Patent Abstract:
One or more mounting registration points provide alignment between a FIMS system and a specimen handling system that delivers a specimen retrieved from a specimen transport box. The specimen handling system includes one or more mounting points, each cooperating with an alignment fixture to immovably secure the specimen handling system to the FIMS system at a corresponding mounting registration point. This mounting technique provides automatic alignment of the specimen handling system to each mounting registration point of the FIMS system.

Full Description:
RELATED APPLICATIONS 
     This application is a division of application Ser. No. 09/612,757, filed Jul. 10, 2000, now U.S. Pat. No. 6,501,070, which is a continuation in part of application Ser. No. 09/352,155, filed Jul. 12, 1999, now U.S. Pat. No. 6,281,516, which claims the benefit of provisional application No. 60/092,626, filed Jul. 13, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to front-opening interface mechanical standard (FIMS) system equipment and, in particular, to a FIMS transport box load interface that facilitates proper registration and accurate, secure positioning of a transport box as the specimens it contains are transferred between a minienvironment and a separate, enclosed specimen transport system. 
     BACKGROUND OF THE INVENTION 
     A system designed to incorporate FIMS permits handling of semiconductor wafers inside and outside of clean room facilities by interfacing a clean semiconductor wafer cassette transport box or pod to a clean environmental housing for semiconductor processing equipment or to other clean environments. The system concept entails mating a box door on a front-opening unified pod (FOUP) or cassette container box to a port door on an equipment enclosure and transferring the cassette into and out of the processing equipment without exposing to outside contamination the semiconductor wafers carried by the pod or wafer cassette. 
     A standard interface is required for cassette transport boxes intended to control the transport environment of cassettes containing semiconductor wafers. The standard interface addresses the proper transport box orientation for material transfer and maintains continuity between the transport box and semiconductor processing equipment environment to control particulate matter. The FIMS specifications are set out in the Semiconductor Equipment and Materials International (SEMI) standard SEMI E47-, E57-, E62-, and E63-0298 (1996-1998). 
     A FIMS system includes minimum volume, sealed front-opening boxes used for storing and transporting semiconductor wafer cassettes and canopies placed over wafer processing areas of semiconductor processing equipment so that the environments inside the boxes and canopies in cooperation with clean air sources become miniature clean spaces. The boxes are made of plastic materials having registration features located relative to one another within and of sizes characterized by relatively wide tolerances that can affect equipment alignment precision. What is needed is a box load interface implemented as part of a transfer mechanism for precise box alignment during loading and unloading of wafer cassettes from a sealed box without external environment contamination of the wafers carried by the wafer cassette. 
     SUMMARY OF THE INVENTION 
     The present invention is a box load interface implemented in a FIMS system. The box load interface comprises a retractable port door that is attachable to the box door of a transport box and that selectively moves the box door toward or away from the box cover of the transport box to thereby open or close it. A port plate has a front surface and a port plate aperture through which the box door can move as the port door moves the box door toward or away from the box cover. A slidable tray slidably mounted to a support shelf positioned transversely of the port plate receives the transport box in a predetermined orientation established by kinematic coupling surfaces located on the top surface of the slidable tray. 
     A slidable tray positioning mechanism selectively moves the slidable tray on the support shelf and thereby moves the transport box toward or away from the port plate. There are three preferred embodiments of a box hold down clamping mechanism mounted to the support shelf. The positioning mechanism is operatively connected to a first embodiment of the clamping mechanism to engage the clamping mechanism to a front clamping feature positioned on the bottom surface of the transport box and thereby apply an urging force to the box cover against the kinematic coupling surfaces while the slidable tray advances toward the port plate to push the front opening of the box cover against the front surface of the port plate. The positioning mechanism is operatively connected to the clamping mechanism also to disengage the clamping mechanism from the front clamping feature and thereby release the urging force from the box cover against the kinematic coupling surfaces while the slidable tray retracts from the port plate to pull the box cover away from the front surface of the port plate. 
     The box hold down clamping mechanism preferably includes a pivot finger pivotally mounted to the support shelf, and the slidable tray includes a push pin. The pivot finger has a recessed area that forms first and second angularly offset push pin contact surfaces that receive the push pin as the slidable tray moves the transport box toward the port plate and thereby rotates the pivot finger in a first rotational sense to engage the pivot finger to the front feature and moves the transport box away from the port plate and thereby rotates the pivot finger in a second rotational sense that is opposite to the first rotational sense to disengage the pivot finger from the front feature. The pivot finger includes a roller bearing that engages the front feature as the pivot finger rotates in the first rotational sense. 
     The positioning mechanism and each of second and third embodiments of the clamping mechanism are fixed with respect to each other so that a clamping mechanism operating under fluidic control engages and disengages from the front clamping feature in the absence of force applied by the sliding motion of the slidable tray. 
     The port plate includes a surface from which two compliant latch keys extend to mate with and operate the latch actuating coupler mechanism within its relatively wide alignment tolerance range, and a latching motor mechanism operatively connected to the compliant latch keys selectively rotates them between first and second angular positions. The latch keys are designed to “wobble” laterally to accommodate the tolerance range of the corresponding mating features on the box door and thereby ensure proper alignment to it. The first angular position secures the port door to and the second angular position releases the port door from the box door when the port and box doors are in matable connection. 
     An alternative embodiment of the two compliant latch keys includes a latch key pull back mechanism operating under fluidic control to securely hold the box door in alignment against the port door when the box and port doors are in matable connection. Maintaining the alignment established to fit the port door latch keys into the box door mating features ensures that there is no post-separation alignment shift between the box door and port door resulting from the loose tolerance range necessitating the wobbly latch key design. 
     The box load interface system also comprises a port door translation mechanism that is operatively connected to the port door to advance it in a forward direction toward the port plate aperture to attach the port door to the box door and then retract it and the attached box door in reverse direction away from the box cover and through the port plate aperture. A port door elevator assembly operates in cooperation with the port door translation mechanism to move the port door in a direction generally parallel to the front surface of the port plate after the box door has been moved away from the box cover and through the port plate aperture. 
     In a first embodiment, the port door translation mechanism and the port door elevator assembly are independent systems operating under coordinated control of separate motor drive assemblies. In a second embodiment, the port door translation mechanism and the port door elevator assembly are combined as a unitary mechanism. The unitary mechanism is implemented with a pivot link structure operating under control of a motor-driven lead screw mechanism to move the port door sequentially in transverse directions of movement that are the same as those accomplished by the translation mechanism and the elevator assembly of the first embodiment. 
     The transport box holds a container in which multiple wafer specimens are stored in spaced-apart, stacked arrangement. The container has an open front side from which the specimens are removed or into which the specimens are inserted. The box load interface comprises a differential optical scanning assembly for detecting positions of the wafer specimens. The scanning assembly scans the wafer specimens in a direction parallel to a facial datum plane, which is defined as a vertical plane that bisects the wafer specimens and is parallel to the open front side where the wafer specimens are removed or inserted. Scanning assembly includes two spaced-apart, pivotally mounted scanner fingers that are operable to center and push back dislodged specimens before determining their orientations in the cassette. 
     A robot assembly is supported by a linear traveling assembly between adjacent port plate apertures for removing and inserting wafer specimens from the transport box. The linear traveling assembly includes a nut mechanism contained within a housing secured to a carriage that supports the robot assembly. The carriage travels along a lead screw between the port plate apertures and is driven by the nut mechanism that includes a lead nut threadably engaged with the lead screw and rotated by a drive motor through a belt and pulley arrangement. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are respective front and rear perspective views of a wafer transport system in which a box load interface of the present invention for use in a FIMS system is implemented. 
     FIGS. 3A-3G show various views of a front-opening wafer carrier box and its components and features. 
     FIG. 4 is a plan view of a front-opening carrier box positioned on the slidable tray mounted to the interface system shelf with its top cover removed to show the slidable tray positioning mechanism components. 
     FIG. 5 is a side elevation view of the front-opening carrier box positioned on the interface system as shown in FIG. 4 but with the side cover of the interface system shelf removed. 
     FIG. 6 is a front side elevation view of the slidable tray and shelf with the carrier box and front cover removed. 
     FIGS. 7A and 7B are plan and side elevation views of the carrier box clamping feature shown in FIGS. 4,  5 , and  6 . 
     FIG. 8 is an enlarged front elevation view of the box load interface with the sheet metal cover removed to show the elevator assembly. 
     FIG. 9 is a left side elevation view of the box load interface of FIG.  8 . 
     FIG. 10 is an exploded view and 
     FIGS. 11A,  11 B, and  11 C are respective side, front, and rear elevation views of the latch key assembly. 
     FIG. 12 is a rear elevation view of the latch key motor mechanism mounted in the port door and the port door translation mechanism mounted on the interior surface of the front plate. 
     FIG. 13 is an enlarged rear elevation view of the latch key motor mechanism shown in FIG.  12  and of the positioning mechanism for the wafer scanning assembly. 
     FIGS. 14 and 15 are respective plan and side elevation views of the wafer scanning assembly mounted on the port plate. 
     FIGS. 16A and 16B are diagrams showing the light beam paths of two sets of light emitters and light sensors. 
     FIG. 17 is a diagram showing a front elevation view of the placement of a wafer cassette on a slidable tray (with the position of a properly registered semiconductor wafer shown in phantom) relative to the crossed beam paths of the light emitters and light sensors shown in FIGS. 16A and 16B. 
     FIG. 18 is a simplified block diagram showing the input signals to and output signals from a central control system that coordinates the operations of the various components of the box load interface mechanism of the invention. 
     FIG. 19 is a side elevation view of a robot assembly mounted to a lead nut assembly. 
     FIG. 20 is a partial side elevation view of the opposite end of the robot assembly. 
     FIG. 21 is a plan view of the lead screw and lead nut assembly. 
     FIGS. 22-24 are respective left end, plan, and right end views of the lead nut assembly. 
     FIG. 25 is a top perspective view of a fluidic pressure controlled pivotable latch for securing a carrier box to the slidable tray. 
     FIG. 26 is an enlarged side elevation view of the pneumatic actuating mechanism of the pivotable latch of FIG. 25 in its carrier box clamping position. 
     FIG. 27 is a sectional view taken along lines  27 — 27  of FIG.  26 . 
     FIG. 28 is an enlarged side elevation view of the pneumatic actuating mechanism of the pivotable latch of FIG. 25 in a carrier box nonclamping, retracted position. 
     FIG. 29 is a top plan view of a fluidic pressure controlled carrier box bottom latch actuating mechanism. 
     FIG. 30 is a cross-sectional view of a latch key rotation mechanism of the bottom latch actuating mechanism of FIG.  29 . 
     FIG. 31 is an enlarged cross-sectional view of a latch key raise/lower mechanism of the bottom latch actuating mechanism of FIG.  29 . 
     FIG. 32 is a sectional view taken along lines  32 — 32  of FIG. 37, showing a latch key pull back assembly that is a modification of the latch key assembly of FIGS. 9,  10 , and  11 A- 11 C. 
     FIG. 33 is a rear elevation view of a fluidic pressure controlled latch key actuating mechanism. 
     FIG. 34 is a sectional view taken along lines  34 — 34  of FIG.  33 . 
     FIG. 35 is a cross-sectional view of the port door of FIG. 33, showing certain pneumatic control components of the latch key actuating mechanism. 
     FIG. 36 is a sectional view taken along lines  36 — 36  of FIG.  33 . 
     FIG. 37 is an enlarged fragmentary view of the latch key actuating mechanism of FIG.  33 . 
     FIGS. 38,  39 , and  40  are side elevation views (with FIG. 38 shown partly in cross section) of a four-bar carriage assembly of unitary construction that combines the functions of the port door translation and port door carriage mechanisms shown in FIGS. 8,  9 , and  12 . 
     FIG. 41 is a fragmentary front side elevation view showing the arrangement of the components of the four-bar carriage assembly mounted to the exterior surface of the front plate of the wafer transport system. 
     FIG. 42 is an enlarged fragmentary isometric view of a pair of bar links pivotably attached to the right-hand side surfaces of the Z and link carriages shown in FIG.  41 . 
     FIGS. 43 and 44 are enlarged fragmentary isometric views of a pair of bar links pivotably attached to the left-hand side surfaces of the Z and link carriages shown in FIG.  41 . 
     FIG. 45 is a side elevation view of a vertical/horizontal port door displacement fluidic control counterbalance mechanism of the four-bar carriage assembly of FIGS.  38 - 44 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a wafer transport system  10  that has an assembly frame  12  to which two front or port plates  14  are attached. Each front plate  14  supports one of two substantially identical box load interface systems  16  for front-opening semiconductor wafer carrier boxes  18  and a linear traveling robot assembly  20  positioned to access the wafers stored in carrier boxes  18  after they have been opened. A right side interface system  16  is shown with a shelf  22  having a slidable tray  24  supporting a carrier box  18 ; and a left side interface system  16  is shown partly disassembled without a carrier box  18 , a shelf  22 , and a sheet metal cover  26  to show the components of an elevator assembly  28 . 
     FIGS. 3A-3G show various views of carrier box  18  and its components and features. 
     FIG. 3A shows carrier box  18  with its box door  30  removed to reveal in the interior of carrier box  18  a wafer cassette  32  with slots spaced apart to accommodate 300 mm diameter semiconductor wafers. Carrier box  18  has a recessed, stepped interior side margin  34  against which the perimeter of an interior surface  36  of box door  30  rests when carrier box  18  is closed. 
     FIGS. 3B and 3C show, respectively, carrier box  18  closed with box door  30  unlocked and interior surface  36  of box door  30  in its unlocked condition; and FIGS. 3D and 3E show, respectively, carrier box  18  closed with box door  30  locked and interior surface  36  of box door  30  in its locked condition. FIG. 3C shows four locking slats  38  fully retracted so that their end tabs  40  remain inside the interior of box door  30 , and FIG. 3E shows locking slats  38  fully extended so that their end tabs  40  extend outwardly of the top and bottom side margins of box door  30 . 
     FIG. 3B shows end tabs  40  positioned outside of slots  42  located in the outermost portion of recessed side margin  34  when box door  30  is unlocked, and FIG. 3D shows end tabs  40  fitted into slots  42  when box door  30  is locked in place. FIGS. 3B and 3D also show two locator pin depressions  44  and two box lock actuating mechanism slots  46  required by the SEMI specification for a FIMS box door. 
     FIGS. 3F and 3G show, respectively, a bottom surface  48  and a box front retaining or clamping feature  50  on bottom surface  48  of a front-opening carrier box  18 . FIG. 3F also shows a center retaining feature  52 , which is an alternative to box front retaining feature  50  for securing carrier box  18  in place on slidable tray  24 . A preferred box  18  is a model F300 wafer carrier manufactured by Integris, Inc., Chaska, Minn. With reference to FIG. 3F, box  18  has on its bottom surface  48  five carrier sensing pads  54 , two advancing box sensing pads  56 , a carrier capacity (number of wafers) sensing pad  58 , a box or cassette information pad  60 , and one each of front end of line (FEOL) and back end of line BEOL information pads  62  required under SEMI E47.1 (Mar. 5, 1998). (FIGS. 25 and 29 show on slidable tray  24  four locations  63  corresponding to the locations of pads  58 ,  60 , and  62  on bottom surface  48  of box  18 . FIG. 25 shows a lockout pin  63   p  placed in the location  63  corresponding to one of the two tray information pads  62 .) Three oblong, inwardly sloped depressions in bottom surface  48  form kinematic pin receiving features  64  that mate with kinematic coupling pins  66  (FIG. 4) fixed in corresponding locations on slidable tray  24  when box  18  is properly installed. Kinematic coupling pins  66  preferably have threaded stem portions that engage threaded holes in slidable tray  24  so that shims can be used as a height adjustment for kinematic coupling pins  66  and thereby facilitate proper alignment of box  18 . When box  18  is placed in proper alignment on slidable tray  24 , sensing pads  54  and  58  and information pads  60  and  62  contact switches mounted in corresponding positions on slidable tray  24  and advancing box sensing pads  56  contact switches mounted in corresponding positions on shelf  22 . 
     With reference to FIGS. 3F and 3G, a depression  68  partly covered by a projection  70  having a beveled surface  72  forms front retaining or clamping feature  50 . Beveled surface  72  provides a ramp along which a wheel or roller can roll up while tray  24  slides box  18  toward an aperture  74  in front plate  14  to mate with a port door  76  (FIGS. 4,  5 ,  8 ,  9 ,  12 , and  13 ) secured to an interior surface  78  of front plate  14 . 
     FIGS. 4,  5 ,  6 ,  7 A, and  7 B show carrier box  18  placed on slidable tray  24  with portions shown in phantom lines to indicate the operation of a slidable tray positioning mechanism  88 . With particular reference to FIGS. 4 and 6, slidable tray  24  has a bottom surface  90  to which two U-shaped guide rails  92  are fixed by bolts  94 . Guide rails  92  extend near the side margins of slidable tray  24  in a direction perpendicular to an exterior surface  96  of front plate  14 . Two guide tracks  98  are bolted to shelf  22  in positions to receive guide rails  92  so that slidable tray  24  can move in a direction toward and away from exterior surface  96  of front plate  14  in response to the operation of tray positioning mechanism  88 . 
     Tray positioning mechanism  88  is mounted to shelf  22  and includes a tray motor  100  from which a shaft  102  extends to a coupler  104  that operatively joins shaft  102  to rotate a lead screw  106  that passes through a nut assembly  108 . Lead screw  106  has an axis  110  and is supported at a proximal end in a tail bearing  112  and at a distal end in a preloaded bearing  114 . Nut assembly  108  is fixed to bottom surface  90  of slidable tray  24  to move it in a direction along lead screw axis  110 . 
     Slidable tray  24  has in its bottom side an open region  120  into which two support members  122  extend in a direction parallel to tray bottom surface  90  to hold at their ends a push pin  124  carrying a cylindrical roller bearing  126 . A first embodiment of a pivotable latch  130  includes a clamping finger  132  mounted to a pivot pin  134  supported between pivot mounting blocks  136  that extend upright from shelf  22  and through open region  120  of tray  24 . Clamping finger  132  has a recessed area  138  that forms a first contact surface  140  and a second contact surface  142  that are angularly offset from each other and a hooked end  144  to which a cylindrical roller bearing  146  is mounted. Push pin  124  is set in a position to contact first and second contact surfaces  140  and  142  as slidable tray  24  moves in response to the operation of tray positioning mechanism  88  so as to, respectively, engage clamping feature  50  with and disengage clamping feature  50  from hooked end  144  of clamping finger  132  in accordance with the following operational sequence. 
     Whenever carrier box  18  is to be positioned against front plate  14  to mate box door  30  with port door  76 , tray motor  100  rotates lead screw  106  in a first lead screw rotational sense to advance nut assembly  108  and thereby translate slidable tray  24  along shelf  22  in a direction toward front plate  14 . This movement of slidable tray  24  causes roller bearing  126  to contact first contact surface  140  and as a consequence cause clamping finger  132  to rotate about pivot pin  134 . As slidable tray  24  continues to advance toward front plate  14 , clamping finger  132  continuously rotates in a first clamping finger rotational sense so that hooked end  144  rolls up beveled surface  72  and fits within box clamping feature  50  and so that roller bearing  126  fits within recessed area  138 . The distances separating roller bearing  126 , pivot pin  134 , and front plate  14  are set so that box door  30  mates with port door  76 , and a front side margin  148  (FIG. 3A) of carrier box  18  is in a sealing relationship with exterior surface  96  of front plate  14  when hooked end  144  fully engages clamping feature  50 . Full engagement of clamping feature  50  urges carrier box  18  against kinematic coupling pins  66  so that it is not dislodged when latch keys  150  extending from port door  76  unlock and remove box door  30 . 
     Whenever carrier box  18  is to be retracted from front plate  14  after box door  30  has separated from port door  76  and sealed carrier box  18 , tray motor  100  rotates lead screw  106  in a second lead screw rotational sense that is opposite to the first lead screw rotational sense to retract nut assembly  108  and thereby translate slidable tray  24  along shelf  22  in a direction away from front plate  14 . This movement of slidable tray  24  causes roller bearing  126  to roll out of recessed area  138  and contact second contact surface  142  and as a consequence cause clamping finger  132  to rotate about pivot pin  134 . As slidable tray  24  continues to retract from front plate  14 , clamping finger  132  continually rotates in a second clamping finger rotational sense that is opposite to the first clamping finger rotational sense so that its hooked end  144  rolls down beveled surface  72  and separates from box clamping feature  50 . Full disengagement of clamping feature  50  releases the urging force applied to carrier box  18  against kinematic coupling pins  66  so that carrier box  18  and its contents (one semiconductor wafer  152  shown in FIG. 4) can be removed from slidable tray  24 . 
     A second embodiment of a pivotable latch  153  is shown in FIGS. 25-28. Unlike pivotable latch  130 , pivotable latch  153  is supported on slidable tray  24  (instead of shelf  22 ) and is actuated by a pneumatic cylinder  154 , instead of by push pin  124  as slidable tray  24  slides along guide rails  92 . 
     With particular reference to FIGS. 25 and 27, pivotable latch  153  includes a clamping finger  155  mounted to a pivot pin  134 ′ fixed between sidewalls  156   a  and  156   b  of a rectangular, open interior mounting block  156  extending upright from slidable tray  24 . Clamping finger  155  is of similar construction to that of clamping finger  132 , except for the omission of recessed area  138 . Components of clamping finger  155  corresponding to those of clamping finger  132  are identified by the same reference numerals followed by primes. Clamping finger  155  has a hooked end  144 ′ to which a cylindrical roller bearing  146 ′ is mounted and a drive pivot pin  155   d  offset from pivot pin  134 ′ and projecting from one side of clamping finger  155 . Clamping finger  155  pivotally moves within the interior space of mounting block  156  so that hooked end  144 ′ projects upwardly outside of and recedes within the interior space bounded by the top surfaces of sidewalls  156   a  and  156   b  when hooked end  144 ′, respectively, engages and disengages box clamping feature  50 . FIG. 28 shows clamping finger  155  in its fully upward position (in phantom lines) and in a downward position (in solid lines). 
     With particular reference to FIGS. 26 and 28, pivotable latch  153  includes a first or top drive link  157  and a second or bottom drive link  158 . Top drive link  157  has an upper end  157   u  pivotally connected to drive pivot pin  155   d , and bottom drive link  158  has a lower end  158   l  pivotally connected to a stationary pivot pin  155   s  fixed inside sidewall  156   b . A lower end  157   l  and an upper end  158   u  of the respective top and bottom drive links  157  and  158  are pivotally connected to a common pivot pin  155   c  fixed in a distal end of an extensible rod  154   r  of pneumatic cylinder  154 . Pneumatic cylinder  154  has a body portion  154   b  into and out from which extensible rod  154   r  moves and which is fixed to slidable tray  24 . Drive pivot pin  155   d  and common pivot pin  155   c  move between their respective positions shown in FIGS. 26 and 28 as extensible rod  154   r  moves between its fully extended and fully retracted positions. Pneumatic cylinder body portion  154   b  includes a cylinder rod extension gas inlet  154   ei  and a cylinder rod retraction inlet  154   ri  to which gas conduits selectively deliver pressurized gas delivered by a switchable gas flow valve to, respectively, engage clamping feature  50  with and disengage clamping feature  50  from hooked end  144 ′ of clamping finger  155  in accordance with the following operational sequence. 
     Whenever carrier box  18  is to be positioned against front plate  14  to mate box door  30  with port door  76 , a user by means of software control actuates a solenoid valve  159 , which in response delivers pressurized gas to cylinder rod extension inlet  154   ei  and, as a consequence, causes clamping finger  155  to rotate about pivot pin  134 ′. As extensible rod  154   r  increases its length of extension from body portion  154   b , clamping finger  155  continuously rotates in a first clamping finger rotational sense (counterclockwise) so that hooked end  144 ′ rolls up beveled surface  72  and fits within clamping feature  50  so that top link  157  and bottom link  158  form between themselves an obtuse included angle that causes an over-center alignment that ensures positive locking action in the clamped position (FIG.  26 ). The distances separating common pivot pin  155   c  in full extension of extensible rod  154   r , pivot pin  134 ′, and front plate  14  are set so that box door  30  mates with port door  76 , and front side margin  148  (FIG. 3A) of carrier box  18  is in a sealing relationship with exterior surface  96  of front plate  14  when hooked end  144 ′ fully engages clamping feature  50 . Full engagement of clamping feature  50  urges carrier box  18  against kinematic coupling pins  66  so that it is not dislodged when latch keys  150  extending from port door  76  unlock and remove box door  30 . Tray motor  100  then rotates lead screw  106  in a first lead screw rotational sense to advance nut assembly  108  and thereby translate slidable tray  24  along shelf  22  in a direction toward front plate  14 . 
     Whenever carrier box  18  is to be retracted from front plate  14  after box door  30  has separated from port door  76  and sealed carrier box  18 , tray motor  100  rotates lead screw  106  in a second lead screw rotational sense that is opposite to the first lead screw rotational sense to retract nut assembly  108  and thereby translate slidable tray  24  along shelf  22  in a direction away from front plate  14 . After carrier box  18  reaches its fully retracted position, the user again by means of software control actuates solenoid valve  159 , which in response delivers pressurized gas to cylinder rod retraction inlet  154   ri  and, as a consequence, causes clamping finger  155  to rotate about pivot pin  134 ′. As extensible rod  154   r  decreases its length of extension from body portion  154   b , clamping finger  155  continually rotates in a second clamping finger rotational sense that is opposite to (clockwise) the first clamping finger rotational sense so that its hooked end  144 ′ rolls down beveled surface  72  and separates from box clamping feature  50 . Full disengagement of clamping feature  50  releases the urging force applied to carrier box  18  against kinematic coupling pins  66 so that carrier box  18  and its contents (one semiconductor wafer  152  shown in FIG. 4) can be removed from slidable tray  24 . 
     A third embodiment of a fluidic pressure controlled bottom latch actuating mechanism  900  is shown in FIGS. 29,  30 , and  31 . Bottom latch actuating mechanism  900  rotates a bottom latch key  902  between first and second angular positions to latch and unlatch center retaining feature  52  (FIG. 3F) of carrier box  18  and thereby hold down carrier box  18  against and release carrier box  18  from slidable tray  24 . Center retaining feature  52  formed in carrier box bottom surface  48  includes a recessed area covered by a top piece having a slot opening of sufficient size to receive a latch key inserted in one angular position and to retain the inserted latch key in another angular position. Like pivotable latch  153 , bottom latch actuating mechanism  900  is supported on slidable tray  24 ; but unlike pivotable latch  153 , bottom latch actuating mechanism  900  does not include a pivotable latch having a clamping finger that engages box clamping feature  50 . As shown in FIG. 29, bottom latch actuating mechanism  900  fits within a recessed area on an interior bottom surface  901  of slidable tray  24 . Bottom latch actuating mechanism  900  includes a latch key rotation mechanism  904  and a latch key raise/lower mechanism  906 . 
     Latch key rotation mechanism  904  is comprised of two pneumatic cylinders  908  and  910  having respective extensible rods  912  and  914  that are connected to different free ends of a timing belt  916 . Timing belt  916  engages a timing pulley  918  to which latch key  902  is attached. Pneumatic cylinders  908  and  910  are contained by a common housing  920 , which is fixed to slidable tray  24  by bolts or other fasteners. Solenoid valves  922  and  924  deliver pressurized gas to gas inlet ports  926  and  927  of the respective pneumatic cylinders  908  and  910  to operate them in push-pull fashion to rotate timing pulley  918  and thereby turn latch key  902  between the first and second angular positions, which are preferably angularly displaced by 90 degrees. FIG. 29 shows latch key  902  in its open (unlatched) position. 
     Latch key raise/lower mechanism  906  is comprised of a pneumatic polygonal piston  928 , the outer surface of which is preferably of octagonal shape that mates with complementary inner surface features of timing pulley  918 , as shown in FIG.  30 . Solenoid valves  930  and  932  (positioned beneath the respective solenoid valves  922  and  924  in FIG. 29) deliver pressurized gas to respective gas inlet/outlet ports  934  and  936  mounted to an inlet housing  938  to selectively raise and lower polygonal piston  928  and thereby raise and lower latch key  902 . A central control system  349  coordinates the operation of solenoid valves  922 ,  924 ,  930 , and  932  to turn latch key  902  between the first (latched) and second (unlatched) angular positions when latch key  902  is present within center retaining feature  52  and turn latch key  902  to its second (unlatched) angular position to insert latch key  902  into or remove latch key  902  from center retaining feature  52 . Latch key  902  in its lower position is set sufficiently low to provide clearance to accommodate an approximately 10 mm side-to-side misalignment tolerance for carrier box  18  during its initial positioning on slidable tray  24 . 
     FIG. 30 is a cross-sectional view of latch key rotation mechanism  904 . With reference to FIG. 30, pneumatic cylinders  908  and  910  are (with one exception noted below) of the same structural design; therefore, the following description of their components and construction is directed only to pneumatic cylinder  908 . Pneumatic cylinder  908  includes an interior chamber  940  that is enclosed by a bushing  942  at one end and an end cap  944  at the other end. A piston  946  pushes against an interior end of extensible rod  912 , and a free end of extensible rod  912  extends through bushing  942  and outside of interior chamber  940  by a length of extension determined by the position of piston  946  in interior chamber  940 . A return coil spring  948   s  having a relatively large spring constant and positioned between bushing  942  and piston  946  of pneumatic cylinder  908  biases extensible rod  912  to retract into interior chamber  940  in the absence of pressurized gas. A return coil spring  948   w  having a relatively weak spring constant and positioned between bushing  942  and piston  946  of pneumatic cylinder  910  takes up the slack in timing belt  916  when extensible rod  912  of pneumatic cylinder  908  is in its fully retracted position to unlatch latch key  902  from center retaining feature  52  in the absence of pressurized gas. A bumper  950  fitted within a recess in piston  946  rests against an end  952  of an adjustment screw  954  secured against end cap  944  by a locking plate  956 . Adjustment screw  954  sets the minimum length of extension of the free end of extensible rod  912  in response to the force applied to piston  946  by return coil spring  948   s.    
     FIG. 31 is a cross-sectional view of latch key raise/lower mechanism  906  showing bottom latch key  902  in its raised position (solid lines) and lowered position (phantom lines). With reference to FIG. 31, latch key  902  includes a shaft  960  supported within a central opening  961  partly of octagonal shape and extending along the length of polygonal piston  928  by an upper bushing  962  and a lower bushing  964  held in place by respective retainer rings  966  and  968 . Shaft  960  is secured to polygonal piston  928  by a retainer ring  970 . Polygonal piston  928  moves in the direction of the length of shaft  960  in a cavity  972  formed within central opening  961  between circular end-of-travel cushions  974  and  976  positioned against the interior faces of the respective retainer rings  966  and  968 . Pressurized gas introduced by way of gas inlet/outlet ports  934  and  936  into cavity  972  moves polygonal piston  928  in the manner described below. 
     A seal  978  fitted within a recess in the outer surface of polygonal piston  928  and a seal  980  positioned between shaft  960  and polygonal piston  928  ensure gas tight separation of the regions in cavity  972  on either an upper face  928   u  or a lower face  928   l  of polygonal piston  928 . Seals  982  positioned between timing pulley  918  and upper bushing  962  and between gas inlet housing  938  and lower bushing  964  ensure that cavity  972  remains gas tight. 
     With reference to FIGS. 29,  30 , and  31 , latch key rotation mechanism  904  rotates latch key  902  between the first (latched) and second (unlatched) angular positions by alternate delivery of pressurized gas to gas inlet ports  926  and  927  of pneumatic cylinders  908  and  910 . Extensible rods  912  and  914  alternately extend from and retract into the respective pneumatic cylinders  908  and  910  in response to the delivery of pressurized gas and thereby impart reciprocating motion to timing belt  916 . Timing pulley  918 , which is journaled for rotation in an upper bearing assembly  986  and a lower bearing assembly  988  that are fixed in slidable tray  24 , rotates back and forth between the first and second angular positions in response to the reciprocating motion of timing belt  916 . Lower bearing assembly  988  is positioned closer than upper bearing assembly  986  to shaft  960  to provide clearance for timing belt  916 . An inner clamp  990  and an outer clamp  992  hold upper bearing assembly  986  within slidable tray  24  and thereby contain within slidable tray  24  the movable components associated with latch key  902 . A rotary seal  994  positioned between shaft  960  and upper bushing  962  forms a gas tight seal for the top end of cavity  972 . Rotary seals  994  positioned between shaft  960  and lower bushing  964  and between inlet housing  938  and timing pulley  918  form a gas tight seal for the bottom end of cavity  972 . 
     With reference to FIG. 31, latch key raise/lower mechanism  906  moves latch key  902  up and down by alternate delivery of pressurized gas to either upper face  928   u  or lower face  928   l  of polygonal piston  928 . Solenoid valves  930  and  932  deliver pressurized gas to gas inlet/outlet ports  934  and  936  of inlet housing  938 . Inlet port  934  is connected to an internal passageway  996  within inlet housing  938  to deliver pressurized gas to lower face  928   l  of polygonal piston  928 . Inlet port  936  is connected to an internal passageway  997  below lower bushing  964  within inlet housing  938  that communicates with a hole  998  drilled along the length of shaft  960  and terminating in a transverse hole  999  through shaft  960  to deliver pressurized gas to upper face  928   u  of polygonal piston  928 . 
     Polygonal piston  928  responds to sequential delivery of pressurized gas by alternate upward and downward movement within cavity  972  and thereby corresponding upward and downward movement of latch key  902 , shaft  960  of which is attached to polygonal piston  928  by retainer ring  970 . Skilled persons will appreciate that each of inlet ports  934  and  936  serves as an exhaust port for the other when it is delivering pressurized gas to cavity  972 . 
     Optical interrupter devices of a type similar to optical interrupter devices  248  and  249  used as sector control end of travel switches can be implemented in latch key rotation mechanism  904  or latch key raise/lower mechanism  906  to detect latch key  902  in, respectively, either of its latched or unlatched angular positions or either of its raised or lowered positions. 
     FIGS. 8 and 9 are respective front and side elevation views of box load interface system  16  showing the spatial relationship of port door  76  and other system components when port door  76  is in a fully elevated position in which it is aligned with and can fit within aperture  74  of front plate  14 . With reference to FIG. 8, port door  76  has a front surface  160  on which two locating pins  162  are positioned to mate with locator pin depressions  44  (FIGS. 3B and 3D) in box door  30  when it and port door  76  are brought into contact by the operation of tray positioning mechanism  88 . A box presence switch  164  may optionally be positioned below each locating pin  162  to provide an electrical signal indicating that box door  30  is properly registered with port door  76  when they are in matable connection. Two pod door latch key assemblies  166  are rotatably positioned within port door  76 . Latch key assemblies  166  include laterally compliant latch keys  150  extending through front surface  160  to fit into spatially aligned slots  46  (FIGS. 3B and 3D) in box door  30  to operate its latching mechanism. 
     FIG. 10 is an exploded view and FIGS. 11A,  11 B, and  11 C are respective side (partly in section), front, and rear elevation views of latch key assembly  166 . With reference to FIGS. 10,  11 A, and  11 C, latch key assembly  166  includes a latch key housing  168  that fits within and is secured by bolts passing through counterbored bolt holes  170  to a component of either a latch key motor mechanism  172  (FIGS. 12 and 13) or a fluidic pressure controlled latch key actuating mechanism  242  (FIGS. 33-37) positioned behind front surface  160  of and within port door  76 . Latch key housing  168  is of cylindrical shape having a neck portion  174  and a base portion  176  of greater diameter. A latch key body  178  has positioned at one end a latch key  150  connected to a shaft that includes concatenated cylindrical portions  180 ,  182 , and  184  of different diameters. Cylindrical portion  184  has located between its ends a hexagonal section  186 . Latch key housing  168  has a centrally located stepped bore  188  that receives latch key body  178  and includes a hexagonal section  190  of complementary shape to the shape of and of the same length as the length of hexagonal section  186 . Neck portion  174  and cylindrical portion  180  are of the same diameter so that they abut each other, and the width (i.e., the distance between opposite sides) of hexagonal section  190  is slightly larger than the width (i.e., distance between opposite faces) of hexagonal section  186  to permit lateral motion of latch key body  178  within latch key housing  168 . A coil spring  192  fitted within a counterbored region  194  in latch key housing  168  and a clip ring  196  fitted around an annular recess  198  in cylindrical portion  184  holds latch key assembly  166  together as a single unit. 
     Latch key housing  168  and latch key body  178  are provided with respective complementary hexagonal sections  190  and  186  to prevent mutual rotation between them. Both latch key assemblies  166  are rotated between first and second angular positions to open and close box door  30 . The widths of hexagonal sections  190  and  186  are slightly different to form a compliant latch key  150  that can “wobble” laterally to accommodate the tolerance range of the corresponding slot  46  in box door  30  and thereby ensure proper alignment to it. 
     With reference again to FIG. 9, port door  76  is shown in matable connection with box door  30 , with latch key  150  turned in secure position within box door slot  46 . Each latch key housing  168  carries on its neck portion  174  a bearing  210  that is supported on an interior surface  212  of port door  76 . 
     Once box door  30  is unlocked, latch keys  150  remain in box door slots  46  and port door  76 , while holding box door  30 , moves away from carrier box  18 . Box door  30  is supported on port door  76  only by latch keys  150 . The loose range of tolerances of the dimensions of box door slots  46  and the design of latch keys  150  allowing them to “wobble” make box door  30  susceptible under its own weight to slippage against front surface  160  of port door  76 . This change in the initial alignment between box door  30  and front plate  14  makes it difficult when re-installing box door  30  to fit its interior surface  36  within the recessed, stepped interior side margin  34  of carrier box  18 . 
     To prevent box door  30  from slipping out of its initial mutual alignment with port door  76 , an alternative embodiment of latch key assembly  166  includes a latch key pull back assembly  199 , which is shown in FIGS. 32 and 34. Latch key pull back assembly  199  pulls box door  30  into a tight relationship with front surface  160  of port door  76  to preserve their initial mutual alignment. Each latch key  150  is non-rotatably mounted within latch key housing  168  through hexagonal sections  186  and  190 , thereby allowing latch key  150  to “wobble” as previously described to accommodate a range of tolerances of box door slots  46 . Cylindrical portion  184  of latch key body  178  and centrally located stepped bore  188  of latch key housing  168  are modified to accommodate a piston  200  that implements the pull back function of pull back assembly  199 . 
     With reference to FIGS. 32 and 34, a piston  200  encircled by an annular seal  201  is secured to a latch key body  178 ′ by screw threads or another suitable attachment method. Piston  200  is slidably movable within a housing  168 ′ to move latch key  150  in either direction along a longitudinal axis  178   a ′ of latch key body  178 ′. Piston  200  is driven by pressurized gas, such as air, supplied to a drive chamber  202  that is formed between an upper bushing  202   a  and a lower bushing  202   b  and sealed gas tight by seals  203   a  and  203   b . Pressurized gas is supplied to drive chamber  202  from a pressurized gas supply (not shown) through a gas supply line  204  connected to a supply housing  205  having a gas passageway  205   a . Passageway  205   a  communicates with intersecting ports  206   a  and  206   b  in latch key body  178 ′, which extends through housing  168 ′ and into supply housing  205  through lower bushing  202   b  and seal  203   b . Port  206   a  is a hole formed along longitudinal axis  178   a ′ of latch key body  178 ′, and port  206   b  is a hole formed in latch key body  178 ′ to intersection port  206   a  in a transverse direction. Port  206   b  opens up into drive chamber  202  to supply pressurized gas that acts on the face of piston  200  to drive it in a direction to pull box door  30  against front surface  160  of and into a tight relationship with port door  76  whenever latch key  150  is in its secure position within box door slot  46 . 
     A return chamber  208  is located on the opposite side of piston  200  where a return coil spring  209  is positioned around latch key body  178 ′ to urge piston  200  and thereby extend latch key  150  to their original positions to permit release of box door  30 . 
     In operation, after each latch key  150  has been rotated to unlock box door  30 , pressurized gas is supplied to drive chamber  202  through passageway  205  and gas inlet ports  206   a  and  206   b . The pressurized gas acts on the face of piston  200 , causing it to move against return spring  209  to retract latch key  150  and thereby draw box door  30  into firm and secure engagement with port door  76 . One of two embodiments of a port door translation mechanism described below moves port door  76  together with box door  30  away from carrier box  18  to open it. 
     When box door  30  is ready to be re-installed to close carrier box  18 , the port door translation mechanism moves port door  76  toward carrier box  18  and box door  30  in alignment with it. Each latch key inserted into a box door slot  46  is rotated to lock box door  30  on carrier box  18 , and pressurized gas is then released from drive chamber  202  through gas inlet ports  206   a  and  206   b  and passageway  205 . Return spring  209  acts in response to the release of pressurized gas to push against the opposite face of piston  200  to return latch key  150  to its original, extended position. The port door translation mechanism can then retract port door  76  away from box door  30  and thereby withdraw latch keys  150  out of box door slots  46  to completely separate port door  76  from a closed carrier box  18 . Skilled persons will appreciate that latch key pull back assembly  199  can be advantageously used in a latch key assembly implemented in the absence of the “wobble” design feature. 
     FIGS. 12 and 13 show latch key motor mechanism  172 , which rotates latch keys  150  between the first and second angular positions to lock and unlock box door  30  of carrier box  18 . With reference to FIGS. 12 and 13, base portion  176  of one latch key housing  168  is fixed to a master disk member  214  by bolts  216  engaging tapped bolt holes  170 , and base portion  176  of the other latch key housing  168  is fixed to a slave disk member  218  by bolts  220  engaging tapped bolt holes  170 . Disk members  214  and  218  and therefore their corresponding latch keys  150  are mounted for rotation about respective axes  222  and  224 . Master disk member  214  includes a worm gear section  226  having worm gear teeth  228  with which a worm gear shaft  230  driven at one end by a motor  232  and terminated at the other end in a bearing  234  engages to move disk member  214  and thereby its corresponding latch key  150  about axis  222  between the first and second angular positions. The operation of motor  232  is controlled to provide a 90° displacement between the first and second angular positions. 
     An elongated coupling or rod member  236  of adjustable length is mounted at its proximal end to disk member  214  for pivotal movement about a first rod pivot axis  238  and at its distal end to disk member  218  for pivotal movement about a second rod pivot axis  240 . Rod member  236  is composed of a spherical joint  236   a  and a turnbuckle portion  236   b  coupled at each of its ends by locknuts  236   c  that after rotary adjustment fix the length of rod member  236 . Disk member  218  is slaved to the motion of disk member  214  and thereby moves its corresponding latch key  150  about axis  224  between the first and second angular positions. Spherical joint  236   a  facilitates the length adjustment of rod member  236  without disassembly by rotation of turnbuckle portion  236   b  but is otherwise not needed to practice the invention. 
     FIGS. 33-37 show a fluidic pressure controlled latch key actuating mechanism  242 , which represents an alternative to latch key motor mechanism  172  and is shown implemented for use with latch key pull back assembly  199 . As does motor mechanism  172 , actuating mechanism  242  rotates latch keys  150  between the first and second angular positions to lock and unlock box door  30  of carrier box  18 . 
     With reference to FIGS. 33-37, base portion  176 ′ of one latch key housing  168 ′ is fixed to a disk member  214  by bolts  216  engaging tapped bolt holes  170 , and base portion  176  of the other latch key housing  168  is fixed to a disk member  218  by bolts  220  engaging tapped bolt holes  170 . Disk members  214  and  218  and therefore their corresponding latch keys  150  are mounted for rotation about respective axes  222  and  224 . Each of disk members  214  and  218  functions as a lever arm that has a coupling end  243  and an opposite end with a protruding vane  244 . Coupling end  243  provides a pivot mounting for a cylinder attachment block  245  that is connected to the distal end of an extensible rod  246  of a pneumatic cylinder  247 . Vane  244  extends from each of disk members  214  and  218  for movement between emitter and sensor legs of respective U-shaped transmissive optical interrupter devices  248  and  249  angularly displaced by 90° on and mounted to port door  76 . The presence of vane  244  in either of optical interrupter devices  248  and  249  causes them to function as sector control end of travel switches that indicate whether either of latch keys  150  is in the first or second angular position. The lengths of extension of each extensible rod  246  between the first and second angular positions is set by hard stop blocks (not shown) positioned in port door  76  to limit the ranges of angular displacement of disk members  214  and  218 . Bumpers made of Delrin® or other suitable material fixed to disk members  214  and  218  can be of selected thicknesses to provide an adjustment of the extent of travel of extensible rods  246 . Each pneumatic cylinder  247  controls, therefore, a key latch mechanism operating as a “bang-bang” device between two angular positions and using end point detection. 
     Extensible rods  246  move disk members  214  and  218  and thereby rotate their corresponding latch keys  150  about the respective axes  222  and  224  between the first and second angular positions. The position and length of extension of each extensible rod  246  provides a 90° displacement between the first and second angular positions. 
     With particular reference to FIG. 33, a pneumatic pressure control system  600  selectively delivers pressurized gas to each pneumatic cylinder  247  in response to latch key position commands provided by central control system  349  (FIG.  18 ). The presence of vane  244  in a corresponding one of optical interrupter devices  248  and  249  provides to central control system  349  initial condition information about the position of each latch key  150 . Pressure control system  600  includes a gas supply line that delivers gas from a pressurized gas source (not shown) to an inlet port  604  of a two-outlet port solenoid valve  606  that controls the operation of pneumatic cylinders  247  and an inlet port  608  of a single-outlet port solenoid valve  610  that controls the operation of latch key pull back assembly  199 . 
     Solenoid valve  606  has outlet ports  620  and  622  that deliver pressurized gas through separate conduits to, respectively, an inlet port  624  of a fluid flow divider  626  and an inlet port  628  of a fluid flow divider  630 . Flow divider  626  has two outlet ports, each connecting through a separate conduit to a cylinder rod extension inlet  632  of a different one of pneumatic cylinders  247 . Flow divider  630  similarly has two outlet ports, each connecting through a separate conduit to a cylinder rod retraction inlet  634  of a different one of pneumatic cylinders  247 . A command signal provided by central control system  349  to an electrical conductor  636  selectively controls the flow path of pressurized gas from inlet port  604  to one of outlet ports  620  and  622  to either extend or retract extensible rods  246  and thereby rotate latch keys  150  between their first and second angular positions. Solenoid valve  606  has gas exhaust ports  638  and  640  corresponding to the gas flow paths produced by the respective outlet ports  620  and  622  to which conduits are connected to release exhaust gases away from the enclosed, clean environmental housing. 
     Solenoid valve  610  has an outlet port  650  that delivers pressurized gas to an inlet port  652  of a fluid flow divider  654 , which has two outlet ports, each connecting through a separate conduit to gas supply line  204  of a different one of latch key pull back assemblies  199 . A command signal provided by central control system  349  to an electrical conductor  658  delivers the flow of pressurized gas from inlet port  608  to outlet port  650  to retract latch keys  150  after they have fit into slots  46  of and opened box door  30  so that it and port door  76  are in secure matable connection. Solenoid valve  610  has a gas exhaust port  660  corresponding to the gas flow path produced by outlet port  650  to which a conduit is connected to release exhaust gases away from the enclosed, clean environmental housing. 
     FIGS. 8,  9 , and  12  show a port door translation mechanism  250  mounted to a port door carriage mechanism  252  to which elevator assembly  28  is operatively connected. Port door  76  has guide tracks  254  that slide along guide rails  256  on port door carriage mechanism  252  so that it can move port door  76  toward or away from interior surface  78  of front plate  14  when port door  76  is aligned with aperture  74 . 
     Port door  76  includes an upper rectangular section  258  that houses latch key motor mechanism  172  and a lower rectangular section  260  that houses port door translation mechanism  250 . Upper section  258  of port door  76  includes a stepped region  262  of a height that defines a surface portion  264  and causes port door  76  to form a sealed connection against interior surface  78  of front plate  14  as surface portion  264  fits within aperture  74  to present latch keys  150  to mate with slots  46  in box door  30 . Lower section  260  of port door  76  supports a motor  270  coupled to a spindle  272  and a lead screw  274  connected at one end to a pulley  276  and supported at the other end in a preloaded bearing  278 . A belt  280  connecting spindle  272  to pulley  276  causes lead screw  274  to rotate and drive a nut assembly  282  to cause port door  76  to slide along guide rails  256  toward or away from interior surface  78 , depending on the direction of lead screw rotation. 
     Because surface portion  264  is sized to fit within aperture  74 , motor  270  is not operated unless elevator assembly  28  has moved port door carriage mechanism  252  to its uppermost position. Elevator assembly  28  moves port door carriage mechanism  252  to its lowermost position after port door translation mechanism  250  has moved port door  76  completely away from interior surface  78  of front plate  14 . 
     FIGS. 13,  14 , and  15  show respective rear elevation, plan, and side elevation views of a differential, transmissive optical scanning assembly  290  mounted within the interior and in a recess near the top side of port door  76 . Scanning assembly  290 , which operates in conjunction with elevator assembly  28 , includes two scanning fingers  292   l  and  292   r , the former having a finger shaft  294   l  mounted for pivotal movement in a bearing  296   l  about a finger pivot axis  298   l  at a proximal end  300   l  and the latter having a finger shaft  294   r  mounted for pivotal movement in a bearing  296   r  about a finger pivot axis  298   r  at a proximal end  300   r . Scanning finger  292   l  supports light sensors  306   a  and  308   a  positioned one on top of the other at a distal end  309   l . Scanning finger  292   r  supports light emitters  306   b  and  308   b  positioned one on top of the other at a distal end  309   r . A light propagation path  310  between light sensor  306   a  and light emitter  306   b  and a light propagation path  312  between light sensor  308   a  and light emitter  308   b  are coplanar in a direction normal to the major surface of wafer  152 . Light propagation paths  310  and  312  cross over at a point  314  (FIG. 17) in the plane. 
     A scanner motor  320  mounted within port door  76  includes a central shaft  322  having an axis of rotation  324  set at an equidistant position between finger pivot axes  298   l  and  298   r . Central shaft  322  carries a disk member  326  to which are mounted two stationary pins  328  and  330  angularly spaced apart from each other to carry out the function described below. A rod member  322   l  is mounted at a proximal end to pin  328  on disk member  326  for pivotal movement about a rod proximal pivot axis  334   l  and at its distal end to a coupling recess mount  336   l  in finger shaft  294   l  for pivotal movement about a rod distal pivot axis  338   l . A rod member  322   r  is mounted at a proximal end to pin  330  on disk member  326  for pivotal movement about a rod proximal pivot axis  334   r  and at its distal end to a coupling recess mount  336   r  in finger shaft  294   r  for pivotal movement about a rod distal point pivot axis  338   r.    
     Scanner motor  320  imparts ±45° reciprocal motion to central shaft  322  and pins  328  and  330  are angularly spaced apart on disk member  326  to pivotally move scanning fingers  292 1 and  292   r  between fully extended positions (shown in solid lines in FIG. 14) and fully retracted positions (shown in phantom lines in FIG.  14 ). Thus, scanning fingers  292   l  and  292   r  move 90° about their respective finger pivot axes  298   l  and  298   r  between the fully extended and fully retracted positions. Skilled persons will appreciate that the extension and retraction of scanning fingers  292   l  and  292   r  can also be accomplished with the use of fluidic cylinders. 
     FIG. 14 shows that the respective distal ends  309   l  and  309   r  of scanning fingers  292   l  and  292   r  in their fully extended positions straddle wafers  152  stored in wafer cassette  32  and that light propagation paths  310  and  312  intersect a chord of each of wafers  152  as they are scanned. 
     When they are fully extended, sensors  306   a  and  308   a  and emitters  306   b  and  308   b  are located inside of the region where a wafer carrier box  18  would occupy and are aligned to form two light propagation paths  310  and  312  that cross each other. The presence of a wafer  152  aligned to intersect one or both light propagation paths  310  and  312  interrupts light propagating from one or both of emitters  306   b  and  308   b  from reaching its corresponding sensor  306   a  and  308   a . Thus, interruption of one or both of light propagation paths  310  and  312  provides information that can be used to position robot assembly  20  for wafer pickup or to determine the presence or absence of a wafer  152  in a slot in wafer cassette  32 , whether two wafers  152  occupy the same slot in wafer cassette  32 , or whether a wafer  152  occupies two slots (i.e., in a cross slot position) in wafer cassette  32 . The mounting configuration and operation of light sensors  306   a  and  308   a  and emitters  306   b  and  308   b  are described below with particular reference to FIGS. 16A and 16B. 
     FIG. 16A shows in greatly enlarged detail a diagram of the placement of sensor  308   a  and emitter  308   b  in the respective scanning fingers  292   l  and  292   r , and FIG. 16B shows in greatly enlarged detail a diagram of the placement of sensor  306   a  and emitter  306   b  in the respective scanning fingers  292   l  and  292   r . With reference to FIGS. 16A and 16B, sensor  306   a  and emitter  306   b  are secured within the respective scanning fingers  292   l  and  292   r  in slightly upwardly beveled mounting surface areas that provide a straight line light propagation path  310  inclined at a +0.75° angle relative to the plane of the top surfaces of scanning fingers  292   l  and  292   r . Sensor  308   a  and emitter  308   b  are secured within the respective scanning fingers  292   l  and  292   r  in slightly downwardly beveled mounting surface areas that provide a straight line light propagation path  312  inclined at a −0.75° angle relative to the plane of the top surfaces of scanning fingers  292   l  and  292   r . FIG. 17 is a diagram showing a front elevation view of the placement of wafer cassette  32  on slidable tray  24  relative to crossed light propagation paths  310  and  312 . Propagation paths  310  and  312  are coplanar in a vertical plane and are angularly inclined in opposite directions to cross over at a point  314  at the midpoint of the distance between scanning fingers  292   l  and  292   r . FIG. 17 also shows in phantom lines a semiconductor wafer  152  positioned above wafer cassette  32  and in a location representing proper registration of wafer  152  in wafer cassette  32 . 
     Light propagation paths  310  and  312  are angularly inclined so that a single wafer  152  properly registered in a slot of wafer cassette  32  and in a specified elevator position interrupts both beams equally. As shown in FIGS. 8 and 15 and described in greater detail below, scanning assembly  290  is supported on elevator assembly  28  that moves a port door carriage  344 , the vertical position of which is measured by an optical position encoder  342 . The movement of port door carriage  344  provides a continuous scan of the contents of wafer cassette  32 . As port door carriage  344  travels past a next specified elevator position, sensors  306   a  and  308   a  produce output signals of equal magnitude for an elevator displacement equal to the wafer thickness. (The same wafer thickness is measured by the corresponding sensors and emitters for light propagation paths  310  and  312  when wafer  152  is registered in its slot.) The magnitudes of the signals will change, but the difference between the signals will not change as port door carriage  344  moves to the next specified elevator position. 
     A wafer  152  in cross slot position will interrupt only one light propagation path for a specified elevator position and thereby cause sensors  306   a  and  308   a  to produce output signals of different magnitudes. The sensor output that indicates the presence of incident light represents the open slot and thus the direction of the horizontal tilt angle of wafer  152 . 
     The common mode rejection properties of differential optical scanning assembly  290  reject signal perturbations caused by mechanical vibrations and provides for an accurate individual wafer thickness measurement. Two wafers  152  occupying the same slot in wafer cassette  32  will interrupt both light propagation paths  310  and  312  for a specified elevator position; however, the magnitudes and difference between the signals will not change for a longer than nominal vertical displacement of port door carriage  344  as it moves to the next specified elevator position. The continuous signal interruption indicates a greater than nominal wafer thickness in a slot and thereby represents double wafer occupancy of a slot in wafer cassette  32 . The above-described crossed light propagation path detection arrangement is described in U.S. patent application Ser. No. 09/141,890, filed Aug. 27, 1998, now U.S. Pat. No. 6,160,265, which is assigned to the assignee of this application. 
     A light beam sensor  346   a  and emitter  346   b  form a light propagation path  348  in a transverse (preferably perpendicular) direction to that of coplanar light propagation paths  310  and  312  described above. Sensor  346   a  and emitter  346   b  are positioned at the top and bottom sides of aperture  74  on exterior surface  96  of front plate  14  and outside of the region where a wafer carrier box  18  would occupy to detect whether a wafer  152  has been dislodged to protrude from its slot in the front opening of carrier box  18 . A dislodged wafer  152  descending out of carrier box  18  would interrupt light propagation path  348  to provide a signal that disables port door carriage  344  from descending farther and thereby prevent the protruding wafer  152  from being clipped by scanning fingers  292   l  and  292   r  as port door  76  is lowered. As indicated in FIG. 18, the output signals of sensors  306   a ,  308   a , and  346   a  and of position encoder  342  are processed by central control system  349  to make the above-described wafer registration determinations. 
     For any of the above-described preferred embodiments of a box hold down clamping mechanism, box load interface system  16  may be equipped with instrumentation indicating carrier box presence and alignment information on slidable tray  24 . With reference to FIGS. 1,  6 ,  19 ,  25 , and  29 , a light beam sensor  390   a  (FIGS. 19,  25 , and  29 ) and a light beam emitter  390   b  (FIGS. 1,  6 , and  19 ) form a light propagation path  392  (FIG. 19) in a transverse direction to exterior surface  96  of front plate  14  and the carrier box mounting surface of slidable tray  24 . Sensor  390   a  and emitter  390   b  are mounted to slidable tray  24  and above aperture  74  on exterior surface  96  of front plate  14  in locations that establish a direction of propagation path  392  that passes through the region occupied by a wafer carrier box  18  when it is placed on slidable tray  24 . Five carrier box placement switches  394  (FIGS. 25 and 29) depressed concurrently by a wafer carrier box  18  indicate its proper registration on kinematic coupling pins  66 . Central control system  349  monitors the continuity of light propagation path  392  and status of placement switches  394 . Central control system  349  causes illumination of an indicator light  396  (FIGS. 1 and 6) to indicate the presence of a carrier box  18  and various combinations of four indicator lights  398  (FIGS. 1 and 6) to indicate the nature of any misalignment of carrier box  18  on slidable tray  24 . 
     FIGS. 1,  8 ,  9 ,  12 , and  15  show elevator assembly  28  supporting port door  76 ; FIG. 12 shows port door  76  in a fully raised position (solid lines)  350  and a fully lowered position (outlined in phantom lines)  352 . Elevator assembly  28  comprises a side drive lead screw mechanism  354  that includes a lead screw  356  driven at a lower end by a smooth running, high torque, DC motor  358  and supported at an upper end by preloaded end bearings  360  for rotation about a longitudinal axis  362 . Numerous servo motors are known in the art, are commercially available, and would be suitable. Motor  358  is in communication with and controlled by an input controller that generates input command voltage signals. The input controller forms a part of central control system  349 , which directs the operation of the interface system of the present invention. Input command signals delivered to motor  358  are converted to rotation of a motor drive output shaft  364 . Motor  358  provides bi-directional rotational output, reflecting the polarity of the voltage input signal. Motor drive output shaft  364  is operatively connected to lead screw  356 . Rotation of motor drive output shaft  364  results in corresponding rotation of lead screw  356 . A lead nut assembly  366  is threaded on lead screw  356  and operatively connected to port door carriage  344  connected to a side surface of port door  76  and lead screw  356 . Rotation of lead screw  356  results therefore in linear displacement of lead nut assembly  366  along the length of lead screw  356 . This results in linear displacement of port door carriage  344  to raise or lower port door  76  to perform a wafer scanning operation. 
     Optical position encoder  342  continuously monitors and provides feedback as to the position of lead nut assembly  366  and thereby the positions of wafers  152  stored in wafer cassette  32  relative to scanning fingers  292   l  and  292   r  mounted to port door  76 . An encoder carriage  372  is mounted in fixed relation to and thus moves in concert with lead nut assembly  366 . Encoder carriage  372  provides a housing for movable components of optical position encoder  342 . Scanning assembly  290  is displaced as a consequence of the displacement of encoder carriage  372  caused by rotation of lead screw  356 . 
     An alternative mechanism for monitoring the position of lead nut assembly  366  can be accomplished by mounting at one of its ends a rotary encoder pair, such as a Model 110514 encoder sold by Maxon for use with a Model 137540 (35 millimeter) or Model 148877 (40 millimeter) Maxon motor. 
     Port door  76  and encoder carriage  372  are slidably mounted on stationary vertical support plates  374  by means of high precision, low friction linear bearing assemblies  378  arranged in parallel to longitudinal axis  362 . Linear bearing assemblies  378  preferably extend for the full length of travel of lead nut assembly  366  and thereby positively guide encoder carriage  372  along the full length of its travel path. Various types of position encoders and devices for continuously monitoring and providing feedback relating to the displacement of lead nut assembly  366  and encoder carnage  372  are known in the art and would be suitable. Optical encoder assemblies are generally preferred, and encoders that operate using Moire fringe pattern principles to continuously monitor the position of encoder carriage  372  are especially preferred. 
     Optical position encoder  342  includes a read head mounting member  380  on which an array of light emitting diodes is mounted. A reference grating is rigidly mounted on read head mounting member  380 , and a stationary grating  382  extends along the full length of travel of encoder carriage  372 . The structural design and functions of read head mounting member  380  and stationary grating  382  that operate using Moire fringe pattern principles are known and described in commonly assigned U.S. Pat. No. 5,382,806. 
     The following summarizes the operational sequence of wafer transport system  10 . An operator or robot mechanism places a carrier box  18  onto slidable tray  24 , and all of the eleven sensors required by SEMI specifications check for proper registration of carrier box  18  on kinematic coupling pins  66 . The operator or program control causes slidable tray  24  to move carrier box  18  relatively rapidly toward aperture  74  in front plate  14 . A controller slows the motion of tray motor  100  to a constant speed when box door  30  reaches the penetration point of latch keys  150  relative to slots  46  in box door  30 . The controller is implemented with a force feedback system that by either sensing tray motor current or following a stored slidable tray position profile detects an obstruction or plastic component out-of-tolerance variation and prevents overpowering slidable tray  24  under conditions that would prevent proper engagement of box door  30  with latch keys  150 . The motor current sense entails sensing an amount of electrical current for a time relative to a distance traveled by slidable tray  24 . The following of the tray position profile entails comparing to a stored position profile a present position derived from a rotary position encoder installed in tray motor  100 . The force feedback system establishes for a valid zone of engagement a low force criterion applied to carrier box  18  that, when exceeded, causes tray motor  100  to stall and thereby allow for a reversal of travel direction of slidable tray  24  before penetration by latch keys  150  could be attempted. 
     When box door  30  mates with port door  76  and front side margin  148  forms a seal with the beveled side margin of aperture  74  in front plate  14 , clamping finger  132  has completed securing carrier box  18  against slidable tray  24  and latch key motor mechanism  172  turns latch keys  150  to lock box door  30  to port door  76 . Port door translation mechanism  250  pulls box door  30  and port door  76  beyond interior surface  78  of front plate  14 . Presence sensor  346   a  determines whether any of the wafers  152  is protruding from wafer cassette  32 . A second presence sensor  347   a  positioned near finger pivot axes  298   l  and  298   r  of scanning fingers  292   l  and  292   r  senses excessive protrusion of a wafer  152  and prevents further downward motion by elevator assembly  28 . 
     Elevator assembly  28  causes port door carriage  344  and thereby port door  76  to descend about 3 cm, and scanning fingers  292   l  and  292   r  flip out of port door  76  to their fully extended positions. Elevator assembly  28  then causes port door carriage  344  to descend to scan the contents of wafer cassette  32 . If presence sensor  346   a  indicates at least one wafer  152  is protruding from wafer cassette  32 , scanning fingers  292   l  and  292   r  retract at each wafer position and flip outwardly to push the protruding wafer  152  back into its slot in wafer cassette  32 . Scanning fingers  292   l  and  292   r  repeat the flipping process for each wafer position until sensor  346 a indicates an obstruction is no longer present. 
     Following completion of a scan, scanning fingers  292   l  and  292   r  retract, elevator assembly  28  moves port door carriage  344  to its lowermost position, and port door  76  remains parked as wafer processing by robot assembly  20  takes place. Upon completion of wafer processing, elevator assembly  28  returns port door  76  to its uppermost position to separate box door  30  from port door  76  and retract carrier box  18  away from front plate  14 . 
     With reference to FIGS.  2  and  19 - 24 , robot assembly  20  is positionable along a linear traveling robot assembly  400 . Linear traveling assembly  400  includes a stationary lead screw  402  supported at either end by a pillow block  404  mounted to a stage base  406 . Each pillow block  404  is bolted or otherwise secured to stage base  406 . A motor-driven rotating nut mechanism  408  is mounted to robot assembly  20  to move it along lead screw  402  between apertures  74  of side-by-side front plates  14 . Nut mechanism  408  is contained within a housing  422  that is secured to a carriage  424 . Carriage  424  is connected to a robot mounting plate  425  that supports robot assembly  20  so that robot assembly  20  along with carriage  424  moves along lead screw  402  between apertures  74 . Carriage  424  includes upper and lower tracks  426  and  428  that travel along upper and lower rails  430  and  432  bolted or otherwise secured to stage base  406 . Stage base  406  is immovably secured to front plates  14  by alignment fixtures  434  that are bolted or otherwise secured at each end. Housing  422  includes a sheet metal covering  436  to prevent dirt and dust from accumulating on nut mechanism  408  and serves as a safety cover to prevent injury that might result from clothing or anything that might get caught in nut mechanism  408  as it travels along lead screw  402 . Nut mechanism  408  is further protected by sheet metal coverings  438  and  440  that are connected to stage base  406  by screws  441  and that extend into slots  442  in carriage  424  and secured by screws  444 . The ends of the sheet metal coverings  438  and  440  cooperate with plastic glides  446  located within carriage  424  to prevent sheet metal coverings  438  and  440  from being bent and to absorb any misalignment and keep them straight. Glides  446  also prevent metal to metal contact between carriage  424  and sheet metal coverings  438  and  440  to reduce contamination. 
     Nut mechanism  408  includes a lead nut  448  rotated by a motor  450  through a belt  452 . Motor  450  is mounted to housing  422  by a motor mount  454 . Motor  450  includes a drive shaft  456  that rotates a motor pulley  458  connected thereto by a conical clamp  460 . Belt  452  is in driving engagement with a lead nut pulley  462  to rotate lead nut  448 . Lead nut pulley  462  is rotated within a bearing  464  that is connected to housing  422  through an inner race bearing clamp  466  and an outer race bearing clamp  468 . Lead nut  448  is connected to lead nut pulley  462  by screw threads at one end and is prevented from rotating within lead nut pulley  462  by a lock nut  470 . Lead nut  448  has resilient fingers  472  at one end that are internally threaded and are forced inwardly by a lead nut sleeve  474  for engagement with lead screw  402 . Wave springs  476  located between lead nut sleeve  474  and lead nut pulley  462  urge lead nut sleeve  474  toward the finger end of lead nut  448 . An internal cam surface  478  on lead nut sleeve  474  acts on an enlarged end  480  of resilient fingers  472  to force them inwardly into a secure threaded engagement with lead screw  402 . 
     Motor  450  receives power from an electrical cable  482  located beneath carriage  424  and supported by a tray  484 . Cable  482  is supported within an articulated track  486  with one end connected to a power source  488  and the opposite end connected to a power housing  490  on carriage  424  so that cable  482  can travel along with carriage  424 . 
     Robot assembly  400  is moved from one position to another by rotating lead nut  448  in the above-described manner to advance carriage  424  along lead screw  402  until the final position is reached. A linear encoder scale  500  is connected to carriage  424  and travels along with it indicate the position of carriage  424 . End stops  502  are connected to stage base  406  at each end of lead screw  402  to stop carriage  424  at the proper location. Robot assembly  20  is positioned to retrieve and return wafers from wafer carrier boxes  18  mated against front plates  14  by box load interface systems  16 . 
     To ensure precise alignment of robot assembly  20 , front plate  14  includes for stage base  406  mounting holes  410  that constitute registration points for readily referencing robot assembly  20  to front plate  14  to ensure vertical and center-to-center alignment. This feature is advantageous because additional subsystems provided in system expansion would be automatically aligned to preassigned registration points. 
     FIGS. 38-45 show a four-bar carriage assembly  510 , which is an alternative embodiment of unitary construction that combines the functions of port door translation mechanism  250  and port door carriage mechanism  252 . Components common to both embodiments are identified by the same reference numerals. 
     With reference to FIGS. 38-44, elevator assembly  28  preferably uses side drive lead screw mechanism  354  in cooperation with a four-bar linkage mechanism  512  to raise and lower port door  76  and to move port door  76  toward and away from aperture  74  of front plate  14 . Linkage mechanism  512  couples port door  76  to lead screw mechanism  354 . Linkage mechanism  512  comprises two pairs of pivot or bar links  516  pivotally mounted to and coupling together a Z carriage  518  and an H-shaped link carriage  520 . Z carriage  518  is rigidly attached to lead nut assembly  366  located proximal to exterior surface  96  of front plate  14 , and link carriage  520  is rigidly attached to port door  76  located proximal to interior surface  78  of front plate  14 . Lead screw  356  driven by motor  358  moves Z carriage  518  vertically on rails  522  that are attached to a backbone structure  524  secured to exterior surface  96  of front plate  14 . The two pairs of bar links  516  have their ends pivotally attached to different, opposite side surfaces of Z carriage  518  and link carriage  520 , the latter of which including a portion extending through an elongated vertical opening in backbone structure  524 . FIG.  42  and FIGS. 43 and 44 show the different pairs of bar links  516  pivotally attached to, respectively, the right-hand side and left-hand side surfaces of Z carriage  518  and link carriage  520  depicted in FIG.  41 . Bar links  516  are positioned to form a parallelogram of changing height as they pivotally move in response to a linear displacement of Z carriage  518 . A travel guide roller  530  mounted on backbone structure  524  operates in part as a mechanical stop that limits the vertical travel of link carriage  520  and port door  76 . The maximum elevation of link carriage  520  set by guide roller  530  aligns port door  76  with aperture  74  of front plate  14 . Guide roller  530  functions, therefore, as a cam surface and follower device. 
     Four-bar carriage assembly  510  operates in the following manner. Elevator assembly  28  causes rotation of lead screw  356  and a corresponding linear displacement of lead nut assembly  366  along the length of lead screw  356 . This results in linear displacement of Z carriage  518  to raise or lower it. Whenever the direction of rotation of lead screw  356  causes Z carriage  518  to move upwardly from its lowest position, which is shown in FIG. 39, link carriage  520  moves upwardly in unison with Z carriage  518  because bar links  516  positioned on either side are aligned parallel to each other in a horizontal direction by operation of a fluidic counterbalance mechanism, the construction and operation of which is described below with reference to FIG.  45 . 
     Bar links  516  maintain their horizontal disposition until an upper surface  532  of link carriage  520  contacts guide roller  530 , which position is shown in phantom lines in FIG.  40 . Link carriage  520  rests against guide roller  530  while Z carriage  518  continues its upward movement. The continued upward movement of Z carriage  518  occurring while link carriage  520  remains stationary in the direction of upward movement causes bar links  516  to pivot as a parallelogram of decreasing height to draw link carriage  520  and therefore port door  76  in a direction perpendicular to the direction of travel of Z carriage  518 . A bottom steering roller  534  is mounted on backbone structure  524  to receive a bottom surface  536  of link carriage  520  as it advances toward interior surface  78  and port door  76  advances toward and in alignment with aperture  74  of front plate  14 . Bottom steering roller  534  prevents rotational motion of link carriage  520  and thereby maintains its straight line inward direction of travel perpendicular to that of Z carriage  518  as it advances toward interior surface  78 . Steering roller  534  also prevents link carriage  520  from falling under fluidic pressure loss conditions associated with the fluidic counterbalance mechanism. Z carriage  518  reaches its highest position, which is shown in solid lines in FIG. 40, when port door  76  fits into and achieves sealed engagement with aperture  74  of front plate  14 . 
     Whenever the direction of rotation of lead screw  356  causes Z carriage  518  to move downwardly from its highest position, bar links  516  pivot to form a parallelogram of increasing height to move link carriage  520  away from interior surface  78  and thereby cause port door  76  to retract from aperture  74  of front plate  14 . Bar links  516  positioned on either side assume a horizontal disposition parallel to each other after upper surface  532  of link carriage  520  no longer contacts guide roller  530  as Z carriage  518  and link carriage  520  continue to descend to the lowest position of Z carriage  518 . 
     With particular reference to FIGS. 43 and 44, a hard stop block  540  is mounted on a side surface  542  of link carriage  520  at a location beneath a surface  544  of the bar link  516  positioned nearer to guide roller  530  on the left-hand side surfaces of Z carriage  518  and link carriage  520 . Hard stop block  540  provides an impact surface  546  against which surface  544  of bar link  516  slides to prevent it (and the remaining three bar links  516 ) from rotating past the horizontal position in a clockwise direction when upper surface  532  of link carriage  520  is not in contact with guide roller  530 , as shown in FIG.  44 . The tendency of bar links  516  to over-rotate results from the operation of a counterbalance mechanism  550 , which is designed to over-counterbalance link carriage  520  and thereby lift port door  76 , as described below. 
     Four-bar carriage assembly  510  is a preferred implementation of a unitary structure that combines the functions of port door translation mechanism  250  and port door carriage mechanism  252 . Skilled persons will appreciate, however, that use of as few as one bar link  516  in a carriage assembly is possible in conjunction with a suitable guide mechanism to effect travel of port door  76  in the two prescribed (i.e., vertical and horizontal) directions. For example, alternative embodiments could include a pair of bar links, one positioned on each of top sides and bottom sides of a Z carriage and a link carriage, or a single bar link implemented with a cam and roller follower mechanism designed to describe the desired motion. Moreover, a two-cylinder fluidic drive mechanism can be substituted for side drive lead screw mechanism  354 . Two fluidic cylinders having extensible rods of the appropriate lengths and connected in series can provide the directional displacements accomplished as described above. 
     With reference to FIG. 45, vertical/horizontal port door displacement fluidic-controlled counterbalance mechanism  550  counterbalances the weight of port door  76  during its sequential translational movement in the upward and downward (i.e., vertical) and inward and outward (i.e., horizontal) directions. In its preferred implementation, counterbalance mechanism  550  slightly over-counterbalances the weight of port door  76  to apply a slight lifting force to it. Counterbalance mechanism  550  includes a fluidic, preferably pneumatic, constant force cylinder  552  having a body portion  554  with a closed end supported by a lower support member  556  fixed to backbone structure  524  and an open end through which an extensible rod  558  protrudes. Cylinder body portion  554  is stationary relative to backbone structure  524 , and extensible rod  558  changes its length of extension from body portion  554  in response to the vertical movement of link carriage  520  and therefore port door  76 . Extensible rod  558  is operatively connected to port door  76  by a belt  560  having one end attached to an upper support member  562  fixed to backbone structure  524  and the other end attached to a free end  564  of a pivot plate  566  pivotally mounted to interior side surfaces of link carriage  520 . Between its ends, belt  560  loops around a roller  572  fixed to the distal end of extensible rod  558  and around two spaced-apart rollers  574  and  576  mounted to upper support member  562 . The positions of the fixed end points of belt  560  and rollers  574  and  576  produce a folded belt configuration that establishes an operational relationship in which 1.0 unit of vertical travel of Z carriage  518  produces 0.5 unit of linear extension of extensible rod  558 . 
     Counterbalance mechanism  550  operates in the following manner. Pneumatic cylinder  552  provides a constant force, F lift , in the direction of travel (i.e., vertical direction) of Z carriage  518  when link carriage  520  is not in contact with guide roller  530 . As Z carriage  518  moves along rails  522 , pneumatic cylinder  552  changes the length of extension of extensible rod  558  by corresponding amounts to take up belt slack and lead out additional belt length as port door  76 , respectively, advances toward or retracts from aperture  74 . Whenever link can age  520  contacts guide roller  530  and Z carnage  518  continues upwardly directed movement, pivot plate  566 , by operation of four-bar links  516 , pivots in a clockwise direction about a pivot axis  580  to provide a closing force, F close =F lift  sin θ, in which θ is the included angle between pivot plate  566  and a segment  582  of belt  560 . Belt  560  pulls pivot plate  566  in a direction that causes it to fold upwardly with a force component directed toward interior surface  78  of front plate  14  to snap shut port door  76  into aperture  74 . FIG. 45 (top) shows link carriage  520  in phantom lines to indicate the extent of horizontal displacement of link carnage  520  and therefore port door  76  for the minimum and maximum values of θ. The pivotal action of pivot plate  566  provides a positive self-locking feature for port door  76 . Whenever link carriage  520  contacts guide roller  530  and Z carriage  518  continues downward directed movement, pivot plate  566  pivots in a counterclockwise direction to provide an opening force of same magnitude but opposite direction of closing force, F close , to retract port door  76  away from aperture  74 . FIG. 45 (bottom) shows in phantom lines the positions of link carriage  520  and pivot plate  566  when Z carriage  518  is in its lowest position. 
     Counterbalance mechanism  550  exhibits several noteworthy features and advantages. There is no applied force required when port door  76  is in a fully open position (in which Z carriage  518  is in its lowest position) or in a fully closed position (in which pivot plate  566  snaps port door  76  shut against front plate  14 ). Pneumatic cylinder  552 , not motor  358 , carries the weight of port door  76 . The counterbalancing implementation creates a stroke multiplier in which the length of the belt is twice the linear distance traveled by Z carriage  518  because of the folded belt configuration. 
     A scanning assembly of a type exemplified by scanning assembly  290  that includes pivotable scanning fingers  292   l  and  292   r  and is designed with either reflective or transmissive beam scanners can also be implemented with four-bar carriage assembly  510 . 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.

Technology Classification (CPC): 7