EP0145755A1

EP0145755A1 - Amplitude modulated coarse position error signal generation in an optical disk storage system employing coarse servo tracks

Info

Publication number: EP0145755A1
Application number: EP19840902390
Authority: EP
Inventors: Majeed K. Abed; Charles A. Farel; Paul M. Romano; Allan L. Samson
Original assignee: Storage Technology Partners II
Current assignee: Storage Technology Partners II
Priority date: 1983-06-13
Filing date: 1984-05-24
Publication date: 1985-06-26
Also published as: CA1218148A; WO1984004965A1

Abstract

Détecteur linéaire (31) destiné à être utilisé dans un système d'asservissement de positionnement approximatif d'un système de stockage à disque optique. Le détecteur linéaire produit un signal d'erreur (67) possédant une amplitude linéairement proportionnelle à la distance sur laquelle le centre de gravité d'une énergie radiante incidente (63) tombe sur une surface réceptrice (62) du détecteur, distance mesurée par rapport à un point de référence fixe sur ladite surface réceptrice. Deux signaux de référence (65, 66) sont dérivés des circuits associés à la surface réceptrice. Un premier signal de référence (65) possède une amplitude proportionnelle à l'intensité de l'énergie lumineuse focalisée et à l'emplacement sur lequel tombe l'énergie lumineuse sur la surface réceptrice par rapport à un premier point de référence. Un deuxième signal de référence (66) possède une amplitude proportionnelle à l'intensité de l'énergie lumineuse focalisée et à l'emplacement sur lequel tombe l'énergie lumineuse sur la surface réceptrice par rapport à un deuxième point de référence. La somme et la différence des amplitudes du premier et du deuxième signaux de référence sont dérivées pour produire des signaux de somme et de différence (Y, X), respectivement. Le signal de différence est divisé par le signal de somme (X/Y) pour produire le signal d'erreur désiré, ce signal d'erreur possédant une amplitude qui est sensiblement indépendante de l'intensité de l'énergie lumineuse focalisée. Des circuits de traitement de signaux (84, 86, 88) et (85, 87, 89) peuvent être utilisés pour démoduler les signaux de référence de sorte que le signal d'erreur désiré ne soit dérivé que de la partie d'énergie radiante incidente réfléchie par une piste d'asservissement approximatif sur le disque optique, et non pas de l'énergie radiante réfléchie par d'autres zones du disque.Linear detector (31) for use in an approximate positioning servo system of an optical disc storage system. The linear detector produces an error signal (67) having an amplitude linearly proportional to the distance over which the center of gravity of an incident radiant energy (63) falls on a receiving surface (62) of the detector, distance measured from at a fixed reference point on said receiving surface. Two reference signals (65, 66) are derived from the circuits associated with the receiving surface. A first reference signal (65) has an amplitude proportional to the intensity of the focused light energy and the location to which the light energy falls on the receiving surface relative to a first reference point. A second reference signal (66) has an amplitude proportional to the intensity of the focused light energy and the location at which the light energy falls on the receiving surface relative to a second reference point. The sum and difference of the amplitudes of the first and second reference signals are derived to produce sum and difference signals (Y, X), respectively. The difference signal is divided by the sum signal (X / Y) to produce the desired error signal, this error signal having an amplitude which is substantially independent of the intensity of the focused light energy. Signal processing circuits (84, 86, 88) and (85, 87, 89) can be used to demodulate the reference signals so that the desired error signal is derived only from the radiant energy portion incident reflected by an approximate servo track on the optical disc, not radiant energy reflected by other areas of the disc.

Description

AMPLITUDE MODULATED COARSE POSITION ERROR SIGNAL GENERATION IN AN OPTICAL DISK STORAGE SYSTEM EMPLOYING COARSE SERVO TRACKS

BACKGROUND OF THE INVENTION

This invention relates to optical disk data storage systems, and more particularly to a system and method for generating a coarse position error signal for use in a coarse servo system of an optical disk data storage system.

Optical data storage systems that utilize a disk to optically store information have been the object of extensive research. Like their counterpart magnetic disk units, these optical disk storage units must have a servo system which controls the positioning of a read/write head to provide direct access to a given track of data recorded on the rotating disk. Further, once a desired track has been accessed, the servo system must cause the read/write head to accurately follow this track while it is being read or when data is initially written thereonto.

Numerous approaches have been proposed in the art for providing the desired access and tracking capability. While a discussion of such prior-art approaches provides interesting background information, applicants do not believe that such a discussion is necessary to teach and understand the operating principles of the invention described herein. Accordingly, no such background discussion is repeated.

Whatever the type of access and tracking system employed, some sort of detection means must be used to generate an error signal that can be used by the appropriate servo system to guide the positioning of the read/write head to a desired radial position with respect to the disk, and to maintain this desired position once reached. For example, a coarse/fine servo system may be used for this purpose. In such a system, coarse servo tracks, selectively placed on the disk, allow the coarse servo system to access and track a relatively large band on the disk. The fine servo system is then used to access and track a desired data track within the band. With respect to the course servo system, a narrow strip of radiant energy^" incident to a detector array can be sensed, and a signal generated having an amplitude proportional to the location at which the strip of radiant energy strikes the array. By selectively placing spaced-apart coarse servo tracks on the disk, and then by illuminating through the read/write head an area of the disk large enough to always include a segment of one of these coarse servo tracks, the reflected radiant energy from the illuminated coarse servo track becomes a narrow strip of radiant energy that may be directed back through the read/write head to the surface of the detector array. The signal generated by the array can then be used as the needed error signal to indicate the location of the read/write head relative to a given coarse track. This error signal is used, in turn, by a coarse positioning servo system to place the read/write head at a desired location so as to provide the requisite coarse access and tracking capability. __

The use of a detector array as described above is not without its drawbacks. An array is by definition a collection of discrete radiation-sensitive elements arranged in a systematic fashion. As such, the output signal generated will have minor discontinuities therein as the radiant energy moves from one element to another. These discontinuities may impact the linearity of the signal thus generated, and are therefore undesirable.

Further, depending upon the size of the array and the number of elements used therein, it may actually be necessary to store the information sensed by each element and serially pass this information out of the array through a single pin or terminal, thereby minimizing the number of input/output pins associated with the detector array. If such is the case, a clock signal, or equivalent,, must be used in order to clock the data out of the device. This imposes a finite processing time during which the sensed position data is serially passed out of the array, reconfigured, and examined. This "processing time" may disadvantageously limit the access speed associated with moving the read/write head from one coarse track to another. Further, the circuitry required to carry out this signal processing is quite complex and expensive to implement, and all the multiplexing involved generates undesirable noise that may adversely impact the data.

As a still further disadvantage, the amplitude of an error signal generated in arrays of the type described above may not only be a function of the sensed position of the radiant energy (as desired), but it may also.be a function of the intensity of the radiant energy as it strikes the array surface. Thus, in order to preserve the integrity of the position error signal, the intensity of the radiation incident to the detector must be held more or less constant. Unfortunately, this is an extremely formidable task when dealing with radiant energy that is reflected off of a rotating disk, which reflected radiant energy may vary a great deal in intensity.

Moreover, more often than not, radiation incident to the array may be reflected from more than just the desired coarse servo track, e.g., from data tracks. Some means is needed therefore to identify and distinguish the desired reflected radiation from any undesired reflected radiation that may be present.

What is needed, therefore, is a detection system that provides a continuous linear output signal that indicates the position of radiant energy incident thereto, that is reflected from only a coarse, servo track, and not from adjacent data tracks, and that is insensitive to variations in the intensity of the incident radiation.

SUMMARY OF THE INVENTION.

It is an object of the present invention to provide a linear detector system for use with a coarse positioning servo system of an optical disk data storage system that generates a position error signal having an amplitude that is proportional to the location of a strip or band of radiant energy incident thereto.

It is a further object of the present invention to provide such a linear detector system that it is especially suited for use with a coarse positioning servo system employing concentric coarse servo tracks on an optical disk, a reflected image of a segment of a coarse servo track being directed through appropriate optical elements to the linear detector system of the present invention.

A still further object of the present invention is to provide such a linear detector system wherein the amplitude of the position error signal is substantially independent of the intensity of the incident radiant energy falling thereon.

An additional object of the present invention is to provide such a linear detector system that responds only to radiation reflected from the coarse servo tracks, and that is insensitive to radiation reflected from other than the coarse servo tracks.

Still another object of the present invention is to provide such a linear detector system wherein the position error signal is continuously generated, and is not dependant upon the use of clock signals, or equivalent, in order to gain access to and process the position information sensed bysaid detector system.

The above and other objects of the invention are realized by employing a linear detector system, described more fully below, as an element in a coarse servo positioning system of an optical disk data storage system.

The optical disk storage system includes means for rotating an optical disk and means for controllably positioning a read/write head radially with respect to said disk, thereby allowing radiant energy, typically laser energy, passing through said- read/write head to be directed to desired locations on the surface of the rotating disk. Such radiant energy is used to selectively mark (write) the disk with desired information, or to read (sense radiant energy reflected from the previously-written marks) the information already on the disk.

Included within the coarse servo positioning system are coarse servo tracks, typically concentrically placed on the disk. As described below, these coarse tracks are used as markers or sign posts to guide the read/write head to a desired radial position with respect to a given coarse track. Coarse illumination means direct radiant energy through the read/write head to the surface of the rotating disk. This radiant energy strikes an area large enough on the surface of the disk to ensure that at least a segment of one coarse servo track is always illuminated. Reflected radiant energy from the surface of the disk therefore includes the coarse track segment within the illuminated area. This reflected energy is directed back through the read/write head to the linear detection system of the present invention.

Advantageously, the coarse servo track has a particular reflectivity pattern associated therewith such that radiation reflected therefrom can be uniquely distinguished from radiation reflected from other portions of the area illuminated by the coarse illumination means. Hence, by using appropriate discrimination means to identify the coarse servo track _.radiation, the linear detection system of the present invention generates an error signal having an amplitude that is linearly proportional to the distance at which this coarse servo radiation falls on a collection surface of a detector used within said system. This distance is measured relative to a fixed reference point on the collection surface. Advantageously, the amplitude of the error signal generated by the linear detection system of the present invention is substantially independent of the intensity of the radiant energy.

In the preferred embodiment, the reflectivity pattern of the coarse servo track is a repetitive on/off (reflectivity high/reflectivity low) sequence such that when the disk is rotated at a constant speed, the reflected radiation from the coarse servo track assumes a pulsed condition having a known frequency. Once converted to a corresponding electrical signal, filtering techniques are used to distinguish this fixed-frequency radiation from other radiation.

Two reference signals are derived from circuitry associated with the collection surface of the detector. A first reference signal has an amplitude proportional to the intensity of the radiant energy and the location that said radiant energy falls on the collection surface relative to a first reference point. A second reference signal has an amplitude proportional to the intensity of the radiant energy and the location that the radiant falls on the collection surface relative to a second reference point. The sum and difference of the amplitudes of these first and second reference signals are derived to produce sum and difference signals, respectively. The difference signal is then divided by the sum signal to produce the desired error signal, ^"which error signal has an amplitude that is substantially independent of the intensity of the radiant energy. Filtering, or other suitable discrimination techniques, may be used at essentially any point prior to dividing the difference signal by the sum signal as these reference signals are processed in order to limit the sensitivity of the detection system to only that radiation reflected from the coarse servo tracks.

The position error signal is used by the coarse servo positioning system as a feedback signal to control the radial position of the read/write head with respect to the disk. In a seek or access mode, the read/write head will be moved radially with respect to the disk until the read/write head is above or near a desired coarse servo track. While so moving, the position error signal assumes a sawtooth waveform, each cycle of which corresponds to the movement from one servo track to an adjacent servo track. Once a desired coarse servo track has been reached, a tracking mode

OMPI is assumed during which the read/write head is held in a fixed position relative to the desired coarse servo track by monitoring the amplitude of the position error signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 is a block diagram of a coarse/fine servo system used in an optical disk data storage system, and illustrates the environment in which the present invention is designed to be used;

FIG. 2 schematically shows the principle elements of FIG. 1;

FIG. 3 is a side view of an optical disk drive and schematically shows the relationship between the optical ^" disk, fixed and moving optics packages, and a linear actuator for controllably positioning the read/write head;

FIG. 4 conceptually illustrates how the intensity of the radiation reflected from the optical disk may vary as a function of disk radial position between two coarse servo tracks;

FIG. 5 is a block diagram of the coarse track detection system of the present invention;

FIG. 6 is an expanded view of a segment of the optical disk surface and conceptually illustrates the reflectivity-high/reflectivity-low pattern placed in the coarse servo tracks;

FIG. 7 is a timing diagram that illustrates the type of radiation signal reflected from the coarse illuminated area 11a cf

O PI the optical disk surface (FIG. 6) when the disk is rotated at a constant angular velocity, both before and after filtering;-

FIG. 8 is a block diagram of an alternative configuration of the coarse track detection system of the present invention; and

FIG. 9 is a schematic diagram of the configuration shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is best understood by reference to the accompanying drawings wherein like numerals will be used to describe like elements or parts throughout.

FIG. 1 shows a block diagram of a coarse/fine servo system of a type with which the present invention could_.be used. The various optical paths associated with the system shown in FIG. 1 are illustrated as bold lines, whereas electrical paths are indicated by fine lines. Mechanical coupling, as occurs between a carriage actuator 24 and the carriage optics 23, is indicated by a dashed line.

Referring next to both FIG. 1 and FIG. 2, the optical disk storage system can be explained. .The system allows reading and writing from and to the surface of a disk 11 having a rotational axis 10 and a plurality of concentric data bands 12-14 (shown in FIG. 2). Each of the data bands includes a plurality of data tracks concentrically spaced about the rotational axis. The surface of the disk 11 has pre-recorded thereon, during manufacture, a plurality of optically readable servo tracks 16-19, concentrically and uniformly spaced about the rotational axis of the disk and positioned between the data bands.

OMPI The disk 11 is rotated about its axis 10 by conventional means. An optical read/write head, depicted by the carriage optics block 23, is positioned adjacent to the surface of the disk 11. Carriage actuator 24 selectively moves the read/write head along a radial axis 20 (FIG. 2), thereby moving the carriage optics 23 in a radial direction with respect to the disk 11 in order to access the data bands thereon. Mechanical motion of the carriage optics 23 is depicted in FIG. 2 as a dotted line 45, with motion being possible in both directions as indicated by the double headed arrow 45".

A fine read/write servo illuminator and detector 25 (FIG. 2) projects read or write light beam(s) 52' to the surface of the disk 11 so as to access data tracks thereon. In order to access the disk surface, this beam 52' is reflected by a fine tracking mirror 26, passes through a beam combiner and separator .27, as well as through the carriage optics 23. Included within the illuminator and detector 25 is a read detector 25b (FIG. 1) that reads light which has been reflected from the accessed recorded data track. This reflected light passes through the carriage optics 23 and beam combiner and separator 27 before reaching the read detector 25b. The read detector converts this light to an equivalent electrical signal(s). This read electrical signal is, in turn, supplied to a data read system 25c, and to a fine access/tracking servo system 25d.

The servo system for access to and tracking of the coarse servo tracks includes a coarse illuminator 30 which projects light, represented as dashed double-dot lines in FIG. 2, through a coarse servo beam separator 36, a beam combiner and separator 27, and the carriage optics 23 onto a relatively broad portion 11a of the disk surface (FIG. 2). An optical detector 31 detects reflected light, represented as dashed single-dot lines in FIG. 2, from the portion 11a of the disk surface. It is noted that the illuminated portion 11a of the disk surface spans at least the distance between two coarse servo tracks, and thereby always illuminates at least one coarse servo track. As' shown in FIG. 2, light is reflected from the portion 11a of the disk 11 between servo tracks 16 and 18 with the servo track 17 being projected onto coarse detector and processing circuitry 31. It is this coarse detector and processing circuitry 31 that comprises the principle element of the present invention, and is described more fully below.

The output of the coarse detector and processing circuitry 31 is a coarse track position error signal (PES), which signal has an amplitude proportional to the location at which the reflected radiation from the il uminated coarse servo track falls on the face of the detector 31. This error signal from the detector 31 is applied to a coarse access/tracking system 34. This system is connected in a servo loop with the actuator^"24, which actuator moves the read/write head (represented schematically by the carriage optics 23) into radial proximity of a selected servo track so that the fine access and tracking system 25d can accurately position read or write beams on a selected data track.

As indicated previously, light reflected from a single data track on the disk is passed by means of the carriage optics 23, beam separator 27, and tracking mirror 26, and is detected by read detector 25b, the output of which is applied to the fine access/tracking servo system 25d. The read or write beams 52' from the illuminator 25a are moved radially with respect to the optical disk 11 by means of the tracking mirror 26, thereby providing for fine selective control of the beam's radial position. The tracking mirror 26, which may be a conventional galvanometer controlled

is -^OMPr__ mirror(s), is controlled by the fine access/tracking servo system 25d.

In order to discriminate radiation reflected from servo tracks from that reflected from data tracks or other areas of the disk surface, the servo tracks have an on/off (reflectivity-high/reflectivity-low) pattern placed therein that may be conceptually thought of as a dashed line, as shown best in FIG. 6. This is explained more fully below. Further, the servo tracks are preferably three to five times the width of the data tracks. The servo tracks provide improved data track following capability by providing coarse tracking control of the read/write head. The coarse tracks are also used to permit rapid random access to a data band, regardless of whether any data has been recorded in the fine track area. (Note, a data band is that region of the disk surface between servo tracks.) This provides the ability to skip to randomly selected data bands for reading or writing. Seeking to a selected band may be accomplished by counting coarse tracks, in conjunction with analog or digital servo techniques commonly used in magnetic disk drives.

FIG. 3 is a side view that schematically shows the relationship between the optical disk 11 and a moving optics package 40 that is driven by the carriage actuator 24 into a read/write relationship with any of the tracks on the disk 11. The carriage actuator 24 may be realized with a linear motor, such as a voice coil motor, that includes a stationary magnet 41 and a moveable coil 49. The optical path for either the read or write light beam(s) to the surface of the disk 11 includes an objective lens 50, mirror 42, telescope lens 43, and mirror 44. Light is transmitted to and from the moving optics package 40 through a suitable optics package 47 mounted to a

OMPI fixed optic plate 48 on which the remainder of the optics are mounted. The details associated with this optics package are not pertinent to the present invention. Any suitable technique could be used within the optics package so long as the radiation reflected from that segment of the coarse track illuminated in the area 11a (FIG. 2) is directed to the coarse detector 31.

Referring next to only FIG. 2, it is seen that the coarse detector 31 comprises a detector 61 having a radiant energy collection surface 62 upon which radiation reflected from the disk surface area 11a is projected. This radiation has a energy centroid or "center-of-mass" 63 associated therewith, which energy centroid represents that point at which a single ray of radiation, having an intensity equivalent to all the radiation falling upon the surface 62, would fall on the surface 62. The detector 61, as explained more fully below, generates two separate output signals that are directed to signal processing circuitry 64 over signal lines 65 and 66. The output from the signal processing circuitry 64, the PES signal, is directed to the coarse access/tracking servo system 34 over signal line 67.

FIG. 4 conceptually depicts the levels of radiation that would be reflected from the surface of the disk along a radial axis thereof as a function of radial position. At a first coarse servo track N, a large amount of radiation is reflected (assuming that the writing of the coarse servo track creates a high-reflectivity condition). In the data band between this first coarse servo track N and an adjacent second coarse servo track N+l, varying amounts of radiation are reflected depending upon the presence or absence of data tracks and the type of data therein. This radiation is typically much lower in intensity than the radiation associated with

_OV.PI the coarse servo tracks because the width of the data tracks is 3-5 times smaller than the width of the coarse servo tracks. Nonetheless, the radiation reflected from the data tracks within the data band can adversely impact the location of the energy centroid of all the radiation reflected from the illuminated area 11a (FIGS. 2 & 6}. Hence, in order to assure consistency in locating the energy centroid regardless of whether data tracks are present or not within the data band, some means must be employed to identify only that radiation reflected from the coarse servo track. A known reflectivity pattern is placed in the coarse servo tracks for this purpose so that the reflected radiant energy therefrom can be d stinguished from reflected radiant energy from the data tracks which may or may not be present.

In the preferred embodiment, the reflectivity pattern selected for the coarse servo tracks is a repetitive όn/off scheme such that the coarse servo track appears as a dashed line. This concept is best illustrated by the coarse servo tracks 17 and 18 in FIG. 6. A small segment of the coarse servo track 18a is written, causing a high reflectivity condition to exist. This high reflectivity segment 18a is followed by a segment 18b where no coarse servo track is written, causing a low reflectivity condition to exist. (In the preferred embodiment, the optical disk 11 exhibits low reflectivity if not written upon, and high reflectivity if written upon. This situation could, of course, be reversed without alterating the basic operating principles of the present invention.) As the disk 11 is rotated at -a constant angular velocity, the coarse servo track illuminated in the coarse illuminated area 11a will alternately reflect high and low amounts of radiation. By making the high reflectivity segments of the coarse servo track equal in length, and by making the low reflectivity segments also equal in length, the reflected radiation from the coarse servo track assumes a periodic pulsed pattern having a known frequency. By selecting the fundamental frequency of the reflected radiation from the coarse servo track to be different from the primary frequency components associated with data that is recorded on the data tracks, this coarse track frequency can then be used as the mechanism for distinguishing the radiation reflected from the coarse servo track from that reflected from the data tracks.

The wavefor A in FIG. 7 conceptually illustrates how the reflected radiation from the illuminated area 11a of the disk's surface appears as a function of time, i.e., as different portions of the disk 11 are rotated into and out of the illuminated area 11a at a constant velocity. The waveform B in FIG. 7 depicts how the waveform A could be "cleaned up" using filtering or equivalent techniques in order to pick out just those components of the waveform A that are attributable to the fixed-frequency radiation reflected from the coarse servo track.

Referring next to FIG. 5 there is shown a block diagram of the coarse detector 31 of the present invention. As explained previously, the detector 61 includes a collection surface 62 upon which reflected radiation from the coarse illuminated area 11a (FIGS 2 & 7) is projected. This collection surface 62 is schematically depicted in FIG. 5 as a current generator because, as explained below, it generates two currents, each having an amplitude proportional to the intensity and location that the radiation falls on the collection surface respective to known reference points thereon. (The collection surface is typically a rectangle having known dimensions associated therewith. In the preferred embodiment,

O PI the reference location points are the respective ends of the collection surface.) As explained, radiation of varying levels actually falls upon much of the collection surface due to the radiation reflected from the data tracks. However, all of this radiation is equivalent to a single ray of radiation 63 falling upon the collection surface at the "energy centroid" location.

As explained, a first signal generated by the detector 61 is a current signal having an amplitude proportional to the intensity of the radiant energy falling upon the collection surface 62 and the distance between a first end of the collection surface 62 and the location where the centroid of radiant energy 63 strikes the collection surface 62. A second output signal from the detector 61 is likewise a current signal having an amplitude proportional to the intensity of the radiant energy incident to the collection surface 62 and the distance between a second end of the collection surface 62 and the point where the centroid of radi-ant energy 63 falls upon the surface 62.

The processing circuitry 64 includes transimpedance amplifiers 70, 71 that respectively convert the current signals from the detector 61 to voltage signals. The outputs of the transimpedance amplifiers 70, 71 are then directed to respective band pass filters 72, 73, which band pass filters are designed to have a center frequency equal to the fixed-frequency of the radiation reflected from the coarse servo tracks. Thus, while the input signals to the band pass filters 72, 73 may be a composite of all the radiation striking the collection surface 62, such as illustrated in waveform A of FIG. 7, the output signals from these band pass filters are limited to only that radiation reflected from the coarse servo tracks, such as_. illustrated in waveform B of FIG. 7. A rectifier and envelope detector circuit 74, 75 is then employed to generate a signal proportional to the amplitude of the signal outputted from the respective band pass filter circuits 72, 73. The output signal from the rectifier and envelope detector circuit 74 is then subtracted from the output signal from the rectifier and envelope detector circuit 75 in a difference amplifier 76. Similarly, the output signal from the rectifier and envelope detector circuit 74 is summed with the output signal from the rectifier and envelope detector circuit 75 in a summing circuit 77. The outputs of the difference amplifier 76 and summing amplifier 77 are then coupled to a divider circuit 78 in such a manner so as to cause the output of the difference amplifier 76 to be divided by the output of the summing amplifier 77. The output signal from the divider circuit 78 is the desired position error signal, or PES.

An analysis of the configuration shown in FIG. 5 reveals that the position error signal will have an amplitude proportional to the distance from one of the ends of the collection surface 62 that the centroid of the radiant energy associated with the coarse servo track falls upon said surface, but substantially independent of the intensity of the radiant energy falling upon said surface 62. (The divider circuit normalizes any energy variations.) Hence, the desired characteristics (proportional to distance but not to intensity) have been realized.

It should be noted that the processing circuit 64 of FIG. 2 could be realized using alternate configurations from that shown in FIG. 5. One such alternate configuration is discussed below in connection with FIG. 8. Another alternate configuration would involve a system that maintains the average intensity of the reflected laser energy as sensed at the detector 61 at a substantially constant level. Such a system would typically include feedback from the processing circuitry 64 to the coarse illuminator 30 (shown as a dotted line 3T in FIG. 2) in order to control the intensity of the incident laser beam. A suitable tolerancing system for controlling the laser beam intensity and disk reflectivity could also be used.

Referring next to FIG. 8, there is shown a block diagram of an alternative configuration from that shown in FIG. 5. In FIG. 8, the order or sequence of processing the signals from the detector 61 has been altered from the processing sequence associated with FIG. 5. In FIG. 8, the use of the detector 61 and transimpedance amplifiers 70, 71 remains unchanged from FIG. 5. However, in FIG. 8 buffer amplifiers 80, 81 are interposed between the transimpedance amplifiers 70, 71 and a sum amplifier 83 and difference amplifier 82. Bandpass filters 84, 85, followed by demodulation circuits 86, 87, and lowpass filter circuits 88, 89, are .then employed to process the outputs from the sum and difference amplifiers 82, 83, respectively, prior to presenting these processed signals to the divider circuit 78.

FIG. 9 is a schematic diagram of the configuration shown in the block diagram of FIG. 8, not including the detector 61 and transimpedance amplifiers 70, 71. The details associated with the bandpass filter 85, demodulator 87, and low pass filter 89 are not shown in FIG. 9 because they are either identical to or easily derived from the circuits of the bandpass filter 84, demodulator 86, and low pass filter 88, respectively. Representative circuit components for the schematic diagram of FIG. 9 are as indicated in Table 1. The components specified in Table 1 assume a detector 61 is used as described below. Table 1

Representative Component values for FIG.

Rl-IOK Cl-8pf CR1-IN4448

R2-30K C2-3pf U1,U2,U3,U4-LF356A

R3-3.3K C3-18pf U5-NE592

R4-60K C4-120pf U6-LF357A

R5-10K C5-0.1uf U7,U8,U9-LF353A

R6-320 C6-500pf U10-AD535

R7-100 C7-820pf

R8-1K C8-0.01uf

R9-35K C9-5pf

R10-1.1K Cl0-6800pf .

- Ll-6800uh Cll-1800pf

The detector 61, including the collection surface 62, may be realized using a commercially available component manufactured by United Detector Technology, Inc., of Santa Monica, California, A United Technology "LSC" position sensing detector is particularly well suited for this use. Specifically, a United Detector Technology part number PIN-LSC/5D has been successfully used by applicants for this function. This device has an active area (collection surface 62) of 0.115 square centimeters. The length of the collection surface is roughly 0.21 inches (0.53 cm.).

Any suitable transimpedance amplifier, available from numerous IC manufacturers, could be employed for the amplifiers 70 and 71. In particular, an operational amplifier HA-5170 manufactured by Harris Semiconductor could be used for this purpose. (As those skilled in the art will recognize, any operational amplifier can be configured to function as a transimpedance amplifier.) Similarly, as described above in conjunction with FIG. 9, the difference and

O Pl ξ NAT\ ^ summing amplifiers 76 and 77 (or 82 and 83) may be realized using commercially available integrated circuit operational amplifiers, such as the LF353 manufactured by National Semiconductor. The divider circuit 78 may be realized with an AD535 Divider, manufactured by Analog Devices.

While a particular embodiment of the invention has been shown and described, various modifications could be made thereto that are within the true spirit and scope of the invention. The appended claims are, therefore, intended to cover all such modifications.

OMPI o*

Claims

CLAIMSWhat is claimed is:

1. A linear detector for generating a position error signal for use in a head positioning servo system of an optical disk storage system having coarse servo tracks pre-written on a rotating disk, said coarse servo tracks bring adapted to modulate light energy reflected therefrom according to a prescribed pattern, said linear detector comprising: signal generating means for generating two signals in response to an incident light beam falling on a collection surface of said generating means, a first of said signals having a signal amplitude proportional to the distance of said light beam from a first end of said collection surface, and the second of said signals having a signal amplitude proportional to the distance that said light beam is from a second end of said collection surface; summation means for adding said first and second signals and producing a sum signal therefrom having an amplitude equal to the sum of the amplitudes of said first and second signals; difference means for subtracting said first and second signals and producing a difference signal therefrom having an amplitude equal to the difference between the amplitudes of said first and second signals; s-i-gna-1—processing means for demodulating said sum and difference signals; and dividing means for dividing said demodulated difference signal by said demodulated sum signal and producing an

OMPI ^ξ NAT\ output signal therefrom, said output signal having a signal amplitude that is proportional to the linear position of said light beam reflected from said coarse servo track on said collector surface as measured relative to one of said ends thereof.

2. The linear detector as defined in claim 1 wherein the amplitude of said output signal is substantially independent of variations in the intensity of the light beam that falls on said collection surface.

3. The linear detector as defined in claim 2 further including respective buffer means for buffering and conditioning said first and second signals prior to processing said signals through said summation and difference means.

4. The linear detector - as defined in claim 3 wherein said first and second signals generated by said generating means comprise current signals, and further wherein said buffer means comprise transimpedance amplifiers for respectively converting said current signals to voltage signals.

5. The linear detector as defined in claim 4 wherein said signal generating means comprises an "LSC" position sensing detector commercially available from United Detector Technology, Inc. of Santa Monica, California.

6. The linear detector as described in claim 2 wherein said beam of light is a laser beam.

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7. A method for generating a linear position error signal for use in an optical disk storage system that indicates the linear position of a centroid of radiant energy incident to a collection surface of a linear detector, said strip of radiant energy corresponding to reflected energy from a segment of a coarse data track written of a rotating disk used within said storage system, said position being measured relative to a known reference point on said collection surface, said method comprising the steps of:

(a) modulating radiant energy reflected from said coarse servo track according to a known pattern;

(b) generating a first reference signal having an amplitude proportional to the intensity of radiant energy and linearly proportional to the location that the centroid of said radiant energy falls upon said collection surface as measured relative to a first reference point thereon; -

(c) generating a second reference signal having an amplitude proportional to the intensity of said radiant energy and linearly proportional to the location that the centroid of said radiant energy falls upon said collection surface as measured relative to a second reference point thereon;

(d) processing said first and second reference signals so as to effectively eliminate therefrom all signal components attributable to sources other than the modulated radiant energy from said coarse servo track;

(e) summing the amplitude of said first and second reference signals to produce a sum signal;

(f) subtracting the amplitude of said first and second reference signals to produce a difference signal; and (g) dividing said difference signal by said sum signal to produce said linear position error signal, said position error signal having an amplitude linearly proportional to the distance that said beam of light falls upon said collection surface as measured relative to one of said first or second reference points, and said position signal amplitude being substantially independent of the intensity of said beam of light.

8. The method as described in claim 7 wherein the radiant energy incident to said collection surface comprises a laser beam that has been reflected from said coarse servo track of said rotating disk of said optical disk storage system, and wherein said linear position error signal is used by a servo system within said optical disk storage system to control! bly position a read/write head with respect to one of a plurality of concentric coarse servo tracks located on said disk.

9. The method of claim 8 wherein said first and second reference signals are processed as described in step (d) prior to generating the sum and difference of said signals as described in steps (e) and (f) .

10. The method of claim 8 wherein said first and second reference signals are processed as described in step (d) after generating the sum and difference of said signals as described in steps (e) and (f) .

11. Apparatus for producing a linear position error signal for use in a servo control system of an optical disk storage system, said linear position error signal being used to controllably position a read/write head of said optical disk storage system with respect to one of a plurality of concentric coarse servo tracks located on a rotating disk, said apparatus comprising: first means for generating a laser beam; second means for directing said laser beam through said read/write head to said rotating disk, said laser beam falling upon a surface of said rotating disk with a spot size sufficiently large to illuminate at least a segment of one of the concentric coarse servo tracks located on said disk, each of said coarse servo tracks on said disk being configured so as to modulate that portion of said laser beam reflected from said coarse servo track with a fixed frequency as said disk rotates at a constant velocity; third means for directing those portions of said laser beam reflected from said disk through said read/write head to a collection surface of a stationarily mounted linear detector, said linear detector comprising: first signal generating means for generating a first reference signal having an amplitude proportional to the intensity of the laser beam energy incident to said collection surface, and linearly proportional to the location at which an energy centroid of said reflected laser beam energy strikes said collection surface as measured with respect to a first reference point on said collection surface, and second signal generating means for generating a second reference signal having an amplitude proportional to the intensity of the laser beam energy incident to said collection surface, and linearly proportional to the location at which the energy centroid of said reflected laser beam energy strikes said collection surface as measured with respect to a second reference point on said collection surface; fourth means for summing the amplitude of said first and second reference signals to produce a sum signal; fifth means for subtracting the amplitude of said first and second reference signals to produce a difference signal; demodulation means for limiting said sum and difference signals to only that portion of said laser beam reflected from said coarse servo tracks; and sixth means for dividing said difference signal by said sum signal to produce said linear position error signal, said linear position error signal having an amplitude that is linearly proportional to the distance that said laser beam energy falls upon said collection surface as measured relative to one of said first or second collection surface reference points, and said linear position error signal amplitude being substantially independent of the intensity of said laser beam energy at said collection surface.

12. A system for generating a position error signal for use in a head positioning servo system of an optical disk storage system having a plurality of data tracks interposed between coarse servo tracks on a rotating disk, said system comprising means for placing a first reflectivity pattern into each of said coarse servo tracks such that when a given coarse servo track is optically read by measuring the radiant energy reflected therefrom, said reflected radiant energy

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O PI ? \θ periodically assumes maximum and minimum values having a known frequency associated therewith when said disk is rotated at a known rotational speed; means for selectively controlling the reflectivity patterns associated with each of said data tracks in accordance with a desired data pattern such that when a given data track is optically read by measuring the radiant energy reflected therefrom, said reflected radiant energy assumes maximum and minimum values in accordance with said data pattern; means for simultaneously sensing the radiant energy reflected from an illuminated area of said disk, said area including a segment of at least one of said coarse servo tracks and segments of a plurality of said data tracks, whereby said radiant energy reflected from said area comprises a composite of the radiant energy reflected from segments of at least one of said coarse servo tracks and a plurality of said data tracks; radiation detection means for detecting the radiant energy reflected from said illuminated area and for generating two signals in response thereto, a first of said signals having a signal amplitude proportional to the distance that a centroid of said reflected radiant energy falls on a collection surface of said radiation detection means relative to a first reference point on said collection surface, and the second of said signals having a signal amplitude proportional to the distance that said centroid of said reflected radiant energy falls on said collection surface relative to a second reference point on said collection surface; first signal processing means for respectively processing said two signals and effectively eliminating therefrom all components except those attributable to the reflected radiant energy reflected from said coarse servo track at said known frequency; second signal processing means for processing said two signals so as to generate therefrom said position error signal, said position error signal having a signal amplitude that is proportional to the distance that the reflected energy from said coarse servo track falls on said collection surface relative to a known reference point on said collection surface.

13. The system as defined in claim 12 wherein said first signal processing means comprises: filtering means for effectively filtering out all frequencies associated with each of said two signals except the known frequency associated with the radiation reflected from said coarse servo track; and amplitude detection means for respectively detecting the amplitude of each of said filtered signals.

14. The system as defined in claim 13 wherein said second signal processing means comprises: summation means for adding said signals and producing a sum signal therefrom having an amplitude equal to the sum of the amplitudes of said signals; difference means for subtracting said signals and producing a difference signal therefrom having an amplitude equal to the difference between the amplitudes of said signals; and dividing means for dividing said difference signal by said sum signal and producing said position error signal therefrom.

15. The system as defined in claim 14 wherein said radiation detection means comprises an "LSC" position sensing detector commercially available from United Detector Technology, Inc., of Santa Monica California.

16. Apparatus for producing a linear position error signal for use in a servo control system of an optical disk storage system, said linear position error signal being used to controllably position a read/write head of said optical disk storage system with respect to one of a plurality of concentric coarse servo tracks located on a rotating disk, said apparatus comprising: first means for generating a laser beam; second means for directing said laser beam through said read/write head to said rotating disk, said laser beam falling upon a surface of said rotating disk with a spot size sufficiently large to illuminate at least a segment of one of the concentric coarse servo tracks located on said disk, each of said coarse servo tracks on said disk being configured so as to modulate that portion of said laser beam reflected from said coarse servo track with a fixed frequency as said disk rotates at a constant velocity; third means for directing those portions of said laser beam reflected from said disk through said read/write head to a collection surface of a stationarily mounted linear detector; said linear detector comprising signal generating means for generating at least one reference signal having an amplitude proportional to the intensity of the laser beam energy incident to said collection surface, and lineraly proportional to the location at which an energy centroid of said reflected laser beam energy strikes said collection surface as measured with respect to a reference point on said collection surface; fourth means for limiting said reference signal to only that portion of said laser beam reflected from said coarse servo tracks; and fifth means for- maintaining the intensity of said reflected laser beam as measured at said collection surface at a substantially constant level, whereby said reference signal assumes an amplitude that is substantially solely a function of the location at which said energy centroid strikes said collection surface.

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