JPH07111819B2 - Servo device for rotating recording media - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a servo system for a rotary recording medium, and more particularly to a servo device suitable for an optical pickup for reading information from an optical disc.

SUMMARY OF THE INVENTION The present invention provides a feedback servo loop for a tracking servo device, a spindle servo device, a focus servo device, or the like, which is required when reading information recorded on a rotary recording medium such as an optical disk. A model of a function corresponding to at least an eccentricity error of the rotating recording medium is included in the above. Therefore, even when the number of rotations of the rotating recording medium changes, the servo device can save power, the steady-state deviation can be reduced, and the servo device can respond stably even to high speed rotation. become able to.

[Conventional technology]

As the rotary recording medium, a magnetic disk, an optical disk, or the like has been put into practical use. Particularly, in the case of an optical disk, the recording areal density is designed to be very high, and it is high as an optical pickup for reading such recorded information. A servo device having responsiveness and a small steady-state deviation is required.

Therefore, a conventional servo device for an optical disc is designed to obtain a high loop gain and stability, and for example, the response of an actuator (biaxial mechanism) for irradiating a recording track of the optical disc with a light beam is improved. In order to improve and to stabilize the secondary characteristic of the actuator due to phase rotation, a complex phase compensation circuit is used to expand the frequency at which the gain of the open loop transfer function becomes zero to a frequency as high as possible. By widening the servo band, the loop gain in the low frequency band is designed to be high as a result.

That is, as shown in FIG. 11, in the case of the tracking servo device of the pickup, the loop transfer gain G of the servo device is increased.
As shown by a curve B, the steady-state deviation is compressed by making it higher than the conventional characteristic A, and the responsiveness is improved by widening the servo band to a high range.

[Problems to be solved by the invention]

However, generally, as shown in the characteristic B, increasing the high-frequency limit of the servo band and increasing the overall loop gain become a factor that impairs stability. The compensating circuit becomes complicated to compensate, and signals with high frequency components that are unrelated to the servo signal leak into the servo device, causing unnecessary heat loss, causing temperature rise of the actuator and power loss of the power supply. There was a problem of doing.

That is, in the servo device constructed by such a conventional design method, the servo band extends to a high region, so that in the case of an optical disc, the RF signal component recorded during reproduction leaks into the servo band. At the same time, unwanted noise is injected into the actuator, increasing power consumption. Further, in order to improve the response of the servo device, it is required to widen the servo band and increase the gain to reduce the steady-state deviation. However, increasing the loop gain ensures stability. It is difficult to reduce the steady deviation as a result.

Therefore, the present applicant has previously proposed to provide a servo circuit for improving the loop gain with respect to the periodic tracking error signal or the focus error signal generated due to the eccentricity of the rotating recording medium (Japanese Patent Laid-Open No. Sho 61-96). 52-12080
No. 5 bulletin).

However, in the case of the above-mentioned servo device, there is a drawback that the effect cannot be exhibited when the rotation speed of the rotary recording medium changes.

In order to solve such a problem, the present invention uses a more realistic method for designing a servo device, and is capable of coping with power saving, high performance, and high speed rotation of the servo device. The present invention provides a servo device applied to.

[Means for solving problems]

The servo device of the present invention employs a servo circuit in which a model corresponding to the control target value is included in the feedback control circuit, and the loop loop gain characteristic of the feedback loop changes following the change of the control target value in the previous period. To configure.

[Action]

When the rotary recording medium is an optical disc and the servo device is a tracking actuator, most of the control target value signals are caused by the eccentricity of the optical disc. Therefore, since the control target value is expressed by a function of the rotation cycle of the optical disk, a control element having a high gain transfer characteristic that changes following the change of the rotation cycle function is installed in the servo loop circuit. Since the steady-state deviation can be compressed to a very small value by providing it, the frequency of the maximum gain of the control element changes corresponding to the periodic function of the target value even when the optical disc rotates at a high speed. The loop gain can always be increased with respect to the control target value. Further, since the servo band is set within a necessary area in accordance with the rotation cycle of the optical disc, the power consumption of the servo device can be reduced.

〔Example〕

As a condition for constructing a servo system capable of responding to a target value without a steady deviation, it is required that a model of a function of the target value is included in a loop of a control system.

"For example, The Internal model principle for linear
Multivaliable Regulators ・ Applied Mathematics & opt
imization ・ Vo12 ・ No2,1975, Spring-verlag 」By the way, in tracking servo when an optical disk is used as the recording medium, the main target value xreff for tracking the spirally formed recording track is the eccentricity of the optical disk. The main component of the above is occupying most, and further, a direct current component for sending the optical pickup in the radial direction of the optical disc is also included. Therefore, when the rotational angular velocity of the optical disc is ω, the target value xreff can be expressed as xreff = ASin ωt + Bt (1)

In the case of an ordinary optical disc player, the Bt component of equation (1) can be handled by the feed motor of the optical pickup, so xreff = ASin ωt (2) is the main target for the actuator. It can be a value.

FIG. 1 is a block diagram of a basic servo device of the present invention constructed on the basis of such conditions. Gc has transfer characteristics The transmission elements divided into the transmission elements described as “Go”, Go is a transmission element having a transmission characteristic capable of appropriately responding to a disturbance or the like as in a conventional servo circuit, and “Ga” is a transmission element of an actuator. The transfer characteristic of the actuator is
Since it is generally a secondary characteristic, Or Indicated by.

The transfer element Gc ₁ has a transfer characteristic such that the transfer element Gc ₁ has an infinite gain with respect to the signal of the rotational speed ωd that constitutes the fundamental wave component of the eccentricity of the optical disk. Ωd is a signal S that detects the number of rotations of the optical disc
It is configured such that it can be changed by the circuit means described later by (ω).

The Bode diagram is designed as shown in FIGS. 2 (a) and 2 (b).

Therefore, the transmission element follows the rotation speed N of the optical disk.
When ωd of Gc ₁ is changed, it has ideally infinite gain with respect to the basic eccentricity amount, and the steady-state deviation can be made zero.

The characteristic a ₂ S ² + a ₁ S + a ₀ of the transfer element Gc ₂ indicates the phase compensation characteristic of the actuator, which is considered to be a secondary characteristic, and its Bode diagram is shown in FIGS. 3 (a) and 3 (b). Thus, for example, it may be the reverse characteristic of an actuator.
{(Ωa) is the resonance frequency of the actuator} The characteristic of the transfer element Gc ₂ may be Gc ₂ = a ₁ S.

Therefore, the Bode diagrams of the combined characteristics of the transfer elements Gc ₁ and Gc ₂ are as shown in FIGS. 4 (a) and 4 (b).

In this case, considering the series of characteristics of each transmission element Gc ₁ · Gc ₂ · Ga, since the rapid phase around in terms of omega = .omega.d is inverted 180 °, the system becomes unstable.

Therefore, as shown in FIGS. 5 (a) and 5 (b), the transfer element Gc ₃ has a first-order advance characteristic. A phase compensator with is added to give a phase margin near ω = ωd as shown by the dotted line in Fig. 4, and Gc when ω = ωd
Securing the stability of the overall characteristics of ₁・ Gc ₂・ Gc ₃・ Ga.

It should be noted that K ₁ and K _{2 in} FIG.
o and the feedback amount by the transfer element Gc adopted in the present invention is set to a predetermined value.

The transfer element Gc ₁ detects the number of rotations of the optical disc, which is one of the rotary recording media, by FG or the like, and changes the resonance peak point ωd corresponding to the rotation frequency ω by the signal S (ω). In this case, it is preferable that the time constants b ₁ and b _{2 of} the transfer element Gc ₃ are also changed at the same time by following the signal S (ω).

As described above, the basic circuit of the servo device of the present invention has a transfer characteristic that responds to the control target value (xreff = ASinωt). Since the transfer element Gc ₁ having the above is included in the servo loop, ideally, the steady deviation can be set to 0 for the eccentric fundamental wave component of the rotating recording medium. Further, even when the speed of the rotary recording medium is increased, the peak point ωd of the transfer element Gc ₁ changes in accordance with the target value, that is, the eccentric fundamental wave component (ωd) of the rotary recording medium. G _O
It is not necessary to increase the gain and the servo band to constantly improve the responsiveness, and the power consumption does not increase.

That is, the loop loop characteristic of the present invention, as shown by the solid line in FIG. 6 (a), has a lower upper limit of the servo band with respect to the loop gain curve A of the conventional servo circuit, so that noise components mixed in in the high frequency range are present. On the other hand, power saving is achieved, and a sufficient loop gain can be given to the eccentric fundamental wave component ωd that occupies most of the target value, and the servo can be applied in a state where the steady deviation is small.

In addition, when the disc rotates at high speed, the transfer element Gc ₁
Since the peak point of moves as shown in FIG. 6 (b) and follows the eccentric fundamental wave component in the high frequency range, the loop gain becomes high,
It can also support high-speed rotation.

FIG. 7 shows a specific circuit example of the transfer element Gc of the present invention. The part A surrounded by the alternate long and short dash line shows the part of the transfer element Gc ₁ , and the transfer characteristic is shown. Form. Further, when the portion B formed by the operational amplifiers A ₅ and A ₆ is included, the composite characteristic of the transfer elements Gc ₁ and Gc ₂ is included. Is formed.

The C part is the transfer characteristic of the phase compensator. The circuit portion of the transfer element Gc ₃ that constitutes

The operational amplifiers A ₁ , A ₂ , A ₃ , and A ₄ that form the transfer element Gc ₁ form a state variable filter.
A ₁ is an adder, A ₂ and A ₃ are integrating circuits, and A ₄ is an amplifier circuit for setting the feedback amount. The feedback amount is set by this amplifier circuit, and ωd of the equation (3) is set. Can be set.

Therefore, the switch S ₁ is switched according to the number of revolutions of the optical disk, and the value of the resistance R ₁ is switched to change the resonance point by following the eccentric fundamental wave component of the optical disk. Loop gain can be given.

The operational amplifiers A ₅ and A ₆ form the frequency characteristic of FIG. 4 (a) by adding the voltages V ₁ ′ and V ₂ ″ of the respective parts of the transfer element Gc ₁ .

Therefore, the resistances r ₉ , r ₇ and r ₈ are adjusted and the coefficients a ₂ , a ₁ and a ₀ are set in relation to the mechanical response characteristics of the actuator.

Switches S ₂ and S _{3 of} transfer element Gc ₃ that form a phase-passing circuit
Similarly, in accordance with the number of rotations of the optical disk, switching is performed similarly to the switch S ₁ to stabilize the phase rotation around ωd.

It should be noted that switching the feedback amount (switch S ₁ ) and time constants b ₁ and b ₂
(Switches S ₂ and S ₃ ) can also be omitted by using a non-linear electronic variable resistor.

FIG. 8 shows another embodiment in which the characteristic switching portions of the transfer elements Gc ₁ and Gc ₂ are formed by a switched capacitor filter.

In the case of this embodiment, the switched capacitors S (r ₁ ), S (r ₂ ), S, which are constituted by the state variable filter as shown in FIG.
(R ₂ ) and S (R ₃ ).

That is, when the switch S constituting each switched capacitor is switched by the FG signal corresponding to the rotation frequency f of the optical disk, the values of the resistors r ₁ , r ₂ , R ₂ and R ₃ in FIG. 7 are changed. It can have the same effect. That is, Therefore, by setting (Kf) ² = ωd ² , a high loop gain can be formed in the stable region with respect to the error signal of the eccentric fundamental wave component of the rotation frequency f (ω) of the optical disc.

FIG. 9 is a block diagram of a servo system showing still another embodiment of the present invention, in which 1 is a high level of an adder 10 which forms a signal (error signal) of a control target value and an actual position value. Low-pass filter for removing the band signal component, 2 is a digital filter formed by A / D converter 2a, digital arithmetic circuit 2b, D / A converter 2c, 3 is a low-pass filter, 4 is a first coefficient unit, 5 is an adder, 6 is a transfer element indicating the transfer characteristic Ga of the actuator, 7 is a transfer element (Go) for applying servo to conventional disturbance noise, 8 is a gate circuit, and 9 is a second coefficient. It is a container (K ₂ ).

In this embodiment, the transmission element Gc in the servo system shown in FIG. 1 is constituted by a digital filter whose peak point frequency .omega.d is variable according to the rotation cycle of the optical disk.

The calculation speed of the digital calculation circuit 2b that constitutes the digital filter may be any as long as there is no time delay with respect to the eccentric frequency (several tens Hz), and a microcomputer or the like can be used as it is.

The gate circuit 8 is provided, for example, when the servo device is applied to a tracking actuator or the like, to prevent an accident that momentarily deviates from the servo pull-in range due to dropout or the like.

That is, in the conventional servo device, the loop gain characteristic is set so that the response becomes as high as possible in the high band as described above. Therefore, if there is a dropout on the recording surface of the optical disc, the capture range of the servo is reduced. However, since the servo device of the present invention has a high gain due to the eccentric fundamental wave component, it has a low responsiveness to such a dropout. Therefore, if the gate circuit 8 is configured to open when there is a dropout, the dropout deactivates the actuator and allows it to pass through the dropout portion according to the inertia of the transfer element Gc of the present invention. Therefore, useless movement of the actuator can be suppressed. Needless to say, the gate circuit 8 can be applied to the basic servo circuit shown in FIG.

FIG. 10 shows a concrete example of a transmission element showing still another embodiment of the present invention. The characteristic of this embodiment is the transmission characteristic. The eccentric fundamental wave component ωd is automatically changed in accordance with the periodic function of the optical disc due to the non-linearity of the circuit.

According to the analysis of the nonlinear resonance element recorded in "Nonlinear Vibration Theory P96: Yoshikazu Sasaki, published by Kyoritsu Shuppansha", the vibration is such that the restoring force is symmetric with respect to the origin and has an odd-order nonlinear characteristic. A forced vibration system when an external force of V = P ₀ Sin ωt is applied to the system is expressed by a motion equation of m + c + kx + βx ³ = P ₀ · Sin ωt.

Here, m is the mass of the actuator, c is the viscous drag, k is the spring constant, and are the second and first derivatives of the displacement x.

Solving such an equation of motion, the resonance frequency f _{0 of the} vibration system
Has a characteristic known as a non-linear synchronization source by being drawn into the vibration frequency f applied from the outside and moving the resonance peak point.

Figure 10 (a) shows the transfer characteristics. FIG. 7 shows an example of a circuit having the above-mentioned non-linear synchronizing action, and the same portions as those in FIG. 7 described above are designated by the same reference numerals.

As shown in FIG. 10 (b), the feedback system based on the non-linear circuit NA composed of the operational amplifiers A ₇ and A ₈ and the diodes D ₁ and D ₂ has the third-order transfer characteristic of the input signal V ₁ and the output voltage V _0. It has a non-linear characteristic of.

Therefore, when the signal V ₁ , that is, the eccentric fundamental wave frequency ω of the optical disk changes, the resonance frequency ωd showing the transfer characteristic of this circuit also follows and changes so that ω = ωd, as shown in FIG. 10 (c). In this way, the eccentric fundamental wave ω is drawn, and ωd also follows and changes within a certain range. (Such a phenomenon is generally known as a "circle phenomenon".) The following range becomes wider as the level of the signal V applied from the outside increases, and changes by about 5 times ω. Following this, ωd of the transfer characteristic Gc also changes.

Therefore, when a servo device is formed by the transmission elements of the circuit shown in this embodiment, a loop loop corresponding to the eccentric fundamental wave component of the optical disc is detected without detecting the rotation speed of the optical disc by, for example, FG (frequency generator). It is possible to construct a servo device having a high gain and to make the steady deviation small.

Further, according to this embodiment, there is an advantage that the phase compensator (Gc ₃ ) for compensating for the phase rotation around the resonance point described above is unnecessary.

A nonlinear circuit NA is added to the filter forming the transfer element Gc ₁ to improve the transfer characteristic. The operational amplifiers A ₁ , A ₂ , A ₃ , A
_{Since 7} and A ₈ originally show non-linear characteristics with respect to a large input level, and a non-linear synchronization phenomenon can be obtained by giving a target value signal xreff of sufficient level, in this case, the non-linear circuit NA is omitted. You can also

In addition, the transfer characteristic of the actuator is 0 type, that is, I mentioned the transfer characteristics The target value is xreff = ASi for the 1-type actuator as well.
By setting nωt + C, it is possible to directly apply the transfer element of each of the above-described embodiments.

In this case, even when the feed motor is small and has a DC offset characteristic, the actuator responds to the DC component with an infinite gain (S → 0, G = ∞), and thus power saving is achieved. Even more effective if.

〔The invention's effect〕

As described above, when the servo device of the present invention is applied to a rotating recording medium, it can give an extremely high loop gain to a control target value corresponding to the number of rotations, so that the steady deviation is extremely low. can do. Further, since the gain curve of the open-loop transfer characteristic of the servo device changes so as to follow the rotation cycle of the rotary recording medium, it is possible to respond stably even at high speed rotation, and at the same time, the rotary recording medium. A signal obtained by connecting in parallel a conventional servo circuit that responds to externally applied shock noise other than the eccentricity component added to, and combining the output signal of this circuit system and the eccentricity component output signal at a predetermined ratio. Since the actuator etc. are controlled by, the effect that the transition to the servo state becomes faster even when the rotating recording medium is started, and even if the servo is disengaged due to disturbance, it is easy to return to the servo lock state. You can

Further, since this conventional servo circuit does not require high gain and high bandwidth, it can be a power-saving servo circuit.

[Brief description of drawings]

FIG. 1 is a basic block diagram of the servo system of the present invention, FIGS. 2 (a) and 2 (b), FIGS. 3 (a) and 3 (b),
4 (a) and 4 (b) respectively show transfer elements Gc ₁ , Gc ₂ , and
Bode diagram showing transfer characteristics of Gc ₁ + G ₂ , FIG. 5 (a),
6B is a Bode diagram showing a transfer characteristic for phase compensation, FIG.
FIGS. 7A and 7B are explanatory diagrams showing the integrated loop gain characteristic of the present invention, FIG. 7 is a circuit diagram showing an embodiment of a feedback transmission element that can be used in the servo device of the present invention, and FIG. 8 is the same. A circuit diagram showing another embodiment of the feedback transfer element, FIG. 9 is a block diagram when a digital filter is used as the transfer element, and FIGS. 10 (a), (b), and (c) use a non-linear resonance circuit. A circuit diagram of the transfer element, a graph showing its characteristics and a resonance frequency, and FIG. 11 are explanatory diagrams of loop gain characteristics in the servo device. In the figure, Gc ₁ is a transfer element that can increase the gain with respect to the eccentric fundamental wave component of the rotating recording medium, Gc ₂ is a transfer element that compensates for the transfer characteristics of the actuator, and Gc ₃ is a transfer element for phase compensation. , Go is a transfer element that mainly responds to disturbances, and Ga is a transfer element that shows the transfer characteristics of the actuator.

Claims (1)

[Claims] 1. A servo system loop for a rotary recording medium, comprising: a transfer characteristic whose peak value changes in response to a change in an eccentric fundamental frequency of the rotary recording medium.
And a second transmission circuit having a gain characteristic that responds to disturbance noise added to the rotating recording medium are connected in parallel, and the outputs of the first and second transmission circuits are provided at a predetermined ratio. A servo device for a rotary recording medium, characterized in that the servo device is composed and supplied to a device to be controlled.