Patent Publication Number: US-6707636-B2

Title: Cartridge picker assembly

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a continuation of copending application No. 09/910,086 filed on Jul. 20, 2001, now U.S. Pat. No. 6,570,735, Which is a divisional of application Ser. No. 09/603,156, filed on Jun. 23, 2000, now U.S. Pat. No. 6,301,072, which is a continuation of application Ser. No. 09/237,516, filed on Jan. 26, 1999, now U.S. Pat. No. 6,157,513, all of which are incorporated herein by reference for all that they disclose. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to data storage systems for handling and storing data cartridges, such as optical disk or magnetic tape cartridges, and more specifically to cartridge access devices for engaging and retrieving the data cartridges stored in data storage systems. 
     BACKGROUND 
     Many different types of data storage systems exist and are being used to store data cartridges at known locations and to retrieve desired data cartridges 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 individual 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. 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 moveable cartridge picker 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 picker assembly or picker may also be provided with a cartridge plunge mechanism or “thumb” assembly for engaging the various data cartridges contained in the cartridge receiving devices and for drawing them into the picker. A picker positioning system associated with the cartridge picker assembly may be used to move the cartridge picker assembly along 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 picker positioning system to move the picker assembly along the cartridge storage racks until the picker assembly is positioned adjacent the desired data cartridge. The cartridge plunge mechanism or “thumb” assembly associated with the picker assembly may then remove the data cartridge from the cartridge storage rack and draw it into the picker assembly. The picker positioning system may then be actuated to move the picker assembly to the appropriate cartridge read/write device. Once properly positioned adjacent the cartridge read/write device, the thumb assembly may 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 thumb assembly may be actuated to remove the data cartridge from the cartridge read/write device. The picker assembly may thereafter return the data cartridge to the appropriate location in the cartridge storage rack. 
     A typical cartridge plunge mechanism or “thumb” assembly is usually slidably mounted to the picker and is provided with a thumb actuator system to move the thumb assembly toward and away from a cartridge access end of the picker. For example, if it is desired to retrieve a data cartridge from a cartridge receiving device, the thumb actuator system moves the thumb assembly toward the cartridge access end of the picker so that the thumb assembly can engage or “grab” the data cartridge. Thereafter, the thumb actuator system may retract the thumb assembly and engaged data cartridge into the picker. If it is desired to load the data cartridge into the cartridge receiving device, then the thumb actuator moves the thumb assembly and data cartridge toward the cartridge access end of the picker, and inserts the data cartridge into the cartridge receiving device. 
     In order to reliably engage a cartridge, the thumb assembly must be positioned adjacent to the cartridge within precise tolerances. As the thumb assembly is moved toward or away from the cartridge access end of the picker, lateral and vertical movement is limited by the thumb actuator system or by guide rails on each side of the thumb assembly. If the thumb assembly is not correctly positioned adjacent to the cartridge, the thumb assembly will be unable to properly engage or disengage the cartridge. 
     Currently known mounting systems are not without their problems. In one currently known mounting system, the thumb assembly is mounted to the cartridge picker assembly on a lead screw which drives and positions the thumb assembly. A lead screw is rotatably mounted to the cartridge picker assembly and runs the length of the picker assembly, passing through the thumb assembly and engaging with a gear or teeth on the thumb assembly. Thus, when the lead screw rotates, the thumb assembly is moved toward or away from the cartridge access end of the picker. The lead screw drive system provides generally good positioning accuracy once aligned, but requires the use of expensive machined elements and is difficult to assemble and align. Furthermore, the teeth and threads tend to wear down over time as they turn against each other, reducing the positioning accuracy. In addition, the minimum gaps required for moving parts may exceed the tolerances required for lateral positioning. For example, if the thumb actuator system is a lead-screw drive system, gaps are required between the lead-screw and the teeth in the thumb assembly to prevent binding. These gaps required for the operation of the drive system may allow lateral movement which exceeds the tolerances required for lateral positioning. A lead-screw drive system may also require a secondary guide member, such as a precision-machined rod, which is relatively expensive and may pose assembly and alignment problems. 
     In addition, positioning errors may accumulate within the picker with each additional part in the guide system. For example, for a lead-screw drive system, the position of the thumb assembly is affected by multiple parts. First, the gap between the lead-screw and the threads in the thumb assembly has a given tolerance. The lead-screw may be held in place by a set of ball bearings which also have a given tolerance. The ball bearings may be held in a casing having a tolerance, which in turn may be attached to the side of the picker which also has a tolerance. Each tolerance adds up, forming a tolerance stack, making it more difficult to maintain the lateral position of the thumb assembly as the tolerance stack deepens. 
     In another currently known mounting system, the thumb assembly is mounted in the picker with guide rails on each side of the picker. Support members extend from the sides of the thumb assembly to engage in the picker guide rails to guide the thumb assembly as it moves toward and away from the cartridge access end of the picker. A guide rail mounting system provides a less-expensive alternative to a lead-screw, and may eliminate some of the expensive machined parts and may facilitate assembly and alignment. However, a guide rail mounting system is subject to a high tolerance stack and large, molded parts which are easily warped. The guide rails are typically molded into the plastic side panels of the picker, which can warp during manufacture and assembly, and which can flex during operation. 
     In addition, the drive system of the picker assembly typically mounts to one side, correctly positioning that side of the picker assembly adjacent a cartridge or a cartridge receiving device. However, guide rails are located on both sides of the picker, and there may be several elements between the two sides of the picker. Therefore, the tolerance stack for the guide rail mounting system includes the tolerances for all elements between the correctly positioned first side and the guide rail on the second side. In other words, correct positioning of the thumb assembly in a picker having a guide rail mounting depends upon correct sizing and placement of multiple elements in the picker. The large tolerance stack and the dependence upon easily warped or flexed components leads to an undesirably high rate of incorrect positioning. 
     Consequently, a need exists for an improved thumb assembly mounting and guide system to control lateral motion. In particular, a need exists for a guide system having a reduced tolerance stack and having more easily controlled tolerances, using relatively inexpensive components. 
     SUMMARY OF THE INVENTION 
     A cartridge picker assembly may include a frame having a first side wall and a second side wall positioned in spaced-apart relation, the first side wall of said frame defining a lateral guide surface that is substantially parallel to a lateral plane. A thumb assembly is slidably mounted to the first and second side walls of the frame so that the thumb assembly is capable of moving toward and away from a cartridge access end of the frame. A guide member mounted to the thumb assembly engages the lateral guide surface on the first side wall of the frame so that the lateral motion of the thumb assembly is controlled as the thumb assembly moves toward and away from the cartridge access end of the frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Illustrative and presently preferred embodiments of the invention are shown in the accompanying drawing in which: 
     FIG. 1 is a plan view of a media access device and cartridge picker assembly according to the present invention 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 side view of a thumb assembly showing the guide member; 
     FIG. 3 is a side view of the thumb assembly of FIG. 2; 
     FIG. 4 is a front view of the thumb assembly of FIG. 2; 
     FIG. 5 is a perspective view of the cartridge picker assembly; 
     FIG. 6 is a perspective view of the left side of the cartridge picker assembly showing the thumb assembly; 
     FIG. 7 is a perspective view of the right side of the cartridge picker assembly showing the thumb assembly; 
     FIG. 8 is an end view in elevation showing the engagement of the thumb and guide rail assemblies; 
     FIGS.  9 ( a-b ) are simplified sectional views in elevation showing the thumb assembly in the retracted and extended positions; 
     FIG. 10 is an enlarged cross-section view in elevation of the thumb and guide rail assembly more clearly showing the engagement of the guide member with the lateral guide surface; 
     FIG. 11 is an enlarged side view of the first flange member of the thumb assembly; and 
     FIG. 12 is an enlarged side view of the second, C-shaped flange member of the thumb assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A cartridge picker assembly or “picker”  10  having an improved thumb referencing and guide system  11  is shown in FIG. 1 as it could be used in a “juke box” data storage system  20  to transfer data cartridges  22  between one or more cartridge receiving devices, such as one or more cartridge storage racks or magazines  24  and one or more cartridge read/write devices  26 . 
     The improved thumb referencing and guide system  11  is used to support and guide a cartridge plunge mechanism or “thumb assembly”  16  as the same is moved back and forth within the picker assembly  10 . The improved thumb referencing and guide system  11  comprises a guide member  12  provided on the cartridge plunge mechanism  16  and a lateral guide surface  14  on the cartridge picker assembly  10 . The improved thumb referencing and guide system  11  thereby provides control of the lateral position of the cartridge plunge mechanism  16  within the cartridge picker assembly  10 . As the cartridge plunge mechanism  16  slides within the cartridge picker assembly  10  to grasp or release a data cartridge  22 , the guide member  12  slides against the lateral guide surface  14  to control lateral position. The guide member  12  and the lateral guide surface  14  thereby substantially reduce the likelihood that the cartridge plunge mechanism  16  will become mis-aligned and fail to grasp a data cartridge  22 . 
     The juke box data storage system  20  in which the improved thumb referencing and guide system  11  may be used is best seen in FIG.  1  and may comprise a generally rectangularly shaped frame or chassis assembly  28  around which are arranged a plurality of cartridge receiving devices, such as the cartridge storage racks or magazines  24  and cartridge read/write devices  26 . In the embodiment shown and described herein, the various cartridge receiving devices (e.g., the cartridge storage racks or magazines  24  and the cartridge read/write devices  26 ) are positioned so that they define a generally U-shaped configuration, as best seen in FIG.  1 . However, other configurations are possible. Alternatively, and as will be described in greater detail below, the cartridge picker assembly or picker  10  according to the present invention may be used in any of a wide range of other types of data storage systems. 
     The cartridge picker assembly  10  may be mounted to a U-shaped guide track  30  provided on the frame or chassis assembly  28 . The U-shaped guide track  30  guides the cartridge picker assembly  10  along a generally U-shaped path  32 , thereby allowing the cartridge picker assembly or picker  10  to access all of the cartridges  22  stored in the various cartridge receiving devices (e.g.,  24 ,  26 ). A picker positioning system  34  operatively associated with the cartridge picker assembly  10  may be used to move the cartridge picker assembly  10  to various positions along the U-shaped guide track  30 . 
     Referring now primarily to FIGS. 2,  4 ,  6 , and  7 , the cartridge picker assembly  10  may be provided with a cartridge plunge mechanism or “thumb” assembly  16  for removing and replacing the data cartridges  22  contained within the various cartridge receiving devices. In one preferred embodiment, the thumb assembly  16  is moveably mounted within the cartridge picker assembly  10  so that the thumb assembly  16  may be moved between a retracted position  112  (shown in FIG. 9A) and an extended position  114  (shown in FIG.  9 B). That is, the thumb assembly  16  may be extended and retracted within the cartridge picker assembly  10  in the directions indicated by arrows  108  and  110 , respectively. The thumb assembly  16  may be provided with a finger assembly  36  to allow the thumb assembly  16  to engage and disengage a data cartridge  22 , as will be described in greater detail below. 
     Referring now primarily to FIGS. 3,  4 ,  6 , and  10 , the lateral position of the thumb assembly  16  within the cartridge picker assembly  10  may be controlled by an improved thumb referencing and guide system  11  (FIG.  10 ), comprising the guide member  12  and the lateral guide surface  14 . The guide member  12  may comprise a plurality of guide ribs that extend from the thumb assembly  16 . In one preferred embodiment, the guide member  12  comprises a front guide rib  40  and a rear guide rib  42 . The front and rear guide ribs  40  and  42  have a height  48  and a width  50  enabling them to engage the lateral guide surface  14 . As seen in FIGS. 6 and 10, the lateral guide surface  14  may comprise a first contact surface  60  and a second contact surface  62 , placed in parallel spaced-apart relation to form a channel  61  (see FIG.  10 ). The guide member  12  engages with the lateral guide surface  14 , as seen in FIG. 8, to prevent side-to-side, or lateral movement (indicated by arrows  64 ) of the thumb assembly. 
     In one preferred embodiment, the first contact surface  60  is substantially parallel to a lateral plane formed by an X axis and a Z axis (FIG.  5 ), and may comprise a first side wall  78  of the cartridge picker assembly frame  76 . The second contact surface  62  may comprise an elongate wall attached to the first side wall  78  of the cartridge picker assembly frame  76 . Hence, the lateral guide surface  14  controls the lateral position of the thumb assembly  16  based upon the first side wall  78  rather than both the first side wall  78  and a second side wall  80  of the cartridge picker assembly frame  76 . The resulting tolerance stack is thereby minimized by removing a dependence upon the second side wall  80  and various other components between the first and second side walls  78  and  80 . 
     This is a substantial advantage over using the guide rails (e.g.,  116  and  118 ) at each side of the thumb assembly  16  for lateral positioning. One benefit of using only the first side wall  78  to control lateral position of the thumb assembly  16  is that the number of elements in the cartridge picker assembly  10  included in the tolerance stack for lateral position is minimized. A second benefit is that the elements remaining in the tolerance stack are more easily controlled. For example, large molded plastic elements such as the second side wall  80  of the cartridge picker assembly  76  and the thumb assembly frame  82 , which are easily warped during the molding process, are kept out of the tolerance stack. 
     Having briefly described the improved thumb referencing and guide system  11  including the guide member  12  and the lateral guide surface  14 , as well as some of its more significant features and advantages, the various embodiments of the improved thumb referencing and guide system  11  according to the present invention will now be described in detail. However, before proceeding with the description, it should be noted that the improved thumb referencing and guide system  11  according to the present invention may be utilized in any of a wide range of cartridge engaging assemblies in any of a wide variety of data storage systems now known or that may be developed in the future. Accordingly, the present invention should not be regarded as limited to the particular data storage system  20  and cartridge picker assembly  10  shown and described herein. 
     With the foregoing considerations in mind, one embodiment of an improved thumb referencing and guide system  11  is shown and described herein as it may be used in a cartridge picker assembly  10 . The cartridge picker assembly  10  may, in turn, be used in a portion of a “juke box” data storage system  20 . In one preferred embodiment, the juke box data storage system  20  may comprise a data storage system of the type shown and described in U.S. Pat. application Ser. No. 09/045,134, filed Mar. 20, 1998, now U.S. Pat. No. 6,025,972, entitled “Multi-Plane Translating Cartridge Handling System,” which is incorporated herein by reference for all that it discloses. Alternatively, the data storage system  20  may be of e 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 also incorporated herein by reference for all that it discloses. 
     Referring now primarily to FIG. 1, the data storage system  20  shown and described in U.S. Pat. No. 6,025,972 referred to above may comprise a generally rectangularly shaped frame or chassis assembly  28  having a pair of opposed side portions  84  and  86  and an end portion  88  around which are positioned various cartridge receiving devices. More specifically, a pair of cartridge storage racks or magazines  24  may be positioned adjacent each opposed side portion  84  and  86  of the frame or chassis assembly  28 , whereas a pair of cartridge read/write devices  26  may be positioned adjacent the end portion  88  of the frame assembly  28 . Accordingly, the various cartridge receiving device (e.g., the cartridge storage racks or magazines  24  and the cartridge read/write devices  26 ) define a generally U-shaped configuration. 
     A picker positioning system  34  may be used to move the cartridge picker assembly  10  along the U-shaped guide track  30 . For example, in the embodiment shown and described herein, the picker positioning system  34  may move the cartridge picker assembly  10  between a first position  90  adjacent the first side portion  84  of the frame assembly  28 , a second position  90 ′ adjacent the end portion  88  of frame assembly  28 , and a third position  90 ″ adjacent the second side portion  86  of frame assembly  28 . 
     The picker positioning system  34  may comprise a rack and pinion drive system having a U-shaped gear rack  92  mounted to the lower plate  94  of frame assembly  28  at a position adjacent the U-shaped guide track  30 . The cartridge picker assembly  10  may be provided with a lower pinion gear  96  (FIG. 5) sized and positioned so that it will engage the lower U-shaped gear rack  92  provided on the lower plate  94 . Similarly, the cartridge picker assembly  10  may also be provided with an upper pinion gear  98  sized and positioned so that it will engage an upper U-shaped gear rack provided on an upper plate  100  (not shown). A drive pinion actuator system  102  mounted to the cartridge picker assembly  10  may be used to drive the lower and upper pinion gears  96  and  98 , thereby allowing the picker assembly  10  to be moved along the U-shaped path  32 . 
     The details of the cartridge picker assembly  10  that may be utilized in one preferred embodiment of the present invention are best seen in FIGS. 5-7. Essentially, the cartridge picker assembly  10  may include a picker frame assembly  76  having a first side wall or member  78  and a second side wall or member  80 . The two side members  78  and  80  define an interior chamber or cavity  104  of sufficient size to receive a data cartridge  22 . The thumb assembly  16  is slidably mounted to the frame assembly  76  so that the thumb assembly  16  may be moved along an X axis toward and away from the cartridge access end  106  of frame assembly  76 , i.e., generally in the directions of arrows  108  and  110 , respectively. See FIG.  6 . More specifically, the thumb assembly  16  is slidably mounted to the frame assembly  76  so that the thumb assembly  16  may be moved between a retracted position  112  (illustrated in FIG. 9 a ) and an extended position  114  (illustrated in FIG. 9 b ). 
     The thumb assembly  16  may be mounted on first and second elongate guide rails  116  and  118  that are provided on the first and second sides  78  and  80  of the picker frame assembly  76 . See FIGS. 10,  11 , and  12 . The first elongate guide rail  116  includes a first horizontal guide surface  128 , whereas the second elongate guide rail  118  includes a second horizontal guide surface  158 . In one preferred embodiment, the first and second horizontal guide surfaces  128  and  158  are non-coplanar, as best seen in FIG.  10 . Alternatively, the first and second horizontal guide surfaces  128  and  158  may be coplanar. In any event, the main body portion  82  of the thumb assembly  16  may include a first flange member  122  sized and positioned to engage the first horizontal guide surface  128  of first elongate guide rail  116 . The main body portion  82  of the thumb assembly  16  may also include a second or C-shaped flange member  124  adapted to engage the second horizontal guide surface  158  of the second elongate guide rail  118 . 
     Referring now primarily to FIGS. 10 and 11, the first flange member  122  may comprise a generally rectangularly shaped member having a bottom surface  160  and a top surface  162 . A first or lower bearing member  164  is provided on the bottom surface  160  of first flange member  122  and contacts the first horizontal guide surface  128 , as best seen in FIG.  10 . The top surface  162  of flange member  122  may also be provided with an upper bearing member  166 . The upper bearing member  166  is positioned so that it may contact a horizontal capture rail  116 . However, upper bearing member  166  does not normally contact horizontal capture rail  116 . 
     The second flange member  124  may comprise a substantially C-shaped member and is best seen in FIGS. 10 and 12. Essentially, the second flange member  124  comprises a lower horizontal member  168  and an upper horizontal member  170 . The bottom facing surface  172  of upper horizontal member  170  may be provided with a pair of bearing members  176  and  178  positioned in spaced-apart relation. The pair of bearing members  176  and  178  contact the second horizontal surface  158  on the second guide rail  118 , as best seen in FIG.  10 . The upper facing surface  174  of lower horizontal member  168  may be provided with a pair of bearing members  180  and  182  positioned so that they may contact the bottom surface  184  of the second guide rail  118 . The bearing members  180  and  182  provided on the lower horizontal member  168  do not normally contact the bottom surface  184  of the second guide rail  118 . 
     In accordance with the structural arrangement described above, the thumb assembly  16  is vertically supported on one side by the bearing member  164  that contacts the first horizontal surface  128 , and on the other side by the bearing members  176  and  178  that contact the second horizontal surface  158 . The three bearing members  164 ,  176 , and  178  allow the thumb assembly  16  to be moved along a defined horizontal plane (not shown), even though the bearing members  164 ,  176 , and  178  themselves may be non-coplanar. 
     The various components just described may be made from any of a wide range of materials, such as metals or plastics, suitable for the intended application. For example, in one preferred embodiment, the first and second side members  78  and  80  comprising the picker frame assembly  76  are molded from a polycarbonate plastic material. Accordingly, the various components and features contained therein, e.g., the guide rail  116 , horizontal guide surface  128 , along with the second guide rail  118 , may be provided as integral components of the respective side members  78  and  80 . Alternatively, the first and second side members  78  and  80  comprising the picker frame assembly  76  may be made from other materials, in which case the various components and features of the side members  78  and  80  may comprise integral components or separate elements, depending on the particular design and material to be utilized. 
     The main body portion  82  of the thumb assembly  16  may also be made from any of a wide range of materials suitable for the intended application. For example, in the embodiment shown and described herein, the main body portion  82  of thumb assembly  16  is made from nylon with a small amount of Teflon® added (e.g., about 15% by weight) as a friction modifier, although other materials could also be used. In one preferred embodiment, the various bearing shoes  126  provided on the first and second flange members  122  and  124  of the main body portion  82  of the thumb assembly  16  comprise integral portions of the main body portion  82 . Alternatively, the bearing shoes  126  could comprise separate elements that may then be affixed to the first and second flange members  122  and  124 . If so, the various bearing shoes  126  may be fabricated from any of a wide range of materials (e.g., Teflon®) suitable for providing a low friction engagement with the first and second guide rails  116  and  118 . 
     The foregoing description of the data storage system  20  and related components (e.g., the cartridge picker assembly  10 , thumb assembly  16 , and picker positioning system  34 ) is provided in order to better understand one environment in which the improved thumb referencing and guide system  11  according to the present invention may be used. However, as was mentioned above, it should be understood that the improved thumb referencing and guide system  11  may be used in any of a wide range of other types of data storage systems or other similar devices having moveable internal components. Consequently, the present invention should not be regarded as limited to the particular data storage system  20  shown and described herein. Also, since detailed descriptions of the data storage system  20  and related components (e.g., the cartridge picker assembly  10  and the picker positioning system  34 ) are not required to understand or practice the thumb referencing invention, the particular data storage system  20  and related components that may be used in conjunction with the improved thumb referencing and guide system  11  will not be described in greater detail herein. 
     Referring now primarily to FIGS. 3,  4 ,  5 , and  10 , the improved thumb referencing and guide system  11  may comprise a guide member  12  and a lateral guide surface  14  enabling the cartridge picker assembly  10  to move longitudinally along an X axis between a retracted position  112  (shown in FIG. 9A) and an extended position  114  (shown in FIG.  9 B). The improved thumb referencing and guide system  11  guides the cartridge picker assembly  10  along a lateral plane formed by an X axis and a Z axis (FIG.  5 ), allowing the finger assembly  36  to reliably grasp a data cartridge  22 . In one preferred embodiment, the finger assembly  36  must be positioned adjacent a data cartridge  22  with a maximum side to side offset of about 1.5 mm. 
     The guide member  12  may comprise a plurality of ribs that extend from the thumb assembly  16 . In one preferred embodiment, the guide member  12  comprises a front guide rib  40  and a rear guide rib  42 . The guide ribs  40  and  42  may be generally rectangular in shape and are attached to the thumb assembly frame  82  so that they extend from the frame  82  to engage the lateral guide surface  14 . 
     The guide member  12  has a width  50  substantially equal to or just smaller than the distance  66  between the first contact surface  60  and the second contact surface  62 , enabling the guide member  12  to slide longitudinally in the cartridge picker assembly  10  along an X axis (FIG.  5 ), but preventing the guide member  12  and the thumb assembly  16  from moving laterally along a Y axis (FIG.  5 ). 
     The lateral guide surface  14  comprises a generally smooth, or otherwise low-friction surface against which the guide member  12  slides as the thumb assembly  16  moves within the cartridge picker assembly  10 . The lateral guide surface  14  may comprise a first contact surface  60  and a second contact surface  62 , placed in spaced-apart, parallel relation to form a channel in which the guide member  12  fits. The first contact surface  60  and the second contact surface  62  are placed apart from each other at a distance  66  which is substantially equal to or just larger than a width  50  of the guide member  12 . This allows the guide member  12  to slide along an X axis but limits side to side movement along a Y axis(FIG. 5) to allow the finger assembly  36  to reliably grasp an adjacent data cartridge (e.g.,  22 ). This provides a second benefit of preventing rotation of the thumb assembly  16  around a Z axis which would cause binding of the thumb assembly  16  and misalignment of the finger assembly  36 . 
     The front guide rib  40  and a rear guide rib  42  have a length  44  and  46 , respectively. The guide ribs  40  and  42  may have any suitable lengths, but in a preferred embodiment lengths  44  and  46  are greater than a distance  66  between the first contact surface  60  and the second contact surface  62 . Along with the lateral alignment benefits during use, this facilitates insertion of the thumb assembly  16  into the cartridge access end  106  of the cartridge picker assembly  10  during the assembly process. As the rear guide rib  42  enters the channel formed by the first and second contact surfaces  60  and  62 , the thumb assembly must be substantially aligned along the X axis before it can be fully inserted. This ensures that the front guide rib  40  will be at or near the proper location to enter the channel, reducing the difficulty of engaging the front guide rib  40  with the lateral guide surface  14 . The guide ribs  40  and  42  may also have curved edges  56  to facilitate assembly and to avoid binding during movement of the thumb assembly  16 . 
     The guide ribs  40  and  42  may alternatively have lengths  44  and  46  less than the distance  66  between the first and second contact surfaces  60  and  62 , thereby reducing material usage, but the thumb assembly  16  will be able to rotate around a Z axis even after the rear guide rib  42  is engaged with the lateral guide surface  14 , making insertion of the front guide rib  40  more difficult. 
     Two or more guide members (e.g.,  40  and  42 ) may provide the necessary control of side to side movement with a minimum of material usage. Alternatively, the guide member  12  may comprise one elongate guide rib having a length greater than the distance  66  between the first and second contact surfaces  60  and  62 . 
     The first contact surfaces  60  of the lateral guide surface  14  may comprise a substantially smooth, elongate member attached to the cartridge picker assembly frame  76 . In one preferred embodiment best seen in FIGS. 6 and 10 the first contact surface  60  comprises the first side wall  78  of the cartridge picker assembly frame  76 , thereby minimizing the tolerance stack by avoiding components and interfaces between the cartridge picker assembly frame  76  and the first contact surface  60 . 
     The second contact surface  62  may comprise a second substantially smooth, elongate member mounted opposite the first contact surface  60  in parallel, spaced-apart relation. In one preferred embodiment the second contact surface  62  comprises a gear rack  134  having a substantially smooth side  62  and an opposite side  186  having gear teeth  188  for operatively engaging the pinion  136  on the thumb assembly  16 . 
     The second contact surface  62  may be mounted to the first contact surface  60  with a horizontal bottom surface  190  between them to form a channel for the guide member  12 . The horizontal bottom surface  190  may comprise a portion of the second contact surface  62  or another component. In one preferred embodiment shown in FIG. 10 the horizontal bottom surface  190  comprises the first side wall  78  of the cartridge picker assembly frame  76 . 
     The second contact surface  62  may be mounted to the first contact surface  60 , or to any components interposed between the first and second contact surfaces  60  and  62 , using any suitable method now known or which may be developed in the future. For example, the second contact surface  62  may be bonded with an adhesive, welded, or molded with the first contact surface  62  as an integral part. In one preferred embodiment, the second contact surface  62  is attached to the first side wall  78  with screws  192 . 
     The guide member  12  is proportioned to allow slidable engagement with the lateral guide surface  14 . The front and rear guide ribs  40  and  42  preferably have a height  48  that allows the guide ribs  40  and  42  to slide against the lateral guide surface  14  without contacting the bottom surface  190 . As a result, the improved thumb referencing and guide system  11  provides lateral position control without interfering with the vertical position control provided by the first and second guide rails  116  and  118 . 
     In one preferred embodiment, the guide member  12  has a width  50  between about 2.07 mm and 2.17 mm (2.12 mm preferred), a height  48  between about 5.2 mm and 5.4 mm (5.3 mm preferred), and a length  44  and  46  between about 6 mm and 12 mm (9 mm preferred). The first and second contact surfaces  60  and  62  have a height between about 5.2 mm and 5.5 mm (5.35 mm preferred) and are separated by a distance between about 2.35 mm and 2.45 mm (2.40 mm preferred). 
     The guide member  12  and the lateral guide surface  14  may be fabricated from any of a wide range of materials, such as polycarbonate plastic or metal. They may be formed as integral parts of the thumb assembly frame  82  and the picker frame  76 , respectively, or may be fabricated as independent components. The guide member  12  and the lateral guide surface  14  are preferably fabricated of a material providing a low-friction surface. In one preferred embodiment, the guide member  12  is made of nylon having about 30% carbon fiber and about 15% Teflon® and the lateral guide surface  14  is made of polycarbonate having about 12% carbon fiber and about 12% Teflon®. 
     The improved thumb referencing and guide system  11  provides a reduced tolerance stack by referencing the lateral position of the thumb assembly  16  from only one side  78  of the picker frame  76 , thereby substantially improving the lateral position control of the thumb assembly  16 . The improved lateral position control also decreases the gap between the teeth  188  of the gear rack  134  and the pinion gear  136 , thereby reducing gear backlash and improving accuracy of the longitudinal plunge motion of the thumb assembly  16  in the cartridge picker assembly  10 . 
     The thumb assembly  16  may be moved between the retracted and extended positions  112  and  114  (FIGS. 9 a  and  9   b ), respectively, by any of a wide range of actuator systems. By way of example, in one preferred embodiment, the thumb assembly  16  may be moved between the retracted and extended positions  112  and  114 , respectively, by a rack and pinion drive assembly of the type shown and described in U.S. Pat. application Ser. No. 09/045,558, filed Mar. 20, 1988, now U.S. Pat. No. 6,160,786, and entitled “Cartridge engaging assembly with Rack Drive Thumb Actuator System,” which is incorporated herein by reference for all that it discloses. Alternatively, other types of thumb actuator systems may be used. 
     The rack and pinion drive system utilized in one preferred embodiment of the present invention is shown and described in U.S. Pat. No. 6,160,786, referred to above, is best seen in FIG.  6  and may comprise an elongate gear rack  134  that is affixed to the first side member  78  of the picker frame assembly  76 . In one preferred embodiment, the gear rack  134  is attached to the first side member  78  to provide the second contact surface  62  described above as part of the improved thumb referencing and guide system  11 . In this embodiment, the smooth rear side  62  of the gear rack  134  faces the first side member  78 , while the toothed front side of the gear rack  134  faces the second side member  80  to engage with the rack and pinion drive system. 
     The gear rack  134  may be made from any of a wide range of materials, such as metals or plastics, suitable for the intended application, as will be described in more detail hereinafter. 
     The main body portion  82  of the thumb assembly  16  may be provided with a pinion gear  136  positioned so that it engages the gear rack  134 . See FIGS. 2,  3  and  4 . The pinion gear  136  is caused to rotate by a drive system  138  which, in one preferred embodiment, includes a motor  140  and a gear reduction system  142 . The motor  140  may be mounted to the main body portion  82  of the thumb assembly  16  and is operatively associated with the gear reduction system  142  so that the motor  140  rotates the pinion gear  136  to extend and retract the thumb assembly  16 . 
     The motor  140  may comprise any of a wide range of motors suitable for the intended application. In one preferred embodiment, the motor  140  comprises a permanent magnet D.C. motor, such as model no. RS-385PH, available from Mobuchi Co. of China, although other types may also be used. 
     The gear reduction system  142  may comprise a conventional gear reduction system utilizing a plurality of spur gears to provide a reduction ratio sufficient to allow the motor  140  to extend and retract the thumb assembly  16  at an appropriate speed and with sufficient force to withdraw and insert the data cartridge  22  into the selected cartridge receiving device. In one preferred embodiment, the gear reduction system  142  provides a reduction ratio of about 11.5:1, although other ratios may be used depending on, for example, the speed and torque characteristics of the particular motor  140  that is selected. The various spur gears comprising the gear reduction system  142  may be made from any convenient material, such as metal or plastic, suitable for the intended application. By way of example, in one preferred embodiment, the various spur gears comprising the gear reduction system  142  are made from brass and stainless steel. Alternatively, the gear reduction system could utilize other types of gears, such as worm gears, to provide the desired reduction. 
     The thumb assembly  16  may also be provided with any of a wide variety of finger assemblies well-known in the art for engaging data cartridges, such as data cartridge  22 . Accordingly, the present invention should not be regarded as limited to any particular type of finger assembly. By way of example, in one preferred embodiment, the finger assembly  36  may comprise an arm  144  having a hook portion  146 , as best seen in FIG.  3 . Hook portion  146  of arm  144  is configured to engage the notch  148  (FIGS. 9 a  and  9   b ) provided on the data cartridge  22 . The arm  144  is pivotally mounted to the main body  82  of the thumb assembly  16  so that the arm  144  is free to pivot about pivot axis  150 . See FIG. 6. A spring  156  may be used to bias the arm  144  toward the engaged position shown in FIG. 9 a . The arm  144  may be provided with a pin  152  (FIG. 6) which engages a guide track  154  provided on the second side  80  of frame assembly  76  (FIG.  7 ). The guide track  154  actuates the arm  144  as the thumb assembly  16  moves back and forth between the retracted position  112  shown in FIG. 9 a  and the extended position  114  shown in FIG. 9 b . However, since finger and track systems, such as finger assembly  36  and guide track  154 , for engaging cartridges 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 of the present invention, the particular finger assembly  36  and guide track  154  used in one preferred embodiment of the present invention will not be described in greater detail herein. 
     It is contemplated that the inventive concepts herein described may be variously otherwise embodied and it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.