CA1200604A

CA1200604A - Optical memory system having an improved long seek capability

Info

Publication number: CA1200604A
Application number: CA000421796A
Authority: CA
Inventors: Der-Chang Hsieh; Edward V. Labudde
Original assignee: Burroughs Corp
Current assignee: Unisys Corp
Priority date: 1982-02-17
Filing date: 1983-02-17
Publication date: 1986-02-11
Also published as: EP0088537A1; DE3375955D1; EP0088537B1; JPS59500245A; WO1983003026A1; US4432082A

Abstract

ABSTRACT OF THE DISCLOSURE
OPTICAL MEMORY SYSTEM HAVING AN IMPROVED LONG SEEK CAPABILITY

A high density optical storage system employing a laser beam for reading data in a track on a rotating optical disk containing a large number of closely spaced concentric tracks. When a long seek operation is to be provided, track following is suspended and a linear motor drives a galvanometer-controlled mirror to a new position corresponding to a new track called for by the long seek operation. Arrival at the new position is determined by detecting when both the velocity of the linear motor and the difference between current and desired positions are below predetermined maximum values indicating that a stable condition has been reached, after which track following operation is resumed. The linear motor also operates during track following in response to the detected angular position of the mirror so as to cause movement of the mirror in a direction which reduces the mirror deflection required to maintain the beam accurately following the track.

Description

o~

TITLE
-OPTICAL MEMORY SYSTEM HAVING AN IMPROVED LONG
SEEK CAPABILITY
CROSS K~ CE TO RELATED P~TEN~ APPLICATIONS
This patent application contains subject matter related to our concurrently filed commonly assigned patent application Serial No. 421,779 for Optical Memory System Having an Improved Short Seek Capa~ility; Serial No. 421,795 for Optical Memory System Having Improved Track Following and Seeking Capabilities; Serial No. 421,788 for Optical Memory System ~aving Improved Track Following; and also to the commonly assigned UOS. patents No. 4,402,061 (8/30/83), No. 4,494,226 (1/15/85); No. 4,417,330 (11/22/83); and No.
4,443,870 (4/17/84). / ' /
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BACKGROUND OF THE INVENTION
The present invention rela~es generally to improve~
metho~s and apparatus for recording and/or rea~ing data in a high density data storage system, and more particularly to improved methods and apparatus for providing reliable and accurate positioning control during the record ~g and reading of data in a high density optical ~orage system.
In recent years considerable effort has been expended to develop improved methods and apparatus for optically recording and reading on a suitable medium because of the unusually high recording density potential offered by optical recording. Examples of various known methods and approaches are revealed in the following references:
U~S. PATENT DOCUMENTS
Patent No~Date Issued Inventor(s) 4,216,5018~ 5/8n Bell 4,222,0719/ 9/80 ~ell, et al.
4,23?,33712/ 4/80 Winslow, et al~
4,243,8~81/ 6/81 Utsumi 4,243,8501/ 6/81 Edwards 4,253,01~2/24/81 ~pheij 4,253,7343/ 3/81 Komurasaki 4,2fi8,7455/1~/81 Okano PUEILICATIONS
R.A. Bartolini, et al., "Optical Disk Systems Emerge", IEEE
Spectrum, Au~ust 1978, ppO 20-28.
G.C. Kenney, et al., ~An Optical Disk Replaces 25 Mag Tapes", IEEE Spectrum, February lg79, pp. 33-38.
K. B~lthuis, et al., "Ten Billion ~its on a niskn, IEEE
Spectrum, August 1979, pp. 26-33.
A.E. Bell, et al., "Antireflection Structures for Optical Recording", IEEE Journal of Quantum Electronics, ~Jol. ~E-14, No. 7, July 1978, pp. 487-495~

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SUMMAXY OF TH~ PPcESENT lN\~;Nl-lON
As is well recognizedt it is of crucial importance in an optical memory system that rel iabl e and accurate positioning control be provided during record in~ and reproduc:ing operations. This requirement is a par~icularly severe one in an optical memory sys~cem because of the very high data recording density typically employed~ as compar~d, for example, to magnetic memory disk systems which store data at a relatively much lower density~ I~c is accordingly the broad object of the present invention to provide significantly impro~ted positioning s~ontrol during the recording and/or reading of data in a high density storage system, such as an op~ical da'ca storage system~ The present in~rention is most particu~arly concerned with the manner in which a long seek function is providetl in such a sys tem .
In a partic~lar pr:eferred embodiment of the presen t invention, in which laser beams are used for recording and r~producin~ data on a rotatable optical disk, significantly improved long seek operation is achieved by employing a plurality of interactive servo loops which accurately and rel iably control the positioning of a galvanometer and a linear motor which together determine the position of the record ing and reproducing laser beams on the optical disX.
The specific nature o the invention as well as other objPcts, advan'cages, features and uses of the invention will become evident from the following description of a preferred embodiment taken in conjunctiorl with the accompanyi ng drawing s .

~S

3~ 04L

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall block diagram of an optical recording and reproducing system in which the present invention may be incorporated.
FIG. 2 illustrates the relative locations of the three laser beams provided by the system of FIG. 1 when focused on a selected track of the optical ~isk.
FI~. 3 is a block and sche~atic representation of the laser optical system shown in FIG. 1.
FI~,. 4 is a schematic diagram generally illustrating ~he arrangement and formatting of data on the optical disk.
FIGo 5 is a schematic ~iagram illustrating details of the header formatting shown in FIG. 4.
FIG. 6 is a block electrical diagram illustrating a preferred implementation of the signal processing electronics of FIG. 1.
FIG. 7 is a cross-sectional view illustrating the construction of an optical disk employed in the system of FIG. 1.
FIG. 8 is a block electrical diagram illustrating a preferred implementation of interactive servo control apparatus employed in the present invention.
FIG. 9 is a block electrical diagram illustrating those portions of FIG. 8 which are pertinent to the performance of trac~ following operations.
~IG. 10 is a block electrical diagram illustrating those portions of FIG. 8 which are pertinent to the performance of a short seek operation.
FIG. 11 is a block electrical diagram illustrating those portions of FIG. 8 which are pertinent to the performance of a long seek operationO
FIGo 12 is a preferred embo~iment of the arrival detector shown in FIG. 11.

~;~spo~

DETAILF:D DESCRIPTION OF THE~ INVENTION
Like numerals and characters designate like elements throughout the fig~res of the drawings.
Reference is initially directed to FIG~ 1, which generally illustrates the ~asic portions of the embodiment of an optical recording and reading system disclosed in the aforementioned commonly assigned copending patent applications~ The data to be recorde~ is first applied to recording circuitry 10 which encodes the applied data using, for example, a conventional encoding format of the type employed for magnetic recording, such as non-return-to-zero, return-~o-zero, etc. Conventional error checking may also be provided for the encoded signal.
The encoded data lOa from the recording circuitry 10 is applied to a laser optical system 12. The laser optical sys~em 12 generates thr~ee laser beams 12a, 12b and 12c which are focused at spaced locatior.s along the center line of the same selected track of a preformatted optical disk 15 supported on a precision spindle 16 for rotation by a mo~or i8. The optical disk 15 may, for example, be a trilayer disk of the type disclosed in the aforementioned U.S
Patent No. 4,222,n71 and in the aforementioned article by Bell, et al. Each laser beam ~ay, for example, be focused to a one micron spot size on the disk 15.
Laser beam 12a is a writing beam which is modulated by the encoded data so as to form optically detectable changes in a selected track of the optical disk 15 representative of the encoded data. It is to be understood that the optically detectable changes produced in the disk by the write laser beam 12a need not be physical changes, such as pits or physical holes. The only requirement is that optically detectable changes be produced in selected areas of the disk in response to the write laser beam 12a which are representative of the encoded data lOa. For the purpose of this description, all of the possible types of optically detectable changes that can be produced will hereinafter be referred to as optical holes.
Laser beams 12b an~ 12c shown in FIG. 1 are reading beams. As typically illustrated in FIG. 2, the reading beam 12b is a read-after-write beam which is accordingly focused behind the writing beam 12a on the center line 17a of a selected track 17 r while the read ing beam 12b is a read before-write beam and is accordingly focused ahead of the writing beam 12a~ The intensity of the read beams 12b and 12c are chosen so that they will not disturb the integrity of previously recorded information~ The read beams are reflected from the d isk 15 back to the optical system 12 which, in response thereto, derives a plurality of detection signals 14a, 14b an3 14c which are applied to signal processing electronics 20. The signal processing electronics 20 also receives the encoded data signal lOa from the recording circuitry 10 for use in checking the accuracy of recorded data, as will hereinafter be considered.
The signal processing electronics 20 uses the detected signals 14a, 14b and 14c to provide an output data signal 20a corresponding to data read from the optical disk 15, along with signals 2t~b and 2nc respectively identifying the track and sector locations on the disk from which the data is read. The signal processing electronics 211 also produces control signals lOb, 21a, 21b, 21c~ 21d, 21e and 21f. More specifically, control siqnal lnb is applie~ to the recording circuitry ln for synchronizing the encoding of data with disk rotation; control signal 21a is applied to the optical d isk motor 18 to prov ide accurate speed control during recording and reading; control signal 21b is applied to the laser optical system 12 or controlling the radial position of the laser beams 12a, 12b and 12c for the purpose of selecting a desired track; control signal 21c is applied to the laser optical system 12 for providing precise track following of the laser beams on the selected track control signal 21d is applied to the laser optical system 12 for providing precise focusing of the laser beams 12a, 12b and 12c; and control signal 21e is applied to the recording circuitry 10 for interrupting recording if the reflected read-before-write beam indicates the possibility of an overwrite recording error because the track ahead contains previously recorded data; and signal 21g is applied to the recording circuitry 10 to interrupt recording if a recording error occurs.
~ Reference is next directed to FIG. 3 which illustrates - a preferred embodiment of the laser optical system 12 generally shown in FIG~ 1. The various components of this laser optical sys~em are illustrated in block and schematic form in FIG. 3 since their implementation can readily be provided by those skilled in the art, as will be evident from the aforementioned references.
As shown in FIGo 3, a laser 30 provides a beam 3na having a wavelength of, or example, 633 nanometers and a power level of, for example, 12 mw. This laser beam 30a is applied to a first beam splitter 32 which splits the beam into a high power beam 32a and a low power beam 32b. The low power beam 32b is applied to a second beam splitter 34 which further splits the beam 32b to provide read-after-write and read-before-write beams 12b and 12c, respectively. It is to be understood that a separate laser co~ld be employed for providing one or more of the above beams if so desired.
The high power beam 32a in FIG. 3 is applied to a high speed light~modulator 36 which modulates the beam 32a in response to the encoded data lOa provided at the output 6C)~

from read beams 12b and 12c pass back to the beam combiner and splitter 38 via the objective lens assembly 4~ and the mirror 40~ The beam combiner an~ splitter 38 directs the reflected beams to op~ical detection circuitry 49 which conver~s the beams into corresponding read-after-write and read-before-write analog electrical signals 14a and 14b which are applied to the signal processing electronics 20 as shown in FIG. 1. Also, at least one of the reflected read beams 12a and 12b is applied to a geometric optical focus detector 47 which provides a relatively low gain, wide capture range signal 14c to the signal processing electronics 20 which is indicative of the quality of focusing of the beams on the selected track.
Nex, to be considered is the preformatting provided for the optical disk 15 in FIG. 1. An example of a typical preformatting arrangement is illustrated in FIGS. 4 and 5.
As generally indicated in FIG. 4, the optical disk 15 in the preferred embodiment being described contains a large plurality of circumferential tracks 17, for example, 40, oon tracks spaced two microns apart on a 14~inch disk.
The disk 15 is also divided into a plurality of sectors 1~.
As indicated in FIG. 4, each track 17 within a sector 19 comprises a header 51 and a data recording portion 520 ~he data recording portion 52 is the portion into which data is written during recording and comprises the greater portion of the track length within each sector 19~ The header 51 of a track 17 is encountered first in each sector 19 and is provided on the disk prior to recording.
A disk provided with such headers 51 i5 typically referred to as being preformatted.
FI~. 5 illustrates an example of a preformatted header 51 provided for each track 17 in each sector 19 of the disk 15 of FIG. 3. Although the optical holes constituting the header 51 need not be physically observable, as mentioned previously, it will be assumed for the purposes of this description that physical holes, such as pits, are employed for the exemplary header shown in FIG. 4. It S will also be assumed that a pit exhibits a relatively high reflectance to an incident beam while other undisturbed disk areas exhibit a relatively low reflectance. It is to be understood that an arrangement may also be employed in which a portion of the optical recording is provided using physical holes, such as pits, and the remaining recorded portions are recorded ~sing optical holes.
Before continuing with ~he description of the header shown in FI~,. 5, reference is directed to FIG. 7 which illustrates a cross-section of a disk 15 which may be employed in the system of FI~,. 1. ~ supporting substrate 90 such as a 0.1 to ~.3 inch thick disk of aluminum is coated with an organic smoothing layer 92 of, for example, 20-60 microns prior to deposition thereon of a hi~hly reflective opaque layer 94 of aluminum which may, for example, have a thickness of 400-800 Angstroms. An inoryanic dielectric layer ~6 such as a 800-12nO Angstrom layer of silicon dioxide which is transparent at the laser freguency is deposited over the aluminum reflector layer 94. An absorbing layer 98 which is absorptive at the laser frequency is then deposited over the dielectric layer 96.
This absorbing layer 98 may for example be a 50 to 300 Angstrom layer of a metal such as tellurium. Finally, the absorbing layer 9~ is overcoated with a protective layer lnO, such as a silicon resin having a thickness of, for example, 150 to 500 microns.
Still with reference to FIG~ 7, an anti-reflection (dark mirror~ condition or a laser beam incident on unrecorded portions of the disk 15 is produced by o~

appropriately choosing the thicknesses and optical characteristics of the layers 94 r 96 and 98~ Recording on such a disk 15 as illustrated in FIG. 7 is then acccmplished by employing an appropriately focused, intensity-modulated recording laser beam (such as laser beam 12a in F~GS. 1 and

2) which records information by forming optically detectahle changes, such as pits 9~a, in the absorbing layer 9~ along a selected track, the spacing and dimensions of these pits 98a being representative of the recorded data. Information is read from the disk 15 using an appropriately focused reading laser beam (such as laser beams 12b and 12c in FIGS. 1 and 2) which is chosen to be of insufficient intensity to affect unrecorded regions 98b of the absorbing layer 98 and has a frequency at which these unrecorded regions 100 exhibit the previously mentioned anti~reflection condition. As a result, the reflected reading beam will be intensity modulate~ by the pits 98a since the reading beam will experience a relatively high reflection when the beam is incident on a pit 9~a, and a relatively low reflection when the reading beam is inciden~ on an unwritten region 98b. It will be understood that dust particles on the upper surface of the protective layer 100 will be far removed from the focal plane of the optical system (that is, they will be out of focus) so as to have a negligible effect on the above described recording and reading operations.
Reference is now directed back to FIG. 5 for a more detailed consideration of the header 51 Since the header 51 is use~ in conjunction with the signal processing electronics 2n in FI~. 1 to provide for reliable and precise operation of the system, it will be helpful to describe the construction and arrangement of the exemplary header 51 shown in FIG. 5 in conjunction with FIG. 6 which illustrates ~2~

an implementation of the signal processing electronics 20 generally shown in FIG. 1. The individual components of FIG. 6 can readily be implemented by those skilled in the art and are thus shown in block form~
Referring to the preformatted heading 51 shown in FIG. 5, it will be seen that immediately following the left sector boundary l9a is a relatively large pit 54 providing a relatively large change in optical reflectance which is used to provide synchronized timing for the signal processing electronics 20. This is acccmplished by applying the detected read-after-write signal 14a in F~G. 3 to a peak detector 73 via a preamplifier 71. The peak detector 73 outputs a narrow pulse 73a corresponding to the pit 54 which it recognizes as the largest peak in the playback signal.
This narrow output pulse 73a produced by the peak detector 73 is then applied as a timing reference to conventional timing circuitry 75 which generates various timing signals l~b, 21a, 75a, 75b, 75c, 75d and 75e for synchronizing the operation of the syste~ with the disk 15. The purposes of 2~ these timing signals will become evident as the description proceed 5 .
Following pit 54 in FI~,. 5 are two pits 56 anc3 5 elongated in a direction parallel to the track 17 and disposed on opposite sides of ~he track center line 17a in a staggered relationship. These pits 5~ and 5~ are used to pro~ide precise track following. This is accomplished in FI~. 6 by applying the amplified read-after-write signal provided at the output of the preamplifier 71 to up~down integrator circuitry 77. mhe up-down integrator circuitry 77 integrates up in response to the detected signal obtained when the read-after-write beam traverses the portion of the track 17 corresponding ~o the elongated pit 56, and integrates down in response to the signal o~tained when the ~z~

read-after write beam traverses the portion of the track 17 corresponding to the elongated pit 5~. It will be understoo~
that ~he difference between these two integrations will be a measure of the preciseness of track following by the laser beams. The dimensions an~ locations of the elongated pits 56 and 58 are chosen in conjunction with the size of the focused beam so that even very small deviations of the beam from the track center line 17a can be detected. This difference provided by integrator circuitry 77 when pits 56 and 58 are traversed during each sector is accordingly used to produce the control signal 21c which is applied to the galvanometer ~2 (FIG~ 3) to provide for precise following of a selected trac~ by the laser beams.
It will be noted in FIG. 6 that the timing circuitry 75 provides timing signals 75a and 75b to the up-down integrator circuitry 77. The timing signal 75a is used to delineate the particular times during traversal of the header 51 of each sector for ~hich up and down integrations should be perforrned so as to appropriately correspond with the locations of the elongated pits 56 and 5~. The timing signal 75b is provided during each sector to the up-down integrator circuitry 77 to serve as a hold signal to hold until the next sector the resultant integrated value obtained after the read-after-write beam has completed traversing the second elongated pit 58.
Following the elongated pits 56 and 58 in the exemplary header 51 shown in FIC7. 5 are a plurality of pits fiO elongated perpendicularly to the track center line 17a.
The locations and dimensions of the pits 60 are chosen so that the reflected signal obtained upon traversing these pits 60 will have a peak value dependent upon the quality of focusing of the incident beam. This may be Ichieved, for ex~nple, by choosing the diarneter of each pit 60 so that it is equal to the diameter of a properly focused beam.
Then, if the incident beam is larger than the thickness of a pit 60 because of improper focusing~ the reflected beam will have reduced power when each pit 60 is traversed, since only a portion of the beam will be reflected. It will also be understood that the spacing between the pits 60 along wi~h the disk rotating speed determine the frequency at which the reflected beam i9 modulated when traversing the pits 60.
Referring again to FIG. 6, it will be understood that the read-after-write beam 14a applied to the preamplifier 71 during the period that the focusing pits 60 are being traversed contains the resulting focusing information.
~ccordingly, a peak detector 64, which is enabled by timing signal 75c during the period that the read-after-write beam is traversing the focusing pits 60, is provided to receive the amplified read-after-write beam at ~he output of the preamplifier 71. The peak detector 64 is adapted to respond to the magnitude of the applied signal within a 2G frequency range determined by the spacing of the pits 6n to produce a relatively high gain output signal 64a which is a measure of the quality of focusing~
The output signal 64a from the peak detector 64 is applied to a signal adder 66 along with the signal 14c provided by the optical focus detector 47 in FI~,. 3O The signal adder 66 appropriately combines these two signals 14c and 64a to produce the resulting signal 21d shown in FI~. 1 which is applied to the focusing motor 46 for maintaining precise focusing of the incident laser beams on the disk.
The purpose of providing a signal 21d to the focusing motor 46 comprised of the signal 14c from the geometric optical focus detector 47 and the signal 64a from the peak detector 64 will now be further considered. It ~3~0~

will be understood that the signal l4c obtained from the geometric optical focus detector 47 in FI~7. 3 provides only a relatively low gain control of the focusing distance which, while providing a wide capture range, typically results in producing a static or low frequency offset error. In accordance with the present invention, as exemplified in FIG. 5, a significantly more precise and of~set free focus signal 21d is achieved by combining the geometric optical detector signal 14c with the peak-detected signal 64a derived from the focusing holes ~0 which is capable of providing a relatively high gain. Of course, as also applies to the track following pits 56 and ~, the header 51 is repeated a sufficient number of times around each circumferential track 17 to obtain ~he desired precise and fast-acting control of focusing as well as of track following.
Continuing with the description of the header 51 sho~ in FIG. 5, the above described focusing pits 60 are followed by pits 72 recorded so as to provide an i~entification of the particular track and sector being traversed by the laser beams. In other ~ords, the pits 72 represent a track and sector address and conventional encoding can be employed for this purpose/ such as is used for identiying tracks and sectors on magnetic disks.
As shown in FIG. 6, the amplified read-after-write signal at the output of the preamplifier 71 is applied along with an appropriate enabling timing signal 75d to a track and sector decoder 7~ which provides track and sector signals 2na and 20b respectively (see also FI~,. 1) indicating the track and sector being traversed by the laser beamsO
The track signal 2nb is also applied to track selection circuitry 8~ along with a track command signal 8na indicative of a selected track to which it is desired thAt the laser ~L2~JO~

beams be positioned. The track selection circuitry 80 compares the track indicated by the track siynal 20b with the track requested by the track command signal 80a and in response thereto produces the signal 21b which is applied to the linear motor 48 in FIG; 3 to center the laser beams over the selected track.
Referring to FIG. 4 along with FIG. 5, it will be understood that, in the exemplary header 51 illustrate~ll the pits 72 which provide tracX and sector address identification are the last portion of the header 51. As pointed out previously, the resulting disk containing these headers is considered to be preformatte~. Such preformatte~ disks will typically be provided ~o users who will make use of the preformatted hea~ers in conjunction with signal processing electronics 20 such as illustrated in FI~.. 6 for recording and reading data in the data recording portion 52 o~ each track 17 in each sector 19.
The amplified read-after-write signal provided at the output of the preamplifier 71 in FIG. 6 is also used for reading data from the data recording portion 51 of each sector 19 (FIGS. 4 and 5)O Accordingly~ the implementation of FIGo 6 includes data read circuitry 82 to which the output of the preamplifier 71 i~ applied for providing a data output signal 20a (see also ~I~,. 1) corresponding to 2S the recorded digital ~ata. rhe data read circ~itry ~2 is enabled during the time period that the read-after-write beam is traversing the data portion 52 (FIG. 4) of each sector 19 by the timing signal 75e~ The resulting data output .signal 2~a rea~ from the ~isk 15 is applie~ to an appropriate utili~ation device (not shown) along with the track and sector signals 2nb and 2~c which identify the track and sector from which data is read.

~Z~ O~

The data output signal ~Oa is also used during data recording to check that data is accurately being recorded.
For this purpose FIG. 6 includes a da~a comparator 83 which receives both the data output signal 2na and ~he encoded data signal lOa from the recording circuitry 10~ The data comparator 83 operates to compare the encoded data lOa with the corresponding data 20a read from the disk 15. ~f the comparator detects an error in the signals lOa and 2na, then the recording error interrupt signal 21f is produced which is applied to the recording circuitry lO to interrupt recording.
An additional feature of the ~resent invention resides in the manner in which ~he read-before-write beam 12c (FI~. 2) is employed. It will be appreciated that the density of recording made possible by the present invention is quite high. ~hus, the possibility exists that an error in positioning of the laser beam.s may arise during recording which coul~ result in destroying previously recorded flata.
Such previously recorded data can be very expensive to replace, if not lost forever if there is no back-up. mhis problem i5 prevented ~y the present invention by making use of the read-before-write beam 12c.
As illustrated in ~I~7~ 6, the read-before-write signal 14b obtained from,the optical detection circuitry 49 in FIG. 3 is applied to a preamplifier 91 whose output 91a is in turn applied to a data detector 95 via filter circuitry 93. The filter circuitry 93 is provided to prevent noise from interfering with the opera~ion of the data detector 35.
The data detector 95 is responsive to the presence of recorded data to produce an interrupt signal 95a which is applied to the recording circuitry lO ~FIG. l) to halt recording, thereby protecting previously recorded data. It is to be understood that the reafl-before-write beam may also be used for other purposes, such as to check the quality of the ~2~3~

track prior to recording, or to provide more precise track following and/or focusing control.
Having described the optical system embo~iment disclosed in the aforementioned patent applications, the improvementS in optical beam positioning control provided in accordance with the present invention will now be considered in detail with reference to FIGS. 8-lQ.
FIG. 8 is an electrical and schematic diagram illustrating apparatus for providing improved optical beam positioning control in accordance with the presen~ invention.
The optical disk 15, the objective lens assembly 44, the galvanometer 42 and the linear motor 48 shown in FIG. 8 correspond to like-numbered elements in FI~7. 3. The remaining portions of FIG. 8 illustrate a preferred implementation for precisely controlling the movement of the galvanometer 42 and linear motor 48 which together control the position of the objective lens assembly ~4 ~and thus the position of the laser beams la, 12b and 12c) relative to the optical disk 15. The movements of the galvanometer 42 and the linear motor 48 are con~rolled in the preferred implementation of FIG. 8 so as to provide for the functions of track following and track seeking.
~ he track following function refers to the operation of the system which causes the laser beams to accurately follow one particular track unless the track is changed as a result of a track seeking operation. It has already been described in connection with FIGS. 5 and 6 how a track~
following signal indicative of the positioning of the laser beams on a track can be obtained. The manner in which the preferred implementation of the present invention makes advantageous use of this track-following signal will become evident hereinafter.

:~2()~
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The track seeking function refers to the operations involved in changing the position of the laser beams from a current track to another selected track. In the preferred implementation of FIG. 8~ track seeking is categorized as a long seek or a short seek; A short seek provides for a relatively small trac~ change o~, for example t 50 tracks or less which can be provided, for example, in an average time of 200-400 microseconds and a maximum time of less than 1.5 milliseconds. A long seek provides for large track changes which, on a ~n,0~0-track disk, can be many tho~sands of tracks.
The specific manner in which the above described track-following and track-seeking functions are advantageo~sly achieved in the preferred implementation of FI~. ~ will now be considered in detail. The structure of FI~. 8 is based on the provision of a plurality of interactive servo loops which provide for unusually accurate and reliable control of the galvanometer 42 and the linear motor 4~ during track-seeking and track-following operations.
In order to simplify the description of F~ and permit its features and advantages to be clearly understood, the above desired functions -- track-following, long seeking, short seeking -- will be separately considered with reference to FIGS. ~, lO and 11, respectively.
~ach of the components illustrated in FIGS. 8-ll are readily providahle by those skilled in the art and are thus not provided in greater detail. Also, in order to make FI~S. 8-ll easier to understand, the switching occurring between these three functions are indicated as being performed using single-pole, double-throw mechanical switches (for example, switches 122 and 12~ in FI~,S. ~ and 4) which switch in response to one or more labeled signals which are shown applied to the movable arm of the switch.

1~3~)604 The contacts of these switches are also labeled to indicate ~he f~nction or functions to which each corresponds. The label "T~" indicates track following, the label "SS"
indicates shor'c seeking, and the label "LSI- indicates long seeking. Those skilled in the ar~ will be able to readily employ appropriate electronic switching circuitry to perform the switching indicated by these switches.
Also note in FI~,S. 8 11 that each of the inputs to an analog summer (such as summers 11~ and 138) is provided with a "+" or "-" to indicate whether the input is being provided in an additive or subtractive polarity relative to the other inputs to the summer.
Track Following (FIG. 9) The track following func~ion will be considered first with reference to FI~. 9 which shows only those portions of FIG. 8 which are pertinent to this function.
As shown in FIG. 9, the galvanome~er 42 is driven by a galvanometer amplifier 110 for controlling the galvanometer mirror 40 (FIG. 3~. The basic servo loop involved in the track-following function illustrated in FIG. ~ is provided using the track following signal 21c derived by the signal processing electronics 20. -The derivation of this signal has been described previously in connection with FIG. ~.
It will be remembered that this track following signal is a measure of ~he preciseness of track following by the laser beams and is accordingly applied as an error si~nal to the galvanometer amplifier 11~ via a switch 114 (which is in the position shown in FIG. 9 during track following) an~ an analog summer 11~ for maintaining the laser beams on a particular track. In the ~referred embodiment, the spacing of the tracks on the disk are chosen in conj unction with the dimensions of che track following pits 56 and S8 in FIG. 5 so that the laser beams will never hang up between - 20 ~

tracks, but will always be following a track. The address of the par~icular track being followed can thus be obtaine~
during track following at the o~tput of the track and sector decoder 78 in FIGo 6, as described previously.
As shown in F~G. 9, the galvanometer 42 includes an angle detector 118 which provides an angle signal 118a representative of the current angular positio^n of the galvanometer mirror 40 (FIG. 3). A suitable angle ~etector is described, for example, in the article "An Infra-Red Based ~eam Scanner Position Sensor", P.M~ ~rethewey, Region fi IEE~ Student ~aper Competitionl Wescon/~ ilton ~otel, San Francisco, California, S~/~. The angle ~etector signal 11~ is employed in two ways in ~ 9 during track-following operations. First~ the angle si~nal 118a is applied to a spriny amplifier l2n (having appropriate spring compensation charac~eristics) whose output 120a is in turn fed hack to the galvanometer amplifier 1' n via the summer 11~ for providing spring compensation feedback for the galvanometer mirror~
Second, the angle signal 118a is applied, via switch 122 (which is in the position shown in FIG. 9 during track following), analog summer 124, switch 127 (which is also in the position shown during track following), to a relatively high gain linear motor amplifier 128~ In response to the angle signal 118a, the amplifier 12~ drives the linear motor 4~ (also see FIG. 3) in a manner such that the galvanometer 42 (which is mechanically coupled to the objective lens assembly 44 in FI~ 3) is moved in a transverse ~irection relative to the track being followed so as to reduce the deflection required to be provided by the galvanometer mirror 40 (from its center or undeflected position) in order to maintain accurate track following.
In other words, during track following, the movement of the i()4 linear motor 4fl in response to the detected mirror angle signal 118a acts to maintain the galvanometer mirror near its center or undeflected position despite the presence of relatively large variations in track eccentricity. This achieves the advantageous result of providing the galvanometer mirror with a much greater effective dynamic range than would otherwise be possible, thereby permitting successful operation over relatively wide track position variations beyond that which the mirror would be capable on ln its own~ The importance of this capability can be appreciated by realizing that on a disk in which the tracks are two microns apart, disk run-out due to spindle eccentricities typically produce track position variations during track foliowing amounting to as much as ~ 35 tracksO
In order to provide even further stability and reliability during track following, the speed of the linear motor 48 is fed back ~o the galvanometer amplifier lln as well as to the linear motor amplifier 1~8. This is accomplished using a tachometer 130 with the linear motor 4 whose output is amplified by a tachometer amplifier 132 for producing a velocity signal 132a which is applied to the linear motor amplifier 12~ via the summer 1~4, and is applied to the galvanome~er amplifier 110 via a buffer amplifier 134, a differentiator 13~ and summers 13~ and 11~.
The advantage of providing the differentiator 136 is that it permits rapid velocity changes to be anticipated hy the galvanometer amplifier lln~ thereby resulting in greater overall loop stability.
Short Seek ~FIG. ln) The short seek function is illustrated in ~IG. 10 and will be seen to comprise the same components as the track following function illu.strated in FIG. 9 with the addition of those components pertinent to the short seek ~L2~
- ~2 -function. These additional components are a waveform generator 14n, a switch 142~ an analog summer 144, and an on-track detector 145. Note also that switches 114, 127 and 142 are shown in their short seek positions. It will be understood that a short seek function is one whïch provides for movement of the laser beams from their current track to a predetermined track which is not greater than a specified number of tracks away from the current track. In the preferred embodiment being considered, the maximum number of tracks that the laser beams can be moved by a short seek operation is, for example, + 50 tracks. If the beams must be moved a greater number of tracks, ~hen a long seek is used which will be described hereinafter.
As illustrated in FIG. 10, a short seek operation is initiated by the application of a short seek command to a short seek waveform generator 140. This short seek command is provided by the track selection circuit 8n (FIG~ 6) whenever a newly selected track is within ~ sn tracks of the current track. The short seek command contains information as to the direction and number of tracks which it is desired that the laser beams he moved. When a short seek command is received by the short seek waveform generator l4nt it produces a short seek signal SS having a duration determined by the number of tracks to be move~ -- that is, a duration ~hich is substantially equal to the expected time required for moving to the number of tracks called for by the short seek commandO This short seek signal ~S serves a nl~ber of purposes: (1) to place switch 142 in its short seek SS position and thereby connect the output of the 3n short seek waveform generator 140 to the gal~anometer amplifier via summers 144, 138 and 116, (2) to place switch 11~ in its short seek SS po~ition so as to open the track following loop by disconnecting the track following signal ~LZ~3~
- 2~ -from the galvanometer amplifier, and (3) to enable the on-track detector ]4S to monitor the track following signal so as to open switch 1~7 when an off-track condition is present ~as during a short seek), thereby preventing movement S of the linear motor 4~ d~ring the short seek operation.
The above described effects procluce~ by the short seek signal .5~ permit the output 140a provided by the waveform generator 14n to be applied to the galvanometer amplifier 110 to exclusively control the galvanometer mirror ln ~and thus the position of the laser beams) during a short seek operation without interference from the track following signal (since switch 114 will be open) or from the linear motor 48 (since switch 127 will be open), thereby permitting accurate control of laser beam movement in accordance with the output 140a provi~le~ hy the short seek waveform generator 14n. In ~he preferred embodiment, the ou~put l4na provided by the shor~ seeX waveform generator 14n is chosen to be a doublet -- that is, a pulse of one polarity followed by a pulse of opposite polarityO ~urinq the first pulse, the linear motor 4~ accelerates and durinq the second pulse it decelerates. It has been found that the durations an~
amplitudes of these opp~site polari~y pulses can be chosen based on the mechanical characteristics of the system and the number of tracks to he moved, ~hereby the galvanometer 42 will rapi~ly and accurately move the galvanometer mirror so that the laser beams are moved the requested number of tracks. When the laser beams arrive in the immediate vicinity of the desired track, the ~short seek signal ~SS ends since, as pointed out above, its duration is chosen to be

3~ substantially equal to the time required for the laser beams to be moved the requeste~ numher of tracks in response to the waveform generator output l4na~

()4 When the short seek signal SS ends, switches 114 and 142 return to their track following positions. Also, switch 127 returns to i~s closed position when the on-track detector 145 indicates that the laser beams are properly on a track, thereby reconnecting the angle detector 118 to the linear motor amplifier 128. The on-track detector 145 then returns to its disabled state until the next short seek operation.
As mentioned previouslyt the track ollowing operations are chosen in conjunction with the track spacing so that the laser beams will not hang up between tracks, hut will always come to rest on a trackO If after the above described short seek operation, the track address provide~ by the track and sector decoder 78 in FIG. ~
indicates that the track on which the laser beams came to rest is not the one requested by the short seek operation, another short seek operation is initiated, and so on until the laser beams come to rest on the correct track. ~t has been found that no more than two short seeks are normally required to position the laser beams on the correct track, and normally only one short seek is required when the laser bea~s are to be moved less than 2 n tracksO
Long Seek (FIG. 11) The operation of the long seek function is illustrated in FIG. 11 which shows those components of FIG. 8 pertinent to this unction. A long seek is initiated by a long seek command containing data which is applied to a long seek control circuit 150. The track selection signal 21b provided by the track selection circuit ~n (FIG. 6) is a long seek command whenever a newly selected track is greater than 50 tracks away from the current track. The long seek control circuit 150 responds to the applied long seek command to produce a long seek signal L~

~2~)6()~

which serves a number of purposes: (1) to place switches 114 and 142 in their long seek LS positions so as to thereby open ~he ~racX following loop and disconnect the short seek waveform generator 140 (FIG. 103, (2) to place switch 1~2 in its long seek position so as to disconnect the angle ~etector signal 118a from the linear motor amplifier 14 and instead connect a long seek waveform generator 152 thereto via summer 124, and ~3) to place switch 154 in its long seek position so as to close a feedbacX loop which ln causes the detected mirror an~le signal provided hy the angle ~etector 118 to be applied to the galvanometer amplifier 42 via a cage amplifier 154 and summers 114, 13 and 116 so as to cause the galvanometer mirror to be driven to its zero angle position in which it remains during the long seek operation; this in effect ~cages" the galvanometer mirror at its zero angle position so as to prevent it from inter~ering with the long seek operation which is perfor~ed using the linear motor 48, and ~4) the long seek signal r.s is also applied to ~he linear motor 2n amplifier 12~ and tachometer a~plifier 132 for changing the gain of these amplifiers to values better suited to the long seek operation, the gain of the linear motor a~plifier 1~
being decreased~ while the gain of the tachometer amplifier 132 is increased.
The long see}; command applied to the long seek control circuit 15~ in FIG. 11 contains ~ata represent~tive of a new track to ~hich the laser beams are to he moved. In response to this lonc; seek command, the long seek control circuit 150 pro~uces an output signal 15na to an anal~g summer 156 having a ma~nitu(1e indicative of the new position to which the linear motor 4~ should be moved in order to correspond to the new track. The summer 156 also receives a signal lhOa indicative of the position of the linear 7o~

motor l3n derived from a linear variable ~iferential transformer (LVDT) 158 coupled to the linear motor 48/ the LVDT signal being applie~ to the summer 156 via an LVDT
amplifier 160.
~he resulting OUtpllt 15ha of the summer lS~ in FI~,. ll is representative of the ~ifference between the current position (indicate~ by Lvnq~ 158) and the desired position indicate~ by the long seek commandO 7rhe summer output l55a is applied to a long seek waveform generator 1~ 162, The long seek waveform generator 162 responds to the summer output 156a to generate an output signal lf 2a having a waveform chosen so that, ~hen applied to the linear motor 48 ~via switch 152, sun.~mers 124 and 126, switch 127 and the linear motor amplifier 148), the linear motor 48 is rapidly and smoothly moved to a position corresponding to the desired selec~ed track. 7rhe arrival of the linear motor 4 at the selecte~ ~osition during a long seek operation in FIG. ll is determined using an arrival ~etector 164 which is responsive to the outputs of the LV~T amplifier 160, the tachometer amplifier 132 an~ the su~ner 15.~ for providing an output signal 164a inclicative of the completion of the long seek operation.
~I~. 12 illustrates a preferred implementation of the arrival detector 164 in FIG. ll~ As indicate~ in FIG. 12 the tachometer amplifier output signal 1~2a is applied to a comparator 170 which comnares the linear motor velocity represented b~ the signal l3?.a with a maximum velocity reference signal. ~hen the linear motor velocity is below the maximum veloci~y reresented by the maximum velocity reference signal, the comparator l7n produces a "true" or "l" output signal 17na which is applied to an A~ gate 176.
Still with reference to FI~.. 12, the summer ou~put l55a in ~IG. ll, which represents the dif~erence between the current and desired positions of the linear m~otor, is o~
~ 27 -applied ~o a comparator 174 which compares this difference signal 15~a with a maximum difference reference signal.
When the difference signal 156a is below the maximum difference value represented by the maximum difference reference signal, the comparator 174 produces a n true" or ~1" output signal 174a which is also applied to the AN~
gate 176.
It will accordingly be understood that the resulting o~tput 164a from the ~N~ gate 176 in ~ 12 will thus be "true" or "1" only when both the linear motor velocity and the difference between current and desired motor positions are below predetermined maximum values, thereby providing a highly stable and reliable detection of the arrival of the linear motor at the position called for by the long seek command (FIG. 11).
Returning now to FIG. 11, the arrival detector output signal 164a is applied to the long seek control circuit l5n which, in response thereto, turns off the long seek signal LS when the arrival detector 164 indicates that the linear motor 48 has arrived at a position corresponding to the selected track, thereby causing operation to return to track following, which has previously been described in connection with FI~,. 9. It will be understood that, if upon return to track following, the track and sector decoder 7 (FI~ 6) determines that the laser beams are on the wrong track, t:hen a short seek o~eration is initiated as ~escribed previously in connection with FIG. ln.
Although the descriptio~ of the invention provided herein has been directed to a particular preferred 3~ embodiment, it is to be understood that many modifications in construction, arrangemen~, components, operation and use are possible without departing from the true scope and 1;~ )4 - 2~ -spirit of the present invention. ~or example, it is to be understood that the present invention is also applicable to other types of hiqh density data s,torage systems. The appended claims are accordingly to be considered as encompassing all such possible modifications and variations.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a high density data storage system including a recording medium having a large plurality of spaced tracks, the combination comprising:
means including a radiation beam for reading data recorded in said track;
means providing for relative movement of said radia-tion beam along a track of said medium;
track following means for causing said beam to ac-curately follow a track on said medium;
long seek position control means for moving said beam a relatively large number of tracks, said long seek posi-tion control means including means for providing a position signal indicative of the position of said beam and a velocity signal indicative of the velocity of said beam, means for disabling said track following means dur-ing operation of said long seek position control means;
means for providing a command signal indicating a new position for said long seek position control means corres-ponding to a new track to which said beam is to be moved;
waveform generating means responsive to said posi-tion signal and said command signal for generating a control signal which is applied to said long seek position control means for causing movement thereof to said new position; and arrival detecting means jointly responsive to said velocity signal and to the difference between said command and position signals for detecting the arrival of said beam at said new position and in response thereto enabling said track following means.

2. The invention in accordance with claim 1, wherein said arrival detecting means comprises means for determining when both said velocity signal and the differ-ence between said position and command signals are both below respective predetermined values.

3. The invention in accordance with claim 1, wherein said track following means comprises:

means for deriving a track following signal indi-cative of deviation of said radiation beam from said track;
track following position control means for provid-ing movement of said radiation beam relative to said track over a relatively small distance;
said track following position control means being responsive to said track following signal for moving said radiation beam to reduce said deviation so that said radi-ation beam follows said track; and detecting means for detecting movement of said track following position control means; and wherein said long seek position control means oper-ates concurrently with said track following position control means in response to said detecting means for providing move-ment of said track following position control means so as to reduce the movement required thereby for maintaining said radiation beam following said track.

4. The invention in accordance with claim 3, wherein said track following position control means provides for relatively fast transverse movement of said radiation beam over a relatively small distance, and wherein said long seek position control means controls movement of said track following position control means so as to provide relatively slower transverse movement of said radiation beam.

5. The invention in accordance with claim 4 in-cluding means providing for control of the movement of said track following position control means in response to the rate of change of the movement provided by said long seek position control means as well as in response to said detect-ing means.

6. The invention in accordance with claim 3 wherein said high density storage system is an optical storage system, wherein said radiation beam is a laser beam, wherein said track felon position control means comprises a galvano-meter controlling an angularly deflectable optical element disposed in the path of said beam and whose angular position affects the transverse position of said beam relative to said track, wherein said detecting means detects the angular de flection of said optical element, and wherein said long seek position control means includes a motor for moving said track following position control means in response to said detect-ing means.

7. The invention in accordance with claim 4 wherein said high density storage system is an optical storage system, wherein said radiation beam is a laser beam, wherein said track following position control means comprises a galvano-meter controlling an angularly deflectable optical element disposed in the path of said beam and whose angular position affects the transverse position of said beam relative to said track, wherein said detecting means detects the angular de-flection of said optical element, and wherein said long seek position control means includes a motor for moving said track following position control means in response to said detect-ing means.

8. The invention in accordance with claim 6, wherein said optical element is a mirror.

9. The invention in accordance with claim 5, includ-ing means for detecting the speed of movement provided by said motor and for producing a velocity signal in response thereto, and means for additionally controlling the movement of said optical element in response to the differentiated velocity signal.

10. The invention in accordance with claim 6, includ-ing means responsive to said detecting means for providing for control of the movement of said optical element in response to the magnitude of the angular deflection thereof.

11. The invention in accordance with claim 6, in-cluding means responsive to said detecting means for driving and maintaining said optical element at substantially its un-deflected position when said long seek position control means is performing a long seek operation.

12. In a high density data storage system including a recording medium having a plurality of spaced tracks and providing a radiation beam for reading data stored in a track as said beam follows said track, an improved method of provid-ing for movement of said beam over a relatively large number of tracks using a long seek positioning means, said method comprising the steps of:
moving said beam in a controlled track following relationship with respect to a track on said medium;
disabling said controlled track following relation ship when said beam is to be moved to a different track;
generating a feedback signal representing the dif-ference between the current and desired positions of said long seek positioning means;
moving said long seek positioning means in response to the generated feedback signal;
detecting the arrival of said long seek positioning means at its final position in response to both the velocity of movement of said beam and the difference between current and desired positions being below predetermined maximum values; and enabling said controlled track following relation-ship after detecting arrival of said long seek positioning means at its final position.

13. The invention in accordance with claim 11, wherein the step of moving said beam in a controlled track following relationship comprises the steps of detecting the deviation of said beam from the track being followed and moving a relatively fast acting track following position control means so as to reduce said deviation, and wherein said method in-cludes the additional steps of detecting the movement of said track following position control means, and moving said long seek position control means concurrently with said track following position control means in response to said detecting so as to reduce the movement required by said position control means for maintaining said radiation beam following said track.

14. The invention in accordance with claim 11 or 12, including the step of moving said track following position control means in response to the rate of change of the speed of moving provided by said long seek position control means as well as in response to said detecting.

15. The invention in accordance with claim 11 or 12, wherein said track seeking position control means includes an angularly deflectable element in the path of said beam, and wherein said method includes the step of moving said long seek position control means in response to the angular deflec-tion of said element.

16. In a high density storage system including a recording medium having a large plurality of closely spaced tracks, the combination comprising:
means including a radiation beam for reading data recorded in a track;
means providing for relative movement of said radi-ation beam along said track;
means for deriving a track following signal indica-tive of the deviation of said radiation beam from a track being followed;
first position control means coupled to said track following signal for moving said radiation beam to provide track following operation such that said radiation beam follows said track;
detecting means for detecting the movement of said first position control means during said track following operation;
second position control means coupled to said detec-ting means and operating concurrently with said first position control means during said track following operation, said second position control means being responsive to said detect-ing means for providing movement of said first position control means so as to reduce the movement required thereby for main-taining said radiation beam following said track; and track seeking control means for causing movement of said beam from a current track being followed to a new track, said track seek control means being operative to disable said track following operation for a temporary time period during which said second position control means is used for moving said beam to a new track.

17. The invention in accordance with claim 16, in-cluding means for also providing control of the movement of said first position control means during said track following operation in response to the rate of change of the velocity of movement provided by said second position control means.

18. The invention in accordance with claim 16, wherein said track seek control means includes:
means for providing a command signal indicating a new position for said track seek position control means corres-ponding to a new track to which said beam is to be moved;
means for producing a position signal indicative of the current position of said beam; and signal generating means responsive to said position signal and said command signal for generating a seek signal which is applied to said second position control means for causing movement of said beam to said new position.

19. The invention in accordance with claim 18, wherein said second position control means includes a motor, an amplifier for driving said motor in response to said de-tecting means during said track following operation and in response to said seek signal when said beam is being moved to a new track, and means operative to provide a significant ly greater gain for said amplifier during said track following operation.

20. The invention in accordance with claim 18, in-cluding means for producing a velocity signal indicative of the velocity of said second position control means, and arri-val detecting means jointly responsive to said velocity signal and to the difference between said position and command signals for detecting the arrival of said beam at a new track position and in response thereto enabling said track following operation.

21. The invention in accordance with claim 20, wherein the arrival of said beam at a new track position is determined based on the velocity signal and said difference both being below respective predetermined values.

22. The invention in accordance with claim 16, 17 or 18, wherein said high density storage system is an optical storage system, wherein said radiation beam is a laser beam, wherein said first position control means comprises a galvano-meter controlling an angularly deflectable optical element disposed in the path of said beam and whose angular position affects the transverse position of said beam relative to said track, and wherein said detecting means detects the angular deflection of said optical element.