Method and apparatus for controlling the position of an object along a radial direction of a rotating body

A position control method for use with a hard disk drive or other rotating body is provided. A radial position of a rotating body corresponding to an object to be controlled is detected as a current object position. Subsequently, a control operation signal is generated which is representative of a deviation between a target object position expressed in terms of the radial position of the rotating body and the current object position detected. In addition, an actuator manipulation-amount signal is output which is derived from the control operation signal. This actuator manipulation-amount signal is formed by selectively summing an integration signal with either a first or a second control signal. The first control signal is a function of the current object position. The second control signal is a function of a predicted object position at a second predetermined time after generation of the control operation signal. At intervals of a first predetermined time, this process is repeated such that movement of the object by an actuator is controlled by the actuator manipulation-amount signal. A position control apparatus and information recording system which implement this position control method are also provided.

FIELD OF THE INVENTION 
The present invention relates generally to a position control apparatus and 
a position control method. More particularly, the present invention 
relates to a position control method of controlling the position of a 
controlled object which is moved to a position corresponding to each 
position along the radial direction of a rotating body by an actuator and 
to a position control apparatus to which that position control method is 
applicable. 
BACKGROUND OF THE INVENTION 
In hard-disk drives (HDDs), a digital closed loop control for controlling 
the position of a magnetic head is performed by detecting the magnetic 
head position at intervals of predetermined time and by controlling a 
current passing through the voice coil of a voice coil motor which moves 
the magnetic head in accordance with a deviation between the detected 
current position of the magnetic head and the target position of the 
magnetic head. FIG. 11 conceptually shows a typical structure of a control 
elements 72, 74, 76, 78, 80, 82, 90, 112 where, in the closed loop control 
system of the HDD, a head position signal y(n) representative of a 
deviation between the current position and target position of the magnetic 
head is input at intervals of predetermined time and, based on the input 
head position signal y(n), a motor control signal u(n) is generated and 
output. 
The transfer function H(z) (ratio of z conversion between an input signal 
and an output signal) of the control elements shown in FIG. 11 is 
expressed by the following Equation 1. In this control elements, a motor 
current control signal u(n) is generated and output according to the 
transfer function of Equation (1), based on the head position signal y(n) 
input at intervals of predetermined time. 
##EQU1## 
In a digital closed loop control system for controlling the position of an 
object, the positioning accuracy for positioning the object relative to 
its target position is enhanced as a cycle becomes shorter (i.e., the 
sampling cycle for detecting the object position). However, it is 
generally known that the sampling cycle cannot be made shorter than a 
predetermined time because of various limitations. In order for the 
positioning accuracy to be enhanced without making the sampling cycle 
shorter, oversampling (where an output is switched at a cycle shorter than 
a sampling cycle) can be performed. For example, oversampling is used when 
a digital speech signal is recorded on a compact disk (CD) for subsequent 
playback as music. 
In the reproduction of the speech signal recorded on the CD, during the 
intermediate period of time between the time that sampling is performed at 
a certain time and the time that next sampling is performed, a signal is 
output by predicting a sampling value with interpolation. In the closed 
loop control system which controls the position of the magnetic head of 
the HDD, if control is performed by using the predicted value of the head 
position during the intermediate period of the sampling cycle, as is the 
above case, then the position of the magnetic head will be able to be 
controlled finely at a cycle shorter than the sampling cycle. However, 
since the predicted value of the head position includes a slight error 
with respect to an actual head position, there is the problem that the 
positioning accuracy as the magnetic head is positioned to the target 
position is not always enhanced due to the influence of the 
above-described error. 
More specifically, even if in the HDD the magnetic head were in a steady 
state where the head position has matched with the target position 
(deviation=0), an external bias (e.g., a bias that the drive circuit of 
the voice coil motor has) would occur at all times. For this reason, when 
the magnetic head is positioned to the target position, an amount of 
manipulation balancing with the external bias must be output even when the 
magnetic head is in the steady state of deviation=0, in order to stop the 
movement of the magnetic head when the head position matches with the 
target position. Therefore, an element for performing an integration 
operation is generally added to the control elements which generate and 
output the motor current control signal u(n). In FIG. 11 a transfer 
element 150, a one-sample delay element 152, and a summing point 154 
correspond to the above-described element for performing an integration 
operation. 
Since, on the other hand, the predicted value of the head position includes 
an error, as described above, a large error occurs between the amount of 
manipulation of the integration operation as the detected value of the 
magnetic head position was used and the amount of manipulation of the 
integration operation as the predicted value of the magnetic head position 
was used, and therefore the amount of manipulation by the integration 
operation changes in an oscillating manner and does not become a fixed 
value. For this reason, the movement of the magnetic head would not be 
stopped even if the magnetic head matched with the target position. Also, 
the movement of the magnetic head would be stopped before the magnetic 
head matches with the target position. Also, the magnetic head, which has 
already matched with the target position, would be moved to a position 
departed from the target position. It is therefore difficult to position 
the magnetic head to the target position with a high degree of accuracy. 
A need exists for a position control apparatus and method which is capable 
of enhancing the positioning accuracy as an object to be controlled is 
positioned to its target position. 
The present invention provides a solution to this and other problems, and 
offers other advantages over the prior art. 
SUMMARY OF THE INVENTION 
A position control method for use with a hard disk drive or other rotating 
body is provided. A radial position of a rotating body corresponding to an 
object to be controlled is detected as a current object position. 
Subsequently, a control operation signal is generated which is 
representative of a deviation between a target object position expressed 
in terms of the radial position of the rotating body and the current 
object position detected. In addition, an actuator manipulation-amount 
signal is output which is derived from the control operation signal. This 
actuator manipulation-amount signal is formed by selectively summing an 
integration signal with one of a first and a second control signal. The 
first control signal is a function of the current object position derived 
from the control operation signal. The second control signal is a function 
of a predicted object position at a second predetermined time after 
generation of the control operation signal which is derived from the 
control operation signal. At intervals of a first predetermined time, this 
process is repeated such that movement of the object by an actuator is 
controlled by the actuator manipulation-amount signal. 
A position control apparatus and information recording system which 
implement this position control method are also provided. 
Alternatively, the preferred embodiment position control apparatus of the 
present invention for overcoming the above problem can be described as 
comprising a detector for detecting a radial position of a rotating body 
that an object to be controlled corresponds to, as a current position of 
said object to be controlled, at intervals of first predetermined time; 
signal output for outputting a control operation signal representative of 
a deviation between a target position of said object expressed in terms of 
the radial position of said rotating body and the current position of said 
object detected by said detector, at intervals of said first predetermined 
time; and controller for controlling a movement of said object by an 
actuator, by outputting, when the control operation signal is output from 
said signal, a manipulation-amount signal representative of an amount of 
manipulation where amounts of manipulation each corresponding to said 
control operation signal, executed by a plurality of control operations 
including an integration operation for matching the position of said 
object with said target position, are summed up, and then by switching an 
amount of manipulation executed by the control operations other than said 
integration operation after a second predetermined time shorter than said 
first predetermined time and outputting the manipulation-amount signal 
representative of an amount of manipulation where amounts of manipulation 
executed by said plurality of control operations are summed up. 
Also, in the alternative description of the present invention preferred 
embodiment apparatus, said rotating body may be a data recording medium 
where a plurality of concentric circular tracks are formed and where data 
can be recorded on each track, and said object to be controlled may be a 
head which is provided with at least a function of reading out said data 
recorded on the tracks of said rotating body. 
Also, in the alternative description of the present invention preferred 
embodiment apparatus, said data recording medium may be a magnetic disk of 
a hard-disk drive, and said head may be a magnetic head of the hard-disk 
drive which is provided with at least a function of reading out said data 
recorded on a track of said magnetic disk. 
Also, in the alternative description of the present invention preferred 
embodiment apparatus, the second predetermined time may be integer 
fraction (e.g., 1/n, where n is an integer) of the first predetermined 
time. 
Also alternatively, the position control method according to the present 
invention can be described as comprising the steps of detecting a radial 
position of a rotating body that an object to be controlled corresponds 
to, as a current position of said object to be controlled; generating a 
control operation signal representative of a deviation between a target 
position of said object expressed in terms of the radial position of said 
rotating body and the current position of said object detected; outputting 
a manipulation-amount signal representative of an amount of manipulation 
where amounts of manipulation each corresponding to the control operation 
signal, executed by a plurality of control operations including an 
integration operation for matching the position of said object with said 
target position, are summed up, at intervals of first predetermined time, 
and also, after a second predetermined time shorter than said first 
predetermined time elapses from the time said manipulation-amount signal 
is output, switching an amount of manipulation executed by the control 
operations other than said integration operation and repeating outputting 
the manipulation-amount signal representative of an amount of manipulation 
where amounts of manipulation executed by said plurality of control 
operations are summed up, thereby controlling a movement of said object by 
an actuator. 
Also, in the alternative description of the position control method 
according to the present invention, the second predetermined time may an 
integer fraction (e.g., 1/n, where n is an integer) of the first 
predetermined time. 
These and various other features as well as advantages which characterize 
the present invention will be apparent upon reading of the following 
detailed description and review of the associated drawings.

DETAILED DESCRIPTION 
In Equation 1, which represents the transfer function of the control 
elements which generate and output a manipulation-amount signal with a 
control operation signal representative of a deviation between the target 
position and current position of an object to be controlled, if the 
integration operation of a plurality of control operations, which the 
transfer function represents, and the control operations other than the 
integration operation are separated, then Equation 1 will be converted 
into Equation 2 which can be expressed as the sum of two transfer 
functions. 
##EQU2## 
In Equation 2, the second term represents a transfer function corresponding 
to the integration operation and the first term represents a transfer 
function corresponding to the operations other than the integration 
operation. For reference, the structure of the control elements, 72, 74, 
76, 78, 80, 82, 88, 90, 112 where the transfer function is expressed by 
Equation 2, is shown in a block diagram of FIG. 12. In Equation 2, since, 
in the steady state where the current position of an object to be 
controlled has matched with the target position, the influence of the 
value of the first term of Equation 2 is small with respect to an amount 
of manipulation obtained by the above-described transfer function, if the 
value of the second term is appropriate, then a controlled object will be 
able to be controlled with a high degree of accuracy so that the position 
of the object can match with the target position. 
Based on the above, in a preferred embodiment of the present invention the 
current position of an object to be controlled is detected at intervals of 
first predetermined time by the detector. From the signal output, a 
control operation signal representative of a deviation between a target 
position of the object and the current position of the detected object is 
output at intervals of the first predetermined time. If the control 
operation signal is output from the signal output, the controller will 
output a manipulation-amount signal representative of an amount of 
manipulation where amounts of manipulation each corresponding to the 
control operation signal, executed by a plurality of control operations 
including an integration operation for matching the position of the object 
with the target position, are summed up. Then, an amount of manipulation 
executed by the control operations other than the integration operation is 
switched after a second predetermined time shorter than the first 
predetermined time, and the manipulation-amount signal representative of 
an amount of manipulation, where amounts of manipulation executed by the 
plurality of control operations are summed up, is output. With this, the 
movement of the object by an actuator is controlled. 
In accordance with the above, the amount of manipulation by the integration 
operation is switched according to the deviation between the target 
position of the object and the detected current position of the object, at 
intervals of the first predetermined time. Therefore, the amount of 
manipulation by the integration operation does not include a predicted 
error, which will be caused by prediction of the current position of the 
object to be controlled, and will become an appropriate value balancing 
with an external bias if the current position of the object matches with 
the target position. Also, when the current position of the object matches 
with the target position, the amount of manipulation by the control 
operations other than the integration operation (to which the result of 
the calculation of the first term of Equation 2 corresponds) has a little 
influence on the above-described amount of manipulation where amounts of 
manipulation by a plurality of control operations are summed up, as 
described above. Therefore, a manipulation-amount signal representative of 
an amount of manipulation substantially equal to an appropriate amount of 
manipulation balancing with an external bias, based on the integration 
operation, is to be output as a manipulation-amount signal at intervals of 
time shorter than the first predetermined time. 
Thus, in the preferred embodiment of the present invention, when the 
current position of a controlled object matches with its target position 
or is in its steady state, since a manipulation-amount signal 
representative of an amount of manipulation equal to or substantially 
equal to an appropriate amount of manipulation balancing with an external 
bias is output at intervals of time shorter than a conventional control 
cycle (first predetermined time), the positioning accuracy as the object 
is positioned to the target position is enhanced. 
If it is considered only to position a controlled object to its target 
position, then the switching of an amount of manipulation, which is 
performed by the control operations other than the integration operation 
after the second predetermined time shorter than the first predetermined 
time elapses from the time that a manipulation-amount signal is output 
according to the control operation signal output by the signal output, may 
be made so that the amount of manipulation by the control operations other 
than the integration operation becomes zero. But, if at the same time 
there is considered a case where, from the state that there is a great 
deviation between the current position and target position of the object 
to be controlled, the object is moved and positioned to the target 
position, then the amount of manipulation by the control operations other 
than the integration operation may be switched to an amount of 
manipulation corresponding to the predicted position of the object as the 
second predetermined time elapses, or the transfer function of the control 
operations other than the integration operation may be preset so that the 
evaluation by a predetermined evaluation function becomes optimum and may 
be switched to an amount of manipulation which is obtained by the 
above-described transfer function. With this, even when there is a great 
deviation between the current position and target position of the object 
to be controlled, the movement of the object can be finely controlled at 
intervals of short time. 
Also, in the preferred embodiment of the present invention, the rotating 
body may be a data recording medium where a plurality of concentric 
circular tracks are formed and where data can be recorded on each track, 
and the object to be controlled may be a head which is provided with at 
least a function of reading out the data recorded on the rotating body. 
Also, the data recording medium may be a magnetic disk of a hard-disk 
drive, and the head may be a magnetic head of the hard-disk drive which is 
provided with at least a function of reading out the data recorded on a 
track of the magnetic disk. 
Also, in a preferred embodiment position control method according to the 
present invention, the current position of an object to be controlled is 
detected, and a control operation signal representative of a deviation 
between the target position of the object and the detected current 
position of the object is generated. A manipulation-amount signal 
representative of an amount of manipulation, where amounts of manipulation 
each corresponding to the control operation signal, executed by a 
plurality of control operations including an integration operation for 
matching the position of the object with the target position, are summed 
up, is output at intervals of first predetermined time, and also, after a 
second predetermined time shorter than the first predetermined time 
elapses from the time the manipulation-amount signal is output, an amount 
of manipulation executed by the control operations other than the 
integration operation is switched. And, outputting the manipulation-amount 
signal representative of an amount of manipulation, where amounts of 
manipulation executed by the plurality of control operations are summed 
up, is repeated. With this, the movement of the object by an actuator is 
controlled. Accordingly, as with the above, the positioning accuracy as 
the object to be controlled is positioned to the target position can be 
enhanced. 
A preferred embodiment of the present invention will hereinafter be 
described in detail with reference to the accompanying drawings. Note that 
the preferred embodiment described with certain numerical values, but is 
not limited to only using the numerical values described hereinafter. 
FIG. 1 shows a hard disk drive 10 according to this embodiment of the 
present invention. The hard disk drive 10 is provided with a drive unit 
14, which will spin a shaft 12 at constant high speed if power is applied. 
The shaft 12 has attached thereto a cylindrical spindle 16 so that the 
axes thereof are vertically aligned with each other. On the outer 
peripheral surface of the spindle 16 there are mounted a disk 18. 
The disk 18 has a disk shape with a predetermined thickness dimension and 
is formed with hard material. Both sides of the disk are coated with 
magnetic material and used as recording surfaces. The central portion of 
the disk 18 is formed with a hole having the substantially same diameter 
as the outer diameter of the spindle 16. The spindle 16 is inserted into 
the center hole of the disk 18, and the disk 18 is fixed to the outer 
peripheral surface of the spindle 16. Therefore, if power is applied to 
the hard disk drive 10 and the shaft 12 is rotated by the drive unit 14, 
then the disk 18 will be rotated together with the spindle 16. 
On each recording surface of the disk 18 a plurality of position detection 
data recorded areas 50 are radially formed along the radial direction of 
the disk 18, as shown in FIG. 2. On the remaining areas there are formed a 
plurality of data track areas 52. In FIG. 3 there are shown a portion of 
the position detection data recorded area 50 and a portion of the data 
track area 52. On the data track area 52 a plurality of data tracks are 
concentrically formed at intervals of pitch P, and FIG. 3 shows the data 
tracks 54A to 54C. Data is written to or read from each data track 54 
along the circumferential direction (indicated by arrow A in FIG. 3) of 
the disk 18 with a magnetic disk to be described later. 
On the other hand, on the position detection data recorded area 50 there 
are provided a track identification data recorded area 50A and a burst 
pattern recorded area 50B. On the track identification data recorded area 
50A, track identification data, which represents the track address of each 
data track in Gray code (cyclic binary code) in correspondence with each 
data track 54, is recorded. Also, on the burst pattern recorded area 50B 
there are formed burst patterns. As shown in FIG. 3, the burst patterns 
consist of four burst pattern rows (burst pattern rows A to D) where 
signal recorded areas (hatched portions in FIG. 3) are arranged in the 
direction of the arrangement of the data track 54, i.e., along the radial 
direction of the disk 18. The length of each signal recorded area in the 
radial direction of the disk 18 and the space between adjacent signal 
recorded areas are equal to the pitch P between adjacent data tracks. 
The signal recorded areas 50a of the burst pattern row A and the signal 
recorded areas 50b of the burst pattern row B are arranged in a zigzag 
manner along the radial direction of the disk 18, and the both ends of 
each signal recorded area in the radial direction of the disk correspond 
to the centers of the data tracks 54 in the width direction thereof. The 
burst pattern rows A and B are formed by recording a signal on each area. 
The signal recorded areas 50c of the burst pattern row C and the signal 
recorded areas 50d of the burst pattern row D are arranged in a zigzag 
manner along the radial direction of the disk 18, and the both ends of 
each signal recorded area in the radial direction of the disk correspond 
to the boundary between adjacent data tracks. The burst pattern rows C and 
D are formed by recording a signal on each area. 
Also, as shown in FIG. 1, the hard disk drive 10 further includes magnetic 
heads 20A and 20B provided in correspondence with the recording surfaces 
of the disk 18. Each of the magnetic heads 20A and 20B includes a read 
element (not shown) which reads data from the recording surface with an MR 
element and also includes a write element (not shown) which writes data to 
the recording surface with a coil. The magnetic head 20A is mounted on one 
end of an access arm 22A and held in a position slightly (for example, 
about 0.1 to 0.2 microns) spaced from the corresponding record surface of 
the disk 18. Likewise, the magnetic head 20B is mounted on one end of an 
access arm 22B and held in a position slightly (for example, about 0.1 to 
0.2 microns) spaced from the corresponding record surface of the disk 18. 
The other end of each of the access arms 22A and 22B is mounted on a drive 
unit 24. 
The drive unit 24 includes voice coil motors 26 (see FIG. 4) which are 
provided in correspondence with the access arms 22A and 22B to move the 
arms. If the voice coil motors 26 are driven by a micro processing unit to 
be described later, the access arms 22A and 22B will be moved so that the 
magnetic heads 20A and 20B move along the radial direction of the disk 18. 
With this arrangement, the magnetic heads 20A and 20B can be positioned 
over desired positions on the recording surfaces of the disk 18. 
The magnetic heads 20A and 20B are connected to each of the circuits shown 
in FIG. 4. That is, the signal output terminal of the magnetic head 20 is 
connected to the input terminal of an amplifier 28 so that the signal 
output from the read element of the magnetic head 20 is amplified with the 
amplifier 28. The output terminal of the amplifier 28 is connected to the 
input terminal of an analog-digital (A/D) converter 30. The output 
terminal of the A/D converter 30 is connected to the signal input terminal 
of a micro processing unit (MPU) 32, so the analog signal output from the 
amplifier 28 is converted into a digital signal with the A/D converter 30 
and output to the MPU 32. 
The MPU 32 decides the position of the magnetic head 20, based on the 
signal input from the A/D converter 30. According to the deviation between 
the decided position of the magnetic head 20 and the target position of 
the magnetic head 20, the MPU 32 generates a digital signal for 
controlling the position of the magnetic head 20 (more specifically, motor 
current control signal for controlling a current passing through the voice 
coil of the voice coil motor 26), as will be described later, and outputs 
the digital signal to a driver 34 connected to the MPU 32. Based on the 
input signal, the driver 34 controls a current passing through the voice 
coil of the voice coil motor 26. With this arrangement, the magnetic head 
20 is moved so that the position of the magnetic head 20 matches with the 
target position. 
The operation of this embodiment will be described. In FIG. 5, among 
various functions the MPU 32 has, there is shown each function of a 
control system for realizing functions which control the position of the 
magnetic head 20. 
The signal output from the A/D converter 30 is input to a head current 
position computing section 60 and a burst pattern detecting section 62. 
The burst pattern detecting section 62 decides if the magnetic head 20 
corresponds to the burst pattern recorded area 50B, based on the input 
signal, and outputs the result of the decision to the head current 
position computing section 60. The head current position computing section 
60 fetches a signal from the A/D converter 30, when the magnetic head 20 
is decided to correspond to the burst pattern recorded area 50B by the 
burst pattern detecting section 62. Based on that signal from the A/D 
converter, the head current position computing section 60 computes and 
outputs a position along the radial direction of the disk 18 that the 
magnetic head 20 currently corresponds to, i.e., the current position of 
the magnetic head 20. Therefore, from the head current position computing 
section 60, the head current positions are output at intervals of 
predetermined cycle (sampling cycle, Ts, corresponds to a first 
predetermined time). 
Also, a head target position setting section 64 sets and outputs the target 
position of the magnetic head 20 which is expressed in terms of a position 
along the radial direction of the disk 18. When there is an offset or 
shift of the longitudinal center position of each of the gaps 
corresponding to the read and write elements of the magnetic head 20, the 
head target position setting section 64 sets and outputs values which are 
different between the time that data is read from the data track 54 and 
the time that data is written to the data track 54 as the target position 
of the magnetic head 20. For example, when data is read out, one value is 
set and output so that the center of the gap of the read element is 
aligned with the center of the data track 54, and when data is written in, 
another value is set and output so that the center of the gap of the write 
element is aligned with the center of the data track 54. 
The head current position output from the head current position computing 
section 60 and also the head target position output from the head target 
position setting section 64 are input to a head position signal generating 
section 66. The head position signal generating section 66 compares the 
input head current position and head target position, and outputs a head 
position signal y(n) which represents the size and direction of the 
deviation of the head current position to the head target position 
(whether the head current position with respect to the head target 
position is shifted toward the inner circumferential side or outer 
circumferential side of the disk 18) in terms of a digital value, at 
intervals of sampling cycle Ts. 
Note that the head position signal y(n) corresponds to the control 
operation signal. The burst pattern detecting section 62 and the head 
current position computing section 60 correspond to the detector. The head 
position signal generating section 66 corresponds to the signal output. 
The head position signal y(n) output from the head position signal 
generating section 66 is input to a main control section 68, which 
corresponds to the controller. FIG. 6 shows the processing of the main 
control section 68 conceptually from the relationship between the input 
and output generated by that processing. 
As shown in FIG. 6, the head position signal y(n) is input to a transfer 
element 72 where the transfer function is k1, through an outgoing point 
70. The signal output from the transfer element 72 is input to a summing 
point 74. Also, the head position signal y(n) is input to a one-sample 
delay element 76 where the transfer function is 1/z, through the outgoing 
point 70. The signal output from the one-sample delay element 76 is input 
to a transfer element 78 where the transfer function is k2. The signal 
output from the transfer element 78 is input to the summing point 74. 
Also, the signals, which are output from a transfer element 80 where the 
transfer function is k3, a transfer element 82 where the transfer function 
is k4, and a transfer element 84 where the transfer function is k5, are 
input to the summing point 74. 
The summing point 74 outputs a first control signal c1(n) equivalent to the 
sum of the signals output from the transfer elements 72, 78, 80, 82, and 
84. This first control signal c1(n) is input to a summing point 88 through 
an outgoing point 86 and also to a one-sample delay element 90 where the 
transfer function is 1/z. The signal output form the one-sample delay 
element 90 is input to the above-described transfer element 80 and a 
transfer element 92 where the transfer function is h3. 
Also, the head position signal y(n) is input to a transfer element 94 where 
the transfer function is h1, through the outgoing point 70. The signal 
output from the transfer element 94 is input to a summing point 96. 
Further, the head position signal y(n) is input to a one-sample delay 
element 98 where the transfer function is 1/z, through the outgoing point 
70. The signal output from the one-sample delay element 98 is input to a 
transfer element 100 where the transfer function is h2. The signal output 
from the transfer element 100 is input to the summing point 96. Also, the 
signals, which are output from the above-described transfer element 92, a 
transfer element 102 where the transfer function is h4, and a transfer 
element 104 where the transfer function is h5, are input to the summing 
point 96. 
The summing point 96 outputs a signal which is equivalent to the sum of the 
signals input from the transfer elements 92, 94, 100, 102, and 104. This 
signal is output as a second control signal c2(n) to a summing point 108 
through an outgoing point 106 and is also input to a one-sample delay 
element 110 where the transfer function is 1/z. The signal output from the 
one-sample delay element 110 is input to the above-described transfer 
elements 84, 104 and a one-sample delay element 112 where the transfer 
function is 1/z. The signal output from the one-sample delay element 112 
is input to the above-described transfer elements 82 and 102. 
Further, the head position signal y(n) is input to a summing point 114 
through the outgoing point 70. The signal output from the summing point 
114 is input to a one-sample delay element 116 where the transfer function 
is 1/z. The signal output from the one-sample delay element 116 is input 
to the summing point 114 and a transfer element 118 where the transfer 
function is ki. Note that the integration operation in the main control 
section 68 is performed by the summing point 114, the one-sample delay 
element 116, and the transfer element 118, and the transfer element 118 
outputs an integration signal in(n) representative of an amount of 
manipulation which is executed according to the head position signal y(n) 
by the integration operation. The integration signal in(n) output from the 
transfer element 118 is input to summing points 88 and 108 through an 
outgoing point 120. 
The summing point 88 outputs a signal which is equivalent to the sum of the 
first control signal c1(n) input from summing point 74 and the integration 
signal in(n) input from the transfer element 118. This signal is input as 
a first motor control signal u1(n) to a switching section 122. Also, the 
summing point 108 outputs a signal which is equivalent to the sum of the 
second control signal c2(n) input from a summing point 96 and the 
integration signal in(n) input from the transfer element 118. This signal 
is input as a second motor control signal u2(n) to the switching section 
122. 
FIG. 6 conceptually shows the switching section 122 as a switch. The 
switching section 122 selectively outputs either the first motor control 
signal u1(n) or second motor control signal u2(n) as a motor current 
control signal u(n) which represents a control amount of motor current in 
terms of a digital value, at timings to be described later. The motor 
current control signal u(n) corresponds to the manipulation-amount signal. 
More specifically, the head position signal y(n) is input to the main 
control section 68 at intervals of sampling cycle Ts, as shown in FIG. 7, 
but if the delay time d caused by calculation elapses from the time the 
head position signal y(n) is input at a certain timing and if the first 
and second motor control signals u1(n) and u2(n) corresponding to that 
head position signal y(n) are input, then the first motor control signal 
u1(n) will first be output as a motor control signal u(n). Then, a signal 
to be output as the motor control signal u(n) is switched to the second 
control signal u2(n) after a predetermined time p1, which is shorter than 
the sampling cycle Ts, elapses from the time the first motor control 
signal u1(n) is output. In this embodiment, the predetermined time p1 is 
Ts/2 and corresponds to a second predetermined time. 
During the period of time that this second motor control signal u2(n) is 
being output, the head position signal y(n+1) of next cycle is input. And, 
a signal, which is to be output as the motor current control signal 
u(n+1), will be switched to the first motor control signal u1(n+1) if a 
predetermined time p2 elapses from the time the second motor control 
signal u2(n+1) is output. In this embodiment, the predetermined time p2 is 
p1=Ts/2 and is a delay time d from the time the head position signal 
y(n+1) is input. 
Note that the block diagram described above has conceptually shown the 
processing which is performed in the main control section 68. In fact, 
predetermined processing routine is executed in the MPU 32, based on the 
head position signal y(n) input from the head position signal generating 
section 66. As a result, a signal equal to the output signal of the 
switching section 122 is generated and output to a driver 34 as a motor 
current control signal u(n). 
In the main control section 68, the first control signal c1(n) output from 
the summing point 74 and the second control signal c2(n) output from the 
summing point 96 are signals representative of an amount of manipulation 
where amounts of manipulation executed by control operations other than 
the integration operation of the controller are summed up. Also, in the 
switching section 122, the first motor control signal u1(n) where the 
integration signal in(n) has been added to the first control signal c1(n) 
and the second motor control signal u2(n) where the integration signal 
in(n) has been added to the second control signal c2(n) are switched at 
intervals of predetermined time p1 (=p2), and one of them is output as the 
motor current control signal u(n). In the signal switching by the 
switching section 122, an amount of manipulation by the control operation 
other than the integration operation is switched after the predetermined 
time p1 shorter than the sampling cycle Ts, and a manipulation-amount 
signal (i.e., second motor control signal u2(n)) representative of an 
amount of manipulation, where amounts of manipulation by a plurality of 
control operations are summed up, is output. 
Incidentally, the first control signal c1(n) and the second control signal 
c2(n) are expressed by the following Equations 3 and 4. 
EQU c1(n)=k1.multidot.y(n)+k2.multidot.y(n-1)+k3.multidot.c1(n-1)+k5.multidot.c 
2(n-1)+k4.multidot.c2(n-2) (eq. 3) 
EQU c2(n)=h1.multidot.y(n)+h2.multidot.y(n-1)+h3.multidot.c1(n-1)+h5.multidot.c 
2(n-1)+h4.multidot.c2(n-2) (eq. 4) 
When in Equations 3 and 4 k1=h1, k2=h2, k3=h3, k4=h4, k5=h5=0, since c1(n) 
and c2(n) of Equation 3 become equal (each matches with the first term of 
Equation 2), the cycle of the change of the motor current control signal 
u(n) matches with the sampling cycle regardless of the signal switching by 
the switching section 122 and becomes equal to a case where oversampling 
is not performed. 
However, in this embodiment of the present invention, the values of the 
constants k1, k2, k3, k4, and k5 of Equation 3 (transfer functions of 
transfer elements 72, 78, 80, 82, and 84) and the values of the constants 
h1, h2, h3, h4, and h5 of Equation 4 (transfer functions of transfer 
elements 94, 100, 92, 102, and 104) have been set by a least squares 
method so that an evaluation value obtained by a certain evaluation 
function becomes minimum (optimum value). With this, in order for the 
magnetic head 20 to match with its target position in accordance with the 
deviation between the current position and target position of the magnetic 
head 20, since the first and second motor control signals u1(n) and u2(n) 
are alternately output (oversampling) as a motor current control signal 
u(n) at intervals of cycle of 1/2 of the sampling cycle Ts (predetermined 
time, p=p1=p2), the positioning of the magnetic head 20 can be finely 
controlled. 
On the other hand, since the integration signal in(n) representative of an 
amount of manipulation executed by the integration operation, which is 
output from the transfer element 118, is switched at intervals of sampling 
cycle Ts in accordance with the head position signal y(n) which is input 
at intervals of sampling cycle Ts, the integration signal in(n) will not 
include a prediction error, which would be caused by predicting the 
position of the magnetic head 20, and will become an appropriate value 
balancing with an external bias if the magnetic head 20 matches with its 
target position. When the magnetic head 20 matches with the target 
position thereof, since the influence on the first motor control signal 
u1(n) by the first control signal c1(n) becomes small and also the 
influence on the second motor control signal u2(n) by the second control 
signal c2(n) becomes small, a signal substantially matching with the 
integration signal in(n) is to be output as a motor current control signal 
u(n) at intervals of cycle of 1/2 of the sampling cycle Ts (predetermined 
time, p). 
Therefore, when the position of the magnetic head 20 matches and has 
matched with the target position (a stable state), since the motor current 
control signal u(n), which represents an amount of manipulation equal to 
or substantially equal to an amount of manipulation balancing with an 
external bias, as in the prior art, is output at a cycle shorter than the 
prior art, the external bias can be compensated accurately and also the 
frequency components other than the external bias are compensated by 
oversampling. As a result, the magnetic head 20 can be positioned to the 
target position with a high degree of accuracy. 
When, in the closed loop control system for controlling the position of the 
magnetic head 20, white noise is input as an external disturbance, 
calculate an optimum solution for making the dispersion of the position of 
the magnetic head minimum. Suppose now that the target position of the 
magnetic head is constant and, in the state that the position of the 
magnetic head has matched with the target position, as shown in FIG. 8, 
white noise v(n) has been added to each of the summing points 88 and 108 
of the main control section 68, and white noise w(n) has been added to a 
summing point 130 which is provided for convenience on the input side of 
the main control section 68. 
Note that a transfer element 132 shown in FIG. 8 is constituted by the 
summing point 114 performing an integration operation in the main control 
section 68, the one-sample delay element 116, and the transfer element 
118, and in this embodiment the transfer function, ki, of the transfer 
element 118 is -0.007795. Also, a controller 134 of FIG. 8 is constituted 
by a plurality of elements of the main control section 68 for generating 
the first and second control signals c1(n) and c2(n). Also, a voice coil 
motor (VCM) 136 is constituted by the VCM 26 and the driver 34. The head 
current position computing section 60, the burst pattern detecting section 
62, the head target position setting section 64, and the head position 
signal generating section 66 are used to detect the magnetic head position 
and generate the head position signal y(n) and do not have an influence on 
the position of the magnetic head, so they are omitted in FIG. 8. 
In this embodiment of the present invention, the first and second motor 
control signals u1(n) and u2(n) are alternately output to the VCM 136 at 
intervals of predetermined time p, which is 1/2 of the sampling cycle Ts, 
as described above. The transfer function of the VCM 136 at that time 
becomes 198, 199 as shown in FIG. 9. In a case (case 3) where the delay 
time, d, of the transfer function shown in FIG. 9 is d=0.3 Ts, the 
predetermined time is P=0.5 Ts, the coefficient is K=4.91, and there is no 
limitation, the optimum solutions of the constants k1, k2, k3, k4, k5, h1, 
h2, h3, h4, and h5 of Equations (3) and (4) were k1=-0.532, k2=1.07, 
k3=-0.0126, k4=-0.277, k5=0.0740, h1=0.093, h2=-0.686, h3=-0.145, 
h4=0.150, and h5=-0.250. 
Also, in a case (case 2) where the constants are limited to 
k4=k5=h1=h2=h3=h4=h5=0, the optimum solutions of k1, k2, and k3 were 
k1=-0.405, k2=2.281, and k3=-0.450. Note that although a least squares 
method can apply to the calculation of the optimum solutions, in the above 
calculation there was used a least steady-state dispersion method that the 
applicant of the present invention has already proposed (see Masashi 
Kisaka, "Proposition of Least Steady-State Dispersion Method," Shingaku 
Journal, vol. J76-A, No. 3(1993), pp 364-371). 
In order to evaluate the above described cases 2 and 3, if the dispersion 
of the control error of the magnetic head to external disturbances is 
calculated for the above-described cases 2, 3 and a case 1 of the 
conventional closed loop control system (k1=h1, k2=h2, k3=h3, k4=h4, 
k5=h5=0) where oversampling is not performed, the dispersion of the 
control error will become as shown FIG. 10. As evident in FIG. 10, even 
the case 2 where the values of the constants are limited is smaller in 
value of dispersion of error than the case 1 where oversampling is not 
performed. From this fact it follows that the positioning accuracy of the 
magnetic head is enhanced. 
While it has been described in the above embodiment that oversampling is 
performed at a cycle of 1/2 of the sampling cycle Ts, the present 
invention is not limited to this. For example, oversampling may be 
performed at a cycle of 1/integer (for example, 1/3 or 1/4) of the 
sampling cycle Ts. 
Also, although the magnetic disk of the hard disk drive has been described 
as a rotating body, the present invention is not limited to this magnetic 
disk. The present invention is applicable to data recording medium where a 
plurality of concentric circular tracks are formed, such as flexible 
magnetic disks other than hard magnetic disks, optical disks, or optical 
magnetic disks. The invention is also applicable to other rotating bodies. 
Thus, the invention is applicable to a wide variety of head positioning 
controls where the head is positioned over a predetermined position on a 
rotating body. 
As has been described hereinbefore, in the position control apparatus, if 
the control operation signal is output a manipulation-amount signal 
representative of an amount of manipulation where amounts of manipulation 
each corresponding to the control operation signal, executed by a 
plurality of control operations including an integration operation for 
matching the position of the object with the target position, are summed 
up, will be output. Then, an amount of manipulation executed by the 
control operations other than the integration operation is switched after 
a second predetermined time shorter than the first predetermined time, and 
the manipulation-amount signal representative of an amount of 
manipulation, where amounts of manipulation executed by the plurality of 
control operations are summed up, is output. With this, the movement of 
the object by an actuator is controlled. Accordingly, the present 
invention has the excellent advantage that the positioning accuracy as the 
object to be controlled is positioned to the target position can be 
enhanced. 
In the preferred embodiment position control method according to the 
present invention, a control operation signal representative of a 
deviation between the target position of the object and the detected 
current position of the object is generated. A manipulation-amount signal 
representative of an amount of manipulation where amounts of manipulation 
each corresponding to the control operation signal, executed by a 
plurality of control operations including an integration operation for 
matching the position of the object with the target position, are summed 
up, is output at intervals of first predetermined time, and also, after a 
second predetermined time shorter than the first predetermined time 
elapses from the time the manipulation-amount signal is output, an amount 
of manipulation executed by the control operations other than the 
integration operation is switched. And, outputting the manipulation-amount 
signal representative of an amount of manipulation, where amounts of 
manipulation executed by the plurality of control operation are summed up, 
is repeated. With this, the movement of the object by an actuator is 
controlled. Accordingly, the present invention has the excellent advantage 
that the positioning accuracy as the object to be controlled is positioned 
to the target position can be enhanced.