Abstract:
A support device for a weighing cell is mounted on one or more support posts resting on support springs that are connected to a stationary chassis and are pretensioned with an upward force. In case of a downward-directed shock, the support springs will yield to the inertial shock force and allow the weighing cell to move downward. The support posts can lift off from their resting places on the support springs, but are constrained by guide members to a limited range of upward vertical movement. A balance equipped with the support device has a low profile height and protects the weighing cell from downward- as well as upward-directed shocks.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention belongs to the field of shock protection devices for sensitive instruments. More specifically, it relates to a certain type of support device for a weighing cell and also to a balance with a stationary housing that incorporates the support device. In known support devices of the kind that the invention aims to improve, the weighing cell has one or more support posts by which the weighing cell is seated on a spring that is anchored at a stationary base or chassis portion, so that the one or more support posts of the weighing cell are movable in relation to the stationary chassis in a substantially vertical direction against the opposing force of the spring.  
           [0002]    Measuring instruments that contain precision mechanisms can suffer damage if they are exposed to shocks in the course of being relocated or shipped, e.g., from being bumped, set down roughly, or even dropped. The risk of shock damage is particularly critical in high-resolution laboratory balances that incorporate levers, flexure pivots, parallelogram linkages and other delicate mechanical elements. Special shock protection measures have been attempted, so as to intercept excessive loads and thereby prevent damage to the sensitive parts of the apparatus. In the case of the aforementioned laboratory balances, the part that is most susceptible to damage is the weighing cell, including the force-introduction mechanism, particularly in regard to shocks in the downward vertical direction of the apparatus, i.e., in the same direction in which the device performs its actual weighing function. The most frequent problem with vertical shocks occurs when the apparatus is set down roughly after a change of place or after it has been lifted to clean the area underneath. In the field of balances, it is therefore a high-priority measure to keep downward-directed vertical shock loads from reaching the weighing cell. The problem is equally critical with shock forces acting in the opposite direction, for example if the balance is transported upside down or if it is turned on its head for cleaning.  
           [0003]    One possibility of how shocks against the weighing cell can be softened is disclosed in CH-A-680877. This reference document describes a balance utilizing a printed circuit board, i.e., a component of the electronic circuitry contained in the balance, as a resilient support for the weighing cell. This design concept is conducive to a compact architecture of the instrument and provides a practical cushion against shock loads. However, in spite of the inherent damping properties of the printed circuit board, the system is susceptible to oscillations. For example, building vibrations can cause oscillations of the weighing cell. To correct this problem in high-resolution balances requires a time-consuming filtering process, and/or it is possible that the displayed weighing result will remain unstable.  
         OBJECT OF THE INVENTION  
         [0004]    The present invention therefore has the object of providing a shock-absorbing means for a weighing cell that is effective against downward- as well as upward-directed vertical shocks and overcomes the aforementioned drawbacks while still allowing a compact, low-profile configuration of the apparatus.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the present invention, the foregoing objective can be realized in a support device which, as described at the beginning, has one or more support posts that are connected to and support the weighing cell, where each support post rests on a support spring that is anchored at a stationary chassis portion, with the support post being movable in relation to the stationary chassis in a substantially vertical direction against the opposing force of the support spring. In addition, the support device of the present invention has the distinguishing features that each of the support springs on which the one or more support posts are resting is biased with an upward pre-tensioning force against a surface portion of the stationary base or chassis, and that the support post can lift off from its resting place on the support spring, but is constrained to a limited range of upward vertical movement by a guide member that is attached to the stationary chassis. Also included within the scope of the invention is a balance with a stationary housing that surrounds the weighing cell, where a bottom or floor portion of the housing is designed to accommodate the inventive support device and, more specifically, where the bottom or floor of the housing constitutes the stationary chassis on which the spring is anchored.  
           [0006]    The inventive concept where the weighing cell is supported by one or more support posts resting on a support spring that is upward-biased with a pre-tensioning force provides a quasi-rigid support base for the weighing cell as long as the downward force transmitted by the support post(s) does not exceed the pre-tensioning force of the support spring. When the pre-tensioning force is exceeded, i.e., when a downward-directed shock is resiliently absorbed, the weighing cell deflects approximately in a mode of parallel motion, because of the guide member(s) constraining the support post(s). In other words, the weighing cell will not tip into a slanted position and, therefore, very little lateral clearance is required, which is again conducive to a compact design. As a further advantage, after the peak forces have subsided, the weighing cell will return to its exact previous position. In addition, the weighing cell can also lift off from its rest position on the support spring, so that an upward-directed vertical shock can likewise be absorbed.  
           [0007]    In an advantageous development of the invention, the guide members are designed as guide springs with the capability of resiliently absorbing upward-directed vertical shocks, without the need for additional parts nor an increase in the profile height of the apparatus.  
           [0008]    A particularly simple and at the same time space-saving configuration is achieved by designing the one or more support springs as leaf springs. Each of the support posts that support the weighing cell can be seated on a leaf spring in a straightforward manner, in particular with a design where the support post has two sections of different thickness, whereby a shoulder is formed by which the post rests on the support spring, with the thinner section of the bolt protruding through the support spring and the bottom end of the post being fastened to the guide member. In a preferred embodiment of the invention, each support spring is arranged to work together with a guide spring as a pair, in which the two springs run parallel to each other, with the advantageous result of a very low profile height.  
           [0009]    By designing the guide members in the form of leaf springs, it is possible to combine multiple guide members that are used together in one support device into an integral die-punched unit. With appropriately shaped bends in the leaf springs, and with a suitable choice of the leaf dimensions, thickness and width in particular, it is possible to match the spring tension to given design constraints, primarily to the mass of the weighing cell and associated parts, and to performance requirements, in particular the magnitude of the shocks that the springs must be capable of absorbing. The design requirements for the support springs are analogous to the guide springs.  
           [0010]    In a balance, the stationary base or chassis to which the support device is attached, is constituted by the stationary balance housing that encloses the weighing cell. In balances with a high measurement resolution, the space inside the housing is often subdivided into compartments in order to shield the weighing cell from being influenced by factors originating from other parts of the balance. Consequently, it is advantageous, for example, to use a design where the volume of the space surrounding the cell is minimized. In a preferred embodiment of a balance with the inventive support device, a minimum-volume configuration is achieved by a split-level design of the floor of the stationary housing, i.e., different parts of the floor being located at different levels. The purpose is to create a hollow space under a raised floor area below the weighing cell. This hollow space, which can be closed by a cover plate, can accommodate the support springs and the guide members in addition to other components of the balance. The raised floor area has openings through which the posts of the support device protrude into the hollow space. The support springs as well as the guide members are preferably of a leaf-spring design and are accommodated inside the hollow space, attached to the stationary housing floor from below.  
           [0011]    In another embodiment, the stationary floor of the balance housing runs in a level plane, where the support springs and guide members are arranged above the floor. In this case it is advantageous to provide the floor with closable openings in the areas where the guide members are attached to the support posts in order to provide access for assembly and disassembly.  
           [0012]    The invention is described below with reference to the drawing figures that illustrate preferred embodiments where the invention is incorporated in a micro-analytical balance and a top-loading precision balance. It must be understood, however, that the invention is not restricted to the specific design configurations described and illustrated herein, but that variations and adaptations that present themselves from the disclosure in its entirety are included within the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    In the attached drawing:  
         [0014]    [0014]FIG. 1 represents a schematically simplified cross-sectional view of a micro-analytical balance;  
         [0015]    [0015]FIG. 2 represents an enlarged detail of FIG. 1;  
         [0016]    [0016]FIG. 3 represents the same detail as FIG. 2, with the support device in a downward-deflected condition;  
         [0017]    [0017]FIG. 4 represents the same detail as FIGS. 2 and 3, with the support device in an upward-deflected condition;  
         [0018]    [0018]FIG. 5 represents a die-punched unit with three support springs in the form of leaf springs;  
         [0019]    [0019]FIG. 6 represents a die-punched unit with three guide members in the form of guide springs; and  
         [0020]    [0020]FIG. 7 represents a schematically simplified cross-sectional view of a top-loading precision balance.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 gives a schematic cross-sectional view of a micro-analytical balance  1  with a stationary housing  2 . The housing  2  encloses a space  7  containing a weighing cell  3  with a load receiver arm  4 . The latter protrudes through an opening into a weighing compartment  6 , where a weighing pan  5  is supported on the load receiver arm. The stationary housing  2  has feet  22  resting on a stationary work surface, such as a weighing table (not shown). In the area below the weighing cell  3 , the floor  21  of the stationary housing  2  is reinforced as well as raised to a higher level in comparison to other floor areas such as, e.g., below the weighing compartment  6 . Details of the weighing cell  3 , such as the weighing mechanism and magnet system, have no bearing on the present invention but can be found in other sources such as, e.g., U.S. Pat. Nos. 3,786,884 and 4,489,800. Attached to the weighing cell  3  are support posts  31  which, in this embodiment, are constituted by cylindrical bolts extending downwards through openings  23  in the housing floor. Each of the bolts rests on a support spring  11 , and the bottom end of each bolt is connected to a guide member  12 . The illustrated embodiment has three support posts  31 . One of the posts is located in front of the cross-sectional plane of FIG. 1 and therefore not visible. The second post is shown in cross-section, while the third lies to the rear of the cross-sectional plane and is shown in frontal view. By raising the housing floor  21  in the area underneath the weighing cell  3 , a hollow space  24  is created, which is closed off by a cover plate  25 .  
         [0022]    [0022]FIG. 2 shows the second of the posts, i.e., the post shown in cross-section in FIG. 1, and its support spring and guide spring in an enlarged and more detailed view. The drawing shows clearly how the bolt-shaped post  31  protrudes downward from the weighing cell  3  and passes with lateral clearance through an opening  23  in the housing floor  21 . The post has two sections, i.e., an upper section  31   a  of larger diameter and a lower section  31   b  of smaller diameter. A shoulder  31   c  at the transition between the two sections serves as the seat surface of the post  31  on the pre-tensioned support spring  11 . The thinner section  31   b  of the post  31  protrudes downward through a hole  13  of the support spring  11 , and the bottom end of the thinner section  31   b  is fastened to the guide spring  12  by means of a screw  36 . Immediately next to the clearance hole  23 , the housing floor  21  has a downward-protruding portion in the shape of a ridge that serves as a resting pad  27  against which the support spring  11  is pre-tensioned. As is self-evident, there are also other ways of designing a feature or device for the support spring to rest against. The opposite end of the support spring  11  is rigidly attached to the housing floor  21 , as is the stationary end of the guide spring  12 . In the preferred embodiment illustrated in FIG. 2, the support spring and the guide spring are configured as a pair of parallel leaf springs that are both attached to the same location  21   a  of the housing floor  21 . At the location  21   a , the housing floor has a downward-protruding mounting pad  28  that is substantially flush with the resting pad  27 . Thus, the support spring  11  in its pre-tensioned rest position will be substantially horizontal and parallel to the housing floor. A bushing  15  of a height that corresponds to the post section  31   b  reduced by the thickness of support spring  11  serves as a spacer for the guide spring  12 . In the illustrated embodiment, the support spring  11  and guide spring  12  with the interposed spacer bushing  15  are attached to the mounting pad  28  by means of a screw  14 . Consequently, the guide spring  12 , too, will run substantially horizontal and parallel to the housing floor  21 .  
         [0023]    The guide spring  12  is likewise pre-tensioned, but in contrast to the support spring  11 , the guide spring  12  is downward-biased. Thus, the guide spring  12  exerts a downward pull on the post  31  and thereby adds to the downward force of the post  31  against the support spring  11 , i.e., to the combined weight of the weighing cell  3  together with its associated dead-weight components and the weighing load on the weighing pan. The pre-tension of the support spring  11  has to be at least sufficient so that the support spring  11  will not separate itself from the resting pad  27  under the force exerted by the post  31  against the support spring  11 .  
         [0024]    [0024]FIG. 3 illustrates the same portion of the support device as FIG. 2 (in a slightly different cross-sectional plane), where the support device is in the process of resiliently absorbing a downward shock force which could occur, e.g., if the balance is set down in an excessively abrupt manner. Yielding to the added inertial forces due to the sudden deceleration, the support spring  11  will deflect downward and thereby separate itself from the resting pad  27 . The guide spring  12  has the effect of constraining the post  31  so that the vertical axis of the post remains parallel to itself during the downward deflection regardless of the magnitude of lateral or asymmetric forces that may act on the post  31  and the weighing cell  3 , for example if the shoulder  31   c  of the support post  31  is not resting evenly on the support spring  11 . After the peak forces have subsided, the support spring  11  returns to its non-deflected condition, with the guide spring ensuring that the post  31  ends up in exactly the same position that it had before the shock.  
         [0025]    [0025]FIG. 4 illustrates how the support device reacts to a shock in the opposite direction of FIG. 3. This is unlikely when the apparatus is in its normal right-side-up position, because an abrupt deceleration of an upward movement hardly ever takes place. However, the condition could occur if the balance is turned upside down, even though FIG. 4 shows the device in upright orientation. The guide spring should be pre-tensioned sufficiently, so that in an upside-down position of the apparatus, the shoulder  31   c  of the support post  31  will not separate itself from the support spring  11  as a result of the weight force acting in a reverse direction on the weighing cell  3 . Additional forces due to shocks are resiliently absorbed by the guide spring  12 . In this situation, too, the guide spring ensures that the support post  31  moves substantially in the direction of the vertical post axis and returns to the same position that it had before the shock.  
         [0026]    As a preferred embodiment of a feature of the invention, FIG. 5 illustrates a die-punched, substantially E-shaped component  40  of the support device with three parallel tongues representing support springs  11 . 1 ,  11 . 2  and  11 . 3  in the form of leaf springs that are connected through a U-shaped attachment portion  41 . The three holes  42  in the legs and at the base of the U serve for the attachment of the component on the stationary base. Measured from the base  44  to the holes  42  the legs  41 . 1  and  41 . 3  are longer than the very short leg  41 . 2 , so that the fixed ends of the leaf springs lie at the corners of a triangle. Each of the leaf springs  11 . 1 ,  11 . 2  and  11 . 3  extends from a hole  42  to the free end of its respective tongue. Next to each hole  42 , the die-punched component  40  is crimped to a permanent slight upward bend, which produces the pre-tension or spring-bias force in the installed condition of the component  40 . Each tongue has a hole  13  through which the section  31   b  of a bolt-shaped post  31  (see FIG. 3) extends downwards. The shoulder  31   c  of the post  31  rests on the rim of the hole  13 .  
         [0027]    As a preferred embodiment of another feature of the invention, FIG. 6 illustrates a die-punched, substantially E-shaped component  50  of the support device with three parallel tongues representing guide springs  12 . 1 ,  12 . 2  and  12 . 3 , which are connected by a U-shaped attachment portion  51 . The three holes  52  in the legs and at the base of the U serve for the attachment of the component on the stationary base at the same locations  21   a  as the attachment portion  41  of the component  40  (see FIGS. 2 and 5). Measured from the base  54  to the holes  52  the legs  51 . 1  and  51 . 3  are longer than the very short leg  51 . 2 , so that the fixed ends of the guide springs lie at the corners of a triangle. Each of the guide springs  12 . 1 ,  12 . 2  and  12 . 3  extends from a hole  52  to the free end of its respective tongue. Next to each hole  52 , the die-punched component  50  is crimped to a permanent slight downward bend which, in the installed condition, produces a downward pre-tension or spring-bias force. Each tongue has a hole  55 , where the respective guide spring  12 . 1 ,  12 . 2  or  12 . 3  is screwed onto its associated post  31  (see FIG. 2).  
         [0028]    In the example illustrated by FIGS. 5 and 6, the two die-punched units  40  and  50 , respectively, each have three leaf springs at analogous locations. The triplet of base-attachment holes as well as the triplet of holes for the posts  31  are at the corners of essentially equilateral triangles. This geometry is not inherently required, although the three-point support recommends itself intuitively as a statically defined type of support arrangement. However, depending on the dimensions and the weight of the weighing system, a single support post may in some cases be sufficient. The arrangement of the posts and the strength of the support springs and guide springs depend on the configuration and weight distribution of the weighing cell. Thus, it is conceivable to use fewer or more than three support posts, although there appears to be no practical reason to have more than four.  
         [0029]    An alternative embodiment of a balance is illustrated schematically in FIG. 7. Those parts that perform the same function-as in the balance of FIG. 1 are identified by the same reference symbols. The balance of FIG. 7 is a top-loading precision balance  1 , which does not have an enclosed weighing compartment. There is free access to the weighing pan  5  that receives the weighing load which, by way of the load receiver  4 , is transmitted to the weighing cell  3 . The weighing cell  3 , together with a support device of the present invention, is arranged within the space  7  that is enclosed by the housing  2 . By using the inventive concept for the supporting device, the profile height of the housing  2  can be kept very low. The balance has feet  22  resting on a support surface (not shown). The topside of the floor  21  has mounting pads  28  where the support springs  11  and the guide springs  12  are attached, as well as rest stops  27   a , against which the support springs  11  are pre-tensioned. Except for the different geometry, the support device of FIG. 7 is analogous to the device of FIG. 2. In the device of FIG. 7, there is no need to have an opening in the housing floor  21  for the support post  31  to pass through. Instead, it is advantageous to provide an access hole  29  to facilitate the attachment of the guide spring to the support post by means of a screw. The access hole  29 , as shown in FIG. 7, can easily be closed with a plug  25   a.    
         [0030]    It is considered self-evident that within the scope and principal concepts of the invention, numerous embodiments of the inventive support device with pre-tensioned support springs and guide springs are possible, and that the described and illustrated embodiments are meant only as examples and not as limitations of the scope of the invention.