Positioning adaptive control method and positioning device as well as information storage device

A positioning adaptive control method and positioning device which allow rapid positioning of a head by speed-controlling a seek operation in an information storage device such as a magnetic disk unit. The seek operation moves the head to a vicinity of a target track and a following operation causes the head to accurately follow a position on the target track when the head is positioned in the vicinity of the target track. These operations are controlled based on data recorded in sectors formed on a recording plane of a disk. A variable gain of an acceleration feed forward signal is controlled sequentially by an adaptive control rule so that a deviation signal between a target speed and an actual speed of the head is reduced when the speed of the head is shifted from an acceleration or constant speed state to a deceleration state under speed control. The variable gain of the acceleration feed forward is used per each sampling for the adaptive control rule and a square value of the speed deviation signal is minimized.

BACKGROUND AND SUMMARY OF THE INVENTION 
Generally, a sector servo method, or a servo method conforming thereto, for 
positioning a data head sector based on servo data is used in information 
storage devices such as a magnetic disk unit to achieve high density 
recording. The servo data is stored in advance at the end of the sector 
formed on a data recording plane. The servo method is suitable for a seek 
operation for rapidly moving the head to a target track, and to a 
following operation for causing the head to follow the center of the 
target track with high precision. 
In the seek operation, the speed of the head is controlled in accordance 
with a predetermined target speed curve in order to move the head to the 
vicinity of the target track. When the head reaches the vicinity of the 
track, the operation is switched to the following operation in order to 
control the head's position and to cause it to follow the center of the 
track. A target speed signal is calculated from the target speed curve 
based on the difference (remaining distance) between a current position 
and a target position of the head. A speed error signal, which is a 
deviation between the target speed signal and an actual speed signal of 
the head, is amplified and is applied as a feedback control input to a 
voice coil motor for driving the head. 
Also, when the target speed curve is given in a ramped manner during 
deceleration, i.e., when decelerated at a constant acceleration, a speed 
deviation is produced in the above-mentioned feedback control. Two methods 
are contemplated for reducing the speed deviation. One method widens a 
control band of the speed loop and the other method reduces an inclination 
of the target speed curve. The former has a problem in that there is a 
limit in the width of the control band due to a mechanical resonance of a 
support mechanism system for supporting and driving the head, and the 
latter has a problem in that it is difficult to keep a head access time 
within a predetermined time. 
A proposed control method of finding a target acceleration curve for 
advance use in the speed control by differentiating the target speed curve 
and a method of obtaining an acceleration feed forward signal from the 
target acceleration curve in order to apply it to a speed error signal has 
been described in Japanese Patent Laid-Open No. 58-182169 for example. Its 
purpose is to compensate the stability through the use of feedback control 
and to improve the response through the use of feed forward control. The 
known method allows the head to follow the target speed curve accurately 
without needing to widen the control band of the speed control loop nor 
reduce the inclination of the target speed curve. Thus, the known method 
causes less speed error in switching to the positional control and 
prevents an overshoot phenomenon in which the head movement exceeds a 
target track position or an undershoot phenomenon in which the head stops 
before reaching the target track position, as well as reducing the access 
time. 
Further, it is possible to accelerate/decelerate the head to cause it to 
follow the center of the target track only by using the positional control 
system without controlling the seek operation and the following operation 
by separate control systems. This method eliminates the switching of the 
control systems and allows the head to be settled on the center of the 
target track smoothly and allows the access time to be reduced. An example 
thereof is described by Hirata et. al in "Head Positioning Control of Hard 
Disk Using H.infin. Control Theory", Papers of the Society of Instrument 
and Control Engineers, Vol. 29, No. 1, pp. 71/77 (1993). In the paper, a 
compensation filter derived from the H.infin. control theory is used as a 
feedback control compensator to suppress resonance/disturbance of the head 
supporting mechanism system. A smooth sinusoidal acceleration signal is 
input in feed forward control to move the head at high speed. 
It is known to design a control system based on dynamic characteristics of 
an object to be controlled so that a given design specification is met. 
For example, a speed control system of a magnetic disk unit is designed so 
that zero-cross frequency in an open loop characteristic meets with the 
design specification by deciding a speed gain of a speed loop by using a 
loop gain of an object to be controlled and by deciding a loop gain of an 
acceleration feed forward compensator by using an inverse gain of the loop 
gain of the object to be controlled. The loop gain of the object to be 
controlled here means a loop gain from a control input to a speed signal 
of the head. The loop gain is given by an amplifier gain of an amplifier, 
a force constant of a voice coil motor, an equivalent mass of the head and 
a speed detection gain, etc. As methods for detecting the speed, there is 
known a method of calculating a position signal by backward differential 
or a method of calculating it by using a speed observer from a position 
signal and a current signal from an amplifier. 
The loop gain of the object to be controlled varies depending on 
manufacturing allowances, operating conditions, operating environments and 
an elapsed change, etc. For example, the force constant of the voice coil 
motor changes depending on the track position where the head is located 
due to magnetic flux leakage from both ends of the voice coil. That is, a 
force gain is small at the inner and outer diameter sides of the disk, and 
is large at the center of the disk. A difference of these gains amounts to 
about 10%. 
A position detection gain of the head depends on the manufacturing 
allowance of the head and the gain changes due to a dispersion of a core 
width of the head. That is, when the core width of the head is larger than 
a designed standard, both the read voltage and position detection gain 
become larger. By contrast, when the core width of the head is smaller 
than the designed standard, both the read voltage and position detection 
gain become smaller. A difference of these gains amounts to about 20% at 
the data head. A fluctuation of the speed detection gain of the head 
depends on the fluctuation of a detected gain of the position signal of 
the head, and is about the same with the detected gain of the position 
signal. 
Mechanical characteristics of the voice coil motor and the electrical 
characteristics of a driving circuit, as well as others, also fluctuate as 
they are influenced by environmental factors such as the temperature and 
humidity within a unit. Further, when such a magnetic disk unit is used 
for a long period of time, the characteristics of the bearing supporting 
the head gradually changes due to abrasion and change in the oil 
characteristics. As a result, the characteristics of an object to be 
controlled become different from those when the unit is first shipped out. 
That is, the characteristics of the entire control system deviate from the 
optimum point due to the dispersion of mechanical characteristics of the 
driving system for driving the head, the dispersion of electrical 
characteristics of the driving circuit and the dispersion of the detected 
gain of the head. These deviations prevent the performance of the rapid 
seek operation and the high precision following operation, thus degrading 
the performance of the unit. The dispersion of the response of the head 
may be suppressed more or less by performing the seek operation by using 
only the feedback control because the gain fluctuation of the object to be 
controlled is suppressed by the feedback characteristic. 
For the case when the head is rapidly sought by using the feed forward 
control as described in Japanese Patent Laid-Open No. 58-182169 and in 
Papers of the Society of Instrument and Control Engineers, Vol. 29, No. 1, 
pp. 71/77 (1993) mentioned above, the dispersion of the gain of the object 
to be controlled turns out directly as a dispersion of the response of the 
head. 
Static and dynamic external forces are applied to the head during the seek 
operation. A signal line for transmitting recording/reproduction signals 
of the head to a circuit board is provided by a cable composed of a copper 
foil pasted to a soft medium called an FPC (flexible printed circuit). The 
cable acts as a spring force, i.e. a force disturbance, to the head 
supporting mechanism. Due to the external force, the head speed signal 
deviates from a target speed signal during the seek operation using speed 
control. On the other hand, the head position deviates from a target 
position during the seek operation using position control. While the head 
turns about an axis of rotation of the head supporting mechanism section, 
a moment force is generated at the supporting mechanism section during the 
seek operation. As a result, the base supporting the supporting mechanism 
section vibrates. Due to the vibration of the base, a disk mounted on the 
base vibrates, thus causing a relative position error between the disk and 
the head as a positional disturbance to the head position. 
Accordingly, it is an object of the present invention to eliminate the 
above-mentioned problems and to significantly increase the recording 
density of the disk unit as well as to rapidly position the head with high 
precision. In practice, it is an object of the present invention to 
provide an adaptive control system and an information storage device 
equipped therewith which can automatically correct the deviation of the 
control characteristics from the optimum point due to the dispersion of 
parts composing the positioning mechanism system and the electrical 
system, while always having good head positioning characteristics. 
It is another object of the present invention to provide an adaptive 
control system and an information storage device equipped therewith which 
controls an acceleration feed forward gain in real-time by an adaptive 
controller in order to reduce the dispersion of the response of the 
position during the seek operation in which speed control is applied. 
It is still another object of the present invention to provide an adaptive 
control system and an information storage device equipped therewith which 
controls an acceleration feed forward gain in real-time by an adaptive 
controller in order to reduce the dispersion of the response of the 
position during the seek operation in which position control is applied. 
It is a further object of the present invention to provide a real-time 
adaptive control system and an information storage device equipped 
therewith which requires no training signal when a gain fluctuation of an 
object to be controlled is estimated. 
In order to achieve the aforementioned objects, a first aspect of the 
present invention provides: 
1) in a positioning adaptive control method for positioning a head for 
reading/writing information either recorded on a disk or to be recorded on 
the disk to a predetermined position of a target track by using a seek 
operation for positioning the head to a track on the disk and a following 
operation for positioning the head while on the track, a control system 
related to the seek operation is controlled sequentially during the seek 
operation so that the head may be positioned to the target track rapidly 
with high precision during the following operation which follows the seek 
operation. 
A second aspect of the present invention for achieving the aforementioned 
objects is that a feed forward control input is controlled sequentially in 
a positioning adaptive control method for moving the head to a target 
position by adding a feedback control input obtained based on a speed 
error between a target speed and an actual speed of the head and a feed 
forward control input obtained based on a target acceleration. The second 
aspect has options as shown in items 2) through 6) below. It is noted that 
the feedback control input and the feed forward control input are used 
with any of the options. 
2) The feed forward control input is controlled sequentially so that the 
speed error is reduced during head movement. 
3) The feed forward control input is controlled sequentially so that a 
square of the speed error is minimized when the speed of the head starts 
to decelerate. 
4) The feed forward control input is controlled sequentially by multiplying 
the feed forward control input with a control gain .theta.(k) which is 
expressed by .theta.(k+1)=.theta.(k)+.eta..multidot.uv(k).multidot.sa(k), 
wherein the feedback control input is denoted as uv(k), the feed forward 
control input as sa(k) and a learning gain as .eta.. 
5) The feed forward control input is controlled sequentially so that a 
value obtained when square values of the speed error are added 
sequentially is minimized when the speed of the head starts to decelerate. 
6) The feed forward control input is controlled sequentially by multiplying 
the feed forward control input with a control gain .theta.(k) which is 
expressed by 
.theta.(k+1)=.theta.(k)+.gamma.(k+1).multidot.uv(k).multidot.sa(k), 
wherein the feedback control input is denoted as uv(k) the feed forward 
control input as sa(k) and a learning gain as .gamma.(k). 
A third aspect of the present invention for achieving the aforementioned 
objects provides a feed forward control input which is controlled 
sequentially in a positioning adaptive control method for moving a head to 
a target position by adding a feedback control input obtained based on a 
position error between a target position and an actual position of the 
head and the feed forward control input obtained based on a target 
acceleration. Similarly to the second aspect, there are options also for 
this aspect as shown in items 7) through 11) below. The feedback control 
input and the feed forward control input are used with any option. 
7) The feed forward control input is controlled sequentially so that the 
position error is reduced during head movement. 
8) The feed forward control input is controlled sequentially so that a 
square of the position error is minimized when the head starts to move. 
9) The feed forward control input is controlled sequentially by multiplying 
the feed forward control input with a control gain .theta.(k) which is 
updated sequentially by 
.theta.(k+1)=.theta.(k)+.eta..multidot.uv(k).multidot.sa(k), wherein the 
feedback control input is denoted as uv(k), the feed forward control input 
as ua(k), a learning gain as .eta. and the feed forward control input 
before a gain is applied as sa(k). 
10) The feed forward control input ua(k) is controlled sequentially so that 
a value obtained when square values of the position error are added 
sequentially is minimized when the head starts to move. 
11) The feed forward control input ua(k) is controlled sequentially by 
multiplying the feed forward control input sa(k) with a control gain 
.theta.(k) which is updated sequentially by 
.theta.(k+1)=.theta.(k)+.gamma.(k+1).multidot.uv(k).multidot.sa(k), 
wherein the feedback control input is denoted as uv(k), the feed forward 
control input as ua (k), a learning gain as .gamma.(k) and the feed 
forward control input before the gain is applied as sa(k). 
A fourth aspect of the present invention for achieving the aforementioned 
objects is that feed forward control inputs are controlled sequentially in 
a positioning adaptive control method for moving the head to a target 
position by adding a feedback control input obtained based on a speed 
error between a target speed and an actual speed of the head or a position 
error between a target position and an actual position of the head, a 
first feed forward control input obtained based on a target acceleration 
and a second feed forward control input obtained based on an acceleration 
disturbance signal. The fourth aspect also has options as shown in items 
12) and 13) below, and the feedback control input, the first feed forward 
control input and the second feed forward control input are used in any of 
the options. 
12) The first and second feed forward control inputs are controlled 
sequentially so that the speed error is reduced during head movement. 
13) The first and second feed forward control inputs are controlled 
sequentially so that the position error is reduced during head movement. 
A fifth aspect of the present invention for achieving the aforementioned 
objects is that a feed forward control input is controlled sequentially in 
a positioning adaptive control method for generating a control input for 
moving the head to a target position by adding a feedback control input 
obtained or a position error between a target position and a head position 
based on a speed error between a target speed and a head speed and the 
feed forward control input obtained based on a target acceleration, and 
has the options as shown in items 14) through 17) below. A variable gain 
is provided at an arbitrary place from the control input for moving the 
head to the target position to the speed signal of the head, and the 
feedback control input and the feed forward control input are used in any 
of the options. 
14) The feed forward control input is controlled sequentially so that the 
speed error is reduced even if the variable gain is increased or reduced 
during head movement to immediately reduce the speed error which has been 
increased due to a gain change of the variable gain. 
15) The feed forward control input is controlled sequentially so that the 
speed error is reduced even if the variable gain is increased or reduced 
in advance in order to immediately return the head speed signal which has 
been fluctuating due to the gain change of the variable gain to the state 
before the gain change of the variable gain. 
16) The feed forward control input is controlled sequentially so that the 
position error is reduced even if the variable gain is increased or 
reduced during head movement in order to immediately reduce the position 
error which has been increased due to the gain change of the variable 
gain. 
17) The feed forward control input is controlled sequentially so that the 
position error is reduced even if the variable gain is increased or 
reduced in advance in order to immediately return the head position signal 
which has been fluctuating due to the gain change of the variable gain to 
the state before the gain change of the variable gain. 
It is noted that in items 4), 6), 9) and 11) described above, an initial 
value of the control gain .theta.(k) may be set at .theta.(0)=1.0 and, 
again in items 4), 6), 9) and 11), a value of a final control gain 
controlled during the previous head movement may be used as an initial 
value of the control gain .theta.(k). 
Signals handled in each positioning adaptive control method described above 
may be either digital signals or analog signals. 
In items 1), 2), 3), 5), 7), 8), 10) and 12) through 17) discussed above, 
the sequential control may be carried out in synchronism with time 
intervals for reading position data in a sector servo. 
A sixth aspect of the present invention for achieving the aforementioned 
objects provides: 
18) a positioning adaptive control device of an information storage device 
comprising a recording medium in which position data is recorded in 
advance; a head for reading the position data; a position signal computing 
element for generating a position signal from the position data reproduced 
by the head; a speed signal computing element for generating a speed 
signal of the head based on the position signal; a target acceleration 
generator for generating a target acceleration signal based on a remaining 
distance between a target position and a current position of the head; a 
target speed generator for generating a target speed signal based on the 
remaining distance; a speed error amplifier for generating a feedback 
control input signal by amplifying a speed error signal of a deviation 
between the target speed signal and the head speed signal; and a feed 
forward controller for generating a feed forward control input signal by 
multiplying the target acceleration signal with a controllable variable 
gain. The head is moved to the target position by a control input signal 
obtained by adding the feedback control input signal and the feed forward 
control input signal. The control device further comprises an adaptive 
controller for sequentially controlling a variable gain of the 
controllable target acceleration signal so that the speed error signal is 
minimized. 
19) In item 18), the adaptive controller may control the variable gain of 
the controllable target acceleration signal sequentially based on a value 
in which the speed error signal, the target acceleration signal and a 
predetermined learning gain are multiplied so that a square of the speed 
error signal is minimized. 
20) In item 18), the adaptive controller may control the variable gain of 
the controllable target acceleration signal sequentially based on the 
speed error signal, the target acceleration signal and a predetermined 
learning gain so that a value in which squares of the speed error signal 
are added sequentially is minimized. 
A seventh aspect of the present invention for achieving the aforementioned 
objects provides: 
21) a positioning adaptive control device of an information storage device 
comprising a recording medium in which position data is recorded in 
advance; a head for reading the position data; a position signal computing 
element for generating a position signal from the position data reproduced 
by the head; a target acceleration generator for generating a target 
acceleration signal; a position controller for generating a feedback 
control input signal by filtering a position error signal of a deviation 
between a target position signal and the position signal; a feed forward 
controller for generating a feed forward control input signal by 
multiplying the target acceleration signal with a controllable variable 
gain. The head is moved to the target position by a control input signal 
obtained by adding the feedback control input signal and the feed forward 
control input signal. The control device further comprises an adaptive 
controller for sequentially controlling a variable gain of the 
controllable target acceleration signal so that the position error signal 
is minimized. 
22) In item 21), the adaptive controller may control the variable gain of 
the controllable target acceleration signal sequentially based on a value 
in which the position error signal, the target acceleration signal and a 
predetermined learning gain are multiplied so that a square of the 
position error signal is minimized. 
23) In item 21), the adaptive controller may control the variable gain of 
the controllable target acceleration signal sequentially based on the 
position error signal, the target acceleration signal and a predetermined 
learning gain so that a sum of squares of the position error signal is 
minimized. 
An eighth aspect of the present invention for achieving the aforementioned 
object provides: 
24) a positioning adaptive control device of an information storage device 
comprising a recording medium in which position data is recorded in 
advance; a head for reading the position data; a position signal computing 
element for generating a position signal from the position data reproduced 
by the head; a speed signal computing element for generating a speed 
signal of the head based on the position signal; a target acceleration 
generator for generating a target acceleration signal based on a remaining 
distance between a target position and a current position of the head; a 
target speed generator for generating a target speed signal based on the 
remaining distance; a speed error amplifier for generating a feedback 
control input signal by amplifying a speed error signal for a deviation 
between the target speed signal and the head speed signal: and a feed 
forward controller for generating a first feed forward control input 
signal in which an acceleration disturbance signal is multiplied with a 
controllable first variable gain, a second feed forward control input 
signal in which the target acceleration signal is multiplied with a 
controllable second variable gain and a feed forward control input signal 
obtained by adding the first feed forward control input signal and the 
second feed forward control input signal. The head is moved to the target 
position by a control input signal obtained by adding the feedback control 
input signal and the feed forward control input signal. The control device 
further comprises an adaptive controller for sequentially controlling a 
first variable gain of the controllable acceleration disturbance signal 
and a second variable gain of the controllable target acceleration signal 
so that the speed error signal is minimized. 
A ninth aspect of the present invention for achieving the aforementioned 
objects provides: 
25) a positioning adaptive control device of an information storage device 
comprising a recording medium in which position data is recorded in 
advance; a head for reading the position data; a position signal computing 
element for generating a position signal from the position data reproduced 
by the head; a target acceleration generator for generating a target 
acceleration signal; a position controller for generating a feedback 
control input signal by filtering a position error signal for a deviation 
between a target position signal and the position signal; a feed forward 
controller for generating a first feed forward control input signal in 
which an acceleration disturbance signal is multiplied with a controllable 
first variable gain, a second feed forward control input signal in which 
the target acceleration signal is multiplied with a controllable second 
variable gain, and a feed forward control input signal obtained by adding 
the first feed forward control input signal and the second feed forward 
control input signal. The head is moved to the target position by a 
control input signal obtained by adding the feedback control input signal 
and the feed forward control input signal. The control device further 
comprises an adaptive controller for sequentially controlling a first 
variable gain of the controllable acceleration disturbance signal and a 
second variable gain of the controllable target acceleration signal that 
the position error signal is minimized. 
26) In items 18), 21), 24) and 25) above, the sequential control is 
preferably carried out in synchronism with time intervals for reading the 
position data in the sector servo. 
A tenth aspect of the present invention for achieving the aforementioned 
objects provides an information storage device comprising a recording 
medium in which position data is recorded, a head for reading the position 
data and a positioning adaptive control device for positioning the head to 
a track on the recording medium. The information storage device is 
equipped with the positioning adaptive control device comprising 
components in any one of the items 18) through 26) discussed above. 
According to the present invention, the acceleration feed forward signal is 
controlled in real-time during deceleration so that the speed error to a 
target speed is reduced when the head, whose speed is controlled, is 
shifted from an acceleration state or a constant speed state to a 
deceleration state in the positioning control system in which the seek 
operation is controlled by the speed control and the following operation 
is controlled by the position control. Thereby, the speed error caused by 
a fluctuation of the gain of an object to be controlled and/or disturbance 
may be minimized. Further, when the control is switched from the speed 
control to the position control in the vicinity of a target track, 
dispersion of responses of the head position and head speed after the 
switch may be fully suppressed, thus giving a favorable settlement 
response and an access time may be shortened. 
The positioning may always be completed rapidly and with high precision by 
performing the sequential control described above during the seek 
operation and by performing the seek operation and the following operation 
ensuing thereto regardless of the fluctuation of gain of the object to be 
controlled. 
According to the present invention, in a positioning control system in 
which the seek operation and the following operation are controlled using 
only the position control, the acceleration feed forward signal is 
controlled in real-time so that a position error with respect to a target 
position is reduced from the start of the seek operation. Thereby, the 
position error caused by a fluctuation of a gain of an object to be 
controlled and/or a disturbance may be minimized. Then, a favorable 
settlement response is given when the head reaches the target track and 
the head access time may be shortened. 
According to the present invention, the previously used training signal 
becomes unnecessary. Further, because the acceleration feed forward signal 
is controlled in real-time during the seek operation, the head can follow 
a target orbit with high precision. Accordingly, the target orbit which 
has been loosely set in the past may be set more sharply and the access 
time may be shortened. 
The specific nature of the invention, as well as other objects, uses and 
advantages thereof, will be clearly apparent from the description and from 
the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
Preferred embodiments of the present invention will be explained below in 
detail with reference to the drawings. 
FIG. 1 is a block diagram showing a positioning adaptive control system 46 
for a magnetic disk unit according to one embodiment of the present 
invention. An object to be controlled 1 is a head driving system of the 
magnetic disk unit and comprises a power amplifier, a voice coil motor, a 
head supporting mechanism, a position signal demodulating system, as well 
as other components. The actual structure of the object to be controlled 
will be explained later with reference to FIG. 3. In the present 
embodiment, the head is moved to a target track by first moving it into 
the vicinity of the target track using a speed control system and then by 
switching the control system to a position control system with a switch 14 
when the head is in the vicinity of the target track. 
In the magnetic disk unit, two phases of position signals are adopted. A 
demodulation circuit demodulates servo data recorded at the head of a 
sector to generate the two phases of triangular discontinuous position 
signals whose phases are shifted by 90.degree. from each other. The center 
of each track is represented by a zero-cross position of the position 
signal. A continuous position signal computing element 2 cuts out linear 
portions from the two phases of position signals to generate a continuous 
position signal. The continuous position signal shall be called a head 
position signal and is represented by y(k), wherein k is a number of steps 
from a seek starting time. 
A head speed signal v(k) is calculated from the head position signal y(k) 
by means of a speed signal computing element 5 using a backward 
differential or the like. A target speed generator 4 refers to a table of 
correspondence stored in advance in a ROM (Read Only Memory) to generate a 
target speed signal tv(k) based on a remaining distance (y(k)-ty) 
generated by an adder 3 between the current head position y(k) and a 
target position ty. Similarly, a target acceleration generator 8 refers to 
a table or correspondence stored in advance in a ROM to generate a target 
acceleration signal ta(k) during deceleration of the speed control based 
on the remaining distance (y(k)-ty) between the current head position y(k) 
and the target position ty. 
A deviation between the target speed signal tv(k) and the head speed signal 
v(k) generated by an adder 7 is referred to as a speed error signal and is 
represented as ev(k). A speed gain kv 6 of a speed loop is decided based 
on a loop gain of the object to be controlled 1 so that a zero-crossing 
frequency of an open loop of the speed control system containing the 
object to be controlled 1 becomes a set value. A feedback control input 
signal uv(k), in which the speed error signal tv(k) is multiplied by the 
speed gain kv, is used as a feedback control signal. 
Hitherto, a feed forward acceleration control signal sa(k), in which the 
target acceleration signal ta(k) is multiplied by the acceleration gain 
ka, has been used as a feed forward control signal. Here, an acceleration 
gain 9 is determined as follows. When the gain from a control input of the 
object to be controlled 1 to a head acceleration is km, the acceleration 
gain ka should be set as an inverse gain of km. However, because it is 
difficult to find km accurately, the acceleration gain ka is set as the 
inverse gain of a nominal gain kmh, i.e. 1/kmh, where kmh is a nominal 
gain as opposed to the actual gain km. The control input u(k) to the 
object to be controlled 1 is a signal in which the feedback speed control 
input uv(k) and the feed forward acceleration control signal sa(k) are 
added by an adder 13. 
Thereby, in an ideal condition in which there is no fluctuation of the gain 
of the object to be controlled nor any disturbance, kmh=km, a deviation of 
the stationary speed as against a target speed may be zeroed 
theoretically. However, when the gain of the object to be controlled 1 
fluctuates, i.e. when kmh.noteq.km, then an acceleration of the head 
deviates from the target acceleration, thus causing a speed error. Thus, 
the head settles less when the speed control system is switched to the 
position control system and the access time is prolonged. 
In that event, the present embodiment takes in a signal in which the target 
acceleration signal ta(k) is multiplied by the acceleration gain ka and a 
signal in which the speed error signal ev(k) is multiplied with the speed 
gain kv. An adaptive controller 12 for sequentially controlling a variable 
gain .theta.(k) of the feed forward control signal per each sampling is 
then provided so that a square of the speed error signal ev(k).sup.2 is 
minimized during the deceleration of the seek operation. 
A feed forward control input signal ua(k) results from the signal sa(k), 
obtained by multiplying the target acceleration speed ta(k) with the 
acceleration gain ka, multiplied with a gain .theta.(k) of a variable gain 
10. When the square of the speed error signal ev(k).sup.2 becomes zero, 
the head moves by a command from only the acceleration feed forward 
signal, and the loop gain of the feed forward control signal 
.theta.(k).multidot.ka(=.theta.(k)/kmh) becomes equal to the inverse loop 
gain of the object to be controlled, i.e. 1/km. 
That is, the adaptive controller 12 estimates the loop gain km of the 
object to be controlled 1. Further, because the variable gain 10 of the 
acceleration feed forward is controlled so that the square of the speed 
error signal ev(k).sup.2 is minimized, the target speed coincides with the 
head speed and the seek operation can be shifted smoothly to the following 
operation. Still further, no training signal needs to be input to a 
control loop in order to control the variable gain 10. 
When the head reaches the vicinity of the target track, the switch 14 is 
changed over to shift to the feedback control using a position controller 
15. Thereby, it becomes possible to follow the target position ty with 
high precision. The position controller 15 can be, for example, a lead/lag 
(phase lead/phase lag) compensator. 
A method for controlling the gain .theta.(k) of the variable gain 10 of the 
acceleration feed forward operation in the adaptive controller 12 will be 
explained below. An evaluation function E(k) is introduced to 
mathematically minimize the square of the speed error signal ev(k).sup.2. 
The evaluation function E(k) is expressed by the following equation 
considering a relationship of uv(k)=kv.multidot.ev(k). That is, the 
feedback control input uv(k) is minimized to minimize the speed error 
ev(k). 
##EQU1## 
An adaptive control rule 11 shown in the following equation (2) is applied 
to control the acceleration feed forward variable gain .theta.(k) in 
real-time by using a gradient method so that the evaluation function E(k) 
is minimized. The variable gain is not controlled when the head is 
accelerated. 
##EQU2## 
Here, .eta. is a learning constant which decides the speed of convergence 
of the variable gain .theta.(k). 
An initial value .theta.(0) of the variable gain will be set here as 
.theta.(0)=1.0 so that the present embodiment can be readily understood. 
It is noted that the variable gain .theta.(k) remains as 1.0 when 
.theta.(0)=1.0 is not updated, and it also means that it is the same with 
the prior art control method. 
Although .theta.(0)=1.0 in the present embodiment, a value of a final 
variable gain controlled during the previous seek operation may be set as 
the initial value of the present variable gain. The equation (3) may be 
derived from the equation (2) by a rule of connection of differential. The 
equation (4) is derived by partially differentiating both sides of the 
relational equation ua(k)=.theta.(k).multidot.s.sub.a (k) by .theta.(k). 
The equation (5) is used when the variable gain is updated by applying the 
rule of gradient. 
A theoretical value of the learning gain .eta.0 is a small value that 
satisfies the following equation (6); 
##EQU3## 
The target acceleration signal ta(k) input during deceleration is always a 
negative value and s.sub.a (k).sup.2 .noteq.0. Accordingly, the learning 
gain .eta. which satisfies equation (6) is obtained and the following 
equation (7) is satisfied: 
##EQU4## 
The speed error is converged to zero by this method. 
A concept for updating the gain .theta.(k) of the variable gain 10 will be 
explained below using the gradient rule applied in equation (2) with 
reference to FIG. 2. In the figure, the horizontal axis represents the 
.theta.(k) component of the variable gain in the acceleration feed forward 
signal and the vertical axis represents the evaluation function given by 
the equation (1). 
A value of the evaluation function for the gain .theta.(k) of the variable 
gain 10 in the k-th step is assumed to be E(k). Because the evaluation 
function curve has a positive gradient (inclination) at point A, the value 
of the evaluation function is reduced if the gain .theta.(k) of the 
variable gainer 10 is corrected in the minus direction. In equation (2), 
the gain .theta.(k) of the variable gain 10 is corrected in the direction 
inverse from the sign of the gradient by finding the gradient from 
computation of .differential.E(k)/.differential..theta.(k) at point A on 
the error curve and by multiplying the gradient with the learning gain 
.eta.. A value at that time is assumed to be .theta.(k+1). The point is 
shifted to point B on the error curve in step (k+1). The greater the 
gradient as opposed to the error curve, the greater is the updated amount 
of the variable gain. The smaller the gradient as opposed to the error 
curve, the smaller is the updated amount of the variable gain. When the 
above operation is executed sequentially, the gradient becomes zero in the 
end and reaches point C. The point C is a point where the evaluation 
function is minimized. 
It is noted that the timing for sequentially executing the above-mentioned 
operation may be synchronized with the timing for reading position data in 
the sector servo. Update intervals in equation (2) may be set as an 
integer number of position data reading intervals, so that it need not 
always be updated every time the position data is read. 
FIG. 3 is a schematic block diagram of an adaptive control device used in 
the inventive magnetic disk unit. In the magnetic disk unit, a disk 
contorller 47 instructs a microprocessor 43 so as to move a head 30 to a 
target track when it receives a recording or reproduction command from a 
host CPU. A magnetic disk 31 is mounted on an axis of rotation driven by a 
spindle motor 32 and the head 30 floats on the surface of the disk 31 with 
a small gap. The head 30 is attached to a head supporting mechanism which 
is, in turn, fixed to a voice coil motor 37. The head 30 moves in the 
direction from the outer diameter side to the inner diameter side, or in 
the reverse direction thereof, in accordance with a movement of the voice 
coil motor 37 in order to read/write information recorded on a track of 
the disk 31. The head 30 transmits information between the disk controller 
47 via a head IC 33 and a R/W (Read/Write) circuit 41. 
In the magnetic disk unit, servo data is recorded at the head of each 
sector of all of the tracks on the disk 31 in order to record information 
with high density. A sector servo method for positioning the head at a 
predetermined position by reading such servo data by the head 30 is 
adopted. Accordingly, all of the heads are data heads. Servo data obtained 
from an individual data head is amplified by the head IC 33 and is 
converted into a position signal by a position signal demodulating circuit 
34. The position signal is converted into a digital signal by an AD 
(analog-digital) converter 35. The servo data includes a section where a 
track number is recorded and it is demodulated by a grey code demodulator 
36. The position signal and the track number are input into the 
microprocessor 43 via a bus line 42. 
The microprocessor 43 within a microprocessor system 46 (which performs the 
adaptive control described which respect to FIG. 1) is connected with a 
RAM (Random Access Memory) 44 and a ROM (Read Only Memory) 45 via the bus 
line 42. Programs used for the speed control system, the position control 
system and the adaptive controller 12 are stored in the ROM 45. The 
variable gain and the like are stored temporarily ln the RAM 44. The 
microprocessor 43 computes the control input u(k). Here, the object to be 
controlled 1 comprises a DA converter 38, a power amplifier 38, the voice 
coil motor 37, the head supporting mechanism system, a detected gain of 
the head position signal and the AD converter 35. 
FIG. 4 is a flowchart for explaining a procedure for executing the seek 
operation according to one embodiment of the present invention. A target 
position ty, the learning gain .eta. in the equation (5), and the initial 
value .theta.(0) of the variable gain 10 of acceleration forward operation 
are set at first in Step 101. Here, .eta. is set at 0.01 and .theta.(0) is 
set at 1.0. The head position y(k) is computed by the continuous signal 
position computing element 2 in Step 102 and the head speed v(k) is 
computed from the head position y(k) by means of the backward differential 
method or the like in Step 103. The target speed generator 4 generates the 
target speed tv(k) by referring to a target speed table stored in advance 
in the ROM 45 based on a difference (remaining distance) between the 
target position and an actual position of the head. A target acceleration 
generator 8 also generates a target acceleration ta(k) similarly based on 
the remaining distance in Step 104. 
In Step 105, the speed error ev(k) is computed from the deviation between 
the target speed tv(k) and the head speed v(k) and in Step 106, this speed 
error ev(k) is multiplied with a gain value kv of the speed gain 6 of the 
speed loop in order to compute the feedback control input signal uv(k). 
Next, from the condition of the speed error ev(k) in Step 107, it is 
determined whether the head is being decelerated, accelerated or at a 
constant speed. When the head is being accelerated or is at constant 
speed, the value ua(k) of the feed forward control input is set at 
ua(k)=0.0 in Step 111. When the head is being decelerated, the feed 
forward acceleration control signal sa(k) is computed by multiplying the 
target acceleration ta(k) with the gain ka of an acceleration feed forward 
acceleration gain 9 in Step 108. 
Referring to FIG. 4a, Step 107 of FIG. 4 is explained. The state of seek 
motion of the head, i.e., decelerating, accelerating, or constant speed, 
is judged on the basis of a seek flag. A seek flag equalling 1 indicates 
that the head is in an acceleration state. A seek flag equalling 2 
indicates that the head is in a constant speed state. A seek flag 
equalling 3 indicates that the head is in a decelerating state. Before 
beginning any seek motion, the seek flag is set equal to 1, i.e., it is 
set for an acceleration state. In Step 107a, the seek motion is determined 
based on whether the head is in a decelerating state, i.e., the flag 
equals 3. If the flag does equal 3, then a decelerating state is 
determined and the process continues at Step 108. However, if the head is 
not determined to be in a decelerating state, then it is determined 
whether the head is in a constant speed state, i.e., flag equalling 2, or 
in an acceleration state. In Step 107b, if the head is not in a constant 
speed state, then Step 107e is executed. In Step 107e, the sign of a speed 
error is compared. If the sign is positive, then the process proceeds to 
Step 111 meaning that an acceleration state was present. If however, the 
speed error e.sub.v becomes negative, namely the head speed approaches and 
reaches the target speed, then Step 107 is executed. In Step 107, the 
remaining distance signifying a difference between the target position and 
the present location of the head is compared to determine whether it is 
greater than a deceleration starting point in a target speed curve which 
denotes a point changing from the constant speed state to the decelerating 
state. If the remaining distance is greater than the decelerating starting 
point, then the flag is set to equal 2, i.e., the constant speed state 
(Step 107h). Otherwise, the seek motion is assumed to be in a decelerating 
state and the flag is set to 3 (Step 107g). 
If the seek motion is determined to be in the constant speed state, then 
the remaining distance is compared with the deceleration starting point in 
the target speed curve (Step 107c). If the remaining distance is greater 
than the deceleration starting point, then the seek motion remains the 
constant speed motion. Otherwise, the flag is set to equal 3 (Step 107d), 
in which case, Step 111 is then executed. 
A response curve for the head speed is shown in FIG. 4b. There, the speed 
of the head v changes from an acceleration motion in the acceleration 
section to a deceleration motion via the constant speed motion. 
FIG. 4c shows another response curve for the head speed without the 
presence of the constant speed section. 
Referring back to FIG. 4, the feed forward control input signal ua(k) is 
computed by multiplying the signal sa(k) with the gain (k) of the variable 
gain 10 in Step 109. In Step 110, the variable gain 10 is updated from 
.theta.(k) to .theta.(k+1) by using equation (5) above so as to minimize 
the speed error ev(k). .theta.(k+1) is used to compute the feed forward 
control input (step 109) in the next (k+1-th) repetition. 
In step 112, the control input u(k) input to the object to be controlled 1 
is found by adding the feedback control input uv(k) found in Step 106 and 
the feed forward control input ua(k) found in Step 109 or 111. Steps 102 
through 112 are repeated sequentially until the head reaches the vicinity 
of the target position in Step 113. When the head reaches the vicinity of 
the target position, the control system is changed from the speed control 
system to the position control system in order to control the head so as 
to follow the target position in Step 114. 
In order to verify the effectiveness of the present embodiment, the results 
of a simulation wherein a loop gain km of the object to be controlled 1 
was increased by +20% and the head was caused to seek a distance over 1/3 
of all of the movable tracks, will be shown below. 
FIGS. 5a through 5d are charts showing the responses of the seek operation 
when the acceleration feed forward signal is controlled using the adaptive 
controller 12 and FIGS. 6a through 6c are charts showing the responses of 
the prior art seek operation wherein no adaptive controller is used. A 
target speed curve and a target acceleration curve of the head were 
conditioned such that the gain of the object to be controlled would not 
fluctuate and such that the speed control is finished in 10 ms and the 
control is shifted to the following operation, i.e. the position control. 
The target acceleration curve was set so as to attenuate exponentially 
just before a target track is reached. 
In FIGS. 5a through 5d, the head position signal y(k) is plotted in FIG. 
5a, the head speed signal v(k) (solid line) and the target speed signal 
tv(k) (broken line) are plotted in FIG. 5b, the head acceleration a (k) 
(solid line) and the target acceleration signal ta(k) (broken line) are 
plotted in FIG. 5c and the gain (k) of the variable gain 10 estimated by 
the adaptive control rule 11 is plotted in FIG. 5d. The control rule of 
equation (5) which minimizes the evaluation function of equation (1) was 
used as the adaptive control rule. 
The estimation is started during deceleration, i.e. from point P in FIG. 
5c, where the head speed reaches to the target speed signal (FIG. 5b). 
Because the loop gain of the object to be controlled fluctuates +20%, the 
acceleration feed forward variable gain .theta.(k) converges to 
0.833(=1/1.2) in about 1 ms. so that it cancels out the fluctuation of the 
gain as shown in FIG. 5d. Due to that, the head speed coincides with the 
target speed, the control is switched from the speed control to the 
position control in 10 ms., and the response of settlement of the head 
position is equal to the condition wherein there is no fluctuation of the 
gain. 
Carefully observing the estimated waveform .theta.(k) shown in FIG. 5d, it 
is estimated to be on the positive side during a period of 0.3 ms. right 
after the start of the estimation. This happens because the variable gain 
of the acceleration feed forward signal is increased so that no overshoot 
is caused when the head speed coincides with the target speed. Thus, the 
evaluation function represented by equation (1) minimizes the evaluation 
function by the current sample value without relying on the past data. 
Thereby, it becomes possible to suppress not only the fluctuation of the 
gain of the object to be controlled, but also the overshoot which might 
otherwise be caused when the head speed is accelerated and reaches the 
target speed. That is, the head is positioned rapidly with high precision 
by one seek operation and the succeeding following operation regardless of 
the fluctuation of the gain of the object to be controlled by sequentially 
controlling it during the seek. 
In the prior art method shown in FIGS. 6a through 6c in which the 
acceleration feed forward signal is not controlled, the head speed signal 
v(k) (solid line) is offset from the target speed signal tv(k) (broken 
line) when the gain of the object to be controlled fluctuates by +20%. The 
head acceleration signal a (k) (solid line) is also unable to follow the 
target acceleration signal ta(k) (broken line). Here, it takes 10.7 ms for 
the speed control to occur and the access time increases because the head 
is settled while overshooting when the control is switched to the position 
control. 
The results of a simulation carried out when the loop gain of the object to 
be controlled was reduced by 20% and a distance of 1/3 of the entire 
movable tracks is sought, will be explained with reference to FIGS. 7a 
through 7d and 8a through 8c. FIGS. 7a through 7d are charts of the 
responses when the acceleration feed forward signal is controlled using 
the adaptive control rule represented by equation (5) above, and FIGS. 8a 
through 8d are charts showing the responses of the prior art seek 
operation in which no adaptive control rule is used. The target speed 
curve and the target acceleration curve are the same with those shown in 
FIGS. 5 and 6. 
In the response of the seek operation of the present embodiment shown in 
FIG. 7, the variable gain .theta.(k) shown in FIG. 7d, estimated by the 
adaptive control rule, converges to 1.25 (=1/0.8) in about 1 ms. because 
the loop gain of the object to be controlled fluctuates by 20%. Due to 
that, the head speed coincides with the target speed and the control is 
switched from the speed control to the position control in 10 ms. A 
settlement response of the head position equal to the nominal state, 
wherein there is no fluctuation of the gain, could be obtained. 
On the other hand, in the method shown in FIG. 8 in which the acceleration 
feed forward signal is not controlled, the head speed signal v(k) (solid 
line) is offset from the target speed signal tv(k) (broken line) when the 
gain of the object to be controlled fluctuates by -20%. The head 
acceleration signal a(k) (solid line) is also unable to follow the target 
acceleration signal ta(k) (broken line). Thereby, the speed control time 
is shorted to about 9 ms. and a rush speed and a rush acceleration become 
large when the control is switched to the position control. Thus, it takes 
more time to settle the head and the access time is increased. 
FIG. 9 shows a force constant which is a constant when a current flowing 
within the voice coil motor 37 for driving the head is converted into 
torque. The voice coil motor 37 is a rotary type motor and a gain of the 
force constant is apt to become small in the tracks located at the outer 
and inner diameter sides of the disk. Assume now that the head is moved 
from a track position A to a track position B at the outermost diameter, 
because the gain is small at the outer diameter side, the capability for 
following the target speed and the target acceleration degrades around the 
time when the seek operation ends and, as a result, the response in 
positioning the head to the target track fluctuates. 
In order to eliminate such fluctuation, the acceleration feed forward 
variable gain is controlled so as to cancel out the decrease of the gain 
of the voice coil motor as shown in FIG. 10. Thereby, the head may be 
positioned favorably at the target track B. 
The adaptive controller 12 of the present invention controls the 
acceleration feed forward variable gain sequentially in real-time during 
the seek operation and allows an increase in the speed error caused by 
environmental changes such as temperature and humidity to be suppressed. 
The adaptive control rule 11 represented by equation (2) is supposed to be 
used in a discrete time control system by means of a data plane servo 
method. The control rule 11 updates the variable gain discretely. For a 
continuous time control system by means of the servo plane servo method, 
an adaptive control rule may be represented by the following equation (8) 
with respect to time t; 
##EQU5## 
Further, because the target acceleration is added only when the speed is 
decelerated, the value of the target acceleration is always negative. Due 
to that, although equation (6) of the learning gain .eta. which guarantees 
the convergence of the estimated value .theta.(k) is always satisfied, 
there is a possibility that sa(k) may become zero in a general unit, so 
that the learning gain .eta. (fixed gain) may be found from the following 
equation (9) as a variable gain .eta.(k); 
##EQU6## 
Here, .lambda. is a positive constant. Apparently, equation (9) satisfies 
equation (6). 
The adaptive control rule 11 of the present embodiment may be used not only 
in equation (5) for minimizing the evaluation function expressed by 
equation (1), but also in the following equation (10). The equation (10) 
controls the acceleration feed forward variable gain .theta.(k) in 
real-time so as to minimize a square sum of the speed error signal; 
##EQU7## 
A minimum value of equation (10) may be obtained by finding .theta.(k) 
which satisfies the following equation (11); 
##EQU8## 
This corresponds to the case in FIG. 2 wherein the value of .theta.(k) at 
point C is found directly. Solving equation (11) for .theta.(k) yields the 
following equation (12); 
##EQU9## 
Because equation (11) is not suited to sequential calculation, it is 
changed so as to be calculated sequentially in real-time during the seek 
operation. The adaptive control rule 11 which minimizes equation (9) turns 
out finally as the following equations (13) and (14); 
##EQU10## 
Here, an initial value of the variable gain is set as .theta.(k)=1.0 and an 
initial value .gamma.(0) of the learning gain, which decides the speed of 
convergence, is set at a positive constant. 
In the present invention, the evaluation function is not confined only to 
those two evaluation functions described above. Rather, various control 
rules which reduce the speed error signal may be used. Further, the 
initial value .theta.(o) of the acceleration feed forward variable gain in 
the adaptive control rule may be set as the final gain value controlled 
during the previous seek operation. Thereby, if the gain of the object to 
be controlled does not fluctuate during the previous and current seek 
operations, a transient response caused by the control of the variable 
gain may be eliminated and the control system may be maintained in the 
optimum condition from the start of the control. 
It is also possible to carry out a trial seek operation off-line and to 
control the gain during the trial seek, without controlling the 
acceleration feed forward variable gain in real-time during the seek 
operation. In practice, the trial seek is carried out when the power is 
turned on at the range of the force constant kf of the voice coil motor is 
flat, and a value .theta.s of the acceleration feed forward variable gain 
is stored in memory. Then, the head is caused to seek several track 
positions wherein the force constant kf changes and a value .theta.f of 
variable gain of the final one is stored in the memory corresponding to 
the destination track and the variable gain, respectively. When the seek 
operation is carried out in the section where the force constant kf is 
flat, a value .theta.s of the variable gain is retrieved from the memory 
and is set in a gain of the acceleration feed forward operation. When the 
head is caused to seek a section where the force constant kf changes, the 
seek operation is carried out by using the value .theta.f of the 
acceleration feed forward variable gain in the vicinity of the destination 
track. 
Although only the acceleration feed forward gain has been controlled in the 
present embodiment, each gain of a filter may be controlled sequentially 
so that the speed error is minimized when the filters are included in the 
acceleration feed forward operation. 
FIG. 11 is a block diagram of another embodiment of a positioning adaptive 
control device of the present invention. 
The present embodiment is different from the above-mentioned embodiment in 
that the acceleration feed forward control input ua(k) is obtained from 
the target acceleration ta(k) and an acceleration disturbance sd(k). Two 
variable gains are attached to the adaptive controller 12 used in the 
above-mentioned embodiment to be able to deal with the acceleration 
disturbance sd(k). That is, beside the target acceleration, the 
acceleration disturbance is also added in the control input. In this case, 
two adaptive control rules 20 are also set. 
An acceleration disturbance signal may be obtained by mounting an 
acceleration pickup sensor on a base and by measuring an acceleration 
vibration of the base during seek operation. It is also possible to 
measure a force disturbance of the FPC for transmitting the head 
recording/reproduction signal to the circuit board to input as the 
acceleration disturbance sd(k). When it is impossible to measure the 
acceleration disturbance, it is possible to set sd(k)=1.0, supposing that 
there is a constant acceleration disturbance acting on the device. 
A method for controlling a gain value .theta..sub.1 (k) of a variable gain 
controller 22 for the acceleration disturbance sd(k) and a gain value 
.theta..sub.2 (k) of a variable gain controller 21 for the target 
acceleration ta(k) will be explained below. An evaluation function for 
minimizing a speed error ev(k) may be expressed by the following equation 
(15); 
##EQU11## 
The evaluation function shown in equation (15) is minimized by using the 
gradient method and a value of uv(k) is substituted into an adaptive 
control rule 20 expressed by the following equations (16) and (17) to 
control the variable gains .theta..sub.1 (k) and .theta..sub.2 (k) in 
real-time. The variable gains are not controlled when the head is 
accelerated. 
##EQU12## 
where, .eta. is a learning constant which decides the speed of convergence 
of the variable gains .theta..sub.1 (k) and .theta..sub.2 (k). 
Initial values .theta..sub.1 (0) and .theta..sub.2 (0) of the variable 
gains are both set at .theta..sub.1 (0)=1.0 and .theta..sub.2 (0)=1.0. 
As described above, because the variable gains which can deal with the 
disturbance are provided anew, the disturbance applied to the object to be 
controlled 1 may be suppressed rapidly with high precision during the seek 
operation and the head access time may be shortened. 
FIG. 12 is a block diagram showing another embodiment of a head positioning 
adaptive control device according to the present invention. 
The present embodiment is different from the embodiment shown in FIG. 1 in 
that the head is moved to a target track only by the position control 
system, not using the speed control system. A target acceleration 
generator 50 generates a tertiary smooth acceleration orbit which is 
expressed by the following equation (18) so that the time change of the 
control input u(k) is minimized for example; 
##EQU13## 
Where, L is a seek moving distance, T is a sampling time and Td is the 
time necessary for the move. 
In FIG. 12, the target acceleration ta(k) is input to a mathematical model 
55 of the object to be controlled in order to generate a target position 
signal ym(k). For example, when the mathematical model is expressed by a 
double integral, the target position is a quintic orbit expressed by the 
following equation (19); 
##EQU14## 
A position controller 57 is equipped with a phase compensating filter or 
the like in order to reduce the position deviation error ye(k) between an 
actual position y(k) and a target position ym(k) of the head. The position 
controller 57 produces a feedback control input uf(k). 
The target acceleration ta(k) is multiplied with a gain value ka of an 
acceleration gain controller 51 comprising an inverse gain of the loop 
gain of the object to be controlled 1 in order to input a signal sa(k) to 
an adaptive controller 54. A gain .theta.(k) of a variable gain controller 
52 of the acceleration feed forward operation is controlled so that the 
feedback control input uf (k) is minimized. When the feedback control 
input uf (k) converges to zero, the position deviation error ye(k) also 
converges to zero. That is, the evaluation function expressed by the 
following equation (20) is minimized; 
##EQU15## 
The above evaluation function is minimized by using the gradient method and 
a value of u.sub.f (k) is substituted into an adaptive control rule 53 
expressed by the following equation (21) to control the variable gain 
.theta.(k) of the acceleration feed forward operation in real-time. The 
variable gain is controlled when the seek operation is started. 
##EQU16## 
where, .eta. is a learning constant which decides a speed of convergence 
of the variable gain .theta.(k). 
The initial value .theta.(0) of the variable gain is set at .theta.(0)=1.0. 
It is possible to update the learning gain .eta. within equation (21) by 
the variable gain .eta.(k) expressed by equation (9). Here, if the object 
to be controlled 1 and the mathematical model 55 of the object to be 
controlled are the same, and the head position y(k) and the target 
position ym(k) are equal, then the position controller 57 is not actuated 
and the feedback control input uf(k) is zero. At this time, the adaptive 
control rule also does not act and the feed forward control input ua(k) 
turns out as a control input u(k) for directly driving the object to be 
controlled 1. 
However, when the object to be controlled 1 is not the same as the 
mathematical model 55 of the object to be controlled, the position 
deviation error ye(k) is produced and the position controller 57 generates 
the feedback control input uf(k) for suppressing the error. The adaptive 
control rule 53 controls the variable gain .theta.(k) of the acceleration 
feed forward operation sequentially by equation (21) so that the feedback 
control input uf(k) is minimized. As the control of the variable gain of 
the acceleration feed forward advances, the feedback control input uf(k) 
converges to zero and the position deviation error ye(k) is eliminated. 
While the head positioning time is delayed by being affected by the 
fluctuation of the gain when the gain of the object to be controlled 
fluctuates in the prior art method as described above, the present 
embodiment allows the head to be moved rapidly with high precision to the 
target position even if the gain of the object to be controlled 
fluctuates. The present embodiment allows an effect similar to the 
embodiment shown in FIG. 1 to be obtained even if the speed control system 
is omitted. 
The results of a simulation in which the loop gain of the object to be 
controlled 1 was increased by +20% and a distance of 1/3 of the whole 
movable tracks was sought by the head is shown in FIGS. 13a through 13c 
and FIGS. 14a and 14b. FIGS. 11a through 13c are charts of the responses 
of the seek operation when the acceleration feed forward signal is 
controlled by the adaptive controller 54 used in the embodiment shown in 
FIG. 12, and FIGS. 14a and 14b are charts of the responses of a prior art 
seek operation in which no adaptive controller 54 is used. A target 
acceleration curve of the head was conditioned so that a gain of the 
object to be controlled would not fluctuate and that the seek operation 
ends in 10 ms. 
In FIGS. 13a through 13c, the head position signal y(k) is plotted in FIG. 
13a, the head acceleration a(k) (solid line) and a target acceleration 
signal ta(k) (broken line) are plotted in FIG. 13b, and the variable gain 
.theta.(k) estimated by the adaptive control rule 53 is plotted in FIG. 
13c. As an adaptive control rule, the control rule expressed by equation 
(21) which minimizes the evaluation function of equation (20) was used. 
The variable gain .theta.(k) is estimated from when the head begins to 
move. Because the loop gain of the object to be controlled fluctuates by 
+20%, the acceleration feed forward variable gain .theta.(k) cancels out 
the fluctuation of the gain and converges to 0.833(=1/1.2) in about 1 ms. 
The head acceleration coincides with the target acceleration and the seek 
operation was finished in 10 ms. The same response of settlement of the 
head position with the nominal state in which the gain does not fluctuate 
in shifting to the following operation was obtained. 
On the other hand, in the prior art method shown in FIGS. 14a and 14b in 
which the acceleration feed forward signal is not controlled, it can be 
seen that when a gain of the object to be controlled fluctuates by +20%, 
an actual acceleration signal a(k) (solid line) is offset from a target 
acceleration signal ta(k) (broken line) of the head as shown in FIG. 14b. 
Thereby, because the seek operation takes 11 ms. and the head is settled 
while overshooting when the control is switched to the position control, 
the access time is increased. 
It is contemplated to add the function for compensating the acceleration 
disturbance in the embodiment shown in FIG. 11 to the embodiment shown in 
FIG. 12. It is also contemplated to obtain the feed forward control input 
from the target acceleration and the acceleration disturbance and to 
control it by the above-mentioned algorithm by providing a variable gain 
to each. 
While the magnetic disk unit has been explained in the embodiments 
described above, the present invention is not confined to it. That is, the 
present invention may be applied to a head positioning control system of 
another information storage device such as a CD-ROM unit and an optical 
disk unit. It may be applied also to those units equipped with a 
positioning control system by switching a control from a speed control 
system to a position control system. It may be also applied to a 
positioning control system only by a position control system. 
According to the present invention, the acceleration feed forward signal is 
controlled in real-time during deceleration so that the deviation between 
the target speed and the actual speed is reduced when the head, whose 
speed is controlled, is shifted from an acceleration state or a constant 
speed state to a deceleration state in the positioning control system in 
which the seek operation of the head is controlled by the speed control 
and the following operation is controlled by the position control. 
Thereby, the speed error caused by a fluctuation of the gain of an object 
to be controlled and/or disturbance may be minimized. Further, when the 
control is switched from the speed control to the position control in the 
vicinity of a target track, dispersion of responses of a head position and 
head speed after the switch may be fully suppressed, thus giving favorable 
settlement responses. 
According to the present invention, in a positioning control system in 
which the seek operation and the following operation are controlled using 
only the position control, the acceleration feed forward signal is 
controlled in real-time so that a deviation between the target position 
and the actual position of the head when the seek operation is started, so 
that a position error caused by a fluctuation of the gain of an object to 
be controlled and/or disturbance may be minimized, thus giving a favorable 
settlement response when the head reaches a target track. 
According to the present invention, the acceleration feed forward signal is 
controlled, so that the head can follow a target orbit with high precision 
and the target orbit which has been loosely set in the past may be set 
more strictly. Still further, a dispersion of the access time of the head 
and head positioning operation may be reduced. 
While preferred embodiments of the present invention have been described, 
variations thereto will occur to those skilled in the art within the scope 
of the present inventive concepts which are delineated by the following 
claims.