Magnetic head-positioning device for magnetic disc drive

In a magnetic head-positioning device using a coil motor, a position sensor detects the median position between one track and an adjacent track so as to decelerate and stop the moving head at the adjacent track. The magnetic head is transferred over a distance of many tracks by acceleration to the median positions and repeating the operation described above. A pair of flip-flops enables the magnetic head to move from one track to another one and a summing amplifier weights inputs from a position sensor, speed sensor and feed circuit to control motion of the voice coil motor.

BACKGROUND OF THE INVENTION 
A conventional magnetic head-positioning device using a voice coil has a 
disadvantage in that the control circuit for controlling the voice coil 
motor is very expensive. A voice coil motor generates a driving force 
operating on the same principles as a dynamic loudspeaker. That is, the 
presence of a current within a conductive material in a magnetic field 
induces a driving force in the conductive material. In general, in a 
conventional magnetic head-positioning device using a voice coil motor, 
the magnetic head detects a signal which is stored on the servo track 
giving information of position so that it is possible to determine the 
position. For transfer between one track and an adjacent track, the 
magnetic head is controlled by changing the control mode in the section to 
the desired track position. That is, a position sensor using a servo disk 
and a magnetic head detects only the track position. Next, a counter 
counts the number of tracks which the magnetic head passes on the way to 
the desired track position. The magnetic head is accelerated or slowed 
down in speed in accordance with the counted value. 
Further, a pulse motor has also been used as a magnetic head-positioning 
device for a magnetic disc drive. However, a magnetic head-positioning 
device having a pulse motor also has the disadvantage of large size. For 
example, with reference to a minifloppy disc drive SA 400 as manufactured 
by the Shugart Company in the United States, the magnetic head is 
positioned by converting the turning motion of a step motor, having a 
diameter of approximately 55 mm and a height of approximately 25 mm, to a 
rectilinear force by means of a cam. Frequently, in the art, the magnetic 
head-positioning device for a mini-floppy disc drive uses a step motor 
having the size described above. The shape and size of a magnetic 
head-positioning device depends on the size of the step motor. Therefore, 
it is difficult to manufacture a magnetic head-positioning device with a 
step motor, and including a cam which is less than 30 mm in one dimension. 
Furthermore, precision of positioning the head is limited to .+-.20 
microns. As a result, a magnetic head-positioning device with a step motor 
is disadvantageous for a small mini-floppy disc device and also for a 
rigid disc device with regard to cost. 
What is needed is a magnetic head-positioning device for a magnetic disc 
drive which is low in cost, small in size and accurate in positioning the 
magnetic head relative to the disc. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention, a magnetic 
head-positioning device for a magnetic disc drive especially suitable for 
small size and low cost production is provided. In accordance with the 
invention, an inexpensive control circuit for controlling a voice coil 
motor is provided and a magnetic head-positioning device for a magnetic 
disc drive in accordance with this invention is described in detail 
hereinafter. 
A position sensor detects the median position between one track and an 
adjacent track so as to stop the head at the desired track. In the 
situation where the magnetic head transfers from one track to another 
track, the magnetic head is forcibly moved close to the median position 
between the desired track and the adjacent track, and then the magnetic 
head moves on to the desired track. Also, when the magnetic head is to be 
transferred over a distance of many tracks, the magnetic head is moved to 
the desired position by repeating the operation described above. Namely, 
in a conventional control circuit for controlling a voice coil motor, the 
position of only the desired track is detected and the number of tracks 
through which the magnetic head passes is counted by a counter. Then, the 
magnetic head is controlled in speed in response to the counted value. 
To the contrary, in a magnetic head-positioning device for a magnetic disc 
drive in accordance with this invention, the position sensor detects the 
median position between tracks. The magnetic head is forcibly accelerated 
close to the median position and decelerated by a speed feedback loop from 
the median position between the track adjacent to the desired track. 
Thereby, the magnetic head is moved from the adjacent track to the 
actually desired track. In the magnetic head-positioning device in 
accordance with this invention, an electric circuit, whose main components 
are a pair of flip-flops, enables the magnetic head to move from one track 
to another one. Thus, by using a flip-flop the control circuit for 
controlling the voice coil in accordance with this invention provides a 
substantial reduction in cost as compared to the conventional circuits. 
Additionally, a magnetic head-positioning device in accordance with this 
invention is favorable as compared to a magnetic head-positioning device 
using a step motor. A magnetic head-positioning device for a magnetic disc 
drive which is less than 30 mm in height is provided. The precision of 
positioning is improved to the precision of the position sensor by having 
a high gain in the position detecting feedback loop. Where the position is 
optically detected by a detection plate with a slit which is produced by 
photoetching, the magnetic head is positioned with a precision of .+-.5 
microns. This is the precision of the detection plate as a result of 
photo-etching. In other words, in a magnetic head-positioning device for a 
magnetic disc device, variation in positioning caused by outside 
interference is reduced by making the gain high in a position feedback 
loop. As a result of this invention, a small voice coil motor comprising a 
coil and a magnetic circuit can be used in a magnetic head-positioning 
device. The magnetic head-positioning device is miniaturized as compared 
with the conventional magnetic head-positioning device with a step motor. 
The magnetic head-positioning device of this invention is very advantageous 
for producing a magnetic disc drive of small size and low cost, and 
especially for a mini-floppy disc drive, a component which is increasingly 
applied to such devices as personal computers and gaging equipment. 
Magnetic mediums for data storage produced by many manufacturers operate 
with a fixed rate of rotation so that there is interchangeability of the 
products with one another. To start reading or writing information, it is 
necessary that the magnetic medium make a half rotation. Therefore, a 
reduction in time for transfer of the head between tracks does not, on the 
average, proportionately reduce access time. For a magnetic 
head-positioning device for a mini-floppy disc drive, a reduction in cost 
and miniaturization are more desired than a quick transfer of the head 
between tracks. On this basis, a magnetic head-positioning device in 
accordance with this invention provides a smaller and less expensive 
magnetic head-positioning device for a magnetic disc drive. 
Accordingly, it is an object of this invention to provide an improved 
magnetic head-positioning device for magnetic disc drive which is 
reliable, small and low in cost to produce. 
Another object of this invention is to provide an improved magnetic 
head-positioning device for a magnetic disc drive which stops the head on 
track with precision equal to the precision of manufacture of the position 
detector. 
A further object of this invention is to provide an improved magnetic 
head-positioning device for magnetic disc drive which provides position 
stability when on track, and current overload protection under heavy load 
conditions of the head-positioning device. 
A further object of this invention is to provide an improved magnetic 
head-positioning device for magnetic disc drive which prevents positional 
error due to variation of brightness of a light source or of sensitivity 
of light receiving elements in a position detector. 
Still another object of this invention is to provide an improved magnetic 
head-positioning device for magnetic disc drive which eliminates problems 
of overshoot in moving from track to track. 
Yet another object of this invention is to provide an improved magnetic 
head-positioning device for magnetic disc drive which uses a voice coil 
motor having a magnetic circuit adapted for uniform speed detection 
sensitivity over the entire range of motion. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises the features of construction, 
combination of elements, and arrangement of parts which will be 
exemplified in the constructions hereinafter set forth, and the scope of 
the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A conventional magnetic head-positioning device using a voice coil has a 
disadvantage in that the control circuit for controlling the voice coil 
motor is very expensive. A voice coil motor generates a driving force 
operating on the same principles as a dynamic loudspeaker. That is, the 
presence of a current within a conductive material in a magnetic field 
induces a driving force in the conductive material. In general, in a 
conventional magnetic head-positioning device using a voice coil motor, 
the magnetic head detects a signal which is stored on the servo track 
giving information of position so that it is possible to determine the 
position. For transfer between one track and an adjacent track, the 
magnetic head is controlled by changing the control mode in the section to 
the desired track position. That is, a position sensor using a servo disk 
and a magnetic head detects only the track position. Next, a counter 
counts the number of tracks which the magnetic head passes on the way to 
the desired track position. The magnetic head is accelerated or slowed 
down in speed in accordance with the counted value. 
With reference to FIG. 1, a conventional circuit for controlling a voice 
coil motor includes an up/down counter 101, a digital-to-analog converter 
102, a desired velocity curve generator 103 and circuits 104 for changing 
control modes. The circuit also includes power amplifier 105 of several 
stages for amplifying the feeble signal, several millivolts in level, read 
from the servo track. Each circuit requires more than several tens of 
integrated circuits, SSI or MSI, so that the cost of a conventional 
magnetic head-positioning device is high. 
Further, a pulse motor has also been used as a magnetic head-positioning 
device for a magnetic disc drive. However, a magnetic head-positioning 
device having a pulse motor also has the disadvantage of large size. For 
example, with reference to a mini-floppy disc drive SA 400 as manufactured 
by the Shugart Company in the United States, the magnetic head is 
positioned by converting the turning motion of a step motor, having a 
diameter of approximately 55 mm and a height of approximately 25 mm, to a 
rectilinear force by means of a cam. Frequently, in the art, the magnetic 
head-positioning device for a mini-floppy disc drive uses a step motor 
having the size described above. The shape and size of a magnetic 
head-positioning device depends on the size of the step motor. Therefore, 
it is difficult to manufacture a magnetic head-positioning device with a 
step motor and including a cam which is less than 30 mm in one dimension. 
Furthermore, precision of positioning the head is limited to .+-.20 
microns. As a result, a magnetic head-positioning device with a step motor 
is disadvantageous for a small mini-floppy disc device and also for a 
rigid disc device with regard to cost. 
What is needed is a magnetic head-positioning device for a magnetic disc 
drive which is low in cost, small in size and accurate in positioning the 
magnetic head relative to the disc. 
Generally speaking, in accordance with the invention, a magnetic 
head-positioning device for a magnetic disc drive especially suitable for 
small size and, low cost production is provided. In accordance with the 
invention, an inexpensive control circuit for controlling a voice coil 
motor is provided and a magnetic head-positioning device for a magnetic 
disc drive in accordance with this invention is described in detail 
hereinafter. 
A position sensor detects the median position between one track and an 
adjacent track so as to stop the head at the desired track. In the 
situation where the magnetic head transfers from one track to another 
track, the magnetic head is forcibly moved close to the median position 
between the desired track and the adjacent track, and then the magnetic 
head moves on to the desired track. Also, when the magnetic head is to be 
transferred over a distance of many tracks, the magnetic head is moved to 
the desired position by repeating the operation described above. Namely, 
in a conventional control circuit for controlling a voice coil motor, the 
position of only the desired track is detected and the number of tracks 
through which the magnetic head passes is counted by a counter. Then, the 
magnetic head is controlled in speed in response to the counted value. 
To the contrary, in a magnetic head-positioning device for a magnetic disc 
drive in accordance with this invention, the position sensor detects the 
median position between tracks. The magnetic head is forcibly accelerated 
close to the median position and decelerated by a speed feedback loop from 
the median position between the track adjacent to desired track. Thus, the 
magnetic head is moved from the adjacent track to the actually desired 
track. In the magnetic head-positioning device in accordance with this 
invention, an electric circuit, whose main components are a pair of 
flip-flops, enables the magnetic head to move from one track to another 
one. Thus, the control circuit for controlling the voice coil in 
accordance with this invention provides a subtantial reduction in cost as 
compared to the conventional circuits. 
Additionally, a magnetic head-positioning device in accordance with this 
invention is favorable as compared to a magnetic head-positioning device 
using a step motor. A magnetic head-positioning device for a magnetic disc 
drive which is less than 30 mm in height is provided. The precision of 
positioning is improved to the precision of the position sensor by having 
a high gain in the position detecting feedback loop. Where the position is 
optically detected by a detection plate with a slit which is produced by 
photoetching, the magnetic head is positioned with a precision of .+-.5 
microns. This is the precision of the detection plate as a result of 
photo-etching. In other words, in a magnetic head-positioning device for a 
magnetic disc device, variation in positioning caused by outside 
interference is reduced by making the gain high in a position feedback 
loop. As a result of this invention, a small voice coil motor comprising a 
coil and a magnetic circuit can be used in a magnetic head-positioning 
device. The magnetic head-positioning device is miniaturized as compared 
with the conventional magnetic head-positioning device with a step motor. 
The magnetic head-positioning device of this invention is very advantageous 
for producing a magnetic disc drive of small size and low cost, and 
especially for a mini-floppy disc drive, a component which is increasingly 
applied to such devices as personal computers and gaging equipment. 
Magnetic mediums for data storage produced by many manufacturers operate 
with a fixed rate of rotation so that there is interchangeability of the 
products with one another. To start reading or writing information, it is 
necessary that the magnetic medium make a half rotation. Therefore, a 
reduction in time for transfer of the head between tracks does not, on the 
average, proportionately reduce access time. For a magnetic 
head-positioning device for a mini-floppy disc drive, a reduction in cost 
and miniaturization are more desired than a quick transfer of the head 
between tracks. On this basis, a magnetic head-positioning device in 
accordance with this invention provides a smaller and less expensive 
magnetic head-positioning device for a magnetic disc drive. 
In the magnetic head-positioning device for a magnetic disc drive in 
accordance with the invention, a voice coil motor is used as the 
electro-mechanical converter which advances rectilinearly in response to 
the force produced by a current bearing conductor in a magnetic field. 
Such a mechanical construction is in practical use in various known 
devices. FIG. 2 is a perspective view of a magnetic head-positioning 
device in accordance with this invention comprising a voice coil motor 
having a coil 201 and magnet 202. The coil 201 is disposed in the magnetic 
field produced by the magnet 202, and is connected to a head holder 206 
whereon a magnetic head 203 is mounted. The holder 206 is movable forward 
and backward along the slides or axes 207 by the force generated in the 
coil 201. Thereby, the magnetic head 203 is transferred onto a track of a 
magnetic disc device (not shown in FIG. 2. A position sensor 204 and a 
speed sensor 205 detect the position and speed of the head holder, that 
is, the magnetic head, respectively. The magnetic head positioning device 
for magnetic disc drive in accordance with this invention positions the 
magnetic head by controlling the voice coil motor as shown in FIG. 2. 
FIG. 3 is a functional block diagram and FIG. 4 is a circuit diagram of a 
magnetic head-positioning device in accordance with the invention. Outputs 
from a position sensor 301, speed sensor 302 and feed circuit 303 are 
summed by a summing amplifier 304, and are subsequently amplified by a 
power amplifier 305 to provide a signal for a voice coil motor 306. A 
plate for detecting the position of the position sensor and a speed 
sensing coil of the speed sensor are mounted on a moving portion of the 
voice coil motor. Therefore, a position control feedback loop includes a 
position sensor, summing amplifier, power amplifier, and a voice coil 
motor. Similarly, the speed feedback loop includes a speed sensor, summing 
amplifier, power amplifier, and the voice coil motor. 
In FIG. 4 portions are outlined with broken lines to indicate those 
functions which correspond with FIG. 3. In FIG. 4, however, the summing 
amplifier 304 and power amplifier 305 are not definitely separated 
although such a circuit structure is also possible. The circuit includes 
resistances 401-403, input terminals 406-408 and output terminal 409 of 
the feed circuit, and D-type flip-flops 410,411 with set-reset capability. 
FIG. 5 shows a metal plate with slits for sensing a position. Light emitted 
from a luminescent element, a light emitting diode 501, arrives at 
receiving light elements, photo transistors 502, 503. A moving plate 505 
with slits, which is connected with the magnetic head carriage 206, moves 
in the directions as shown by an arrow 506. 
As shown in FIG. 6, the moving plate 505 is provided with slits for 
transmitting light in two rows between which there is a 180.degree. phase 
shift. On account of the 180.degree. out-of-phase relationship, outputs 
from photo transistors 502, 503 represent waveforms as shown in FIGS. 7a, 
b, respectively, against variations of position x of the moving plate 505 
with slits. As shown in FIGS. 7a, b, the waveforms are 180.degree. 
out-of-phase against the variation of position x. The output from a 
position sensor presents the waveform as shown in FIG. 7c by amplifying 
the outputs from the transistors as shown in FIGS. 7a, b by means of a 
differential amplifier. 
According to the supply voltage and the amplifying levels of the position 
sensor, the output from the position sensor is saturated as shown in FIG. 
7(c). The voice coil motor is driven by a signal outputted from the 
position sensor through a summing amplfiier and a power amplifier. In FIG. 
7(c), provided that the direction to the center of rotation of a magnetic 
disc drive is "forward" and the opposite direction is "backward", a voice 
coil motor goes forward as shown by an arrow 701 when the output from the 
position sensor is at low level. The direction "backward" is shown by an 
arrow 702. As for a position controlling feedback loop consisting of a 
position sensor, summing amplifier and voice coil motor, each of the 
positions x1, x2, . . . xn (n is the total number of tracks) in FIG. 7(c), 
indicates a position where the voice coil motor is stopped. The positions 
indicated by x'1, x'2, . . . x'n are the median positions between tracks. 
In the present specification, the "adjacent track positions" means the 
track positions which adjoin each other such as x1 and x2, or x2 and x3. 
The magnetic head is controlled to be transferred and stopped at the 
nearest track position to the median position between adjacent tracks. 
That is, the magnetic head is controlled to stop at the position where the 
outputs from the two photo-transistors coincide in level. 
An object of the invention is to prevent the occurrence of positional error 
caused by variation of the brightness of the luminescent element and the 
receiving photo-sensitivity of the light-receiving elements. This is 
achieved by receiving the light emitted from only one luminescent element 
by means of two light-receiving elements through a detection plate. In 
FIG. 8, the outputs PTa and PTb from the two photo-transistors intersect 
in a point P. Therefore, the position xP is a track position where the 
voice coil motor is stopped. That is, the position xP corresponds to one 
of x1, x2, . . . xn of FIG. 7. If the light density emitted from the light 
emitting diode is increased by a change of the supply voltage due to 
temperature characteristics, and so on, or the photo-sensitivities of the 
two photo-transistors become high on account of temperature 
characteristics, or the like, the output PTa from the photo transistor 
changes to PTa', and the output PTb changes to PTb'. Consequently, the 
intersecting point where the outputs from the two photo transistors 
coincide changes from P to Q. Nevertheless, the positions xP and xQ 
respectively, corresponding to the intersecting points P and Q are not 
changed. In the present invention, therefore, positional error does not 
occur due to the variation of the brightness of the light-emitting element 
and the photo-sensitivity of the light-receiving elements. 
In FIG. 5, a slit 507 of a stable plate 504 is smaller than a slit 508 of 
the moving plate 505. The result is that the brightness passed through the 
slits of the plates 504 and 505 is determined by the mechanical accuracy 
of a perpendicular side of the slit in the direction in which the plate 
505 transfers as indicated by the arrow 506. A variation in this direction 
perpendicular to the direction in which a plate 505 transfers does not 
affect the brightness of the light passed through slits. Namely, a 
magnetic head-positioning device according to this invention is effective 
in that variation in the side perpendicular to the direction in which the 
movable plate transfers at the time of, or after mounting, does not affect 
the positioning accuracy. It may as well make a stable plate 504 and a 
mounting of a photo-transistor or a light-emitting diode in a body. 
As shown in FIGS. 5 and 6, only two slits 509 and 510 which are disposed 
near the longitudinal ends of the movable plate 505 are larger in the 
moving directions 506 than the other slits 508. This provides large zones 
where the voice coil motor is not stopped other than at x1 and xn in FIG. 
7. The voice coil motor is prevented from stopping at an unsuitable 
position by providing a stopper mechanically in the zone where the 
magnetic head is not stopped electrically. That is to say, a zone in which 
the last stopper of a voice coil motor is provided is extremely expanded 
by making the two slits nearest the edge of movable plate larger than the 
other slits in the transferring direction. 
Thus, the present invention permits a reduction in cost for manufacturing a 
magnetic disc drive. The movable plate 505 is produced from a thin metal 
plate using a photolithographic process or a pressing process. Such 
methods for manufacturing a plate for detection bring about a reduction in 
cost and a shortening of the space between a light-emitting element and a 
light-receiving element. On a floppy disc drive, a standard track pitch is 
529 .mu.m. The interval between x1 and x2 is 529 .mu.m in FIG. 7 and 
consequently the width of a slit 508 of a movable plate is 265 .mu.m in 
FIGS. 5 and 6. 
The light emitted from the luminescent element gets to a light receiving 
element through a slit of so narrow a width of about 265 .mu.m. On the 
other hand, the brightness of the emitting light is gradually decreased 
because of the energization of the light-emitting diode. To decrease the 
current energizing in a light-emitting diode provides a longer diode life. 
However, the reduction in current which flows in a light-emitting diode 
justly indicates the decrease in brightness of the emitting light. Also, 
it becomes slow to respond as making the sensitivity of a photo-transistor 
high. Under these conditions, it is difficult for the output voltage to be 
large at the position of a photo-transistor. 
What is most able to eliminate the above mentioned disadvantages is to 
narrow the space between the light-emitting diode and a photo-transistor. 
This is achieved by means of the metallic plate for detection which can be 
produced to be some ten .mu.m in thickness. 
If a slitted plate whose thickness is some ten .mu.m is made of glass, this 
glass plate is easily broken by a slight stress. Though a glass plate is 
widely used, it is not suitable for such a position sensor as in an 
embodiment of this invention. On the other hand, a metallic plate with 
slits according to this invention is neither broken nor bent because it 
moves in a small space along the gap. 
Furthermore, a position sensor according to the present invention requires 
no adjustment for a standard track detector by making a position sensing 
plate and a standard track detecting plate in a unitary body. 
A standard track is standardized for a track-seek operation in a magnetic 
disc device. A track on the most outer periphery is usually considered as 
a standard track, and it is 00 track with reference to a floppy disc 
device. That is, a floppy disc device is provided with a 00 track 
detector. And a 00 track detector is adjusted in position at the position 
of the 00 track so as to detect the 00 track. In FIG. 6, a 00 track is 
detected by a light-emitting diode 601, a photo-transistor 602 and a slit 
plate 603. The plate with slit 603 is united with a position sensing plate 
with slits 505 in a unitary body. The position sensing plate with slits 
(FIG. 6) not only determines track position, but detects the 00 track. 
Accordingly, there is no necessity for adjusting the position of a 00 
track detector, which provides an extreme reduction in cost. 
Next, the speed sensor and speed feedback loop are explained. The 
conductive material moving in the magnetic field has induced an 
electromotive force which is proportional to speed. The speed sensor 
output is obtained by amplifying the above-mentioned electromotive force. 
FIG. 9 shows an example of a speed sensor which is composed of a speed 
sensitive coil 901, a magnet 902 and a magnetic circuit 903. The speed 
sensitive coil 901 is transferred in the directions of the arrow 904 
together with the voice coil motor, and produces the electromotive force 
which is proportional to speed. 
The above-mentioned speed sensor is shown as only an example. For example, 
a construction wherein the speed sensitive coil is fixed, and the magnet, 
or the magnet and the magnetic circuit move is also practically used. 
In the speed sensor shown in FIG. 9, the magnetic flux concentration of the 
gap portions 905 at the ends of the magnet is lower than that in the 
center of the magnet. Thus, the sensitivity to speed decreases at both 
ends of the magnet. 
In a conventional speed sensor, uniform sensitivity to speed is obtained by 
making the magnet longer and by moving the speed sensitive coil within a 
region which has uniform magnetic flux concentration in the gap. However, 
this method has such shortcomings that a large size magnet is needed and 
the speed sensor increases in dimension. 
On the other hand, the speed sensor in accordance with the present 
invention has a uniform magnetic flux concentration in the gap and uniform 
sensitivity to speed at both ends of the magnet. Such a speed sensor is 
obtained by forming the magnetic circuit 903 inside the speed sensitive 
coil shown in FIG. 9 in a manner such that cross-sectional area is larger 
at both ends in the transverse direction with respect to the moving 
direction 904 of the speed sensitive coil shown in FIG. 9. In addition, 
sensitivity distribution to the desired speed can also be obtained. When 
the voice coil motor moves on the track and the inductance of the coil at 
both ends is relatively high, it sometimes passes through a target 
position because of a delay of speed reduction. That is, "overshoot" is 
produced. In accordance with the present invention, overshoot is 
eliminated by controlling the maximum speed with the above-mentioned 
magnetic circuit wherein the magnetic flux concentration is higher and the 
speed feedback is stronger at both ends. The target speed in the speed 
feedback loop is 0, that is, viscous resistance is produced toward the 
move of the voice coil motor. 
The speed feedback loop has two objects. One of them is to achieve 
stability of the voice coil motor in the stop position. It is well known 
to achieve stability by providing a speed feedback loop in addition to the 
position controlling feedback loop, so a detailed explanation is omitted 
here. The speed feedback loop in a magnetic head-positioning device for 
magnetic disc drive in accordance with the present invention has another 
object as follows. The objects is to reduce the speed from the median 
position between adjacent tracks to the adjacent track at the time of 
moving on track. As stated above, the voice coil motor moves to the 
nearest track position from the median position between adjacent tracks by 
means of the position control loop. At the time of moving on tracks, the 
voice coil motor, which is accelerated to the median position between 
adjacent tracks, is decelerated as needed at positions up to the next 
median position between adjacent tracks in the moving direction. As 
illustrated in FIG. 7, at the time of moving from track X.sub.1 to track 
X.sub.2, the voice coil motor can be decelerated from the median position 
X.sub. 1 ' between adjacent tracks to the next adjacent track position 
X.sub.2 ' in the moving direction. Thus, in the magnetic head-positioning 
device for magnetic disc drive in accordance with the present invention, 
the speed feedback loop has open loop gain by which the speed can be 
reduced with the above-mentioned position. When the position sensor output 
or the speed sensor output is saturated, the summing amplifier has the 
function for weighted-summing in order to provide the greater weight to 
the speed sensor output. In FIG. 7, at the time of moving from X.sub.1 to 
X.sub.2, the position control loop has the function of accelerating the 
voice coil motor from X.sub.1 ' to X.sub.2. On the other hand, the speed 
control loop has the function for always decelerating the voice coil 
motor. At the time of moving on tracks, if the voice coil motor is not 
decelerated enough from X.sub.1 ' to X.sub.2, it raises a possible problem 
that the voice coil motor moves to X.sub.3. Accordingly, the magnetic 
head-positioning device for magnetic disc drive in accordance with the 
present invention has speed feedback, making is possible, to decelerate 
enough from X.sub.1 ' to X.sub.2, or the sum amplifier for providing the 
greater weight to the speed sensor output than that to the position sensor 
output. 
Next, the feed circuit and movements on tracks are explained. The feed 
circuit has the function for moving the magnetic head to the adjacent 
track position. An example of a feed circuit is shown at 303 of FIG. 4. 
Operation of the feed circuit is explained with reference also to FIG. 11. 
In FIG. 11, graph a shows change of the position sensor output versus 
change of position. That is, FIG. 11a shows the same information as FIG. 
7c. In FIG. 11a, the positions X.sub.1, X.sub.2 and X.sub.3 are stop 
positions, namely, track position points on the magnetic disc, and the 
positions X.sub.1 ', X.sub.2 ' and X.sub.3 ' are median points between 
adjacent tracks. When the magnetic head is transferred in the direction 
wherein the value of X increases, the position sensor output alternately 
and repeatedly is increased and decreased. In this case, the position 
sensor output is increasing in the neighborhood of the track position and 
is decreasing in the neighborhood of the median point between adjacent 
tracks. 
On the other hand, when the magnetic head is transferred in the direction 
wherein the value of X decreases, the position sensor output is increased 
in the neighborhood of the median point between adjacent tracks and is 
decreased in the neighborhood of the track position. This is opposite to 
the above-mentioned case. That is, when the moving direction of magnetic 
head is known, the median position between adjacent tracks and the 
adjacent track position can be discriminated in FIG. 11a showing the 
position sensor output. The voice coil motor is compulsorily driven to the 
median point between adjacent tracks by using the above-mentioned 
discrimination method in the feed circuit, and thus, the magnetic head is 
transferred to the adjacent track. 
In FIG. 4, D-type flip-flops 410, 411 have set-reset capability and change 
on a rise of the clock input. When the magnetic heads stops, the 
flip-flops 410, 411 are set in the state of turning on the power by the 
input terminal 408, and the output 409 of the feed circuit is in a 
floating state. 
A case where the magnetic head is transferred forward by one track is 
explained using an example of being transferred from X.sub.1 to X.sub.2 in 
FIG. 11. A reset pulse b of FIG. 11 is inputted to the flip-flop 410 from 
the input terminal 406 shown in FIG. 4. The flip-flop 410 is reset, and 
Q.sub.1 goes to a High-level. The output from Q.sub.1 is shown in FIG. 
11c. When the transistor with its gate connected to Q is ON, the feed 
circuit output 409 comes to a Low-level and a signal for transferring the 
voice coil motor forward (in the direction toward X.sub.2) is outputted. 
When the voice coil motor is transferred forward and reaches the median 
point between adjacent tracks, which is X.sub.1 ' shown in FIG. 11a, a 
position comparator 416 within the position sensor changes state. Thus, 
Q.sub.1 from the flip-flop 410 comes to the Low-level, being clocked by 
the output of the comparator 416, and the output from the feed circuit is 
in the floating condition again. That is, the feed circuit outputs a 
signal by which the voice coil motor is compulsorily transferred to the 
median point between adjacent tracks. Above, the case where the voice coil 
motor is transferred in the forward direction was explained as an example. 
In case of transfer in the backward direction, quite the same operation is 
accomplished by means of a flip-flop 411. 
In summarizing, the above described circuitry provides a regulation system 
for the magnetic head which performs properly to bring the head to the 
desired track under three very probable conditions. Namely, as stated 
above, the head is accelerated from its position on the first track 
X.sub.1 to the median point X.sub.1 ' and thereafter it is decelerated as 
required to bring the head to rest over the next, that is, the adjacent 
track X.sub.2. In the process of deceleration the head may stop in any of 
three positions. Namely, the head in being decelerated may first stop in 
the region between X.sub.1 ' and X.sub.2, that is, it is short of its 
target position. The control system will then bring the head to the 
position X.sub.2 with further accelerations and decelerations as 
neccessary. 
Secondly, in being decelerated from the median position X.sub.1 ', the head 
may stop for the first time right on the target track position X.sub.2. No 
further operation is required. However, in a third condition the head may 
stop beyond the target track X.sub.2, that is, the head may first stop 
between X.sub.2 and the next median position X.sub.2 '. In such a case the 
circuits will provide a deceleration signal, that is, a reverse signal, 
which will bring the head back to the target track X.sub.2, recognizing 
that this may require several accelerations and decelerations. 
In the undershoot condition, the head is accelerated again by the position 
feedback loop and reaches the target target track position and stops. In 
the overshoot case where the head passes through the target track 
position, and does not pass through the next median position X.sub.2 ', 
the head is accelerated in the return direction by the position feedback 
loop and reaches the target track position X.sub.2 and stops there. Thus, 
as stated above, when head speed becomes zero for the first time between 
the median position X.sub.1 ' and the next median position X.sub.2 ' 
through deceleration, the head reaches the target track position X.sub.2 
and finally stops there because the position feedback loop is always 
operating between the two median positions X.sub.1 ' and X.sub.2 '. The 
term deceleration is used herein, therefore, to refer to the entire 
process of bringing the head to rest at the target track position after 
initial acceleration from the first track position X.sub.1 to the next 
median position X.sub.1 '. 
In FIG. 4, flip-flops with set-reset capability are used in the feed 
circuit 303. This prevents the flip-flop from inverting before the voice 
coil motor starts moving by keeping the time width T of the reset pulse 
provided to the flip-flops 410, 411 (FIG. 11b) wide to a certain extent in 
case the voice coil motor is vibrating slightly because of a disturbance, 
and so forth, in the track position, namely, the balanced point of the 
position control loop. 
In the presented example, the track position and the median position 
between tracks are discriminated by the flip-flop of the feed circuit. 
However, it is possible to have the above-mentioned discrimination 
function within the position sensor by providing a logical circuit for 
discriminating within the position sensor. The logical circuit in the 
position sensor can be obtained in the same way of thinking as the feed 
circuit, which is explained above, so its explanation is omitted here. 
As stated above, a signal from the feed circuit for accelerating the voice 
coil motor to the median position between adjacent tracks is outputted. In 
the magnetic head-positioning device for magnetic disc drive in accordance 
with the present invention, the voice coil motor is transferred between 
tracks by properly setting the weight of each input to the summing 
amplifier for weighted-summing each output in addition to each level of 
outputs from the feed circuit, the speed sensor and the position sensor, 
and by accelerating or decelerating the voice coil motor. FIGS. 11b 
through g illustrate waveforms in each position when the voice coil motor 
is transferred from X.sub.1 to X.sub.2. A signal for accelerating the 
voice coil motor from X.sub.1 to X.sub.1 ' is outputted from the feed 
circuit in response to step pulse b. Output from the feed circuit, whose 
level is large, has the largest weight in the summing amplifier. 
Resistance 403 is smaller than resistance 401 in FIG. 4, thus, the voice 
coil motor is accelerated in the direction toward X.sub.2. When the voice 
coil motor reaches X.sub.1 ', output from the feed circuit is put in the 
floating condition and the voice coil motor is decelerated in response to 
output from the speed sensor shown in FIG. 11f. As explained above, the 
voice coil motor is set in open loop gain of deceleratable speed feedback, 
and resistances are set in the state of (resistance 403+resistance 
402)&gt;resistance 401. As a result, acceleration or deceleration of the 
voice coil motor is controlled. FIG. 11d illustrates the output from the 
position comparator 416. FIG. 11e illustrates the output from the position 
sensor. 
Moveover, voltage for acceleration can be reduced to a certain extent and 
maximum speed can be controlled in the neighborhood of the median position 
by properly setting the ratio of resistance 403 to resistance 401 as shown 
in drive waveform of FIG. 11g, which is the waveform at the voice coil 
motor terminal 404 (FIG. 4). That is, a range where the voice coil motor 
is transferred at almost constant speed in the neighborhood of the median 
position can be available by selecting resistance values. 
This has the advantages of removing malfunction at the time of moving on 
track, and of ensuring the movement on track against mechanical 
disturbance as follows. That is, it is probable that the voice coil motor 
passes over to the next track from the target track position, i.e., in the 
previous example, from position X.sub.2 to X.sub.3, because of delay of 
current inversion responsive to the inductance of the coil, namely, 
because of delay of deceleration. In this case, initial speed of 
deceleration movement is reduced and speed can be certainly reduced to 0 
before reaching the neighborhood of X.sub.2 by lowering acceleration 
voltage and controlling the maximum speed in the previous region of median 
position, as explained above. Thus, malfunction at the time of moving on 
track is removed. Moreover, the above-mentioned movements mean that the 
voice coil motor is controlled with respect to its speed at the time of 
passing through the median position between adjacent tracks (namely, the 
maximum speed) by feedback with the output signal from the speed sensor. 
Therefore, in case the voice coil motor does not speed up fully in the 
acceleration region because of an increase in friction load, and so forth, 
the maximum voltage is automatically applied to the voice coil motor until 
reaching the median position between adjacent tracks. On the other hand, 
in the opposite case, of low friction, the voltage signal applied to the 
voice coil motor is limited. That is, movement of the voice coil motor on 
track is further ensured against mechanical disturbance. 
FIG. 15 illustrates an example of experimental results with respect to the 
voice coil motor drive waveform in order to concretely explain the 
above-mentioned concepts. FIG. 15a shows the waveform in a case where the 
load on the voice coil motor is heavier than the force generated from the 
voice coil motor. The maximum voltage is continuously applied in the 
acceleration region, since the voice coil motor does not speed up fully. 
FIG. 15b shows the drive waveform in the normal condition. FIG. 15c shows 
the waveform when the supply voltage is raised above normal. In this case, 
the applied voltage in the acceleration region is more limited in time, 
since the force generated from the voice coil motor increases and speed 
goes up. 
Next, a median position detector is explained. The median position detector 
exactly detects that the voice coil motor is passing over the median 
position between adjacent tracks by means of the position detector output 
and the speed detector output. As previously explained, in the magnetic 
head-positioning device in accordance with the present invention, the 
voice coil motor is accelerated to the median position between adjacent 
tracks, and is decelerated from there. Therefore, error in detecting the 
median position causes a possibility of hunting or seek error. The median 
position detector in accordance with the present invention detects that 
the voice coil motor passes over the median position by three conditions, 
namely, an instruction direction on the tracks, the change direction of 
the position sensor output, and the output polarity of the speed sensor, 
or by any of these conditions, and outputs to the feed circuit. 
The schematic diagram of FIG. 12 shows an example of a median position 
detector 1201. The median position detector is placed between the position 
sensor 301 and the feed circuit 303 shown in FIG. 4, which is considered 
together with FIG. 12, hereafter. Logical signal movements within the 
median position detector shown in FIG. 12 are simple, so that a detailed 
explanation is eliminated here. In short, only when three outputs are 
proper, namely, the level input outputted from the magnetic disc drive 
control circuit side indicating the direction of step movements, polarity 
of the speed sensor output indicating the direction in which the voice 
coil motor actually moves, and the direction to which output of the 
position sensor (which is the position comparator 416 in FIG. 12). The 
median position detector detects that the voice coil motor passes over the 
median position between adjacent tracks in the moving direction, and 
outputs a narrow pulse to the feed circuit in order to invert the 
flip-flop within the feed circuit. As a result, the median position is 
detected exactly. 
Next, a timer circuit is explained. The timer circuit is important in terms 
of safety of the magnetic head-positioning device in accordance with the 
invention. As previously explained, in case of moving on track, current 
compulsorily flows to the voice coil motor until it reaches the median 
position between adjacent tracks in response to the feed circuit output. 
When the voice coil motor is mechanically stuck because of dust and so 
forth, there is a possibility that current flows continuously to the voice 
coil motor and the coil burns out. In order to remove this, a maximum time 
during which current can compulsorily flow to the coil by means of the 
feed circuit is set by the timer circuit. In a case of exceeding the 
above-mentioned maximum time, the feed circuit output automatically 
changes to the same condition as when the voice coil motor stops. 
An actual example is illustrated in the timer circuit 1202 of FIG. 12. The 
circuit includes monostable multivibrators 1204, 1205, and what is called 
a re-triggerable one shot multivibrator 1202, for example, MC 14538 
produced by Motorola, Inc. 
The period of the monostable multivibrator 1204 is set longer than the step 
pulse rate. When the head is continuously moving on tracks, a narrow 
pulse, which is set with the period of the monostable multivibrator 1205, 
is outputted after the time which is set with a period of the 
re-triggerable one shot multivibrator 1204 from that the last step pulse. 
Then, the flip-flop of the feed circuit, which has not been inverted, 
failing to detect the median position between adjacent tracks, is inverted 
in response to the above-mentioned pulse. Even when moving on tracks in 
order, a pulse is outputted from the timer circuit. However, output of a 
pulse does not affect control of the voice coil motor at all due to the 
construction of the feed circuit as explained above. 
Next, a capacitor short circuit is explained. In the conventional position 
control feedback loop, a compensating circuit, called a phase lag 
compensation, is put within the loop, and gain in the low frequency region 
is increased in order to minimize the stationary position error. However, 
when a large capacitor for phase lag compensation is used, the time 
required for controlling the voice coil sometimes becomes extremely longer 
by variation of the initial electric charge or other parameters of the 
capacitor, because of time delay in charge-and-discharge of the capacitor. 
In the capacitor short circuit in accordance with the present invention, 
the above-mentioned shortcomings, namely, that the period required for 
controlling the voice coil becomes extremely longer, are removed. In order 
to accomplish that, both terminals of the capacitor for phase lag 
compensation or a capacitor having a relatively large size, which is put 
within the position control feedback loop for other purposes, are 
short-circuited for a very short period at the time of reaching the target 
track position, and zero voltage on both terminals of the capacitor is 
obtained. Thus, effects of residual electric charge on the capacitor are 
removed. 
An actual example of a capacitor short circuit is illustrated in FIG. 12. 
It is comprised of circuit 1203 and a short-circuit switch for capacitor 
shorting within the position sensor illustrated in FIG. 12. Change in 
output of position comparator 416 is differentiated and the capacitor is 
short-circuited for a short period to a certain degree wherein stability 
of the voice coil motor is not affected in the stationary state. 
The effect of the capacitor short circuit is shown in FIG. 13, which shows 
results of experimentation. FIG. 13a shows performance with a circuit 
having the capacitor short circuit, and FIG. 13b shows performance in the 
circuit not having the capacitor short circuit. In both cases, the load is 
relatively heavy. Curves 1301 and 1303 show displacement of the voice coil 
motor, which is measured optically without contact. Curves 1302 and 1304, 
which are shown for reference, illustrate the voice coil motor driving 
waveforms, which correspond to FIG. 11g. In comparison of FIG. 13a with 
FIG. 13b, it is clear that control of position is greatly improved by the 
capacitor short circuit. As stated above, in the magnetic head-positioning 
device for magnetic disc drive in accordance with the invention, the 
capacitor short circuit has a large effect when the load on the voice coil 
motor is heavy. 
Next, the voice coil motor is explained. FIG. 14a shows a schematic view of 
the voice coil motor. A magnetic flux concentration is generated in the 
gap by a magnetic 1402 and magnetic circuit 1403. Force is generated by 
feeding current to a coil 1401. It is desired that the ratio L/R of 
inductance L and resistance R of the coil 1401 is small in order to speed 
up response of the voice coil motor. That is, it is desirable that 
inductance of the coil 1401 is reduced. In the voice coil motor in 
accordance with the invention, inductance is reduced by providing a cavity 
portion within the coil of the magnetic circuit and by reducing the volume 
of magnetic material within the coil as shown in FIGS. 14b, c, d, as 
examples. FIGS. 14b, c, d illustrate only the magnetic circuits. In FIGS. 
14b, c, d, magnetic flux concentration within the magnetic circuit is 
equalized by providing a cavity or gap portion which is inversely 
proportional to the magnetic flux produced by the magnet in consideration 
of the flow of magnetic flux by magnet. Owing to this, only the inductance 
of the coil can be reduced without affecting the magnetic flux 
concentration in the gap. 
As stated above, in the magnetic head-positioning device for magnetic disc 
drive in accordance with the invention, highly precise magnetic 
head-positioning and track-transferring are available by means of a very 
simple circuit construction. In particular, when applied to a floppy disc 
drive, a super-thin and less expensive floppy disc device is available and 
useful. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in the above constructions without 
departing from the spirit and scope of the invention, it is intended that 
all matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all of the generic and specific features of the invention herein described 
and all statements of the scope of the invention which, as a matter of 
language, might be said to fall therebetween.