Servo control apparatus

A servo control apparatus for performing a feedback control to an analog controlled system, includes an error generating device that generates a digital error corresponding to a difference between a desired value and a feedback value corresponding to a controlled variable of the analog controlled system, a control device that generates a digital manipulated variable, and a digital-to-analog converting device that converts the digital manipulated variable into an analog manipulated variable for driving the analog controlled system. The control device includes a disturbance estimating device that estimates a disturbance applied to the analog controlled system by carrying out a digital estimating process by using the digital manipulated variable and the digital error and generates a digital compensated variable corresponding to the estimated disturbance, and a manipulated variable generating device that generates the digital manipulated variable by using the digital error and the digital compensated variable.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a servo control apparatus which controls a
 controlled system by a feedback control, and more particularly, to a servo
 control apparatus which estimates a disturbance applied to the controlled
 system, and carries out a feedback control while compensating an actual
 disturbance on the basis of the estimated disturbance.
 2. Description of the Related Art
 In recent years, the following research and development have been actively
 made. When controlling a controlled system by a feedback control, a
 disturbance applied to the controlled system is estimated, and a
 manipulated variable considering the estimated disturbance is applied to
 the controlled system, and thereby, a feedback control is carried out
 while compensating an actual disturbance on the basis of the estimated
 disturbance.
 Thus, a state observer has recently attracted interest as a preferable
 device for estimating the disturbance.
 The principle of the state observer will be described below.
 The observer is a device for estimating a state incapable of being actually
 detected, from a detectable state. The state incapable of being actually
 detected is a state where a disturbance is actually applied to the
 controlled system, for example. The observer estimates a disturbance
 applied to the controlled system, and then, computes a compensated
 variable capable of canceling the actual disturbance, and further, adds
 the compensated variable to a feedback control object so as to compensate
 the actual disturbance.
 Next, a disturbance estimating process by the observer will be described
 below with reference to FIG. 13. FIG. 13 shows the case of applying the
 observer with respect to a focus servo control object which controls a
 focus actuator included in an optical disk reproducing apparatus such as a
 CD (Compact Disk) player as a controlled system. In particular, FIG. 13
 shows a feedback servo loop formed in the focus servo control system.
 The focus actuator (hereinafter, referred simply to as actuator) is an
 actuator for moving an objective lens in the direction perpendicular to an
 information recording surface of an optical disk, to focus a light beam on
 the information recording surface of the optical disk.
 In FIG. 13, a controlled object U(s) is set as an actuator, and a
 controlled variable y is set as a position of the actuator in the
 direction perpendicular to the optical disk.
 Now, a transfer function of the actuator is expressed as a second-order lag
 control system as follows.
EQU U(s)=A.times.wa.sup.2 /(S.sup.2 +2.times.ka.times.wa.times.s+wa.sup.2) (1)
 where, A is a gain (m/Ampere) of actuator, ka is a viscous braking
 coefficient of the actuator, and wa is a natural vibration frequency
 (rad/sec) of the actuator.
 Next, supposing a conversion sensitivity for a focus error signal output in
 the actuator as a positional detection sensitivity Ke (Volt/m), the
 following equation (2) is formed. Incidentally, the conversion sensitivity
 is determined by a sensitivity of photo-detector and an amplification
 factor of an error generating amplifier included in the optical disk
 reproducing apparatus.
EQU REF-y.times.Ke=er (2)
 where, REF is a desired value on which an actuator should be positioned, er
 is an error in the aforementioned feedback control system. As shown in
 FIG. 13, the error er obtained by the above equation (2) is supplied to
 one input terminal of an observer.
 In FIG. 13, a relationship between a manipulated variable (voltage value) u
 and a drive current i for driving the actuator is expressed as follows.
EQU i=Kdr.times.u (3)
 where, Kdr (Ampere/Volt) is a voltage/current conversion sensitivity of a
 driver. The driver is controlled by the manipulated variable u, thereby
 generating the drive current i. The drive current i is converted into a
 drive voltage v by a current/voltage converter having a current/voltage
 conversion sensitivity Kiv (Volt/Ampere) as shown in the following
 equation (4), and then, is supplied to the other input terminal of the
 observer.
EQU V=Kiv.times.i (4)
 where, the current/voltage conversion Kiv is equivalent to a conversion
 sensitivity with respect to the feed back of the drive current i to the
 observer. Namely, the current/voltage conversion Kiv corresponds to a
 so-called return resistance.
 Next, in order to simplify an explanation, a disturbance applied to an
 actuator is regarded as only disturbance with respect to a certain
 position. Then, as shown in FIG. 13, if a disturbance variable is set as
 d, the following equation (5) is formed.
EQU i.times.U(s)+d=y (5)
 Here, in the above equation (2), when the desired value REF is set as zero
 (REF=0), the equation (2) becomes the following equation.
EQU y.times.Ke=-er (2A)
 Further, the following equation is obtained from the above equation (4).
EQU i=v/Kiv (4A)
 Further, if i and y in the equation (5) are eliminated by using the
 equations (2A) and (4A), the following equation is formed.
EQU (v/Kiv).times.U(s)+d=-er/Ke
 When arranging the this equation, the disturbance variable d is expressed
 as shown in the following equation (6), using the input voltage v and the
 error er supplied to the observer.
EQU d=-er/Ke-(v/Kiv).times.U(s) (6)
 In this case, parameters schematically showing an interior of the observer
 is expressed as nominal values, and an additional character n is appended
 to each nominal value in expression in order to distinguish actual control
 elements from the nominal value. Namely, the positional detection
 sensitivity Ke is expressed as a positional detection sensitivity nominal
 value Ken, the voltage/current conversion sensitivity Kdr is expressed as
 a voltage/current conversion sensitivity nominal value Kdrn, the
 current/voltage conversion sensitivity Kiv is expressed as a
 current/voltage conversion sensitivity nominal value Kivn, and the
 controlled system U(s) is expressed as a controlled system nominal value
 Un(s).
 Incidentally, the nominal value is a torque rated value of a spindle motor
 for rotating the optical disk in the optical disk reproducing apparatus,
 and is indicative of a value shown by a performance indication of the
 optical disk reproducing apparatus. If there is no performance indication,
 the nominal value is determined by an experiment or the like, or is
 computed (calculated) from a theoretical calculation. Incidentally, there
 is the case where the nominal value and an actual control element are not
 always equal to each other due to factors such as inaccurate determination
 or calculation, aged deterioration, temperature change or the like.
 On the basis of the above equation (6), when an estimated disturbance
 variable DOBS, which is an estimated variable of the disturbance d, is
 expressed using each nominal value, the following equation (7) is
 obtained.
EQU DOBS=-er/Ken-(v/Kivn).times.Un(s) (7)
 Thus, as is evident from the above description, it is possible to estimate
 and compute the estimated disturbance variable DOBS from the input voltage
 v and the error er by using the observer without detecting the actual
 disturbance d.
 Moreover, in FIG. 13, the estimated disturbance variable DOBS is converted
 into a compensated variable h by a robust filter R(s), and then, the
 compensated variable h is added to a variable which is obtained by
 phase-compensating the error er with a phase compensator C(s), and thus, a
 manipulated variable u is generated so as to suppress the disturbance d.
 On the other hand, in recent years, it is general that the total process of
 feedback control system including the observer is digitally carried out by
 one DSP (Digital Signal Processor) at a high speed and high accuracy.
 However, it is general that the above controlled system U(s) is driven by a
 drive current i which is usually an analog signal. Thus, it is a frequent
 case that a desired value REF, which is a target of control, is also an
 analog signal.
 Therefore, in the case of treating these analog signals with the DSP which
 is a digital circuit, in FIG. 13, at least an A/D converter for digitizing
 the error er, an A/D converter for digitizing the drive voltage v, and a
 D/A converter for analogizing the manipulated variable u are needed. As a
 result, the structure of the servo control device becomes complicate.
 Further, if the servo control device is realized as an IC chip, an
 occupied area increases, and a production cost becomes high.
 Moreover, the above description has shown the case where the controlled
 system U(s) is a focus actuator. In the case where the controlled system
 U(s) is a tracking actuator, a slider motor or a spindle motor, the
 problems similar to above take place. Incidentally, the tracking actuator
 is a device for finely adjusting a light beam irradiation position in the
 radial direction of the optical disk by moving the objective lens in the
 direction parallel to the information recording surface of the optical
 disk. The slider motor is a device for roughly adjusting a radial position
 of light beam irradiation position by moving a pickup. The spindle motor
 is a device for controlling a rotational frequency of the optical disk.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a servo
 control apparatus which can digitally estimate a disturbance applied to a
 controlled system, and can realize an operation for suppressing the
 disturbance with a simple structure, and further, can achieve a device
 miniaturization and a cost reduction.
 A servo control apparatus in accordance with the present invention performs
 a feedback control to an analog controlled system. The servo control
 apparatus includes: an error generating device that generates a digital
 error corresponding to a difference between a desired value and a feedback
 value corresponding to a controlled variable of the analog controlled
 system; a control device that generates a digital manipulated variable;
 and a digital-to-analog converting device that converts the digital
 manipulated variable into an analog manipulated variable for driving the
 analog controlled system. The control device includes: a disturbance
 estimating device that estimates a disturbance applied to the analog
 controlled system by carrying out a digital estimating process by using
 the digital manipulated variable and the digital error, and generates a
 digital compensated variable corresponding to the estimated disturbance;
 and a manipulated variable generating device that generates the digital
 manipulated variable by using the digital error and the digital
 compensated variable.
 As can be understood from the above configuration, in the servo control
 apparatus of the present invention, a disturbance applied to the analog
 controlled system can be estimated in a digital process without using the
 analog manipulated variable. Therefore, an analog-to-digital converter for
 converting the analog manipulated variable into the digital manipulated
 variable is not needed. Accordingly, the number of components of the servo
 control apparatus can be reduced, and thus, the structure of the servo
 control apparatus can be simplified. Hence, cost reduction and chip
 miniaturization can be achieved.
 To improve the performance of estimating the disturbance, the disturbance
 estimating device may estimate the disturbance at a current sample timing
 by using the digital manipulated variable at a previous sample timing and
 the digital error at the current sample timing.
 To improve the accuracy of estimating the disturbance, the control device
 may further include an estimated manipulated variable generating device
 that generates a digital estimated manipulated variable by multiplying the
 digital manipulated variable at a previous sample timing by a nominal
 value corresponding to a sensitivity of the analog-to-digital device, and
 a disturbance estimating device may estimate the disturbance by using the
 digital estimated manipulated variable and the digital error at a current
 sample timing.
 In the case where a drive device for driving the analog controlled system
 by using the analog manipulated variable is added to the servo control
 apparatus, the control device may further include an estimated manipulated
 variable generating device that generates a digital estimated manipulated
 variable by using a product of the digital manipulated variable at a
 previous sample timing, a nominal value corresponding to a sensitivity of
 the analog-to-digital device and a nominal value corresponding to a
 sensitivity of the drive device.
 In the servo control apparatus in accordance with the present invention,
 the manipulated variable generating device may generate the digital
 manipulated variable by using the digital compensated variable at a
 previous sample timing and the digital error. By using the digital
 compensated variable at a previous sample timing, the disturbance can be
 also estimated accurately.
 In the servo control apparatus in accordance with the present invention,
 the disturbance estimating device may estimate the disturbance on the
 basis of an internal state thereof at a previous sample timing. On the
 basis of an internal state of the disturbance estimating device, the
 disturbance can be also estimated accurately.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Next, the preferred embodiments of the present invention will be described
 with reference to the accompanying drawings.
 Each embodiment described below shows the case where the present invention
 is applied to an information reproducing apparatus which reproduces
 information recorded in an optical disk. In the information reproducing
 apparatus, a feedback control system including an observer is used for a
 rotation servo control of an optical disk, a focus servo control, a
 tracking servo control, and a slider servo control with respect to a
 pickup.
 (I) First Embodiment
 A first embodiment of the present invention will be described below with
 reference to FIG. 1 to FIG. 5.
 The first embodiment described below shows the case where the present
 invention is applied to a rotation servo control of an optical disk in an
 information reproducing apparatus.
 First, a construction of an information reproducing apparatus according to
 the first embodiment will be described below with reference to FIG. 1 and
 FIG. 2. FIG. 1 is a block diagram schematically showing a construction of
 an information reproducing apparatus according to a first embodiment of
 the present invention, and FIG. 2 is a block diagram showing a detailed
 construction of a rotation detector shown in FIG. 1.
 As shown in FIG. 1, an information reproducing apparatus S of the first
 embodiment includes a pickup 1, a pickup controller PC, a spindle motor
 10, and a spindle motor controller SC.
 The spindle motor controller SC includes a rotation detector 11, an analog
 to digital (A/D) converter 12, a desired value generator 13, a DSP
 (Digital Signal Processor) 14, a RAM (Random Access Memory) 15, a ROM
 (Read Only Memory) 16, a digital to analog (D/A) converter 17, and a drive
 circuit 18.
 As shown in FIG. 2, the rotation detector 11 includes a pulse generator 20,
 an edge detector 21, an F-V (frequency to voltage) converter 22, and a
 subtracter 23.
 Moreover, as shown in FIG. 1, the DSP 14 is provided with a phase
 compensator 14a and an observer 14b. The phase compensator 14a and the
 observer 14b are realized as a function of the DSP 14 when the DSP 14 is
 operated on the basis of a control program stored in the ROM 16. In this
 case, these observer 14b and phase compensator 14a may be individually
 realized as an independent circuit.
 FIG. 1 shows only part relating to servo control according to the present
 invention in the information reproducing apparatus S. In addition to the
 members shown in FIG. 1, the actual information reproducing apparatus S
 includes a reproduction processing section for reproducing information
 recorded in an optical disk DK on the basis of a detection signal from the
 pickup 1, a display section for displaying an operating state of the
 information reproducing apparatus S or an operating section for inputting
 a process for executing the information reproducing apparatus S.
 Next, the following is a description on a basic operation.
 First, the pickup 1 irradiates an information recording surface of the
 optical disk DK with a light beam B, and then, generates a detection
 signal on the basis of a reflected light of the light beam.
 At this time, the irradiation position of the light beam B and the focal
 position of the light beam B are controlled by the objective lens (not
 shown) installed in the pickup 1. The pickup controller PC moves the
 objective lens in the direction perpendicular to and parallel to the
 information recording surface of the optical disk DK. Then, the pickup
 controller PC adjusts the focal position of the light beam B. In such a
 manner, a focus servo control and tracking servo control are carried out.
 The spindle motor 10 rotatably drives the optical disk on the basis of a
 drive signal Si while generating a rotational frequency signal Srp
 indicative of a rotational frequency, and then, outputs the rotational
 frequency signal Srp to the rotation detector 11. In this case, the
 spindle motor 10 is equivalent to a controlled system U(s) shown in FIG. 3
 and FIG. 5.
 Subsequently, the rotation detector 11 detects a rotational frequency of
 the spindle motor 10 on the basis of the rotational frequency signal Srp.
 An output voltage value of the rotation detector 11 is changed when the
 rotational frequency of the spindle motor 10 is changed by a unit
 rotational frequency. This output voltage value indicates a rotational
 frequency detection sensitivity in the rotation detector 11, and this
 rotational frequency detection sensitivity is equivalent to a rotational
 detection sensitivity Ke (Volt/Hz) shown in FIG. 3 and FIG. 5.
 Simultaneously, the desired value generator 13 generates and outputs a
 desired value signal Sref indicative of a desired value of the rotational
 frequency of the spindle motor 10. The desired value signal Sref is
 equivalent to a desired value REF shown in FIG. 3 and FIG. 5.
 The A/D converter 12 converts a rotational frequency error, which is an
 analog signal obtained by subtracting an output signal value of the
 rotation detector 11 from the desired value signal Sref, into a digital
 signal, thereby generating a digital error signal Sedig, and then, outputs
 it to the DSP 14. The rotational frequency error is equivalent to an error
 er shown in FIG. 3 and FIG. 5. Moreover, the digital error signal Sedig is
 equivalent to a digital error EDIG shown in FIG. 3 and FIG. 5. A
 conversion sensitivity of the A/D converter 12 (i.e., a digital value
 corresponding to a unit voltage in rotational frequency error) is
 equivalent to a conversion sensitivity Kad (digit/Volt) shown in FIG. 3
 and FIG. 5.
 Next, the DSP 14 generates a manipulated signal Su for driving the drive
 circuit 18 by a digital operation of the phase compensator 14a and the
 observer 14b on the basis of the digital error signal Sedig, and then,
 outputs the manipulated signal Su to the D/A converter 17. At this time,
 the DSP 14 reads out a control program stored in the ROM 16 as a ROM
 signal Sro in advance, and then, performs a function as the phase
 compensator 14a and the observer 14b on the basis of the control program.
 Further, data required for performing the above function is used while
 being temporarily stored in the RAM 15 as a RAM signal Sra.
 The D/A converter 17 converts the manipulated signal Su from a digital
 signal into an analog signal so as to generate an analog manipulated
 signal Sau, and then, outputs the analog manipulated signal Sau to the
 drive circuit 18. In this case, a conversion sensitivity of the D/A
 converter 17 (i.e., a voltage value corresponding to one digital value) is
 equivalent to a conversion sensitivity Kda (Volt/digit) shown in FIG. 3
 and FIG. 5.
 Then, the drive circuit 18 amplifies the analog manipulated signal Sau
 which is a voltage signal while generating the above drive signal Si
 having a current value corresponding to the voltage value, and then,
 outputs the drive signal Si to the spindle motor 10. Accordingly, the
 spindle motor 10 is driven. In this case, the drive signal Si is
 equivalent to a drive current i shown in FIG. 3 and FIG. 5. Moreover, a
 conversion sensitivity of the drive circuit 18 (i.e., a current value of
 the drive signal Si corresponding to a unit voltage in the analog
 manipulated signal Sau) is equivalent to a conversion sensitivity Kdr
 (Ampere/Volt) shown in FIG. 3 and FIG. 5.
 Next, an operation of the rotation detector 11 will be described in detail
 with reference to FIG. 2.
 As shown in FIG. 2, the pulse generator 20 generates a pulse signal Spu
 whose pulse number per unit time is proportional to a rotational frequency
 of the spindle motor 10 shown by the rotational frequency signal Srp, and
 then, outputs the pulse signal Spu to the edge detector 21.
 Subsequently, the edge detector 21 detects any one of a rise edge or fall
 edge in each pulse contained in the pulse signal Spu, thereby generating
 an edge signal Seg, and then, outputs the edge signal Seg to the F-V
 converter 22.
 Then, the F-V converter 22 generates a rotational frequency voltage signal
 Srv having a voltage value corresponding to the number of rise edge per
 unit time or the number of fall edge per unit time, on the basis of a rise
 edge cycle or fall edge cycle in the edge signal Seg, and then, outputs
 the signal Srv as a feedback signal to one terminal of the subtracter 23.
 The subtracter 23 subtracts the rotational frequency voltage signal Srv
 from the desired value signal Sref obtained from the desired value
 generator 13, thereby generating an error signal Ser indicative of a
 rotational frequency error, and then, outputs the error signal Ser to the
 A/D converter 12.
 Next, the following is a description on a feedback control according to the
 present invention in a control system including the spindle motor
 controller SC having the aforementioned construction and the spindle motor
 10, and this explanation is made with reference to FIG. 3 to FIG. 5.
 FIG. 3 is a block diagram showing the whole of feedback control in a
 control system including the spindle motor controller SC and the spindle
 motor 10, FIG. 4 is a flowchart showing a process by an observer in the
 control system, and FIG. 5 is a block diagram showing the whole of the
 control system and including a block diagram showing internal elements of
 the observer 14b. In FIG. 3 and FIG. 5, the same reference numerals are
 used to designate the same elements as those in the conventional feedback
 system shown in FIG. 13, and the detailed explanation is omitted with
 respect to the same elements.
 First, in the first embodiment, the controlled system U(s) is the spindle
 motor 10; therefore, the controlled variable is a rotational frequency of
 the spindle motor 10.
 Now, when a transfer function of the spindle motor 10 approximates to a
 first-order lag system, the following equation is formed.
EQU U(s)=Km/(1+Tm.times.s)
 where, Km (Hz/Ampere) is a gain of the spindle motor 10, and Tm(sec) is a
 time constant of the spindle motor 10.
 Next, Kmn is set as a nominal value of the gain Km and Tmn is set as a
 nominal value of the time constant Tm, and the transfer function of the
 spindle motor 10 is converted into a digital system as follows.
EQU Un(z)=Kmn.times.(1-Kpn)/(1-Kpn.times.z.sup.-1)
 where, Kpn is a coefficient relating to a pole frequency, and more
 specifically, is expressed as exp (-T/Tmn). Moreover, T(sec) is a sampling
 period in digital processing, and an operation expressed by
 ".times.z.sup.-1 " is an operation for obtaining a variable before one
 sample timing.
 On the other hand, a nominal value of the rotation detection sensitivity Ke
 (Volt/Hz) in the rotation detector 11 is set as Ken, a nominal value of
 the conversion sensitivity Kad (digit/Volt) in the A/D converter 12 is set
 as Kadn, a nominal value of the conversion sensitivity Kda (Volt/digit) in
 the D/A converter 17 is set as Kdan, and a nominal value of the conversion
 sensitivity Kdr (Ampere/Volt) in the drive circuit 18 is set as Kdrn. In
 this case, in the feedback control system of the first embodiment, a
 digital manipulated variable u1 which is a digital manipulated variable u
 before one sample timing is obtained, and then, the digital manipulated
 variable u1 is multiplied by the nominal values Kdan and Kdrn of the
 conversion sensitivity of the D/A converter 17 and the drive circuit 18,
 thereby obtaining an estimated manipulated variable in1, and then, the
 estimated manipulated variable in1 is used as one input of the observer,
 and, on the other hand, the digital error EDIG is used as the other input
 of the observer.
 Next, an operation of the observer 14b will be described in detail with
 reference to FIG. 4 and FIG. 5. In an example shown in FIG. 4 and FIG. 5,
 an operation based on an inverse function of characteristic equation
 indicating a controlled system is realized in the feedback control system
 of the first embodiment. The above inverse function is indicative of an
 inverse transfer characteristic of the controlled system U(s), and is
 expressed as 1/U(s).
 In the case of realizing the above observer by this inverse function, an
 operation as shown in FIG. 4 is carried out in the feedback control system
 including the observer 14b.
 More specifically, as shown in FIG. 4, first, an operation to obtain the
 estimated manipulated variable in1 is performed in accordance with the
 following equation, using the digital manipulated variable u1 before one
 sample timing, the nominal value kdan of the conversion sensitivity Kda of
 the D/A converter 17, and the nominal value Kdrn of the conversion
 sensitivity Kdr of the drive circuit 18 (step S1).
EQU in1=Kdrn.times.Kdan.times.u1
 At this time, the digital manipulated variable u1 before one sample timing
 is calculated as the following equation.
EQU u1=z.sup.-1.times.u
 In this equation, u is a digital manipulated variable of current sample
 timing (input value of the D/A converter 17). Moreover, an operation
 ".times.z.sup.-1 " is equivalent to carrying out an operation exp
 (-s.times.T). Actually, this operation is realized by storing the digital
 manipulated variable u1 before one sample timing in the RAM 15, and
 reading it therefrom.
 As shown in FIG. 5, a controlled variable y is converted into the digital
 error EDIG by using the following equation, and then, is output from the
 A/D converter 12.
 REF-y.times.Ke=er
EQU Kad.times.er=EDIG
 Next, an operation to obtain a compensation current iw is performed in
 accordance with the following equation, using a digital nominal value
 Un(z) of the controlled system U (s), the nominal value Ken of the
 conversion sensitivity Ke of the rotation detector 11, the conversion
 sensitivity Kadn of the A/D converter 12, the digital error EDIG and
 estimated manipulated variable in1 (step S2).
EQU iw=in1-EDIG/(Ken.times.Kadn.times.Un(z))
 At this time, the compensation current iw contains the estimated
 disturbance variable DOBS (FIG. 13).
 Subsequently, an operation to obtain a compensated variable w (compensated
 variable of manipulated variable u) is performed in accordance with the
 following equation, using the compensation current iw, the nominal value
 Kdan, the nominal value Kdrn and R(z) indicating a robust filter (not
 shown) included in the DSP 14 (step S3).
EQU w={1/(Kdan.times.Kdrn)}.times.R(z).times.iw
 Finally, an operation to obtain the digital manipulated variable u of the
 current sample timing is performed in accordance with the following
 equation, using the digital error EDIG, the compensated variable w and
 C(z) indicating a transfer characteristic of the phase compensator 14a
 (step S4).
EQU u=EDIG.times.C(z)+w
 This digital manipulated variable u is supplied to the D/A converter 17.
 Thereafter, the digital manipulated variable u is converted into an analog
 signal by the D/A converter 17 so as to generate an analog manipulated
 signal Sau, and then, the drive circuit 18 is driven using the analog
 manipulated signal Sau so as to generate a drive signal Si. The drive
 signal Si is output to the spindle motor 10; as a result, the spindle
 motor 10 is rotated.
 In this case, the following equation is formed as an operation in the
 robust filter, for example.
EQU R(z)=Krg/(1-Krp.times.z.sup.-1)
 where Krg is a gain constant of the robust filter, and Krp is a pole
 constant (0&lt;Krp&lt;1) thereof.
 Moreover, the following equation is formed as an operation in the phase
 compensator 14a, for example.
EQU C(z)=Kcg.times.(1-Kc0.times.z.sup.31 1)/(1-Kcp.times.z.sup.-1)
 where Kcg is a gain constant of the phase compensator 14a, Kc0 is a
 coefficient at a zero point thereof, and Kcp is a pole coefficient (0&lt;Kc0
 &lt;Kcp&lt;1) thereof.
 In the feedback system shown in FIG. 3 and FIG. 5, the estimated
 manipulated variable in1 has been operated using the digital manipulated
 variable u1 before one sample timing. This principle will be described
 below.
 To obtain the ideal estimated manipulated variable in1, the operation is
 performed using the digital manipulated variable u of the current sample
 timing. However, in fact, it is impossible to obtain the digital
 manipulated variable u of the current sample timing when the operation to
 obtain the estimated manipulated variable in1 is actually performed,
 because the digital manipulated variable u of the current sample timing is
 obtained after all of the operations (step 1 through step 4 in FIG. 4)
 end. For this reason, the digital manipulated variable u1 before one
 sample timing is used in place of the digital manipulated variable u. This
 is based on the premise that the digital manipulated variable u has no
 rapid change during one sample timing.
 In the feedback control system, assuming that the manipulated variable u at
 the current sample timing is directly used in the observer, an A/D
 converter for digitizing the drive current i must be needed. On the
 contrary, in the first embodiment, since the digital manipulated variable
 u1 before one sample timing is used in place of the digital manipulated
 variable u of the current sample timing, the A/D converter for digitizing
 the drive current i is not needed. In the case where a sampling frequency
 is sufficiently higher as compared with a servo area of the feedback
 control system, a one sample timing lag is no problem.
 As described above, according to the first embodiment, the spindle motor
 controller SC carried out a feedback control while estimating a
 disturbance applied to the spindle motor 10 on the basis of the digital
 manipulated variable u1 and the digital error EDIG. This serves to
 dispense the A/D converter for digitizing the drive current i.
 Accordingly, it is possible to simplify the construction of the spindle
 motor controller SC, and to reduce a production cost, and further, to
 reduce an integration area.
 Further, the disturbance corresponding to the current sample timing is
 estimated using the digital manipulated variable u1 before one sample
 timing and the error EDIG of the current sample timing; therefore, an
 actual disturbance can be effectively suppressed.
 Furthermore, the disturbance is estimated on the basis of the estimated
 manipulated variable in1 generated by multiplying the nominal value Kdan
 of the conversion sensitivity Kda of the D/A converter 17 and the nominal
 value Kdrn of the conversion sensitivity Kdr of the drive circuit 18 with
 respect to the digital manipulated variable u1 before one sample timing;
 therefore, the disturbance can be accurately estimated.
 The above first embodiment has described the case where the rotation
 detector 11 and the A/D converter 12 are used in order to generate the
 digital error signal Sedig on the basis of the rotational frequency signal
 Srp from the spindle motor 10. In addition, as shown in FIG. 6, the
 digital error signal Sedig may be generated from the rotational frequency
 signal Srp by a rotation detector 11' including a pulse generator 20, an
 edge detector 21 and a period detecting counter 25 and by a
 period-frequency converter 26.
 More specifically, as shown in FIG. 6, the rotational frequency signal Srp
 is generated according to the same manner as the aforementioned case (FIG.
 2). Then, the period detecting counter 25 detects any one of the rise edge
 cycle and the fall edge cycle of the rotational frequency signal Srp,
 thereby generating the cycle signal Stdig, and then, outputs this signal
 to the period-frequency converter 26.
 Then, the period signal Stdig is converted into a frequency by the
 period-frequency converter 26 so as to generate the corresponding
 frequency signal Sfdig, and the frequency signal Sfdig is supplied to one
 input terminal of a digital subtracter 23'. Further, the frequency signal
 Sfdig is subtracted from a desired value signal Srefdig (,which is a
 digital signal,) from the desired value generator 13, and then, the
 digital error signal Sedig, which is a difference of the above
 subtraction, is generated and output.
 In this manner, the pulse signal Spu may be directly converted into a
 digital value so that the digital error signal Sedig can be obtained.
 In the above first embodiment, the desired value REF has been applied as an
 analog signal. The desired value REF is converted into a digital signal in
 advance, and thereafter, may be applied as a desired value signal Srefdig
 in a manner shown in the following equation.
EQU y.times.Ke=er
EQU refdig-Kad.times.er=EDIG
 Moreover, in the above first embodiment, the manipulated signal Su (digital
 manipulated variable u) has been converted into an analog manipulated
 signal Sau using the D/A converter 17. Alternatively, the analog
 manipulated signal Sau may be generated from the manipulated signal Su by
 a so-called PWM (Pulse Width Modulation) circuit or the like.
 (II) Second Embodiment
 Next, a second embodiment of the present invention will be described with
 reference to FIG. 7 and FIG. 8. FIG. 7 is a flowchart showing a process by
 an observer of a second embodiment, and FIG. 8 is a block diagram showing
 a construction of a control system including a block diagram showing
 internal elements of the observer of the second embodiment.
 The above first embodiment has described the case where the observer 14b is
 operated on the basis of an inverse function of a controlled system. In
 this second embodiment, a so-called Gopinath minimal-order observer is
 used as the observer 14b.
 An operation of feedback control system of the second embodiment is the
 same as the case of the spindle motor controller Sc of the first
 embodiment except an internal operation of the observer 14. Therefore,
 explanation with respect to the same elements is omitted.
 The above Gopinath minimal-order observer is one of observers designed
 according to a Gopinath method which is general as an observer design
 method.
 The following is a description on a construction and operation of the
 feedback control system of the second embodiment in the case where the
 Gopinath minimal-order observer is used as the observer 14b.
 In the feedback control system including the observer 14b functioning as
 the Gopinath minimal-order observer, as shown in FIG. 7, first, in the
 same manner as the case of the first embodiment, an operation to obtain an
 estimated manipulated variable in1 is performed in accordance with the
 following equation, using the digital manipulated variable u1 before one
 sample timing (step S1).
EQU in1=Kdrn.times.Kdan.times.u1
 Next, a controlled variable y is converted into the digital error EDIG by
 using the following equation, and then, is output from the A/D converter
 12.
EQU REF-y.times.Ke=er
EQU Kad.times.er=EDIG
 Subsequently, based on the above digital error EDIG and the estimated
 compensated variable in1, an operation to obtain a compensated controlled
 variable yw is performed in accordance with the following equation, using
 the robust filter R(z) included in the DSP 14, the nominal value Kmn of a
 gain Km of the spindle motor 10, the nominal value Tmn of the time
 constant Tm of the spindle motor 10, the above nominal values Ken, Kadn,
 Kdrn and a time constant Tp of the robust filter (step S10).
EQU yw=R(z).times.{in1.times.Kmn-EDIG.times.(Tmn/Tp)/
 (Ken.times.Kadn)}+EDIG.times.(Tmn/Tp)/(Ken.times.Kadn)
 In this case, the following equation shows a relationship between the time
 constant Tp of the robust filter and the pole constant Krp of the robust
 filter in the first embodiment.
EQU Krp=exp(-T/Tp)
 At this time, the compensated controlled variable yw contains the estimated
 disturbance variable DOBS (FIG. 13).
 Next, on the basis of the above compensated controlled variable yw, the
 nominal value Kmn, the nominal value Kdan of the conversion sensitivity
 Kda of the D/A converter 17 and the nominal value Kdrn of the conversion
 sensitivity Kdr of the drive circuit 18, an operation to obtain a
 compensated variable w is performed in accordance with the following
 equation (step S11).
EQU w={1/(Kmn.times.Kdan.times.Kdrn)}.times.yw
 Finally, on the basis of the digital error EDIG and the above compensated
 variable w, an operation to obtain a digital manipulated variable u of the
 current sample timing is performed in accordance with the following
 equation, and then, is output to the D/A converter 17 (step S4).
EQU u=EDIG.times.C(z)+w
 Thereafter, the digital manipulated variable u is converted into an analog
 signal by the D/A converter 17 so that an analog manipulation signal Sau
 is generated, and then, the drive circuit 18 is driven by the analog
 manipulation signal Sau so as to generate the above drive signal Si, and
 outputs it to the spindle motor 10. Accordingly, the spindle motor 10 is
 rotated.
 According to the second embodiment, in the information reproducing
 apparatus including the feedback control system using the Gopibath
 minimal-order observer, the similar effect as the first embodiment can be
 obtained.
 Moreover, the second embodiment has described the case of estimating the
 disturbance corresponding to the current sample timing using the digital
 manipulated variable u1 before one sample timing and the error EDIG of the
 current sample timing. In addition, the digital manipulated variable u has
 been delayed for one sample timing so as to obtain the digital manipulated
 variable u1. In place of this construction, the substantially same effect
 as the above second embodiment can be also obtained from the following
 construction; more specifically, as shown in FIG. 8, a delay equipment
 (shown by "z.sup.-1 ") for delaying each operated value for one sample
 timing is inserted between the subtracter in the observer and operation
 points (between controlled elements) up to the digital manipulated
 variable u.
 (III) Third Embodiment
 Next, a third embodiment of the present invention will be described with
 reference to FIG. 4, FIG. 9 and FIG. 10. The following third embodiment
 shows the case where the present invention is applied to a process by a
 pickup controller PC for carrying out a focus servo control of the pickup
 1 included in the information reproducing apparatus S shown in FIG. 1.
 First, a construction of the pickup controller PC of the third embodiment
 will be described with reference to FIG. 9. FIG. 9 is a block diagram
 schematically showing a construction of the pickup controller PC of the
 third embodiment.
 As shown in FIG. 9, the pickup controller PC of the information reproducing
 apparatus S includes a focus error detector 30, a subtracter 31, a desired
 value generator 32, an A/D converter 33, a DSP 34, a RAM 35, a ROM 36, a
 D/A converter 37, and a drive circuit 38.
 The DSP 34 is provided with a phase compensator 34a and an observer 34b in
 the same manner as the case of the first embodiment.
 Moreover, the pickup (PU) 1 is provided with an actuator (ACT) 1a for
 moving an objective lens (not shown) in the direction perpendicular to an
 information recording surface of an optical disk DK.
 Next, an operation of the pickup 1 will be described below.
 First, the pickup 1 irradiates the information recording surface of the
 optical disk DK with a light beam B, while carrying out a tracking servo
 control and a focus servo control based on the drive signal Si. Further,
 the pickup 1 generates a detection signal Spp on the basis of a reflected
 light of the light beam, and then, outputs the detection signal Spp to a
 focus error detector 30. In this case, the actuator 1a is equivalent to a
 controlled system U(s) shown in FIG. 10.
 The detection signal Spp is supplied to the focus error detector 30, and is
 used in an information reproducing process of the reproduction processing
 section described in the first embodiment.
 Next, the focus error detector 30 generates a focus error signal Sfe
 indicative of a shift in the direction perpendicular to the information
 recording surface of the optical disk DK between the focal position of the
 light beam B and the position on the information recording surface, on the
 basis of the detection signal Spp, and then, outputs the focus error
 signal Sfe to one terminal of the subtracter 31. In this case, an error
 signal detecting sensitivity of the focus error detector 30 (i.e., an
 output voltage value of the focus error detector 30, which is changed when
 the objective lens is moved by a unit distance) is equivalent to a
 position detecting sensitivity Ke (Volt/m) shown in FIG. 10.
 Moreover, as a focus error detecting method in the focus error detector 30,
 a so-called SSD (Spot Size Detection) method or astigmatism method may be
 used, for example.
 Simultaneously, the desired value generator 32 generates and outputs a
 desired value signal Sref indicative of a position in which the objective
 lens should be positioned (i.e., a position in the direction perpendicular
 to the information recording surface on which the objective lens should be
 positioned so that the focal position of the light beam B is positioned
 onto the information recording surface). In this case, the desired value
 signal Sref is equivalent to a desired value REF shown in FIG. 10.
 Then, the subtracter 31 generates an error signal Ser indicative of a
 positional error obtained by subtracting a value of the focus error signal
 Sfe from a value of the desired value signal Sref, and then, outputs the
 error signal Ser to the A/D converter 33. This positional error is a
 positional error relating to a focal position of the light beam B, and is
 equivalent to an error er shown in FIG. 10.
 The A/D converter 33 converts the error signal Ser from an analog signal
 into a digital signal so that a digital error signal Sedig is generated,
 and then, outputs the digital error signal Sedig to the DSP 34. In this
 case, a conversion sensitivity of the A/D converter 33 (i.e., a digital
 value corresponding to a unit error in the positional error) is equivalent
 to a conversion sensitivity Kad (digit/m) of the A/D converter 33 in FIG.
 10. Moreover, the digital error signal Sedig is equivalent to a digital
 error EDIG shown in FIG. 10.
 Next, in the same manner as the case of the DSP 14 of the first embodiment,
 the DSP 34 generates a manipulated signal Su for driving the drive circuit
 38 by a digital operation of the phase compensator 34a and the observer
 34b based on the digital error signal Sedig, and then, outputs the signal
 Su to the D/A converter 37. At this time, the DSP 34 performs a function
 as the phase compensator 34a and the observer 34b on the basis of a
 control program while reading the control program stored in the ROM 36 as
 a ROM signal Sro in advance. Moreover, a data required for performing this
 function is temporarily stored as a RAM signal Sra in the RAM 35.
 The D/A converter 37 converts the manipulated signal Su from a digital
 signal into an analog signal so as to generate an analog manipulated
 signal Sau, and then, output the signal Sau to the drive circuit 38. In
 this case, a conversion sensitivity of the D/A converter 37 (i.e., a
 voltage value corresponding to one digital value) is equivalent to a
 conversion sensitivity Kda (Volt/digit) of the D/A converter 37 shown in
 FIG. 10.
 The driver circuit 38 amplifies the analog manipulated signal Sau which is
 a voltage signal, and generate a drive signal Si having a current value
 corresponding to the voltage value of the amplified manipulated signal
 Sau, and then, outputs the drive signal Si to the actuator 1a, and thus,
 the actuator 1a is driven so as to move the objective lens. In this case,
 the drive signal Si is equivalent to a drive current i shown in FIG. 10.
 Moreover, a conversion sensitivity of the driver circuit 38 (i.e., a
 current value of the drive current Si corresponding to a unit voltage of
 the analog manipulated signal Sau) is equivalent to a conversion
 sensitivity Kdr (Ampere/Volt) of the drive circuit 18 shown in FIG. 10.
 Next, a feedback control according to the present invention in a control
 system including the pickup controller PC having the above construction
 and the actuator 1a will be described below with reference to FIG. 4 and
 FIG. 10.
 FIG. 10 is a block diagram showing the whole construction of feedback
 control in the control system including the pickup controller PC and the
 actuator 1a, and including a block diagram showing internal elements of
 the observer 34b. In this third embodiment, a disturbance estimating
 process by the observer 34b is basically the same as the observer 14b of
 the first embodiment, and therefore, the flowchart shown in FIG. 4 is used
 as a flowchart for explaining the process by the observer 34b. In FIG. 10,
 the same reference numerals are used to designate the same controlled
 elements as those in the conventional feedback control system shown in
 FIG. 13, and therefore, the details with respect to the same elements are
 omitted.
 In the third embodiment, a controlled system U(s) is the actuator 1a;
 therefore, a controlled variable y is a position of the actuator la (or
 the objective lens) in the direction perpendicular to the information
 recording surface of the optical disk DK.
 Now, when a transfer function of the actuator la approximates to a
 second-order lag system, the following equation is formed.
EQU U(s)=A.times.w.sup.2 /(s.sup.2 +2.times.k.times.w.times.s+w.sup.2)
 where, A is a gain (m/Ampere) of the actuator 1a, k is a viscous braking
 coefficient of the actuator 1a, and w is a natural vibration frequency
 (rad/sec) of the actuator 1a.
 Further, Kgn is set as a nominal value of a gain constant Kg of the
 actuator 1a, K1 n is set as a nominal value of a primary coefficient K1 of
 the actuator 1a, and K2n is set as a nominal value of a secondary
 coefficient K2 of the actuator 1a. Then, the transfer function of the
 actuator 1a is converted into a digital system as follows.
EQU Un(z)=Kgn/(1+K1n.times.z-1+K2n.times.z.sup.-2)
 On the other hand, a nominal value of the position detecting sensitivity Ke
 as an error detecting sensitivity in the focus error detector 30 is set as
 Ken, a nominal value of the conversion sensitivity Kad (digit/Volt) in the
 A/D converter 33 is set as Kadn, a nominal value of the conversion
 sensitivity Kda (Volt/digit) in the D/A converter 37 is set as Kdan, and a
 nominal value of the conversion sensitivity Kdr (Ampere/Volt) in the drive
 circuit 38 is set as Kdrn. In this case, in the feedback control system of
 the third embodiment, a digital manipulated variable u1 which is a digital
 manipulated variable u before one sample timing is obtained, and then, the
 digital manipulated variable u1 is multiplied by the nominal values Kdan
 and Kdrn, so that an estimated manipulated variable in1 is obtained. This
 the estimated manipulated variable in1 is used as one input of the
 observer. The digital error EDIG is used as the other input of the
 observer.
 Next, an operation of the observer 34b will be described in detail with
 reference to FIG. 4 and FIG. 10. In an example shown in FIG. 4 and FIG.
 10, an operation based on the same inverse function of the first
 embodiment is realized in the feedback control system of the third
 embodiment.
 In the feedback control system including the observer 34b which is
 adaptable to the use of an inverse function of controlled system, as shown
 in FIG. 4, first, an operation to obtain the estimated manipulated
 variable in1 is performed in accordance with the following equation, using
 the digital manipulated variable u1 before one sample timing, the nominal
 value kdan of the conversion sensitivity Kda of the D/A converter 37, and
 the nominal value Kdrn of the conversion sensitivity Kdr of the drive
 circuit 38 (step S1).
EQU in1=Kdrn.times.Kdan.times.u1
EQU u1=z.sup.-1.times.u
 where u is a digital manipulated variable of the current sample timing
 (input value of the D/A converter 37).
 In this case, a controlled variable y is expressed as a digital error EDIG
 from the A/D converter 33 as follows.
EQU REF-y.times.Ke=er
 Kad.times.er=EDIG
 Next, an operation to obtain a compensation current iw is performed in
 accordance with the following equation, using a digital nominal value
 Un(z) of the controlled system U (s), the nominal value Ken of the
 conversion sensitivity Ke of the focus error detector 30 and the nominal
 value Kadn of the conversion sensitivity Kad of the A/D converter 33, the
 digital error EDIG and estimated manipulated variable in1 (step S2).
EQU iw=in1-EDIG/(Ken.times.Kadn.times.Un(z))
 At this time, the compensation current iw contains the estimated
 disturbance variable DOBS shown in FIG. 13.
 Subsequently, an operation to obtain a compensated variable w is performed
 in accordance with the following equation, using the compensation current
 iw, the nominal value Kdan, the nominal value Kdrn and R.sub.2 (z)
 indicating a robust filter included in the DSP 34 (step S3).
EQU w={1/(Kdan.times.Kdrn)}.times.R.sub.2 (z).times.iw
 Finally, an operation to obtain the digital manipulated variable u of the
 current sample timing is performed in accordance with the following
 equation, using the digital error EDIG, the compensated variable w, and
 C.sub.2 (z) indicating a transfer sensitivity of the phase compensator 34a
 (step S4).
EQU u=EDIG.times.C.sub.2 (z)+w
 Then, the digital manipulated variable u is supplied to the D/A converter
 37.
 Thereafter, the digital manipulated variable u is converted into an analog
 signal by the D/A converter 37 so as to generate an analog manipulated
 signal Sau, and then, the drive circuit 38 is driven using the analog
 manipulated signal Sau so as to generate a drive signal Si. The drive
 signal Si is output to the actuator 1a; as a result, the actuator 1a is
 driven.
 In this case, the following equation is formed as an operation in the
 robust filter, for example.
EQU R.sub.2 (z)={Krg/(1-Krp.times.z.sup.-1)}.sup.2
 where Krg is a gain constant of the robust filter, and Krp is a pole
 constant thereof.
 Moreover, the following equation is formed as an operation in the phase
 compensator 34a, for example.
EQU C.sub.2 (z)=Kp+Ki/(1-z.sup.-1)+Kd(1-z.sup.-1)
 where Kp is a proportional term, Ki is an integral term, and Kd is a
 differential term.
 As described above, according to the third embodiment, the pickup
 controller PC can obtain the substantially same effect as the first
 embodiment in the focus servo control with respect to the objective lens
 included in the pickup 1.
 The above third embodiment has described the case where the desired value
 REF is applied as an analog signal. In addition, similarly to the case of
 the first embodiment, the desired value REF may be applied as a digital
 value after being converted from an analog signal to a digital signal in
 advance.
 Further, the above third embodiment has shown the construction such that
 the manipulated signal Su is converted into the analog manipulated signal
 Sau by the D/A converter 37. In addition, the analog manipulated signal
 Sau may be generated from the manipulated signal Su by using a PWM circuit
 or the like.
 Furthermore, the above third embodiment has described the case where the
 present invention is applied to a focus servo control with respect to the
 objective lens included in the pickup 1. In addition, the present
 invention may be applied to a tracking servo control using a tracking
 servo control using DPD (Differential Phase Detection) method, a
 three-beam method or the like. Moreover, the present invention may be
 applied to a slider servo control for moving the pickup 1 to a radial
 direction of the optical disk DK.
 (IV) Fourth Embodiment
 Next, a fourth embodiment of the present invention will be described below
 with reference to FIG. 11 and FIG. 12.
 The aforementioned embodiments have described the case where the estimated
 manipulated variable in1 is operated using the digital manipulated
 variable u1 before one sample timing in the pickup controller PC or
 spindle motor controller SC included in the information reproducing
 apparatus S. In this fourth embodiment, the estimated manipulated variable
 in1 is obtained by using the compensated variable w1 before one sample
 timing.
 In a feedback control of this fourth embodiment, the physical construction
 is the same as the aforementioned pickup controller PC or spindle motor
 controller SC, and therefore, the details with respect to the same
 elements are omitted.
 FIG. 11 is a flowchart showing a process by the observer in a control
 system of the fourth embodiment, and FIG. 12 is a block diagram showing
 the whole construction of the control system, and including a block
 diagram showing internal elements of the observer of the fourth
 embodiment.
 First, an operation of the observer of the fourth embodiment will be
 described with reference to FIG. 11 and FIG. 12. In an example shown in
 FIG. 11 and FIG. 12, an operation based on the same operation by an
 inverse function as the first embodiment is realized in a feedback control
 system.
 In the feedback control system of the fourth embodiment including an
 observer which is adaptable to the use of an inverse function of the
 controlled system, as shown in FIG. 11, first, an operation to obtain a
 digital manipulated variable u of the current sample timing is performed
 in accordance with the following equation, using the phase compensator
 C(z) on the basis of a compensated variable w1 output from the observer
 before one sample timing and the digital error EDIG (step S15).
EQU u=EDIG.times.C(z)+w1
 where w1=z.sup.-1.times.w (in this case, z.sup.-1 =exp (-s.times.T))
 Then, the digital manipulated variable u is supplied to a pre-stage D/A
 converter of the drive circuit.
 Subsequently, an operation to obtain an estimated manipulated variable in1
 is performed in accordance with the following equation, using the digital
 manipulated variable u (input signal of the D/A converter), a nominal
 value Kdan of the conversion sensitivity Kda of the D/A converter and a
 nominal value Kdrn of the conversion sensitivity of the drive circuit
 (step S16).
EQU in1=Kdrn.times.Kdan.times.u
 Subsequently, an operation to obtain a compensation current iw is performed
 in accordance with the following equation, using a controlled system U
 n(z), the nominal value Ken of the conversion sensitivity Ke of an error
 detector for detecting a rotational frequency error or focus error, and a
 nominal value Kadn of the conversion sensitivity Kda of the above A/D
 converter for converting the digital manipulated variable u into an analog
 signal, the aforementioned digital error EDIG and estimated manipulated
 variable in1 (step S17).
EQU iw=in1-EDIG/(Ken.times.Kadn.times.Un(z))
 Finally, an operation to obtain a compensated variable w from the
 compensation current iw is performed in accordance with the following
 equation, using the nominal value Kadn of the conversion sensitivity Kda
 of the above D/A converter, the nominal value Kdrn of the conversion
 sensitivity Kdr of the drive circuit, and the robust filter included in
 the DSP (step S18).
EQU w1={1/(Kdan.times.Kdrn)}.times.R(z).times.iw
 Then, the compensated variable w is stored as a compensated variable w1 in
 the RAM connected to the DSP in order to be used in the next sample
 timing.
 Thereafter, the computed digital manipulated variable u is converted into
 an analog signal by the D/A converter so that an analog manipulated signal
 is generated. Then, the drive circuit is driven by the signal so as to
 generate a drive signal, and outputs the drive signal to the actuator or
 spindle motor so that these actuator and spindle motor are driven.
 As described above, in the feedback control system of the fourth
 embodiment, a feedback control is carried out while estimating a
 disturbance applied to the controlled system on the basis of the digital
 manipulated variable u and the digital error EDIG. Accordingly, it is
 possible to dispense a conventionally required A/D converter for
 digitizing the drive current i, and to simplify the construction of the
 servo control apparatus, and further, to reduce a production cost and an
 integration area.
 Further, the disturbance corresponding to the current sample timing is
 estimated using the digital compensated variable w1 before one sample
 timing and the error EDIG of the current sample timing; therefore, it is
 possible to accurately suppress a disturbance.
 Furthermore, a disturbance after one sample timing is estimated on the
 basis of the estimated manipulated variable inl generated by multiplying
 the digital manipulated variable u by the nominal value Kadn of the
 conversion sensitivity Kda of the above A/D converter and the nominal
 value Kdrn of the conversion sensitivity Kdr of the drive circuit, and the
 error EDIG of the current sample timing. Therefore, it is possible to high
 accurately estimate a disturbance.
 The above fourth embodiment has described the case where the disturbance
 corresponding to the current sample timing is estimated using the digital
 compensated variable w1 before one sample timing and the error EDIG of the
 current sample timing. Further, the digital compensated variable w is
 delayed for one sample timing so as to obtain the digital compensated
 variable w1. In addition, in place of the construction, the substantially
 same effect as the above fourth embodiment can be also obtained from the
 following construction; as shown in FIG. 12, a delay equipment for
 delaying each operated value for one sample timing is inserted between the
 subtracter in the observer and operation points (between controlled
 elements) where the compensated variable w is output.
 The invention may be embodied in other specific forms without departing
 from the spirit or essential characteristics thereof. The present
 embodiments are therefore to be considered in all respects as illustrative
 and not restrictive, the scope of the invention being indicated by the
 appended claims rather than by the foregoing description and all changes
 which come within the meaning and range of equivalency of the claims are
 therefore intended to be embraced therein.
 The entire disclosure of Japanese Patent Application No. Hei 11-34808 filed
 on Feb. 12, 1999 including the specification, claims, drawings and summary
 is incorporated herein by reference in its entirety.