Abstract:
The optical servo system for a tape drive that functions to align a read/write head with the data tracks written on a recording surface of a tape by reading optical servo tracks that are formed on the back side of the tape. This process decouples the magnetic recording of data on the recording surface of the tape from the optical servo system which makes use of servo tracks formed on the back side of the tape. The data storage capacity of the tape is increased since the entire recording surface of the tape is filled with data tracks and the precise alignment of the read/write head makes it possible to place the data tracks closer together. Regions of contrasting reflectivity or phase are also provided on a surface of the read/write head to enable the optical servo system to view an image of both the read/write head and the entire back side of the tape to thereby align the movable read/write head with the data tracks.

Description:
This is a continuation of copending application Ser. No. 09/775,893 filed Feb. 5, 2001 which is a continuation of Ser. No. 09/577,017 filed on May 22, 2000 (U.S. Pat. No. 6,236,529), which is a continuation of 08/980,723 filed on Dec. 1, 1997 (U.S. Pat. No. 6,084,740) 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to servo systems for use with tape media and, in particular, to a servo system that makes use of optical tracking features formed on the back side of the tape to enable the system to precisely align the read/write heads with the recorded data located on the front side of the tape. 
     Problem 
     It is a problem in the field of tape data storage media to accurately position the read/write heads with respect to the tracks of data written on the tape. In longitudinal tape media, the tape typically contains a single recording surface on which is written along the length of the tape a plurality of parallel aligned tracks of data. In systems using high track densities (&gt;200 tracks per inch), the precise alignment of these data tracks with the read/write heads of the associated tape drive is accomplished by the use of a plurality of servo tracks written on the tape recording surface, interspersed with the tracks of data. The servo tracks, typically written during the tape manufacturing process, function as a physical reference for the placement of the data tracks when the data tracks are written on the tape and for reading previously written data tracks from the tape. The position information derived from the servo tracks is used by the tape drive to adjust the position of the movable read/write head to enable the accurate reading and writing of data to and from the data tracks. 
     A problem with this tracking system is that the number of data tracks written on the tape is limited by the need for servo tracks written on the recording surface to provide position information. There is a need to facilitate the development and use of future tape systems with increased data capacity. This is often accomplished by the increase in the number of data tracks and the amount of data placed in a given track. Due to the mechanical instability of tape media, higher data track densities require a decreased physical spacing between data and servo tracks to ensure the accurate alignment of the read/write heads with the data tracks. The decreased spacing drives the need for increased numbers of servo tracks which, in turn, must share the tape area with data tracks. Furthermore, it is highly desirable for future systems to retain the ability to read the tapes made on earlier systems—this is termed “backward compatibility”. This backward compatibility requires the head positioning servos to be able to work on tapes with varying numbers of tracks and track configurations. This presents a design challenge and can force future drive designs to trade off between performance enhancements and backward compatibility. A further problem is that magnetically written servo tracks are susceptible to track erasure. Bulk erasure of the tape can erase servo tracks, drive system failure can result in the servo tracks being overwritten and corrupted—either of which can render the tape and its data useless. 
     There are numerous servo track systems in use in the field of rewritable data storage media. Some of these are illustrated by the disclosures of the following patents. 
     U.S. Pat. No. 4,958,245, titled “Apparatus And Method For Optical Servo Control With Media Having Information Storage And Servo Control Regions Of Different Reflectivities” discloses an optical servo head to read position information from a disk on which data is magnetically recorded. The disk has a plurality of optical servo tracks formed thereon in the form of relatively nonreflective regions comprising concentric grooves formed in the reflective surface of the magnetic disk. The servo system illuminates a plurality of the reflective and non-reflective regions and uses a quadrature photodetector array to achieve tracking. 
     U.S. Pat. No. 5,067,039 titled “High Track Density Magnetic Media With Pitted Optical Servo Tracks And Method For Stamping The Tracks On The Media” discloses a method for mechanically stamping the servo tracks on the optical disk during the disk manufacturing process. 
     U.S. Pat. No. 5,279,775 titled “Acousto-Optic Intensity Control Of Laser Beam During Etching Of Optical Servo Information Of Magnetic Media” discloses a system that etches servo tracks on a magnetic disk. Track following during the etching process is accomplished by the use of an acoustic-optical device to maintain the beam in concentric patterns, while a laser beam is used to etch the servo tracks, with the laser beam intensity being controlled by the acoustic-optical device. 
     U.S. Pat. No. 5,283,773 titled “Steering Laser Beam While Etching Optical Servo Tracks For Magnetic Disks” discloses a system that etches servo tracks on a magnetic disk. Track following during the etching process is accomplished by the use of an acoustic-optical device to maintain the beam in concentric patterns, while a laser beam is used to etch the servo tracks. 
     U.S. Pat. No. 5,462,823, titled “Magnetic Recording Materials Provided With A Photosensitive Layer” discloses a magnetic recording element that comprises a support layer coated with a magnetic recording layer and a photosensitive layer. Optical tracking information is formed on the photosensitive layer by the exposure of the photoreactive surface using a servo track mask. 
     The above noted servo systems all make use of servo tracks that are formed on the rewritable media on the same surface as is used to store the data. The servo information is typically in the form of servo tracks that are formed coextensive with the data tracks and interspersed among the data tracks. Therefore, the servo tracks occupy space on the tape that can be used for the storage of data. Furthermore, there is an inherent interaction between the use of servo tracks and the writing of data tracks such that the system cannot optimize the data recording function without impacting on the servo function. Conversely, the system can not optimize the servo function without impacting the data recording function. 
     Solution 
     The above described problems are solved and a technical advance achieved by the present optical servo system for a tape drive that functions to align a read/write head with the data tracks written on a recording surface of a tape by reading optical servo tracks that are formed on the back side of the tape. This process decouples the magnetic recording of data on the recording surface of the tape from the optical servo system which makes use of servo tracks formed on the back side of the tape to position the read/write head. For example, the recording formats of the data can be altered and the number of data tracks can be changed without impacting the optical servo system. The servo system can accommodate a wide range of recording format changes within its signal processing algorithms without modifying its servo tracks. In addition, the data storage capacity of the tape is increased since the entire recording surface of the tape is filled with data tracks and precise alignment of the read/write head with the data tracks makes it possible to place the data tracks closer together. 
     The tape used in this system has magnetic data tracks recorded on the front side of the tape and-optical servo tracks, comprising regions of differing reflectivity or phase, formed on the back side of the tape. Although current magnetic media types could be utilized, the servo track reading and writing processes explained below are optimized by the use of a media with a second side optically tuned to have high contrast or phase change at the read lumination wavelength and high write sensitivity at the servo track writing wavelength. A magnetic read/write head that is positioned juxtaposed to the front side of the tape reads data from and writes data to the data tracks while the optical servo system reads servo data from the servo tracks that are formed on the back side of the tape. The requirement for close data track-to-servo track spacing is met by having the servo tracks located immediately behind the data tracks. Regions of contrasting reflectivity or phase are also provided on a surface of the read/write head to enable the optical servo system to view an image of both the read/write head and the entire back side of the tape to thereby align the movable read/write head with the data tracks. The head&#39;s optical features may be formed by numerous means known to those familiar with the art including integration into the head structure itself or affixing a secondary structure to a head surface. An optical sensor array generates electrical signals indicative of the received image which are then used by a digital signal processor to determine the required alignment of the read/write head with the data tracks. Once the proper alignment is determined, the digital signal processor generates a position error signal that in turn is fed to the servo amplifier which drives an actuator to align the movable read/write head with the data tracks. 
     The use of the two sources of optical data from the read/write head and the tape media improves the accuracy, performance and reliability of the data track to read/write head alignment while simplifying the entire servo system. Using this approach, all optical components can be fixed in place. Since the servo system “closes the loop” around the tape and head optical feature alignment, system alignment and calibration requirements are eased. Fault tolerance to damaged tape and/or head optical features is facilitated by the availability of redundant optical information—a plurality of optical features exist on both the tape and the head. Furthermore, because this system locates the entire width of the tape with respect to the head, servo information is always available to quickly re-establish head-to-tape alignment (i.e. “track following”) should it be lost during drive operations. Interchange, the ability to read a given tape on a population of tape drives, is facilitated by the servo system&#39;s ability to image the mechanical relationship between tape and head immediately after the tape is loaded into the drive and make appropriate offset adjustments in the head&#39;s static position. The immediate availability of this offset information results in a reduced tape load time. This reduction, in turn, results in higher overall job throughput for the drive in a repetitive tape loading environment—such as is commonly seen when robots are used to mount and dismount tapes in the drive. 
     The optical servo tracks are not subject to magnetic erasure. Accidental magnetic damage is eliminated thus increasing data recovery reliability. Bulk magnetic erasure of the tapes facilitates their reuse by reducing old data noise sources, improving data security by eliminating old data and providing an economic benefit over new tapes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in block diagram form the present optical servo system for a tape drive; 
     FIG. 2 illustrates the waveform signal output from the image system of the present optical servo system for a tape drive; 
     FIG. 3 illustrates in block diagram form the system for forming the servo tracks on the tape for use by the present optical servo system for a tape drive; and 
     FIG. 4 illustrates in flow diagram form the operation of the present optical servo system for a tape drive. 
    
    
     DETAILED DESCRIPTION 
     The present optical servo system for a tape drive operates with a tape that has data tracks written on a recording surface (first side) of the tape and optical servo tracks formed on the back side (second side) of the tape. An optical servo system reads the servo tracks on the second side of the tape and optical features on the read/write head to determine the alignment between a read/write head with data tracks on the first side of the magnetic tape. The digital signal processor generates control signals indicative of the adjustments needed to align the read/write head with the data tracks, the head actuator responds to the amplified signals and moves the read/write head to the desired position. 
     Optical Servo System For A Tape Drive 
     FIG. 1 illustrates in block diagram form the present optical servo system for a tape drive  600  and FIG. 4 illustrates in flow diagram form the operation of this system. The present optical servo system for a tape drive  600  operates to align tape  100  and read/write head  605  of the tape drive as tape  100  passes over read/write head  605 . The remaining mechanical elements of the tape drive are not illustrated herein for the purpose of simplicity of description. The read/write head  605  is movable and its position is determined by actuator  611 , which operates under control of the present optical servo system for a tape drive  600  as directed by the drive processor  612 . The tape  100 , for the purpose of this description, is a magnetic tape that has a recording surface on a first side  102  and a second side  101  that is used for positioning information. This process begins with the drive processor  612  issuing a track position request at step  901 . 
     The tape  100  has formed on the second side  101  thereof a plurality of parallel aligned servo tracks  110  that are used to precisely identify the position of the data tracks written on the first side  102 , of the tape  100 . The optical servo system for a tape drive  600  reads one or more of the servo tracks  110  at the point that tape  100  is directly over read/write head&#39;s optical features  650 . This is accomplished by the Light Emitting Diodes (LEDs)  601 - 604  each generating a beam of light that is directed onto tape  100  and read/write head  605  by mirror  120 . The servo tracks  110  formed on the second side  101  of tape  100  represent areas of differing reflectivity or phase and the image of the second side  101  of tape  100  and the optical features  650  on the face of the read/write head  605  extending beyond tape  100  is reflected back onto mirror  120 . This reflected image is directed by mirror  120  through lens  606  onto sensor array  607 . Sensor array  607  is an imaging device, such as a linear Charge Coupled Device (CCD) imaging array, that functions to capture the reflected image and convert the reflected image into an electrical signals at step  902 . The generated electrical signal, indicative of the image content, is transmitted to interface circuit  615 , which produces an analog signal that is transmitted to an analog to digital converter  608  to create a digital representation of the image at step  903 . A digital signal processor  609  receives this digital representation of the image and algorithmically determines the alignment of the read/write head  605  with the data tracks from the digital image at step  904 . The digital signal processor  609  compares this current head position alignment with the drive processor&#39;s requested position from step  901  and then determines the direction and the distance that the read/write head  605  must be moved to meet the new position request. The digital signal processor  609  at step  906  generates a position error control signal that defines this movement and transmits the signal to a servo amplifier  610  at step  907  to control the operation of actuator  611  to move the read/write head  605  in the direction needed to align the movable read/write head  605  with the data tracks at step  908 . 
     While the read/write head  605  approaches the desired track position, the sensor array  607  continues to periodically update the tape-head image. This image signal is sent through processing blocks  615 ,  608  to the digital signal processor  609 . The digital signal processor  609  makes a new determination of read/write head alignment then reduces its position error signal to the servo amplifier  610  accordingly to slow the actuator&#39;s movement of the head. This sampling process (steps  902 - 908 ) repeats itself until the head is in the desired position. At this point, the system “track follows” by continuing to detect small misalignments between the head and the desired track and signaling the actuator  611  to make corrections to the head&#39;s position to keep it precisely aligned (while still continuing to repeat steps  902 - 908 ). This process continues until a new request (step  901 ) is received from drive processor  612  to move to a new track position. The drive processor&#39;s new position request is compared by the digital signal processor  609  (at step  904 ) to the current read/write head position and a new position error signal is generated (at step  906 ) and sent to the servo amplifier  610  (step  907 ). The actuator  611  begins to move the read/write head  605  (step  908 ) while the sensor array  607  allows the servo system to monitor progress toward achieving the new head position by periodically updating the tape-head image. The image is sent through processing blocks  615 ,  608  to the digital signal processor  609 , where the position error signal is updated—this process repeats until the new head position is attained and track following begins (steps  902 - 908 ). 
     As is known in the art, for fastest performance, the actuator&#39;s acceleration is dependent upon the distance it is required to travel—large repositioning creates the largest acceleration/deceleration. Small track following re-positioning results in the smallest acceleration/decelerations and the greatest positional precision. In addition, the configuration disclosed herein is illustrative of the inventive concept and other optical configurations are well within the design capabilities of one skilled in the art. 
     Image Content 
     An example of the image  700  captured by the present optical servo system for a tape drive  600  is illustrated in FIG.  2 . Image  700  represents a sample of all of the pixels that are generated by sensor array  607 . For the purpose of illustration, the image size is selected to be 5,000 pixels, with the horizontal axis of the diagram of FIG. 2 representing the individual pixels, and the vertical axis representing the signal magnitude of the selected pixel. The diagram also includes notations along the top of the pixel chart to indicate the typical extent of the tape  100  in the image  700 . In addition, the image  700  is divided into five regions: read/write head image  701 , boundary between read/write head and tape image  702 , tape image  703 , boundary between read/write head and tape image  704 , read/write head image  705 . These various regions are individually discussed below. 
     Areas  701  and  705  are region of the read/write head image and comprise approximately the first and last 500 pixels of image  700 . The variations in signal strength illustrated by the continuous curve of FIG. 2 represents the presence of the contrasting markings  650  located on the surface of read/write head  605 . Since the read/write head  605  is the sole source of this portion of the image, the variations are regular and map to the contrasting markings  650 . Areas  702  and  704  represent the boundary between read/write head and tape image. An expanded view of area  702  illustrates the image received from the edge of tape  100 . Within area  702 , from about pixel  500  to about pixel  700 , the contrasting regions are from the contrasting markings  650  located on the surface of read/write head  605  near the edge of tape  100 . Region  712 , from about pixel  700  to approximately pixel  900 , indicates the edge region of tape  100  that does not contain optical data. Area  713  from approximately pixel  900  to approximately pixel  1200  has intermittent regions of high and low reflectivity or phase. In this example, the regions of low reflectivity indicate individual tracks of the servo track group  110 . Finally, area  703  includes pixels from approximately pixel  1200  through approximately pixel  3800  and is an image of contrasting reflectivities or phases representing the servo tracks  110  formed on the second side  101  of tape  100 . 
     Method of Aligning 
     Applying conventional pattern recognition methods to image data from step  903 , the digital signal processor  609  calculates the position error signal  906  by first establishing accurate measurements of the relative positions, in pixels, of the optical features found on the magnetic head and tape. Because of the tracking accuracy required by the tape drive may be more stringent than the pixel-to-pixel resolution in the image data, sub-pixel measurement accuracy is needed. This accuracy is achieved by averaging the contributions to a given position measurement from as many features in the image data as possible. The digital signal processor  609  must utilize all of the image data for the magnetic head optical features,  701  and  705 , to yield an accurate measurement of the magnetic head position. Similarly, many optical tracks  703  must be read and the data averaged to obtain an accurate measurement of a given magnetic track on tape. One method of utilizing all the data in an image subset is to use the correlation algorithm where an image subset is compared to a reference signal stored in memory. The resulting correlation coefficient indicates a best fit when the reference signal is optimally aligned with the image subset. The reference signals can be based on typical signals experienced by many such tape drives and stored in non-volatile memory or can be based on actual signals obtained by a given tape drive during initial machine calibration or periodic re-calibrations between tape loads. 
     Once the optical features are adequately determined, the magnetic head position is calculated by interpolation from those features,  701  and  705 . The magnetic track position is computed from the position of a plurality of the closest optical servo tracks from the set  703 . The position error signal sent to the servo amplifier  610  at step  907  is the difference between the calculated magnetic head position and the desired magnetic track position. 
     Additional Features 
     The optical servo system for a tape drive  600  can provide additional capabilities beyond the provision of read/write head positioning information. Auxiliary information formed on the second side  101  of tape  100  can include encryptionlauthentication data, tape identification data or even maintenance information and read only data. For example, reflectivity or phase parameters can be recorded on tape  100  to indicate the initial state of tape  100  as well as manufacturing data. The optical servo system for a tape drive  600  can then measure the present optical characteristics of tape  100  to thereby obtain a measure of the wear on tape  100 . Furthermore, optical servo system for a tape drive  600  can view the image or read/write head  605  in its entirety between loads of tape  100  to verify head integrity and identify any optical feature defects. 
     The servo tracks  110  can perform a simple read/write head positioning function or can be coded to provide data relating to longitudinal positioning of tape  100  to enable high speed searching of tape  100 , which data can also be used for tape velocity determination. This auxiliary information is encoded into the servo tracks during the servo track writing process using modulation that can be separated out from the basic servo signals by the digital signal processor  609 . The auxiliary information is then supplied to other drive or system processes in step  909 . 
     Servo Track Write Svstem 
     A system for writing optical servo tracks  400  on tape  100  is illustrated in FIG.  3 . The system for writing optical servo tracks  400  writes the set of optical servo tracks  110  in one pass of tape  100  through the system for writing optical servo tracks  400 . The system for writing optical servo tracks  400  includes a laser  415  that focuses a beam of light into beam expander  412 . The expanded beam output by beam expander  412  is extended through hologram  411  which splits the beam into a plurality of individual beams  404 - 409 , which are focused onto tape  100  by lens  410 . The exact number of beams is a design choice. However, the accuracy of the alignment of the read/write head  605  and the second side  101  of tape  100  improves as the number of optical servo tracks  110  increases. Therefore, it is desirable to place as many servo tracks  301 - 306  on tape  100  as possible. Increasing the number of servo tracks to be simultaneously written on the media is facilitated by the use of media whose second side  101  is optically tuned to laser  415 &#39;s wavelength. The plurality of beams  404 - 409  forms parallel aligned, optical servo tracks  301 - 306  onto the second side  101  of tape  100  as tape  100  is passed under the beams  404 - 409 . Rollers  401 - 403  move tape  100  under the focused beams  404 - 409  at a constant speed on the second side  101  of tape  100  to ensure that the optical servo tracks  110  have the same consistency. 
     The laser beams  404 - 409  can be used to write auxiliary data on the servo tracks  110  by programming laser  415  to intermittently extinguish. The intermittent extinguishing (modulation) of the beam causes the servo optical tracks  110  to be written intermittently in a specified pattern on side  101  of tape  100 . This auxiliary data can be used by the present invention to determine tape speed, alignment, defect detection and other attributes of the tape being read, as noted above. In the alternative, a plurality of lasers can be used to write the optical servo tracks. The lasers could also be of different intensities to change the reflectivity or phase of individual tracks or the beams from the lasers can be of varying widths to change the width of the formed servo tracks to allow recognition of different sections of tape.