Abstract:
A method whereby a sample handler moves and positions racks, containing sample tubes, in an analytical instrument. The sample handler has an in-feed and out-feed that advance sample tube racks using a walking beam mechanism. The racks are seated within the in-feed and are transported onto a cross-feed. The racks and tubes contained thereon are moved past detection devices that identify the samples and measure various properties thereof. Thereafter, the cross-feed moves the racks to a position behind the out-feed where the walking beam mechanism moves the tube racks out of the analytical instrument.

Description:
This application is a divisional application of U.S. Ser. No. 09/115,391, filed Jul. 14, 1998 now U.S. Pat. No. 6,331 437. 
    
    
     FIELD OF THE INVENTION 
     This application relates to an automated sample handler for an analytical instrument in which racks holding capped or uncapped test tubes or other containers are input into and output from the instrument. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following U.S. patent applications/patents, commonly assigned to the Bayer Corporation of Tarrytown, N.Y., and incorporated by reference herein: 
     (a) design patent application Ser. No. 29/090,683, filed Jul. 14, 1998, now U.S. Pat. No. D444,157; application Ser. No. 29/090,547, filed Jul. 10, 1998, now U.S. Pat. No. D420,747; application No. 29/089,359, filed Jun. 15, 1998, now U.S. Pat. No. D421,130; and application Ser. No. 29/088,045, filed May 14, 1998, now U.S. Pat. No. D411,307; 
     (b) utility patent application Ser. No. 09/113,643, filed Jul. 10, 1998, now U.S. Pat. No. 6,156,275; application Ser. No. 09/097,790, filed Jun. 15, 1998, now U.S. Pat. No. 6,065,617; application No. 08/985,759, filed Dec. 5, 1997, now U.S. Pat. No. 6,043,097; application Ser. No. 09/115,393, filed Jul. 14, 1998, now U.S. Pat. No. 6,227,053; application Ser. No. 09/115,777, filed Jul. 14, 1998, now U.S. Pat. No. 6,257,091; application Ser. No. 09/113,640, filed Jul. 10, 1998, now U.S. Pat. No. 6,074,617; and abandoned application Ser. No. 09/115,080, filed Jul. 14, 1998. 
     BACKGROUND OF THE INVENTION 
     Many different types of sample handlers have been used in various analytical instruments to feed multiple test tubes into and out of the instrument. Several manufacturers have utilized a sample handler system whereby the sample handler comprises an input queue, an output queue and a cross-feed. The input queue consists of an area in which racks of test tubes are input into the instrument and are transported toward the cross-feed. The racks are then transferred to the cross-feed, where one or more racks may be at a given time. The racks are indexed at set positions along the cross-feed where operations are performed on the test tubes, such as aspirating samples from the test tubes, and the racks are then moved to the end of the cross-feed adjacent the output queue where they are output to the output queue. One such system is described in U.S. Pat. No. 5,207,986. Various methods are used to transport the racks within the input queue and output queue. In some instruments, like the Chem I system sold by the Bayer Corporation, the input queue and output queue are indexed and walking beams are used to lift the base of the racks and translate them from one indexed position to an adjacent indexed position. 
     It is desirable to provide a sample handler that handles containers of various types, diameters and heights, whether capped or uncapped, and to permit a robotic arm to transport the containers to and from the sample handler for faster processing elsewhere without have to return the containers to a particular rack or position on the rack. 
     These prior art instruments do not provide this flexibility. First, they only handle a single type and style of test tube within a particular instrument. Second, these sample handlers are not designed to work in conjunction with a robot that removes containers, such as test tubes, individually from the racks for transport either within the instrument or between the instrument and a laboratory automation transport line. An entire rack would likely be lifted if a robot were to attempt to lift a test tube from a rack in the prior art instruments. Third, the input queue and output queue generally are not designed to handle uncapped test tubes because they do not stabilize the racks sufficiently and samples in open test tubes may spill. Fourth, the positions of the test tubes within a particular rack must be maintained or the instruments will be unable to track and perform the proper operations on the test tubes. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an automated handler for feeding test tube racks, which may hold capped or uncapped test tubes, into an analytical instrument and output uncapped test tubes (also referred to as “open test tubes”) from the instrument after the contents of the test tubes have been sampled. 
     It is a further object of this invention to provide an automated handler from which individual test tubes and other containers can be retrieved from racks and returned to racks individually by a robotic arm. 
     It is a further object of this invention to provide an automated handler for an analytical instrument that is operable in either a freestanding mode, in which racks of test tubes are manually inserted into and removed from the handler, or as a subsystem in a laboratory automation system in which test tubes are retrieved from or returned to a transport line containing test tubes. 
     A first aspect of the present invention is directed to a sample handler for an analytical instrument having a feeder for handling a rack, which may hold containers. The feeder comprises left and right side walls of a substantially identical height, a walking beam mechanism, and a tray, having walls of a substantially identical height, that is moved by the walking beam mechanism. When the walking beam mechanism is activated, the tray lifts a rack, which has tabs on the left and right side of the rack at a substantially identical height, from the side walls of the feeder. The feeder may be an infeed or an outfeed of a sample handier. The tray in the feeder has asymmetric guide rails to prevent the rack from skewing in the tray. 
     Another aspect of the present invention is directed toward an analytical instrument having a sample handler that interacts with a robotic arm on the instrument. The sample handler has an infeed, cross-feed and outfeed. A rack is input to the instrument in the infeed and is then transferred to a track on a cross-feed of the sample handler. Pusher fingers beneath the track push the rack from behind the infeed to another position, preferably behind the outfeed, where the robotic arm removes containers for transport elsewhere in the instrument. An ultrasonic range sensor detects whether a rack has been inserted into the infeed and whether the rack is skewed when it is placed on the cross-feed track behind the infeed. A reader of machine-readable code, such as a bar code reader, and an ultrasonic liquid level sensor are positioned adjacent the track to identify the container and profile the containers before the robotic arm removes the containers from the rack. 
     Another aspect of the present invention is directed to a sample handler having an outfeed with a walking beam mechanism to move the racks with a movable tray. A rear area of the tray has side walls that have a plurality of detents separated by ridges to capture a rack within the detents and hold the rack in a fixed position for the return of containers to the racks. 
     Another aspect of the present invention is directed toward a sample handler having an infeed, cross-feed, outfeed, and stat shuttle. The stat shuttle provides for the inputting of containers on a priority basis, including containers that may otherwise be input on a rack placed in the infeed. The stat shuttle also permits the inputting and outputting of a variety of containers. Like the cross-feed, the stat shuttle has a bar code reader and ultrasonic liquid level sensor to identify and profile containers in the stat shuttle. Thus, containers that are unidentified or not properly profiled in the cross-feed may be transferred to the stat shuttle for another attempt at identification and profiling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The inventions and modifications thereof will become better evident from the detailed description below in conjunction with the following figures in which like reference characters refer to like elements in which: 
     FIG. 1A is a perspective view of the sample handler of the present invention for an analytical instrument and some adjacent components of the instrument with several panels and doors of the instrument situated above the sample handler; 
     FIG. 1B is a top view of the sample handler of FIG. 1A; 
     FIG. 1C is a perspective view of the sample handler of FIG. 1A without the panels and doors of the instrument situated above the sample handler; 
     FIG. 2A is a perspective view of the bottom of the test tube rack; 
     FIG. 2B is an elevational view of the rack holding test tubes and of the pusher fingers, shown in dotted lines, positioned within openings on the bottom of the rack after the rack is placed onto the cross-feed track behind the infeed; 
     FIG. 3A is a perspective view of portions of the infeed and cross-feed of the sample handler with a test tube rack in a front operator-accessible area; 
     FIG. 3B is a perspective view of portions of the infeed and cross-feed with a test tube rack in a rear area of the infeed that is not accessible to the operator; 
     FIG. 3C is a perspective view of portions of the infeed and cross-feed with the test tube rack positioned in the infeed end of the cross-feed; 
     FIG. 3D is a perspective view of portions of the outfeed and cross-feed with the test tube rack positioned in the outfeed end of the cross-feed; 
     FIG. 3E is a perspective view of portions of the outfeed and cross-feed with the test tube rack positioned in the rear area of the outfeed which is inaccessible to an operator; 
     FIG. 3F is a perspective view of portions of the outfeed and cross-feed with the test tube rack positioned in the forward-most position in the rear area of the outfeed with tabs on the rack positioned under clamps that are in their open position; 
     FIG. 4A is a top view of the infeed with the tray removed; 
     FIG. 4B is a perspective view of the walking beam mechanism and several cross-beams of the infeed attached to only the right wall of infeed, the walking beam mechanism of the outfeed being similar; 
     FIG. 4C is a cross-sectional view along line C—C of FIG. 4B of the slider block of the walking beam mechanism with a shoulder screw of infeed tray, shown in FIG. 5C, rested within a channel of the slider block; 
     FIG. 5A is a top view of infeed tray, 
     FIG. 5B is a side view of infeed tray; 
     FIG. 5C is a cross-sectional view of a portion of the infeed tray along line C—C of FIG. 5B; 
     FIG. 6A is front view of the cross-feed; 
     FIG. 6B is a perspective view of the cross-feed from the rear of the cross-feed, 
     FIG. 6C is a perspective view of the cross-feed of FIG. 6B from the rear of the cross-feed with the main floor, rear wall, rack endstop, mount bracket and track removed; 
     FIG. 6D is a perspective view of the cross-feed of FIG. 6C with the front wall removed, 
     FIG. 6E is a front perspective view of the cross-feed with the ultrasonic liquid level sensor positioned above a rack with containers; 
     FIG. 6F is a perspective view of the gimbal in which the ultrasonic liquid level sensor is mounted; 
     FIG. 6G is a perspective view of the sensor holder to which the gimbal is mounted, 
     FIG. 6H is a perspective view of the platform; 
     FIG. 7A is a top view of the outfeed tray; 
     FIG. 7B is a side view of the outfeed tray; 
     FIG. 7C is a cross-sectional view of a portion of the outfeed tray along line C—C of FIG. 7B; 
     FIG. 8A is a front isometric view of the laboratory automation adapter; 
     FIG. 8B is an exploded view of the laboratory automation adapter of FIG. 8A; 
     FIG. 9 is an isometric view of a stat shuttle that may included in the sample handler; 
     FIG. 10A is a side elevational view of the cam profile for the infeed walking beam mechanism; and 
     FIG. 10B is a side elevational view of the cam profile for the outfeed walking beam mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1A-C,  2 A and  2 B, an analytical instrument  10  has a sample handler  20  according to the present invention to input and output containers to instrument  10 . Sample handler  20  comprises an infeed (or “input queue”)  80 , a cross-feed  95 , and an outfeed  100  (or “output queue”). Infeed  80  and outfeed  100  are positioned parallel to one another along their length. Cross-feed  95  is positioned behind infeed  80  and outfeed  100  and extends at least from behind the leftmost wall of infeed  80  to behind the rightmost wall of outfeed  100 . 
     Instrument  10  has one or more modules (not shown) in addition to sample handler  20  to perform various operations, including analyses, on the contents of a test tube. Various panels  30 ,  40 , including doors  35 ,  45  and a tower  50  for electronic controls are positioned above sample handler  20  and prevent access by an operator to the rear of sample handler  20 , including a rear area  82  of infeed  80  and the rear area  102  of outfeed  100  as well as the entire cross-feed  95  during operation of the sample handler. If doors  35  or  40  are opened, sample handler  20  (and one or more robotic arms that may interact with the sample handler) stops. The operator may access a front area  81  of infeed  80  and a front area  101  of outfeed  100 , however, while instrument  10  is operating. 
     Multiple microcontrollers control the operation of instrument  10  and communicate with one another over a CAN bus. One of these controllers is a sample handler controller, which may comprise a control board based on the Intel 386EX microprocessor. Sample handler controller communicates with and serves as a master controller for a separate controller for cross-feed  95  as well as separate controllers for the robotics which operate in conjunction with sample handler  20 . Cross-feed  95  may be a CAN node and the cross-feed controller may comprise a Phillips 8051 microprocessor to control the high current stepper motor of cross-feed  95 . Software in the sample handler controller provides a user interface to permit the user to control various aspects of sample handler  20 . 
     Preferably, in order to save on processing time by the controllers, a grid of all of the potential “registration locations” from and to which a container may be moved is mapped out in workstation software before instrument  10  is first activated. In the disclosed embodiment, these registration locations include eight locations in the outfeed side of cross-feed  95 , one location per tube receptacle  66  on one of racks  60 , and  72  locations in rear area  102  of outfeed  100 , including  8  possible tube receptacle locations on each of  9  possible racks in rear area  102 . 
     A control keypad is incorporated into tower  50  on the front of sample handler to permit the operator to stop the motion of infeed  80 , cross-feed  90 , or outfeed  100  in the event of a jam or to clean a spill. 
     Test Tube Racks 
     Test tubes or inserts, such as Microtainers®, or tubes with Ezee-Nest® inserts (generically referred to below as “test tubes”) are placed into test tube racks  60  (FIG. 2B) designed specifically for transporting the test tubes through sample handler  20 . A bar code label  70 , or some other form of machine-readable identification code, is affixed to each of racks  60  and, similarly, a bar code label  71 , or some other form of machine-readable identification code, is affixed to each test tube to allow instrument  10  to identify the racks  60  and test tubes and are used to identify, through a work order generally entered by the operator at the workstation or downloaded from a hospital laboratory system, what must be done with the test tubes. Custom-designed racks  60  are the subject of the referenced patent Ser. No. 09/087,780, now U.S. Pat. No. 6,065,617 application. 
     Each of racks  60  may hold as many as eight test tubes, which may be test tubes of various types, heights, and diameters, in individual tube receptacles  66  separated by side walls  64 . A lateral front wall  61  of each rack has openings  63  in front of each test tube location that are sufficiently large to expose the bar code label  71  on each test tube to be read by a bar code reader  55  (FIG. 1B) (or, if a machine readable identification code other than bar codes are used, a device suitable for reading that code) while a lateral rear wall  65  of each rack is closed. Test tubes are placed in the rack  60  by the operator and held in place with a spring, preferably a vertical leaf spring  67 , in each tube receptacle  66 . The test tubes must be firmly seated in the tube receptacles  66  to hold the test tubes securely, to prevent collisions of an improperly seated test tube with various obstructions (such as panel  30 ), and to provide precise positioning of the test tubes to permit bar code reader  55  to identify each test tube and an ultrasonic liquid level sensor  90  to determine the level of liquid therein and to detect the presence of caps on test tubes. 
     Tabs  110 ,  111  (or “ears”) on each side of racks  60  are located at the same height on each side of racks  60  and are used to hold racks  60  upright and to lift and advance the position of rack  60  in infeed  80  and outfeed  100  as explained below. Tabs  110 ,  111  are also used by sensors  92 ,  93  (FIGS. 3A and 3E) in cross-feed  95  to detect the presence of a rack  60  at either side of cross-feed  95  and to provide a reference level for profiling by ultrasonic liquid level sensor  90 . Recesses  115 ,  116  on each of respective tabs  110 ,  111  are provided to allow a pair of clamps  103 ,  104  in outfeed  100  to hold rack  60  in place. 
     Two openings  68 ,  69  are provided at the bottom of racks  60  (FIG. 2A) for racks  60  to travel over guide rails  130 ,  131  on infeed tray  120  as further described below. Openings  68 ,  69  in the bottom of racks  60  have a width W sufficient to fit pusher fingers  94   a ,  94   b  within openings  68 ,  69  with the pusher fingers in the raised position without contacting the rack and to prevent the rack from camming on guide rails  130 ,  131  on tray  120  and guide rails  500 ,  501  on outfeed  100  (as indicated by pusher fingers shown as dotted lines in FIG.  2 B). On the right side of each opening is a respective window  72 ,  74  to be engaged by respective pusher fingers  94   a ,  94   b  (FIG. 6C) on cross-feed  95 . On the left side of and continuous with openings  68 ,  69  are internal voids  76 ,  78  that provide the additional clearance necessary for fingers  94   a ,  94   b  to first disengage from windows  72 ,  74  before being pivoted downward to the right as the platform  410  to which they are attached moves to the left of cross-feed track  336  (described below) when pusher fingers  94   a ,  94   b  hit respective walls  79   a ,  79   b  on racks  60 . Openings  68 ,  69  are positioned asymmetrically along the length of the rack  60  (as are guide rails  130 ,  131  in infeed tray  120 ) to intuitively guide the operator to insert racks  60  into infeed  80  in only one direction with the front wall  61  of racks  60 , the bar code labels  70 ,  71  on racks  60  and test tubes, respectively, facing the front of infeed  80  to be read by bar code reader  55  on cross-feed  95 . 
     A ballast (not shown) weighing approximately 35-40 grams is incorporated within each of racks  60  during assembly and is located between windows  72 ,  74  to stabilize racks  60 . 
     The movement of racks  60  within sample handler  20  will be described in detail below. 
     Infeed 
     An operator inserts test tubes into racks  60  and inserts racks  60  into infeed  80 . Infeed  80  holds multiple racks, each of which may contain one or more test tubes or, in one particular situation to be explained, may intentionally contain no test tubes. In a preferred embodiment, infeed  80  holds as many as 21 racks. 
     Infeed  80  uses a bidirectional “walking beam” mechanism mounted above a chassis  57  (FIG. 1A) to move racks within infeed  80  and outfeed  100  and to move racks  60  to and from cross-feed  95 . The walking beam mechanism is somewhat similar to the mechanism for moving racks  60  in input and output queues as described in U.S. application Ser. No. 08/822,585, filed Mar. 20, 1997, now U.S. Pat. No. 5,861,563 and commonly assigned to the Bayer Corporation, which is incorporated by reference herein. However, among various differences, in infeed  80  of the present invention, the walking beam mechanism has walking beams that are of substantially equal height to stabilize racks  60 . Moreover, in the present invention, the walking beam mechanism moves racks  60  generally to the rear of infeed  80 , rather than to the front, by moving infeed tray  120 , in which racks  60  are placed, sequentially in an upward motion, followed by a rearward motion, a downward motion and a forward motion. 
     FIG. 4A illustrates infeed  80  with infeed tray  120  removed. Infeed  80  comprises two parallel side walls  121 ,  122  connected together with cross-beams, such as beams  123 - 126 . Side walls  121 ,  122  are of equal height so that tabs  110  on racks  60  may hang from the top rims of respective side walls  121 ,  122 . Infeed  80  has no front and rear walls to permit easy insertion of racks  60  into infeed  80  and the transfer of racks  60  to cross-feed  95 . A drip tray  140  is attached to the front of infeed  80  to catch spills. (FIG. 1B) 
     Referring to FIGS. 5A-5C, infeed tray  120  is a movable tray placed in infeed  80 . Tray  120  has a bottom  150  and side walls  151 ,  152  (the “walking beams”) but is open at its front and rear like infeed  80  so as not to obstruct the front and rear openings of infeed  80 . A middle section  153  on the rear of each of side walls  151 ,  152  slopes toward the front of tray  120  and the bottom section  154  of side walls  151 ,  152  then drops vertically to meet bottom  150 . Thus, the tops of side walls  151 ,  152  extend above cross-feed  95  as tray  120  moves rearward above cross-feed  95  without tray  120  hitting cross-feed  95 . This also results in the rearmost racks in tray  120  not being positioned above the bottom  150  of tray  120  as they reach the back of tray  120 . A short lip  155  projects upward at the rear of tray  120  to contain spills without impeding the movement of racks  60  out of the rear of infeed  80  and a drip tray  156  is attached to the front of tray  120 . 
     Side walls  151 ,  152  are slightly lower, by approximately 1½ mm in the preferred embodiment, than, and do not overlap the tops of, side walls  121 ,  122  of infeed  80  when the walking beam mechanism is in the home position. The width of infeed  80  and tray  120  must be somewhat larger than the width of racks  60  such that some skewing of the racks  60  will not cause racks  60  to cam between side walls  151 ,  152  of tray  120 . A U-shaped bracket  160  is mounted to the bottom of tray  120  and a shoulder screw  165  is mounted within bracket  160 . 
     Two stationary guide rails  130 ,  131  run from the front to the back of tray  120 . Guide rails  130 ,  131  are each narrower than openings  68 ,  69  on racks  60  to allow openings to move over guide rails  130 ,  13   1 . Racks  60  do not actually sit on guide rails  130 ,  131  or on the bottom of tray  120  but rather, as indicated above, are suspended above the bottom of tray  120 , hanging from tabs  110 ,  111  which rest either on the top of side walls  151 ,  152  of tray  120  or on the top of side walls of infeed  121 ,  122 . Openings  68 ,  69  on racks  60  key with guide rails  130 ,  131  to guide racks  60  along infeed  80  while preventing them from skewing or twisting more than slightly within infeed  80 . Openings  68 ,  69  leave adequate clearance for racks  60  to pass over guide rails  130 ,  131  to permit some skewing so the operator does not have to insert racks  60  into infeed  80  with extreme precision. These features on tray  120  and racks  60  are significant because racks  60  may contain uncapped test tubes whose contents may spill if racks  60  were not prevented from falling down into tray  120 . 
     As explained above, guide rails  130 ,  131  are situated asymmetrically along the width of tray  120  to insure that racks  60  may only be inserted into infeed  80  in a proper orientation with front wall  61  of each of racks  60  facing the operator to expose bar code labels  70 ,  71  of racks  60  and each test tube on racks  60  to bar code reader  55 . As a result, the operator is intuitively guided by guide rails  130 ,  131  to not insert racks  60  in the reverse orientation. The top rims of side walls  151 ,  152  of tray  120  are smooth so that the operator may slide racks  60  freely towards the back of infeed  80  or toward the front of infeed  80  when racks  60  are still in front area  81  which is accessible to the operator. 
     The walking beam mechanism is shown in FIG. 4B with tray  120  removed and with various other components, including right wall  122  of infeed  80  and cross-beam  123 , cut away to show more clearly how the walking beam mechanism operates. A first lift bar  170  is mounted toward the rear of infeed  80 . Lift bar  170  comprises a rod  172 , the ends of which sit in holes in each of side walls  121 ,  122  and which defines a first pivot axis around which lift bar  170  pivots, an I-shaped bar  174  and a second rod  176  to which three rollers  177 , one roller adjacent each end of I-shaped bar  174  and one roller midway between the ends of I-shaped bar  174 , are mounted. A plastic tubular spacer  173  surrounds second rod  176  and keeps rollers  177  spaced at the desired intervals. Second rod  176  may move up and down in a slot  178  on each of side walls. (Only slot  178  on left wall  121  is shown but the slot on right wall  122  is identical.) A third rod  179  is connected between a bracket  180  on the bottom of lift bar  170 . A roller  182  is mounted to third rod  179  below the pivot axis of lift bar  170 . 
     A second lift bar  190  is mounted toward the front of infeed  80 . This lift bar  190  also comprises a rod  192 , the ends of which sit in holes in each of side walls  120 ,  121  and which defines a second pivot axis around which second lift bar  190  pivots, an I-shaped bar  194  and a second rod  196  to which three rollers  197 , one adjacent each end of I-shaped bar  194  and one midway between the ends of I-shaped bar  194 , are mounted. A second plastic tubular spacer  193  surrounds second rod  196  and keeps rollers  197  spaced at the desired intervals. Second rod  196  may move up and down in a slot  198  on each of side walls  121 ,  122 . (Only the slot on left side wall  121  is illustrated.) A third rod  199  is connected between bracket  200  on the bottom of second lift bar  190  but no roller is mounted to third rod  199 . A long link  230  serves as a tie rod connecting third rod  199  on front lift block  190  to third rod  179  on rear lift block  170 , thereby driving second lift bar  190  in synchronization with first lift bar  170 . 
     A motor  210 , preferably a single gear brushless DC motor, is mounted in front of rear lift bar  170 . Motor  210  has integrated control electronics that interface to the sample handler controller. A disk cam  220 , having a profile as shown in FIG. 10A, is mounted to a drive shaft on motor  210  at the center of cam  220 . Cam  220  is coupled to roller follower  182  on lift bar  170 . 
     A slider block  240  slides above long link  230  and is trapped around long link  230  with a keeper plate  250  mounted to slider block  240  beneath long link  230  (FIG.  4 C). One end of a second, shorter link  260  mounts to the left side of slider block  240 , generally toward the rear of slider block  240  to minimize the length of short link  260  and not interfere with the placement of tray  120  within infeed  80  by the pulling slider block  240  rearward over long link  230 . Where sample handler  20  is designed to accommodate racks  60  according to the preferred embodiment, in which racks may be moved 25 mm per cycle of the walking beam mechanism, the opposite end of short link  260  mounts to the right side of cam  220  at a point 12½ mm away from the center of cam  220  so as to cause tray  120  to advance 25 mm toward the rear of infeed  80  with a 180° turn of cam  220 . The precise amount of rearward movement of racks  60  caused by each rotation of cam  220  is not significant in infeed  80  as long as racks  60  move relatively quickly toward the rear of infeed  10 . 
     A channel  270  running sideways through the center of slider block  240  provides a means for locating tray  120  within infeed  80  by inserting shoulder screw  165  into channel  270 . U-shaped bracket  160  fits around the sides of slider block  240  and helps to locate and stabilize tray  120 . When tray  120  is inserted in infeed  80 , with the walking beam mechanism turned off, the top of side walls  151 ,  152  of tray sit preferably 1½ mm below the top of side walls  121 ,  122  on infeed  80 . 
     With tray  120  positioned within infeed  80  and sitting in its proper position on slider block  240 , the operator may place one or more of racks  60  into infeed  80 . A long-range ultrasonic sensor  280  is positioned on cross-feed  95  behind infeed  80 . Range sensor  280  emits ultrasonic waves that travel toward the front of infeed  80 . Racks  60  are made from a material that reflects an echo back toward range sensor  280  if racks  60  are inserted into infeed tray  120 . An emitted wave that is reflected back to and detected by range sensor  280  as an echo signals that one or more racks are in tray  120 . 
     Range sensor  280  may point directly toward the front of infeed  80  but does not in the preferred embodiment because it may be desirable to position other components of instrument  10  behind cross-feed  95  and because it is desirable to also use range sensor as a skew sensor as well to determine if the right side of a rack has been placed on cross-feed  90  skewed away from sensor  280 . Therefore, in the preferred embodiment, range sensor  280  is positioned sideways along the axis of cross-feed  95  pointing toward outfeed  100  and into a custom-designed acoustic mirror  290  which is mounted to the back wall  332  of cross-feed  95  and which is off-center to the right side of infeed  80 . Acoustic mirror  290 , a plastic passive reflector, is constructed from polycarbonate, or any plastic that has a reflective surface. 
     A preferred range sensor  290  is manufactured by Cosense Sensors Inc. of Hauppauge, N.Y. as Model No. 123-10002. That sensor is enclosed in. a shielded body that is 0.425″ diameter by 0.75″ long. Where the sensor emits a wave at a preferred frequency of 0.5 MHz for 150 milliseconds to have a sufficient range to detect racks  60  inserted at the front of infeed  80 , the dead zone, which equals the distance from sensor  280  in which the 0.5 MHz wave cannot be sensed is approximately 2 inches. (The length of the dead zone equals the distance the wave travels before range sensor  280  resumes listening for an echo from the wave.) Therefore, acoustic mirror  290  is approximately 2.5 inches long in the preferred embodiment. The leftmost 2 inches  292  of acoustic mirror  290  accounts for the dead zone within which movement directly within acoustic mirror  290  in front of range sensor  280  cannot be detected. A 0.5 inch angled portion  294  on the right of acoustic mirror  290  has a reflective surface which is angled at a 45° angle toward the front of infeed  80 . This bends by 90° the wave emitted by sensor  280  after it has passed the dead zone and focuses the wave toward the front of infeed to detect the presence of racks in tray  120 . 
     In order to best detect a skew of the right side of a rack in cross-feed  90  while performing range sensing, acoustic mirror  290  should be mounted on cross-feed  90  behind infeed  80  with a bias to the right side of infeed as much as possible but the angled portion  294  should be positioned so as to reflect wave toward the front of infeed  80  between guide rails  130 ,  131 . 
     Software in instrument  10  may determine the distance of the object from the rear of cross-feed  95  based on the time it takes for the sound to be reflected back to range sensor  280 . However, there is no need for the software to track the precise position at which the rack that triggers the walking beam mechanism is inserted, although software could be included to determine this information. If range sensor  280  is configured and operated to detect objects beyond the front of infeed  80 , the software may also be programmed to reject signals detected by sensor  280  that are generated by objects more than a certain maximum distance from acoustic mirror  290 , such as a person walking in front of the infeed  80 , to prevent the activation of the walking beam mechanism by signals outside of infeed  80 . 
     The walking beam mechanism is activated by the detection by range sensor  280  of racks  60  in infeed  80 , unless there is a rack in the infeed side of cross-feed  95 . Upon activation of the walking beam mechanism, cam  220  begins rotating and rolling against roller follower  182 , causing lift bar  170  to pivot about rod  179  with rod  176  moving upward within slot  178 . Because front lift bar  190  is linked to rear lift bar  170  via long link  230 , the pivoting of rear lift bar  170  also causes front lift bar  190  to pivot in the same direction. This causes tray  120  to move upward a total of 3 mm with the top of side walls  121 ,  122  of tray  120  raised 1½ mm above the top of side walls  151 ,  152  of infeed  80  when tray  120  is fully raised. As tray  120  moves upwards, tabs  110  on each of racks  60  are picked up off the top of side walls  121 ,  122  of infeed  80  and transferred onto the top of side walls  151 ,  152  on tray  120 . In the event that range sensor  280  fails and does not activate the walking beam mechanism, the walking beam mechanism may be manually activated. The speed at which the walking beam is preferably activated is 25 rpm +/−2 rpm. This speed, as well as the lift of cam  220  is selected to minimize the noise generated by the transfer of racks  60  between side walls  121 ,  122  and side walls  151 ,  152 . The position of the walking beam mechanism for infeed  80  (and for outfeed  100 ) is controlled by activating motor  210  for a given time at a known speed. 
     As tray  120  nears completion of its upward motion and after racks  60  have been transferred to the top of side walls  151 ,  152  on tray  120 , short link  260  pulls slider block  240  rearwards, as provided for by the positioning of the mounting of short link  260  to cam  260 , thereby moving tray  120  with racks  60  rearwards approximately 25 mm. Cam  220  begins lowering lift bars  170 ,  190  as tray  120  nears completion of its rearward movement, thereby lowering tray  120 . As the top of side walls  151 ,  152  of tray  120  move below the top of side walls  121 ,  122  of infeed  80 , tabs  80  on racks  60  are again transferred from being supported on the top of side walls  151 ,  152  of tray  120  to the top of side walls  121 ,  122  of infeed  80 . As tray  120  is lowered, cam  220  causes slider block  240  to move tray  120  toward the rear of infeed  80  approximately 25 mm to return tray  120  to its original position. As long as the walking beam mechanism is activated, tray  120  continues moving in accordance with the up-rear-down-forward directions with racks  60  being passed back and forth between the top of side walls  121 ,  122  and the top of side walls  151 ,  152 . This multidirectional motion of tray  120  causes racks  60  to move rearwards in infeed  80 , with some racks  60  pushing the racks  60  behind them backwards toward cross-feed  95  to compact the racks  60  in the rear of infeed  80 . Thus, even if racks  60  were placed into tray  120  somewhat skewed, the compacting motion will make them parallel to side walls  151 ,  152  of infeed tray  120 . 
     Vertical panel  30  covering the front of instrument  10  is positioned above infeed  80  and extends downward to limit operator access to rear area  82  of infeed  80 . Panel  30  provides clearance for the tallest test tubes with the tallest caps which are properly seated in racks  60  and gives a visual cue to the operator to reseat any improperly seated test tubes. Infeed  80  has a front area  81  in front of panel  30  which is accessible to the operator and, although rear area  82  is not accessible to the operator, the operator could push racks  60  in front area  81  toward rear area  82 , causing racks  60  in rear area to be pushed backward. The operator may remove a rack  60  or shuffle the order of racks  60  before they pass behind panel  30  above infeed  80 . 
     Test tubes on racks  60  must be seated properly in racks  60  by the operator not only to insure the stability of the test tubes but also to position bar code labels  71  on test tubes so they may be read by bar code reader  55  along cross-feed  95 , and to insure that the test tubes may pass under the armature  91  for ultrasonic liquid level sensor  90  extending above cross-feed  95  so that the level of liquid in the test tubes is properly determined by the ultrasonic liquid level sensor  90 . 
     A gross height sensor  320  may be optionally mounted to the side of infeed  80  behind panel  30  to detect test tubes that are not fully seated but pass under panel  30  or whether some test tubes are taller than the specifications of instrument  10  permit it to handle. Gross height sensor  320  comprises an optical infrared through-beam sensor  320  having a transmitter and receiver mounted on brackets  321 ,  322 , respectively, and should be calibrated to be sensitive enough to detect clear glass test tubes. Bracket  321  for the transmitter for gross height sensor  320  is mounted on one side of infeed  80  and bracket  322  for the receiver is mounted to the opposite side, both being mounted so that the transmitter and receiver detect test tubes positioned at a height slightly higher than the tallest expected test tube with a cap to be placed in sample handler  20  with tray  120  fully raised. If gross height sensor  320  detects that a particular test tube in a rack is seated too high, the movement of the walking beam mechanism for infeed  80 , which causes racks  60  to move toward the rear of infeed  80 , is stopped and the walking beam mechanism is activated in the reverse direction (cam  220  causes tray  120  to move back, up, forward, down) to move the rack with the improperly seated test tube back into the operator-accessible from front area  81  of infeed  80  to enable the operator to reseat the test tube or to transfer a sample in a test tube which is too tall for instrument  10  to a test tube which meets the specifications. An empty rack (which normally would be filled with one or more test tubes) is shown in FIG. 3B in a position after it has passed panel  30  and gross height sensor  320 . 
     The walking beam mechanism continues cycling and moving racks  60  rearward to the back of infeed  80  until at least one of racks  60  reaches the back of tray and the cycling of tray  120  lifts the rearmost rack  60  in infeed  80  and transfers it onto a stationary track  336  that is formed around the inside perimeter of cross-feed  95  (the distance separating the rear of infeed  80  from track  336  being preferably approximately 25 mm) where cam  220  causes a rearward movement in a single cycle of 25 mm (FIGS.  1 B and  6 B). FIG. 3C shows the rack seated in cross-feed  95 . This transfer to cross-feed  95  is detected by the left tab  110  of rack being placed on sensor  92  so as to block the infrared beam on optical sensor  92 . Once rack is moved to cross-feed  95 , the walking beam mechanism cycles two additional times, which causes chamfered edges  157  on the top rear of tray  120  (FIG. 5B) to hit the front of tabs  110 ,  111  and thereby pushes the rack rearward before catching the tabs  110 ,  111  on side walls  151 ,  152  and again placing the rack on track  336 . This insures that the rack on track  336  of cross-feed  95  is perpendicular to cross-feed  95 . The walking beam mechanism then turns off. 
     The walking beam mechanism will automatically stop sooner if a rack is not deposited in cross-feed  95  after a certain amount of time, during which the walking beam mechanism is cycled a maximum number of times. This would indicate that the movement of racks  60  has probably been obstructed. In the embodiment where the walking beam mechanism moves racks  60  25 mm per cycle and tray  120  holds  21  racks each 23 mm wide, the cycling may be automatically stopped after a time sufficient for the walking beam mechanism to cycle  25  times because only  21  cycles should have been necessary to move a rack inserted at the front of tray  120  to cross-feed  95 . 
     During the operation of the walking beam mechanism, the operator may insert additional racks  60  into infeed  80  even though tray  120  is moving. The operator may also push racks  60  toward the rear of infeed  80  as far as possible without disturbing the operation of sample handler  20 . 
     As explained above, in addition to detecting racks  60  in tray  120 , range sensor  280  also assists in detecting if a rack  60  is inserted into cross-feed  95  by tray  120  is skewed. Only limited skewing is possible due to guide rails  130 ,  131  in tray  120  which transfers rack to cross-feed  95 . However, a high degree of accuracy is required when a rack is placed on cross-feed  95  because test tubes must be properly positioned to be removed by a robotic arm (not shown). The proper placement of the left side of a rack into cross-feed  95  is detected by left tab  110  on the rack being placed above sensor  92 . At the same time, range sensor  280  detects if the right side of rack is skewed by calculating that readings across range sensor  280  are within a small limited allowable range away from range sensor  280 , the maximum limit preferably being 0.1 inches. The rack is determined to be skewed if the right side of rack is further than this maximum limit. 
     Homing means, such as those known to those skilled in the art, should be provided to accurately home the walking beam mechanism for infeed  80  (and for outfeed  100 ). 
     Cross-Feed 
     Cross-feed  95  is designed to firmly grab racks  60  placed on track  336  of cross-feed  95  by the walking beam mechanism of infeed  80 , one rack at a time, to push the rack linearly to the opposite side of cross-feed  95  behind outfeed  100 , and to hold that rack downward and as vertically as possible to both position each test tube in one of the eight predetermined registration positions on cross-feed  95 , which the robotic arm recognizes, to allow a robot to remove test tubes individually, without disturbing other test tubes in the rack  60 , and without accidentally pulling up the rack along with the test tube due to friction between the test tube and the rack. Once the test tubes have been removed from the rack  60 , outfeed  100  removes the rack from cross-feed  95 . 
     Referring to FIGS. 6A-6E, in addition to track  336 , cross-feed  95  has a front wall  330 , a rear wall  332  (or fence), a linear transport mechanism  335  positioned under track  336  and a rack transport connector subassembly that comprises a platform  410  connected to the top of linear transport mechanism  335  for gripping the rack on cross-feed  95 . Front wall  330  is short where it is situated behind infeed  80  and outfeed  100  to provide clearance for a rack to be placed on cross-feed  95  by infeed  80  and to be removed from cross-feed  95  by outfeed  100 . The center portion of front wall  330  that is not located behind infeed  80  or outfeed  100  is taller and has a preloading means for providing a force against the front of the rack as it moves through cross-feed to maintain the perpendicularity of the rack to track  334 . However, this center portion is lower than the level of openings  63  on rack to permit the reading of bar code labels  70 ,  71 . In one embodiment, the preloading means comprises four pressure springs  336  on the back of front wall, each comprising a short metal link  337  parallel to front wall  330  and a spring  338  between each end of link  337  and mounting points  339  on front wall  330 . Rear wall  332  also helps properly seat the rack on cross-feed  95  perpendicularly to track  336 . Rear wall  332  is raised in the area behind infeed  80  to prevent rack  60  from tilting backwards as it is passed by tray  120 , when tray  120  is a raised position, to cross-feed  95 . 
     The linear transport mechanism of cross-feed  95  comprises two pulleys  340 ,  341 , one pulley mounted to each end on a bottom  334  of linear transport mechanism  335 , and a belt  345  surrounding pulleys  340 ,  341 . The linear transport mechanism is driven by a stepper motor  350 , that is preferably controlled by the microprocessor in the cross-feed controller, located beneath belt  345  behind the outfeed  100  side of sample handler  20 . Stepper motor  350  is electrically coupled to the cross-feed controller. The gear head output shaft  360  on motor  350  is coupled to a pulley  370  which is in turn coupled to pulley  341  with drive belt  380 . A rail  390  is mounted along the top of assembly bottom  334  on linear transport mechanism  335  and extends between pulleys  340 ,  341 . Two bearing blocks  400 ,  401 , which may be any bearing block that fits, slide along guide way  390  and are also attached to and move with belt  334 . A platform  410  is mounted to bearing blocks  400 ,  401 . 
     Two L-shaped pusher fingers  94   a ,  94   b  are pivotally mounted at pivot points  427  to the top of platform  410  and each of fingers  94   a ,  94   b  is preloaded with a spring  405   a ,  405   b  (FIG. 6H) to a raised position. The upper ends of pusher fingers  94   a ,  94   b  are angled upward towards the outfeed  100  side of cross-feed at an angle in the approximate range of 20-45° to cam into windows  72 ,  74  on racks  60  and the top end  425  of each of fingers  94   a ,  94   b  is chamfered on both front and back sides to bias the rack against track  336 . The back chamfer on fingers  94   a ,  94   b  also biases the rack  60  against rear wall  332  to ensure that the test tubes are properly in the registration locations for robot access. 
     A rack  60  may be placed in cross-feed  95  when platform  410  is positioned under the arriving rack. In this case, with platform  410  in position behind infeed  80 , pusher fingers  94   a ,  94   b  are in the raised position and fit within openings  68 ,  69  without contacting windows  72 ,  74 . At other times, a rack  60  may be placed by tray  120  on cross-feed  95  when platform  410  is still holding another one of racks  60  behind outfeed  100  or returning from the opposite side of cross-feed  95 . In this case, as platform  410  moves under the rack  60  behind infeed  80 , pusher fingers  94   a ,  94   b  are pivoted downward to the right by the force of the rack and then return to the raised position as they arrive within openings  68 ,  69 . 
     Once a rack is placed securely on cross-feed  95 , i.e. after it has been placed on cross-feed  95  and two additional  360  degree movements of cam  220 , platform  410  begins moving to the opposite side of cross-feed  95  and, in the process, pusher fingers  94   a ,  94   b  cam within windows  72 ,  74 , respectively, to push the rack across track  336 . The rack should not accelerate to more than approximately 0.3 g to avoid spilling the liquid in open test tubes. 
     Bar code reader  55  is mounted adjacent cross-feed  95  a short distance beyond the inner side of infeed  80  and reads bar code labels  70 ,  71  on the rack and test tubes as rack and test tubes are transported along cross-feed  95  in front of bar code reader  55 . If a label cannot be read, such as when the bar code label on the test tube is not oriented toward bar code reader  55 , the test tubes which were not identified are not extracted from the rack for processing by instrument  10  (or are sent to the stat shuttle  600  for a second attempt at container identification). 
     An ultrasonic liquid level sensor  90  is positioned above cross-feed  95  within a sensor holder  408  mounted to a bracket  91 . (FIGS. 6E-6G) The sensor  90  is preferably mounted in a gimbal  407  that fits within sensor holder  408 . A preferred sensor  90  is height sensor (“transponder”) manufactured by Cosense as Model No. 123-10001. Sensor  90  should be positioned on bracket  91  approximately  5  inches from the bottom of the rack to allow for a 0.75 inch dead zone immediately beneath sensor  90 . The data provided by sensor  90  may be used to provide a profile of the type of test tubes in the rack, the level of liquid in open test tubes, and whether test tubes have a cap which must be removed. The rack is also profiled to provide a height reference. This profiling is the subject of the referenced application entitled Dynamic Noninvasive Detection of Analytical Container Features Using Ultrasound. If the profiling indicates that a cap is present, instrument  10  instructs a robotic arm to transport the capped test tubes to an automatic decapper, which is preferably a component on instrument  10  and may be included in the sample handler module. After the decapper removes the cap, another ultrasonic liquid level sensor (not shown) in the decapper determines the liquid level in the now uncapped test tube. 
     Ultrasonic liquid level sensor  90  is mounted upstream from bar code reader  55  along cross-feed  95  to provide the necessary distance for the rack  60  on platform  410 , which is initially at rest behind infeed  80 , to accelerate up to the slew speed that allows ultrasonic liquid level sensor  90  to take a sufficient number of equally spaced data points and profile the test tubes in the rack before passing under ultrasonic liquid level sensor  90 . For example, in one embodiment, the required slew speed may be 2 inches/second so ultrasonic liquid level sensor  90  must be placed far enough along cross-feed  95  to allow the rack  60  to reach that slew speed. Profiling requires a smooth motion of the rack and test tubes under sensor  90 . Test tubes cannot accelerate too quickly or samples in test tubes will be disturbed. 
     The data collected by ultrasonic liquid level sensor  90  is also used in conjunction with a homing sensor (not shown) for platform  410  built into the linear transport mechanism of cross-feed  95  to verify that the rack is fully seated. 
     Track  336  of cross-feed  95  must maintain the perpendicularity of the rack  60 , to insure the accuracy of a critical datum point for the height reference set by tabs  110 ,  111  on the rack as measured by the ultrasonic liquid level sensor  90  and to maintain the registration positions for the robotic arm. Should sensor  90  malfunction, sample handler  20  could still be used but the test tubes would all have to be uncapped and be filled to substantially the same height. 
     As soon as the rack clears the area of cross-feed  95  behind infeed  80 , if additional racks are in tray, they are detected by range sensor  280  and the walking beam mechanism starts cycling again and continues moving until another rack is placed on track  336  of cross-feed  95 . 
     When a rack reaches the opposite side of cross-feed  95 , which is the unloading position shown in FIG. 3D for unloading test tubes from rack to be transported elsewhere in instrument  10 , the right tab of rack is positioned above sensor  93 , which is an optical sensor similar to sensor  92 . A hard mechanical stop  440  is also provided at the outfeed end of cross-feed  95  adjacent rail  390  to stop bearing blocks  400 ,  401  in a precise position for unloading of the test tubes and subsequent transfer of the rack  60  to outfeed  100 . Hard stop  440  is adjustable to accommodate some slight variations in the positioning of cross-feed  95  in different instruments. After sensor  93  is triggered, software instructs stepper motor  350  to advance  2  additional steps to tension pusher fingers  94   a ,  94   b  to bias the rack against hard stop  440 . 
     While in the unloading position, pusher fingers  94   a ,  94   b  remain engaged in windows  72 ,  74  and a robotic arm located on instrument  10  above sample handler  20  may extract each of the test tubes from the rack. The robotic arm is able to extract test tubes positioned in cross-feed  95  as long as the test tubes are within one of the registration locations. Allowance is made for some slight variation in position. The engaged pusher fingers  94   a ,  94   b  mechanically constrain rack during extraction of the test tubes by robotic arm to prevent friction between the test tubes and rack from pulling the rack out of cross-feed  95  along with the test tubes. 
     An optical through beam sensor (not shown) may be added to cross-feed  90  to detect if there is a rack in the cross-feed during the initialization of instrument  10  after a power outage. Generally, this will not occur if an uninterrupted power supply is attached to instrument  10  to allow an orderly power down, including moving racks  60  out of cross-feed, to insure that no racks in cross-feed  95  remain undetected upon the restoration of power. 
     Outfeed 
     Referring to FIG. 3E, rack is moved to outfeed  100  after it has been emptied of test tubes by the robotic arm. Like infeed  80 , outfeed  100  comprises a bidirectional walking beam mechanism mounted above the chassis  57  similar to the walking beam mechanism as described and shown in FIG. 4B above with reference to infeed  80  (except that cam  220 ′ has a different cam profile, a preferred profile being shown in FIG.  10 B). Outfeed  100  has side walls  510 ,  511  which are joined together with cross-beams. 
     Outfeed  100  has a front area  101  which is always accessible to the operation for removing racks from the system and a rear area  102  which is inaccessible to the operator during operation of instrument  10 . The operator is prevented from inserting a hand in rear area  102  by panel  40  and door panel  45  (FIGS. 1A and 1B) on instrument  10 . A drip tray  590  is attached to the front of outfeed  100  to catch any spills. 
     Sitting within outfeed  100  is an outfeed tray  450  which has side walls  505 ,  506  and a bottom  507  but is open at the front and rear of tray  450 . (FIGS. 7A and 7B) Tray  450  preferably holds a total of 20 racks with 10 racks in rear area  102  and the remaining racks in front area  101 . Like infeed tray  120 , the top of side walls  505 ,  506  of outfeed tray  450  extend farther back toward cross-feed  95  than the bottom of side walls  505 ,  506 , sloping forward along a middle section at the rear of side walls  505 ,  506  so that the bottom  507  of tray  450  does not hit cross-feed  95  when tray  450  rotates backward over cross-feed  95 . 
     Tray  450  has a shoulder screw  460  attached to a U-shaped bracket  461  on the bottom of tray  450  (FIG. 7C) which sits in a channel on sliding block that is identical to sliding block  240  and causes the backwards and forward movements of tray  450 . Two guide rails  500 ,  501  extend from the front to back of the top of tray  450  but are asymmetrically positioned across the width of the tray, with the same asymmetry as in infeed tray  120 , to accommodate and prevent skewing of racks  60 . Tray  450  is sufficiently wider than racks  60  to prevent camming of racks against side walls  505 ,  506 . Outfeed tray  450  has a lip  580  in the back (FIG. 7B) and a drip tray  600  attached to the front of tray  450  for spill containment. 
     There are two primary differences between infeed  80  and outfeed  100 . The first difference is that the top of side walls  510 ,  511  on outfeed  100  and top of side walls  505 ,  506  on outfeed tray in rear area  102  have trapezoidal detents  531 - 539  (on outfeed side walls  510 ,  511 ) and detents  540 - 549  (on tray side walls  505 ,  506 ). Tabs  110  on racks  60  may sit in detents  531 - 539  and  540 - 549  in order to precisely position each of racks  60 . This allows the robotic arm to locate the tube receptacles in racks  60  to which the test tubes are to be returned using the predefined grid of  72  registration locations where test tubes may be inserted in outfeed  100 . The software tracks which of detent positions have racks and which tube receptacle positions in those racks are available for the insertion of test tubes. In the embodiment illustrated in FIGS. 3D-3F, there are nine detents  530  on outfeed side walls  510 ,  511  and ten detents  531  on side walls  510 ,  511  of tray  450 . When tray  450  is in its rest position in outfeed  100 , nine rear detents  540 - 548  in tray  450  are aligned with the nine detents  531 - 539  on outfeed  100 . Detents  531 - 539 ,  540 - 549  are identical in shape and size. They are approximately 2 mm larger than the width of tabs  110  to provide a small amount of clearance for tabs  110 . Thus, where detents  531 - 539 ,  540 - 549  are approximately 25 mm, tabs  110  are made approximately 23 mm wide. While the precise distance that tray  120  in infeed  80  must move rearward to translate racks  60  along infeed  80  may vary, the distance which tray  450  must move must be precise, 25 mm for the preferred specifications, to move racks  60  from one detent to another. 
     Detents are separated by ridges  550  which maintain a separation between racks  60 . Ridges  550  are designed to be high enough to maintain racks  60  in the registration positions within the detents. The cam profile of outfeed  100  must be designed to lift racks  60  high enough and far enough so as to clear ridges  550  when being moved between the detents. 
     If racks  60  are initially not centered within the detents as they are moved within tray  450 , the trapezoidal shape of detents pushes racks  60  into the center of the detents. The trapezoidal shape of the detents and 2 mm clearance also allows racks  60  to “float”, i.e., tilt slightly forward or backward, when a robotic arm inserts a test tube in a tube receptacle in the rack should the robotic arm or test tube be slightly angled when the tube is inserted in the rack. 
     The second primary difference between infeed  80  and outfeed  100  is in the cam profile. The outfeed cam causes outfeed tray  450  to be raised and lowered a larger distance than infeed  80 , the total distance between the highest and lowest points being preferably 7½ mm. When tray  450  is fully lowered in outfeed  100 , side walls  505 ,  506  sit 4 mm below side walls  510 ,  511 . The cam raises tray  450  3½ mm, so as to lift tray  450  above ridges  550  between detents. 
     Raising tray  450  higher in outfeed  100  does not create the same problem it would create in infeed  80  because the up and down movement of racks  60  only occurs in the rear area  102  of tray  450 , which is enclosed behind panel  40  and therefore is less noisy and disturbing to the operator than the movement of racks in infeed  80  where almost ⅔ of the tray is exposed to the operator. 
     Outfeed  100  both removes the rack, which has been emptied of test tubes from cross-feed  95  and moves racks  60  from one detent position to a second adjacent detent position closer to the front of outfeed  100  to generally output racks  60  toward the front of outfeed  100 . As with the walking beam mechanism on infeed  80 , the movement of the walking beam mechanism on outfeed  100  is accomplished by the rotation of tray  450  in conjunction with the transfer of tabs  110 ,  111  on rack between the top of side walls  510 ,  511  on outfeed  100  and the top of side walls  505 ,  506  on tray  450 . 
     To remove a rack  60  from cross-feed  95  after the test tubes have been removed from the rack  60  by the robotic arm, as tracked by the software, the motor on the outfeed walking beam mechanism is activated for a predetermined length of time to rotate the outfeed cam in a counterclockwise direction approximately a quarter of a turn. This causes outfeed tray  450 , in a continuous motion, to first move backward approximately 25 mm, which is the distance between two adjacent detents, such that the rearmost detent  540  is positioned under tabs  110 ,  111  and to thereby capture and cradle the rack between side walls  505 ,  506  of tray  450 . At that point, the outfeed walking beam mechanism momentarily stops for a fixed time and holds tray  450  in a fixed position, while pusher fingers  94   a ,  94   b  are extracted from windows  72 ,  74  on the rack  60  in cross-feed  95 , which has been emptied of test tubes, to allow platform  410  to return to the opposite side of cross-feed  95  behind infeed  80 . As the platform  410  begins moving, the left side of pusher fingers  94   a ,  94   b  contact walls  79   a ,  79   b  and are thereby pushed downward to move out from under the rack  60 . By cradling the rack as pusher fingers  94   a ,  94   b  are extracted from windows, outfeed  100  prevents the rack  60  from returning toward infeed  80  along cross-feed  95 . After the timeout for pusher fingers  94   a ,  94   b  to clear the rack, the rack  60  is captured within detent  540  on tray  450  and the outfeed walking beam mechanism is again activated, causing tray  450  to move the extracted rack upward approximately 7½ mm, side walls  505 ,  506  of tray  450  rising approximately 3½ mm above the top of side walls of outfeed  100  and thereby transferring tabs on racks from the top of side walls  510 ,  511  of outfeed  100  to the top of side walls  505 ,  506  of tray  450 . Tray  450  then moves forward 25 mm and downward 7½ mm, transferring tabs  110 ,  111  on racks  60  to side walls  510 ,  511  of outfeed  100 , depositing the rack removed from cross-feed  95  into rearmost detent position  531  on outfeed 25 mm closer to the front of outfeed  100 . 
     After removal of the first rack from cross-feed  95 , the cycling of the walking beam mechanism on outfeed  100  is repeated to remove other racks  60  after they are emptied of test tubes in cross-feed  95 . FIG. 3E shows a rack after it has been moved forward  3  detent positions and is suspended from detent  533 . Tray  450  cannot rotate while a rack is in cross-feed  95  behind outfeed  100  before the test tubes are removed from the rack  60  because the rack  60  must remain seated in platform  410  during that time, but cycling resumes after the test tubes have been extracted from that rack  60 . As tray  450  picks up a rack  60  from cross-feed  95 , it also picks up 
     any other racks  60  in the rear area  102  of outfeed  100  and moves them towards the front of outfeed  100  one detent position at a time. Detent positions  531 - 539  are generally filled with racks  60  before the frontmost rack is output into the user-accessible area of outfeed  100  when a tenth rack is picked up by tray  450 . 
     Test tubes are output from other modules in instrument  10  after processing and placed in the frontmost rack by robotic arm as they are output until that rack is full of test tubes. After the frontmost rack is filled, the remaining racks are filled with test tubes, with a rack  60  that has an empty tube receptacle  63  and is closest to the front of outfeed  100  being filled first. 
     In the front area of tray  450 , side walls  510 ,  511  have smooth top rims and the top of si de walls  505 ,  506  have an undercut  560  such that the top of side walls  505 ,  506  of tray  450  in this front area are always lower than the side walls  510 ,  511  of outfeed  100 , even when tray  450  is fully raised by the walking beam mechanism. This prevents tray  450  from lifting and moving racks which are fed out into front area  101  of the tray. Racks  60  are output into this front area  101  may be manually removed by the operator. If not immediately removed by the operator, the currently-outputted rack pushes and compacts the previously-outputted racks in front area  101  along the smooth rims at the top of side walls  510 ,  511  toward the operator. A sensor  595  at the front of tray detects if tray  450  is filled with racks and turns off the motor for the walking beam mechanism on outfeed until some of racks  60  are removed. There is no front wall on tray  450  to make it easier to remove racks  60  by the operator sliding one hand under several racks and simultaneously lifting those racks with the other hand. 
     If a test tube which has been returned to the outfeed  100  is needed by the operator immediately and the operator cannot wait until all nine detent positions  531 - 539  are filled before the frontmost rack is output, sample handler  20  may be instructed by the operator with software at the user interface of instrument  10  to output the frontmost rack immediately. Upon receiving this instruction, sample handler  20  cycles outfeed  100  to move racks forward toward the front of instrument  10  until the frontmost rack is output and then the walking beam mechanism is cycled backwards in the reverse direction to move. racks  60  remaining in rear area  102  of outfeed  100  one at a time back toward cross-feed  95  to their original positions. Undercut  560  on tray  450  prevents racks  60  in front area  570  from being fed backwards into the rear area  102  during this reverse movement of racks back toward cross-feed  95 . 
     As a result of moving some racks  60  with empty tube receptacles  66  out from outfeed rear area  102  to front area  101  for the operator to immediately remove a test tube from a particular rack, there may not be sufficient space in the remaining racks  60  in instrument  10  for outputting all of the test tubes in instrument  10 . To return sufficient racks  60  into sample handler  20 , the operator may insert empty racks  60  into infeed  80 . 
     Several means are provided to prevent an operator from moving racks  60  in rear area of outfeed  100  from their proper detent positions and away from the registration locations specified in the software which would result in problems with the robotic arm&#39;s placement of test tubes into precisely-positioned tube receptacles. A horizontal finger stop  502 , ie., a raised horizontal rail, extends horizontally from the bottom of output tray  450  so the operator cannot, by tilting the bottom of a rack toward the back of outfeed  100  during removal of the rack, hit racks in rear area  102 . Finger stop  502  rises high enough to block a tilted rack but low enough so that it does not block the movement of rack forward from rear area  102  to front area  570 . 
     Also preventing operator interference are pneumatically-operated clamps  310 ,  311  mounted to shafts  312 ,  313  respectively in respective clamping cylinders  314 ,  315 . Air lines supply air to open and close clamping cylinders  314 ,  315 . Whenever tray  450  is moving, and at most other times, shafts  312 ,  313  are raised above outfeed  100 . However, when software in instrument  10  determines that a rack is positioned in the frontmost detent  539  on outfeed  100  as in FIG.  3 F and tray  450  is not moving, clamp cylinders  314 ,  315  will be pneumatically operated to pull clamps  310 ,  311  down into recesses  115 ,  116  in tabs  110 ,  111  on this rack to hold it in this detent  539 . 
     As mentioned above, door panel  45  is also situated above outfeed  100 . If door panel  45  is opened by the operator while instrument  10  is operating and the operator inserts a hand above rear area  102 , an optical sensor  570 , comprising a transmitter mounted to bracket  571  to side wall  510  and receiver mounted to bracket  570  to side wall  511 , detects the intrusion and immediately stops instrument  10 , including movement of outfeed  100  and the robotic arm, to prevent the operator from being injured by a moving walking beam or robotic arm. Thus, sensor  570  operates as a “light curtain”. 
     Stat Shuttle 
     Sample handler  20  may also be provided with a stat shuttle  600  mounted parallel to and between infeed  80  and outfeed  100 . (FIGS. 1A and 1B) Test tubes and other containers, may be fed into the instrument using the stat shuttle  600  to process these containers on a priority basis, with the instrument interrupting the normal operation of processing containers input via infeed  80 . Stat shuttle  600  also enables the feeding of other types of containers, such as reagent and diluent packages, into the instrument on the stat shuttle  600 . Stat shuttle  600  may also be used to output containers from the instrument. 
     Referring to FIG. 9, stat shuttle  600  comprises a linear transport mechanism  610 , similar to the linear transport mechanism for cross-feed  95 , coupled to a microprocessor-controlled stepper motor  615 , such as motor  350 , via similar pulleys and drive belts. A platform (not shown) is connected to the linear transport mechanism  610  and an adapter  605 , as described in the referenced application entitled Stat Shuttle Adapter and Transport Device, may be mounted to the platform. One of racks  60  may be inserted into adapter  605  to transport test tubes into and out of sample handler  20 , either because one or more samples must be analyzed on a high priority or where infeed  80  is broken. Other adapters, such as container-specific adapters like the reagent package and diluent package adapters, may be inserted into adapter  605  to transport containers on stat shuttle  600 . As with cross-feed  95 , a bar code reader  623  (FIG. 1C) is placed alongside stat-shuttle  600  to read bar code labels on racks  60 , adapters, test tubes and other containers and an ultrasonic liquid level sensor  625  is positioned above the path of adapter  605  and is mounted in a bracket  635  adjacent the stat shuttle  600 . Due to space constraints, in a preferred embodiment, bar code reader  623  is not positioned directly at containers in stat shuttle  600  but instead bar code reader  623  reads the bar codes as reflected by mirror  627  positioned at a 45 degree angle between the right side and rear of sample handler  20 . 
     Containers, such as test tubes, may be inserted into stat shuttle  600  by an operator in a front area  600   a  of stat shuttle  600  and stat shuttle  600  transports the containers to a rear area  600   b  of stat shuttle  600  where a robotic arm may retrieve the containers from preferably predefined registration positions. Similarly, the robotic arm may return the containers to one of the predefined registration positions on stat shuttle  600  to output the containers. 
     Stat shuttle may also be used in a situation where reader  55  along cross-feed  95  was unable to read the machine-readable code on the test tube or other container or sensor  90  was unable to obtain usable level information from sensor  90 . In this situation, the robotic arm may transport the affected container to an awaiting rack in the rear area  600   b  of stat shuttle  600 . Stat shuttle  600  may then output the container to the front area  600   a  of stat shuttle  600  and then move the container back to rear area  600   b . The container thus has another opportunity to pass another reader  623  and sensor  625  to attempt to obtain usable data. 
     Laboratory Automation 
     Instrument  10  may be used as a subsystem in a laboratory automation system, such as the Lab Cell system from Bayer Corporation or the automated apparatus described in U.S. Pat. No. 5,623,415, which is assigned to the SmithKline Beecham Corporation. When used in this manner, test tubes are input into instrument from a transport line  700  carrying test tubes adjacent instrument, such as to the left of sample handler  20 , rather than from racks  60  in infeed  80 . (FIG.  1 B). Test tubes in the transport line are individually held in packs which are moved adjacent instrument  10  via diverter gates (not shown) and may be rotated in a specified angular position in the pack. Test tubes are removed from transport line  700  with the robotic arm and transported by robotic arm to instrument  10  for processing. 
     As with test tubes input into instrument via racks  60 , test tubes input into instrument  10  must be identified by a bar code reader  55  and an ultrasonic level sensor  90  before being processed by instrument  10 . The test tubes are therefore inserted into a lab automation adapter  710  (FIG. 8A) that is attached to a modified platform (not shown) on cross-feed  95 . Adapter  710  comprises an upper rack portion  512  that is similar to racks  60 . Upper rack portion  712  has tube receptacles  713  separated by intermediate walls  714 , each of tube receptacles  713  having a base  711 . Each tube receptacle  713  preferably also has a spring  717 , such as a leaf spring, for holding the test tube in the respective tube receptacle. 
     The adapter  710  has a cover  705 , similar to the cover on racks  60 . (FIG. 8B) The top of cover  705  is positioned at the same height as the top of one of racks  60  and the base  711  of each tube receptacle  713  is at the same distance from the top of upper rack portion  712  as the base of tube receptacles  63  when one of racks  60  is sitting on track  336 . This positions the test tubes to allow bar code reader  55  and ultrasonic liquid level sensor  90  to function properly and positions the test tubes at the proper height for retrieval and placement of the test tubes by a robotic arm on instrument  10 . Cover  705  has tabs  110 ′,  111 ′ that are used to provide the reference level for profiling of the rack with sensor  90 . For similar reasons of detection and for placing of the test tubes in the same registration positions on cross-feed  95  for retrieval by the robotic arm, there are preferably a similar number of tube receptacles  713  as there are tube receptacles  63  in racks  60  (in the illustrated embodiment, eight tube receptacles). 
     A front wall  715  of adapter  710  has openings  716  to permit bar code reader  55  to read machine identifiable code such as bar code labels on the test tubes as well as a bar code label  718  on adapter  710 . The diverter gates in transport line  700  are used to angularly position each test tube so that the robotic arm inserts test tubes in adapter  710  with the bar code labels positioned in openings  716 . 
     Upper rack portion  712  is connected to a lower rack portion  720  that may form a separate component to which upper rack portion  712  is removably mounted by any conventional means. Lower rack portion  720  has a mounting means  725 , such as the illustrated bayonet, to mount adapter  710  to a mount, such as a standard bayonet interlock mount (not shown), on the modified platform, which is preferably substantially the same platform as platform  410  plus the bayonet mount, on cross-feed  95 . Thus, unlike racks  60 , adapter  710  is snapped in firmly to connector and cannot be pulled up by the robotic arm when test tubes are removed from adapter  710 . Mount  735  is positioned between pusher fingers  94   a ,  94   b , which are not used in this mode, and lower rack portion  720  does not come into contact with or utilize the pusher fingers. The modified platform may always be used instead of platform  410  since the modification of the platform does not interfere with the operation of pusher fingers  94   a ,  94   b.    
     When adapter  710  is connected to the modified platform, adapter  710  converts cross-feed  95  to a bidirectional test tube shuttle to transport test tubes removed from transport line along cross-feed  95  in front of bar code reader  55  and under liquid level sensor  90  to the opposite side of cross-feed  95  and may be used to transport test tubes outputted by other modules of instrument  10  back to transport line  700 . 
     The modified platform also has an electrical sensor  740  to detect when the adapter  710  is connected to the modified platform so that software disables the walking beam mechanisms of infeed  80  and outfeed  100 . 
     Before outputting the test tubes back to transport line, the robotic arm may place the test tubes into a holding area  1000  (FIG. 1A) to provide the instrument with an opportunity to perform reflex testing, i.e., to test the sample again if a particular value was obtained in the first test. After the tests are complete, the robotic arm transports and reinserts the test tubes back in the transport line  700 . It is preferable to include two robotic arms on instrument  10  where instrument  10  will be used with a laboratory automation system to increase the throughput instrument  10 . 
     One skilled in the art will recognize that the present invention is not limited to the above-described preferred embodiment, which is provided for the purposes of illustration and not limitation. Modifications and variations, in particular, to dimensions of components (e.g., size of tubes and racks), the number of components within a subassembly (e.g., number of racks or tubes in a rack) and to the walking beam mechanisms, may be made to the above-described embodiment without departing from the spirit and scope of the invention.