A POSITIONING APPARATUS FOR A TAPE LIBRARY AND A METHOD OF CONFIGURING THE POSITIONING APPARATUS

TECHNICAL FIELD This invention relates to a positioning apparatus for a tape library and a method of configuring a positioning apparatus for a tape library. The invention has particular, but not exclusive, application in the storage and retrieval of digital tape cartridges in a digital tape library.

BACKGROUND

A tape library typically comprises an array of rows and columns of tape slots for storing tape cartridges. A single tape cartridge may, for example, hold 50GB of digital data. The tape library usually has a robotic gripper for retrieving a particular tape cartridge from or loading the tape cartridge to the corresponding tape slot. After retrieving the tape cartridge from its corresponding tape slot, the robotic gripper loads the retrieved tape cartridge onto a tape drive for subsequent recording/playback operations. The robotic gripper moves rotatably and longitudinally relative to a tray. The tray moves vertically relative to the tape library.

Typically, inventory management of the tape cartridges is performed by a controller.

The controller commands and controls the positioning of the robotic tray and the robotic gripper during the retrieval/loading of tape cartridges from/to their corresponding tape slots. To avoid positioning errors, adjacent tape slots are conventionally arranged apart in a spaced relationship from each other so as to ensure adequate clearance between them. However, the need for ensuring adequate clearance between adjacent tape slots tape consequentially constrains the capacity of the tape library to store tape cartridges.

SUMMARY

According to a first exemplary aspect, there is provided a positioning apparatus for a tape library. The positioning apparatus comprises a tape cartridge loader movable relative to the tape library along an axis. At least one sensor on the tape cartridge loader is operable to detect a reference position along the axis. A controller is operable to track a distance moved by the tray along the axis.

Preferably, the controller is configured to encode a first position of the tray based on a first pre-determined offset distance from the reference position detected by the sensor. The controller may encode a second position of the tray along the axis based on a second pre-determined offset distance from the reference position detected by the sensor. The reference position may be defined by at least one edge of a flag.

According to another exemplary aspect, there is provided a positioning apparatus for a tape library. The positioning apparatus comprises a tray movable along an axis relative to the tape library. At least one sensor is operable to detect a reference position along the axis. At least one flag is mounted to and extending from the tape library towards the tray. Each flag is operatively aligned with one of the sensors and has at least one edge for defining the reference position.

For both aspects, the at least one edge of the flag may be substantially perpendicular to the axis. The flag may comprise a lower edge for defining the reference position when the tray is generally moving in a first direction along the axis. Alternatively, the reference position may be defined by an upper edge of the flag when the tray is generally moving in a second direction along the axis. In this alternative example, the second direction is opposite to the first direction.

The tape may be for locating a tape slot from at least one column of tape slots of the tape library. Optionally, the at least one column of tape slots is substantially parallel to the axis. Further, the flag may be for locating an upper tape slot and a lower tape slot from the at least one column of tape slots. The flag may extend laterally outwardly from the tape library from between two of the columns of tape slots.

In addition, the positioning apparatus may comprise a plurality of flags above and substantially identical to the flag. The plurality of flags may all extend laterally outwards from the tape library from between the two columns of tape slots. :

The at least one sensor may be a photo-interrupter sensor. The photo-interrupter sensor may be a U-shaped member through which the flag passes.

The controller may be an encoder. The encoded may be a linear encoder strip.

For both aspects, a foremost edge of the flag may be substantially perpendicular to the axis. The foremost edge of the flag may define the reference position when the tape cartridge handler is moving along the axis. Both aspects may further comprise at least one vertical-marking flag and at least one horizontal-marking flag, and may also further comprise a vertical-sensing sensor and a horizontal-sensing sensor. The at least one sensor may be a photo micro reflective sensor. A window panel may be provided having at least one slit window for aligning with the at least one sensor, the slit window having a width narrower than a foremost edge of the one flag. The foremost edge of the flag may comprise a reflective surface.

According to a further exemplary aspect, there is provided a method of configuring a positioning apparatus for a tape library, the positioning apparatus having at least one sensor, a controller and a tray movable relative to the tape library along an axis. The method comprises determining a first offset distance of the tray from a reference position along the axis; tracking a distance moved by the tray in a direction along the axis after the at least one sensor detects the reference position; and encoding a first position of the tray along the axis by the controller based on the tracked distance and the first offset distance of the tray from the reference position.

The method may further comprise encoding a second position of the tray along the axis by the controller based on a second offset distance of the tray from the reference position.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings: Fig. 1 is a perspective view illustrating a first exemplary embodiment of a positioning apparatus and four columns of tape slots of a tape library;

Fig. 2 is a close-up view of a section of Fig. 1 illustrating a column of flags extending laterally outwardly from between two columns of tape slots;

Fig. 3 is a close-up view of a photo-interrupter sensor of the first exemplary embodiment of Fig. 1 ;

Fig. 4 is a close-up side view of the positioning apparatus of Fig. 1 ; Fig. 5 is another close-up side view of the exemplary embodiment of Fig. 1 illustrating the use of flags for locating tape slots of the tape library;

Fig. 6 is a further side view of the exemplary embodiment of Fig. 1 illustrating the use of a flag for locating a top-most tape slot; Fig. 7 is a perspective view illustrating a second exemplary embodiment of a positioning apparatus and four columns of tape slots of a tape library; Fig. 8 (a) is a close-up view of a section of Fig. 7 illustrating a column of flags extending laterally outwardly from a column of tape slots; Fig. 8 (b) is a close-up front view of the flags of Fig. 8 (a);

Fig. 9 is a close-up exploded perspective view of the positioning apparatus of Fig. 7; Fig. 10 is a close-up exploded perspective view of a robotic gripper of the positioning apparatus of Fig. 7;

Fig. 11 is a close-up front view of the robotic gripper of the positioning apparatus of Fig.

7; Fig. 12 is a close up perspective view of the robotic gripper of the positioning apparatus of Fig. 7 showing guiding and resting surfaces for a tape cartridge; Fig. 13 (a) is a close-up side view of a photo micro reflective sensor of the robotic gripper of Fig. 10; Fig. 13 (b) is a close-up side view of a photo micro reflective sensor and a reflective surface with a slit window therebetween;

Fig. 13 (c) is a close-up side view of the photo micro reflective sensor and the reflective surface of Fig. 13 (b) without a slit window therebetween;

Fig. 14 (a) is a side view of a calibration set-up for the robotic gripper of the positioning apparatus of Fig. 7;

Fig. 14 (b) is a perspective view of the calibration set-up of Fig. 14 (a); and Fig. 15 is a chart of sensing distance versus sensor voltage of the photo micro reflective sensor of Fig. 13.

DETAILED DESCRIPTION OF THE EMBODIMENT

Fig. 1 is a perspective diagram of a first exemplary embodiment of a positioning apparatus 101 for a tape library 103 for storing tape cartridges. The positioning

apparatus 101 comprises a tape cartridge loader 102. The tape cartridge loader 102 comprises a robotic tray 105 that is movable relative to the tape library 103 along a vertical axis 107 and a robotic gripper 111 that is supported by the robotic tray 105 and moves vertically together with the robotic tray 105. The robotic gripper 111 is for inserting a tape cartridge into a tape slot as well as removing a tape cartridge from a tape slot. The robotic gripper 111 includes a deck 112 onto which a tape cartridge is loaded during movement of the tape cartridge to and from a tape slot.

At least one sensor 20 is arranged or mounted on a (hidden) side of the tray 105 of the tape cartridge loader 102 that faces the tape library 103. For clarity, an identical or substantially identical tape library on a corresponding opposite side of the tape cartridge loader 102 has not been shown in Fig. 1. However, it can be clearly envisaged that the at least one sensor 20 (two of which are shown in Fig. 1) on this corresponding opposite side of the tape cartridge loader 102 operates in a same or similar manner with the tape library on the corresponding opposite side of the tape cartridge loader 102 that it faces.

The sensor 20 is preferably a photo-interrupt sensor and is operable to detect a starting position and a reference position along the vertical axis 107 as the tape cartridge loader 102 moves along the vertical axis 107.

In the first exemplary embodiment of Fig. 1 , two columns Q of flags 10 are arranged between four columns 119 of tape slots in the tape library 103. Preferably, each tape slot is substantially perpendicular to the vertical axis 107. Each flag 10 extends laterally outwards from between the two columns 119 of tape slots. A close-up view of the arrangement of the flags 10 with respect to the tape library 103 is shown in Fig. 2. The flags 10 are preferably identical. The flags 10 are used for accurately locating all the tape slots of the tape library 103 by serving as reference positions along the vertical axis 107 during calibration of the positioning apparatus 101. Fig. 5 shows a close-up side view of the flags 10, each flag 10a, 10b having a lower flag edge 11a, 11b respectively. Preferably, at least the lower flag edges 11a, 11 b are substantially perpendicular to the vertical axis 107.

The positioning apparatus 101 comprises a controller 115 operable to track a position of or a distance moved by the tape cartridge loader 102 in a direction along the vertical axis 107 from the starting position. The starting position may be on a lower end of the vertical axis 107 or on an upper end of the vertical axis 107. The controller 115 preferably comprises a linear encoder strip 115 having a scale and a readhead. In the preferred embodiment, the scale is vertically mounted to a fixed reference frame 116 while the readhead is attached to the tray 105 such that the readhead moves vertically along the scale as the tape cartridge loader 102 moves vertically relative to the tape library. Each location of the readhead on the scale corresponds to a specific count along the scale. As the tape cartridge loader 102 moves upwards or downwards, the count read by the readhead changes in accordance with the distance moved by the tape cartridge loader 102. By tracking the count read by the readhead during movement of the tape cartridge loader 102, the controller 115 is able to determine the actual vertical distance moved by the tape cartridge loader 102.

Fig. 3 is a close-up view illustrating the sensor 20 of the exemplary embodiment. Specifically, a photo-interrupt sensor 20 is shown comprising a U-shaped member 203 that operates by transmitting an infrared beam across opposite sides 205, 207 of the U- shaped member 203. In use, the photo-interrupt sensor 20 detects a lower flag edge 11 of a flag 10 when the lower flag edge 11 just passes through the U-shaped member 203 and interrupts the infrared beam transmission across the opposite sides 205, 207, as shown in side view in Fig. 4. When the infrared beam is interrupted, the controller 115 is notified to store a location of the readhead on the scale corresponding to a particular count on the scale.

Use of the flags 10 to serve as reference positions for locating the tape slots 30 will now be described with reference to Fig. 5. For proper insertion into a tape slot 30, the deck 112 of the robotic gripper 111 must be aligned at a same level as a slot shelf 31 of that tape slot 30. A constant distance B is pre-determined using known dimensioning techniques such as those used in precision engineering and stored in the controller 115. The distance B is a vertical separation between the sensor 20 and the deck 112 of the robotic gripper 111 and is stored as a specific number of counts on the scale by the controller 115. For every tape slot 30, a vertical separation A1 between the slot shelf 31 and the lower flag edge 11 (reference position) of that tape slot 30 is also pre-

determined and stored in the controller 115 as a specific number of counts. The vertical separation A1 thus serves as a pre-determined offset distance of the position of the tray 105 (when the deck 112 is aligned with the slot shelf 31) and therefore the position of the tape cartridge loader 102 from the reference position 11. The controller 115 thus stores a plurality of specific vertical separation values (as counts), one value A1 for each tape slot 30 and its corresponding lower flag edge 11 , and the constant value B for the fixed separation between the sensor 12 and the deck 112.

Where the apparatus 101 is calibrated prior to actual use, during calibration, the tape cartridge loader 102 is first brought to its lowest possible position, known as a Home position. The tape cartridge loader 102 is then raised along the vertical axis 107. When the sensor 20 detects the lower flag edge 11 of a flag 10, the tape cartridge loader 102 is immediately stopped. The tape cartridge loader 102 is then lowered by a distance that is a sum of B and A1 so that the deck 112 on the robotic gripper 111 becomes aligned with the slot shelf 31 of the tape slot 30 corresponding to that flag 10. The robotic gripper 111 is now at a precise position for proper insertion of a tape cartridge into the tape slot 30. This vertical position of the tape cartridge loader 102 is stored in the controller 115 as a specific position-count along the scale for that particular tape slot 30.

In similar manner, the tape cartridge loader 102 is raised, stopped and lowered accordingly for each tape slot 30 in a column 119, so that every tape slot 30 in a column 119 has a specific position-count stored in the controller 115. This is performed for all the tape slots 30 in all the columns 119 so that a position-count database is created and stored in the controller 115 for all the tape slots 30 in the tape library 103.

In an alternative calibration method particularly suited for a topmost tape slot 3Oe as shown in Fig. 6, a vertical separation A2 between the slot shelf 31 e and the lower flag edge 11d of a tape slot 3Od below is first pre-determined and stored in the controller 115. As the tape cartridge loader 102 is raised, the sensor 20 detects the lower flag edge 11 of a flag 10d corresponding to a tape slot 3Od. To align the deck 112 with the slot shelf 31 e of the tape slot 3Oe above, the tape cartridge loader 102 is moved up by a distance that is a difference between A2 and B. This position is also stored in the

controller 115 to mark the position of the trayi 05 for loading/unloading with respect to the topmost tape slot 3Oe.

With the position-count database stored in the controller 115 for all the tape slots 30, during actual use of the positioning apparatus 101 for tape loading/retrieving, the robotic gripper 111 can be accurately positioned at a particular tape slot 30 by simply vertically moving the tray 105 and stopping when the readhead is at location on the scale at a count corresponding to the position-count for that particular tape slot 30, as stored in the position-count database.

In an alternative embodiment, calibration need not be performed prior to actual use of the apparatus 101. During operation, the tape cartridge loader 102 is first lowered to the Home position. To access a particular tape slot 30 as shown in Fig. 5, the tape cartridge loader 102 is raised until the sensor 12 detects the lower flag edge 11 of the flag 10 corresponding to that tape slot 30, whereupon the tape cartridge loader 102 is immediately stopped. The tape cartridge loader 102 is then lowered by a distance corresponding to a sum of A1 (as already pre-determined for that particular tape slot 30) and B so that the deck 112 on the robotic gripper 111 becomes aligned with the slot shelf 31 of the tape slot 30. Since the vertical separation A1 is known for every tape slot 30 in all the columns 119, this method may be used to precisely position the deck 112 with respect to each tape slot 30 in the library 103 without prior calibration of the apparatus 101. For a topmost top slot 3Oe, a vertical separation A2 between the slot shelf 31 e and the lower flag edge 11 d of a tape slot 3Od below may first pre-determined and stored in the controller 115. To access the topmost tape slot 3Oe, as the tape cartridge loader 102 is raised, the sensor 20 may first detect the lower flag edge 11 of a flag 10d corresponding to a tape slot 3Od whereupon the tape cartridge loader 102 is immediately stopped. To align the deck 112 with the slot shelf 31 e of the tape slot 3Oe above, the tape cartridge loader 102 may then be moved up by a distance that is a difference between A2 and B.

Fig. 7 is a perspective diagram of a second exemplary embodiment of a positioning apparatus 201 for a tape library 203 for storing tape cartridges. The positioning apparatus 201 comprises a tape cartridge loader 202. The tape cartridge loader 202 comprises a robotic tray 205 that is movable relative to the tape library 203 along a

vertical axis 207 and a robotic gripper 211 that is supported by the robotic tray 205 and moves vertically together with the robotic tray 205. The robotic gripper 211 is for inserting a tape cartridge into a tape slot as well as removing a tape cartridge from a tape slot. The robotic gripper 211 includes a deck 212 onto which a tape cartridge is loaded during movement of the tape cartridge to and from a tape slot.

The positioning apparatus 201 also comprises a controller 215 operable to track a position of or a distance moved by the tape cartridge loader 102 in a direction along the vertical axis 207 and in a direction along the horizontal axis 209.

At least one position sensor 702 is arranged or mounted on a (hidden) side of the robotic gripper 211 of the tape cartridge loader 202 that faces the tape library 203. For clarity, an identical or substantially identical tape library on a corresponding opposite side of the robotic tray 205 has not been shown in Fig. 7. However, it can be clearly envisaged that the at least one sensor 702 on the robotic gripper 211 operates in a same or similar manner with the tape library on the corresponding opposite side of the robotic tray 205.

In the second exemplary embodiment of Fig. 7, two columns R of vertical-marking flags 71 OV and horizontal-marking flags 71 OH are arranged between four columns 219 of tape slots in the tape library 203. Each flag 710V and 710H comprises a preferably rectangular portion that extends laterally outwards from between the two columns 219 of tape slots. A close-up view of a column R of flags 710V and 710H with respect to the tape library 203 is shown in Fig. 8 (a). The vertical-marking flags 710V are preferably identical with each other and serve as reference positions for accurately locating all the tape slots of the tape library 203 along the vertical axis 207. The horizontal-marking flags 710H are preferably identical with each other and serve as reference positions for accurately locating all the tape slots of the tape library 203 along the horizontal axis 209. Each tape slot (e.g. 704) is preferably provided with one vertical-marking flag 701V and one horizontal-marking flag 710H. The flags 710V and 710H are preferably configured to be snap-fitted onto a receiving portion 706 of each corresponding tape slot 704.

Fig. 8 (b) shows a close-up front view of the vertical-marking flag 701V and the horizontal-marking flag 710H of an exemplary tape slot (e.g. 704) of Fig. 8(a) having a tape cartridge 208 inserted therein. Each flag 710V and 710H preferably has a gloss chrome reflective surface 712V and 712H respectively. The gloss chrome reflective surfaces 712V and 712H are preferably located at a foremost edge of each flag 710V, 710H that extends laterally outwards from between the two columns 219 of tape slots. The gloss chrome reflective surface 712V of the vertical-marking flag 710V preferably has a width of 0.6mm while the gloss chrome reflective surface 712V of the vertical- marking flag 710V preferably has a width of 1 mm.

The vertical-marking flags 710V are preferably positioned so that their gloss chrome reflective surfaces 712V appear as horizontally-oriented strips. Each vertical-marking flag 710V is preferably positioned on its tape slot 704 such that a centerline 714V of its gloss chrome reflective surface 712V is downwardly displaced by a predetermined Y mm from a bottom edge of a tape cartridge 208a stored in a tape slot 704a immediately above the tape cartridge 208 in its tape slot 704. The horizontal-marking flags 710H are preferably positioned so that their gloss chrome reflective surfaces 712H appear as vertically-oriented strips. Each horizontal-marking flag 710H is preferably positioned on its tape slot 704 such that a centerline 714H of its gloss chrome reflective surface 712H is leftwardly displaced by a predetermined X mm from a left edge of the tape cartridge 208 in its tape slot 704.

The at least one position sensor 702 on the robotic gripper 211 is preferably a low cost photo micro reflective sensor such as an OMRON™ EE-SY110 Photomicrosensor (Reflective). Preferably, as shown in Figs. 9 and 10, two such photo micro reflective sensors are provided: a vertical-sensing sensor 702V and a horizontal-sensing sensor 702H. The sensors 702V, 702H are preferably mounted on a camera board 720 on the robotic gripper 211 that preferably spans a width of the robotic gripper 211. The camera board 720 is located on the side of the robotic gripper 211 that faces the tape slots 704, with the sensors 702V, 702H on the side of the camera board 720 facing the tape slots 704.

A window panel 730 having at least one slit window 732 therethrough for aligning with the at least one position sensor 702 is also provided on the robotic gripper 211 of the

tape cartridge loader 202 as shown in Figs. 9 to 12. The window panel 730 is also located on the side of the robotic gripper 211 that faces the tape slots 704. When assembled, the camera board 720 is sandwiched between the window panel 730 and a push panel 220 on a front edge of a moving deck 222 of the robotic gripper 211.

Preferably, a vertical-sensing slit window 732V is provided on the window panel 730 and positioned such that it is aligned with the vertical-sensing sensor 702V on the camera board 720. Accurate positioning of the vertical-sensing slit window 732V with respect to the vertical-sensing sensor 702V is preferably achieved by means of four locating bosses 734V. The vertical-sensing slit window 732V preferably comprises a horizontally-oriented slot having a width of 0.4mm as shown in Fig. 13 (a).

Accurate alignment or positioning of the vertical-sensing slit window 732V with respect to the vertical-sensing sensor 702V means that an infrared beam emitted by the vertical-sensing sensor 702V will pass through the vertical-sensing slit window 732V and be reflected by the gloss chrome reflective surface 712V of a vertical-marking flag 710V on the tape slot 704 only when the tape cartridge loader 202 is moved along the vertical axis 207 such that the vertical-sensing slit window 732V is at a same vertical location as the vertical-marking flag 710V, as shown in Fig. 13 (b). Detection of the reflected infrared beam by the vertical-sensing sensor 702V through the vertical- sensing slit window 732V thus allows the position of the vertical-marking flag 71 OV to be captured by the vertical-sensing sensor 702V and stored by a controller 215.

Accurate vertical position capture is ensured by configuring the width of the vertical- sensing slit window 732V to be at most the same as or smaller than the width of the gloss chrome reflective surface 712V. In this way, oblique reflections of the emitted ray are blocked by the window panel 730 from returning to the vertical-sensing sensor 702V. The window panel 730 also serves to filter off and prevent detection of other nearby objects having low reflectivity and large surface area. By providing the window panel 730 with the vertical-sensing slit window 732V, only substantially perpendicular reflection of the emitted beam by the reflective surface 712V can be detected through the vertical-sensing slit window 732V as shown in Fig. 13(b), thereby ensuring that correct position capture only takes place when the tape cartridge handler 202 is properly vertically aligned with a corresponding tape slot 704. On the other hand,

without the window panel 30 and slit window 732V, obliquely reflected beams will be inadvertently captured by the sensor 702V even when the reflective surface 712V is not in line with the sensor 702V, as shown in Fig. 13 (c), resulting in inaccurate positioning of the tape cartridge loader 202 with respect to each tape slot 704.

Similarly, a horizontal-sensing slit window 732H is provided on the window panel 730 and positioned such that it is aligned with the horizontal-sensing sensor 702H on the camera board 720. Accurate positioning of the horizontal-sensing slit window 732H with respect to the horizontal-sensing sensor 702H is preferably achieved by means of four locating bosses 734H. The horizontal-sensing slit window 732H preferably comprises a vertically-oriented slot having a width of 0.4mm.

Accurate alignment or positioning of the horizontal-sensing slit window 732H with respect to the horizontal-sensing sensor 702H means that an infrared beam emitted by the horizontal-sensing sensor 702H will pass through the horizontal-sensing slit window 732H and be reflected by the gloss chrome reflective surface 712H of a horizontal- marking flag 710H on the tape slot 704 only when the robotic gripper 211 is moved along the horizontal axis 207 with respect to the tray 205 such that the horizontal- sensing slit window 732H is at a same horizontal location as the horizontal-marking flag 710H. Detection of the reflected infrared beam by the horizontal-sensing sensor 702H through the horizontal-sensing slit window 732H thus allows the position of the horizontal-marking flag 710H to be captured by the horizontal-sensing sensor 702H and stored by the controller 215.

Accurate horizontal position capture is ensured by configuring the width of the vertical- sensing slit window 732H to be at most the same as or smaller than the width of the gloss chrome reflective surface 712H. In this way, oblique reflections of the emitted ray are blocked by the window panel 730 from returning to the horizontal-sensing sensor 702H. The window panel 730 also filters off and prevents detection of other nearby objects having low reflectivity and large surface area. Only substantially perpendicular reflection of the emitted beam by the reflective surface 712H can be detected through the horizontal-sensing slit window 732H, thereby ensuring that correct position capture only takes place when the tape cartridge handler 202 is properly horizontally aligned with the corresponding tape slot 704.

As shown in Figs. 11 and 12, guiding surfaces 740 may be provided on either side of the robotic gripper 211 to control tape cartridge position along the horizontal axis 209. Preferably, two guiding surfaces 740 are provided, each having a longitudinal axis parallel to the direction of tape cartridge movement with respect to the robotic gripper 211. The guiding surfaces 740 are provided to control tape cartridge position in a direction parallel to the horizontal axis 209. Location of the horizontal-sensing slit window 732H on the window panel 730 may be at a pre-determined displacement XX mm from one of the guiding surface 740 as shown. Resting surfaces 742 may also be provided on the robotic gripper 211 to ensure that the tape cartridge is at a specific vertical displacement from the robotic gripper 211. Location of the vertical-sensing slit window 732V on the window panel 730 may be at a pre-determined displacement YY mm from the resting surfaces 742 as shown.

Where the apparatus 201 is calibrated prior to actual use, during calibration, as shown in Figs. 14(a) and (b), a first spring loaded probe 810 having a representative horizontal-marking flag 810F attached thereto is used to measure the exact displacement XX mm of the horizontal-sensing slit window 732H from one of the guiding surface 740 as mentioned above. A second spring loaded probe 812 having a representative vertical-marking flag 812F attached thereto is used to measure the exact displacement YY mm of the vertical-sensing slit window 732V from the resting surfaces 742 as mentioned above. The values XX and YY are then stored in the controller 215 for appropriate computations to be made with respect to the actual vertical and horizontal positions of the vertical-marking flags 710V and the horizontal- marking flags 710H respectively, as captured by the vertical-sensing sensor 702V and horizontal-sensing sensor 702H respectively during movement of the tape cartridge loader 102. In this way, exact positions of the tape cartridge loader 202 for accurate alignment of a tape cartridge 208 with respect to each and every tape slot 704 in the tape library 203 may be computed and stored in the controller 215 as the controller 215 tracks the distance moved by the tape cartridge loader 102 along each axis 207, 209. The exact stored positions of the tape cartridge loader 102 therefore allow subsequent accurate re-positioning of the robotic gripper 211 at any desired tape slot 704 in the tape library.

Fig. 15 is a chart of sensing distance versus sensor voltage of the photo micro reflective sensor of Fig. 11. Sensing distance refers to a distance between a slit window 732V or 732H in the window panel 730 and the respective gloss chrome reflective surface 712V or 712H of a respective flag 710V or 710H on a tape slot 704. As shown, peak sensing performance in detecting the reflective surface 712 is achieved at a distance of less than 3.3 mm as shown by the two darker curves for flags mounted on an upper tape slot edge and a lower tape slot edge. The lightest curve shows sensing performance in detecting a white surface such as paper. The chart shows that the sensor 702 does not detect at distances of less than 1.7mm.

It should be appreciated that the invention has been described by way of example only and that various modifications in design and/or detail may be made without departing from the spirit or scope of this invention. For example, for the apparatus 101 of the first exemplary embodiment, an upper flag edge 41 of the flag 10 (as shown in Fig. 5 ) may be used as a reference edge instead of the lower flag edge 11 as described above. Flags may also be provided only on alternate tape slots in a column so that one flag is used as a reference for two vertically adjacent tape slots. Also, the method for calibrating and or accessing a topmost tape slot may also be used to calibrate and access other tape slots. For the apparatus 201 of the second exemplary embodiment, the displacements XX mm and YY mm may take reference from other fixed structures on the robotic gripper 211 instead of from the guiding surface 740 and resting surfaces 742 respectively. Other than a gloss chrome surface 712 for the flags 710, other sufficiently reflective surfaces may be provided.