Abstract:
A method of generating a composite signal in a closed loop servo signal system of a data recording system used to drive a recording head to any given position within a dynamic range of a servo tracks includes an optical pickup system for generating optical spots focused on a recording medium, the spots separated by equal distances across a track, the optical pickup system receiving a set of reflectances from the spots, a media system for providing the servo tracks responsive to optical spot illumination, an electric system for generating a set of filtered signals from the reflectances and for generating a set of S-curves by pair wise subtraction of the filtered signals, a processing system to generate a composite servo position signal from the S-curves and filtered reflectances, and a servo system for driving the recording head to a desired position by comparing the desired position to a measured position from the composite servo position.

Description:
BACKGROUND 
     This invention relates generally to optical tracking systems. Increased data storage capacity and retrieval performance is required of commercially viable mass storage devices and media, such as linear magnetic tape. Linear magnetic tape systems, for example, have moved toward multi-head, multi-channel fixed head structures with narrowed recording gaps and track widths. Such narrow recording heads allow many linear tracks to be formed on a tape medium of predetermined width, such as one-half inch width tape. Tape substrates arc also being made thinner, with increased tape lengths being made possible in smaller diameter reels. 
     Because of a relatively high linear tape velocity and because tape substrates continue to be made thinner and thinner, guiding tape past a tape head structure, such as a magnetic recording head, along an accurate invariant linear path is difficult and can lead to errors in reading and writing to the tape. One such error is referred to as “lateral tape motion,” commonly referred to as LTM. LTM refers to the lateral motion of the tape as it travels across the magnetic recording head and is inherent in mechanical transport systems such as linear magnetic tape systems. LTM is a source of tracking errors in linear tape recording systems. 
     SUMMARY 
     In a general aspect of the invention, a method of generating a composite signal in a servo loop of a data recording system to drive a recording head to any given position within any servo track includes an optical pickup system for generating optical spots focused on a recording medium, the spots separated by equal distances across a track, the optical pickup means receiving, a set of reflectances from the spots, a media system for providing the servo tracks responsive to optical spot illumination, an electronic system for generating a set of filtered signals from the reflectances and for generating a set of S-curves by pair wise subtraction of thee filtered signals, a processing system to generate a composite servo position signal from the S-curves and filtered reflectances, and a servo system for driving the recording head to a desired position by comparing the desired position to a measured position from the composite servo position. 
     Embodiments of the invention may have one or more of the following advantages. 
     The invention reduces the amount of space required on the magnetic side of the tape to provide position information. One can use relatively large track pitch and correspondingly large optical spots to achieve tracking positioning accurate to a small fraction of track pitch. Utilizing the back surface of the tape with the ability to track at any position on the optical pattern increases the efficiency of data storage on the magnetic side of the tape. 
     The invention overcomes the difficulty of accurately aligning a chosen position on a magnetic head !with an optical spot from an optical pickup system. This difficulty is particularly severe when optical tracks are on the non-recording side of the media. With this invention any alignment offset between head and optics is compensated by tracking an equal but opposite offset. In so doing the recording head is always aligned to the pre-determined track position of the tape, making tape interchange possible. Utilizing the back surface of the tape with the ability to track at any position on the optical pattern increases the efficiency of data storage on the magnetic side of the tape. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The foregoing features and other aspects of the invention will be described further in detail by the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a linear magnetic tape system. 
         FIG. 2A , and  FIG. 2B  are a block diagrams of a magnetic tape. 
         FIG. 3  is a block diagram of the read/write head assembly of FIG.  1 . 
         FIG. 4  is a block diagram showing one example of using an optical pickup system to generate three optical spots. 
         FIG.5  is a block diagram of an optical chip combining the functions of the laser diode and the detectors. 
         FIG. 6  is a block diagram of one optical track made of a series of marks and the three laser spots illuminating the optical track. 
         FIG. 7  is graph of three reflectance curves generated by the three spots as they traverse across the optical track, and the S-curve generated by the top and bottom spots. 
         FIG. 8  is a graph of the time progression as the three optical spots traverse three optical servo tracks. 
         FIG. 9  is a graph of three reflectance from three spots traversing the servo tracks, and three S-curves due to a pair. 
         FIG. 10  is a graph illustrating a composite servo position curve obtained from  FIG. 9  by inverting and shifting selected S-curves. 
         FIG. 11  is a block diagram of the servo control loop to achieve precise positioning of the recording head. 
         FIG. 12  is flow chart of a triple push-pull process residing in the triple push-pull algorithm engine. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     In the following detailed discussion, specific component examples are used to describe an overall system concept. For example, a linear magnetic tape is used to represent a recording media, a series of equally spaced marks used to represent an optical track, two detector elements used to receive the reflectance from each optical spot, and the backside of the magnetic tape is used for the servo tracks. However, the invention described herein applies to any recording media, such as an optical disk or a magnetic disk, for an optical track defined by a solid line, whether one uses one detector per optical spot or multiple detectors per optical spot, and whether the servo tracks are on the magnetic recording side or on the back side of the magnetic tape. 
     Referring to  FIG. 1 , an exemplary linear tape system  10  includes a delivery system  12 , a read/write head assembly  14  and a pickup system  16 . The delivery system  12  houses a magnetic tape  18 . The magnetic tape  18  travels past a recording head  20  and an optical pickup system  22  contained in the read/write head assembly  14  and is delivered to the reel pickup system  16 . The recording head  20  reads and writes information, generally referred to as data, to the magnetic tape  18  as it travels from the delivery system  12  to the pickup system  16 . As the magnetic tape  18  passes over the recording head  20  the magnetic tape  18  may become misaligned with respect to the intended track position due to, for example, lateral tape motion (LTM). If left un-corrected, one is forced to use a recording track pitch much larger than the LTM. However, changes in lateral position can be detected by the optical pickup system  22  when servo tracks are engraved on the tape and be compensated via a closed servo control loop. 
     Referring to  FIG. 2 , a block diagram of the magnetic tape  18  using one optical servo track to control the read/write of multiple magnetic heads is shown. Referring to  FIG. 2A , four magnetic recording elements  26 ,  28 ,  30  and  32  are enclosed in the recording head  20  that read and write to five different magnetic tracks labeled  34   a-e ,  36   a-e ,  38   a-e  and  40   a-e  on the front side  42  of the magnetic tape  18 . Each of the four elements  26 ,  28 ,  30  and  32  write or read in parallel to an individual track  34 - 40  during one pass of the tape, to tracks  26   d ,  28   d ,  30   d  and  32   d  for example. 
     Referring to  FIG. 2B , a backside  44  (also referred to a non-magnetic side) of the magnetic tape  18  includes five optical tracks labeled  46   a ,  46   b ,  46   c ,  46   d  and  46   e.  Each of the optical tracks  46   a-   46   e  is permanently burned into the backside  44  of the magnetic tape  18 . 
     The optical pickup assembly  22  is aligned with magnetic recording element  34  in the recording head  20 . When a servo loop is closed, each of the five optical tracks  46   a-   46   e  is responsible for the read/write action of four magnetic tracks. In this way the five optical tracks multiplied by four read/write heads generates twenty magnetic tracks that span the tape. 
     The read/write head assembly  14  uses the optical pickup system  22  and the optical tracks  46  to detect position errors and compensate for effects of lateral tape movement (LTM), fully described below. 
     Referring to  FIG. 3 , the read/write head assembly  14  includes the recording head  20  and the optical pickup system  22 . An actuator  50  is shown connected to the read/write head assembly  14 . In operation, the magnetic tape  18  moves across the magnetic recording head  20  and a set of four data tracks (not shown) are recorded or read from the tape. In one example of a write, the front side  42  of the magnetic tape  18  receives data on its recording tracks  34   d ,  36   d ,  38   d  and  40   d (of  FIG. 2A ) from a series of recording channels  26 ,  28 ,  30  and  32  residing on the recording head  20 . The optical pickup system  22  utilizes a servo track  46   d  on the backside  44  of the magnetic tape  18  to detect LTM of the magnetic tape  18  along an axis  52 . Compensation is then done by positioning of the read/write head assembly  14  via movement of the actuator  50 . Any of the individual optical tracks  46   a - 46   e  is used one at a time for “track following” during a recording event. 
     Referring to  FIG. 4 , an optical pickup system  22  that focuses three spots on the recording medium  18  is shown. The present invention uses three optical spots to provide three servo push pull signals. The optical pickup system  22  includes a laser diode  60 , two segmented detectors  62   a  and  62   b , a hologram unit  64 , and a lens unit  66 . A divergent beam out of the laser diode  60  is focused on the tape  18  via the action of the lens unit  66 . The hologram unit  64  divides the single laser beam into multiple beams such that one can derive three beams  68   a-c  to focus on the tape  18 . The hologram unit  64  also allows the three beams  68   a-c  reflected off the tape to focus on the two segmented detectors  62   a  and  62   b.    
     Referring to  FIG. 5 , a block diagram of the laser and detectors are shown integrated on a silicon die  70 . The detectors  62   a  and  62   b  each contain three detector segments, identified as  62   a - 1 ,  62   a - 2 ,  62   a - 3 , and  62   b - 1 ,  62   b - 2 ,  62   b - 3 . The reflectance due to optical spot  80  (of  FIG. 4 ) would fall on detectors  62   a - 1 , and  62   b - 1 , the reflectance due to spot  82  (of  FIG. 4 ) would fall on  62   a - 2 , and  62   b - 2 , and the reflectance due to spot  84  (of  FIG. 4 ) would fall on  62   a - 3  and  62   b - 3 . The photocurrents from the corresponding -detector pair such as  62   a - 1  and  62   b - 1  may be combined to represent the reflectance from spot  80 . Likewise,  62   a - 2  and  62   b - 2  can be combined to represent the reflectance from spot  82 , and  62   a - 3  and  62   b - 3  can be combined to represent the reflectance from spot  84 . In an embodiment, these three reflectance signals are combined to produce signals used in this invention. 
     Referring to  FIG. 6 , a single servo track represented by a low of marks  90  on the backside  44  of magnetic tape  18  is shown. As the magnetic tape  18  streams by the optical pickup system  22 , each of the marks on row  90  passes under the laser spots  80 ,  82  and  84 . And detector pairs,  62   a - 1  and  62   b - 1 ,  62   a - 2  and  62   b - 2 , and  62   a - 3  and  62   b - 3  capture the reflectance from the three spots respectively. The row of marks  90  passes under the laser beam  60  at such a speed during magnetic tape movement so as to be seen, in effect, as a solid line by the detectors  62   a  and  62   b . When the laser  60  is perfectly aligned with the servo track  90 , the spot  82  will be totally immersed in the solid line caused by the movement of the row of marks  90 , while the spots  80  and  84  will be partially immersed in the solid line since they are spaced equal distance from the center spot  82 . In a preferred embodiment, the spacing is one third of a track pitch. When the reflectance from spots  80  and  84  are subtracted, a new signal is generated which is referred to as a push-pull signal. In this instance, when the laser is perfectly aligned with the servo track, the push pull value is zero. However, if the three spots are allowed to traverse through the servo track, then the push pull signal defined by spot  80  and  84  will generate a curve traditionally called an S-curve. 
     Referring to  FIG. 7 , a graph of individual reflectance curves  102 ,  104 , and  106  from the three spots as they traverse through the single optical track in  FIG.6  is shown, along with an S-curve  108  formed by spots  80  and  84 . The S-curve  108  does possess a limited linear dynamic range, extending less than one third of a track pitch. Such an S-curve  108  is used to allow the central spot  82  to stay on the center of the optical track. However, as shown below, the dynamic linear range is extended to cover the entire track pitch by using three pairs of push-pulls. 
     Referring to  FIG. 8  a graph is shown to illustrate a time progression as the three optical spots  80 ,  82 , and  84  traverse across a multitude of tracks  112 - 116 . Specifically, track  112  is represented by a row of marks in a solid track  118 , track  114  is represented by a row of marks in a solid track  120 , and track  116  is represented by a row of marks in a solid track  122 . As discussed above, each row of marks appears as a solid line, i.e., zone, as the magnetic tape streams past the recording head  20  and the optical pickup system  22 ; this solid line of marks is seen as the shaded solid tracks  118 ,  120  and  122 . 
     Each vertical group of three spots indicates a potential laser position relative to a track position, and thus a recording channel position, over time. The optical pickup system  22  samples and obtains a set of three spots as time passes. For example, at time  7 , the middle spot  82  is fully immersed in the row of marks representing track  114 , i.e., zone  120 , with the top spot  80  and bottom spot  84  only partially immersed in solid zone  120 . The detector pairs from the corresponding segments in  62   a  and  62   b  pick up the reflectance from the spots  80 ,  82 , and  84 . 
     The reflectance changes depending on the position of the spots, i.e., at time instant  7 , small amplitude is seen from the top spot  80  and bottom spot  84  and larger amplitude is seen from the middle spot  82 . 
     At sample time  6 , the bottom spot  84  is not immersed in zone  120 , while the central spot  82  is totally immersed in zone  120  and the top spot  80  is partially immersed in zone  120 . At sample time  8 , the bottom spot  84  is partially immersed in zone  120 , the center spot  82  is totally immersed in zone  120 , and the top spot  80  is not immersed in zone  120 . 
     As mentioned previously, a single S-curve from the push pull signal of one pair of optical spots provides limited useful position information of the laser relative to any given optical servo track. However,  FIG. 8  shows that three S-curves from three pairs of push pulls allow one to derive position information within any position of any given track. The reflectance from an optical spot that is saturated at its maximum or minimum amplitude is discarded, leaving two optical spots having reflectance with the largest amplitude gradient. The difference of these reflectance constitute linear portion of the S-curve in an individual zone. As will be seen below, six such zones provide useful position information within the entire track. 
     Referring to  FIG. 9 , a graph  130  of S-Curves derived from the repeating optical zones tri of  FIG. 8  includes spot reflectances  132 ,  134  and  136  as dashed lines. A set of three pairs of optical reflectance generates the three S-curves  138 - 142  shown in bold in FIG.  9 . The linear sections  144 ,  146 ,  148 ,  150  and  152  correspond to useful position information within any individual zone. At any given time period, a triple push-pull engine calculates a difference from the two signals in a three-signal group having the greatest amplitude gradient. Transitions from one difference curve to another are accomplished via a merging, i.e., blending, process where the S-curves intersect. The three S-curves and the transition between them form a basis of a triple push pull method. The blending of one S-curve to another uses a weighted average algorithm. 
     Referring to  FIG. 10 , a graph  150  rendering a piecewise linear position  152  is shown. The graph  150  is obtained from the linear portion of the S-curve ( 144  for example) within any zone. The S-curves  138 - 142  of  FIG. 9  are inverted and shifted to generate the linear position  152  estimates for the entire offset range at a given point in time. Once the offset is obtained from the linear position  152 , the offset is fed to a servo loop for appropriate actuator actions. 
     Referring to  FIG. 11 , a block diagram of the servo control loop system  160  is shown as a closed loop control system that includes a compensation engine  162 , an actuator control  164 , the optical pickup system  22 , a demodulator circuit  166  and a triple push-pull engine  168 . The calculated error between the actual measured position and the desired tracking position is fed to the compensation engine  162 . The compensation engine  162  compromises the response between the mechanical actuator  164  and an electronic system, and delivers an actuator command based on the compensated position error. The actuator control  164  actuates the actuator  50  in response to the error command and moves the read/write head assembly  14 . The optical pickup system  22  picks up updated values of reflectance from the magnetic tape  18 , and sends them to the demodulator  166 . The function of the demodulator  166  is to convert the sampled reflectance signals due to the series of marks into a continuous analog signal. The three analog signals are then sent to the triple push-pull engine  168  to generate a new position estimate, and so the loop goes on until the actuator converges to the desired position. 
     Referring to  FIG. 12 , a triple push-pull process  200  residing in the triple push pull engine  76  includes receiving  202  a set of three digitized reflectance values from the optical servo track, and generating  204  amplitudes for two spots having the greatest amplitude gradient. The amplitudes are used to generate  206  S-curves. A linear position estimate is generated  208  by inverting and shifting the s-curves as required. The linear position estimate is used  210  to align the magnetic recording head to the linear magnetic tape. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.