Note: Descriptions are shown in the official language in which they were submitted.
PHN 7219
1055156
The invention relates to an apparatus for read-
ing a disc-shaped record carrier in which signals are
recorded in tangential tracks, which apparatus includes
an optical read unit with a radiation source, a direct-
ing system and a read detector, which radiation source
emits a radiation beam, which is projected onto the re-
cord carrier as a read spot of radiation and which via
the directing system transfers the information contain-
ed in the scanning point of the record carrier to the
read detector, a control system for controlling the
radial position of the scanning point on the desired
track, which control system includes a drive means for
the directing system and a first measuring system for
measuring the radial position of the scanning point and
supplying a corresponding first control signal to the
drive means, which first measuring system employs a first
pattern of radiation spots projected onto the record car-
rier, which pattern is imaged onto a first measuring -
detector and which first measuring system supplies a
first control signal, which during a radial movement
of the scanning point over a number of track distances
contains a periodic alternating component whose period
equals the track distance, the control system being
stable for one half period of said alternating component
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and being unstable for the other half period. In this res-
pect the terms stable and unstable denote a decrease and
increase respectively of the kinetic energy in the system
as the scanning point passes through the relevant periods.
Such an apparatus is known for our Canadian
Patent 1,003,961 which issued on January 18, 1977. The
radial control system used therein serves to ensure that
when the information recorded on the record carrier is
being read the scanning point always accurately follows
the information track on said record carrier. Said record
carrier may be provided with a multiplicity of concentric
tracks, but generally the information track has a spiral
shape. The method of modulation of the recorded signal
and the method in which said signal is recorded on the
record carrier is not essential for the present invention,
so that it will not be discussed in further detail. As
an example of a method of recording reference is made
to our pending Canadian Patent Application No. 136,033
which was filed on March 2, 1972.
When the record carrier is provided with a
spiral information track, the scanning point should be
moved radially at approximately uniform speed. Said
uniform movement is generally obtained by moving the
read unit in a radial direction. In addition, it should
be possible to effect relatively small but rapid radial
movements of the scanning point because, for example
as a result of an eccentricity of the pivot relative to
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the centre of the record carrier, there may be radial de-
viations in the position of the information track.
Said radial displacements of the scanning point
are effected with the aid of said control system, which
consists of the drive means for the directing system and
the first measuring system. The directing system usually
consists of a reflecting element, which under the in-
fluence of the drive means can be subjected to a rota-
tion and can thus reflect an incident beam of radiation
at a variable angle. Said directing system, like in our
Canadian Patent 1,003,961, can influence the direction
of the radiation beam before said beam impinges on the
record carrier or after said beam impinges on the record
carrier and has already been modulated by the information
contained therein. The record carrier itself may be both
radiation-transmitting and radiation-reflecting.
The first measuring system for measuring the
radial position of the scanning point may have different
embodiments. For example, use can be made of a first
pattern of radiation spots projected onto the record
carrier by a radiation source, which pattern consists
of two radiation spots which viewed in the radial direc-
tion are situated at either side of the scanning point,
each of said radiation spots being imaged onto a sepa-
rate part of the first measuring detector. The mutual
magnitude of the signals produced by the images of the
A.
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two radiat;on spots on the separate parts of the measur-
ing detector will change in accordance with the position
of the scanning point relative to the information track.
By subtraction of the signals supplied by the separate
parts of the first measuring detector, a first control
signal is obtained which is unambiguously representa-
tive of the position of the scanning point relative to
the information track. Another possibility is to image
a number of tracks with the aid of a relatively large
radiation spot onto a grating-shaped measuring detector
as described in our Canadian Patent No. 957,067 which
issued on October 29, 1974.
In all said measuring systems a control sig-
nal is produced, which upon a radial displacement of the
scanning po;nt over a number of track distances comprises
a periodic alternating component whose period equals the
track distance. In this respect track d;stance is to be
understood to mean the distance measured in the radial
direction between the centre lines of two adjacent tracks.
The said alternating component is used as a control sig-
nal for the drive means of the directing system.
It has been found that in the case of a radial
displacement of the scanning point over a number of
track distances, the control system is in a stable state
for half a period of the periodic alternating component,
and in an unstable state during the other half period
of said alternating component. This is because the al-
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ternating component, which is used as a control signal for
the drive means of the directing system, has a positive
slope during one half period and a negative slope during
the other half period so that in the event of a change
of the radial position of scanning point the variation
of the control signal which is applied to the drive means
is of opposite sign.
Normally, this property of the control system
presents few problems, because the system once it is in
the stable range, will always remain in said stable
range under normal conditions. However, if the control
system leaves the stable range owing to a disturbance,
for example owing to a shock or a flaw in the record
carrier, said property may cause a relatively large,
substantially undamped oscillation, so that the scan-
ning point may effect an oscillation over a number of
track distances in a radial direction and a correct
reading of the information is not possible until after
a certain, sometimes comparatively long, time after the
occurrence of the disturbance.
It is an object of the invention to provide
an apparatus in which said problem is eliminated. The
invention is therefore characterized in that for as-
suring the stability of the control system a second
measuring system is provided, which employs a second -
pattern of radiation spots which is projected onto the
record carrier and which is imaged onto a second measur-
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_~ PHN 7219
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ing detector, which second measuring system supplies a
second control signal, which upon a radial movement of
the scanning point over a number of track distances
comprises a periodic alternating component whose period
equals the track distance, but which alternating compo-
nent as a result of a deviating radial position of the
second pattern of radiation spots relative to that of
the first pattern of radiation spots exhibits a phase
shift of at least substantially a quart period relative
to the alternating component of the first control signal,
and that the control system has a variable transfer
function which is controlled by a control unit which re-
ceives a control signal derived from the second control
signal, and which control unit upon a movement of the
scanning point over a number of track distances con- -
trols the transfer function so as to stabilize the
control system.
By means of the second measuring system a second
control signal is obtained which provides an ind;cation
on whether or not the control system is in the stable
range. Since the alternating component of said second
control signal relative to the alternating component of
the first control signal exhibits a phase shift of at
least substantially a quarter period, the half period
of the one polarity of said alternating component cor-
responds to a half period of the alternating component
of the first control signal with a slope of a first
PHN 7219
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sign, and the other half period of the opposite polarity
of said alternating component of the second control sig-
nal with the other half period of the alternating component
of the first control signal with a slope of opposite
sign.
Said second control signal can be used to pro-
duce a control signal, which influences the transfer
function of the control system when said system comes
into its unstable range in such a way that as said con-
trol system passes through a stable and an unstable
range the kinetic energy exchanged during the passage
of the unstable range is smaller than the energy ex-
changed during the passage through the stable range. For
this purpose, various possibilities are available. First
of all, the gain of the control system may be reduced as
the control system passes through the unstable range. A
second possibility is the reduction of the damping term
in the control system, which gives rise to a negative
damping in the unstable range. Finally, the sign of
said damping term may be reversed in the unstable range,
so that it provides a positive damping in said range.
Moreover, the second control signal may be
used for deriving a control signal which provides an ind-
ication of the direction in which the scanning point is
moved in a radial sense. Said control signal can be ob-
tained by differentiation of the second control signal.
Thus, a signal is obtained which is in phase or in phase
opposition relative to the first control signal depend-
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ing on the direction of movement of the scanning point. By
controlling the transfer function with the aid of said sig-
nal an unambiguous stabilizing action results, because de-
pending on the direction of movement the transfer function
is varied either during the positive or during the negative
period of the first control signal.
It is also possible to multiply the differen-
tiated second control signal by the first control signal,
so that a signal is obtained whose polarity is a direct
indication of the direction of movement of the scanning
point. Said informatio~ may then be used for influencing
the transfer function either during the positive or dur-
ing the negative period of the first control signal.
In addition to said two possibilities, a num-
ber of complementary steps may be taken, which will be
further outlined in the description.
The second measuring s~stem may have different -~
embodiments in accordance with the desired accuracy. In
a preferred embodiment the second pattern of radiation
spots consists of two radiation spots which are radially
shifted relative to each other by a distance equal to
half the track distance and which are each imaged onto
a separate part of the second measuring detector, a sec-
ond control signal being obtained by subtraction of the
signals supplied by said separate parts of the second
PHN 7219
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measuring detector. Said embodiment has the advantage that
the second control signal thus obtained merely conta;ns an
alternating component, whilst the d.c. component normally
present in the signal formed by imaging a radiation spot
on a measuring detector is eliminated by the subtraction.
Said d.c. component is both time-dependent as a result of
noise in the radiation energy supplied by the radiation
source and location-dependent as a result of differences
in the radiation absorption and/or reflection as a function
of the location on the record carrier and as a function of
the location where the radiation beam in the read unit is
transmitted, in particular of the lens systems used.
In a second preferred embodiment the second
measuring signal is obtained by measuring the signal re-
ceived by the read detector itself, specifically the low-
frequency variations of said signal. Said embodiment has
the advantage that no additional measuring detector is
required, but on the other hand it has the drawback that
the supplied control signal contains a d.c. component
which may give rise to a less accurate operation.
The invention will now be described in more
detail, by way of example, with reference to the draw-
ing, in which: ~ -
Fig. 1 shows an embodiment of the read apparatus
according to the invention, and
Figs. 2 and 3 show the associated control sig-
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nals,
Figs. 4 and 5 show two methods of obtaining the
desired control signal, and
Fig. 6 shows a number of signal characteristics
in illustration of the method of Fig. 5,
Figs. 7 and 8 show two methods of influencing
the transfer characteristic of the servo-unit in accord-
ance with the control signal,
Fig. 9 in conjunction with Fig. 10 shows an
alternative method of obtaining the desired stabiliza-
tion,
whilst Figs. 11 and 12 show two embodiments
of the differentiation circuit employed for this, in
order to obtain a special effect.
In Fig. l the reference number 1 designates a
disc-shaped record carrier, which at its underside is
provided with a multiplicity of concentric or quasi-
concentric tracks, not shown. In said tracks the infor-
mation can be recorded in various known manners. As an -
example, reference is made to our pending Canadian
Patent Application No. 136,033, in which the information
track contains blocks and areas whose length represents
the stored information. Said blocks and areas have
a different effect on a radiation beam which is pro-
jected onto the information track, so that said radiation
beam is modulated in accordance with the recorded infor-
mation. For example, the transmission or reflection co-
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PHN 7219
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efficient of the blocks may differ from that of the areas,
so that a read beam of radiation is amplitude modulated.
In a reflecting record carrier it is equally possible to
dispose the blocks and areas at different levels, the dif-
ference in depth being one fourth of the wavelength of the
radiation used for reading. The read beam of radiation is
then phase modulated. As the method of modulation and the
method of recording the signal which is recorded on the
record carrier are only of secondary importance for the
present invention, there will not be discussed in more
detail.
The record carrier 1 is rotated by a motor M
via a shaft 2 which extends through a central opening
in said record carrier. The information stored in said
record carrier 1 is read with the aid of a beam of ra-
diation, which after interaction with the record carrier
is detected with the aid of an optical read device,
which is accommodated in a housing 3. Said optical
read apparatus, which substantially corresponds to that ~ -
described in our Canadian Patent No. 1,003,961 includes
a light source 6, which emits a read beam of radiation
a. Via a semi-transparent mirror 7 said beam of rad-
iation a reaches a plane mirror 8, so that the radiation
beam is reflected in the direction of the record carrier
1. Said reflected beam of radiation a is focussed in a
scan spot S at the lower surface of the record carrier -
1 by a lens 11. The
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radiation beam which is reflected by said record carrier is
again reflected towards the semi-transparent mirror 7 by
the plane mirror 8 and by the mirror 7 to a read detector
12, on which the scanning point S of the record carrier
is thus imaged and which detects the information contain-
ed in the beam of radiation. Said detected information is
then eventually available for further processing at an
output terminal 14.
To guarantee a continuous reading of the infor-
mation stored on the record carrier the scanning point S
which is imaged on the read detector 12 should continual-
ly follow the information track of the record carrier. If
said information track is spiral shaped, this means that
the scanning point S must first of all be moved in a ra-
dial directi~on at a speed which corresponds to the pitch
of said spiral information track. Furthermore, the scan-
ning point S must be capable of following possible radial
movements of the information track, for example those re-
sulting from an accentricity of the "central" opening of
the record carrier.
This necessary control of the radial position
of scanning point S is established by co-operation of
two control systems, namely a coarse control which can
only effect a slow radial displacement of the scanning
point and a fine control which can only perform a rela-
tively small but rapid radial displacement of the scan-
ning point, In the embodiment shown the coarse control
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is obtained with the aid of a motor M2, which via a servo-
amplifier 16 receives a control signal and which by means
of a transmission, for example the worm 4 and the gear rack
5, can move the housing 3 in the radial direction. Fine
control is effected with the aid of the plane mirror 8
which is rotatable about a spindle 9.
Rotation of the mirror 8 is effected by a drive
element 10, for which various elements may be employed,
a number of possibilities being given in our Canadian
Patent No. 1,003,961. Said drive element receives a
control signal from a servo-unit 15.
The information concerning the radial position
of the scanning point S relative to the desired track
which is required for said control is obtained with the
aid of a radiation beam b, which is produced by the ra-
diation source 6, and which also impinges on the lower :
surface of the record carrier 1 via the semi-transpa- -rent mirror 7, the mirror 8 and the lens 11. After re-
flection said radiation beam b via the mirror 8 and the
semi-transparent mirror 7 reaches a control detector 13
whose output signal is fed to the servo-unit 15. As a
control signal for coarse control, which is to be applied
to the servo-amplifier 16, a signal is employed which
is a measure of the average deviation of the mirror 18
relative to a central position. Such a signal can be
obtained in various manners, which are obvious to those
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PHN 7219
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skilled in the art, for example with the aid of capacitive
or inductive transducers, which is the reason why the Fi-
gure only schematically indicates how the desired control
signal is taken from the drive element 10.
Depending of the composition of the radiation
beam b the control detector 13 may have different embodi-
ments. As a first example reference is made to our Canadian
Patent No. 957,067. The control detector described here
consists of a grating of radiation-transmitting and rad-
iation-absorbing strips, onto which via the radiation beam
b a number of tracks of the record carrier are imaged. The
position of said grating shaped image of the track pattern
relative to the grating-shaped detector by means of suit-
able transducer elements provides a control signal which
is representative of the position of the scanning point S
relative to the desired track. As a second example reference
is made to our Canadian Patent No. 987,029 which issued on
April 6, 1976. In this embodiment two radiation spots are
projected at either side of the scanning point by the rad-
iation beam b and the control detector consists of two
separate sub-detectors, on each of which one of the radiation
spots is imaged. The intensity of the two imaged rad-
iation spots varies in accordance with the radial position
of the scanning point and subtraction of the output sig-
nals of the two sub-detectors yields a suitable con-~rol
signal.
Generally, a control signal is obtained with
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the aid of the control detector 13, which signal as a
function of the radial position of the scanning point
is substantially as shown in Fig. 2. Apart from a pos-
sible d.c. component said control signal contains an a.c.
component whose period equals the track distance, i.e.
the distance between the centre lines of two adjacent
tracks. Fig. 2 shows a purely sinusoidal shape. However,
it will be evident that depending on the detector system
used said sinusoidal shape may deviate. For simplicity,
only said sinusoidal control signal will be discussed
hereinafter.
Fig. 2 represents the control signal which is
obtained upon a displacement of the scanning point over
3 track distances. The control system is assumed to be
in the stable state at the points where the control sig-
nal is zero and the slope is positive, i.e. points s1, s2
and S3, which consequently correspond to the centres of
said adjacent tracks. For example, if the scanning point
moves from the track sl towards a higher value of the
radius r, a positive control signal is produced, depending
on which the control system is assumed to force the scan-
ning point back to said position sl. For a displacement
of the scanning point towards a smaller value of r it is
obvious that the reverse applies.
However, if the scanning point enters a range
Q, which corresponds to a negative slope of the control
signal, the control system suddenly becomes unstable.
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PHN 7219
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As it passes through the range Q, the system gains energy,
which implies that if the scanning point traverses a full
track distance owing to a disturbance, it will automatical-
ly traverse more track distances.
As a result of an extraneous disturbance the
scanning point S may be moved over a number of track
distances, which number is first of all determined by
the deviation of the mirror at the instant of the dis-
turbance. As the coarse control is also actuated for
a rotation of the mirror S which corresponds to said
displacement, a substantially undamped oscillation
of the scanning point over a number of track distances
may occur owing to interaction of the fine and the
coarse control, so that at least for a certain time
reading of the information stored on the record car-
rier becomes impossible.
In order to remedy this, the radiation source
6 in the read apparatus of Fig. 1 emits a third radia-
tion beam c, which via the semi-transparent mirror 7,
the mirror 8 and the lens 11 impinges on the record
carrier. The portion of said radiation beam that is re-
flected by the record carrier is again reflected to an
additional detector 17 via the mirror 8 and the semi- ~
transparent mirror 7. Said additional detector 17 sup- ~ -
plies an additional control signal, whose shape sub-
stantially corresponds to the shape of the control sig- ~ -
nal supplied by the control detector 13. As a result of
a deviating radial position of the pattern of radiation
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spots which is projected onto the record carrier by the
radiation beam c relative to the pattern of radiation
spots which is projected onto the record carrier by the
radiation beam b, the additional control signal exhibits
a phase shift of 1/4 period relative to the control signal
supplied by the control detector 13.
For clarity the two control signals are shown
in Fig. 3. The control signal Il, which is supplied by
the control detector 13 corresponds to the control sig- ~`
nal shown in Fig. 2. The control signal I2 supplied by
the additional detector 17 has the same shape as the
control signal Il as a function of r, but is phase shif-
ed by 1/4 period. From said control signal I2 a control
signal I2' can be derived in a simple manner, which has
a first positive value when the control signal I2 is
positive and a second negative value when said control
signal I2 is negative. It is then evident from the Figure
that the positive value of the control signal I2' corres-
ponds to the stable ranges P of the control signal Il
and the negative value to the unstable ranges Q. For
this, it is assumed that a positive value of the first -
control signal Il causes a displacement of the scanning -
point towards smaller values of r and a negative value
a displacement towards higher values. The control sig-
nal I2' thus provides an indication whether the control : -
system is in a stable range P or in an unstable range Q.
Said information is used for influencing the control
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system during at least a part of the time that said control
system is an unstable range.
The control signal I2' derived from the addi-
tional control signal of the detector 17 by a converter
18, in this case a squarewave generator, is fed to the
servo-unit 15 which is included in the control system
and which depending on the value of said control signal
I2' may have two possible transfer functions. Obviously,
the first transfer function, which is operative in the
stable control range P, serves to obtain an optimum
control behaviour within said stable range. The second
transfer function, which is operative in the unstable
range Q, has such a deviating transfer function, that
whilst the scanning point passes through a number of
stable and unstable ranges P and Q respectively, the
energy exchanged by the servo-unit 15 during passage
through the unstable ranges Q is smaller than the energy
exchanged during passage through the stable ranges P. -
This ensures that upon the occurrence of an external
disturbance a damped oscillation is produced in any case,
so that the scanning point is at any rate restored to a : -
stable setting. How fast this happens of course de-
pends on the degree and the ~anner in which the trans-
fer functions of`the servo-unit 15 are varied. Nat-
urally, if desired, the signal I2 can also be used di-
rectly as a control signal and continuously vary the
transfer function by means of sa1d signal, at least
during the unstable periods.
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However first of all some possibilities of ob-
taining the desired control signals Il and I2 will be
described. A first possibility is represented in Fig. 4.
By sl, s2 and S3 a part of three adjacent tracks is de-
signated, which tracks are spaced at a track distance
~ from each other. S is the scanning spot, i.e. the
area of the record carrier which is eventually imaged
onto to the read detector 12. The pattern of radiation
spots of the radial control system basically consists
of two radiation spots Rl and R2, which are situated
at either side of the scanning spot S in the radial
direction at a distance of 1/4 ~ . The additional
pattern of radiation spots obtained with the aid of -
the radiation beam c comprises the two radiation spots -;
Tl and T2. The radiation spot Tl is radially shifted
relative to the radiation spot Rl by a distance of 1/4
I , and the radiation spot T2 is shifted by the same
distance relative to the radiation spot R2. Each of
the radiation spots is eventually imaged onto a sep-
arate detector or subdetector.
Generally, the signal Idet supplied by such -
a detector as a result of the radiation spot imaged
thereon may be written in terms of: -
Idet = Io (1 + m sin r~ . 2 1r) (1~
Io being the energy supplied by the radiation beam at
the location of the radiation spot, m being the ampli- -~
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tude of the a.c. component upon a radial displacement of
the radiation spot, r being the radial position of said
radiation spot, and ~ being the track distance.
Assuming that the image of the radiation spot
R2 actually results in the said signal, i.e.
IR = Io (1 + m sin rj~ 2 ?f) (2)
the image of the radiatlon spot Rl will yield a signal
IR = Io (1 + m sin r ;~/2 ~ 2 1r) = Io (1 - m sin r~ 2 ~) (3)
Subtracting the signal IR from the signal IR yields the
following control signal
Il = 2 m sin r~ . 2 1~ (4)
which is entirely in accordance with the waveform I
shown in Fig. 3.
Similarly, the image of the radiation spot T
yields the signal
IT = Io (1 + m sin r ~ 4 ~ 2 ~) = Io (1 + m cos r~ 2 ~) (5)
and the image of the radiation spot T2 gives the signal
IT = Io (1 + m sin r ~/4 i~ 21~) = Io (1 - m cos r~ 21~) (6)
Subtracting the signal IT from the signal IT yields as
additional control signal
I2 = 2 m cos r~ 2 lr (7)
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which is fully in accordance with the signal waveform I2
of Fig. 3.
If desired, the number of radiation spots for
obtaining the desired control signal may be increased.
F;g. 4 by way of example shows two addit;onal radiation
spots R3 and R4, the radiation spot R3 being located at
a distance ~ from the radiation spot Rl and the radiation
spot R4 at a distance ~ from the radiation spot R2. The
signals IR and IR resulting from the radiation spot R
and R4 respectively are therefore fully in phase with
the signals IR and IR resulting from the images of
the radiation spots Rl and R2 respectively. When taking
( R2 IR4) ~ (IRl ~ IR3) as a control signal, the re-
sulting signal is entirely in phase with the control sig-
nal Il but has twice the amplitude.
The use of the additional radiation spots has
some substantial advantages. First of all, as previously
stated, the amplitude of the control signal thus obtain-
ed is doubled. Secondly, the effect of deviation in the
distance of the tracks is reduced, because the addi- -
tional radiation spots result in a certain averaging.
Furthermore, the influence on the control signal of the
information which is contained in the tracks is reduced.
Normally, said high-frequency information is removed
with the aid of filters. Since the additional radiation
spots also result in a certain averaging with respect
to this parameter, the influence of this information
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component is automatically reduced. Naturally, it is also
possible to use more than two additional radiation spots,
so that the said effects become more marked. Further,
the radiation spot R3 may be situated at a track distance
~ from the radiation spot R2, i.e. between the tracks
S2 and S3 and the radiation spot R4 at a track distance
~ from the radiation spot Rl, i.e. between the tracks
sl and s2. The desired control signal is then represent-
ed by (IR + IR ) ~ (IR + IR )
A second possibility of obtaining the desired
control signals is represented in Fig. 5, the associated
waveforms being shown in Fig. 6. Fig. 5 shows a track s,
on which a scanning spot S is projected. At either side
of said scanning spot two radiation spots Rl and R2 are
projected, which are radially shifted by a distance of
1/4 ~ relative to the scanning spot S. By means of the
radiation spots Rl and R2 the control signal Il is ob-
tained in a manner identical to that described with re-
ference to Fig. 4, which signal again complies with the
formula (4) and which ~s shown in Fig. 6a. - -
However, in this case the additional control sig-
nal is not obtained with the aid of additional radiation
spots, but is derived from the signal obtained by imaging
the scanning spot S on the read detector. Generally, the
information contained in the information track will have
a higher frequency than the maximum control frequency. As
a result, said low-frequency signal component can be ex-
tracted from the signal supplied by the read detector with
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the aid of filters. Said signal component, except for a
phase shift, complies with the general formula (1) and
may be written in terms of
IT = Io (1 + m sin ~ . 21r) = Io (1 + m cos r~ 2 ~) (8)
Said signal IT, shown in Fig. 6b, contains an a.c. compo-
nent which is suitable for deriving the desired control
signal from it. With the aid of a separator stage the d.c.
component Io can be extracted, after which the desired
control signal IT' (o), shown in Fig. 6c, can be deriv-
ed from the remaining a.c. component, whose periods again
accurately correspond to the stable and unstable ranges of
the control system.
A problem associated with this embodiment is the
presence of the d.c. component in the control signal IT.
Said d.c. component Io is not entirely constant, but is
both time and location dependent. The time dependence is
mainly caused by variations in the intensity of the ra-
diation emitted by the radiation source. The location
dependence is caused by a difference in absorption and/
or reflection in accordance with the position of the re-
cord carrier and the location where the beam of radia-
tion is transmitted through the optical system, in par-
ticular the lenses.
In order to obtain an insight into the conse-
quences of these two variations of the d.c. component Io
in the control signal IT, may be represented by variations
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of the d.c. level which ;s extracted from said control sig-
nal IT. Indeed, said extraction may give rise to additional
variations. Assuming that instead of the correct d.c. level
Io the D.C. level D.C.(l) is extracted, which is lower than
Io~ a control signal IT'(l) is derived from the residual
a.c. component, which control signal is shown in Fig. 6d.
This reveals that only a part of the unstable ranges Q
is actually identified as an unstable range by said con-
trol signal IT'(l). This is still acceptable because at
any rate during a part of said unstable ranges the sta-
bilizing steps can be taken, so that a possible oscilla-
tion is damped anyway. However, assuming that instead
of the d.c. level Io the higher d.c. level D.C.(2) is
extracted, a control signal IT' (2) as shown in Fig. 6e
will be derived from the residual a.c. component. Said
control signal IT'(2) even identifies parts of the stable
ranges P as unstable regions, so that during a part of
the stable ranges the transfer function destined for the
unstable ranges is rendered effective. Said transfer
function may then cause an unstable behaviour in the
relevant part of the "stable" range, which of course is
highly undesirable. However, by a suitable choice of
said transfer function, it can be achieved that even for
this uncorrect identification of the stable and unstable
ranges a stable system is retained in the stable range
under all conditions. This will be discussed in more
detail when describing the embodiments of the servo-
PHN 7219
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unit shown in Figs. 7 and 8.
A method of eliminating said undesired d.c. com-
ponel,t of the contro, signal IT is the formation of the
sum of the signals produced by the radiation spots Rl
and R2, i.e. the sum of the signals which are represent-
ed by the formulas (2) and (3). Said sum signal equals
2Io and therefore only contains the d.c. component. This
allows said sum signal to be used for the compensation
of the d.c. component Io contained in the control signal
IT, because said two components are always substantial-
ly subject to the same variations.
The embodiment of the apparatus according to
the invention shown in Fig. 7 includes a servo unit 15,
which receives its input signal from the control detec-
tor 13 and supplies a control signal for the drive means
10 of the mirror. The control signal supplied by the
control detector 13 is applied both to an adder unit
21 and to a differentiator 19. Via an amplifier 20 the
output signal of said differentiator 19 is inverted and
applied to the adder unit 21. The output signal of the
adder unit is finally amplified by the amplifier 22 and
then serves as an output signal of the servo-unit. The
differentiator 19 together with the amplifier 20 provides
the desired damping term in the transfer function of the
servo-unit 15.
The transfer function of the servo-unit is in-
fluenced by a control signal which via a squarewave shaper
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18 ;s derived from the control signal I2 which is supplied
by the detector 17 and which consequently corresponds to
the control signal I2' shown in Fig. 3. Said control sig-
nal may be applied to either or both amplifiers 20 and 22.
If the control signal is fed to the amplifier 22, steps
can be taken to reduce the gain factor of the transfer
function of the servo amplifier in the unstable ranges
of the control system. If the control signal is applied
to the amplifier 20, the absolute value of the damping term
in the unstable ranges of the control system can be reduc-
ed. In an unstable range, because of the opposite slope
of the control signal which is supplied by the control
detector 13 as a function of the radius r said damping
term gives rise to a negative damping whose influence
is diminished by said reduction of the gain factor of
amplifier 20.
In the method of obtaining the control signal
described with reference to Figs. 5 and 6 said method of
influencing the transfer function of the servo-unit is -
preferred. The reduction of the gain factor of the am~
plifiers may then be selected so that the resulting re-
duced transfer function remains stable, even when switch- -~
ing over occurs in a stable range p of the servo system.
Fig. 8 shows a second embodiment of the appa-
ratus according to the invention. The servo-unit 15 is
of substantially the same design as that shown in Fig.
7. The output signal of the amplifier 20, however, is
.
. `, , '`, ` ' ~ .
PHN 7219
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now applied to a switch 23, which in a first position
directly supplies said output signal to the adder unit 21
and in a second position via an inverting amplifier stage
24. Said switch 23 is actuated by a control unit 25, which
receives the control signal I2' supplied by the squarewave
shaper 18. When the control system enters an unstable
range, the sw;tch 23 ;s set to the second position, so
that the generated damp;ng term is add;t;onally invert-
ed. As a consequence, said damping term also causes a
pos;t;ve damping in the unstable range.
Sa;d method of influencing the transfer function
of the servo-unit 15 is not part;cularly su;ted to be
employed in the method of obtaining the control signal
described with reference to Figs. 5 and 6. This is be-
cause if the control signal were changed over during a
stable range, caus;ng the s;gn of the damping term to be
reversed, the control behav;our in said stable range
would seriously be d;sturbed.
F;g. 9 shows a third embodiment of the apparatus
according to the ;nvention. The servo-amplifier 15 is
identical to that of Fig. 7. However, in th;s case the
control signal suppl;ed by the detector 17 is not fed to
a squarewave shaper, but to a d;fferent;ator 26. In the
case of a first control signal Il (see Fig. lOa) which
satisfies formula (4) and a second control signal I2
(see Fig. lOb) wh;ch satisfies formula (7), said dif-
ferentiator 26 supplies a signal dt2 which satisfies
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the following formula:
dt2 = _ ~ ~lt . (sin r~ 2 1r ) dt (9)
It follows from said formula that said differentiated
dI2
signal dt is in phase or in phase opposition with the
first control signal Il, depending on the sign of dt ~
i.e. the d~rection of the radial movement. Said datum
is used for stabilizing the control system.
I~ is, for example assumed that the differen-
dI2 dI
tiated signal dt has the response dt2(1) as shown in
Fig. lOc, which signal is then exactly in phase oppo-
sition with the first control signal and thus denotes
a positive value of ddt, i.e. a movement of the scanning
point towards a greater value of r. From this signal
dI
dt2 (1) a squarewave s;gnal I3(1) can be derived (see
Fig. lOd) with the aid of a threshold value detector
27, which detects when said differentiated signal ex-
ceeds a positive threshold D. The gain factor of the
amplifier 22 is controlled with said squarewave control
signal I3(1) in such a way that said gain factor is re-
duced if the control signal is positive.
This means that the gain is reduced during the
first part of the negative half period of the first con-
trol signal Il. Assuming again that a positive value of
said control signal Il tends to cause a movement of the
scanning point towards a smaller value of r and a nega-
tive value a movement towards a greater value, this re-
duction of the gain during the negative half period
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will result ;n a stabilization of the control system. When
passing through an even number of positive and negative
half periods of the control signal Il, the energy which
is exchanged during the positive half period which coun-
teracts the movement towards a greater value of r which
still persists at that instant, is greater than the energy
which is exchanged during the negative half periods, which
tends to maintain the said movement. As a result, said
prevail;ng movement is damped.
If the movement of the scanning point is exactly the op-
posite, i.e. dt ~ ~ the signal dt2 (2) shown in Fig. lOc
is produced as a differentiated second control signal,
from which via the threshold detector 27 the squarewave
signal I3(2) (Fig. lOd) is derived. The gain of the
transfer function is then reduced during a part of the
positive half periods of the first control signal Il,
so that the desired stabilizing action is obtained
again.
If the threshold D (Fig. lOc) of the threshold
detector 27 is selected to be zero, the gain factor will
be reduced either during the entire positive or the entire
negative half period of the first control signal Il,
irrespective of the speed of the radial movement. This
is less desirable, because in this case the gain factor
would also be switched continually if the scanning
point would move in the stable range around the desired
stable settings (sl, s2, S3 etc).
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This is prevented by selecting the threshold value
D to differ from zero. The amplitude of the differentiated
second control signal dt2 ;s proportional to the magnitude
of the speed of movement of the scanning point. At a higher
amplitude of the differentiated second control signal
(see for example dt2 (3) in Fig. lOc) a squarewave signal
(see I3(3) in Fig. lOe) is obtained with the aid of a
threshold value D which is not zero, whose positive pulse
width increases approximately proportionally to said am-
plitude~ This means that the stabilizing action is pro-
portional to the magnitude of the speed.
If the speed decreases owing to said stabiliz-
ing action, the pulse width of the squarewave signal I3
also decreases ? which means that the switching points
of the gain factor further recede from the desired set-
tings (sl, s2, s3 etc), so that automatically the desired
range with a fixed transfer function is obtained around
said settings. In the extreme case the speed is so small,
dI
that the amplitude of the signal dt2 is smaller than the
threshold value D, so that no squarewave signal I3 is
produced any longer. However, said situation is merely
hypothetic because before such a situation can occur
the control system is already locked in a stable range.
In a modification of the embodiment described
hereinbefore the differentiated second control signal
dI2
dt of formula (9) is again multiplied by the first con-
trol signal, which yields
-- 31 -- -- .
`' ` ' ' ' ' '. " ' ` , ' '
PHN 7219
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2 I = ~ ( sin r~ 2 Ir )2 dr (10)
The polarity of this signal is a direct indication of the
direction of the radial movement. By reducing the gain
depending on said polarity, either during the positive
or during the negative half period of the first control
signal Il, the desired stabilizing action is obtained
again. Thus, in this case a threshold value detector
is required to which the first control signal Il is ap-
plied and which in accordance with the polarity of sig-
nal expressed by the formula (10) either during the po-
sitive or the negative half period of said first con-
trol signal supplies a control pulse to the amplifier
22 in the servo-unit. By giving said threshold value
detector a threshold which is not zero, a range which
is situated around the desired settings (sl, s2, S3 etc.)
is excluded, so that a fixed transfer function is opera-
tive in this range under all conditions.
In a second modification the differentiator
26 is designed in a special manner, for example as
shown in Fig. 11. Said embodiment of the differentiator
includes an operational amplifier V, whose inverting
input is connected to earth via a capacitor C and via
a resistor R to the output. The second control signal
I2 supplied by the detector 17 is fed to the non-invert-
ing input of the operational amplifier V.
If it is assumed that the signal I2 satisfies
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the equation of formula (7), the output signal I4 of the
operational amplifier V will be:
I4 = I2 + CR dt2 = 2 m cos r~ 2 ~r - 4 ~ CR (sin r~ 2 ~ dt
(11)
For a rapid radial movement of the scanning point the
second term is predominant, so that in that case the
system operates substantially in accordance with the
stabilising method described with reference to Figs. 9
and 10, because the signal I4 then substantially corres-
ponds to the signal dt2 of formula (9). However, if the
speed ddt f the radial movement decreases, the influence
of the first term of formula (11) increases, until it
finally becomes predominant. In this last borderline
case the signal I4 fully corresponds to the control
signal I2, so that the system then operates in accord- -
ance with the stabilizing method given in Figs. 7 and ~3 -
in combination with Fig. 3. Thus, depending on the speed
one stabilizing method gradually merges into the other.
An additional advantage of said stabilizing - -
method is the fact that for rapid movements the thres-
hold value for the system may be zero without causing
any problems, so that maximum stabilization is pos-
sible. This because at low speed the other stabilizatio~ _
method is adopted.
A similar behaviour can also be obtain~ with
the aid of a "poor" differentiator, for example as
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.. . . .. . . . .. . . . .. . .. ..
PHN 7219
1055156
shown in Fig. 12. The differentiating network then con-
sists of the parallel connection of a resistance Rl and
a capacitance Cl in series with a resistance R2. The
transfer function of said network is
R2 (1 + pc Rl)
(Rl R2) pc Rl R2 (12)
where p is the imaginary angular frequency. If
CR~ < P ~ CR~ R~ + R2 formula (12) may be ap-
proximated by
CRl R2
1 R2 (13)
which implies a pure differentiation. For P <Cl ~ how-
ever, the transfer function may be reduced to
R2
1 R2 (14)
which is a normal voltage division. At relatively high
frequencies, i.e. at high speed, the signal I2 is dif-
ferentiated, so that the stabilizing method of Fig. 9
is obtained, whilst at lower frequencies, i.e. at low
radial speed, the stabilizing method of Figs. 7 and 8
is obtained with a continuous transition, between these
two methods.
It is obvious, that the embodiment of the
servo-unit is not limited to the embodiments shown in
Figs. 7, 8 and 9. For example, it is also possible to
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PHN 7219
1055156
realize a damping term with the aid of suitable trans-
ducers at the drive element 10 or the mirror 8. There
are numerous embodiments of and the methods in which
the amplifier 20 and/or 22 can be influenced by the con-
trol signal, because this type of amplifier is known in
many versions and selecting a suitable amplifier in ac-
cordance with the requirements imposed and influencing
the gain factor thereof in the correct manner will not
present any problem to those skilled in the art.
, .. .... .
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