Patent Publication Number: US-2005133660-A1

Title: Method and apparatus for guiding a moving tape

Description:
BACKGROUND  
      Information is recorded on and read from a moving magnetic tape with a magnetic read/write head positioned next to the tape. The magnetic “head” may be a single head or, as is common, a series of read/write head elements stacked individually and/or in pairs within the head unit. Data is recorded in tracks on the tape by moving the tape lengthwise past the head. The head elements are selectively activated by electric currents representing the information to be recorded on the tape. The information is read from the tape by moving the tape longitudinally past the head elements. Magnetic flux patterns on the tape create electric signals in the head elements as the tape moves along. These signals represent the information stored on the tape.  
      Data is recorded on or read from each of the parallel tracks on the tape by positioning the head elements at different locations across the tape. Head elements are moved from track to track, as necessary, either to record or to read the desired information. A head position actuator operatively coupled to servo control circuitry controls movement of the head according to servo information recorded on the tape. A tape drive usually includes head positioning actuators. A head positioning actuator often includes a lead screw driven by a stepper motor, a voice coil motor, or a combination of both. The head is supported by a carriage that is driven by the actuator along a path perpendicular to the direction of tape travel. The head elements are positioned as close to the center of a track as possible based upon the servo information.  
      Servo circuitry is better able to position a head properly with respect to a tape if the lateral position of the tape is suitably restricted. Tape guides with flanges often are used to restrict the position of the tape. Flanges, however, can cause excessive wear on the edge of the tape. Conversely, the sharp edges of the tape can, over time, cause excessive wear on the flange, itself. The tape sometimes curls at the edges when it touches the flange. This curling further destabilizes the lateral position of the tape.  
      As the speed of tape drives continues to increase, another factor has been noted that contributes to lateral tape motion. This factor is a “ground-effect” that results from a film of air that can form between the tape and the guide. This film of air acts to decrease the friction between the tape and the guide. The tape then tends to float and to wobble laterally. In some cases, the reduction in friction even causes the tape to ripple across the lateral dimension of the tape.  
     SUMMARY  
      Method and apparatus for guiding a moving tape having a tape edge parallel to a direction of motion of the tape wherein the tape is received tangentially on a curved surface having an edge. Force applied to the tape is increased as the tape drifts farther away from a nominal position so as to move the tape away from the edge of the surface. Lateral motion of the tape is dampened by breaking up an air-cushion between the tape and the curved surface.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Several alternative embodiments will hereinafter be described in conjunction with the appended drawings and figures, wherein like numerals denote like elements, and in which:  
       FIG. 1  is a flow diagram that summarizes a representative embodiment of a method for guiding a moving tape;  
       FIG. 1A  is a diagram of a mathematical model of one representative embodiment of a corner of a tape guide;  
       FIG. 1B  is a diagram of a mathematical model of an alternative embodiment of a corner of a tape guide;  
       FIG. 2  is a pictorial diagram of an exemplary embodiment of a tape guide;  
       FIG. 2A  is a diagram showing detail of an exemplary embodiment of a dampening apparatus; and  
       FIG. 3  is an edge view of a representative embodiment of tape roller guide.  
    
    
     DETAILED DESCRIPTION  
       FIG. 1  is a flow diagram that summarizes a representative embodiment of a method for guiding a moving tape. The tape in this embodiment has an edge parallel to the direction of motion of the tape. According to the present method, the tape is received tangentially on a curved surface (step  5 ). A force is applied to the tape edge to counter drift as the tape drifts from a nominal position (step  10 ). The applied force increases as the tape drifts farther away from its nominal position. The present method still further comprises dampening the lateral motion of the tape by breaking up an air-cushion that can form between the tape and the curved surface (step  15 ). Lateral motion of the tape is motion of the tape in a direction perpendicular to the direction of motion of the tape as the tape moves over the curved surface.  
       FIG. 1A  is a diagram of a mathematical model of one representative embodiment of a corner of a tape guide. According to one alternative variation of the present method, a force is applied to the tape edge that increases approximately linearly with the distance of the tape from its nominal position. According to another alternative embodiment, a surface  25  of a tape guide is represented by an x-axis  27 . A flange  30  disposed at the edge of the surface  25  of the tape guide is represented by a y-axis  32 . A linear transition  35  joins the surface  25  of the tape guide to the flange  30 . According to another alternative embodiment, a linear transition  35  results when the corner between the flange  30  and the surface  25  of the tape guide comprises a chamfer. Mathematical equation 37 
   y=d−x    (37)  
 represents the straight line corresponding to the linear transition  35 . When a magnetic tape moves to a position  40  where the edge of the tape has left the surface  25  of the tape guide and has begun to ride up on the linear transition  35 , mathematical equation 37 indicates to what level of elevation the edge of the tape will rise above the surface  25  of the tape guide. If the edge of the tape is more than a distance d  60  from the edge of the flange  30 , then the tape does not rise up at all. For distances less than d  60  from the edge of the flange, then mathematical equation 37 applies. 
 
      According to one embodiment, a flange is disposed on the edge of the curved surface. The position of the flange is a convenient reference point for defining the location of the edge of the tape. For example, with the tape in position  40 , the edge is a distance x 1   46  from the flange. Mathematical equation 37 states that the edge of the tape rises to an elevation 
 
 y   1 = d−x   1  
 
 (y 1  has reference designator  48  in  FIG. 1A ) above the surface  25  of the tape guide. The tape in a tape drive normally is kept under tension, so when the tape edge rides up on a transition like the linear transition  35  in this example, the tape stretches slightly. This stretch causes a slight increase in the tension of the tape that produces a reaction force f R    41  that acts downward toward the surface  25 . As the edge of the tape rides upward along the linear transition  35 , the edge experiences two force components ( 42 ,  44 ) that are applied at right angles to each other at the edge of the tape. The lengths of the arrows representing forces ( 42 ,  44 ) in  FIG. 1A  are not intended to be proportional to the force represented. The horizontal component  44  of the force at the edge of the tape tends to direct the tape back into its proper position as the reaction force f R    41  acts to drive the tape back against the surface  25 . In the case of the horizontal force component  44 , the tape has not moved very far from its proper position, so only a small force is applied to the edge of the tape. 
 
      When the tape moves to a position  50  farther from its nominal position than position  40 , then the same considerations apply. The edge of the tape in this example is a distance x 2   56  from the flange  30  where x 2  is less than x 1 . Mathematical equation 37 states that the edge of the tape now rises to an elevation 
 
 y   2 = d−x   2  
 
 (y 2  has reference designator  58  in  FIG. 1A ) above the surface  25  of the tape guide. Elevation y 2   58  is greater than y 1   48 , so the tape undergoes a stretch greater than the stretch corresponding to y 1   48 . Accordingly, the increase in the tension in the tape is greater, and the forces ( 52 ,  54 ) applied to the edge of the tape are greater than the forces ( 42 ,  44 ) corresponding to y 1 . The horizontal component  54  of the force applied to the edge of the tape tends to direct the tape more forcefully toward its proper position than was the case corresponding to y 1   48 . 
 
       FIG. 1B  is a diagram of a mathematical model of an alternative embodiment of a corner of a tape guide. According to this alternative embodiment, a curved transition  65  joins the surface  25  of the tape guide to the flange  30 . One example embodiment of a curved transition comprises a circular arc  65  defined by the mathematical equation 67 
   y=d−{square root}{square root over (d     2     −(x−d)     2     )}.    (67)  
 Mathematical equation 67 indicates to what level of elevation the edge of the tape will rise above the surface  25  of the tape guide according to this example embodiment. As was true for the linear transition, if the edge of the tape is more than a distance d  60  from the edge of the flange  30 , then the tape does not rise up at all. For distances less than d  60  from the edge of the flange, then mathematical equation 67 applies. 
 
      The circular arc  65  tends to provide more gentle treatment for an out of position tape than does the linear transition  35  shown dotted in  FIG. 1B  for convenience. For example, with the tape in position  70 , the edge is a distance x 1   76  from the flange. Mathematical equation 67 states that the edge of the tape rises to an elevation 
 
 y   1 = d−{square root}{square root over (d     2     −(x 1 −d)     2     )} 
 
 (y 1  has reference designator  78  in  FIG. 1B ) above the surface  25  of the tape guide, less than the rise in elevation corresponding to linear transition  35 . Consequently, the increase in tension of the tape is less than the increase obtained with the linear transition  35 . This results in less reaction force f R    41  acting downward toward the surface  25 . The resulting forces ( 72 ,  74 ) applied to the edge of the tape with the curved transition are correspondingly less than the forces ( 42 ,  44 ) applied with the linear transition  35 . 
 
      When the tape moves to a position  80  farther from its nominal position than position  70 , then the same considerations apply. The edge of the tape in this example is a distance x 2   86  from the flange  30  where x 2   86  is less than x 1   76 . Mathematical equation  67  states that the edge of the tape now rises to an elevation 
 
 y   2 = d−d   2 −{square root}{square root over (( x   2 − d ) 2 )}  (67) 
 
 (y 2  has reference designator  88  in  FIG. 1B ) above the surface  25  of the tape guide. Elevation y 2   88  is greater than y 1   78 . Therefore, the tape undergoes a stretch greater than the stretch corresponding to y 1   78 . Accordingly, increase in the tension in the tape is greater, and the forces ( 82 ,  84 ) applied to the edge of the tape are greater than the forces ( 72 ,  74 ) corresponding to y 1   78 . The horizontal component  74  of the force applied to the edge of the tape tends to direct the tape more forcefully toward its proper position than was the case corresponding to y 1   78 . The force  74  corresponding to the circular arc  65  is less than the force  44  corresponding to the linear transition  35 , again demonstrating the more gentle treatment of the tape with the circular arc  65 . 
 
      Other types of transitions besides the linear transition  35  and the circular arc  65  are possible. The examples presented here are only for illustration and should not be interpreted as an intention to limit the scope of the appended claims. For example, transitions combining both straight and curved portions are contemplated. Further, the transition could comprise multiple straight sections or curvatures with increasing or decreasing degrees of slope or curvature, respectively.  
      As already described, a film of air (i.e. air-cushion) can form between the tape and the curved surface in some embodiments of high-speed tape drives. Consequently, the tape may have a tendency to “float” above the curved surface. This floating effect reduces the friction between the curved surface and the tape thereby allowing lateral motion of the tape. The present method dampens lateral motion of the tape by breaking up air-cushion between the tape and the curved surface. According to one example embodiment, this breaking up of the air-cushion is accomplished by directing or channeling air away from the surface of the tape guide at a plurality of locations. According to one alternative embodiment, the number of locations is two. According to another alternative embodiment, the number of locations is three. According to yet another alternative embodiment, the number of locations is four. According to still one more alternative embodiment, the number of locations is five. These locations and corresponding structure are discussed in more detail with reference to  FIGS. 2 and 3 .  
       FIG. 2  is a pictorial sectional diagram of an exemplary embodiment of a tape guide. This embodiment of a tape guide is capable of guiding a moving tape. This embodiment comprises a curved surface  125  that is capable of tangentially receiving a moving tape. The embodiment further comprises restrictors capable of restricting the position of the tape on the curved surface. One example of restrictors comprises a first flange  131  and a second flange  132  disposed on opposite ends of the curved surface. The flanges ( 131 ,  132 ) form nominal right angles with the curved surface  125 . The flanges ( 131 ,  132 ) operate to impede the lateral motion of a tape moving in a direction  135  parallel to the flanges ( 131 ,  132 ).  
       FIG. 2A  is a diagram showing detail of an exemplary embodiment of a dampening apparatus. This detail is a close-up view of the corner that defines the intersection between the curved surface  125  and the first flange  131  described in the discussion of  FIG. 2 . The corner is not square, but, rather, has the shape of a curved transition  150  capable of applying a force to an edge  155  of a tape  160  as the tape moves away from its nominal position and approaches first flange  131 . As the tape moves farther away from its nominal position and farther up the transition  150 , the force applied to the tape increases. It should be understood that a substantially similar corner defines the intersection between the curved surface and second flange  132 .  
      As described in the discussion of  FIG. 1A  and  FIG. 1B , other types of curved transitions  150  are available. One embodiment of the tape guide comprises a linear transition or “chamfer.” The linear transition operates as described in the discussion of  FIG. 1A . Another embodiment of the tape guide comprises a circular arc. The circular arc operates as described in the discussion of  FIG. 1B . Other types of curved transitions are possible as discussed above in connection with  FIGS. 1, 1A  and  1 B. The linear and circular examples presented herein should not be interpreted as a limitation on the appended claims.  
       FIG. 2  further illustrates that one alternative embodiment further comprises friction enhancers between the tape and the curved surface. Friction enhancers operate in one embodiment by partitioning an air-cushion layer that can form between the tape and the curved surface. According to one alternative embodiment, friction enhancers comprise grooves  140  disposed in the curved surface  125  in a direction  135  nominally parallel to the direction of motion of the tape. According to one particular alternative embodiment, the grooves  140  are V-shaped and act to channel air away from the curved surface at three locations. According to another alternative embodiment, two grooves are provided. According to yet another alternative embodiment, four grooves are provided. Still one more alternative embodiment of the tape guide comprises five grooves disposed in the curved surface  125 .  
       FIG. 3  is an edge view of a representative embodiment of tape roller guide. This representative embodiment is capable of guiding a moving tape. The present embodiment of the tape roller guide  200  comprises a hub  205 . The hub  205  has a cylindrical curved surface  210 . The cylindrical curved surface  210  is capable of tangentially receiving a tape. The embodiment further comprises first and second flanges ( 230 ,  232 ). Flanges ( 230 ,  232 ) function as range restrictors capable of restricting the position of the tape edge on the cylindrical curved surface  210  of the hub  205 .  
      The present embodiment further comprises dampeners  250  (depicted in  FIGS. 1A, 1B  and  2 A), said dampeners  250  comprising corners that define the intersection of the cylindrical curved surface  210  and the flanges ( 230 ,  232 ). The corners are configured to apply progressively more force to an edge of the tape as the edge of the tape moves through a corner toward a flange. One example embodiment of the dampeners  250  is substantially identical to the transition  150  described in the discussion of  FIGS. 1B and 2A . In another alternative embodiment of the dampeners, the transition  150  comprises a circular or rounded corner formed with a radius substantially in the range of 0.03 mm to 0.5 mm. This exemplary range is provided to illustrate, but not limit the scope of the appended claims. In yet another alternative embodiment of the dampeners, each transition  250  comprises a chamfer disposed substantially as described in the discussion of  FIG. 1A . As already described, other curved transitions  250  are possible, and the examples presented herein are not intended to limit the scope of the appended claims.  
      The first flange  230  extends out from a first end  207  of the hub. Likewise, the second flange  232  extends out from a second end  208  of the hub opposite the first end  207 . Together, first and second flanges ( 230 ,  232 ) act to restrict the position of the tape edge on the cylindrical curved surface of the hub.  
      According to one embodiment, the tape guide roller  200  further comprises a plurality of grooves  240  disposed in the curved surface  210  of the hub  205 . The grooves  240  act as friction enhancers. The grooves  240  act to enhance friction between the tape and the curved surface  210 . According to one alternative embodiment, grooves  240  are V-shaped having a width substantially in the range of 0.2 mm to 0.6 m and a depth substantially in the range of 0.1 mm to 0.3 mm. One alternative embodiment of the tape guide roller comprises two grooves. Another alternative embodiment comprises three grooves. Yet another alternative embodiment comprises four grooves. Still one more embodiment of the tape guide roller comprises five grooves. Again, any ranges stated herein are for the purposes of illustration and are not intended to limit the scope of the appended claims.  
      While the present method, tape guide, and tape guide roller have been described in terms of several alternative methods and exemplary embodiments, it is contemplated that alternatives, modifications, permutations, and equivalents thereof will become apparent to those skilled in the art upon a reading of the specification and study of the drawings. It is therefore intended that the true spirit and scope of the appended claims include all such alternatives, modifications, permutations, and equivalents. One such variation would include the introduction of spiral grooves or other geometrically shaped grooves. Such grooves are intended to be included in the scope of the appended claims.