provided for on / off control of the switch 9, and the switch 9 is turned off as required to disconnect the servo loop.
This is used to break the loop at the time of a track jump operation in the special reproduction mode. The light receiving elements 5 and 6 and the differential amplifier 7 constitute error signal generating means.

Here, the tracking mirror 11 is generally supported by an elastic body made of rubber or the like at its pivot neutral point, and the mirror 11 is pivotally controlled in response to a signal applied to the coil 12 against the restoring force of the elastic body. Thus, the spot light 2 is always set to the target value (on the center of the track 1). The frequency-displacement amount (amplitude) characteristic of the transducer (which receives the servo signal and converts it into the positional displacement of the information detection point) including the coil and the mirror is a secondary characteristic
The changes shown in FIG. 2 are shown.

That is, the device shown in FIG. 1 has a drawback that an abnormal operation occurs because it has a resonance characteristic at a frequency of ₀ , and the characteristics of the entire transducer change due to changes in temperature and physical properties of the elastic body for supporting the mirror 11. It has the drawback of becoming unstable.

Such a phenomenon is common not only to the tracking servo system but also to the tangential servo system.

Therefore, an object of the present invention is to provide a servo device for controlling the position of an information detection point, which enables stable servo operation.

The servo apparatus according to the present invention is a servo apparatus that controls the information detection point to a target position by a transducer that makes a positional displacement of the information detection point according to a servo signal, and determines the amount of deviation of the information detection point from the target position. Error signal generating means for generating an error signal having a corresponding level, and compensation signal generation for generating a compensation signal containing a frequency component of a level corresponding to the level of the frequency component of the band of the error signal near the resonance frequency of the transducer. Means for subtracting the compensation signal from the error signal to compensate the frequency characteristic, and the error signal having the frequency characteristic compensated by the subtracting means is used as the servo signal.

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

FIG. 3 is a circuit block diagram of an embodiment of the present invention.
The same parts as those in the figure are designated by the same reference numerals, and the description thereof will be omitted. In this example, the spot light 2 which is the information detection point
The characteristic of the transducer is compensated by using the output of the light receiving element 13 which receives the disk reflected light. The light receiving element 13 is generally a so-called four-division type photodetector whose light receiving surface is divided into four by two orthogonal lines as shown in the figure, and a light receiving portion which opposes with a dividing line parallel to the tangential direction of the track 1 narrowed. The respective output sums of 13a, 13b and 13c, 13d are obtained by the adders 15, 16, and the respective addition outputs are applied to the differential amplifier 17. The difference between the two addition outputs is arithmetically amplified by the differential amplifier 17 and becomes the negative input of the subtracter 19 via the equalizer 18. A servo signal is applied to the positive input of the subtractor 19. The transducer formed by the mirror 11 and the coil 12 is driven by the subtraction output of the subtractor 19. In addition, in the adder 20, all outputs of the four-division type photodetector 13 are added to form a reproduction RF signal.

Next, the output signal of the differential amplifier 17 will be described in detail.

The light receiving elements 5 and 6 and the light receiving element 13 normally receive the return light from the disk at the center position of each light receiving element when the transducer is at the neutral position as shown in FIGS. 4 and 5. It is arranged to do. In the tracking control by the three-beam method, as shown in FIG. 4, the reflected light from the disc of the spot light 3 and 4 is received by the light receiving elements 5 and 6, and the difference in the output is obtained from the differential amplifier 7. This is a tracking error. Then, the transducer is driven based on this error signal, and control is performed so that the spot light 2 always traces on the track 1.

The transducer is controlled within the range of the deflection angle of the tracking mirror or the viewing angle of the objective lens,
Since each light receiving element is fixed, the transducer is controlled (that is, the tracking mirror 11 is displaced from the neutral position) so that the center of the light receiving element corresponding to the optical axis of each reflected light is as shown in FIG. It will be displaced (hereinafter referred to as lens offset).

Therefore, the light receiving area of the light receiving elements 5 and 6 is usually set to be sufficiently large with respect to the spot diameter of the reflected light of the spot light 3 and 4 from the disk. By doing so, even if the tracking mirror 11 is deviated from the neutral position due to a tracking control or a disturbance causes a lens offset and the optical axis of the reflected light is slightly deviated from the centers of the light receiving elements 5 and 6, the tracking control is performed. Error signal (that is, the output of the differential amplifier 7) is 0, and the spot light 2 for signal reading traces on the track 1.
Since the RF signal is obtained by the total sum of the four-division light receiving element 13, it can be detected even if the beam spot of the reflected light deviates from the center of the light receiving element.

Here, considering a push-pull error due to the beam spot on the light receiving element 13, the tracking servo is locked in a state in which the tracking mirror 11 is deviated from the rotation neutral point. Therefore, as shown in FIG. The optical axis of the reflected light of the spot 2 and the center position of the light receiving element 13 are displaced to the left and right with respect to the direction of the track 1 similarly to the reflected light of the spot lights 3 and 4, and correspond to this displacement. An error signal (push-pull error) will be obtained.

Since the tracking servo by the three-beam method is locked, a push-pull error, that is, a tracking error component is not included in the output of the differential amplifier 17, and a signal proportional to the amount of deviation from the rotation neutral point of the tracking mirror 11 is generated. Will be obtained.

As described above, even when the error signal output from the differential amplifier 7 is 0 (that is, the tracking servo is locked), the light receiving amount on the light receiving element 13 may be unbalanced, Therefore, it can be seen that a signal proportional to the displacement amount of the transducer in the direction orthogonal to the track is obtained at the output of the differential amplifier 17.

Since this signal is a signal indicating the displacement versus frequency of the transducer, this signal characteristic also agrees with the characteristic shown in FIG. Since the signal having such characteristics is fed back to the servo system via the equalizer 18 as described below, the characteristics of the transducer to which this feedback is applied are as shown in FIG. Up to the frequency of _0, the resonance characteristics at ₀ are compensated by being substantially flat.

As a characteristic of the equalizer 18, for example, a BPF centered around ₀
With the (band-pass filter) characteristic, it is possible to compensate only in the vicinity of the resonance frequency _{0 of} the transducer. In this way, the signal of the level corresponding to the displacement amount from the neutral point of the movable body in the transducer is transmitted through the equalizer 18 to the level corresponding to the level of the frequency component of the band near the resonance frequency of the transducer of the tracking error difference signal. Is obtained by the compensation signal generating means composed of the light receiving element 13, the differential amplifier 17, and the equalizer 18, and the compensation signal is subtracted from the tracking error signal by the subtractor 19 which is the subtracting means to obtain the frequency of the error signal. The characteristics are compensated.

Although it is similarly applied to the tangential servo system, in this case, the light receiving portions 13 facing each other with a dividing line parallel to the track orthogonal direction of the four-division type photodetector 13 narrowed.
It suffices to obtain the respective output sums of a, 13c and 13b, 13d and feed back the difference between the two output sums.

FIG. 7 is a circuit block diagram of another embodiment of the present invention,
The same parts as those in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted. This example is made in view of the fact that the mechanical impedance of the transducer (impedance in the vicinity of the resonance frequency) corresponds to the amount of displacement of the transducer from the neutral point of the movable body. And resistors 21 to 23 form an impedance bridge circuit. Coil 12 and resistance
The potential difference υ between the series connection point with 21 and the series connection point with the resistors 22 and 23 is derived in the differential amplifier 17, and this difference output υ
Through the equalizer 18 and the subtractor 19
The negative input of is fed back to the servo system.

The equivalent circuit of the transducer is shown in FIG. 8 (A). E is an input signal voltage, F is a force, I is a current, and V is a velocity, which are vector quantities. If the transducer is a so-called voice coil type of speaker, in the magnetic flux density B, the force F acting on the coil when the current I flows through the coil having an effective length l is F = BlI (1) Is represented. Now, assuming that Bl = A (conversion coefficient from electric system to mechanical system), the equation (1) becomes F = AI ... (2). Further, from the equivalent circuit of FIG. 8 (A), both equations F = ZmV ... (3) E = ZeI + AV ... (4) are obtained. Here, Zm is the mechanical impedance of the transducer, and Ze is the impedance measured with the vibrating part fixed. Since AI = ZmV ... (5) is obtained from the equations (2) and (3), if this is substituted into the equation (4), E = (Ze + A ² /Zm)I...(6), and FIG. It can be transformed into the equivalent circuit shown in (B). Mechanical impedance Zm at this time of the transducer is represented by _{Zm = r m + j (ωm} -1 / ωc) ... (7). Here, r _m is equivalent mechanical resistance, m is equivalent mass, and c is equivalent compliance.

Since the mechanical dynamic impedance Z _M of the transducer is expressed by Z _M = A ² / Zm (8), when Z m has the minimum value, Z _M
Is the maximum, and from equation (7) In consisting angular frequency Zm takes a maximum value which is A ² / _{r m.} That is, the mechanical dynamic impedance Z of the transducer
It can be seen that the frequency characteristic of _M is as shown in FIG.

Therefore, if the change in the mechanical dynamic impedance Z _M of the transducer is detected and fed back to the servo system, the characteristic shown in FIG. 2 is improved and the characteristic shown in FIG. 6 can be obtained. Therefore, an impedance bridge circuit is provided, and the mechanical dynamic impedance Z _M can be detected by detecting the voltage change generated at both ends of the coil 12. The equalizer 18 is centered at ₀ (ω ₀ / 2π)
The object of the present invention can be achieved by using the BPF structure and feeding back only the characteristics of the region shown by the dotted line in FIG. 9 to the servo system.

FIG. 10 is a circuit block diagram of another embodiment of the present invention,
The same parts as those in FIG. 3 are designated by the same reference numerals, and the description thereof will be omitted. In this example, the signal indicating the displacement amount from the neutral point of the movable body in the transducer, that is, the output signal of the differential amplifier 17 is differentiated by the differentiating circuit 24, and the differentiated output is fed back to the servo system as a compensation signal. Is. Therefore, the differentiating circuit 24 functions as the equalizer 18 in FIGS. 3 and 7.

Here, if the transfer function of the transducer is A (s) and the feedback amount of the feedback system by the differential output is Ks, The following formula is obtained. Now, A (s) is expressed by the following equation.

Here, ω _n is a natural angular frequency, and ξ is a damping coefficient. Therefore, equation (10) becomes Therefore, the characteristics of the transducer including the feedback system can be obtained. That is, the constant K of the feedback section is introduced into the first-order term, and as a result, the Q (sharpness) of the transducer becomes controllable by the value of K, and the damping factor can be controlled and ₀ in FIG. It is possible to cancel the peak and obtain the characteristic shown in FIG.

It goes without saying that the examples shown in FIGS. 7 and 10 can also be applied to the tangential servo system. Further, the present invention is not limited to the optical information reading device, but can be applied to other types such as a capacitance type.

As described above, according to the present invention, an error signal having a level corresponding to the deviation amount of the information detection point defined by the transducer from the target position is generated, and the frequency component of the band of the error signal in the vicinity of the resonance frequency of the transducer is generated. Generates a compensation signal containing a frequency component of a level corresponding to the level of, the frequency characteristic is compensated by subtracting the compensation signal from the error signal, and the error signal whose frequency characteristic is compensated is used as an input servo signal to the transducer. Therefore, there is an advantage that the resonance characteristic of the transducer can be removed and stable characteristics can be obtained with a simple configuration.

[Brief description of drawings]

FIG. 1 is a block diagram of a conventional tracking servo system, FIG. 2 is a characteristic diagram of a transducer in tracking servo, FIG. 3 is a block diagram of an embodiment of the present invention, and FIGS. 4 and 5 are of FIG. Differential amplifier 1 in the apparatus of the embodiment
7 is a schematic diagram for explaining the output signal of FIG. 7, FIG. 6 is a characteristic diagram of the block of FIG. 3, FIG. 7 is a block diagram of another embodiment of the present invention, and FIG. 8 is an equivalent circuit diagram of the transducer. FIG. 9 is a characteristic diagram of the transducer, and FIG. 10 is a block diagram of another embodiment of the present invention. Description of main part code 2 …… Spot light for information detection 11 …… Tracking mirror 12 …… Mirror drive coil 13 …… Quadrant photodetector 18 …… Equalizer 24 …… Differentiation circuit

Claims (1)

[Claims] 1. A servo device for controlling the information detection point to a target position by a transducer that displaces the information detection point in accordance with a servo signal, the servo device depending on the deviation amount of the information detection point from the target position. Error signal generating means for generating an error signal having a level; and compensation signal generating means for generating a compensation signal including a frequency component of a level corresponding to a level of a frequency component of the error signal in a band near the resonance frequency of the transducer. An information detection point, comprising: subtraction means for compensating the frequency characteristic by subtracting the compensation signal from the error signal, and the error signal having the frequency characteristic compensated by the subtraction means is used as the servo signal. Servo device for position control.