Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 2
TITLE OF THE INVENTION
A Disk Recording/Reproduction Device and a Method ofOperation Thereof
BACKGROUND OF THE INVENTION
S Field of the Invention
The present invention relates to a disk
recording/reproduction device which records information on
a disk such as an optical disk and a magnetic disk and
which reproduces recorded information, and a method of
operating such a disk recording/reproduction device. ;
Description of the Background Art
In a conventional disk recording/reproduction device,
a structure is widely used where the head for recording
and reproducing information is positioned by being moved
to an arbitrary radial position of the disk using a linear
motor or a swing arm. ~;
Fig. 12 is a block diagram schematically showing an
example of structure of a conventional optical disk
recording/reproduction device. A disk 7 is rotatably
driven by a spindle motor 9. A plurality of tracks 8 are
provided concentrically or spirally in disk 7. An optical
head 1 directs an optical beam 3 to an arbitrary track 8
to carry out recording and reproduction of information.
Optical head 1 is fixed on a linear motor 2. Linear motor
2 includes a bearing portion 10 of a rolling or sliding
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structure in contact with the bottom floor inside th0 main
body of the device. Optical he~d 1 is driven radial of
disk 7 by linear motor 2. -
Optical head 1 is provided with a tracking error
detecting unit 4 by its optical system. Tracking error
detecting unit 4 detects the relative position deviation
amount of the actual irradiated position with respect to
the reference irradiated position b~ optical beam 3 in
track 8 to provide a tracking error signal TES
representing the position deviation amount.
Tracking error signal TES is provided to a driver 6
via a phase compensating unit 5. Phase compensating unit
5 carries out a predetermined phase compensation for ;
tracking error signal TES. Driver 6 provides a driving ~--
si~nal to linear motor 2 to cancel the tracking error .:~.
according to the phase compensated signal from phase ;
compensating unit 5. Linear motor 2 is driven according `~-
to a driving signal provided from driver 6. As a result, :
optical head is driven radial of disk 7, whereby optical
beam 3 is properly positioned on track 8.
Fig. 13 is a block diagram of a structure of a
tracking servo loop which is a control loop for carrying
out the position control of head 1. The tracking servo
loop includes the elements of tracking error detecting
unit 4, phase compensating unit 5, driver 6, and linear
--2-- .
. .: , . . . : .. . ,, . :~.
2 t~
motor 2. The tracking servo loop is a control system of
following a position Xo of optica] head 1 with a
displacement Xi of track 8 as a desired value. If the
product of all the gains of each element in the tracking
servo loop is G(s), a followed error Xe is represented by
the following equation (l):
Xe = {1 / (l + G(s))} Xi ... (l)
The behavior of optical head 1 and the position
relationship between optical beam 3 and track 8 will be
described hereinafter when disturbance vibration is
applied to the tracking servo loop of the above-described
optical disk recording/reproduction device.
Fig. 14 is a kinematic model diagram of optical head
l and its vicinity shown in Fig. 12. Assuming that the
displacement of disturbance vibration is Y, the
displacement of optical head 1 (or linear motor 2 driving ~ ;
optical head 1) is X, the mass of the movable portion of
linear motor 2 including optical head 1 is M, and the
equivalent spring constant and coefficient of viscosity
generated in bearing lO of linear motor 2 are K and D,
respectively, the following equation of (2) is obtained by
an equation of motion according to Laplace transformation:
Ms X + Ds (X - Y) + K (X - Y) = 0 ... (2)
where s represents Laplace operator.
By transforming equation (2), the ratio of
~ O ~ ~ ~j 9 2
displacement X of optical head 1 to displacement Y of
disturbance vibration (X/Y) is obtained by the following
equation (3):
(X/Y) = (Ds + K) / (Ms + Ds + K)
= 1 - {(Ms2) / (Ms2 ~ Ds + K)}
= l - {K / (Ms + Ds ~ K)} {(Ms )} / K}
{Il)o2 / ( 52 + 2~o~1)0S + ~lo ) ~ ( S / ~I)o )
= 1 - Go(s) (S2 / ~o2) ... (3)
~O(s) in the above equation (3) is a normalized form .
of the transfer function of linear motor 2 representing
the spring and mass system, and is well known as
represented by the following equation (4). :
Go(s) = K / (Ms + Ds + K)
= ~o2 / (S2 + 2~o~0 S + ~2~ ~-- (4)
lS ~0 is the resonance angular frequency of linear motor
2 and ~0 is the damping value of linear motor 2, and are
expressed by the following equations of (5) and (6),
respectively.
~0 = (K / M)s ................................................ (5)
~o = D / {2 (MK) } ................................................. (6)
Referring to Fig. 14, it is appreciated -that there is
no influence of disturbance vibration to the tracking
s~rvo loop of the optical disk recording/reproduction
device if displacement X of optical head 1 is equal to
displacement Y of disturbance vibration when disturbance
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vibration is e~erted. In other words, the difference
between displacement Y and displacement X, (Y-X) is the
component of the disturbance signal mixed into the
tracking servo loop.
The ratio of the mixed disturbance vibration
component (Y-X) to the exerted disturbance vibration is
defined in the following equation (7) as a dis~urbance
transmission rate B(s). This defined disturbance
transmission rate B(s) is used in the following
description.
B(s) = (Y - X) / Y = 1 - (X / Y) (7)
Referring to equations (3) and (7), disturbance
transmission rate B(s) is represented by the following
equation (8).
B(s) = Go(s) ~ (s / ~0) -- (8)
Therefore, it can be considered that disturbance
transmission rate B(s) represents the deviation amount of
optical head 1 with respect to track 8 generated by
disturbance vibration. ~ :
The characteristics of transfer function Go(s) and
disturbance transmission rate B(s) are graphed in Fig. 15.
~ig. 15 shows the characteristics of transfer function
Go(s) of the spring-mass system and the characteristics of
disturbance transmission rate B(s), where the gain is
plotted along the ordinate and the angular frequency
: ~ . ,. : .. , . . : :
plotted along the abscissa. The transfer function Go~s) is
indicated by the solid line and the disturbance
transmission rate B(s) is indicated by the broken line.
It can be appreciated from Fig. 15 that the characteristic
of the transfer func-tion Go(s) is a second order low pass
filter type with the resonance angular frequency ~0 as the
breakpoint frequency. The characteris~ics of disturbance
transmission rate B(s) is a second order high pass filter -
type where the characteristics of transfer function Go(s)
is rotated counter clockwise about resonance angular
frequency ~0.
Consider the case where there is deviation in the
position between optical head 1 and track 8 due to
disturbance vibration introduced into the tracking servo
loop, resulting in a offset of optical head 1. Viewing `~
the position of optical head 1 and the position of track 8
in a relative manner, it can be seen that the position of .
track 8 is deviated due to the disturbance vibration. In
other words, the position of track 8 which is the followed
desired value of the tracking servo loop is disturbed by
disturbance vibration.
In view o~ the foregoing, a tracking servo loop is
represented by a block diagram in Fig. 16. Referring to
Fig. 16, displacement Y of disturbance vibration is
multiplied by disturbance transmission rate B(s), which
:f, ' ~ ' . - ., - - ~ :
. 3 ~ 2
yields the introduced amount of disturb~nce signal into
the tracking servo loop. This B(s) Y is added to
followed error Xe so as to disturb displacement Xi of
track 8 which is the followed desired value.
In accordance with Fig. 16, followed error Xe is
represented by the following equation (9).
Xe = {1 / (1 + G(s))~ Xi + {B(s) / (1 + G(s))} Y ~ -
-- (9)
The second ter~. of the right hand side of equation
(9) represents the increment of followed error Xe caused
by disturbance vibration. In order to transform
displacement Y of disturbance vibration into an
acceleration dimension expression generally used from the
displacement dimension, a Laplace operator 52 indicating a
repeated differentiation is added to the numerator and
denominator of the second term in equation (9) to obtain
the following equation (10).
Xe = ~1 / (1 + G(s))} Xi + {3(s) / (1 + G(s)) s2}
Ys .,. (10)
By extracting the transfer function to followed error
Xe from disturbance vibration acceleration yS2 in the
second term of the right hand side of equation (10), and
defining that transfer function as disturbance suppression
characteristics D(s), the following equation (11) is
obtained.
2 ~ 9 ~ ~
D(s) = {B(s) / (1 ~ G(s)) s2} ... (11)
The disturbance suppression characteristics D(s) of
the above equation (11) represents the charactexistics
suppressing the disturbance vibration in optical head l.
Followed error Xe ganerated by disturbance vibration
acceleration Ys becomes lower as the value of disturbance
suppression characteristics D(s) is smaller. This means
that it is greatly immune to disturbance vibration. A
smaller disturbance suppression characteristics D(s)
results in a tracking servo loop superior to anti-
vibration performance.
It can be appreciated from the above equation (11)
that ~he value of servo gain G(s~ is increased or the
value of disturbance transmission rate B(s) reduced to
lower disturbance suppression characteristics D(s) in
order to improve anti-vibration performance.
A possible consideration to increase the servo gain
G(s) to lower disturbance suppression characteristics D(s)
is to enlarge the servo band. However, this method has a
disadvantage of possibility of deviation in the servo by a
defect in disk 7, or posing severe requirements to the
characteristic strain of the machine system such as linear
motor 2, and is not applicable in this case.
Another method is to raise the servo gain of the low
frequency region where disturbance vibration is easily
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~91~92
generatzd by means of phase lag compensation which is a
kind of integral compensation. This is based on the fact
that the frequency components o~ clisturbance vibration are
easily generated normally in the range below several ten
H~. This method will be describecl hereafter.
Fig. 17 shows an example of a galn curve of a servo
gain G(s) of the tracking servo loop using the linear
motor. In Fig. 17, the gain is plotted along the
ordinate, and the angular frequency is plotted along the
abscissa. By obtaining disturbance suppression
characteristics D(s) according to equation (11) while
taking into consideration the gain curve of Fig. 17 and
~he disturbance transmission rate B(s) characteristics o
Fig. 15, the characteristics gain curve shown in Fig. 18 -
is established.
Fig. 19 is a graph showing the frequency
characteristics of a phase lag compensation L~(s) where a
predetermined angular frequency ~v at the low frequency
region is the upper limit angular frequency. In the graph
of Fig. 19, the gain is plotted along the ordinate and the
angular frequency plotted along the abscissa. By adding
the element of phase lag compensation L~(s) haviny the
frequency characteristics of Fig. 19 into the tracking
servo loop having the gain curve of Fig. 17, the gain
curve of that tracking servo loop is as shown in Fig. 20.
_g_ '~-
20~1~92
It is appreciated from Fig. 20 that the servo gain of
an angular frequency below the upper limit angular
frequency ~v is higher than the gain shown in Fig. 17 due
to phase lag compensation L~(s). The addition of such a
phase lag compensation Ll(s) results in the gain curve of
disturbance suppression characteristics D(s) as shown in
Fig. 21. Because the servo gain in the low frequency -~
region rises as shown in Fig. 20 due to addition of phase
lag compensation Ll(s), it is appreciated from Fig. 21 that
the gain of disturbance suppression characteristics D(s)
in the low frequency region below the upper limit
frequency ~y is reduced. As a result, disturbance
suppression characteristics D(s) is reduced in the lower ;~
frequency region where disturbance vibration is easily
generated to allow improvement in anti-vibration
performance.
However, if the upper limit angular frequency ~v
phase lag compensation Ll(s) is further set higher to .
further improve the anti-vibration performance at the
lower frequency resion taking advantage of the effect of
such phase lag compensation, the phase margin in cut off
angular frequency ~c Of the servo gain will be reduced,
resulting in a problem of an unstable tracking servo loop.
A possible consideration to further improve anti- - -
vibra~ion performance at the low frequency region is to
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simply apply at least two stages of phase lag compensation
to increase the servo gain at low frequency regions. In
this case, the characteristics of the original linear
motor will be a second order integral system in regions
greater than resonance angular frequency ~0, resulting in
an integral system of not less than the total of fourth
order. Fig. 22 is a graph showing the vector locus of the
servo gain when two stages of phase lag compensation are
added. Referring to Fig. 22, the vector locus of the
servo gain makes one rotation about point (-1, jO)
clockwise. This will produce the problem that the
tracking servo loop is not stable due to Nyquist stability
criterion.
Thus, there is a limit in the method of reducing ~ `
disturbance suppression characteristics D(s) by simply
applying phase lag compensation to increase the servo gain
in low frequency regions.
A method of increasing disturbance suppression -
characteristics D(s) can be considered by reducing
disturbance transmission rate B(s) to improve anti-
vibration performance. The method of reducing disturbance
transmission rate B(s) of the mechanical behavior of
linear motor 2 with respect to disturbance vibration will
be described hereinafter.
If pre-load towards bearing unit 10 of linear motor 2
--11--
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: .. .. :. . ; . . . . .
.: . .. ,, .. . : ~ . ~ -
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2~5~2
is increased to raise the equivalent spring ability,
resonance angular frequency ~0 takes a higher value. As
described in Fig. 15, the characteristics of disturbance
transmission rate Bts) takes a high pass fllter type that
decreases where the angular frequency is below the
resonance angular frequency ~0. Therefore, if the
resonance angular frequency ~0 is set to a resonance
angular frequency ~0' higher than a predetermined resonance
angular frequency ~0, disturbance transmission rate B(s) in
the low frequency region is reduced as shown in Fig. 23.
Fig. 23 is a graph showing the change in disturbance
transmission rate B(s) according to the change in
resonance angular frequency ~0. In Fig. 23, the gain is
plotted along the ordinate and the angular frequency is
plotted along the abscissa, wherein disturbance -;~
transmission rate B(s) by the original resonance angular
frequency ~0 and by the higher resonance angular frequency
~0' are indicated by a solid line and a chain dotted line,
respectively.
It is appreciated from Fig. 23 that a higher ~ .
resonance angular frequency causes the characteristics of
disturbance transmission rate B(s) to be shifted towards
the high frequency side, whereby disturbance transmission
rate B(s) in the low frequency region is reduced.
However, such a rise of the resonance angular
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2~592
frequency to ~0' causes the servo gain G(s) of an angular
re~uency below resonance angular frequency ~0' to become
lower than the original servo gain G(s), as shown in Fig.
24. Fig. 24 is a graph showing the change in servo gain
S according to the change of resonance angular frequency.
In Fig. 24, the gain is plotted along the ordinate and the
angular frequency plotted along the abscissa. The gain
curve according to the original resonance angular
frequency ~0 and that by the higher resonance angular .
frequency ~0' are shown by a solid line and a chain dotted
line, respec~ively.
It can be appreciated from Fig. 24 that the gain
curve by the higher resonance angular frequency ~0' has the
servo gain G(s) reduced in the angular frequency below
resonance angular frequency ~0' in comparison to the
original gain curve because the servo gain G(s) below the `
resonance angular frequencies ~0 and ~0' is constant. ;'
Therefore, disturbance suppression characteristics
D(s) determined by disturbance transmission rate B(s) and
servo gain G(s) is not improved by the above-described
method. Also, increase in the pre-load to bearing unit lO
for carrying out the above-described method will promote
abrasion of bearing unit 10, resulting in the problem of ~-
reducing the mechanical lifetime of the device.
Thus, it was difficult to improve disturbance
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2~ .3~
suppression characteristics D~s) i-rom the conventional
improving methods of anti-vibration performance. There
were many problems regarding improvement of anti-vibration
performance of a disk recording/reproduction device.
SUMMARY OF THE INVENTION
An object of the present invention is to improve the
anti-vibration performance of a disk
recording/reproduction device.
Another object of the present invention is to improve
lQ the anti-vibration per~ormance of a disk
recording/reproduction device without degrading stability
of the servo loop and lifetime of the mechanical system.
A disk recording/reproduction device according to the
present inven~ion is a disk recording/reproduction device
for recording and reproducing information to and from a
disk having tracks that can record information, and
includes a head, a moving velocity detecting circuit, a
tracking error detecting circuit, a phase compensating ~ -
unit including a phase lag compensating circuit, and a
head driving circuit. The head is provided in a movable
manner to record/reproduce information at an arbitrary
radial position of the disk. The moving velocity
detecting circuit detects the moving velocity of the head.
The tracking error detecting circuit detects error of the
recording/reproduction position of the head with respect
~:,
`.: ~ . . . , ' ~ .: .:
: ~ . ., ~ . . : .. . .
2g~5~2
to a track posi~ion of the disk, whereby ~he detected
result is provided as a tracking error signal. The phase
compensating unit applies phase compensation to a tracking
error signal. The head driving circuit moves the head to
cancel error of the recording/reproduction position of the
head with respect to the track position according to the
phase lag compensated result of the phase lag compensating
circuit while applying damping to the head accordLng to
the detected result of the moving velocity detecting
circuit.
Because damping is applied to the operation of the
:. .
head according to the detected result of the moving -
velocity detecting circuit, the disturbance transmission
rate in the head is reduced. Also, although the servo `~
gain regarding the control of the head is reduced by the ~-
addition of this damping, the phase lag compensating
circuit of the compensating unit compensates for phase lag
of the tracking error signal, so that the lowered servo
gain is raised.
Therefore, the disturbance transmission rate is
reduced, and reduction of the servo gain caused by
reduction of the disturbance transmission rate will be
compensated for. Thus, the anti-vibration performance of
the device can be improved without degrading stability of
the servo loop and lifetime of the mechanical system
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2~ 0 ~ 1 r~ ~ 2
because the disturbance transmission rate can be reduced
without decreasing the servo gain.
A disk recording/reproduction device according to
another aspect of the present invention is a disk
S recording/reproduction device for recording and
reproducing information to and from a disk having tracks
that can record information, and includes a head, a first
control circuit, and a second control circuit.
The head is provided in a movable manner to
record/reproduce information at an arbitrary radial
position of the disk. The first control circuit applies `~
damping to the operation of the head according to the
moving velocity of the head. The second control circuit
controls the position of the head to follow the position
of the track and compensates for the transfer ~ `
characteristics of the head to compensate for the servo
gain that is reduced by damping.
Because damping is applied to the operation of the
head according to the moving velocity of the head by the
first control circuit, the disturbance transmission rate
of the head is reduced. Also, although the sexvo gain is
reduced regarding the operation of the head due to load of
this damping, the second control circuit compensates for
the mechanical characteristics of the head to compensate
-for reduction in the servo gain, whereby the servo gain is
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raised.
Therefore, the disturbance transmission rate isreduced, and reduction of the servo gain is compensated
for according to decrease of the disturbance transmission
rate Thus, the anti-vibration performance of the device
can be improved without degrading the stability of the
servo loop and the lifetime of the mechanical system
because the disturbance transmission rate is reduced
without decreasing the servo gain.
The foregoing and other objects, features, aspects
and advantages of the present invention will become more
apparent from the following detailed description of the
present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram schematically showing a
structure of an optical disk xecording/reproduction device
according to an embodiment of the present invention.
Fig. 2 is a side view of an example of a velocity
sensor.
Fig. 3 is a block diagram schematically showing
another example of a velocity sensor.
Fig. 4 schematically shows an example of a position
sensor used in the velocity sensor of Fig. 3.
Fig. 5 is a block diagram of the periphery of a
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velocity feedback loop.
Fig. 6 is a graph showing the characteristlcs of the
transfer function of a linear motor when velocity feedback
is applied.
Fig. 7 is a graph showing characteristics of the
disturbance transmission rate of a linear motor where ,
velocity feedback is applied.
Fig. 8 is a graph showing the characteristics of the
transfer function of a linear motor where velocity
feedbacX and phase lag compensation are combined.
Fig. 9 is a graph showing the disturbance suppression
characteristics where velocity feedback and phase lag
compensation are combined. ~`
Fig. 10 is a graph showing the vector locus of servo
gain where velocity feedback and two stages of phase lag
compensation are combined.
Fig. 11 is a graph showing the disturbance
suppression characteristics where velocity feedback and ;~
two stages of phase lag compensation are combined.
Fig. 12 is a block diagram schematically showing a
structure of a conventional optical disk
recording/reproduction device.
Fig. 13 is a block diagram of a tracking servo loop.
Fig. 14 is a kinematic model diagram of an optical
head and its vicinity.
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: . ..
2 ~ 2
Fig. li is a g:raph showing the transfer function
characteristics of a spring-mass system and disturbance
- transmission rate characteristics.
Fig. 16 is a block diagram of a tracking servo loop -
taking into consideration disturbance vibration.
Fig. 17 is a graph showing a gain curve of a tracking
servo loop.
Fig. 18 is a gxaph showing disturbance suppression
characteristics.
Fig. 19 is a graph showing fxequency characteristics
of phase lag compensation.
Fig. 20 is a graph showing a gain curve of a tracking
servo loop where phase lag compensation is applied.
Fig. 21 is a graph showing disturbance suppression
characteristics where phase lag compensation is applied.
Fig. 22 is a graph representing a vector locus of a
servo gain where two stages of phase lag compensation are
applied.
Fig. 23 is a graph showing change in disturbance
transmission rate according to change in resonance angular
frequency.
Fig. 24 is a graph showing change in servo gain
according to change in resonance angular frequency.
DESCRIPTION OF THE PREFERRED EM~ODIMENTS
Fig. l is a block diagram schematically showing a
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structure of an optical disk recording/reproduction device
according to an em~odiment of the present invention. The
components in Fig. 1 similar to those shown in Fig. 12
have the same reference numbers denoted and their
description will not be repeated. The structure of Fig. 1
differs from the structure of Fig. 12 in that a velocity
sensor 11 for detec~ing velocity of an optical head 1 (or
a linear motor 2 moving optical head 1) is provided, a
velocity feedback loop is formed where the output of
velocity sensor 11 is fed back negatively from driver 6 to
linear motor 2 via an amplifier 13 and a substrator 22,
and a phase lag compensating circuit 12 is included in
phase compensating circuit 5.
Velocity sensor 11 is of the types shown in Figs. 2-
4. Fig. 2 is a side view of an example of a velocity
sensor 11. Velocity sensor 11 has either of a magnet 14
or a coil 15 fixed to optical head 1 (or linear motor 2)
and the other fixed to the main body side of the optical
disk recording/reproduction device. In velocity sensor 11
of such a structure, electromotive force is generated in
coil 15 due to the relative motion of magnet 14 and coil
15. This electxomotive force has a constant relationship
with respect to the velocity of relative motion.
Therefore, the electromotive force generated in coil 15 by
this relative motion is provided as a velocity detection
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signal.
Fig. 3 is a block diagram schematicall~ showing
another example of a velocity sensor 11. This velocity
sensor 11 includes a position sensor 16 for detecting the
position (displacement) of optica:l head 1 (or linear motor
2), and a differentiator 17 for d:ifferentiating the output
of position sensor 16. Velocity sensor 11 has the
position of optical head 1 detected by position sensor 16,
whereby the detected position information is
differentiated by differentiator 17. The output of
differentiator 17 represents the velocity of optical head
1. !
A specific example of position sensor 16 of Fig. 3 is
shown in Fig. 4. Referring to Fig. 4, a light emitting
diode 18 is provided in optical head 1 (or linear motor
2). A PSD (Position Sensitive Device) 19 which is a~ -~
photodiode for detecting the position of optical head 1 is
provided along the moving direction of optical head 1 in
the main body of the device. Light emitting diode 18 is
provided at a position to direct the emitted light towards
the detecting face of PSD 19. Therefore, the position of
incident light 20 into PSD 19 varies according to the
position of optical head 1. PSD 19 has output terminals A
and B respectively provided at the either ends in the
longitudinal direction. Photoelectric current flows from
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the irradiated position by incident light 20 towards
output terminals A and B. The photoelectric current from
output terminals A and B vary accordin~ to the resistance
between the irradiated position of incident light 20 and
5 the respective output terminals A and B. This resistance
increases according to the distance betw~en the irradiated
position and the output terminal. By obtaining the
; difference of the photoelectric current outputs, the
irradiated position by incident light 20 can be defined to
identify the position of optical head 1.
The photoelectric current provided from output
terminals A and s are provided to a differential amplifier
21. Differential amplifier 21 differential-amplifies
these photoelectric current outputs to provide the same to
differentiator 17 shown in Fig. 3 as the information
representing the position of optical head 1.
The change of disturbance transmission rate B~s) will
be described hereinafter where a velocity feedbacX loop as
shown in Fig. 1 is formed. Fig. 5 is a block diagram of
the periphery of the velocity feedback loop. A driving
input U which is a signal supplied to driver 6 from phase
compensating unit 5 is converted into a displacement X0 of
optical head 1 (or linear motor 2) via a gain KD of driver
6, a thrust constant K~ of linear motor 2, and mechanical
characteristics of linear motor 2 1/~Ms +Ds~K).
, ~, . . ~ , . , : .
~3~
Displacement XO is negatively fed back to driver 6 via a
sensitivity ~y-S of velocity sensor 11 and a gain A of
amplifier 13. M in the mechanical characteristics of the
linaar motor is the mass (including mass of optical head
l) of the movable portion of linear motor 2, D is the
equivalent coeficient of viscosity generated in bearing
unit 10 of linear motor 2, and K is the equivalent spring
constant generated in bearing unit lO of linear motor 2.
The transfer function (XO/U) from driving input U to
displacement XO in such a velocity feedback loop is
expressed by the following equation (12).
(XO / U) = (KF KD) / {MS + (D + A KV KF KD) S
+ K}
= (KF ' KD / K) [K / {MS2 + (D + A KV
KF KD) S + K}]
= (KF KD / K) GO/ (S) .. (12)
Go~(s) of the above equation ~12) is a normalized form
of the lin~ar motor transfer function after the velocity
feedback loop foxmation, and is expressed by the following
equation (13).
&Ot(S) = K / {MS + (D + ~ KV K~ KD) S + K}
2 / (SZ + 2~o' ~o 5 + ~-)o ) ... (13)
~ 0 is the resonance angulax frequency of linear motor
2, and ~0~ is the damping value after velocity feedback
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loop formation, and are represented by the following
equations of (14) and (15), respectively.
= (K/M) ... (14)
~0' = (D + A ~ KV KF KD) ~ {2 (~IK) } ... (15)
The damping value where a velocity feedback loop is
not formed, i.e. ~0 of equation (6) is determined by the
equivalent coefficient of viscosity D of bearing unit 10
of linear motor 2, spring constant K and mass M of the
movable portion of linear motor 2. The damping value is
generally in the range of 0.1-0.3, and approximately 0.5
at maximum.
However, the damping value of ~0' where a velocity
feedback loop is formed is affected by the product of gain
KD Of driver 6, thrust KF of linear motor 2, sensitivity Kv
of velocity sensor 11, and gain A of amplifier 13
amplifying the output thereof according to the above
equation of (15). Therefore, if thrust constant KF~ gain
KD and sensitivity KV are determined, the damping value ~0'~
can be changed by varying gain A. Furthermore, because ~
the equivalent coefficient of viscosity D of the bearing ~ -
portion in the numerator of equation (15) is a small value
that is generated by rolling resistance and the like,
(A-KV-K~-KD>>D) can be set by the value of gain A. In this
case, ~0' is represented by the following equation ~16):
~0~ = (A Kv K~ KD) / {2 (~K) '5} . . . ( 16)
-24~
-~ . , -. ~: . . . -
.. . . . . .
2 ~ r ~1 r~
When the value of gain A is set so that ~o'>l, a
breakpoint of characteristics appears in the
characteristics of the normalized transfer function Go'(s)
of linear motor 2 that is represented by equation (13).
5 Fig. 6 is a graph showing the characteristics of transfer
function Go'(s) of linear motor 2 where a velocity feedback
is applied. When the value of A is set so that ~o'>l, as
described above, transfer function Go~(s) shows a
breakpoint at angular frequency ~A lower than ~0 and at
angular frequency ~B higher than ~0 with the resonance
angular frequency ~0 as the center. Thus, the gain in the
region between angular frequencies ~A and ~B iS reduced
from the original gain.
The characteristics of disturbance transmission rate
B'(s) takes a high pass filter type where the ~
characteristics of transfer function Go~(s) ls rotated ~ -
counter clockwise about resonance angular frequency ~0,
resulting in the characteristics shown in Fig. 7. Fig. 7
is a graph representing the characteristics of disturbance
transmission rate B'(s) of the linear motor where velocity ~ -
feedback is applied. As a result of the gain in the
region between angular frequencies ~A and ~B reduced as
shown in Fig. 6, the characteristics of disturbance
transmission rate B'(s) is accordingly reduced in the
region between angular frequencies ~A and ~B as shown in
-25-
e~
Fig . 7 . Therefore, the angular frequency components in
the region between angular frequencies ~A and ~B in
disturbance vibration will not easily be introduced into
the tracking servo loop.
A control similar to such a velocity feedback is
carried out in a conventional disk recording/reproduction
device. An example is described in "Development of CD-ROM
Drive System" Sanyo Technical View Vol. 19, No. 1, pp. 35-
45, February 1987. However, there is no description of
the object and meaning of a velocity feedback in this
document. There are also disclosures in Japanese Patent
~aying-Open No. 61-227692, Japanese Patent Laying-Open No.
2-263367 and Japanese Patent Laying-Open No. 3-118481.
However, the control of velocity feedback described in
these applications are all provided for the purpose of
improving stability of control of the servo system. There `~
is no study nor description of employing the control for ;~
improving anti-vibration performance. It is not possible
to improve anti-vibration performance with only the
described velocity feedback due to reasons set forth in
the following. ~ `
As already described with reference to the above
equation of (11), disturbance suppression characteristics
D(s) is determined by servo gain G(s) and disturbance
transmission rate B(s). Therefore, the following problems
-26-
- . : ~ . - ~` , . . .;
2 ~ 9 ~
will be generated by just applying velocity feedback.
When transfer function Go/(s) of linear motor 2 which
- becomes the ba~is of servo gain G(s) is reduced in the
region between angular frequencies ~A and ~B as shown in
S Fig. 6, servo gain G(s) is also reduced in the same region
of angular ~requency due to the ~elocity feedback.
Regardless of how much disturbance transmission rate B'(s)
is reduced in the same region of angular fre~uency to
lower the introducing amount of disturbance vibration,
disturbance suppression characteristics D'(s) determlned
by servo gain G'(s) and disturbance transmission rate
B'(s) where a velocity feedback is applied will show no
change with respect to the disturbance suppression -
characteristics D(s) of Fig. 18 where velocity feedback is
not applied. There is no change, whether good or bad, in
the characteristics.
The invention of the present application is `~-
characterized by including a phase lag compensating ~ -
circuit 12 in the tracking servo loop for raising the gain
of an angular frequency region below angular frequency ~B :
to a resonance angular frequency ~0, for example, in `
addition to the above-described velocity feedback loop.
Figs. 8(a) - (c) are graphs showing the characteristics of
the transfer function of linear motor 2 where a velocity
feedback and phase lag compensation are combined. Figs.
-~7-
~ ;.. .... .. .
: , ~ - :.- .
2~9~ 5~2
8(a) - 8(c) show the charactexistics of phase lag
compensation L2(s), of transfer unction G~'(s) of linear
motor 2 where velocity feedback is applied, and of
transfer funstion L2(s) ~ Go'(s) of linear motor 2 where
velocity feedback and phase lag compensation are applied~
respectively. By introducing into the tracking servo loop
a phase lag compensation Lz(s) having the characteristics
so as to raise the gain in the angular frequency region
below angular frequency ~3 to resonance angular frequency
~0 as shown in Fig. 8(a), transfer function Go'(s) of
linear motor 2 where velocity feedback is applied has the
gain in the region below angular frequency ~B compensated
for as shown in Fig. 8(c). Accordingly, reduction of
servo gain G'(s) is also compensated for.
lS Because there is no change in disturbance
transmission rate B'(s) even when a phase lag compensating -
circuit 12 is introduced into the tracking servo loop,
disturbance suppression characteristics D'(s) has the `
characteristics in the low frequency region improved in
comparison with that of Fig. 18, as shown in Fig. 9.
Fig. 9 is a graph showing the characteristics of
disturbance suppression characteristics D'(s) where
velocity feedback and phase lag compensation are combined.
It can be appreciated from Fig. 9 that the disturbance
suppression characteristics D'(s) in the low frequency
-28-
2 ~ 9 ~
region of below angular fre~uency ~B is reduced by vixtue
of reduction of disturbance transmission rate B'(s) by
velocity feedback and compensation of servo gain G'(s) by
phase lag compensation L2(s).
Thus, according to the present invention, the
disturbance suppression characteristics is reduced to
improve anti-vibration performance by reducing the
disturbance transmission rate by a velocity feedback and
compensating for only the servo gain reduced by that
velocity feedback by phase lag compensation.
The inclination of the gain will not exceed -40
(dB/dec) even in the case where a phase lag compensation ~-~
L2(s) is applied as described above. Because the --
characteristics of the tracking servo loop shows only a
maximum of a second order lag characteristics, the phase
lag compensation Ll(s) described with reference to Fig. l9 ~`
can further be applied to the tracking servo loop to
further improve anti-vibration performance. Fig. 10 i5 a . ~;
graph representing the vector locus of servo gain G(s)
where a velocity feedback and phase lag compensation of
two stages are combined. In the case where a velocity
feedback and a phase lag compensation L~(s) is combined and
then further combined with a phase lag compensation Ll(s),
the characteristics of the tracking servo loop is limited ,
to a third order lag characteristics. In such a case, the
-29-
.. ... , . ., ... .... . , ,, ~
:.. , . .. - . .. ,, .. .,, , ..... ~....... .
: - . .. - .
-,~
2 0 ~
vector locus of the servo gain does not make one rotation
clockwise about point (-l, jO) as shown in Fig. 10.
Therefore, the tracking servo loop is stable by Nyquist
stability criterion.
Fig. ll is a graph representing the disturbance
suppression characteristics D"(s) where a velocity
feedback and two stages of phase lag compensation are
combined. As described above, there is no change in
disturbance ~ransmission rate B'(s) even if a phase lag
compensation Ll(s) is further added, and only servo gain
G'(s) in the region between angular frequencies ~u and ~v
show an increase. Therefore disturbance suppression
characteristics D"(s) is further reduced in the region
between angular frequencies ~u and ~, whereby the anti-
vibration performance of the device is further improved.
Although an optical disk recording/reproduction
device of a structure where optical head 1 is linearly -~
moved to be positioned radial OI disk 7 by linear motor 2
in the present embodiment, the invention is not limited to
this, and can be applied to a magnetic disk device of a
structure where the head carrying out
recording/reproduction of information is positioned by
being moved along the circumferential track radial of the
disk by a rotary arm.
In this case, a control as in the present embodiment
-30-
~ .. . .
J15~
can be carried out by providing a velocity sensor such as
to detect the rotation velocity (angular velocity) o the
rotary arm.
Although the present invention has been described and
illustrated in detail, it is clearly understood that the
same is by way of illustration and example only and i5 not
to be taken by way o limitation, the spirit and scope of .
the present invention being limited only by the terms of -
the appended claims. ::~
'
-31-
;
.,, , , . , , , ~
. ' : ` ~ ' ' ' .' '.
