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

Abstract: A linear detector (61) for use in a coarse positioning servo system of an optical system. The linear detector produces an error signal having an amplitude linearly proportional to the distance that a relatively narrow strip of light energy (63) falls on a collection surface (62) of the detector as measured relative to a fixed reference point on said collection surface. Two reference signals (65, 66) are derived from circuitry associated with the collection surface. A frist reference signal (65) has an amplitude proportional to the intensity of the focussed light energy and the location that said light energy falls on the collection surface relative to a first reference point. A second reference signal (66) has an amplitude proportional to the intensity of the focussed light energy and the location that said light energy falls on the collection 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 different signals, respectively. The difference signal is divided by the sum signal to produce the desired error signal (67), which error signal has an amplitude that is substantially independent of the intensity of the focused light energy.

Description: 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. Prior art approaches relating to access and tracking systems are discussed in Patent Cooperation Treaty (PCT) International Application No. WO/84/01849, published 10 May 1984 by the World Intellectual Property Organization (WIPO). While a discussion of such prior-art approaches provides interesting background information, applicant does 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 OMPI 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. In the above-cited application, a detector array is disclosed for this purpose. According to the teachings therein, a narrow strip of radiant energy incident to the 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 position servo system to place the read/write head at a desired location so as to provide the requisite access and tracking capability. While the detector array disclosed in the above-cited application adequately performs its intended function, and represents the best mode of carrying out the invention disclosed therein at the time the invention was made, such a detector array 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 continuities 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. As a still further disadvantage, the amplitude of the error signal generated in arrays of the type disclosed in the above-cited application 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. Also, detector arrays of the type disclosed in the above-described application must be realized from somewhat complex circuits, employing a large number of discrete components. Such complex circuits are expensive (in terms of both time and money) to build and maintain. OMPI What is needed, therefore, is a simple, less-expensive detection system that provides a continuous linear output signal that indicates the position of a narrow strip of radiant energy incident thereto, and t at 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 linearly proportional to the location of a strip 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 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 optics 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. 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 by said detector system. A still further object of the present invention is to provide OMPI such a linear detector system that is simple and inexpensive to build, yet that provides repeatable, reliable performance. 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. Course 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 insure that at least a sector of one coarse servo track is always illuminated. Reflected radiant energy from the surface of the disk therefore includes the location of the coarse track sector within the illuminated area. This reflected energy is directed back through the read/write head to the linear detection system of the present invention. The linear detection system generates an error signal having an OMPI amplitude that is linearly proportional to the distance at which the narrow strip of radiant energy (reflected from the coarse track on the surface of the disk) falls on a collection surface of a detector used within said system as measured relative to a fixed reference point on said collection surface. The amplitude of the error signal is substantially independent of the intensity of the radiant energy. Two reference signals are derived from circuitry associated with the collection surface. 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. 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 said disk. In a seek or access mode, the read/write head will be moved radially with respect to said 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 is a block diagram of the coarse track detection system of the present invention; and FIG. 5 shows the waveform of the output signal from the detection- system shown in FIG. 4 during radial movement of the read/write head across the disk. 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. OMPI tø . WIPO .Λ*J 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. 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 a read or write Tight beam(s) 52' to the surface of the disk 11 so as to access data tracks thereon. In order to access the OMPI 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 lla 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 lla of the disk surface. It is noted that the illuminated portion lla of the disk surface spans at least the distance between two servo tracks, and thereby always illuminates at least one servo track. As shown in FIG. 2, light is reflected from the portion lla 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 it will be described more fully below. The output of the coarse detector and processing circuitry 31 is a coarse track position error signal, which signal has an amplitude proportional to the location at which the reflected strip of light OMPI from the illuminated 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 through the data read system 25c. 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 galvenometer controlled mirror(s), is controlled by the fine access/tracking servo system 25d. In order to allow the servo tracks to be easily discriminated from the data tracks, the servo tracks are preferrably 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 (represented schematically in FIG. 2 by the carriage optics 23). 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 Tl. 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 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 a strip of light, or narrow strip of radiant energy, representing that segment of the coarse track illuminated in the area lla (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 the strip of light 63, reflected from the appropriate coarse servo track, is projected. 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 position error signal, the output from the signal processing circuitry 64, is directed to the coarse access/tracking servo system 34 over signal line 67. _ OMPI W , WIPO v4TIθ -IE- Referring next to FIG. 4, there is shown a more detailed block diagram of the coarse detector- 31 of the present invention. As explained in the preceding paragraph, the detector 61 includes a collection surface 62 upon which a strip of radiant energy 63 from a coarse track is projected. The collection surface 62 has a known length L associated therewith. (This collection surface also has a width associated therewith, but the width is not an important consideration for purposes of the present invention.) The strip of radiant energy 63 reflected from the coarse track has a width w associated therewith. This width w is, of course, related to the actual width of the coarse servo tracks 16-19 (FIG. 2) that are pre-written on the disk 11. In practice, the width w is small in comparison to the length L. 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 d between a first end of the collection surface 62 and the location where the strip 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 L - d between a second end of the collection surface 62 and the point where the strip of radiant 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 voltage output signal from the transimpedance amplifier 70 is then subtracted from the output voltage signal from the transimpedance amplifier 71 in a difference amplifier 72. Similarly, the voltage output signal from the transimpedance amplifier 70 is summed with the voltage output signal from the transimpedance amplifier 71 in a summing circuit 73. The outputs of the difference amplifier 72 and summing amplifier 73 are then coupled to a divider circuit 74 in such a manner so as to cause the output of the difference amplifier 72 to be divided by the output of the summing amplifier 73. The output signal from the divider circuit 74 is the desired position error signal. An analysis of the configuration shown in FIG. 4 reveals that the position error signal will have an amplitude proportional to the distance d (or the distance L - d), but independent of the intensity of the radiant energy falling upon the collection surface 62. Hence, the desired characteristics (proportional to distance but not to intensity) have been realized. Referring to FIG. 5, there is shown a waveform representing the shape of the position error signal as the read/write head is radially moved past several coarse tracks. At one extreme, as the strip of radiant energy corresponding to the coarse servo track first falls upon one end of the collection surface 62, the error signal assumes a large value, such as -A (FIG. 5). For example, in the preferred embodiment, a large negative amplitude corresponds to the strip of light 63 falling upon the bottom of the collection surface 62 as shown in FIG. 4. As the read/write head moves, thereby causing the strip of radiant energy to move towards the top of the collection surface 62, the amplitude of the position error signal linearly increases from the negative amplitude -A to the positive amplitude A. When the strip of light 63 is falling upon a center point of the collection surface 62 (which center point is indicated by a dashed line C in FIG. 4), then the amplitude of the position error signal will be zero. For the actual situation shown in FIG. 4, the strip of radiant energy 63 is falling on the collection surface 62 a distance d from the top end thereof. This distance d is roughly one half of the distance to the center line C. Therefore, the position error signal would assume a value of approximately A/2. Hence, the position error signal provides a continuous signal whose amplitude is proportional to the location of the strip of radiant energy 63 on the surface 62 of the detector 61. 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 applicant for this function. This device has an active area (collection surface 62) of 0.115 square centimeters. The dimension L shown in FIG. 4 is roughly 0.21 inches (0.53 cm.), while the dimension w shown in FIG. 4, the width of the strip of radiant energy, is typically 0.085 inches (0.033 cm.) in the preferred embodiment. Any suitable transimpedance amplifier, available from numerous IC manufacturers, could be employed for- he 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, the difference and summing amplifiers 72 and 73 may be realized using commercially available integrated circuit operational amplifiers, such as the LF353 manufactured by National Semiconductor. The divider circuit 74 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.

Claims: ' CLAIMS What 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 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; and dividing means for dividing said difference signal by said sum signal and producing an output signal therefrom, said output signal having a signal amplitude that is proportional to the linear position of said light beam 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 from 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, said laser beam being reflected from coarse servo tracks on a rotating disk and being directed to said linear detector so that the reflected image of said servo track falls upon said collection surface of said signal generating means, the output signal from said linear detector being used as an error signal within a servo system of said optical storage system that controllably positions a read/write head with respect to said disk. 7. A method for generating a linear position error signal for QMPI__ WIPO •N>j use in an optical disk storage system that indicates the linear position of a narrow strip 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 band 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) generating a first reference signal having an amplitude proportional to the intensity of said strip of radiant energy and linearly proportional to the location that said incident strip of radiant energy falls upon said collection surface as measured relative to a first reference point thereon; (b) generating a second reference signal having an amplitude proportional to the intensity of said strip of radiant energy and linearly proportional to the location that said strip of radiant energy falls upon said collection surface as measured relative to a second reference point thereon; (c) summing the amplitude of said first and second reference signals to produce a sum signal; (d) subtracting the amplitude of said first and second reference signals to produce a difference signal; and (e) 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 said strip of 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 controllably position a read/write head with respect to one of a plurality of concentric coarse servo tracks located on said disk. 9. 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 change the reflectivity characteristics of said disk wherever said coarse servo tracks are placed; 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 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 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; 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.

Inventors: ABED MAJEED K, ABED, MAJEED, K.
Applicants: STORAGE TECHNOLOGY PARTNERS, STORAGE TECHNOLOGY PARTNERS II
Application year: 1984
Main classification: G11B21
IPC codes: G11B7