Abstract:
In a mass storage library, a module storage array includes a back plate having apertures patterned to form a cartridge storage together with cell blocks that mate with the openings in the back plate. A column of cell blocks are held in place between a shelf and a spring load and include calibration targets to align the cell blocks to the library accessor and reader. One stripe the length of the storage cell stack contains a bar-code. The bar-code extends the entire length of the stripe and identifies the individual cell blocks when empty. A plurality of columns of cell blocks provide the complete storage array. Each column of the cell blocks include one stripe with a bar-code along its length that is directly fastened to the back plate with each cell of the column having a separate bar-coding on the strip. The accessor includes a sensor that reads the coding to provide an indication of an empty cell block.

Description:
RELATED APPLICATIONS 
     This application is being filed on the same date together with patent application Ser. No. 09/713,336 TUC9-2000-0022, entitled “STORAGE CELL MOUNTING AND ALIGNMENT FOR CARTRIDGE SYSTEM LIBRARIES” and patent application Ser. No. 09/713,327 TUC9-2000-0043, entitled “LIBRARY SYSTEM HAVING CALIBRATION TARGETS FOR PRECISELY LOCATING ALIGNMENT OF COLUMNS OF CARTRIDGE STORAGE CELL ARRAYS RELATIVE TO THE CARTRIDGE ASSESSOR UNIT”, all assigned to the same assignee. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an information storage library and more specifically to the coding of empty storage cell using a coded label stripe fastened to the panel mount of the storage cells in the library. 
     BACKGROUND OF THE INVENTION 
     Information storage libraries include cartridges that are selected and loaded to a drive unit for data recording and reproduction. The tape library apparatus comprises a storage cell unit having a plurality of cells each accommodating a cartridge, a magnetic drive unit for recording and reproducing data, and an accessor for automatically switching cartridges between the cell unit and the drive unit. The library apparatus further comprises an input/output unit for entering cartridges into the library apparatus. The accessor transports cartridges through the drive unit, storage cell unit, and the input/output unit. Library apparatus of this kind are widely used as an external mass storage devices for computers. 
     In previous library storage systems, a bar-code reader or scanner is used to inventory the data storage media within the storage array of the library. This inventory method can also identify and locate empty storage cells within the storage array. All the storage cells are provided with a bar-code label that the accessor recognizes as denoted an empty cell. When there is a cartridge in any particular cell, and that cell is scanned by the bar-code reader, the reader either reads the cartridges&#39; bar-code label, if it is supplied with one, or it reads no label. If the storage cell is empty, the reader reads an “empty cell” label, and it recognizes the cell as empty. In the previous library systems, each storage cell had its own “empty cell” bar-code label attached to the cell at some point during the manufacture of the library system. With ever increasing storage capacities, this could mean there are several thousand storage cells in the storage array, each with its own bar-code label. What is needed is the elimination of the need for individual bar-code labels on every storage cell in the storage array. 
     It is, therefore, an object of the present invention to provide an enhanced storage cell mounting system for information storage libraries. 
     Another object of the present invention, therefore, is to provide a mounting for storage cells that is easy to assemble while providing a means for the library system to recognize an empty cell. 
     There is an ongoing need for an easy method of providing a coding label that provides each storage cell with its own empty cell label while providing an ease of manufacturing and assembling the storage array of cartridges that can provide the location of every empty cell in the array to the robotic picker. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are achieved by providing an automated storage library having a media storage cell array system for a plurality of data cartridges, at least one drive mechanism, a transport mechanism or accessor for moving the cartridges between the drives and the storage cell area, the transport mechanism or accessor includes a robotic picker, wherein a coding label stripe is applied to the back plane of the storage array. This label stripe is applied to the back plane vertically, allowing the full height of the storage array, using one stripe containing a continuous code striping on each label stripe for each column of storage cells in the array. Each storage cell has an opening at its back end that allows the reader to read the coded labels stripe whenever there is no cartridge media in that particular storage cell. The coding of each individual cell position on the stripe is either a bar-code striping or other coding information readable and translated to identify a special storage cell and is readable by a scanner for transmitting the information to the library system. 
     In accordance with another aspect of the present invention, there is provided a library system comprising: a plurality of cartridges for storing data information located on media within each of the plurality of cartridges, a plurality of data recorder modules for receiving and reading information from and writing information to a selected cartridge for information storage, at least one storage cell module array having a plurality of storage cell cavities for retaining the cartridges, a data directory for maintaining a directory of the information stored in the mass storage library, an accessor including a picker mechanism and associated with the storage module for retrieving cartridges from its storage cell cavity and for transporting cartridges between its storage cell and one of the plurality of data recorders and for returning the cartridges to its storage cell cavity after the operation and a gripper means mounted on the picker mechanism accessor for grasping the cartridge, the improvement wherein each storage cell module array includes a back plate mounting means having a plurality of stripes, one for each column of storage cells with the stripes including a code marking means along the entire length of each stripe. The code marks on the stripes are read by the accessor and denote the identification of an empty storage cell when no cartridge is in the cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an automated storage and retrieval data processing library employing the storage cell mounting means of the present invention; 
     FIG. 1A is an exploded view of the accessor  18  of FIG. 1; 
     FIG. 1B is a perspective view of the picker  19  of FIG. 1A; 
     FIG. 2 is a frontal view of the library system of FIG. 1; 
     FIG. 3 is a perspective illustration of a storage cell mount according to the present invention; 
     FIG. 4 is a perspective view of the mount of FIG. 3 including several storage cells and bar-code label stripes; 
     FIG. 5 is a perspective view of a close-up of FIG. 4; 
     FIG. 6 is a perspective view of the label stripes of FIG. 4 with bar-code markings; 
     FIG. 7 is a frontal view of the mounting plate including “T” shaped apertures for the storage cells; 
     FIG. 8 is a side view of the storage cell mount with the cell blocks; 
     FIG. 9 is a perspective view of the resilient spring means used to hold down the stack of storage cells as shown in FIG. 8; 
     FIGS. 10A to  10 D are perspective views of the top and bottom storage cell as shown in FIG. 5; 
     FIGS. 11A to  11 D are perspective views of the middle storage cells which are positioned between the top and bottom storage cells of FIGS. 10A to  10 D as shown in FIG. 8; 
     FIG. 12 shows a pictorial representation of the scan mechanism for the storage cell array; 
     FIG. 13 is a frontal view of a column of storage cells with the alignment fiducials; 
     FIG. 14 shows the center calculation alignment fiducial; 
     FIG. 15 is a frontal view of a storage cell array with the positioning of the top and bottom fiducials for each column as shown in FIG. 13; 
     FIG. 16 shows the scan procedure for the column fiducials of FIG. 15; and 
     FIG. 17 is a flowchart of the process steps to precisely position the gripper assembly to each storage cell. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the illustrations, like numerals correspond to like parts depicted in the figure. The invention will be described as embodied in an automated magnetic tape library storage subsystem for use in a data processing environment. Although the invention shown uses magnetic tape cartridges, one skilled in the art will recognize that the invention equally applies to optical disk cartridges or other removable storage media. Furthermore, the description of an automated magnetic tape storage system is not meant to limit the invention to data processing applications as the invention herein can be applied to storage and cassette handling systems in general. 
     FIG. 1 is a perspective view of one type of an automated information storage and retrieval library  10  employing an input/output station  13  mounted on a door  11 . The library  10  includes one or more data recorder modules or drives  12 , a plurality of cartridges  14  stored in a bank of cavities comprising storage cells  16  and forming a storage module, an accessor  18  for transporting a selected cartridge between a storage cell and a drive  12 , see FIG.  1 A. The accessor  18  includes a cartridge picker  19 . The accessor  18  can access cartridge either from the storage cells  16  or from the input/output station  13 . Only a few cartridges  14  and storage cells  16  are shown. It should be noted that the entire open section of the library  10  in operation is generally completely lined with storage cells  16  each containing one cartridge  14  and comprising several storage cell mounting arrays  26 . Each storage cell  16  is fastened to the back plate  22 . Columns of storage cells  16  rest on a shelf  28  as will be described further. 
     The drives  12  can be optical disk drives or magnetic tape drives and the cartridge can contain optical or magnetic media, respectively. The storage cells  16  are supported by the mounted plate  22  to hold the cartridges  14  in correct alignment with the cartridge picker  19  of the accessor  18  (see FIG. 1A) and also in alignment with the drives  12 . The library  10  also includes a library data director or controller that connects with, and controls the actions of the drives  12  and the accessor  18 . 
     FIG. 1B shows the picker mechanism  19  with two gripper mechanisms  20  and  21 . A bar-code scanner  17  is shown mounted to the picker  19 . The scanner  17  is used to scan the bar-code identifying marks for an empty cartridge storage cell  16  as shown in FIGS. 4-6 and discussed later. 
     FIG. 2 shows a frontal view of the library system  10  with the input/output station  13  positioned for easy access by an operator to input cartridges required by the library system and to remove cartridges after the reading and/or writing of the media in the cartridges is completed. The input/output station  13  is shown positioned next to the mounting plate  22 , see FIG. 1, which, together with the storage cells  16 , form one of a plurality of storage cell mounting arrays  26 . 
     The controller, which includes at least one computing processor, is further interconnected with a host processor from which it receives access commands. Information to be recorded on, or to be read from, selected cartridges  14  is transmitted between the drives  12  and the host through the library controller or directly between the drives  12  and the host. The storage cell mounting array  26  of the present invention includes the mounting plate  22  and the storage cells  16  and is shown in FIGS. 3,  4  and  5 . 
     Referring now to FIGS. 3,  4  and  5 , the storage cell mounting array  26  is shown as including the back or mounting plate  22  with some of the storage cell blocks mounted to the back plate  22 . The back plate  22  includes the shelf  28  supporting the bottom array of storage cell blocks. A plurality of essentially “T” shaped openings  30  and  31  are formed in the back plate  22  to support the storage cell blocks. The storage cell blocks include a top cell block  32  and a bottom cell block  34 . The top and bottom cell blocks  32  and  34  are the same and will be discussed later with respect to FIGS. 10A to  10 D. The storage cell blocks also include a series of intermediate cell blocks  36  which comprise the remaining storage cell blocks mounted between the top cell block  32  and the bottom cell block  34 . The intermediate cell blocks  36  are further discussed later for FIGS. 11A to  11 D. Resilient means shown as springs  38  press against the top cell blocks  32  and hold all of the storage cell blocks against the shelf  28 . Continuous bar-code label stripes  40  are provided as shown in FIGS. 4 and 6, one stripe  40  for each column of storage cell blocks  32 ,  34  and  36 . Each continuous stripe  40  includes a plurality of bar-code or similar identifying markings  41  (see FIG.  8 ). 
     In FIG. 6, five label stripes  40  are shown. The five stripes  40  would cover five columns of cell blocks (see FIG. 4) for one storage cell mounting array  26 . Each continuous stripe  40  includes a code marking  41  extending the entire length of each stripe  40 . The code marking  41  is viewable through each of the cell blocks in each column. The single stripe  40  and the single code marking  41  on each stripe  40  replaces the separate pieces for each cell block as required in the prior art. In the preferred embodiment, the code marking  41  is a bar-code arrangement as shown in FIG. 6. A front view of the back panel  22  of the storage cell mounting array  26  without the storage cell blocks is shown in FIG.  7 . 
     Referring now to FIG. 7, the storage cell mounting array  26  shows the back plate  22  and the positioning of the shelf  28  with respect to the “T” shaped openings  30  and  31 . Slots  42  are provided in the back plate mounting means  22 . The slots  42  support one end of the springs  38  (see FIG.  4 ), with the other end of the springs  38  pressing against the top cell block  32 . Further discussion of this placement of the spring  38  in relation to the back plate  22  will be given later when the framework  43  is discussed in relation to FIG.  8 . 
     The back plate  22  of the storage cell mounting array  26  is a single sheet of metal larger than the storage cell array  26 . Into this back plate  22  is punched a series of single “T” shaped aperture or holes  30  and double “TT” shaped holes  31 . The single “T” shaped holes  30  are spaced along the outer vertical edges of the back plate  22  and hold the outside column of the storage cells structure. The double “TT” shaped holes  31  form the inside columns of the storage cells with each “T” of the double “TT” shaped holes  31  supporting one edge of the storage cell structure. Thus, each double “TT” shaped holes support one edge of two storage cells structures. Further discussion of the holes  30  and  31  will be given later when the building of the back plate  22  to support the storage cell mounting array  26  is given. 
     The back plate  22  also supports the shelf  28  which in turn will support the storage cell blocks when the storage array  26  is mounted vertically into the library storage frame. The back plate  22  can be firmly mounted onto the library storage frame by welding, for instance, to the framework  43 . The punching of all of the openings in the back plate  22  such as the “T” shaped openings  30  and  31  and the slots  42  and the mounting of the shelf  28  can be done very accurately on a numerically controlled punch system, for instance, at the time of manufacture. Each of the storage cell blocks to be mounted onto the back plane  22  has two “T” shaped protrusions on the back of each storage cell block that clip into the back plate “T” shaped openings  30  and  31 . This provides the lateral location for the storage cells. The storage cell blocks sit and interlock one upon the other and are supported by the shelf  28  as shown in FIG.  8 . 
     Referring now to FIG. 8, the storage cell mounting array  26  includes the top cell block  32 , the bottom cell block  34 , and a plurality of intermediate cell blocks  36  supported between the top cell block  32  and the bottom cell block  34 . The bottom cell block  34  in turn is supported by the shelf  28 . The shelf  28  is held in position by a shelf support  47  which is firmly fastened to the back plate mounting means  22 . The spring  38  is shown pressing against the top cell block  32  while having its lower end protruding through one of the notches shown as slots  42 . A detail of the shape of spring  38  is shown in FIG.  9 . The framework  43  presses against the spring  38  to provide spring resilient force at the top end  48  against the top cell block  32  thereby holding all of the storage cell blocks  32 ,  34  and  36  against the shelf  28 . The interplay between each of the cell blocks into their supporting blocks and the formation of the storage cells  16  by the cell blocks will be further discussed with the structure of the cell blocks as shown in FIGS. 10A to  10 D and FIGS. 11A to  11 D. The configuration of the top cell block  32  and the bottom cell block  34  of the storage cell mounting array  26  is shown in FIGS. 10A to  10 D. The storage cell block shown in FIGS. 10A to  10 D is useful for either the top cell block  32  or the bottom cell block  34  as shown in FIG.  8 . FIGS. 10A and 10C show the back perspective view of the top cell block  32  showing the connection to the back plate  22  while FIGS. 10B and 10D show the frontal view of the top cell block  32  as viewed from the accessor  18  or robotics side of the library  10 . 
     FIGS. 10A and 10B show the topside of the top cell block  32  and FIGS. 10C and 10D show the underside of the same cell block. For purposes of this discussion, it will be assumed that the top cell block  32  is being described but it should be understood that the cell block of FIGS. 10A and 10B could equally be the bottom cell block  34 . In FIGS. 10A and 10B, the top of cell block  32  has a series of panels  50  which mainly are support structures to provide a structure to allow the spring  38  to apply a force which holds the cell stack against the shelf  28 . The storage cell blocks of this construction are of the size and shape to support the tape cartridge. The top cell block  32  includes two “T” shaped protrusions  52  and  54  which are shaped to mate with the “T” shaped openings  30  and  31  of the back panel  22  as shown in FIG.  7 . FIGS. 10C and 10D show the underside of the top cell  32 . The underside of the top cell block  32  includes an open area  56  that forms a cavity which provides one-half of the sleeve-like construction of the total storage cell that eventually will hold the cartridges  14 . There is essentially no difference between the top cell block  32  and the bottom cell block  34 , the only difference being is that there is one cell block mounted at the top of the storage cell array  26  (see FIG. 8) against which the spring  38  presses against while the bottom cell block  34  rests on the shelf  28 . Supported between the top cell block  32  and bottom cell block  34  are the intermediate cell blocks  36  as shown in FIG.  8 . The top and bottom cell blocks  32  and  34  also include a calibration target  49  which will be discussed later. The structure of the intermediate cell blocks  36  are shown in FIGS. 11A to  11 D. 
     FIG. 11A shows the topside of the intermediate storage block  36  while FIG. 11B shows the bottom side of the same intermediate cell block  36 , both views from the back side. The intermediate cell blocks  36  likewise include the “T” shaped portions  58  and  60  which are of the same construction to the “T” shaped  52  and  54  of the top cell block  32 . The bottom side of the top cell block  32  of FIGS. 10C and 10D includes projections  55  and  57 . The top side of the intermediate cell block  36 , as shown in FIG. 11A, includes mating locations  62  and  64  which mate with the projections  57  and  55  of the top cell block  32  (as shown in FIG. 10C) as well as the projections  68  and  70  of an adjacent intermediate cell block  36 . The intermediate cell block  36  also includes an opening  66  which together with the opening  56  of the top cell block  32  form one storage cell  16 . As shown in FIG. 11B, the intermediate cell block  36  also includes projections  68  and  70  and an opening  72 . All of the projections on the cell blocks  32 ,  34 , and  36  such as projections  55 ,  57 ,  68  and  70  interlock with mating structures of the intermediate blocks  36 , such as the mating structures  62  and  64  shown in FIG.  11 A. FIGS. 11C and 11D show the underside views of FIGS. 11A and 11B, respectively. 
     Referring again to FIG. 8 for the building of the storage cell mounting array  26 , together with FIGS. 10A to  10 D and  11 A to  11 D, the bottom cell block  34  is placed on the shelf  28  with its support structure panels  50  facing upward with its protrusions  52  and  54  placed into the bottom adjacent “T” shaped openings  30  and  31 . The projections  68  and  70  of the first intermediate cell block  36  lock with the mating structure  53  and  51 , respectively, of the bottom cell block  34 . The second intermediate cell block  36  is placed on the first intermediate cell block with its protrusions also placed into the next higher “T” shaped openings  30  and  31  of the back plate  22  and its projections  68  and  70  mating with the mating structures  64  and  62 , respectively, of the first intermediate cell block. The opening  66  of the first intermediate cell block and the opening  72  of the second cell block form a cavity  23  as shown in FIG. 8, which becomes one storage cell  16  of the library  10  of FIG.  1 . At the top of the stack, as shown in FIG. 8, the top storage cell  32  is placed on the last intermediate storage cell of the stack again with the projections  55  and  57  (FIG. 10C) mating with the mating surfaces  62  and  64  (FIG. 11A) and its protrusions into the appropriate “T” shaped openings of the back plate  22  thereby forming a cavity  24  which becomes the top storage cell  16  of FIG.  4 . The protrusions  52 ,  54 ,  58  and  60  of the storage cells lock into the “T” shaped openings  30  and  31  of the back plate  22  to hold the storage cells against the back plate  22  while permitting vertical movement of the stack of storage cells with the clamp spring  38  firmly holding the entire stack in contact with the shelf  28 , see FIG.  8 . 
     FIG. 9 shows the shape of the spring  38  together with its lower end  46  and the top end  48 . The spring clamp  38  can be made of any resilient material such as spring steel to form the holding structure as shown in FIG.  8 . 
     Referring now to FIG. 3 for an explanation of the construction of the storage cell mounting array  26  according to the present invention. After the back panel mounting means  22  is punched to contain all of the “T” shaped openings  30  and  31  and the slots  42 , see FIG. 7, together with the mounting openings for the shelf  28 , the shelf  28  and the shelf support  47  (see FIG. 8) are mounted to the back plate  22 . Then the bottom cells  34  are placed onto the shelf  28 . The protrusions such as protrusions  52  and  54  of FIG. 10A are locked into the “T” shaped openings  30  and  31 . After a row of the bottom cell blocks  34  are placed onto the shelf  28 , the first column of intermediate cell blocks  36  are placed on top of the bottom cell block  34 . The protrusions  68  and  72  of the intermediate cell block  36  of FIG. 11A then is contacted with the mating structures  51  and  53  of the bottom cell block such as shown in FIG.  10 B. The “T” shaped protrusions  58  and  60  of the intermediate cell block  36  mate with the “T” shaped openings  30  and  31 . These intermediate cell blocks are then placed one on top of the other until the entire height of the storage cell array is completed. A top cell  32  is placed at the top end such that protrusions  55  and  57  likewise mate with the mating structures  62  and  64  of the intermediate cell  36  which is just underneath the top cell block  32 . The spring  38  then has its lower end  46  inserted into the notch  42 , see FIG. 7, while the top end of the spring  38  is directed against the top cell block  32  as shown in FIG.  8 . Each column of cell blocks from the bottom cell  34  through the intermediate cell blocks  36  to the top cell block  32  form one column of the entire storage cell array. The openings in each of the storage blocks provide one half of the eventual storage cell openings  16  to contain one cartridge  52 . The horizontal portion of the “T” shaped openings  30  and  31  provide an easy slot for entering the cell blocks  32 ,  34  and  36  into the back panel  22  while the vertical portion of the “T” shaped openings  30  and  31  provide a restriction against any horizontal movement of the individual cell blocks. Thus, all of the cell blocks are inserted into the “T” shaped openings  30  and  31  via their “T” shaped protrusions. The bottom cell  34  rests against the shelf  28 . The top cell  32  and the intermediate cells  36  are firmly held into position with their “T” slots and “T” shaped protrusions interlocking and all are held firmly down against the shelf  28  by virtue of the springs  38 . Each column of cell blocks interlock and are therefore held in perfect alignment in both the vertical and horizontal position thereby providing accurate one time alignment of the storage array. The back plate  22  can be firmly mounted within the library system  10  by welding for instance to retain the accurate cell location for the life of the system. 
     The height of the storage cell array  26  will vary with not only the manufacturing tolerances of the individual cell blocks  32 ,  34  and  36  but also with temperature and humidity. This is accommodated by the top of the storage cell array being held down by the spring  38 . In this manner the storage cell array can grow and shrink and the individual cell blocks can still be held in close proximity one to the other. The robotic gripper  19  can be programmed to calibrate the location of both the bottom and the top of the storage cell array as will be discussed later. Thus the robotic gripper  19  can compute the exact location of every storage cell  16  despite the manufacturing tolerances and the temperature and humidity variations. 
     The calibration mechanism for the storage cell alignment is shown in FIGS. 12,  13 ,  14 ,  15 , and  16 . As shown in FIG. 12, the calibration mechanism includes the light emitting structure  74  and the optical sensor  72 . Both are mounted between the two gripper mechanisms  20  and  21  of the accessor  18  (see also FIGS.  1 A and  1 B). The light emitter  74  emits a beam of light towards the calibration target  49  of the cell stack  70 . The fiducial or calibration target  49  either returns a reflection that is sensed by the optical sensor  72  or no reflection is returned indicating that the calibration target  49  has been located. A digital response to this signal is used to determine the reflective state of the cell stack  70 . Locating the absolute positions of the cartridges in the storage cells in a library requires being able to sense the precise positions of each storage cell in a cell stack and the precise position of the drives for the cartridges. Using a calibration sensor that is mounted on the accessor  18  of the robotics mechanism, the four sides of the calibration target  49  can be determined with precision. The light emitter  74  images a spot of light onto the target fiducial  49  and the optical sensor  72  senses the reflected energy. When over the target  49  which is a hole in the top cell block  32  and the bottom cell block  34 , there is no reflected light. The hole to no hole transitions can be precisely determined. Finding the four edges of the target  49  provides the precise location of the center of the target  49 . 
     Knowing the center of both the top and bottom targets  49  in the X direction, see FIG. 13, the mast tilt of the robotics relative to the cell stack  70  can be determined. Knowing the Y center of both the top and bottom targets  49 , the Y location of each cell can be computed. All of the blocks comprising the cell stack, top cell block  32 , bottom cell block  34  and intermediate cell blocks  36  are the same height since each are molded. The center of each cell in the cell stack  70  can be calculated and used to position the accessor  18  and the gripper  20  and  21  for a pick or place operation. 
     Referring to FIG. 14, the process for finding each cell stack includes the following steps. 
     Nominal X and Y locations are used to position the tape library robotics in an area in which the fiducial exists in open space. A two pass vertical sweep is made with the calibration sensor in order to determine the Y center. Once the Y center is determined, the robotic is placed in that vertical center and a two pass horizontal sweep is made to determine the X center. Since it is impossible to stop the robot at the exact transition of the sensor, the two pass scan is performed and an average is taken to provide a more precise location of the fiducial center. The vertical center is calculated as YCenter=(Et(Pass 1 )+Eb(Pass 1 )+Eb(Pass 2 )+Et(Pass 2 ))/4. The horizontal center is calculated as Xcenter=(El(Pass 1 )+Er(Pass 1 )+Er(Pass 2 )+E 1 (Pass 2 ))/4. 
     Once the top and bottom column fiducial centers are determined, the precise location of each cell can then be determined. First the vertical pitch of each cell can be calculated by: 
       Y pitch=(( Y TopCenter−Offset of Top Cell to Top Fiducial)−( Y BotCenter+Offset of Bottom Cell to Bottom Fiducial))/Number of Cells 
     The relative mast tilt of the robot is determined by 
     
       
         Mast Tilt=(Cell Number  X*Y Pitch* X ( X BottomCenter− X TopCenter))/( Y TopCenter− Y BottomCenter)) 
       
     
     The relative mast tilt of the robot is then applied to the calculation of the X position of the cell by the following: 
     
       
           X CellCenter= X TopCenter+Mast Tilt 
       
     
     The Y position of the cell is calculated by the following: 
     
       
           Y CellCenter=( Y TopCenter− Y Offset of Top Cell to Top Fiducial)−((Cell Number−1)* Y Pitch)+ Y Offset of picker palm to sensor 
       
     
     The fiducial target  49  at the top and bottoms of each storage column cell stack  70  are meant to be just black holes in space that can be found by using a nominal location of this hole. They are positioned on both the top and bottom of each storage column, drive, and I/O station. The process for storage columns, drives, and I/O stations are nearly the same except that drives only contain one fiducial. The fiducial located on the storage columns will be discussed here. They are shown in FIGS. 12,  13 ,  14 ,  15 , and  16 . Once the top and bottom fiducial centers are located, then any absolute location of the storage cells can be resolved. A hunting process is also used in order to allow a large tolerance off the initial or nominal locations. This concept eliminates a strict manufacturing build spec. This hunting process allows, for example, + or −9 mm off the nominal fiducial location. The fiducial center find process is as follows: 
     Position robot calibration sensor to top column fiducial position Stv from FIG.  16 . This is 14 mm above the nominal Y location of the fiducial. This allows for a wider Y window in the top column fiducial. The robot does a continuous motion downward until the calibration sensor switches to the on state. This is the indication that the optical spot is now positioned on the fiducial reflective space. The robot starts a negative sweep downward while monitoring the calibration sensor. This process monitors the state change of the sensor from on to off and back to the on state. This registers the state change of the fiducial hole edges as VE 1 N (off) and VE 2 N (on). These two edges found in the negative sweep are then compared to a window of 2 to 8 mm. This guarantees that this is the fiducial hole vertically. If the edges are determined not to be within the fiducial limits or a VExN edge is not found, then the robot is once again positioned to STv with a +3 mm offset in the X position and the scan process is repeated. This process is called hunting for the fiducial hole in the negative Y direction and is then −3 mm in the X direction continued with increments of +3 mm in the X direction. If VE 1 N and VE 2 N are found to within the fiducial hole limits, then a positive scan is made upward with the robot monitoring for fiducial edges VE 2 P and VE 1 P. No hunting is performed in the Y positive direction since it is assumed that we are within the fiducial hole. Now an average of the locations of these four edges can be performed to determine the Y center (Yc) of the fiducial hole. 
     Once the Y center (Yc) is found, then the robot is positioned to position STh from FIG.  16 . The X location of STh is derived as being the nominal X position+11 mm. This should start the scanning of the robot from within the fiducial hole. The robot then creeps in the negative direction until the calibration sensor indicates the on state. This state now puts the robot sensor on the reflective space of the fiducial. The robot starts a positive sweep across the fiducial while monitoring the calibration sensor. This process monitors the state change of the sensor from on to off and back to the on state. This registers the state change of the fiducial hole edges as HE 1 P (off) and HE 2 P (on). These two edges found in the positive sweep are then compared to be with a window of 6 to 15 mm. This guarantees that this is the fiducial hole horizontally. If HE 1 P and HE 2 P are found to be within the fiducial hole limits, then a negative scan is made across the fiducial with the robot monitoring for fiducial edges HE 2 N and HE 1 N. Now an average of the locations of these four edges can be performed to determine the X center (Xc) of the fiducial hole. 
     The vertical center of the column fiducial is calculated as the following: 
     
       
           Yc =( YE   1   P+YE   2   P+YE   1   N+YE   2   N )/4 
       
     
     The horizontal center of the column fiducial is calculated as the following: 
     
       
           Xc =( HE   1   P+HE   2   P+HE   1   N+HE   2   N )/4 
       
     
     This process is repeated for the bottom column calibration target  49 . In order to get a more precise location of the STv position of the bottom fiducial, the delta of the nominal top fiducial is used to obtain a more precise starting location. These then eliminate unnecessary hunting in the Y direction. The Yc and Xc (Y Center and X Center) for both the top and bottom column fiducials are saved for future cell locations as the following in FIG.  15 . XTnYTn and XBnYBn is found by the calibration sensor, whereas X is the X center location, Y is the Y center location, T is the top column fiducial, B is the bottom column fiducial and n is the column number or cell number. Since environment changes tend to change the height of columns, this process allows the dynamic calculation of the Y pitch of the storage cell locations based upon the top and bottom fiducial location data. Y pitch as shown in FIG. 12 is calculated as the following: 
     
       
           Y Pitch=(( YTn−YT Offset)−( Ybn+YB Offset))/NumCells, 
       
     
     whereas Ytn is the stored Y location of the top column n fiducial, Ybn is the stored Y locations of the bottom column n fiducial, YTOffset is the Y offset from the top column fiducial to the first storage cell, YBOffset is the Y offset from the bottom column fiducial to the last storage cell, and NumCells is the number of storage cells within the storage column. 
     Since manufacturing build tolerances vary from one machine to another, the X location of the top storage cell may be different than that of the bottom storage cell. This may be the result of the different placements of the storage array within the frame side. Also the vertical mast  18  of the picker assembly may lean or tilt as shown in FIG.  13  and this may cause the X location of the top column fiducial to be different from that of the bottom column fiducial. In both cases, this mast tilt can be calculated based upon the top and bottom fiducial location data as the following: 
     
       
         MastTilt=(NumCells* Y Pitch*( XBn−XTn ))/( YTn−YBn ) 
       
     
     whereas NumCells is the number of storage cells within the storage column, Ypitch is the calculated pitch of the storage cells as determined above, Xbn is the bottom column fiducial X location, Xtn is the top column fiducial X location, Ytn is the top column fiducial Y location, and Ybn is the bottom column fiducial Y location. 
     Once knowing the top and bottom X Y locations, mast tilt of the gripper assembly and frame array, and the Ypitch of the storage cells, an absolute storage cell location can be derived from the following: 
     
       
           Y Coord=( YTn−YS )−(Cell#* Y Pitch)− Y hand 
       
     
     whereas Ytn is the top column fiducial location, YS is the Y offset from the top column fiducial to the first storage cell, Cell# is the cell number with the storage column in which the absolute location is to be derived, Ypitch is the Y pitch of the storage cells with the storage column, and the Yhand is the offset from the center of the gripper mechanism to the calibration sensor as shown in FIG.  12 . 
     
       
           X coord= XTn +MastTilt+ X hand 
       
     
     whereas XTN is the top column fiducial location and MastTilt is derived as described above. 
     The process for precisely positioning the gripper assembly relative to each cell in the cell assembly, therefore, is shown in FIG.  17 . The process for finding the storage cell alignment, as discussed above for FIGS. 12 to  16 , block  100 , starts with the scanning of the calibration target, as shown in block  102  to find the four edges of the calibration target as shown in block  103 . The steps in blocks  102  and  103  are repeated until all four edges of the calibration target have been found, as shown in block  104 . Once all four edges have been found, the next step is to determine the precise location of the target center, as shown in block  105 . The next step in block  106  determines the mast tilt of the robotic accessor relative to the cell stack. In block  108 , the center of each storage cell in the cell column is calculated. Then, with the advantages of the calibration targets, as shown in block  110 , the gripper assembly can be accurately positioned according to the center of each cell in the cell assembly. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail maybe made therein without departing from the spirit and scope of the invention. For instance, the back plate may be made of steel for a rigid construction and the shaped openings in the back plate  22  can be formed by a numerical controlled process control computer which will provide accurate positioning of the “T” shaped openings  30  and  31  relative to each other and relative to the openings which hold the shelf  28  and the spring  38 . The cell blocks, likewise, can be made of plastic or any strong material to hold the shape for storing the cartridges within. The appended claims are the only limitation on the described invention.