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
  • G11B7/085—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam into, or out of, its operative position or across tracks, otherwise than during the transducing operation, e.g. for adjustment or preliminary positioning or track change or selection
  • G11B7/0857—Arrangements for mechanically moving the whole head
  • G11B7/08582—Sled-type positioners
  • G11B7/08588—Sled-type positioners with position sensing by means of an auxiliary system using an external scale

Definitions

• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the intensity of the radiation incident to the detector must be held more or less constant. Unfortunately, this is an extremely daunting 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.
• 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.
• 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 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.
• radiant energy typically laser energy
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the read/write head 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.
• a tracking mode 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.
• the position error signal 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
• 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.
• 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
• FIG. 8 is a block diagram of an alternative configuration of the coarse track detection system of the present invention.
• FIG. 9 is a schematic diagram of the configuration shown in FIG. 8.
• 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.
• the optical disk storage 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 projects read or write light beam(s) 52' to the surface of the disk 11 so as to access data tracks thereon.
• 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.
• 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.
• 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.
• PES coarse track position error signal
• 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.
• 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
• the servo tracks 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.
• 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.
• 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.
• 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).
• 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
• 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.
• the reflectivity pattern selected for the coarse servo tracks is a repetitive ⁇ n/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.
• the coarse servo track illuminated in the coarse illuminated area 11a will alternately reflect high and low amounts of radiation.
• the reflected radiation from the coarse servo track assumes a periodic pulsed pattern having a known frequency.
• 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.
• 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.
• the reference location points are the respective ends of the collection surface.
• 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.
• 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.
• 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.
• 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.
• FIG. 8 there is shown a block diagram of an alternative configuration from that shown in FIG. 5.
• the order or sequence of processing the signals from the detector 61 has been altered from the processing sequence associated with FIG. 5.
• the use of the detector 61 and transimpedance amplifiers 70, 71 remains unchanged from FIG. 5.
• 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
• 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.
• an operational amplifier HA-5170 manufactured by Harris Semiconductor could be used for this purpose.
• any operational amplifier can be configured to function as a transimpedance amplifier.
• O Pl ⁇ NAT ⁇ ⁇ summing amplifiers 76 and 77 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.

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• Moving Of The Head To Find And Align With The Track (AREA)
• Optical Recording Or Reproduction (AREA)

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• 1984
  • 1984-05-24 EP EP19840902390 patent/EP0145755A1/de not_active Withdrawn
  • 1984-05-24 WO PCT/US1984/000808 patent/WO1984004965A1/en unknown
  • 1984-06-12 CA CA000456409A patent/CA1218148A/en not_active Expired

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