Abstract:
A lift system for effecting a vertical parallel movement of a platform relative to a base, in particular for moving a sample changer platform of a sample changer in a weighing system, has at least three substantially identical and mutually interchangeable mechanical lift units supporting the platform. The lift units occupy an at least triangular flexible layout on the base. The lift system is equipped with a drive system that includes a transmission device and a drive element, wherein the transmission device connects the lift units among each other and to the drive element. The arrangement produces a simultaneous vertical movement of the lift units with the platform always staying aligned in a plane that is substantially orthogonal to the gravity force.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a lift system that produces a vertical parallel movement of a platform relative to a base, and it also relates to a weighing system that has a sample changer equipped with the inventive lift system that produces the vertical parallel movement of a sample changer platform. 
   Lift systems are used in different technical fields where a vertical movement, e.g. of a platform or an object relative to a base is required. 
   For example in the field of weighing technology, a lift system in conjunction with a sample changer serves to transfer one or more weighing object units or weighing containers to a balance pan. In a frequently used arrangement, the balance pan is located above or below a sample changer position of a sample changer, where the balance pan and the sample changer are vertically movable in relation to each other. 
   A piece-counting apparatus with a balance is disclosed in U.S. Pat. No. 5,883,336. A rotating sample changer has receiver openings for a container into which the pieces are placed that are to be counted. An empty container is brought by the sample changer into the filling position above the balance. By means of a pneumatic lifting mechanism, a weighing platform is moved vertically to lift the container off the sample changer—by reaching through the receiver opening of the sample changer—in order to perform the weighing. The lifting mechanism in this apparatus has a pneumatic lifting cylinder arranged centrally below the weighing platform and, in addition, requires three guiding elements arranged around the lifting cylinder, each of which consists primarily of a pin guided in a sleeve. 
   Known technical solutions are in practical use for balances, in particular for comparator balances, where a sample changer working together with the balance is equipped with a single lift unit arranged in the middle of a platform that is vertically movable in relation to the weighing pan. This device is suitable for balances that are specified for smaller loads, e.g., in a range from a few grams up to a few kilograms because, due to the small overall dimensions of the sample changer and the balance pan, the vertical travel distance between the sample changer and the balance pan for the transfer of the weighing object is likewise relatively small. It is therefore possible to use a simple mechanical device, such as for example an eccentric, as a lifter unit, although the drive torque as well as the speed of the vertical movement are not constant over the vertical lifting range of an eccentric. 
   For balances with a fine resolution of the weighing result, it is of critical importance that after the transfer of one or more weighing object units onto the weighing pan, the combined center of gravity of the weighing object units should lie on a vertical line passing through the area where the load is introduced into the weighing cell. It is therefore a requirement that the vertical movement, e.g. for seating the weighing object on the sample changer, occurs in exact parallel alignment relative to the weighing pan and furthermore in a sufficiently gentle and jolt-free manner so that the weighing object units will not shift their positions relative to each other during the transfer. 
   As a principal observation, in the case of low-capacity balances that have a sample changer equipped with a centrally arranged lift unit, the small amounts of torque occurring in the transverse direction are not significant enough to present a problem. Nevertheless, if a balance of the same type is designed for larger loads, a single lift unit arranged in the middle of a platform that is vertically movable relative to the weighing pan can prove to be a problem. When the relatively heavy platform is raised and lowered, transverse forces can occur that have an adverse effect on maintaining exact parallelism in the movement of the platform. Constraining the movement in conformance with the existing state of the art by means of several guiding elements arranged around a central lift unit requires a large amount of space and allows little flexibility. Furthermore, in the case of pneumatic as well as hydraulic lifting systems, preventive measures have to be taken against an accidental lowering of the platform from the lifted position, e.g., if there is a malfunction. 
   OBJECT OF THE INVENTION 
   It is therefore the object of the present invention to provide a lift system that is capable of sustaining the transverse torques, in particular for a relatively heavy platform, e.g., for a sample changer in a high-capacity weighing system, and that is also capable of vertically moving the platform in a jolt-free manner and maintaining parallelism. At the same time, the objective also calls for a space-saving and flexible arrangement of the lift system. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a lift system for the vertical parallel movement of a platform relative to a base is equipped with at least three mutually interchangeable mechanical lift units of the same kind carrying the platform. The lift units are arranged in an at least triangular flexible layout on the base. The lift system has a drive system with a transmission device and a drive unit, wherein the transmission device connects the lift units to each other and to the drive unit. As a result, the lift units move up and down simultaneously and, consequently, the platform is always aligned in a plane that is substantially orthogonal to the direction of the gravity force. 
   With a lift system that has a plurality of lift units acting on the platform on more than one point or one line, it is possible to prevent the occurrence of transverse forces. The parallelism of the platform movement relative to a base that carries the lift units is assured, because there is a common drive system for all of the lift units and because the lift units are of the same kind and are mutually interchangeable. It is particularly advantageous if the lift units are to a large extent identical. 
   The arrangement of the lift units on the base is flexible, the only condition being that they form at least a triangle. This allows spaces to be freed up below the platform where other assembly groups can be accommodated and arranged; for example, a space can be freed up in a weighing system to accommodate the weighing unit that includes the weighing cell. This leads to a compact configuration of the overall system. 
   In contrast to a lift system consisting of several mechanical lift units where each of the lift units has its own drive unit, the lift system according to the invention has the advantage that it does not require any measures for the synchronization of the lift units. The inventive lift system is designed to produce a simultaneous vertical movement of all lift units. The lift units to be used in the inventive lift system are preferably of a kind in which the driving force or driving torque as well as the speed of the vertical movement are substantially uniform over the entire vertical lifting range. 
   In an advantageous embodiment of the invention, the lift system is driven by an endless drive belt, in particular a spur belt, where each lift unit is equipped with a gear pulley and the spur belt is in form-fitting engagement with the gear pulley. The drive unit is preferably an electric motor. 
   In a particularly advantageous embodiment, each lift unit is configured as a lifting cylinder with a cylinder tube and a cylinder rod, where the cylinder rod is connected to the movable platform and is movable axially by means of a spindle that is guided in a threaded nut. This provides the lift system with an inherent intrinsic safety with regard to a malfunction. If the drive unit fails or the spur belt breaks, the lift units and the platform resting on them will be kept in their current position and will not drop. The spindles of all of the lift units have external threads of substantially the same pitch. 
   In view of the variance due to manufacturing tolerances in the production of mechanical components, it is a particularly significant feature of the invention that each lift unit is individually adjustable and that the individual adjustability of each lift unit remains available even after the lift unit has been installed in the lift system. 
   The lift system according to the invention is particularly well suited for use in a weighing system, especially if the weighing system is equipped with a sample changer. Thus, according to the invention, a weighing system that has a weighing unit with a weighing pan and a sample changer for transferring the weighing object from the sample changer to the weighing pan has a lift system equipped with at least three mechanical lift units that are of the same kind, mutually interchangeable, carrying a sample changer platform, and occupying an at least triangular flexible layout on a base. The lift system further includes a drive system with a transmission device and a drive unit, wherein the drive system effects a simultaneous vertical movement of the lift units. At all positions of vertical displacement, the sample changer platform is always aligned in a plane that is substantially orthogonal to the direction of gravity. 
   Since this kind of weighing system is preferably equipped with a weighing cell that works according to the principle of electromagnetic force compensation, the lift system is made substantially of non-magnetic materials. 
   In a preferred embodiment of the inventive weighing system, the sample changer has a tray that is rotatable on the platform and that is configured to be raised and lowered together with the platform. 
   The weighing system is modular, which means that the weighing unit can easily be removed from the sample changer for servicing. The weighing unit in this arrangement has an understructure that allows the weighing unit to be set either on rollers so that it can be pulled out from an opening in the sample changer or to be set on feet when the weighing unit is installed inside the sample changer. 
   A lift system according to the invention can be employed particularly in situations where a lifting mechanism for large and heavy objects is needed and where at the same time a high degree of precision is required in the control of the lifting movement. The flexibility of the inventive lift system is also of advantage where platforms have to be lifted that are for example asymmetric, because the lift units can be arranged to support the platform at the places where they are required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Details of the invention are described below based on an example from the field of weighing technology embodied in a comparator balance with a sample changer which is illustrated in drawings using a largely schematic drawing format, wherein: 
       FIG. 1  shows an overall view of the weighing system in a perspective representation, wherein the weighing system has been taken apart to show the individual assembly groups; 
       FIG. 2  shows the lift system in a three-dimensional view; 
       FIG. 3  shows the drive system in a three-dimensional view; 
       FIG. 4  shows a lift unit in a three-dimensional view; and 
       FIG. 5  shows a lengthwise section through a lift unit. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the weighing system which consists substantially of a sample changer  1  and a weighing unit  2 , shown taken apart in a three-dimensional view. Facing the viewer is the weighing unit  2 , which is accommodated in a separate housing. To the right and more towards the background of  FIG. 1  is the sample changer  1 . The latter has an opening  4  for the weighing unit  2 . To move the weighing unit into position, the weighing unit  2  has an understructure  5  which allows the weighing unit to be selectively set either on rollers  6 , e.g., to move the weighing unit out of the opening  4  for servicing, or on feet  7 . When the weighing unit  2  is installed in its operating position within the opening  4 , it is set on three feet  7  (only one being visible in the drawing) by raising the rollers. By means of the fastener elements  3 , the weighing unit  2  is attached to the sample changer  1 , so that it can no longer change its position. The weighing unit  2  is enclosed at the front by a cover panel  8 . A force-transmitting rod member  10  that connects to the weighing cell (not shown) protrudes from the top of the weighing unit  2 , set back in a recess  9  for protection from air drafts. 
   The sample changer  1  stands on three cylindrical pillars  11 , each of which is height-adjustable at its bottom end for the purpose of leveling the sample changer. A circular tray  12  that is rotatable about its center point has sample receivers  13  arranged in four positions spaced at angles of 90° from each other. The sample receiver  13  is configured as a grate, so that a weighing pan  14  with protruding ridges can reach through the grate to pick up one or more weighing object units from the sample receiver  13  that is currently in the position above the weighing pan  14 . In  FIG. 1 , the sample receiver  13  and the weighing pan  14  in the forward-facing sample changer position are shown taken apart. The ring  15  serves for additional protection of the weighing pan  14  from the influence of air drafts. Two drive units  16 ,  17 , on the one hand for a rotary movement (drive unit  17 ) of the tray  12 , and on the other hand for an up and down movement (drive unit  16 ) are connected to the sample changer  1 . 
     FIG. 2  shows the lift system  21  of a sample changer  1  in a three-dimensional representation, viewed at a slight downward angle. The pillars  11  on which the sample changer  1  is supported have a horizontal step at about three fourths of their total height, where the profile shape of a hollow cylinder changes into a post  22  with the profile of a circular arc with the same radius as the pillar  11 . The step in the pillar profile supports a base plate  23  that serves as mounting base for the lift system  21 . 
   Four lift units  24  (three of which are visible in  FIG. 2 ) are bolted onto the base plate  23 . The transmission device  25 , which connects the lift units  24  to each other and to the drive unit  16 , is likewise accommodated on the base plate  23 . The platform  26 , extending parallel to the base plate  23 , is supported by the four lift units  24  which can raise and lower the platform  26  in relation to the base plate  23 . Besides the lift units  24 , there is no other connection between the platform  26  and the base plate  23 . 
   The platform  26  has a circular passage opening  27  for the weighing pan  14  (see FIG.  1 ). The platform  26  is of a circular shape and has a step  28  along its circumference that serves to guide the rotary movement of the tray  12  (see FIG.  1 ). The tray  12  is supported by rollers that run along the step  28  as the tray turns in a circle. Furthermore,  FIG. 2  also shows the opening  4  for the weighing unit  2 . 
     FIG. 3  represents a three-dimensional illustration of the base plate  23  with the platform  26  taken off, viewed at a slightly downward-directed angle. As shown in this drawing, the transmission device  25  for the lift system  21  is mounted on the base plate  23 . In the interest of clarity, the rotation mechanism, which is likewise mounted on the base plate  23  and is driven by the drive unit  17 , has been omitted in the drawing. The drive unit  16 , preferably an electric motor, serves to drive the lift system  21 . The four lift units  24  are connected to each other and to the drive unit  16  by an endless spur belt  18 . The spur belt  18  is trained over six guide pulleys  20  that are installed on the base plate  23  by means of pulley mounts  19 , so that the spur belt  18  runs without crossings in a closed loop parallel to the surface of the base plate  23 , connecting the drive unit  16  and all four lift units  24 . This assures that all lift units  24  are driven simultaneously. The concept of driving the lift units simultaneously and the substantially identical design of the lift units  24  among each other are prerequisites for maintaining parallelism in the raising and lowering of the platform  26  (not shown in  FIG. 3 ) in relation to the base plate  23  by means of the lift units  24 . 
   The pulley mounts  19  for the guide pulleys  20  are bolted onto the base plate  23 . By slightly loosening the screw  50  of any of the pulley mounts  19 , the respective pulley mount with its pulley  20  can be swiveled slightly about the axis of the bolt, whereby the spur belt is loosened or tightened. The drive unit  16 , preferably an electric motor, can reverse its sense of rotation for the up and down movement of the lift units  24 . 
   The base plate  23  and the platform  26  are aligned in a plane that extends orthogonal to the direction of the gravity force, so that a precise transfer of one or more weighing object units can be performed from the sample receiver  13  to the weighing pan  14  (see FIG.  1 ). 
     FIG. 4  shows a lift unit  24  in a perspective representation. It has a mounting socket  29  standing on four stilts  30 , each of which has a screw hole  31  to hold a screw for fastening the mounting socket  29  to the base plate  23 . Between the stilts below the mounting socket  29 , there is a gear pulley  32  designed for a form-fitting engagement with the spur belt  18 . The gear pulley  32  has a groove  33  around its circumference to receive a precisely fitting ridge of the spur belt  18 . 
   Connected to the top of the mounting socket  29 , the cylinder tube  34  extends vertically upward. A cylinder rod  35 , shown protruding from the cylinder tube in  FIG. 4 , is guided inside the cylinder tube and constitutes the vertically movable part of the lift unit  24 . The cylinder rod  35  has a narrower section at its upper end forming a horizontal ledge  36 . The ledge  36  supports the platform  26  (not shown here), which can be clamped between the ledge  36  and a clamping disk  37  that can be screwed tightly onto the lift unit  24 . All four lift units  24  are held in this manner between the base plate  23  and the platform  26 . It is important for the lift units  24  to be in exact vertical alignment on the base plate  23 , so that the platform  26  can be aligned parallel to the base plate  23  and orthogonal to the direction of the gravity force. 
     FIG. 5  shows a lengthwise section through a lift unit  24 . The drawing also illustrates at the same time how a lift unit  24  is installed as the connecting element between the base plate  23  and the platform  26 . Each lift unit  24  has at its lower end an axial bearing  38  in which a spindle  39  is rotatably supported. The axial bearing  38 , which serves to take up axial forces, is accommodated in an appropriately configured recess  52  of the base plate  23 . The recess  52  has a further set-back portion  53  at the center, so that the rotating spindle  39  has no contact with the base plate  23 . 
   The gear pulley  32 , which is surrounded as well as covered at the top by the mounting socket  23 , is fixed on the spindle  39  by two set screws (not shown in drawing) for which the tapped holes  40  are provided. The gear pulley  32  has a groove  33  around its circumference to receive the spur belt  18  in a form-locking engagement. The cylinder tube  34  is inserted in the mounting socket  29  from above and is seated on the top surface of the mounting socket  29  by means of a step  41 . The cylinder tube  34  forms the housing for the lift unit  24  and also serves as a guide for the cylinder rod  35  that moves inside the cylinder tube  34 . The bottom end of the cylinder tube  34  holds a radial bearing  42  that absorbs the radially directed forces exerted on the spindle  39  by the spur belt. The section  43  of the spindle  39  that extends above the radial bearing  42  has an external thread. As a counterpart to the external thread, a threaded nut  44  is fixedly installed in the cylinder rod  35  which runs coaxially inside the cylinder tube  34 , where the threaded nut  44  is interposed between the cylinder rod  35  and the spindle  39 . The external thread of the spindle  39  runs in the internal thread of the threaded nut  44  whereby the nut, and thus the cylinder rod  35 , is moved up and down relative to the cylinder tube  34 . An upper polymer bearing  46  and a lower polymer bearing  47  are interposed between the cylinder rod  35  and the cylinder tube  34  to guide the movement of the cylinder rod  35 . 
   The cylinder rod  35  contains a hollow space  45  to receive the spindle  39  as the lift unit  24  moves downward. At the top, the cylinder rod  35  protrudes from the cylinder tube, and the upper end of the cylinder rod  35  has a stepped-down section  48  forming a horizontal ledge  36 . The stepped-down section  48  of the cylinder rod  35  is inserted in the platform  26  which rests on the ledge  36 , with a rubber shim  49  inserted for damping. The platform  26  is attached to the lift unit  24  by inserting a clamping disk  37  in the recess  54  of the platform  26  and fastening the clamping disk  37  to the cylinder rod  35  with a screw from the top. As a damping measure, it is recommended to also insert a rubber shim  51  between the platform  26  and the clamping disk  37 . 
   As the spur belt  18  connects all of the lift units  24  to each other and to the drive unit  16 , all four lift units  24  are moved together at the same time. With the lift units  24  being substantially identical, in any event at least of the same type and mutually interchangeable, the arrangement performs a synchronous vertical movement of the cylinder rods  35  of all four lift units  24 , whereby the platform  26  remains parallel to the base plate  23 . The movement is jolt-free, and the transfer of the weighing object units from the sample receiver  13  of the sample changer  1  to the weighing pan is gentle enough so that the weighing object units will not change their positions relative to each other. 
   Differences between the lift units  24  that are due to manufacturing tolerances, e.g., variations between the pitches of the spindles  39  and possibly other components of the lift system, can be compensated by a height adjustment. To allow this adjustment, the cylinder rod has a hexagonal hole  55  above the tapped hole  56  that serves to fasten the clamping disk  37 . By inserting a matching key into the hexagonal hole  55  and turning the cylinder rod up or down, the lift system can be level-adjusted even in the installed condition of a lift unit  24  and with the spur belt  18  pulled tight. 
   The mechanical lift units  24  that are driven by a spindle  39  running in a threaded nut  44  put a substantially constant torque load on the drive unit and produce a lift movement of substantially uniform speed over the entire vertical lifting range. Furthermore, the intrinsic safety is inherently assured for this type of lift drive. This means that in case of a power failure of the drive unit  17  or if the spur belt  18  breaks, the lift units  24 , and thus the platform  26 , are held at their current positions, and an abrupt fall of the platform is not possible.