CA1055156A - Apparatus for reading a disc-shaped record carrier - Google Patents
Apparatus for reading a disc-shaped record carrierInfo
- Publication number
- CA1055156A CA1055156A CA212,960A CA212960A CA1055156A CA 1055156 A CA1055156 A CA 1055156A CA 212960 A CA212960 A CA 212960A CA 1055156 A CA1055156 A CA 1055156A
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- CA
- Canada
- Prior art keywords
- control signal
- signal
- control
- detector
- radiation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0948—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following specially adapted for detection and avoidance or compensation of imperfections on the carrier, e.g. dust, scratches, dropouts
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0901—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following only
- G11B7/0903—Multi-beam tracking systems
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0946—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following specially adapted for operation during external perturbations not related to the carrier or servo beam, e.g. vibration
Landscapes
- Optical Recording Or Reproduction (AREA)
- Moving Of The Head For Recording And Reproducing By Optical Means (AREA)
- Holo Graphy (AREA)
- Recording Measured Values (AREA)
Abstract
ABSTRACT
An apparatus for reading a disc-shaped record carrier on which information is stored in a preferably spiral-shaped track. Said information is read optically with the aid of a radiation beam, which via a directing system transfers the information contained in the scan-ning point of the record carrier to a read detector. The radial position of the scanning point is controlled with the aid of a control system which via a first pattern of radiation spots which is projected onto the record carrier receives information about said radial position and by means thereof controls the directing system. In order to ensure that the system is stable under all conditions use is made of a second pattern of radiation spots which are projected onto the record carrier, which pattern is radially shifted relative to the first pattern, and from which a control signal is derived with which the trans-fer function of the control system is varied so as to stabilize the control system.
An apparatus for reading a disc-shaped record carrier on which information is stored in a preferably spiral-shaped track. Said information is read optically with the aid of a radiation beam, which via a directing system transfers the information contained in the scan-ning point of the record carrier to a read detector. The radial position of the scanning point is controlled with the aid of a control system which via a first pattern of radiation spots which is projected onto the record carrier receives information about said radial position and by means thereof controls the directing system. In order to ensure that the system is stable under all conditions use is made of a second pattern of radiation spots which are projected onto the record carrier, which pattern is radially shifted relative to the first pattern, and from which a control signal is derived with which the trans-fer function of the control system is varied so as to stabilize the control system.
Description
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
- 2 --lOSS~S6 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 . ,~
, 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.
. .
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-.', , 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-, : . . . ............. . .......... .
. .
_~ PHN 7219 1~55156 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 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-, " , ,, ~ PHN 7219 ~OS5156 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 lOS51S6 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-~055156 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-,, 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 ; . , , , ~, ......... .
.
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 . . . . .
.,,;,, , . '', ~ ' ' ' l~SSlS6 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 A
~" ,. ,, ~ . .
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 , . . ..
. . .
,' , . - . .
.
lOS5156 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
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 . ,~
, 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.
. .
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-.', , 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-, : . . . ............. . .......... .
. .
_~ PHN 7219 1~55156 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 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-, " , ,, ~ PHN 7219 ~OS5156 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 lOS51S6 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-~055156 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-,, 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 ; . , , , ~, ......... .
.
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 . . . . .
.,,;,, , . '', ~ ' ' ' l~SSlS6 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 A
~" ,. ,, ~ . .
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 , . . ..
. . .
,' , . - . .
.
lOS5156 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.
. . .
, ., . , , , , , , . ' . ,, , ' .
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 .
~ PHN 7219 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 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.
- 19 - :
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- -~
, "
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) 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 ' 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 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 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-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 lOS5156 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 .
. `, , '`, ` ' ~ .
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 ~ 055156 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 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).
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 -- -- .
`' ` ' ' ' ' '. " ' ` , ' ' 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 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 .. . . .. . . . .. . . . .. . .. ..
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 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 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.
, .. .... .
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.
. . .
, ., . , , , , , , . ' . ,, , ' .
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 .
~ PHN 7219 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 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.
- 19 - :
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- -~
, "
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) 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 ' 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 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 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-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 lOS5156 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 .
. `, , '`, ` ' ~ .
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 ~ 055156 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 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).
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 -- -- .
`' ` ' ' ' ' '. " ' ` , ' ' 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 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 .. . . .. . . . .. . . . .. . .. ..
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 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 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.
, .. .... .
Claims (12)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for reading a disc-shaped record carrier on which signals are stored in tangential tracks, which apparatus includes an optical read unit with a ra-diation source, a directing system and a read detector, which radiation source emits a radiation beam, which is projected onto the record carrier as a read spot of ra-diation and which via the directing system transfers the information contained in the scanning point of the record carrier to the read detector, a control system for con-trolling the radial position of the scanning point on the desired track, which control system includes a drive means for the direction system and a first measuring sys-tem for measuring the radial position of the scanning point and supplying a corresponding first control sig-nal to the drive means, which first measuring system employs a first pattern of radiation spots projected on the record carrier, which pattern is imaged onto a first measuring detector and which first measuring sys-tem supplies a first control signal, which during a ra-dial movement of the scanning point over a number of track distances includes a periodic a.c. component whose period equals the track distance, the control system being stable for one half period of said a.c.
component and being unstable for the other half period of the control system, characterized in that for assuring 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 pattern is imaged onto a second measur-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 con-tains a periodic a.c. component whose period equals the track distance, but which a.c. component as a result of a deviating radial position of the second pattern of ra-diation spots relative to that of the first pattern of radiation spots exhibits a phase shift of substantially a quarter period relative to the a.c. component of the first control signal, and that the control system has a variable transfer function which is controlled by a con-trol unit which receives a third control signal which is derived from the second control signal, and which control unit upon a movement of the scanning point over a number of track distances controls the transfer function so as to stabilize the control system.
component and being unstable for the other half period of the control system, characterized in that for assuring 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 pattern is imaged onto a second measur-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 con-tains a periodic a.c. component whose period equals the track distance, but which a.c. component as a result of a deviating radial position of the second pattern of ra-diation spots relative to that of the first pattern of radiation spots exhibits a phase shift of substantially a quarter period relative to the a.c. component of the first control signal, and that the control system has a variable transfer function which is controlled by a con-trol unit which receives a third control signal which is derived from the second control signal, and which control unit upon a movement of the scanning point over a number of track distances controls the transfer function so as to stabilize the control system.
2. An apparatus as claimed in Claim 1, characteriz-ed in that the control signal is a squarewave signal, which is substantially in phase with the second control signal.
3. An apparatus as claimed in Claim 1, charac-terized in that the third control signal is obtained with the aid of a differentiation circuit, to which the second control signal is applied.
4. An apparatus as claimed in Claim 3, characteriz-ed in that the third control signal is a squarewave sig-nal, which is derived from the differentiated second con-trol signal with the aid of a threshold value detector, which detects the periods in which the differentiated second control signal exceeds a certain absolute thres-hold value and which during said period supplies a square-wave pulse the polarity of which at each moment is related to the polarity of the differentiated second control sig-nal.
5. An apparatus as claimed in Claim 1, charac-terized in that the third control signal is a squarewave signal which is supplied by a threshold value detector, which detects the periods in which the first control signal has specific polarity and at the same time ex-ceeds a certain threshold value and during said periods supplies a squarewave pulse, selection between the two possible polarities of the first control signal to be detected being effected depending on the polarity of a product signal, which is the product of the first control signal and the differentiated second control signal.
6. An apparatus as claimed in Claims 1, 2 or 3, characterized in that the gain factor of the transfer function is varied in accordance with the control sig-nal.
7. An apparatus as claimed in Claim 1 or 2, characterized in that the damping term contained in the transfer function is varied depending on the third con-trol signal.
8. An apparatus as claimed in Claim 2, charac-terized in that the transfer function may contain two mutually inverse damping terms depending on the control signal.
9. An apparatus as claimed in Claims 1, 2 or 3, characterized in that the second pattern of radiation spots consists of two radiation spots which are mutually shifted in a radial direction by a distance equal to half the track distance and which are each imaged onto a sep-arate part of the second measuring detector, the second control signal being obtained by subtraction of the sig-nals supplied by the individual parts of the second measuring detector.
10. An apparatus as claimed in Claims 1, 2 or 3, characterized in that the second control signal is ob-tained by measuring the low-frequency component of the signal which is supplied by the read detector.
11. An apparatus as claimed in Claim 3, characteriz-ed in that the differentiation circuit includes an opera-tional amplifier, to the non-inverting input of which the second control signal is applied and whose inverting in-put is connected to a point of constant potential via a capacitance and to the output of the operational am-plifier via a resistance.
12. An apparatus as claimed in Claim 3, charac-terized in that as differentiation circuit a network is used which for comparatively low frequencies operates at least substantially proportionally and at compara-tively high frequencies operates at least substantially differentiating.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL7315419A NL7315419A (en) | 1973-11-10 | 1973-11-10 | DEVICE FOR READING A DISK-SHAPED REGISTRATION CARRIER. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1055156A true CA1055156A (en) | 1979-05-22 |
Family
ID=19819976
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA212,960A Expired CA1055156A (en) | 1973-11-10 | 1974-11-04 | Apparatus for reading a disc-shaped record carrier |
Country Status (17)
| Country | Link |
|---|---|
| JP (1) | JPS5433724B2 (en) |
| AR (1) | AR204426A1 (en) |
| AT (1) | AT336298B (en) |
| BE (1) | BE822040A (en) |
| BR (1) | BR7409349A (en) |
| CA (1) | CA1055156A (en) |
| CH (1) | CH579807A5 (en) |
| DE (1) | DE2452815C2 (en) |
| DK (1) | DK135070B (en) |
| ES (1) | ES431776A1 (en) |
| FR (1) | FR2251067B1 (en) |
| GB (1) | GB1490726A (en) |
| IT (1) | IT1024778B (en) |
| NL (1) | NL7315419A (en) |
| NO (1) | NO144501C (en) |
| SE (1) | SE394335B (en) |
| ZA (1) | ZA746767B (en) |
Families Citing this family (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2396379A1 (en) * | 1977-07-01 | 1979-01-26 | Thomson Brandt | OPTICAL INFORMATION DISC READER EQUIPPED WITH AN AUTOMATIC INFORMATION ACCESS DEVICE |
| NL7803517A (en) * | 1978-04-03 | 1979-10-05 | Philips Nv | RECORD CARRIER WITH AN OPTICALLY READABLE PHASE STRUCTURE AND DEVICE FOR READING. |
| JPS626580Y2 (en) * | 1979-11-17 | 1987-02-16 | ||
| NL8001974A (en) * | 1980-04-03 | 1981-11-02 | Philips Nv | TRACKING DEVICE. |
| JPS57113428A (en) * | 1980-12-29 | 1982-07-14 | Pioneer Video Corp | Focus servo device |
| NL8105347A (en) * | 1981-11-26 | 1983-06-16 | Philips Nv | Apparatus for the optical scanning of a disc-shaped record carrier. |
| NL8105346A (en) * | 1981-11-26 | 1983-06-16 | ||
| NL8200208A (en) * | 1982-01-21 | 1983-08-16 | Philips Nv | DEVICE FOR READING A DISC REGISTRATION CARRIER. |
| JPS58143472A (en) * | 1982-02-22 | 1983-08-26 | Hitachi Ltd | Tracking servo controller of recording information reproducing device |
| JPS58169350A (en) * | 1982-03-31 | 1983-10-05 | Hitachi Ltd | Optical track shift detecting system |
| GB8824888D0 (en) * | 1988-10-24 | 2002-08-14 | British Aerospace | Servo actuation systems |
-
1973
- 1973-11-10 NL NL7315419A patent/NL7315419A/en not_active Application Discontinuation
-
1974
- 1974-01-01 AR AR256416A patent/AR204426A1/en active
- 1974-10-24 ZA ZA00746767A patent/ZA746767B/en unknown
- 1974-11-04 CA CA212,960A patent/CA1055156A/en not_active Expired
- 1974-11-07 DE DE2452815A patent/DE2452815C2/en not_active Expired
- 1974-11-07 CH CH1490774A patent/CH579807A5/xx not_active IP Right Cessation
- 1974-11-07 NO NO744015A patent/NO144501C/en unknown
- 1974-11-07 SE SE7413962A patent/SE394335B/en not_active IP Right Cessation
- 1974-11-07 GB GB48309/74A patent/GB1490726A/en not_active Expired
- 1974-11-07 JP JP12763274A patent/JPS5433724B2/ja not_active Expired
- 1974-11-07 DK DK580974AA patent/DK135070B/en not_active IP Right Cessation
- 1974-11-07 IT IT70277/74A patent/IT1024778B/en active
- 1974-11-07 BR BR9349/74A patent/BR7409349A/en unknown
- 1974-11-08 BE BE150377A patent/BE822040A/en not_active IP Right Cessation
- 1974-11-08 AT AT895674A patent/AT336298B/en not_active IP Right Cessation
- 1974-11-08 ES ES431776A patent/ES431776A1/en not_active Expired
- 1974-11-08 FR FR7437019A patent/FR2251067B1/fr not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB1490726A (en) | 1977-11-02 |
| FR2251067A1 (en) | 1975-06-06 |
| SE7413962L (en) | 1975-05-12 |
| DK135070C (en) | 1977-08-01 |
| SE394335B (en) | 1977-06-20 |
| DK580974A (en) | 1975-07-07 |
| DK135070B (en) | 1977-02-28 |
| DE2452815C2 (en) | 1982-10-28 |
| FR2251067B1 (en) | 1983-08-05 |
| JPS5433724B2 (en) | 1979-10-23 |
| AT336298B (en) | 1977-04-25 |
| ATA895674A (en) | 1976-08-15 |
| ES431776A1 (en) | 1976-12-16 |
| BE822040A (en) | 1975-05-09 |
| DE2452815A1 (en) | 1975-06-26 |
| ZA746767B (en) | 1976-05-26 |
| BR7409349A (en) | 1976-05-18 |
| AU7515574A (en) | 1976-05-13 |
| NO144501C (en) | 1981-09-09 |
| AR204426A1 (en) | 1976-02-06 |
| NO744015L (en) | 1975-06-09 |
| IT1024778B (en) | 1978-07-20 |
| JPS5081218A (en) | 1975-07-01 |
| NO144501B (en) | 1981-06-01 |
| CH579807A5 (en) | 1976-09-15 |
| NL7315419A (en) | 1975-05-13 |
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