Low tolerance positioning system in an automated handling system

An automated storage system transports data cartridges between a storage device and a plurality of storage cells via a picker assembly. The storage cells are formed of one or more removable magazines. The picker assembly accurately locates each data cartridge within a storage cell by first establishing a zero position reference and locating a home position relatively therefrom. The home position is a predetermined point on each magazine. An infrared detector and infrared sensor are situated on the picker assembly for precisely locating the home position. Each cartridge is known to be located within a predetermined tachometer count range from the home position. The detector/sensor is used to precisely locate each cartridge. The precise tachometer position is stored and the cartridge is returned to the known location. As a result, tolerance accumulation effects are eliminated in the system.

FIELD OF THE INVENTION 
The present invention relates generally to robotic handling systems, and 
more particularly, to a storage system having a picker assembly with a 
positioning system for accurately locating objects located within storage 
cells. 
BACKGROUND OF THE INVENTION 
Many business and science applications use computers to record data, with 
much of the data being stored on removable media. The removable media may 
take the form of magnetic tape cartridges, optical disk cartridges, floppy 
diskettes, or floptical diskettes. The advantages of storing data on 
removable media are numerous. These advantages include a capability of 
storing large amounts of data (additional cartridges can be used to store 
additional data), providing a vehicle for long term storage and archival, 
backing up data which resides on non-removable media, and allowing for 
easy transfer of data between computers. Removable media typically 
provides the most economical means of retaining the stored data. 
In the past, when a request for a specific removable medium (hereinafter 
referred to as data cartridge) was made, an operator needed to retrieve 
the data cartridge and physically load the selected data cartridge into 
the storage device. This manual mode involved a significant delay while 
the data cartridge was being retrieved by the operator before the data on 
that data cartridge could be processed. Additionally, the operator could 
easily make an error and load an incorrect data cartridge. 
With advancements in data storage products, the data cartridges were 
reduced in size and robots were designed and incorporated to automatically 
retrieve data cartridges and load those data cartridges into a storage 
device. The robot is housed within an automated storage library that also 
contain one or more storage devices and a plurality of data cartridges 
placed within storage cells. The robot replaced the operator resulting in 
improved access time and reliability. However, an initial alignment with 
each of the data cartridges within the storage cells would have to be 
established to allow for the robot to retrieve a chosen data cartridge. 
Often a camera is mounted to the robot to provide the feedback for the 
robot to be aligned with the desired data cartridge. Furthermore, the 
storage cells would be fixed and accurately located within the automated 
storage library to improve the reliability of the robot. Removal or 
shifting of the storage cells (as is possible with magazine type storage 
cells) requires realigning the robot. 
The difficulties and expense that exist with the camera positioning system 
have been addressed, to some degree, by using a strip encoder to provide 
positioning feedback. The strip encoder is affixed or aligned to the 
storage cells and is detected by sensors placed on the picker assembly. 
Both the strip encoder and the storage cells require identical 
cell-to-cell spacing (pitch) to accurately locate the picker assembly with 
each corresponding storage cell. Moreover, the strip encoder needs to be 
precisely affixed or aligned to the storage cells. 
Thus, what is need is a positioning system having a capability to directly 
detect data cartridges residing in the storage cells. Consequently, the 
complexity and expense of a camera feedback system is desired to be 
eliminated; likewise, the location tolerances between a strip encoder and 
the storage cells could also be eliminated. A positioning system which 
directly detects the data cartridges should precisely align the picker 
assembly with the storage cells without requiring the storage cells to be 
accurately located. Thus the storage cells could easily be removed and 
replaced within the automated storage library without hindering the 
alignment method. Furthermore, the data cartridges would not need to be 
accurately located within the storage cells, which would require less 
precision when manufacturing the storage cells. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved positioning 
system for locating objects within a plurality of storage cell locations. 
Another object of the present invention is to provide an automated storage 
system having a picker mechanism for randomly accessing storage media 
while minimizing time to access such storage media and minimizing the 
alignments required within the automated storage system. 
According to a first embodiment of the present invention, an automated 
storage system for transporting a storage medium from a storage cell to a 
storage device and vice versa, includes a picker assembly mounted to an 
accessor which translates the picker assembly between the storage medium 
and the storage device. Mounted to the picker assembly are an infrared 
emitter and an infrared detector used for detecting a predetermined 
feature on the storage cells and the edges of said storage medium. 
Assembled within the accessor are a motor for driving the accessor and a 
tachometer used to provide counts during positioning. A driver card 
contains the motor drivers, a position compare register, and a position 
counter. 
In another embodiment of the present invention, a method for positioning 
the picker assembly to be aligned with one of the plurality of storage 
medium is utilized. The method includes the steps of (a) moving the picker 
assembly to a first position at the end of the accessor travel; (b) 
translating the picker assembly a predetermined distance to a second 
position near the extended tab located on the storage magazine; (c) 
detecting the extended tab of the storage magazine to establish a "home" 
reference for said magazine; (d) saving the tachometer count of the 
magazine's "home" reference in a position register; (e) moving the picker 
assembly a predetermined distance to third position approximately in line 
with one of the storage medium; (f) detecting the edge of the storage 
medium; and (g) saving the tachometer count of the storage medium's edge 
within a position register. 
The foregoing and other objects, features, and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention, as illustrated in the accompanying 
drawing.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawing, like numerals correspond to like parts 
depicted in the figures. The invention will be described as embodied in an 
automated magnetic tape storage system for use in a data processing 
environment. Although the invention is shown using magnetic tape 
cartridges, one skilled in the art will recognize that the invention 
equally applies to optical disk cartridges or other storage media. 
Furthermore, the description of an automated magnetic tape storage system 
is not meant to limit the invention to data storage applications as the 
invention described herein can be applied to robotic handling systems in 
general. FIG. 1 shows an automated storage system 1 for managing removable 
media, for example magnetic tape cartridges. The removable media is 
contained within a plurality of data cartridges 5. Each data cartridge 5 
is located within a magazine 4, wherein each magazine 4 includes a 
plurality of storage cells for holding the plurality of data cartridges 5. 
The magazine 4 provides a means of retaining each data cartridge 5 while 
also allowing access to retrieve the plurality of data cartridges 5 via a 
picker assembly 3. 
The automated storage system 1 also includes at least one storage device 2, 
for example an IBM 3494 Magnetic Tape Drive. The magazine 4 is located 
across from and parallel to the storage device 2. A magazine may also 
reside parallel and adjacent to the storage device 2 (not shown). The 
picker assembly 3 transports a data cartridge 5 from the magazine 4 to a 
storage slot in the storage device 2. The picker assembly 3 is attached to 
an accessor 6 that translates the picker assembly parallel to the storage 
device 2 and magazine 4. The accessor 6 contains a motor 7 and a 
tachometer 8. The translation allows the picker assembly 3 to be 
positioned directly in line with any of the plurality of data cartridges 5 
and/or the storage slot. 
Upon being positioned in line with a selected data cartridge 5, the picker 
assembly 3 retrieves the data cartridge 5 from the magazine 4. The data 
cartridge 5 is retained within the picker assembly 3 while the picker 
assembly 3 is translated to the storage device 2 via the accessor 6. At 
the storage device 2, the picker assembly 3 transfers the data cartridge 5 
into the storage device 2. After the storage device 2 has processed the 
data cartridge 5, the picker assembly 3 removes the data cartridge 5 from 
the storage device 2 and is translated back to the originating storage 
cell (or an empty storage cell) within the magazine 4, where the picker 
assembly 3 transfers the data cartridge 5 back into the magazine 4. 
Referring now to FIG. 2, a tandem relationship of fore and aft grippers 20 
and 21, respectively, of the picker assembly 3 is shown. The fore gripper 
20 faces the magazine 4, and the aft gripper 21 faces the storage device 
2. The fore and aft grippers 20, 21 each include upper and lower fingers 
which close for gripping an article. The tandem relationship of the fore 
gripper 20 to the aft gripper 21 enables a data cartridge 5 to "pass-thru" 
the picker assembly 3, hence the picker assembly 3 operates as a pass-thru 
picker. Both the fore and aft grippers 20, 21 extend beyond an envelope of 
the data cartridges 5 when a data cartridge 5 is present on either side of 
the picker assembly 3. For the picker assembly 3 to be translated, the 
fore and aft grippers must be in an open position (as shown) to avoid an 
interference with any residing data cartridges 5. In order to retrieve a 
data cartridge 5 from the magazine 4 or the storage device 2, the 
appropriate fore or aft gripper 20, 21 must close and contact the given 
data cartridge. The pass-thru picker is advantageous for system time and 
space efficiency, but other gripper mechanisms are available and may be 
suitable. For example, in a storage system handling objects other than 
data cartridges, a larger or smaller gripping mechanism may be desired. 
FIG. 3 shows the picker assembly 3 in greater detail, including the 
infrared emitter 30 and infrared detector 31 used to position the picker 
assembly. While the preferred embodiment of the present invention uses 
infrared light for detecting, one skilled in the art will recognize that 
other detector forms may be incorporated, including but not limited to 
optical, proximity or capacitive, and mechanical. Both of the infrared 
sensors are wired to an interconnect card 33 by cables 34 and 35, 
respectively. An infrared beam 32 couples the emitter 30 and the detector 
31, and spans the area in which the cartridges are stored. Any object 
within this area, such as a cartridge or a magazine, will block the 
infrared beam 32 and interrupt the couple between the emitter 30 and the 
detector 31. 
FIG. 4 is a block diagram of the primary components of the system used to 
position the picker assembly. The infrared emitter 30 is coupled to the 
infrared detector 31 and senses if the spanned area is open or blocked. 
The infrared emitter 30 and the infrared detector 31 are cabled to an 
interconnect card 33 which resides on the picker assembly. The 
interconnect card 33 is cabled to a driver card 40 which contains motor 
drivers, position compare registers, and a position counter. Additionally, 
the driver card 40 is connected via forward and reverse direction lines to 
the accessor motor 7. Phase "A" and phase "B" lines provide feedback from 
the accessor motor tachometer 8 to the driver card 40. A processor resides 
within the tape drive electronics 41 and provides the motion control 
instructions used to position the picker assembly. The tape drive 
electronics 41 additionally contains the circuits and mechanics for 
writing data to and reading data from the data cartridges 5 as is well 
known in the art. The particular form of data storage may vary, e.g., 
linear, helical, analog, etc., and is not dispositive of the present 
invention. 
FIG. 5 depicts the spatial relationship between the picker assembly 3 and 
the magazines 4. In this plan view, the infrared detector 31 can be seen; 
the infrared emitter is located directly beneath the detector 31 in a 
plane below the data cartridges 5. As the picker assembly 3 is translated, 
moving left to right, the infrared beam between the emitter and detector 
would be blocked by data cartridges 5 and the side tabs of the magazines 
50 and 51. 
FIG. 6 shows the magazines 4 and the magazine's holder 61. Each magazine 4 
contains a plurality of storage cells, defined by dividers 60 which 
separate the plurality of data cartridges 5. The magazine 4 is removable 
from the magazine's holder 61 as indicated by the arrows. The lateral 
location of the magazine 4 is determined by the fit between the magazine 
and the magazine holder 61. Likewise, the lateral location of each data 
cartridge 5 is set by the fit of the data cartridge 5 within the storage 
cell. 
METHOD OF OPERATION 
Referring again to FIG. 1 and FIG. 5, the automated storage system 1 will 
establish a zero position reference upon being powered on. Initially, the 
accessor 6 moves the picker assembly 3 to a left end of the accessor 6, 
thus establishing a zero position reference for the automated storage 
system 1. The zero reference position can alternately be established at a 
right end of the accessor 6 or at a limit switch position. From the 
storage system 1 zero position reference, the accessor 6 will translate 
the picker assembly 3 a predetermined distance to the right, to an 
approximate area where the extended tabs 50 and 51 of the left and right 
magazines 4 are expected. While the picker assembly 3 is being moved, the 
number of counts generated by the tachometer 8 is being monitored. As the 
picker assembly 3 traverses the area, the infrared beam is blocked first 
by the extended tab 50 of the left magazine 4 (if the magazine has been 
inserted), and next by the extended tab 51 of the right magazine 4 (also 
if that magazine has been inserted). The locations of the extended tabs 50 
and 51 are noted and establishes a "home" reference for that specific 
magazine 4. Travel to any storage cell within a magazine 4 is made as a 
delta from the magazine's "home" reference. 
Using the positioning method described herein eliminates any magazine 
location error that exists because of tolerances of the magazine 4, 
tolerances of the magazine holder 61, and clearance between the magazine 4 
and the magazine holder 61. Therefore, the magazine 4 does not need to be 
held in a tight nominal position because the automated storage system is 
able to determine where the magazine 4 is located and establish a "home" 
reference for moving to storage cells within the magazine 4. 
Again referencing FIG. 1 and FIG. 5, the positioning method used to 
establish the magazine's "home" reference is extended further to eliminate 
the cumulative effect of tolerances within the magazine 4. Because narrow 
gaps exists between the data cartridges 5, the infrared emitter 30 and 
infrared detector 31 are able to locate the edge of any cartridge 52 
within the magazine 4. To detect the edge 52, the picker assembly is 
translated a predetermined distance from the magazine's "home" reference 
50 or 51, to an approximate area where the gap to the left of the chosen 
cartridge 5 is expected. Referencing the tachometer count at the 
magazine's "home" reference, the delta to the counts generated by the 
tachometer 8 is monitored. As the picker assembly 3 traverses the 
cartridge's edge 52 the infrared beam is blocked, and the tachometer count 
is saved in a register. Noting the tachometer count at this time allows 
the data cartridge 5 to be removed, brought to the storage device 2, 
returned from the storage device 2 to the same tachometer count, and 
placed into the magazine 4 in essentially the same position. This 
positioning can be repeated for any of the plurality of data cartridges 5. 
Positioning may also be accomplished via an encoder or stepping motor. The 
only positioning tolerances that remain are in the lead accuracy of the 
device that translates the picker assembly 3. All of the storage cell 
tolerances have been eliminated from positioning the picker assembly 3. As 
a result, storage cells, and hence data cartridges 5, can be packed more 
tightly since picker assembly 3 positioning is more precise. Conversely, 
data cartridges may be packed more loosely (manufacturing tolerance 
relaxed) due to the improved precision. Packing data cartridges 5 more 
tightly or relaxing manufacturing tolerances is a design choice based in 
part upon cost and space considerations. 
The positioning method described herein provides a means of transporting a 
data cartridge from a magazine to a storage device, and vice versa, while 
eliminating the tolerance accumulation that is present in the system. 
Mounted to the picker assembly are an infrared emitter and detector which 
span the data cartridges and magazines. The infrared beam between the 
emitter and detector is used to locate the extended tab on each magazine, 
and establish a "home" reference for each magazine. Consequently the 
magazine need not be held in a tight nominal position, and can easily be 
removed without hindering the alignment. The infrared beam is also used to 
detect the edge of the data cartridges residing in the magazine, thereby 
eliminating the cumulative effect of tolerances within the magazine. By 
doing so, the data cartridges do not need to be accurately located within 
the storage cells and the magazine may be more readily manufactured. 
In summary, an automated storage system for randomly selecting one storage 
medium from a plurality of storage media and transporting the selected 
storage medium therefrom has been described. The automated storage system 
includes a storage device for receiving the selected storage medium for 
retrieving data therefrom and/or storing data thereto. A plurality of 
storage cells contain the storage medium therein, the plurality of storage 
cells located across from and substantially parallel to the storage 
device. The storage media are oriented in the plurality of storage cells 
for direct transport to the storage device. A picker mechanism is used to 
retrieve the storage medium from the storage cells and the storage device. 
An accessor is positioned for carrying the picker mechanism and moving the 
picker mechanism between one of the plurality of storage cells and said 
storage device. An infrared emitter and an infrared detector are 
implemented to detect the magazine and the storage medium, and provide the 
positioning references for accurately locating any one of the storage 
medium. 
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 details may be made 
therein without departing from the spirit and scope of the invention. For 
example, various changes may be made to certain materials as long as the 
critical requirements are met, for example, weight, strength, etc. The 
detectors described herein are infrared detectors, but the detector types 
may vary so long as object and home position detection is effectively 
accomplished. Still further, means for attaching structures together may 
be changed without departing from the scope of the invention.