Control system for positioning head in disk device by estimating and correcting actuator parameters based on head position and actuator drive current obtained by vibrating the head

A control system rotates disks as information storage media to position a head in a disk drive that records or reproduces data. An actuator swings the head in the radial direction of the disk. Positioning of the head is performed by a servo control system adjusting a drive current supplied to the actuator based on the value of the drive current and head position information. Parameters for the actuator are estimated based on the head position information and the value of the drive current obtained when the head is vibrated at one or more frequencies in a plurality of positions. The estimated parameters are used to adjust input/output signals for the servo control system to correct the preset parameters for the control system.

FIELD OF THE INVENTION 
The present invention relates to a control system for a disk device and in 
particular to a head control system for moving and positioning a head. 
BACKGROUND OF THE INVENTION 
Today, magnetic disk drives, and in particular hard disk drives, are 
essential peripheral devices as external storage devices for computers. In 
the field of personal computers, those with a small size and a large 
capacity are widely used. 
A magnetic disk drive is in general a component integrally incorporating a 
plurality of magnetic disks, a disk rotation mechanism, a 
recording/reproduction head, and a head positioning servo mechanism. It 
uses the disk rotation mechanism to rotate the plurality of magnetic disks 
at a high speed, and moves the recording/reproduction head in the radial 
direction above the disk to reach a target track for recording or 
reproduction. 
The head is provided at the tip of a head arm that is swung by an actuator 
around a rotation shaft. The actuator is controlled by a servo system to 
perform head seeking and target track following operations. A 
recording-reproduction-separated head that is a combination of an MR 
(magnetoresistive effect) head for reproduction and a thin film inductive 
head for recording is used as the recording/reproduction head. 
Head control for such a magnetic disk drive generally has speed control and 
position follow-up modes. In the speed control mode, a speed profile 
(reference speed pattern) is generated according to the difference between 
a position of the head and the target position. A drive current in 
proportion to the difference between the speed profile and head speed is 
supplied to the actuator to match the head speed with the speed profile, 
allowing the head to move to the target track at a high speed. In the 
position follow-up mode, a control signal in proportion to the difference 
between the position of the head and the target position, and a control 
signal in proportion to the head speed are used to drive the actuator to 
control the head so that it will not deviate from the target track. 
Such a proportional gain set for each head control mode is determined by 
the design values of the actuator. However, since dynamic parameters vary 
from one actuator and another, the gain is eventually adjusted and 
determined before shipment. For the position follow-up mode, an automatic 
gain controller has been proposed to maintain the loop gain constant by 
measuring the loop gain for the entire closed loop (Japanese patent 
unexamined publication No. 4-219801). 
In addition, the disk drive requires a servo device to suppress the 
vibration of the head caused by the resonance of a head arm, a carriage, 
disks, etc. 
In a disk drive employing an observer in a control system, however, if 
actuator parameters vary due to temperature changes or the characteristics 
of each actuator, attempts to compensate the variations with the automatic 
adjustment of the loop gain do not result in sufficient compensation for 
the servo characteristics because these parameters are built into in the 
observer. 
In addition, if parameter variations due to the characteristics of the 
actuator are eventually adjusted before shipment, it is difficult to 
compensate variations due to secular changes. 
With the automatic loop gain control device (the unexamined publication No. 
4-219801), gain adjustment for the observer is difficult because loop gain 
adjustment depends solely upon output gain adjustment. Particularly, 
varying actuator parameters affect the speed profile following 
characteristics of the head in speed control mode and the automatic loop 
gain control device cannot eliminate this effect. 
Furthermore, a disk drive that digitally controls the head is subject to 
alias noise as well as lost bits. To reduce the effect of alias noise, a 
detected positional error signal is conventionally used as an error 
control signal after it is caused to pass through an anti-alias filter or 
a lowpass filter. This prevents the generation of positional error signals 
for control if the head moves at a high speed and the high-frequency 
components in the positional error signals increase. 
It is an object of the present invention to provide a control system that 
can stabilize the servo characteristics of a disk drive even when there 
are variations and secular changes in parameters for an actuator that 
moves a head. 
It is another object of this invention to provide a control system that can 
detect positional error signals well to precisely control head positioning 
even when the head is moving at a high speed, such as in access operation. 
SUMMARY OF THE INVENTION 
The present invention is a control system for positioning a head in a disk 
drive which rotates disks to perform at least data reproduction, 
particularly a head control system which can estimate parameters for an 
actuator that moves a head and use them to compensate parameters preset by 
a control system accordingly, thereby allowing servo characteristics to be 
kept stable even if the actuator parameters vary due to the 
characteristics of the actuator or secular changes. 
The control system for controlling the actuator for moving said head in the 
substantially radial direction of said disks is comprised of: a positional 
information generator for generating positional information representing 
the position of the head on the disks; a servo controller for controlling 
a drive current to the actuator so that the position of the head can match 
a given target position on the basis of the preset parameters for the 
actuator, the detected value of a drive current supplied to the actuator, 
and the head position information; a parameter estimation circuit for 
estimating current parameters of the actuator, based on the value of the 
drive current supplied to the actuator and the positional information 
obtained by vibrating said head at one or more predetermined frequencies 
in one or more predetermined positions on the disks; and a parameter 
correction circuit for correcting the preset parameters in the servo 
controller by using the estimated parameters. 
The parameter estimation circuit is preferably comprised of: a reference 
wave generator for generating reference waves with one or more 
predetermined frequencies; a superposing circuit for superposing the 
reference waves on a control signal generated by the servo controller with 
the head located at one or more predetermined head positions; and a 
calculator for calculating the current parameters of the actuator by 
discrete Fourier transformation using the detected value of the drive 
current supplied to the actuator, the head position information obtained 
by the positional information generator, and the reference waves. 
The parameter correction circuit is preferably comprised of: a multiplier 
for multiplying the detected drive current value to be inputted to the 
servo controller by the estimated parameter; and a divider for dividing a 
control signal outputted from the servo controller by the estimated 
parameter. 
The control system for controlling the actuator for moving said head in the 
substantially radial direction of said disks is comprised of: a positional 
information generator for generating positional signals representing the 
position of the head, based on servo data read out from the disks by the 
head; a switch for selecting one among the positional signals before 
passing a lowpass filter or anti-alias filter and the positional signals 
after passing the lowpass filter, depending on the dynamic state of the 
head; a servo controller for controlling a drive current to the actuator 
so that the position of the head can match a given target position on the 
basis of the preset parameters for the actuator, the detected value of a 
drive current supplied to the actuator, and the selected positional 
signals; a parameter estimation circuit for estimating current parameters 
of the actuator, based on the value of the drive current supplied to the 
actuator and the selected one of the positional signals obtained by 
vibrating the head at one or more predetermined frequencies in one or more 
predetermined positions on the disks; and a parameter correction circuit 
for correcting the preset parameters in the servo controller by using the 
estimated parameters. 
Preferably, the switch selects the positional signals before passing the 
lowpass filter when the head exists without a predetermined distance from 
the target position or the head speed is greater than a predetermined 
valise, and selects the positional signals after passing said lowpass 
filter when the head exists within the predetermined distance or the head 
speed is smaller than said predetermined value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment according to this invention is explained in detail below with 
reference to the drawings. 
FIG. 1 is a block diagram illustrating the control system of a magnetic 
disk drive. The magnetic disk drive has a mechanism wherein, for example, 
three magnetic disks rotate around a spindle. These magnetic disks have 
thereon one servo face 1 and five data faces 2. 
Servo data are written in advance into the servo face 1 using the Servo 
Track Writer (STW). The servo-face servo data 12 are read by a servo head 
3 and input to a servo-face positional error signal generator 5, which 
generates servo-face positional error signals 14 (see FIG. 2). 
A recording-reproduction-separated head 4 records or reproduces data on or 
from the data face 2. As described below, however, servo data are also 
written into the data faces 2 and this data-face servo data 13 are read by 
the head 4. The data-face servo data 13 are inputted to a data-face 
positional error generator 6, which generates data-face positional error 
signal 15 (see FIG. 4). 
A passed track counter 7 uses the servo-face positional error signal 14 to 
output its count 16 indicating the number of tracks passed to a digital 
signal processor (DSP) 8 while the head is moving. 
The servo-face positional error signal 14 and data-face positional error 
signal 15 are input to the DSP 8, which then generates a hybrid positional 
error signal. This will be explained later referring to FIG. 6. 
A current detector 11 detects a drive current 19 when a power amplifier 9 
drives the actuator 10, and the DSP 8 uses the detected current value 17 
and the positional error signals described above to perform speed and 
position follow-up controls, and parameter estimation and correction. 
These operations will be described in detail later referring to FIG. 7. 
This constitution allows the-heads 3 and 4 moved by the actuator 10 to be 
controlled stably. 
As described above, the resonance component of the mechanism including the 
actuator 10 is superposed on the servo-face positional error signal 14 and 
data-face positional error signal 15 generated from the servo data 12 and 
13 read by the heads 3 and 4. 
In addition, the servo-face positional error signal 14, the data-face 
positional error signal 15, and a detection signal 17 from the current 
detector 11 are outputted as digital signals after conversion. 
FIG. 2 shows a wave form chart for the servo-face positional error signal 
14. The servo-face positional error signal 14 is a triangular wave with a 
cycle corresponding to four tracks as well as two phases that are at 90 
degrees to each other, and consists of an N-position signal 2 and a 
Q-position signal 21. When the head is moved in the inner direction, the 
Q-position signal 21 follows the N-position signal 20 with a delay of 90 
degrees. 
FIG. 3 is a schematic sectional view illustrating the configuration of the 
recording-reproduction-separated head 4. It has thin film inductive heads 
25 and 26 as data recording heads and an MR (magnetoresistive effect) head 
28 as a data reproduction head which is provided between magnetic 
interference prevention shields 27 and 29. The thin film inductive heads 
25, 26 and the MR head 28 are integrated and bonded to one side of a 
slider 24, and are protected by a protect film 30 for preventing the 
degradation and crash of the heads. 
In addition to the servo-face servo data 12, the data-face servo data 13 
are used to reduce the effect of off-tracking in each disk and head. The 
data-face servo data 13 are written as shown in FIG. 4. Specifically, 
half-track offset mode causes the data recording head to follow the servo 
track with shifting by 1/2 track and to write as data-face servo data 13, 
A and B burst signals into the data face in every other track. During 
reproduction, the data-face servo data 13 are read by the head 4 and the 
data-face positional error signal generator 6 uses the difference between 
the gains of the A and B burst signals to generate a triangular wave with 
a cycle corresponding to two tracks, as the data-face positional error 
signal 15. Both the servo head 3 and the recording-reproduction-separated 
head 4 are moved by the same actuator 10 comprising the rotary voice coil 
motor (VCM). 
It is desirable that for data recording and reproduction, only the 
data-face positional error signal 15 be used to determine the position of 
the head. However, if much data-face servo data 13 is written, the amount 
of data on the data faces 2 is reduced. Therefore, methods combining the 
data-face positional error signal 13 with the servo-face positional error 
signal 14 to generate a hybrid positional error signal have been adopted. 
Among them is a frequency division method. This method is described below 
with reference to FIG. 6. 
In FIG. 6, the actuator 10, the servo head 3, the 
recording-reproduction-separated head 4, the servo-face positional signal 
generator 5, and the data-face positional error signal generator 6 are 
identical to those shown in FIG. 1 but are simplified. In addition, a 
control positional error signal generator 63 is provided in the digital 
signal processor (DSP) 8 of FIG. 1 as described below. 
It should be noted that FIG. 6 is a functional diagram showing a method for 
generating frequency division hybrid positional error signals, and that, 
in this embodiment, a circuit consisting of an anti-alias filter and a 
switch is provided in the front stage of the control positional error 
signal generator 63 as explained in FIG. 7. 
In the control positional error signal generator 63 in FIG. 6, the 
servo-face positional error signal 14 and the data-face positional error 
signal 15 are sampled and held by sample and hold circuits 36 and 37, and 
the outputs 57 and 58 are input to a highpass filter 38 and a lowpass 
filter 39, respectively. Signals 59 and 60 output from the highpass filter 
38 and the lowpass filter 39 are added together by an adder 61 to generate 
a hybrid positional error signal 40. Either the hybrid positional error 
signal 40 or servo-face positional error signal 14 is selected by the 
switch 62 and outputted as a control positional error signal 41. 
The switch timing for the switch 62 depends upon the moving speed of the 
head 4. That is, if the head is moving fast, the reliability of the 
data-face positional error signal 15 becomes very low and the servo-face 
positional error signal 14 must be selected as a control positional error 
signal 41 instead of the hybrid positional error signal 40. 
Using the control positional error signal 41 obtained in this manner, a 
controller 34 and an observer 35 perform the control according to the 
present invention as described below. 
FIG. 7 illustrates an embodiment of a servo device according to this 
invention. This embodiment is a magnetic disk drive to which this 
invention is applied and the control positional error signal 41 is 
generated using a frequency division method as shown in FIG. 6. 
The positional error signal generators 5 and 6 in FIG. 7 refer to the 
servo-face positional error signal generator 5 and the data-face 
positional error signal generator 6, and are represented as one block. 
Therefore, there are two circuits composed of an anti-alias filter 49 and 
a switch 70 and the circuits are connected to the servo-face positional 
error signal generator 5 and the data-face positional error signal 
generator 6, respectively. The control positional error signal generator 
63 is as explained in FIG. 6. 
First, input and output signals for the DSP 8 are described. The servo-face 
positional error signal 14, the data-face positional error signal 15, and 
the detection signal 17 detected by the current detector 11 are converted 
to digital signals, which are then inputted to the DSP 8. 
The anti-alias filter 49 is a lowpass filter that has a cutoff frequency of 
twice the sampling frequency, and reduces alias noises in the servo-face 
positional error signal 14 and the data-face positional error signal 15 
after they are converted to digital signals. 
However, if the head is moving at a high speed, the anti-alias filter 49 
compresses the positional error signals 14, 15. Thus, in speed control 
mode, the positional error signals 14, 15 are used before passing the 
anti-alias filter 49. While in the position settling and following modes, 
the positional error signals 14, 15 are used after passing the filter 49. 
In addition, the detection signal 17 is virtually saturated when the head 
is accelerated but has a much smaller value in position follow-up mode. 
Therefore, the current detector 11 that detects the actuator drive current 
19 requires a very wide range. The current detector 11 thus switches the 
bit range between the speed control mode and the position following mode. 
After loading A/D converted values, the DSP 8 internally expands their 
lower bits in consideration of the effect of operational errors. 
In addition to the above signals, signals taken in the DSP 8 include 
external instruction commands, the count 16 of the number of passed track 
generated by a passed track counter 7 from the servo-face positional error 
signal 14, and a comparison signal for two-phase positional error signals 
shown in FIG. 2. The comparison signal is used in the speed control mode 
to select between the positional error signals. 
Head Speed Estimation 
The controller 34 and the observer 35 perform control in three modes: speed 
control, position settling, and position follow-up. The controller 34 
requires a head speed in each mode but the magnetic disk drive does not 
allow the direct detection of the head speed. Thus, the drive current 
detection signal 17, the count 16 of the number of passed tracks, and the 
control positional error signal 41 are inputted to the observer 35, which 
determines the position of the head to estimate the head speed. Methods 
for determining the position of the head vary according to control modes 
and are therefore described in conjunction to individual control modes. 
The observer 35 has a two-stage configuration to reduce the effect of the 
resonance component superposed on the control positional error signal 41. 
The first-stage observer inputs the drive current value 17, the count 16 
of the number of passed tracks, and the control positional error signal 41 
to estimate the head speed, and the second-stage observer then uses the 
first estimated value and the drive current value 17 to estimate the final 
head speed. 
The observer 35 recognizes as a disturbance and estimates the effect of the 
spring element of a flexible print cable (FPC) connected to the actuator 
10 as well as a magnet or spring that serves to move the head to the 
innermost circumferential side. The observer 35 uses an actuator model to 
calculate a disturbance estimated value assuming that the disturbance 
joins the drive current for the actuator 10 as a step input. The 
disturbance estimated value is subtracted from a control signal 18 that is 
the output of the DSP 8 to cancel the effect of the disturbance. 
It should be noted that the output 42 of the observer 35 in FIG. 7 includes 
the head speed and the disturbance estimated values. 
Speed Control Mode 
In the speed control mode, the actuator 10 is driven based on the 
difference between a target speed profile 55 and the speed estimated value 
42 and moves the heads 3 and 4 to the target tracks. The target speed 
profile 55 is generated by a speed profile generator 43 in response to a 
speed profile generation instruction 54 from the control unit 34. 
In the speed control mode, the observer 35 uses as positional information 
the head position determined based on the count 16 of the number of passed 
tracks and the servo-face positional error signal 14 selected depending 
upon a combination of comparison signals for the positional error signals. 
As an example of the speed profile generation circuit 43, the target speed 
profile 55 is expanded on memory in advance. If an inexpensive memory with 
a smaller number of bits is used for the speed profile, the bit range is 
changed depending upon the target speed profile value before the speed 
profile is expanded, and when the speed profile is loaded, the bits are 
shifted to reduce the effect of lost bits in the speed profile value. 
As an example of the target speed profile 55, the head speed is specified 
such that the head achieves the maximum speed when accelerated, then 
maintains a constant speed, and reduces its speed in proportion to the 
square root of the remaining distance when decelerated. This speed profile 
generation method allows the speed profile to be loaded with a small 
number of steps because the remaining distance and the addresses of the 
speed profile can be correlated. 
Bang-bang control by feed-forward is used to reduce the access time for the 
one-cylinder access operation in the speed control mode. 
Position Settling Mode 
The position settling mode is a control mode that replaces the speed 
control mode when the head approaches within a specified distance from the 
center of the target track or when the value of the head speed becomes 
smaller than the specified value. In the position settling mode, settling 
operation is performed by PD or PID control. In the PD control, a 
proportional item in proportion to the positional difference between the 
head and the center of the target track as well as a differential item in 
proportion to the speed estimated value 42 determined by the observer 35 
are used, while in PID control, an integral item that is an integral of 
the positional errors is used in addition to the preceding two items. The 
initial value of the integral item must be the final value of the last 
following operation, that is, the value when the speed control mode is 
entered. 
In the position settling mode, the observer 35 uses the positional 
information generated from only the positional error signal 40 determined 
by the number of the target tracks. The internal conflict of the observer 
35 caused by the switching of positional information is corrected by 
removing the number of passed tracks for the estimated position. 
Position Follow-up Mode 
The position follow-up mode is a control mode that replaces the position 
settling mode after a certain time and is controlled by the same system as 
the position settling mode. However, the position settling mode uses a 
gain focusing on a quick response to execute proper settling operation 
while the position follow-up mode uses a gain focusing on the compression 
rate to execute proper following operation. 
In each of the above control modes, the second-stage observer is used to 
reduce the effect of the resonance of the actuator 10. However, to 
directly reduce the effect of resonance, control outputs to the actuator 
10 may be made to pass a digital notch filter to reduce the signal outputs 
of the resonance frequency before outputting from the DSP 8. 
The actuator parameters, however, may vary in accordance with temperature 
changes, the position of the head and so on, resulting in an excess 
response in the position settling mode or the like. Therefore, parameters 
estimation and correction are required. 
Parameter Estimation 
Estimation of parameters of the actuator 10 is performed by a parameter 
estimation unit 45 in the following procedure. 
1) Since the parameters for the actuator 10 vary according to the positions 
of the heads 3 and 4, the controller 34 drives the actuator 10 to move the 
heads 3 and 4 to positions where measurements are desired and to make them 
perform following operation. 
2) The parameter estimation unit 45 then outputs a parameter estimation 
start instruction 51 and closes a switch 68. This causes a reference sine 
wave signal 52 with a specified frequency generated by a sine wave 
generator 44 to join control outputs 64 from the controller 34 via the 
switch 68, resulting in generation of a DSP control signal 18. According 
to the control signal 18 that contains a sine wave component, the power 
amplifier 9 drives the actuator 10 to vibrate the heads 3 and 4 at the 
specified frequency. 
3) The drive current detection signal 17 for several cycles at the 
specified frequency, the control positional error signal 41 generated by 
the vibrating heads 3 and 4, and the reference sine wave signal 52 are 
inputted to the parameter estimation unit 45, which performs the discrete 
Fourier transformation to calculate the input/output gains of the actuator 
10. Taking the ratio of the input/output gains provides a parameter 
estimated value 53 for the specified frequency, which is then stored in 
memory 46. 
To reduce the effect of eccentricity, it is desirable to select the 
frequency of the reference sine wave signal 52 while avoiding the 
rotational frequency of the disk. The period of the reference since wave 
signal 52 is determined to be integer times of the sampling period of the 
DSP 8. To remove the effect of noise around a specific frequency, it is 
also desirable to carry out similar measurements at different frequencies 
so as to take an average of these values. To generate reference sine waves 
for this purpose, an integral multiple of frequencies is generated by 
selectively reading the values from a sine wave table in a reference sine 
wave signal generator 44. 
Parameter Correction 
The parameter correction is performed by a multiplier 47 and a divider 48 
using the estimated parameter value Kf for the actuator 10 stored in a 
memory 46. That is, the drive current value 17 detected by the current 
detector 11 is multiplied by the parameter estimated value Kf and the 
result 65 is inputted to the observer 35. Thus, in the head speed 
estimation, the disturbance estimation, and the operation in each mode 
described above, the observer 35 uses as a drive current detection value a 
signal 65 that is a multiple of a drive current value and a parameter 
estimated value Kf. In addition, the control output of the control unit 34 
is divided by the parameter estimated value Kf and the result is outputted 
to the power amplifier 9 as a DSP control signal 18 to control the 
operation of the actuator 10. 
Thus, actuator parameter variations due to the characteristics of the 
actuator 10 or secular changes are corrected based on parameter estimated 
values in the memory 46 updated each time the parameter estimation is 
performed. 
Operational Sequence of DSP 8 
(1) Start-up 
When the power to the disk drive is turned on and the drive receives an 
instruction to cause the spindle to rotate the disks, it causes the 
maximum current to flow through the spindle motor, which then starts 
rotating. Once the speed reaches 90% of the reference rotational speed, a 
signal from the external microcomputer which observes the rotational speed 
of the spindle through a Hall encoder causes the DSP 8 to start 
controlling the rotational speed of the spindle. 
When the rotational speed of the spindle follows the reference rotational 
speed and the servo-PLO (Phase-locked Oscillator) is locked, the sampling 
frequency of the DSP 8 is switched from the clock frequency of a 
oscillator to the clock frequency obtained from the disk face to match the 
frequency of the servo signal obtained from the disk face with the 
frequency of servo signal input timing for the DSP 8. In other words, the 
DSP 8 enters into zero mode. 
The zero mode is an operation mode which initializes variables and flows 
currents to the actuator 10 to move the heads 3 and 4 to the innermost 
circumference side. While the actuator 10 does not receive currents, the 
spring element of the FPC and the magnet cause the heads 3 and 4 to be 
located on the innermost circumference as described above. In the zero 
mode, the actuator 10 further applies pressure to the heads 3 and 4 to 
secure them. 
Then, the control of the actuator starts in parallel to the control of the 
rotational speed of the spindle. 
(2) Return-to-zero Operation 
The return-to-zero operation is performed by moving the head to the 
cylinder 0. In this case, the cylinder 0 shall be where the following 
operation is performed in response to the -N position signal for the 
outermost circumference of the data region. The return-to-zero operation 
is described below with reference to FIG. 5. 
When the DSP 8 receives a return-to-zero instruction, it switches to the 
speed control mode with the target speed profile set at a constant speed 
of 100 mm/sec or below, moves the head at the constant speed, and monitors 
the input of an outer guard band (OGB) signal obtained in an outer guard 
band region 31 outside a data region 32. It starts reducing the speed 
after obtaining the OGB signal. Once it completes deceleration and 
confirms that the head has performed following operation in a certain 
cylinder, it starts moving the head to the cylinder 0. In moving the head 
back in the speed control mode, it uses a slow speed value of 10 mm/sec or 
below so that a deceleration profile will not be required as a target 
speed profile. After it determines based on the absence of the OGB signal 
that the head reaches the cylinder 0, it performs following operation in 
response to the -N position signal and completes the return-to-zero 
operation to enter the command acceptance state. 
(3) Parameter Estimation and Update Operations 
As an initial operation, the parameters for the actuator 10 are estimated 
as described above and the estimated values 53 are stored in the memory 46 
for updating. In this case, the number of points where parameter 
estimation is executed depends upon the characteristics of the actuator 
10. For example, a three zone method vibrating the heads 3 and 4 on the 
innermost circumference, center, and outermost circumference for parameter 
estimation is available. Other methods performing estimation at several 
points and supplementing these estimated values to obtain sequential 
values are also available. Before parameter estimation, the head is moved 
in the speed control and the position settling modes using the nominal 
values of parameters. 
(4) Access Operation 
Once a parameter estimated value is thus updated, the updated value is used 
to perform access operation in the speed control and the position settling 
modes. However, the bang-bang control mentioned above is used in the speed 
control mode with one track access operation. 
When a composite head with separate reproduction and recording heads is 
used, offset occurs in the center of the composite head because there is a 
gap between the two heads. Since the amount of offset varies according to 
the position of the heads, offset correction is required for the position 
of each head. Therefore, the amount of offset for the head is determined 
based on the position of the target track and the type of the head before 
initiating access operation, and access operation is executed according to 
the target position with the amount of offset considered. The amount of 
offset must also be considered in switching between the recording and the 
reproduction heads during the following operation. 
However, the head information for the observer 35 must always be free from 
the offset regardless of the difference of the heads. 
A series of operation controls by the DSP 8 has been described. The 
switching operation for hybrid positional error signals in each mode is 
described below. 
Switching Operation for Positional Error Signals 
As described above, the switch 62 causes the control positional error 
signal generator 63 shown in FIG. 6 to select either the servo-face 
positional error signal 14 or hybrid positional error signal 40 and 
outputs it as a control positional error signal 41. For example, in the 
speed control mode, the servo-face positional error signal 14 is used as 
the head position information to calculate distances and the mode switch 
points to generate a target speed profile 55. While in the position 
settling and the follow-up modes, the hybrid positional error signal 40 is 
used as the head position information to calculate the proportional and 
integral items. However, the observer 35 always uses the servo-face 
positional error signal 14. 
It has been noted that the frequency division method can be used as a 
method for generating the hybrid positional error signals in this 
invention but a reference servo method is also available. As shown in FIG. 
5, this method writes the servo data 12, 13 into the guard band regions 
31, 33 outside the data region 32 on the data face and uses the servo data 
to measure the amount of off-tracking on the data face 2 and the servo 
face 1. It then determines the amount of off-tracking in the data region 
32 using the amount of off-tracking in the guard band regions 31, 33 on 
the inner circumferential side 31 and the outer circumferential side 33. 
Finally, it adds the determined amount to the servo-face positional error 
signal 14 to generate a hybrid positional error signal 40. Since this 
method allows the amount of off-tracking on each data face to be measured 
in advance to execute the operation using only the servo-face positional 
error signal 14, it does not require the switching of the hybrid 
positional error signal 40 and the servo-face positional error signal 14 
as in the frequency division method. In addition, since the amount of 
off-tracking is known, the switching of the amount of off-tracking for 
access operation is executed similarly to the compensation for the 
recording-reproduction-separated head. In this method, the observer 35 
always uses only the servo-face positional error signal 14 with no offset 
because the amount of off-tracking varies according to the position of the 
head. 
Also, in this embodiment, the switch 70 works in such a way that the 
positional error signal is used before passing the anti-alias filter 49 in 
the speed control mode, while a similar signal is used after passing the 
anti-alias filter 49 in position settling and following modes. 
It is also possible to execute switching based on the head speed that 
generates the positional error signals 14, 15 at the frequency determined 
by the bands of the lowpass filter that is an anti-alias filter 49. 
However, the head speed is determined by estimated values from the 
observer 35. 
FIG. 8 is a block diagram of the head control system illustrating the 
parameter correction part of the actuator 10 in this embodiment. FIG. 9 is 
a block diagram of the control system of the disk drive with respect to 
FIG. 8. The block configuration in FIG. 8 corresponds to the parameter 
update and correction part in the configuration of this embodiment shown 
in FIG. 7, and FIG. 9 is a simplified version of the configuration in FIG. 
1. 
In FIG. 9, a head 101 reads servo information 102 from the disk 1 and a 
positional error signal generator 103 generates positional error signals 
104. A positional error signal generator 105 in FIG. 8 includes a 
positional error signal generator 103 and a passed track counter 7 and 
generates positional error signals 104, the number of passed tracks 16, 
and head position information 67. Parameter estimation and correction 
operations have already been described. 
As described above, the head control system according to this invention can 
estimate parameters for the actuator that drives the head and use them 
accordingly to correct the preset parameters for the control system. 
Therefore, even if the actuator parameters vary due to the characteristics 
of the actuator or temperature changes, servo characteristics can be kept 
stable. 
In addition, the position or speed of the head can determine whether or not 
the head positional error signal should be made to pass the anti-alias 
filter (lowpass filter) before inputting to the controller, thereby 
allowing the position of the head to be detected precisely regardless of 
head control mode.