CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/429,438, filed May 8, 2006, now U.S. Pat. No. 7,832,921, which claims the priority of Swiss patent application CH 00813/05, filed May 9, 2005, the contents of each of which are incorporated herein in their entireties 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a storage device for laboratory samples comprising at least one storage rack and a shaker drive. The invention also relates to climate controlled cabinets having such a storage device. 
     In automated essays, e.g. in microbiology, samples are stored in so-called micro titer plates having a large number of small wells. Because of the large number of probes to be investigated, a large number of plates are used in a test run. Typically, the plates are stored in tower-shaped storage racks. Each storage rack is adapted to receive a plurality of micro titer plates (or similar laboratory samples) on top of each other. For certain applications, it is necessary to shake the samples during certain time periods. 
     EP 1443 101 describes a storage device in a climate controlled cabinet that comprises a shaker drive, which shakes a storage rack carrier. A number of storage racks, e.g. two, are mounted to the rack carrier. Since the storage racks are comparatively high, undesired oscillatory movements of the upper ends of the storage racks can occur during shaking. This limits the shaking frequency and amplitude, makes it necessary to use a very sturdy setup, and leads to undesired noise and vibrations as well as to a shaking amplitude that is different for different storage locations. 
     BRIEF SUMMARY OF THE INVENTION 
     Hence, it is a general object of the invention to provide a storage device or a climate, controlled cabinet that allows an efficient shaking of even high storage racks. 
     Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the storage rack, in a first aspect of the invention, is manifested by the features that it comprises 
     a shaker drive, 
     a rack carrier being driven by said shaker drive for performing a shaking motion with horizontal component, 
     at least one storage rack arranged on the rack carrier, which storage rack comprises storage locations for a plurality of laboratory samples on top of each other, and 
     a counterweight assembly connected to an upper end section of the storage rack and being driven to exert a force opposite to the shaking motion in order to counteract an oscillation of the upper end section of the storage rack. 
     Hence, the counterweight assembly is connected to the storage rack(s) at an upper end section thereof and is driven such that it exerts an oscillatory force opposite to the shaking motion to counteract the undesired oscillations. In this manner, undesired excessive motion of the upper end of the storage rack(s) is reduced. 
     In an advantageous embodiment the counterweight assembly comprises a moving part that is driven opposite to the shaking motion of the storage racks and can thus generate the desired force. 
     In a second aspect of the invention, the storage device comprises 
     a shaker drive, 
     a rack carrier driven by said shaker drive for performing a shaking motion, and 
     at least one storage rack arranged on the rack carrier, said storage rack comprising a plurality of storage locations for laboratory samples on top of each other, 
     wherein said shaker drive is adapted to shake the storage carrier in a vertical direction. 
     Hence, the object is achieved by a vertical shaking motion. This prevents a lateral oscillation of the storage rack and leads to highly homogeneous acceleration forces within the whole storage rack. A further advantage of this arrangement is the fact that an improved mixing within the small fluid volumes is achieved because of the vertical shaking motion. 
     Advantageously, the rack carrier can be moved upwards and downwards by the shaker drive without being tilted such that the acceleration forces act in vertical direction only. 
     In an advantageous embodiment the shaker drive comprises a rotating body driven for rotation. The rotating body has a non-horizontal end face. At least one follower member abuts against the end face and performs an up and down motion (in respect to the rotating body) while the rotating body rotates. In this matter a rotational motion can be efficiently transformed into a vertical up and down movement. 
     In order to achieve a homogenous vertical shaking motion, several rotating bodies and follower members can be provided, wherein the rotating bodies are driven for rotation by a common belt. 
     The invention also relates to a climate controlled cabinet with such a storage device, where the storage racks can be stored under controlled temperature and, where necessary, under controlled gas composition, in particular under controlled humidity. 
     In all aspects of the invention large acceleration forces can be achieved such that even small amounts of fluid can be mixed efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein: 
         FIG. 1  is a climate controlled cabinet with a shaker drive for vertical shaking, 
         FIG. 2  is a detail of the shaker drive of  FIG. 1 , 
         FIG. 3  is a second embodiment of a climate controlled cabinet with a shaker drive for vertical shaking, 
         FIG. 4  is a third embodiment of the invention, 
         FIG. 5  is the embodiment of  FIG. 4  seen from a somewhat different angle, 
         FIG. 6  is the embodiment of  FIG. 4  seen from a further angle, with an additional protecting plate, 
         FIG. 7  is a top view of the base plate of the embodiment of  FIG. 4  with the belt pulleys, wherein the course of the belt and the arrangement of the storage racks are shown in dashed lines, 
         FIG. 8  shows the upper strut of the counterweight assembly, 
         FIG. 9  shows a holder rail for a storage rack, and 
         FIG. 10  shows a variant of the embodiment of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The climate controlled cabinet of  FIG. 1  comprises an interior space  2  holding a storage device  3  with shaker drive  4 . The climate controlled cabinet comprises means to generate a controlled climate in its interior space  2 . It can be equipped for manual and/or automatic access. The climate controlled cabinet of  FIG. 1  e.g. corresponds to the device shown in U.S. Pat. No. 6,478,524 and has to storage racks  5  arranged in a “V”-shaped configuration. The shaker drive of  FIG. 1  is, however, also suited for storage devices with only one or substantially more storage racks. It can also be used for storage devices that are not arranged in a climate controlled cabinet. The same is true for the shaker drives  4  and storage devices  3  described further below. 
     Each storage rack  5  comprises two vertical side walls  6 , each having a plurality of ledges  7 . The ledges  7  form a corresponding number of storage locations arranged above each other for receiving the laboratory samples. In a particularly important application, the laboratory samples are micro titer plates. These have a plurality of small wells for receiving fluids. Micro titer plates have, as a rule, a standard size with a footprint of 127.76 mm×85.48 mm (each +/−0.5 mm) according to standard ANSI/SBS 1-2004, Jan. 8, 2004, “for microplates—footprint dimensions”. The storage locations are adapted to these dimensions. 
     The storage racks  5  are mounted to a common rack carrier  8  and are affixed thereto in suitable, releasable manner. For this purpose, rack carrier  8  provides an horizontal mounting surface with holder rails as described below. 
     Rack carrier  8  can be moved vertically and is fixedly held in horizontal direction. In the embodiment of  FIG. 1 , this is achieved by means of two plate springs  9  arranged between rack carrier  8  and a stationary base body  10 . The plate springs are arranged horizontally such that they allow a vertical motion of the rack carrier  8  but prevent it from moving horizontally. For this purpose, mounting blocks  11  are arranged to both, base body  10  and rack carrier  8 , each pair of mounting blocks  11  holding one plate spring in horizontal orientation. 
     To generate the shaking motion, four bearings having rotating bodies with inclined end faces are provided. Four follower members  13  are mounted to the rack carrier, each of which rests against a non-horizontal end-face  15  of a rotating body  14  of the shaker drive. 
     As described in the following and as can in particular be seen in  FIG. 2 , each rotating body  14  is driven for rotation about a vertical rotation axis  16 . Each end face  15  is flat and inclined in respect to, rotation axis  16  of its respective rotating body  14 , the respective follower member  13  touches the end face  15  at a distance from the rotation axis. 
     If rotating body  14  is rotated about rotation axis  16 , the respective follower member  13  carries out an up and down movement in respect to rotating body  14 . The plate springs ensure that the follower members  13  do not lift off the end surfaces  15  even at high shaking frequencies. 
     As shown in  FIG. 1 , the rotating bodies  14  are mounted to base body  10 . Each rotating body  14  is connected to a driving gear  18 . A gear belt  19  is led around all driving gears  18  and driven by a motor  20 . This ensures that the rotating bodies are driven in synchronicity such that the follower members  13  move synchronously. 
     As can in particular be seen in  FIG. 2 , each follower member  13  of the present invention is a roller running on end face  15  and touching the same at only a distance from rotating axis  16 . By using rollers, frictional losses can be small. It is, however, also possible to use low friction gliders as follower members instead of rollers. 
     Instead of a flat, inclined end surface  15 , a corrugated or bent end surface  15  can be used. It is also possible to mount the follower members  13  to base plate  10  and the rotating bodies  14  (with the shaker motor  20 ) to rack carrier  8 . 
     Instead of the shaker drive shown in  FIGS. 1 and 2 , another type of shaker drive can be used by means of which rack carrier  8  can be moved up and down. Advantageously, a purely vertical movement is generated in order to fully avoid the problematic, lateral displacement of the storage racks  5 . 
     For example, a direct, purely electromagnetic vibrational drive can be used. 
     Another variant of a shaker drive is shown in  FIG. 3 . 
     This figure again shows a storage device  3  in a climate controlled cabinet  1 . In this embodiment, however, storage device  3  comprises more than two storage racks  5 , which are arranged rotatably about a vertical axis on a carousel. Such an assembly is e.g. described in WO 02/059251. The carousel in  FIG. 3  is formed by rack carrier  8 . 
     In this embodiment the shaker drive  4  is designed not, only to shake rack carrier  8  vertically, but also to rotate it about its vertical central axis  22 , i.e. both movements are generated by the same motor  20 . 
     Also in this embodiment, the storage racks rest on rack carrier  8 , which can be moved vertically but not horizontally. In addition to this, rack carrier  8  can be rotated about its vertical central axis. For this purpose, it is held in a rotational bearing, which can e.g. comprise a cylindrical bushing  70  located at central axis  22 . 
     Rack carrier  8  rests on three rollers  23 , which are arranged symmetrically about central axis  22  of the carousel. Each roller is held eccentrically on a horizontal shaft  24 , wherein the horizontal shaft extends radially in respect to central axis  22 . Each shaft  24  is mounted to stationary base body  10  and holds a first, concentric bevel gear  26 , which forms an angled gearing together with a second bevel gear  27 . The second bevel gears  27  are synchronously driven by means of a common gear belt  19  and a motor  20 . 
     When gear belt  19  is set in motion, rollers  23  start to rotate eccentrically about the roller shafts  24  and convey a substantially constant rotational movement and, at the same time, an alternating vertical up and down movement to rack carrier  8  such that the storage racks  5  are shaken. 
     In a variant of this embodiments, only one of the rollers  23  is driven. 
     By means of the embodiment of  FIG. 3 , it becomes possible to rotate and shake the carousel of the storage racks  5  with a single motor only. 
     It is, however, also possible to combine a different type of shaker drive, such as a shaker drive with bearings as shown in  FIG. 1 , with a carousel drive. In this case, the device has a rotational drive for rotating the storage racks about vertical axis  22  and a shaker drive for generating an up and down motion of the storage racks. This is, constructively, somewhat more complicated, but it allows to choose the shaking frequency independently from the rotational speed. 
     In order to obtain sufficient mixing with standard micro titer wells, the shaking motion of the embodiments shown so far should have an amplitude between 0.1 and 10 mm. 
     As mentioned initially, the present invention also relates to a storage device where the storage racks are shaken horizontally. A corresponding embodiment is shown in  FIGS. 4 to 7 . 
     This is also a storage device  3  with two storage racks  5  arranged with “V”-shaped footprint, which can e.g. be located in a climate controlled cabinet  1  (as indicated in  FIG. 4 ). However, this storage device can also be used without a climate controlled cabinet. In addition, the number of the storage racks  5  can be adapted to the respective needs, and the concept can also be used in carousel shaped geometries of the storage racks, but also for a single storage rack only. 
     The two storage racks  5  are, as in  FIG. 1 , arranged on a rack carrier  8 , which is set into motion by a shaker drive  4 . 
     In contrast to the embodiment of  FIG. 1 , the shaker drive of  FIGS. 4-7  generates a horizontal shaking motion, or at least a shaking motion having a horizontal component of movement. For this purpose, a motor  20  drives, directly or via a gear belt  19  shown in dashed lines in  FIGS. 6 and 7 , three gears  30 ,  31 ,  32 . Motor  20  and gears  30 ,  31 ,  32  are arranged on stationary base body  10 . 
     Pins  33  extending upwards are eccentrically mounted to the two outer gears  30 ,  31  and engage corresponding openings of rack carrier  8  to convey their eccentric motion thereto. When gears  30 ,  31  rotate, an eccentric horizontal shaking motion of rack carrier  8  is generated. 
     Rack carrier  8  is held by the pins  33  as well as two frictional bearings  34 , namely in such a manner that it can move horizontally but cannot tilt. An additional vertical shaking motion is not achieved with the embodiment of  FIGS. 5-7 , but could be generated by other shaker drives. 
     As can be seen from  FIG. 7 , an arm  36  is mounted to one gear, the position of which can be detected by a sensor. This allows to determine the absolute position of rack carrier  8 , such that it can be brought into a defined position (e.g. for being accessed by an automated robot system). 
     To keep the lateral oscillations of the storage racks small, in particular in the upper end sections thereof, the embodiment of  FIGS. 4-7  comprises a counterweight assembly  40  between the two storage racks  5 . It comprises a moving part, which is located between a lower support  41  and an upper support  42 . Lower support  41  is located at the height of rack carrier  8  and extends, in order to achieve a stable and compact design, through an opening or recess  48  of the same. It is mounted to base body  10 . Upper support  42  is located at the height of upper end section  50  of the storage racks and is mounted to a strut  49 , which in its turn is rigidly but releasably connected to the top end of both storage racks  5 . 
       FIG. 8  shows strut  49  from below. Two pins  58  can be seen, which releasably engage the storage racks  5  as well as upper support  42 . 
     Each support  41 ,  42  has a rotating table  43 ,  44 , which is rotatable about a vertical counterweight axis  45 . Lower rotating table  43  is driven by means of gear  32  and gear belt  19 , namely with the same rotational speed as and in synchronicity with the gears  30 ,  31 . Upper rotating table  44  is not actively driven but can rotate freely about counterweight axis  45 . 
     A rod-shaped counterweight carrier  47  extends vertically between the two rotating tables  43 ,  44 , approximately from the height of rack carrier  8  to the height of upper end section  50  of the storage racks  5 . It is arranged eccentrically in respect to counterweight axis  45 . A counterweight  52  is mounted to counterweight carrier  47 , namely at a height between rack carrier  8  and upper end section  50 . Advantageously, counterweight  52  can be displaced&#39;along counterweight carrier  47  and can be locked in position at the desired height. 
     As mentioned, the gears  30 ,  31  and  32  are driven synchronously by means of gear belt  19  and have the same speed of rotation (i.e. rate of rotation). Hence, a fixed phase relation exists between the position of the eccentric pins  33  and the one of lower rotating table  43 . This phase relation is advantageously chosen such that there is no mutual phase difference between the positions of the pins, but that there is a 180° phase difference between the positions of the pins and the one of lower rotating table  43  (i.e. of counterweight carrier  47  mounted thereto). When motor  20  is set into motion, counterweight carrier  47  is moving oppositely to rack carrier  8 , such that it counteracts the oscillations caused by shaking to storage racks. 
     It is advantageous if the mass of the moving part of counterweight assembly  40  multiplied with the amplitude of motion of the moving part is substantially equal to the sum of the masses of all storage racks  5  and the rack carrier  8  multiplied with the amplitude of motion of rack carrier  8 . As can in particular be seen in  FIG. 7 , the shown embodiment uses a much larger amplitude of motion of the moving part of counterweight assembly  40  than the amplitude of motion of the pins  33  and therefore of rack carrier  8 , for which reason the mass of the moving part of counterweight assembly  40  can be comparatively small. 
     Counterweight assembly  40  can be covered by a protective plate  55 , as it is e.g. shown in  FIG. 6 . 
     To save space, counterweight assembly  40  of the shown embodiment is arranged between the two storage racks  5 . In a star-shaped or carousel-shaped arrangement of the type of  FIG. 3 , counterweight assembly  40  can be located in the center of the carousel. 
     In view of the shaking motion of all the embodiments shown here, the storage racks  5  should be well affixed to rack carrier  8 . For this purpose, a holder rail  59  is arranged for each storage rack  5  on rack carrier  8 , as shown in  FIG. 4  and in particular in  FIG. 9 . Each holder rail  59  is received in a recess  60  at the foot of each storage rack  5  and is held in centered manner by means of horizontal grub screws  61  (see  FIG. 4 ), by means of which the storage racks  5  can be clamped to the guide rails  59 . The holder rails  59 —as well as the corresponding recesses  60  at the feet of the storage racks  5 —comprise an inclined face  62  at a site opposite to the grub screws  61  (see  FIG. 9 ) for centering and fixing the storage racks  5  on rack carrier  8 . 
     It the embodiments shown so far all storage racks are shaken. It is, however, also possible to shake only one set of the storage racks while leaving another set at rest, even when the shaker drive is activated. A corresponding arrangement is illustrated in  FIG. 10 . It shows a device which is basically built in the same manner as the one of  FIG. 7 , but which comprises a third additional storage rack  5 ′. The latter does not rest on the (shaking) rack carrier  8 , but on the stationary base body  10 . Hence, the third storage rack  5 ′ remains stationary even when shaker drive  4  is active. 
     It is also possible to provide two rack carriers which can be shaken selectively, either by means of two separate shaker drives of by means of an selectable transmission between the two rack carriers to a common shaker drive. 
     While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.
The invention provides a storage device with storage racks and a shaker where the undesired horizontal oscillations of the upper ends of the storage racks are small. The storage device for laboratory samples includes a shaker drive, a rack carrier being driven by said shaker drive for performing a shaking motion with horizontal component, at least one storage rack arranged on the rack carrier, which storage rack comprises vertical side walls, each side wall having a plurality of ledges arranged vertically above each other and forming storage locations for a plurality of laboratory samples on top of each other, and a counterweight assembly connected to an upper end section of the storage rack and being driven to exert a force opposite to the shaking motion in order to counteract an oscillation of the upper end section of the storage rack.