Abstract:
Apparatus for controlling the position of a plunge mechanism which is mounted to a cartridge engaging assembly is disclosed herein. The apparatus may be embodied in a cartridge engaging assembly having a plunge mechanism that is moveable between a retracted position and an extended position. The plunge mechanism is provided with a reference mark that moves with the plunge mechanism. A detector is mounted to the cartridge engaging assembly, wherein the detector senses an image of the reference mark and produces an output signal related thereto. A control system is operatively associated with the detector and is responsive to the output signal produced thereby. The control system is also operatively associated with the plunge mechanism so that it regulates the position of the plunge mechanism based on the output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuation of application Ser. No. 09/799,316 filed on Mar. 5, 2001 U.S. Pat. No. 6,661,748, which is hereby incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The invention pertains to the field of cartridge engaging assemblies (i.e., “pickers”) of the type used in multiple cartridge data storage systems, and more specifically, to a method and apparatus for controlling the position of a plunge mechanism which is mounted to a cartridge engaging assembly. 
   BACKGROUND OF THE INVENTION 
   There are many types of data storage systems available today. Some of these systems store data cartridges at known locations, and retrieve desired data cartridges from the known locations so that data may be written to or read from the data cartridges. Such data storage systems are often referred to as “juke box” data storage systems, particularly if they can accommodate a large number of data cartridges. 
   A typical juke box data storage system may include one or more different types of cartridge receiving devices for holding the various data cartridges. For example, one type of cartridge receiving device may comprise a cartridge storage rack or “magazine”, while another type of cartridge receiving device may comprise a cartridge read/write device or “drive”. The cartridge storage racks or magazines serve to provide storage locations for the data cartridges and are often arranged so that they form one or more vertical stacks, although other configurations are possible. The cartridge read/write device may be located at any convenient location within the data storage system. 
   The data storage system may also be provided with a movable cartridge engaging assembly or “picker” for transporting the data cartridges between the various cartridge receiving devices (e.g., between the cartridge storage racks and the cartridge read/write devices). A typical cartridge engaging assembly may also be provided with a plunge mechanism or “thumb assembly” for engaging the various data cartridges contained in the cartridge receiving devices and for drawing them into the cartridge engaging assembly. A positioning system associated with the cartridge engaging assembly may be used to move the cartridge engaging assembly between the various cartridge receiving devices. 
   Data storage systems of the type described above are usually connected to a host computer system which may be used to access or store data on the data cartridges. For example, if the host computer system issues a request for data contained on a particular data cartridge, a control system associated with the data storage system will actuate the positioning system to move the cartridge engaging assembly along the cartridge storage racks until the cartridge engaging assembly is positioned adjacent the desired data cartridge. The plunge mechanism associated with the cartridge engaging assembly may then extend to remove the data cartridge from its cartridge storage rack, and then retract to draw the cartridge into the cartridge engaging assembly. The positioning system may then be actuated to move the cartridge engaging assembly to an appropriate cartridge read/write device. Once properly positioned adjacent the cartridge read/write device, the plunge mechanism may extend to insert the selected data cartridge into the cartridge read/write device so that the host computer may thereafter read data from or write data to the data cartridge. After the read/write operation is complete, the plunge mechanism may be actuated to remove the data cartridge from the cartridge read/write device. The cartridge engaging assembly may thereafter return the data cartridge to its cartridge storage rack. 
   A typical plunge mechanism is usually slidably mounted to the cartridge engaging assembly and is provided with a drive system so that the plunge mechanism may be moved toward and away from a cartridge access end of the cartridge engaging assembly (i.e., to an extended or retracted position). For example, if it is desired to retrieve a data cartridge from a cartridge receiving device, the drive system moves the plunge mechanism toward the cartridge access end of the cartridge engaging assembly so that the plunge mechanism may engage (or grab) the data cartridge. The drive system then retracts the plunge mechanism and pulls the engaged data cartridge into the cartridge engaging assembly. If it is desired to load the data cartridge into the cartridge receiving device, the drive system then moves the plunge mechanism and data cartridge toward the cartridge access end of the cartridge engaging assembly, thus inserting the data cartridge into the cartridge receiving device. 
   Typically, the position of the plunge mechanism must be controlled within fairly tight tolerances, especially when the plunge mechanism is moved to its extended position. Otherwise, the plunge mechanism may not be able to adequately engage a data cartridge, or the plunge mechanism may fail to seat a data cartridge in a cartridge receiving device. One method of controlling the position of a plunge mechanism is disclosed in U.S. Pat. No. 6,104,693, entitled “Mounting System for Cartridge Plunge Mechanism”, which is hereby incorporated by reference for all that it discloses. The method disclosed therein uses a detector assembly to read a plurality of index marks on an elongate linear reference member. The detector assembly is mounted to the body of a plunge mechanism, and the reference member is mounted to the frame of the cartridge engaging assembly. In this manner, movement of the plunge mechanism causes the detector assembly to pass by the plurality of index marks on the reference member. The position of the plunge mechanism may therefore be controlled, for example, by counting the number of index marks which the plunge mechanism passes, and controlling the drive system for the plunge mechanism in response thereto. 
   SUMMARY OF THE INVENTION 
   New methods and apparatus for controlling the position of a plunge mechanism which is mounted to a cartridge engaging assembly are disclosed herein. 
   In one embodiment, a cartridge engaging assembly comprises a plunge mechanism which is mounted to the cartridge engaging assembly. The plunge mechanism is movable between a retracted position and an extended position. A reference mark is provided on the plunge mechanism so that the reference mark moves with the plunge mechanism. A detector is also mounted to the cartridge engaging assembly. As the plunge mechanism is extended, the detector detects the reference mark and produces an output signal which is related thereto. A control system is operatively associated with the detector and is responsive to the output signal produced thereby. The control system is also operatively associated with the plunge mechanism. As a result, the control system can regulate the position of the plunge mechanism based on the output signal produced by the detector. 
   In another embodiment, a method for regulating a position of a plunge mechanism which is associated with a cartridge engaging assembly comprises 1) providing a reference mark on the plunge mechanism, 2) detecting a size of the reference mark on the plunge mechanism, and 3) determining the position of the plunge mechanism based on the size of the detected reference mark. 
   In another embodiment, a method for moving a plunge mechanism to an extended position within a cartridge engaging assembly comprises initiating movement of the plunge mechanism from a retracted toward an extended position in accordance with an open loop control schedule. A detector is monitored to determine when a reference mark provided on the plunge mechanism is detected. Control of the movement of the plunge mechanism is then switched to a closed loop control schedule when the detector detects the reference mark provided on the plunge mechanism. 
   In yet another embodiment, a method for moving a plunge mechanism to an extended position within a cartridge engaging assembly comprises initiating movement of the plunge mechanism from a retracted toward an extended position in accordance with an open loop control schedule. After the plunge mechanism is moved for a predetermined distance, control of the movement of the plunge mechanism is switched to a closed loop control schedule, and an output signal of a detector which detects a reference mark provided on the plunge mechanism is monitored. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Illustrative and presently preferred embodiments of the invention are illustrated in the drawings, in which: 
       FIG. 1  is a plan view of a cartridge engaging assembly as it may be used in a data storage system to access and transport data cartridges contained within the data storage system; 
       FIG. 2  is a first perspective view of the cartridge engaging assembly shown in  FIG. 1 ; 
       FIG. 3  is a second perspective view of the cartridge engaging assembly shown in  FIG. 2 , wherein a side member is removed to show the arrangement of a rack and pinion drive system for moving the assembly&#39;s plunge mechanism; 
       FIG. 4  is a third perspective view of the cartridge engaging assembly shown in  FIG. 2 , wherein a side member is removed to show the arrangement of a guide track for the assembly&#39;s plunge mechanism and finger assembly; 
       FIG. 5  is a simplified rear elevational view of the cartridge engaging assembly shown in  FIG. 2 , wherein the assembly&#39;s frame is broken away to reveal the rear of the assembly&#39;s plunge mechanism; 
       FIGS. 6   a  &amp;  6   b  are simplified side elevational views of the cartridge engaging assembly shown in  FIG. 2 , wherein  FIG. 6   a  shows the assembly&#39;s plunge mechanism in its retracted position, and wherein  FIG. 6   b  shows the assembly&#39;s plunge mechanism in its extended position; 
       FIG. 7  is an enlarged cross-sectional, elevational view of the  FIG. 2  plunge mechanism and its guide rail assembly, wherein the engagement of the bearing members of first and second flange members with various guide rails is more clearly shown; 
       FIG. 8  is an enlarged side view of the first flange member of the  FIG. 2  plunge mechanism; 
       FIG. 9  is an enlarged side view of the second, C-shaped flange member of the  FIG. 2  plunge mechanism; 
       FIG. 10  is an illustration of how the reference mark provided on the  FIG. 2  plunge mechanism is detected by a detector; 
       FIG. 11  is a first exemplary embodiment of a reference mark which may be provided on the  FIG. 2  plunge mechanism; 
       FIG. 12  is a second exemplary embodiment of a reference mark which may be provided on the  FIG. 2  plunge mechanism; and 
       FIG. 13  is a block diagram illustrating a control system&#39;s response to the detector and plunge mechanism shown in FIG.  4 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A cartridge engaging assembly or “picker”  10  is shown in  FIG. 1  as it could be used in a “juke box” data storage system  12  to transfer data cartridges  14  between one or more cartridge receiving devices, such as one or more cartridge storage racks or “magazines”  16 , and one or more cartridge read/write devices or “drives”  18 . The various cartridge receiving devices (e.g., the cartridge storage racks  16  and the cartridge read/write devices  18 ) may be positioned at various locations within the data storage system  12  so that they define a generally U-shaped configuration, as best seen in FIG.  1 . However, the cartridge engaging assembly  10  may be used in any of a wide range of other types of data storage systems. For example, the cartridge engaging assembly  10  may also be utilized in a data storage system of the type shown and described in U.S. Pat. No. 5,596,556, entitled “Linear Displacement and Support Apparatus for use in a Cartridge Handling System”, which is hereby incorporated by reference for all that it discloses. 
   The cartridge engaging assembly  10  is adapted to 1) engage a data cartridge  14  contained in a cartridge receiving device  16 ,  18 , and then 2) draw the data cartridge  14  into the cartridge engaging assembly for transport to a different cartridge receiving device  16 ,  18 . Movement of the cartridge engaging assembly  10  may be achieved via an actuator system  36  which moves the cartridge engaging assembly  10  along a U-shaped path of a cartridge positioning system  22 . 
   Referring primarily to  FIGS. 2-4 , the cartridge engaging assembly  10  may comprise a frame assembly  38  that defines a chamber or cavity  44  sized to receive the data cartridge  14 . A plunge mechanism or “thumb assembly”  40  is slidably mounted to the frame assembly  38  so that the plunge mechanism  40  may be moved toward and away from the cartridge access end  46  of the frame assembly  38 , generally in the directions indicated by arrows  54  and  55 , respectively (FIG.  3 ). More specifically, the plunge mechanism  40  may be moved between a retracted position (shown in  FIGS. 3-5  and  6 ( a )) and an extended position (shown in FIG.  6 ( b )). 
   In one preferred embodiment, the mounting system used to slidably mount the plunge mechanism  40  to the frame assembly  38  comprises a first guide rail  79  positioned on a first side member  70  of the frame assembly  38 , and a second guide rail  76  positioned on a second side member  72  of the frame assembly  38  (FIGS.  3  and  4 ). The first guide rail  79  comprises a first horizontal surface  80 , and the second guide rail  76  comprises a second horizontal surface  88 . The plunge mechanism  40  comprises a first flange member  78  for slidably engaging the first horizontal surface  80  of the first guide rail  79 , and a second, C-shaped flange member  84  for slidably engaging the second horizontal surface  88  of the second guide rail  76 . The mounting system guides the plunge mechanism  40  along a horizontal plane and allows it to be moved toward and away from the cartridge access end  46  of the frame assembly  38 , generally in the directions of arrows  54  and  55 , respectively. 
   A rack and pinion drive assembly  42  is used to move the plunge mechanism  40  back and forth between its retracted position and the extended position. Essentially, the rack and pinion drive assembly  42  may comprise a gear rack  48  mounted to the first side member  70  of the frame assembly  38 , and a pinion gear  50  mounted to the plunge mechanism  40 . The pinion gear  50  is mounted to the plunge mechanism  40  so that the pinion gear  50  engages the gear rack  48 , as best seen in  FIGS. 3 ,  6 ( a ) and  6 ( b ). A drive system  52  connected to the pinion gear  50  rotates the pinion gear  50  to move the plunge mechanism  40  toward and away from the cartridge access end  46  of the frame assembly  38  (i.e., generally in the directions indicated by arrows  54  and  55 , respectively). 
   The engagement of the pinion gear  50  with the gear rack  48  defines a vertical guide surface or plane  97 , as best seen in FIG.  7 . This vertical guide surface or plane  97 , together with the second or C-shaped flange member  84  on the plunge mechanism  40  and the second side member  72 , limits the side-to-side movement (indicated by arrows  21 ) of the plunge mechanism  40 . 
   The plunge mechanism  40  may also be provided with a finger assembly  56  which is configured to engage a notch  15  or other similar feature provided on a data cartridge  14 , as best seen in  FIG. 6   b . The finger assembly  56  allows the plunge mechanism  40  to engage and withdraw a data cartridge  14  from a cartridge receiving device  16 ,  18 . In one preferred embodiment, the finger assembly  56  is actuated by a guide track  58  in the frame assembly  38 , as best seen in FIG.  4 . 
   The cartridge engaging assembly  10  may also be provided with a position control system  62  for providing an output signal from which information may be derived about the position of the plunge mechanism  40  in the frame assembly  38 . The position control system  62  comprises a reference mark  66  which is applied to the plunge mechanism  40  so that it moves with the plunge mechanism  40  (see  FIGS. 5 ,  6 ( a ) and  6 ( b )). A detector  68  mounted to the frame assembly  38  detects the reference mark  66  on the plunge mechanism  40  and produces an output signal which is indicative of the size of the reference mark  66 . The size of the reference mark may then be used by a picker control system  64  to determine the position of the plunge mechanism  40 . The picker control system  64  may then control the drive system  52  to precisely move the plunge mechanism  40  to a desired position. 
   The cartridge engaging assembly  10  may be operated as follows to retrieve a data cartridge  14  from a selected cartridge receiving device (e.g., either the cartridge storage rack  16  or the cartridge read/write device  18 , as the case may be). Assuming that the cartridge positioning system  22  ( FIG. 1 ) has positioned the cartridge engaging assembly  10  adjacent the selected data cartridge  14 , the picker control system  64  may actuate the drive system  52  to rotate the pinion gear  50  so as to move the plunge mechanism  40  toward the cartridge access end  46  of the frame assembly  38 . That is, the drive system  52  moves the plunge mechanism  40  in the direction of arrow  54 . As the plunge mechanism  40  approaches the data cartridge  14 , the finger assembly  56  engages the notch  15  or other similar feature on the data cartridge  14  (FIG.  6 ( b )). By detecting the reference mark  66  as the plunge mechanism  40  approaches the data cartridge  14 , the position control system  62  may be used to precisely position the plunge mechanism  40  via the drive system  52 . The arrangement of the plunge mechanism  40  and finger assembly  56  is such that the finger assembly  56  engages the notch  15  on the data cartridge  14  when the plunge mechanism  40  is at or near its fully extended position. This condition is illustrated in  FIG. 6   b . After the data cartridge  14  has been engaged by the finger assembly  56 , the picker control system  64  may operate the drive system  52  to reverse the direction of rotation of the pinion gear  50 . This causes the plunge mechanism  40 , along with the engaged cartridge  14 , to move in the opposite direction (i.e., in the direction of arrow  55  (FIG.  3 )), thereby retracting the plunge mechanism  40  and data cartridge  14  into the cavity  44  defined by the frame assembly  38 . The drive system  52  continues to retract the plunge mechanism  40  and engaged data cartridge  14  until the data cartridge  14  is drawn into the cavity  44  of the frame assembly  38  by an amount sufficient to allow the positioning system  22  ( FIG. 1 ) to move the cartridge engaging assembly  10  to another location. The positioning system  22  may thereafter move the cartridge engaging assembly  10  to a new location. 
   Once the cartridge engaging assembly  10  has been properly positioned adjacent the new location, the picker control system  64  may again actuate the drive system  52  to move (i.e., extend) the plunge mechanism  40  in the direction of arrow  54 . As the plunge mechanism  40  moves toward the cartridge access end  46  of the frame assembly  38 , surface  60  pushes against the data cartridge  14 , thereby pushing the data cartridge  14  into the selected cartridge receiving device (e.g., the cartridge storage rack  16  or the cartridge read/write device  18 , as the case may be). At the same time, the guide track  58  actuates the finger assembly  56  to disengage the finger assembly  56  from the notch  15  provided in the data cartridge  14 . If the plunge mechanism  40  is appropriately extended, the data cartridge  14  will be inserted fully into a cartridge receiving device  16 ,  18  so that it properly “seats” in the cartridge receiving device  14 ,  18 . Appropriate extension of the plunge mechanism  40  is once again controlled by the position control system  62 . After insertion of the data cartridge  14  into a cartridge receiving device  16 ,  18 , the picker control system  64  may actuate the drive system  52  once again, to thereby retract the plunge mechanism  40  back into the cartridge engaging assembly  10 . The cartridge engaging assembly  20  is then ready to withdraw another data cartridge  14  and transport it to a new location. 
   Having briefly described the cartridge engaging assembly  10 , as well as some of its more significant features and advantages, the cartridge engaging assembly  10  will now be described in detail. However, before proceeding with the description, it should be noted that the cartridge engaging assembly  10  may be utilized in any of a wide range of data storage systems now known in the art, or that may be developed in the future, for storing and accessing one or more data cartridges. Accordingly, the present invention should not be regarded as being limited to the particular data storage system  12  shown and described herein. It should also be noted that while the cartridge engaging assembly  10  is shown and described herein as it could be used to store and retrieve a DLT (digital linear tape) data cartridge  14  having a standard size and configuration, it is not limited to any particular type or style of data cartridge. Indeed, the cartridge engaging assembly  10  could be used with any type of data storage device comprising any type of data storage medium (e.g., LTO cartridges, magnetic discs or tapes, optical discs, etc.). Consequently, the cartridge engaging assembly  10  should not be regarded as limited to use with the DLT type of data cartridge  14  shown and described herein. 
   With the foregoing considerations in mind, one embodiment of a cartridge engaging assembly or “picker”  10  is shown in  FIG. 1  as it could be used in a “juke box” data storage system  12  to transfer data cartridges  14  between one or more cartridge receiving devices, such as one or more cartridge storage racks or “magazines”  16 , and one or more cartridge read/write devices or “drives”  18 . The various cartridge receiving devices (e.g., the cartridge storage racks  16  and the cartridge read/write devices  18 ) may be positioned at various locations within the data storage system  12  so that they define a generally U-shaped configuration, as best seen in  FIG. 1 , although other arrangements are possible. By way of example, the cartridge engaging assembly  10  may be utilized in a data storage system of the type shown and described in U.S. Pat. No. 6,025,972, entitled “Multi-Plane Translating Cartridge Handling System”, which is hereby incorporated by reference for all that it discloses. Alternatively, the cartridge engaging assembly  10  may be utilized in a data storage system of the type shown and described in U.S. Pat. No. 5,596,556 (referred to supra). 
   The cartridge engaging assembly  10  is adapted to 1) engage a data cartridge  14  contained in a cartridge receiving device  16 ,  18 , and then 2) draw the data cartridge  14  into the cartridge engaging assembly for transport to a different cartridge receiving device  16 ,  18 . Movement of the cartridge engaging assembly  10  may be achieved, in part, via an actuator system  36  which moves the cartridge engaging assembly  10  along a U-shaped path of a cartridge positioning system  22 . 
   By way of example, the cartridge positioning system  22  may be of the type shown and described in U.S. Pat. No. 6,025,972 (referred to supra). The cartridge positioning system  22  disclosed therein may comprise a generally rectangularly shaped structure having a pair of opposed side portions  24  and  26  and an end portion  28 . A pair of cartridge storage racks  16  may be positioned adjacent the two opposed sides  24  and  26  of the cartridge positioning system  22 . Similarly, a pair of cartridge read/write devices  18  may be positioned adjacent the end  28  of the cartridge positioning system  22 . 
   The cartridge positioning system  22  may also comprise a lower plate  23  having a U-shaped guide member or channel  30  formed therein for guiding the cartridge engaging assembly  10  along a generally U-shaped path  32  so that the cartridge engaging assembly  20  may access the data cartridges  14  contained in the various cartridge storage racks  16  and cartridge read/write devices  18 . 
   As previously stated, the cartridge engaging assembly  10  may be moved along the U-shaped guide member  30  by an actuator system  36 . For example, the actuator system  36  may move the cartridge engaging assembly  10  between a first position  17  adjacent the first side portion  24  of the positioning system  22 , a second position  17 ′ adjacent the end portion  28  of the positioning system  22 , and a third position  17 ″ adjacent the second side portion  26  of the positioning system  22  (i.e., the cartridge engaging assembly  10  may be moved along the generally U-shaped path  32 ). 
   The actuator system  36  may comprise a rack and pinion drive system having a U-shaped gear rack  34  mounted adjacent the U-shaped guide member  30  in the lower plate  23 . A lower pinion gear  31  may be mounted to the cartridge engaging assembly  10  so that it engages the U-shaped gear rack  34 . A pair of lower bearing members  33 ,  35  mounted to the cartridge engaging assembly  10  may be configured to be received by the U-shaped guide member  30 . The cartridge engaging assembly  10  may also be provided with a pair of upper bearing members  37 ,  39  and an upper pinion gear  41  which engage an upper U-shaped guide member (not shown) and an upper U-shaped gear rack (not shown) provided on an upper plate (not shown) associated with the cartridge positioning system  22 . A drive pinion actuator system  43  may then be used to drive the lower and upper pinion gears  31 ,  41  to thereby move the cartridge engaging assembly  10  along the U-shaped path  32 . 
   The foregoing description of the data storage system  12  and cartridge positioning system  22  is provided in order to better understand one environment in which the cartridge engaging assembly  10  may be used. However, it should be understood that the cartridge engaging assembly  10  may be used in any of a wide range of other types of data storage systems, and in conjunction with any of a wide range of cartridge positioning systems now known or that may be developed in the future. Consequently, the cartridge engaging assembly  10  should not be regarded as being limited to use with the particular data storage system  12  and cartridge positioning systems  22  shown and described herein. Also, since detailed descriptions of the data storage system  12  and cartridge positioning system  22  are not required to understand or practice the cartridge engaging assembly  10 , the particular data storage system  12  and cartridge positioning system  22  that may be used in conjunction with the cartridge engaging assembly  10  will not be described in further detail herein. 
   The details of one embodiment of the cartridge engaging assembly  10  are best seen in  FIGS. 3-5 . Essentially, the cartridge engaging assembly  10  may include a frame assembly  38  having a first side member  70  and a second side member  72 . The two side members  70 ,  72  of the frame assembly  38  define a chamber or cavity  44  of sufficient size to receive a data cartridge  14 . The plunge mechanism  40  is slidably mounted to the frame assembly  38  so that the plunge mechanism  40  may be moved toward and away from the cartridge access end  46  of the frame assembly  38 , generally in the directions of arrows  54  and  55 , respectively (FIG.  3 ). More specifically, the plunge mechanism  40  is slidably mounted to the frame assembly  38  so that the plunge mechanism  40  can be moved between a retracted position (shown in  FIGS. 3 ,  4 , and  6 ( a )) and an extended position (shown in FIG.  6 ( b )). 
   In one preferred embodiment, the plunge mechanism  40  may be mounted on first and second elongate guide rails  79  and  76  that are respectively provided on the first and second sides  70 ,  72  of the frame assembly  38 . The first elongate guide rail  79  includes a first horizontal guide surface  80 , whereas the second elongate guide rail  76  includes a second horizontal guide surface  88 . In one preferred embodiment, the first and second horizontal guide surfaces  80 ,  88  are non-coplanar, as best seen in FIG.  7 . Alternatively, the first and second horizontal guide surfaces  80 ,  88  may be coplanar. In any event, the main body portion  96  of the plunge mechanism  40  may include a first flange member  78  which is sized and positioned to engage the first horizontal guide surface  80  of the first elongate guide rail  79 . The main body portion  96  of the plunge mechanism  40  may also include a second or C-shaped flange member  84  which is adapted to engage the second horizontal guide surface  88  of the second elongate guide rail  76 . 
   Referring now primarily to  FIGS. 7 and 8 , the first flange member  78  may comprise a generally rectangularly shaped member having a bottom surface  25  and a top surface  27 . A first or lower bearing member  75  is provided on the bottom surface  25  of the first flange member  78  so that it may contact the first horizontal guide surface  80 , as best seen in FIG.  7 . The top surface  27  of the flange member  78  may be provided with an upper bearing member  77 . The upper bearing member  77  is positioned so that it may contact a horizontal capture rail  74 . The first flange member  78  may also be provided with a pair of vertical bearing members  71 ,  73  which are positioned so that they may contact the first side member  70 . However, as will be explained in greater detail below, the upper bearing member  77  and pair of vertical bearing members  71 ,  73  do not normally contact their respective adjacent surfaces. 
   The second flange member  84  may comprise a substantially C-shaped member and is best seen in  FIGS. 7 and 9 . Essentially, the second flange member  84  comprises a lower horizontal member  45  and an upper horizontal member  47 . The bottom facing surface  93  of the upper horizontal member  47  may be provided with a pair of bearing members  89 ,  91  which are positioned in spaced-apart relation. The pair of bearing members  89 ,  91  contact the second horizontal surface  88  on the second guide rail  76 , as best seen in FIG.  7 . Additionally, the lower horizontal member  45  may be provided with a plurality of bearing members. More specifically, the lower horizontal member  45  may be provided with a pair of vertical bearing members  81 ,  83  which are positioned so that they may contact the second side member  72 . The upper facing surface  95  of the lower horizontal member  45  may be provided with a pair of bearing members  85 ,  87  which are positioned so that they may contact the bottom surface  90  of the second guide rail  76 . The various bearing members  81 ,  83 ,  85 ,  87  which are provided on the lower horizontal member  45  do not normally contact their respective adjacent surfaces. 
   In accordance with the structural arrangement described above, the plunge mechanism  40  is supported on one side by the bearing member  75  that contacts the first horizontal surface  80 , and on the other side by the bearing members  89 ,  91  that contact the second horizontal surface  88 . The three bearing members  75 ,  89 ,  91  allow the plunge mechanism  40  to be moved along a defined horizontal plane (not shown), even though the bearing members  75 ,  89 ,  91  themselves may be non-coplanar. 
   The side-to-side movement of the plunge mechanism  40  (e.g., movement in the directions indicated by arrows  21  (FIG.  7 )) is limited by a combination of the engagement of the pinion gear  50  and gear rack  48  on the one side, and on the other side by the pair of vertical bearing members  81 ,  83  and the second side member  72 . More specifically, the engagement of the pinion gear  50  and the gear rack  48  defines a vertical guide surface or plane  97  which limits the maximum leftward (looking at  FIG. 7 ) movement of the plunge mechanism  40 . The maximum rightward movement of the plunge mechanism  40  is limited by the pair of vertical bearing members  81  and  83  contacting the second side member  72 , wherein the second side member  72  defines a second vertical guide surface or plane. As will be described in greater detail below, it is generally preferred that spaces or gaps be provided between the pair of vertical bearing members  81 ,  83  and the side member  72  to prevent binding of the pinion gear  50  and gear rack  48  (FIG.  7 ). 
   Recognizing that the pinion gear  50  and gear rack  48  will wear with use, the first flange member  78  may be provided with the pair of vertical bearing members  71 ,  73  which may eventually contact the first side member  70 . The pair of vertical bearing members  71 ,  73  thereby limit the maximum leftward movement of the plunge mechanism  40  even if the pinion gear  50  and gear rack  48  experience excessive wear. Put another way, the maximum side-to-side movement (arrows  21 ) of the plunge mechanism  40  generally will not exceed the sum of the spaces or gaps provided between the pair of vertical bearing members  71 ,  73  and the first side member  71  on the one side of plunge mechanism  40 , and the pair of vertical bearing members  81 ,  83  and the second side member  72  on the other side of plunge mechanism  40 . 
   The various spaces or gaps provided between the first pair of vertical bearing members  71 ,  73  and the first side member  71 , and provided between the second pair of vertical bearing members  81 ,  83  and the second side member  72 , may comprise any of a wide range of sizes depending on the amount of horizontal positioning accuracy that is to be provided for the plunge mechanism  40 . In one preferred embodiment, the spaces or gaps provided between the vertical bearing members  71 ,  73  and the first side member  70  may be in the range of about 0.10 mm to 0.60 mm (0.35 mm preferred). Similarly, the spaces or gaps provided between the second pair of vertical bearing members  81 ,  83  and the second side member  72  may be in the range of about 0.01 mm to 0.61 mm (0.3 mm preferred). 
   In one preferred embodiment, the first side member  70  may also be provided with the capture rail  74  to prevent excessive upward vertical movement of the first flange member  78 . Excessive upward vertical movement of the second or C-shaped flange member  84  is provided by the lower surface  90  of the second guide rail  76 . 
   The bearing member  77  on the first flange member  78 , and the bearing members  85 ,  87  on the lower horizontal member  45  of the second flange member  84 , will contact the capture rail  74  and lower surface  90  of the second guide rail  76 , respectively, should the condition develop wherein the plunge mechanism  40  is urged upward. 
   It is generally preferred that spaces or gaps be provided between the bearing member  77  and the capture rail  74  on the one side of the plunge mechanism  40 , and the bearing members  85 ,  87  and the lower surface  90  of the second guide rail  76  on the other side of the plunge mechanism  40 . The various spaces or gaps may comprise any of a wide range of sizes depending on the amount of vertical positioning accuracy that is to be provided for the plunge mechanism  40 . In one preferred embodiment, the spaces or gaps provided between the bearing members  77  and the capture rail  74  may be in the range of about 0.05 mm to 0.45 mm (0.25 mm preferred). Similarly, the spaces or gaps provided between the bearing members  85 ,  87  and the lower surface  90  of the second guide rail  76  may be in the range of about 0.05 mm to 0.45 mm (0.25 mm preferred). 
   The various components of the mounting system just described may be made from any of a wide range of materials which are suitable for an intended application. For example, in one preferred embodiment, the first and second side members  70 ,  72  of the frame assembly  38  are molded from polycarbonate plastic, although other materials may also be used. It is preferred, but not required, that the various guide rails (e.g., the first and second guide rails  79 ,  76 , as well as the capture rail  74 ) associated with the first and second side members  70 ,  72  be provided as integral components thereof, thus eliminating the need to separately attach and align these components to the two side members  70 ,  72 . For example, in one preferred embodiment wherein both of the side members  70 ,  72  are molded from polycarbonate plastic, the first guide rail  79 , second guide rail  76 , and capture rail  74  are molded into the first and second side members  70 ,  72  of the frame assembly  38 , as best seen in  FIGS. 3 and 4 . 
   The main body portion  96  of the plunge mechanism  40  may also be made from any of a wide range of materials which are suitable for an intended application, such as metals or plastics. For example, in one preferred embodiment, the main body portion  96  of the plunge mechanism  40  is molded from nylon, although other materials could also be used. The various bearing members  71 ,  73 ,  75 , etc. provided on the first and second flange members  78 ,  84  of the main body  96  of the plunge mechanism  40  may comprise integral portions of the respective flange members  78 ,  84 . Alternatively, the various bearing members  71 ,  73 ,  75 , etc. may be provided as separate components which are fixedly attached to the first and second flange members  78 ,  84  at appropriate positions. If so, the various bearing members  71 ,  73 ,  75 , etc. may be fabricated from any of a wide range of materials (such as Teflon.RTM.) which are suitable for providing a low friction engagement with the various surfaces of the various guide rails. 
   As mentioned above, the plunge mechanism  40  is moved between its retracted and extended positions by a rack and pinion drive assembly  42 , as best seen in FIG.  3 . Essentially, the rack and pinion drive assembly  42  comprises an elongate gear rack  48  that is affixed to the first side member  70  of the frame assembly  38  at a convenient position. For example, in one preferred embodiment, the gear rack  48  is affixed to the first side member  70  at a position which is immediately below the horizontal guide surface  80 . Alternatively, the gear rack  48  may be affixed elsewhere on the first side member  70 , or to the second side member  72 . 
   The gear rack  48  may be made from any of a wide range of materials which are suitable for an intended application, such as metals or plastics. In the embodiment shown and described herein, wherein the first side member  70  of the frame assembly  38  comprises molded plastic, the gear rack  48  may comprise an integral component of the first side member  70  (i.e., the gear rack  48  comprises a molded portion of the first side member  70 ). Alternatively, the gear rack  48  may comprise a separate component which is fixedly mounted to the first side member  70 . 
   The main body portion  96  of the plunge mechanism  40  is provided with a pinion gear  50  which is positioned so that it engages the gear rack  48 . See  FIGS. 3 ,  6 ( a ) and  6 ( b ). The pinion gear  50  is caused to rotate by a drive system  52  which, in one preferred embodiment, includes a motor  98  and a gear reduction system  94 . The motor  98  may be mounted to the main body portion  96  of the plunge mechanism  40  such that it is operatively associated with the gear reduction system  94 . In this manner, the motor  98  rotates the pinion gear  50  to extend and retract the plunge mechanism  40 . 
   The motor  98  may comprise any of a wide range of motors which are suitable for an intended application. In one preferred embodiment, the motor  98  comprises a permanent magnet d.c. motor such as Model No. RS-385PH, available from Mobuchi Corp. of China. However, other motor types may also be used. 
   The gear reduction system  94  may comprise a conventional gear reduction system which utilizes a plurality of spur gears to provide a reduction ratio sufficient to allow the motor  98  to extend and retract the plunge mechanism  40  at an appropriate speed and with sufficient force to withdraw and insert the data cartridge  14  into a selected cartridge receiving device  16 ,  18 . In one preferred embodiment, the gear reduction system  94  provides a reduction ratio of about 11.5:1, although other ratios may be used depending on the speed and torque characteristics of a particular motor  98 . The various spur gears of the gear reduction system  94  may be made from any material which is suitable for an intended application, such as metal or plastic. In one preferred embodiment, the various spur gears of the gear reduction system  94  are made from brass and stainless steel. Alternatively, the gear reduction system  94  could utilize other types of gears to provide the desired reduction, such as worm gears. 
   The plunge mechanism  40  may also be provided with any of a wide variety of finger assemblies which are well-known in the art for engaging data cartridges such as data cartridge  14 . Accordingly, the finger assembly should not be regarded as limited to any particular type of finger assembly. By way of example, in one preferred embodiment, the finger assembly  56  may comprise an arm  57  having a hook portion  59 . The hook portion  59  of the arm  57  is configured to engage a notch  15  (FIGS.  6 ( a ),  6 ( b )) which is provided on a data cartridge  14 . The arm  57  is pivotally mounted to the main body  96  of the plunge mechanism  40  so that the arm  57  is free to pivot about a pivot axis  61 . A spring  63  biases the arm toward the engaged position shown in FIGS.  3  and  6 ( a ). The arm  57  may be provided with a pin  65  which engages the guide track  58  provided on the second side  72  of the frame assembly  38  (FIG.  4 ). The guide track  58  actuates the arm  57  as the plunge mechanism  40  moves back and forth between its retracted position ( FIGS. 3 ,  4  &amp;  6 ( a )) and its extended position (FIG.  6 ( b )). 
   However, since finger and track systems for engaging data cartridges  14  are well-known in the art and could be easily provided by persons having ordinary skill in the art after having become familiar with the teachings disclosed herein, the particular finger system  56  and guide track  58  disclosed above will not be described in greater detail herein. 
   The cartridge engaging assembly  10  may also be provided with a position control system  62  for providing an output signal from which information may be derived about the position of the plunge mechanism  40  in the frame assembly  38 . The position control system  62  comprises a reference mark  66  which is applied to the plunge mechanism  40  so that it moves with the plunge mechanism  40 . A detector  68  mounted to the frame assembly  38  detects the reference mark  66  on the plunge mechanism  40  and produces an output signal which is indicative of the size of the reference mark  66 . The size of the reference mark may then be used by a picker control system  64  to determine the position of the plunge mechanism  40 . The picker control system  64  may then control the drive system  52  to precisely move the plunge mechanism  40  to a desired position. 
   The reference mark  66  may comprise any of a wide range of marks. By way of example, the mark  66  in one preferred embodiment may comprise a pair of horizontal bars separated by a gap (i.e., a pair of bars positioned in spaced-apart relation; FIG.  11 ). However, the reference mark  66  could also consist of a single bar (FIG.  12 ), or comprise any number of bars. Likewise, the mark could comprise indicia other than bars. 
   The reference mark  66  may be applied to the plunge mechanism  40  in a number of ways. In a preferred embodiment, the reference mark  66  may be applied to a sticker which, in turn, is applied to the plunge mechanism  40  (FIG.  5 ). In another embodiment, the reference mark  66  may be formed by a recess or boss on the plunge mechanism  40  (not shown). In yet another embodiment, the reference mark  66  may comprise any identifiable edge, mark, etc. which appears on the plunge mechanism  40  as a result of its manufacture. (Note that the reference mark  66  may be generally referred to in the claims as a “reference means”.) The reference mark  66  is preferably constructed and applied to the plunge mechanism  40  such that at a least a portion of the reference mark  66  may be easily and clearly detected by the detector  68 . 
   To enable detection of the reference mark  66  by the detector  68 , a light source  67  may need to illuminate the reference mark  66 . Ideally, the reference mark  66  receives enough illumination from ambient light sources (e.g., through the cartridge access end  46  of the cartridge engaging assembly  10 ). However, it is envisioned that such is not often the case. A light source  67  may therefore be mounted to the cartridge engaging assembly  10  such that light is projected onto the reference mark  66 . In  FIG. 4 , the light source  67  is mounted on the second side member  72  of the frame assembly  38 . However, the light source  67  may be variously mounted, and may, for example, be mounted in either a stationary position (as shown in  FIG. 4 ) or in a location which travels with the plunge mechanism  40  (e.g., a light source might be mounted directly on the plunge mechanism (not shown)). A preferred embodiment of a light source  67  may comprise a plurality of light emitting diodes (LEDs)  69  which emit light through a lens  99 . 
   The detector  68  is mounted to the cartridge engaging assembly  10  such that it is capable of sensing the reference mark  66 . One way in which the detector  68  may sense the reference mark  66  is via a charge coupled device (CCD)  102 . Depending on where the detector  68  is mounted, the detector  68  may require a lens  104  or lenses for aligning and/or focusing an image of the reference mark  66  onto a device such as the afore-mentioned CCD  102 . 
   In  FIG. 4 , the detector  68  is mounted on the second side member  72  of the frame assembly  38 . Although the detector  68  may be variously mounted, it should be mounted such that travel of the plunge mechanism  40  causes the reference mark  66  to be sensed differently by the detector  68 . For example, movement of the plunge mechanism  40  with respect to the detector  68  may cause an image of the reference mark  66  which is sensed by the detector  68  to change in size (i.e., the image is sensed by a varying number of pixels of a CCD  102 ). 
   If the detector  68  comprises a CCD  102  and lens  104  as illustrated in  FIG. 4 , then the position of the plunge mechanism  40  in the cartridge engaging assembly  10  may be determined as a function of the distance between the lens  104  and the reference mark  66 . The distance between the lens  104  and the reference mark  66  may be determined using the following formula (FIG.  10 ):
 
 M=H″/H=S″/S   (1)
 
   In the above formula, M is the magnification of the lens  104 . The quantity H is a known measurement of the reference mark  66  (e.g., the space between the two horizontal bars of the reference mark  66  (FIG.  11 )). The quantity H″ is a calculable measurement of the image which is sensed by the detector  68  (e.g., the number of pixels representing the space H between the two horizontal bars of the reference mark  66 ). The quantity S″ is the distance between the lens principle point and a predetermined point of the image sensed by the detector  68  (wherein the predetermined point of the image corresponds to a similar point of the reference mark  66 ). The quantity S is an unknown representing the distance between the lens principle point and a predetermined point on the reference mark  66 . Given that all quantities but S are either known or are independently calculable, the unknown S can be computed using the following formula:
 
 S=HS″/H″   (2)
 
   Once S has been computed, then 1) a known position of the detector  68  with respect to the cartridge engaging assembly  10 , and 2) a known position of the reference mark  66  with respect to the plunge mechanism  40 , may be used to compute the position of the plunge mechanism  40  with respect to the cartridge engaging assembly  10 . 
   The above computations may be performed by either the detector  68  or the picker control system  64 . If the computations are performed by the detector  68 , then the detector  68  may generate an output signal which is indicative of the position of the plunge mechanism  40 . However, if the computations are performed by the picker control system  64 , then the detector  68  may generate an output signal comprising image pixel data. In either case, the picker control system  64  may use the output signal of the detector  68  to derive information about the position of the plunge mechanism  40 . The position information may then be used to control operation of the drive system  52 , and thus control movement of the plunge mechanism  40  to a desired position. 
   The detector  68  may comprise any of a wide range of CCD and/or other sensor types that are well-known in the art and readily commercially available. Preferably, the CCD  102  is a linear CCD or bar code reader. As such, the bar code reader can not only be used to detect the reference mark  66 , but to also read a bar code label which is applied to a data cartridge  14  located adjacent the cartridge access end  46  of the cartridge engaging assembly  10 . A bar code reader for reading bar code labels which are applied to data cartridges  14  is disclosed in U.S. patent application Ser. No. 09/716,878, entitled “Dual Bar Code Reading System for a Data Storage System”, which is hereby incorporated by reference for all that it discloses. 
   The data storage system  12  may also be provided with a picker control system  64  ( FIG. 13 ) for coordinating the function and operation of the cartridge engaging assembly  10 , as well as the function and operation of the cartridge positioning system  22  and actuator system  36 . In  FIG. 13 , the detector  68  is shown to receive the reference mark  66  (i.e., in the form of an image). The detector  68  then generates an output signal which is provided to the control system  64 . The control system  64  uses the output signal to determine the position of the plunge mechanism  40 . The control system  64  then provides a signal (e.g., a voltage of limited duration) to the plunge mechanism  40  to control movement thereof (i.e., to regulate the position of the plunge mechanism  40 ). Detection of the reference mark  66  and control of the plunge mechanism&#39;s movement continues until the control system  64  determines that a detected size of the reference mark  66  is indicative of the plunge mechanism  40  being in a final and desired position (e.g., an extended position). 
   Since picker control systems  64  are well-known in the art and could be easily provided by persons having ordinary skill in the art after having become familiar with the teachings disclosed herein, a particular picker control system  64  will not be described in further detail herein. 
   The cartridge engaging assembly  10  may be operated as follows to retrieve and load a data cartridge  14  from and to a selected cartridge receiving device (e.g., either the cartridge storage rack  14  or the cartridge read/write device  18 , as the case may be). Assuming that the cartridge positioning system  22  has positioned the cartridge engaging assembly  10  adjacent a selected data cartridge  14 , the picker control system  64  may actuate the drive system  52  to rotate the pinion gear  50  to move the plunge mechanism  40  toward the cartridge access end  46  of the frame assembly  38  (i.e., in the direction of arrow  54 ). As the plunge mechanism  40  approaches the data cartridge  14 , the finger assembly  56  engages a notch  15  or other similar feature on the data cartridge  14  (FIG.  6 ( b )). By detecting the reference mark  66  as the plunge mechanism  40  approaches the data cartridge  14 , the position control system  62  may be used to precisely position the plunge mechanism  40  via the drive system  52 . The arrangement of the plunge mechanism  40  and finger assembly  56  is such that the finger assembly  56  engages the notch  15  on the data cartridge  14  when the plunge mechanism  40  is at or near its fully extended position. This condition is illustrated in  FIG. 6   b . After the data cartridge  14  has been engaged by the hook portion  59  of the finger assembly  56 , the picker control system  64  may operate the drive system  52  to reverse the direction of rotation of the pinion gear  50 . This causes the plunge mechanism  40 , along with the engaged cartridge  14 , to move in the opposite direction (i.e., in the direction of arrow  55 ), thereby retracting the plunge mechanism  40  and data cartridge  14  into the cavity  44  defined by the frame assembly  38 . The drive system  52  continues to retract the plunge mechanism  40  and engaged data cartridge  14  until the data cartridge  14  is drawn into the cavity  44  of the frame assembly  38  by an amount sufficient to allow the cartridge positioning system  22  to move the cartridge engaging assembly  10  to another position. The cartridge positioning system  22  may thereafter move the cartridge engaging assembly  10  to a new location. 
   Once the cartridge engaging assembly  10  has been properly positioned adjacent a new location, the picker control system  64  may again actuate the drive system  52  to move (i.e., extend) the plunge mechanism  40  in the direction of arrow  54 . As the plunge mechanism  40  extends, a surface  60  of the plunge mechanism  40  pushes against the data cartridge  14 , thereby pushing the data cartridge  14  into a selected cartridge receiving device (e.g., the cartridge storage rack  16  or the cartridge read/write device  18 , as the case may be). At the same time, the guide track  58  actuates the finger assembly  56  to disengage the hook portion  59  of the finger assembly  56  from the notch  15  provided in the data cartridge  14 . If the plunge mechanism  40  is appropriately extended, the data cartridge  14  will be inserted fully into a cartridge receiving device  16 ,  18  so that it properly “seats” in the cartridge receiving device  14 ,  18 . Appropriate extension of the plunge mechanism  40  is once again controlled by the position control system  62 . Once the plunge mechanism  40  is fully extended, the data cartridge  14  will have been inserted fully into the cartridge receiving device  16 ,  18 . The picker control system  64  may then actuate the drive system  52  to retract the plunge mechanism  40  back into the cartridge engaging assembly  10 . The cartridge engaging assembly  10  is then ready to withdraw another data cartridge  14  and transport it to a new location. 
   During the above cartridge retrieval and loading operations, the position of the plunge mechanism  40  when it is in its extended position is often critical. For example, under- or over-extension of the plunge mechanism  40  during cartridge retrieval can lead to the finger assembly  56  being unable to engage the notch  15  on a data cartridge  14 . Likewise, under- or over-extension (and particularly under-extension) can lead to a data cartridge  14  failing to properly seat within a selected cartridge receiving device  16 ,  18 . Such failures to seat can 1) lead to a data cartridge  14  being misread by a cartridge read/write device  18 , or 2) lead to a data cartridge  14  becoming dislodged from a cartridge receiving device  16 ,  18  (e.g., falling out of the cartridge receiving device  16 ,  18 ). As a result of these problems, operation of the drive system  52  may be controlled by the picker control system  64 , as previously described, to precisely control the position of the plunge mechanism  40 . 
   Preferably, the drive system  52  first moves the plunge mechanism  40  from its retracted position toward its extended position in accordance with an open loop control schedule. Either 1) in response to monitoring the detector  68  to determine when the reference mark  66  is detected, or 2) in response to having moved the plunge mechanism  40  for a predetermined distance, control of the drive system  52  is switched to a closed loop control schedule wherein the plunge mechanism  40  is precisely positioned in response to the detector&#39;s output of a signal which is indicative of the position of the plunge mechanism  40 . Preferably, the response time of the detector  68  and/or accuracy of the drive system  52  provide for switching the drive system  52  to its closed loop control schedule during the last five millimeters (5 mm) of plunge mechanism plunge. Thus, a relatively small amount of image data needs to be processed by the detector  68  and/or control system  64 . 
   As was previously and briefly discussed, one embodiment of the cartridge engaging assembly  10  uses the detector  68  to fulfill multiple functions. One function is the detection of the reference mark  66 . Another function is the reading of bar code labels which are applied to the edges of data cartridges  14  adjacent the cartridge access end  46  of the cartridge engaging assembly  10 . When the plunge mechanism  40  is in its retracted position, the light source  67  illuminates the labels on the data cartridges  14 . In this manner, prior to retrieving a data cartridge  14 , the cartridge engaging assembly  10  may verify or detect which data cartridge  14  it is retrieving. As the plunge mechanism  40  moves to its extended position, the light source  67  begins to illuminate the reference mark  66  which is applied to the plunge mechanism  40 . In this manner, the detector  68  can detect the reference mark  66  and provide an output signal to the picker control system  64  for the purpose of precisely positioning the plunge mechanism  40  with respect to a data cartridge  14 . 
   While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed, and that the appended claims are intended to be construed to include such variations, except as limited by the prior art.