Abstract:
The disclosed embodiments describe force-transmitting devices for use in gravimetric measuring instruments. The force-transmitting devices scale the force from a calibration weight to facilitate calibration and weighing. The devices comprise unidirectional coupling elements. The unidirectional coupling element comprises coupling element parts. The elements may be adapted to transmit only a tensile force or only a compressive force to a measurement transducer. Adapting the unidirectional coupling element to transmit one type of force or the other may be done by selecting an appropriate arrangement of coupling element parts. The coupling element parts are adapted to transmit force along a midline axis by either a projection and v-shaped groove coupling or projections on the first part mated with surfaces on the second part adapted to receive and guide the first part.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to, and claims, benefit of a right of priority under 35 USC §119 from European patent application 09180142.3 filed on 21 Dec. 2009, the content of which is hereby incorporated by reference as if fully recited herein. 
     TECHNICAL FIELD 
     The disclosed embodiments are generally in the field of gravimetric measuring instruments and more particularly gravimetric measuring devices with force-transmitting devices. 
     BACKGROUND 
     Conventional gravimetric weighing instruments include weighing cells based on a variety of different operating principles such as for example weighing cells with strain gauges, weighing cells with string oscillators, or electromagnetic force-compensation (EMFC) weighing cells. Gravimetric measuring instruments with string oscillator weighing cells or electromagnetic force-compensation weighing cells deliver measurement results with a very high resolution. 
     In EMFC weighing cells, the weight of the load is transmitted—either directly or through one or more force-transmitting levers—to a measurement transducer which produces an electrical signal corresponding to the weighing load. By means of a weighing-oriented electronic module, the electrical signal is further processed and displayed on an indicator. 
     The mechanical design arrangement of weighing cells with string oscillators is largely analogous to EMFC weighing cells, with the difference that an oscillating string transducer is used instead of an electromagnetic measurement transducer. The load translates into an amount of tension of the oscillating string. The resultant frequency change of the string&#39;s oscillation, in turn, represents a measure for the load placed on the weighing pan. At the point in time when the measurement value is being captured, the mechanical system of an EMFC weighing cell is in a state of equilibrium similar to a mechanical beam balance with counterweights. In contrast, the load-receiving part of a string oscillator weighing cell undergoes a small vertical displacement in relation to the stationary part, as the string is set under tension by the load, whereby its length is slightly increased. String oscillator weighing cells are therefore also referred to as small-displacement force-measuring cells. 
     Both types of weighing cells are used for example in precision balances and analytical balances in the milligram range or in microbalances in the microgram range and need to be periodically recalibrated in order to ensure that the measurement values delivered by them lie within a prescribed tolerance range in accordance with the manufacturer&#39;s specifications and legal requirements. These periodic calibrations serve to compensate for the influence of factors that affect the weighing cell, for example changes of the ambient temperature or barometric pressure. 
     The calibration is made by periodically setting a load of known weight on the load-receiving part. Based on the difference between the weight value that was established in the final test prior to delivery of the weighing cell and the currently measured value, a correction value can be calculated by means of which the subsequent measurement results of the weighing cell can be corrected. In order to obtain the most accurate calibration value possible, the calibration weight should equal the maximum capacity load of the weighing cell. This can mean that very large calibration weights will be necessary. 
     Among the prior art, a variety of different gravimetric measuring instruments are known which include a built-in calibration weight. 
     A gravimetric measuring instrument of this kind which is based on the principle of electromagnetic force compensation and has a built-in rod-shaped calibration weight is disclosed in EP 0 955 530 B1. The rod-shaped calibration weight is set on a calibration weight carrier arm which is coupled to the load-receiving part and serves as a ratio lever. Due to this lever advantage, the mass of the calibration weight, and thus its dimensions, can be kept small. As the calibration weight arm is always coupled to the load-receiving part, it serves only for the purposes of receiving and leveraging the calibration weight during the calibration process, but is not part of the calibration weight itself. Consequently, the calibration weight carrier arm is part of a force-transmitting device, more specifically of a lever mechanism for transmitting and reducing the force before it reaches the measurement transducer, and remains permanently connected to the load-receiving part also while the balance operates in normal weighing mode. 
     As disclosed in CH 661 121 A5, the force-transmitting device can also include a multi-stage lever mechanism, wherein the individual levers are suitably connected to each other by means of coupling elements, so that a force reduction is achieved between the load-receiving part and the measurement transducer. Formed on one of the coupling elements are suitably designed receiving elements on which to set a calibration weight. 
     In JP 3761792 B2 a weighing cell equipped with strain gauges is disclosed which has a calibration weight with a ratio lever. A coupling element is arranged between the ratio lever and the load-receiving part. By raising the calibration weight and the coupling element, a bearing block which is formed on the coupling element is separated from a knife edge pivot which arranged on the load-receiving part, whereby the ratio lever is uncoupled from the load-receiving part. 
     All of the foregoing conventional solutions are equipped with calibration-weight-loading devices that are familiar to professionals in the weighing equipment field. 
     The precise determination of the correction value depends not only on the resolution capabilities of the measurement transducer but also to a substantial degree on the level of precision at which the geometric relationships are maintained. Even the smallest deviations in the seating position of the calibration weight from its specified position on the calibration weight carrier arm described in EP 0 955 530 B1, or on the coupling member described in CH 661 121 A5, or the smallest shifts in the position of the bearing block relative to the knife edge pivot in JP 3761792 B2, will cause a lengthening or shortening of the effective lever arm and thus to an error in the correction value. Consequently, the points of contact between the calibration weight and the calibration weight carrier arm, or between the knife edge pivot and the bearing block, are finished with the highest precision at a correspondingly high cost. 
     SUMMARY 
     The disclosed embodiments concern a force-transmitting device for a gravimetric measuring instrument, wherein the force-transmitting device has a load-receiving part and a stationary part. A weighing pan which is connected to the load-receiving part receives the load that is to be weighed. The force which the load exerts on the weighing pan is transmitted—either directly or by means of a force-reducing lever mechanism—to a measurement transducer. Together with a parallelogram-type guiding arrangement serving to guide the weighing pan and the load-receiving part in vertical movement, the force-transmitting device and the measurement transducer substantially constitute a weighing cell of a gravimetric measuring instrument. 
     An object of the disclosed embodiments is to create a force-transmitting device for a gravimetric measuring instrument with a measurement transducer, wherein a calibration weight can be brought into operating contact with the force-transmitting device in a way that minimizes the degree to which the calibration weight force acting on the measurement transducer is influenced by shifts in the geometry. 
     A force-transmitting device of a gravimetric measuring instrument has a stationary part and a load-receiving part. The load-receiving part is force-transmitting linked either directly or by way of at least one coupling element and at least one lever to a measurement transducer which is arranged on the stationary part. The force-transmitting device according to the disclosed embodiments further includes a calibration lever whose fulcrum is based on the stationary part. The calibration lever includes a first and a second calibration lever arm, wherein the first calibration lever arm is solidly connected to a calibration weight, while the second calibration lever arm is connected by way of a unidirectional coupling element to the load-receiving part, or to the at least one coupling element, or to a lever arm of the at least one lever. The unidirectional coupling element is divided in two, i.e. into a first coupling element part and a second coupling element part. The first coupling element part is connected through a first flexure pivot to a fixed place on the load-receiving part or on the coupling element or on the lever arm, while the second coupling element part is connected through a second flexure pivot to a fixed place on the second calibration arm. The flexure pivots of both coupling element parts are designed to have the greatest possible flexibility, so as to minimize bending moments generated by pivoting movements, but to still be able to perform their generic function of transmitting tensile or compressive forces in the direction of their central longitudinal axis. Furthermore, an unstable bending tendency of the flexure pivots must not be allowed to be so strong that the coupling element parts could be deflected by the force of gravity or by inertial and acceleration forces during operation of the force-transmitting device. 
     The coupling element parts are aligned to each other in such a way that, due to the unidirectional coupling element being divided into two parts, the force that can be transmitted from the second flexure pivot to the first flexure pivot is either exclusively a tensile force or exclusively a compressive force. This concept allows the calibration lever with the solidly connected calibration weight to be force-transmitting linked to the load-receiving part, to a coupling element, or to a lever arm of the force-transmitting device, or to completely unlink the calibration weight force. The steps of linking or unlinking can be performed by means of the previously mentioned calibration-weight-loading device by raising or lowering the calibration weight. The feature of a “unidirectional coupling element” in the sense of the disclosed embodiments thus implies that due its special design, the linking allows either only a compressive force or only a tensile force to be transmitted in a defined direction along its central longitudinal axis. Under a load in the opposite direction, the coupling element parts separate themselves from each other, so that no force transmission can take place between them. Depending on the actual configuration of the coupling element parts, this separation can also be in effect only for a specific range of displacement of the first coupling element part relative to the second coupling element part, in which case only this specific range of displacement is to be considered as separation in the sense of the disclosed embodiments. 
     In contrast to conventional devices, the transmission of the force is not directed through the seating contacts of the calibration weight but through the unidirectional coupling element or, more specifically, through its flexure pivots. Consequently, the geometrical relationships always remain unchanged, because minute shifts in position between the first and the second coupling element parts are evened out and are not taking place in the actual points where the force is introduced, which are defined by the unchangeable locations of the flexure pivots. As the gathering of measurement results in weighing cells with electromagnetic force compensation takes place in a balanced state of the system, the bending moments in the pivots of the force-transmitting device are approximately equal to zero. The calibration lever, more specifically its fulcrum pivot, is preferably designed accordingly, so that the lever fulcrum is free of bending moments during the calibration process. With good approximation, the foregoing comments also apply to string oscillator weighing cells, because a load placed on the load-receiving part causes only a minute amount of stretching of the string, and the bending moments occurring as a result in the flexure pivots of the force-transmitting device are therefore very small. 
     The respective central longitudinal axes of the first and the second flexure pivot preferably coincide with each other, so that no destabilizing moments occur within the unidirectional coupling element. However, a small parallel offset of the two central longitudinal axes can be allowed if the design of the at least one contact area between the first coupling element part and the second coupling element part provides an intrinsic stability to keep the two coupling element parts from deflecting sideways. This stability is inherently present in the transmission of tensile forces, while in the case of compressive forces only certain design configurations will lend an intrinsic stability to the unidirectional coupling element. 
     In a first embodiment, the second coupling element part has, as a means for the transmission of compressive forces, a surface which faces towards the first coupling element part, with two projections that protrude from the surface and are arranged mirror-symmetrically relative to the central longitudinal axis of the second flexure pivot. Further, the first coupling element part has at least one receiving surface which faces towards the second coupling element part and against which the two projections can seat themselves. With this configuration, two contact points which are arranged in a plane that extends orthogonal to the central longitudinal axes of the two flexure pivots are lending an intrinsic stability to the unidirectional coupling element, enabling the latter to transmit a compressive force. 
     As a means for centering the first coupling element part and the second coupling element part relative to each other during the calibration process and to prevent them from shifting their mutual positions orthogonal to the central longitudinal axes of the flexure pivots, there can be two receiving surfaces for the projections slanted against each other at a shallow angle. However, the slope angle of the receiving surfaces is subject to certain limits due to the requirement for intrinsic stability of the unidirectional coupling element. 
     The limits for the slope angle of the receiving surfaces depend on the geometric proportions of the unidirectional coupling element, wherein the angle α between a receiving surface and the central longitudinal axis of the flexure pivot has to satisfy the following condition:
 
90°≧α≧arccos( b/s ),
 
wherein b stands for the perpendicular distance of a contact point from the central longitudinal axis of the flexure pivot and s stands for the distance of a contact point from the bending axis of the flexure pivot. According to the foregoing definition of the angle α, intrinsic stability is attained if the lines of action of the forces transmitted at the contact points are not intersecting within the confines of the coupling element part.
 
     In a configuration that is suitable for the transmission of tensile forces, the first coupling element part can have a first traction element with a projection, and the second coupling element part can have a second traction element with a V-shaped bearing, wherein the first coupling element part and the second coupling element part are hooked into each other, the projection is aligned with the V-bearing, and when tensile forces are being transmitted, the projection is in force-transmitting contact with the V-bearing. 
     In another embodiment, the first coupling element part and the second coupling element part can be connected to each other by means of a flexible hinge. Although the two coupling element parts in this embodiment are physically connected to each other, the coupling element is nevertheless divided into two parts, consistent with the disclosed embodiments. The opening direction of the flexible hinge is arranged in accordance with the central longitudinal axis of the first and second flexure pivots. For the transmission of tensile or compressive forces, a projection is formed on the first coupling element part and a contact surface, oriented to cooperate with the projection, is formed on the second coupling element part. 
     The force-transmitting device can also include a parallel-guiding arrangement, so that in the operating mode of the device, the load-receiving part is guided in vertical movement by being linked to the stationary part by means of at least one upper parallel guide and at least one lower parallel guide. 
     The force-transmitting device can be composed of different individual components which are joined together by means of connecting elements to form a unit. Two or more of these elements can also be joined through a materially integral connection. Preferred are design configurations where at least the load-receiving part, the stationary part, the calibration lever and the unidirectional coupling element are monolithically connected to each other. 
     As mentioned hereinabove, the force-transmitting device according to the disclosed embodiments can be used in combination with a measurement transducer and a calibration weight as a weighing cell in a gravimetric measuring instrument. A gravimetric measuring instrument is normally calibrated in such a way that the weight force produced by the calibration weight is transmitted to the measurement transducer by way of the load-transmitting device in the same direction as a force acting on the load-receiving part. Consequently, the load resulting from a calibration force is introduced into the measurement transducer in the same way as a normal load is introduced in the normal operating mode of the device. In string oscillator weighing cells, the measuring force can in most cases only be applied in the direction of the load. The foregoing embodiment is therefore suitable for weighing cells of the string oscillator type as well as the EMFC type. 
     As another possibility, the weight force of the calibration weight can also be applied by the force-transmitting device to the measurement transducer in the opposite direction compared to a force acting on the load-receiving part. This concept is particularly suitable for EMFC weighing cells which have a measurement transducer with push/pull action as described in detail in US 20080218303 A1. Due to this design of the measurement transducer the calibration weight can be used as a compensation weight to expand the load range of the weighing cell. In the uncoupled condition of the calibration weight, a weighing cell of this type could for example weigh loads in the range of 0 to 100 grams, and by engaging the calibration weight it could switch to an extended load range and weigh loads from 100 to 200 grams, as the weight force of the calibration weight acts against the force of the load on the load-receiving part. 
     The calibration weight is normally made of a corrosion-proof material with a uniform density of ρ=8.0 kg/dm 3 . However, the calibration weight can also be made in part of the same material as the force-transmitting device and combined with a supplemental mass of higher density. In order to improve the accuracy of the calibration value, there can be a pressure sensor serving to measure the ambient barometric pressure of the gravimetric measuring instrument. The measurement value produced by the pressure sensor can be used to calculate a buoyancy correction for the calibration weight, as the buoyant force of the air displaced by the calibration weight counteracts the weight force of the calibration weight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Different design variations embodying the force-transmitting device according to the disclosed embodiments as well as their application in a gravimetric measuring instrument will hereinafter be described in more detail with the help of the drawings, wherein elements that are the same from one drawing to another are identified by the same reference symbols, and wherein 
         FIG. 1  shows a cross-section representation of a weighing cell with a first embodiment of a force-transmitting device in side view, with a lever mechanism arranged between the load-receiving part and the measurement transducer for the reduction of the force acting on the load-receiving part, wherein a unidirectional coupling element capable of transmitting only compressive forces is arranged between the lever mechanism and the calibration lever. 
         FIG. 2  shows a cross-section representation of a weighing cell with a second embodiment of the force-transmitting device in side view, with a lever mechanism arranged between the load-receiving part and the measurement transducer for the reduction of the force acting on the load-receiving part, wherein a unidirectional coupling element is capable of transmitting only tensile forces is arranged between the lever mechanism and the calibration lever. 
         FIG. 3  shows a cross-section representation of a weighing cell with a third embodiment of the force-transmitting device in side view, with a lever mechanism arranged between the load-receiving part and the measurement transducer for the reduction of the force acting on the load-receiving part, wherein a unidirectional coupling element capable of transmitting only tensile forces is arranged between the load-receiving part and the calibration lever. 
         FIG. 4  shows a cross-section representation of a weighing cell with a forth embodiment of the force-transmitting device in side view, with a lever mechanism of two levers arranged between the load-receiving part and the measurement transducer for the reduction of the force acting on the load-receiving part, wherein a unidirectional coupling element capable of transmitting only compressive forces is arranged between a second coupling element of the two levers and the calibration lever. 
         FIG. 5  shows a cross-section representation of a weighing cell with a fifth embodiment of the force-transmitting device in side view, with a lever mechanism of three levers arranged between the load-receiving part and the measurement transducer for the reduction of the force acting on the load-receiving part, wherein a unidirectional coupling element capable of transmitting only tensile forces is arranged between the lever mechanism and the calibration lever. 
         FIG. 6  shows a cross-section of a monolithically designed force-transmitting device in side view, wherein a lever mechanism, a coil lever, a calibration lever, coupling elements connecting the levers, as well as a parallel-guiding linkage are formed by means of narrow linear cuts, and wherein the unidirectional coupling element arranged between the lever mechanism and the calibration lever has two coupling element parts which are connected to each other through a flexible hinge. 
         FIG. 7  shows an embodiment of a unidirectional coupling element that is capable of transmitting only a tensile force. 
         FIG. 8  shows an embodiment of a unidirectional coupling element that is capable of transmitting only a compressive force. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic representation of a weighing cell  100  with a first embodiment of a force-transmitting device  110 . The force-transmitting device  110  has a stationary part  111  and a load-receiving part  112 . The load-receiving part  112  is guided in parallel movement by being linked to the stationary part  111  through a first parallel guide  114  and a second parallel guide  115 . All of the pivotal connections  113  of the force-transmitting device  110  are symbolized in the drawing by solid black circles and can be realized with any of the designs known in the art. These pivotal connections  113  may be designed as flexure pivots of the kind shown in  FIGS. 5 to 7 . 
     Connected to the load-receiving part  112  is a load receiver  140  in the form of a weighing pan. Further, a measurement transducer  130  capable of producing a force-dependent weighing signal is rigidly mounted on the stationary part  111 . The measurement transducer  130  shown in the drawing has a coil  131  and a magnet  132  as well as a position sensor  133 . The magnet  132  is solidly connected to the stationary part  111 . The force that is to be measured is acting on the coil  131 , which is arranged inside the magnet  132  in such a way that the coil  131  does not touch the magnet  132 . The force acting on the coil  131  causes the latter to shift its position relative to the magnet  132 , and this change in position is detected by the position sensor  133 . The signal produced by the position sensor  133  is sent to an electronic system (not illustrated here) of the balance, which continuously regulates a current flowing through the coil  131 , so as to restore the original position of the coil  131  relative to the magnet  132  against the action of the force. The current flowing through the coil  131  is measured, and the raw measurement signals are processed in the electronic system into a measurement value which, in turn, is passed on to a display unit (likewise not shown in the drawing) where the measurement value is presented in a visual format. Of course, instead of the measurement transducer  130  shown here one could also use other kinds of transducers, for example string oscillators, inductive or capacitative transducers, strain gauge transducers and the like. 
     Since the measurement transducer  130  shown in the drawing can only produce a compensation force of limited magnitude for the compensation of the force acting on the load-receiving part  140 , a lever mechanism with one or more levers is often used for the reduction of the force that is to be measured. This lever mechanism is arranged as a force-transmitting conduit between the load-receiving part  112  and the measurement transducer  130 . The lever mechanism shown in the drawing has a lever  116  which is pivotally supported on the stationary part  111  and whose short lever arm  117  is connected by way of a first coupling element  119  to the load-receiving part  112 . The long lever arm  118  of the lever  116  is connected to the coil  131 . 
     The force-transmitting device  110  further includes a calibration lever  120  which is likewise pivotally supported on the stationary part  111  and whose first calibration lever arm  121  is rigidly connected to a calibration weight  123 . The function of transmitting the weight force of the calibration weight  123  is performed by a unidirectional coupling element  124  whose first coupling element part  125  is connected to the long lever arm  118  by way of a first flexure pivot  127  and whose second coupling element part  126  is connected to the second calibration lever arm  122  by way of a second flexure pivot  128 . The first and second flexure pivots  127 ,  128  stand as conceptual representations for any possible kind of pivotal connections that have a certain amount of intrinsic stiffness or whose pivoting angle is limited. These properties of the pivotal connections, specifically the flexure pivots  127 ,  128 , is absolutely essential at least for the upstanding pivot, in this case the flexure pivot  128 , so that the two coupling element parts  125 ,  126  remain aligned with each other in their uncoupled state and will not deflect sideways under the force of gravity. 
     As shown in  FIG. 1 , a calibration-weight-loading device  150  is mounted on the stationary part. Depending on the position of the cam disk  151 , the calibration-weight-loading device  150  either keeps the calibration weight  123  supported on the stationary part  111  or sets the calibration weight  123  free from its support. While the calibration weight  123  is in the supported state the two coupling element parts  125 ,  126  are separated from each other, while in the released state the two coupling element parts  125 ,  126  are in force-transmitting contact with each other, whereby the weight force of the calibration weight  123  is transmitted to the lever  118  and thus to the coil  131  of the measurement transducer  130 . As is clear from the Figure, the unidirectional coupling element  124  as illustrated here allows only compressive forces to be transmitted, because when the calibration weight  123  is in the released state the second coupling element part  126  is pushed against the first coupling element part  125 , acting against the reactive force that is generated by the measurement transducer  130 . 
       FIG. 2  shows a side view of a cross-section of a weighing cell  200  with a second embodiment of a force-transmitting device  210 . With the exception of the calibration lever  220  and the unidirectional coupling element  224 , all of the elements illustrated in  FIG. 2  are analogous to those of  FIG. 1  and will therefore not be described again in the following. 
     The calibration lever  220  shown in  FIG. 2  has likewise a first calibration lever arm  221  and a second calibration lever arm  222 . The first calibration lever arm  221  extends from the lever fulcrum  113  to the calibration weight  123 , and the second calibration lever arm  222  extends between the lever fulcrum  113  and the second flexure pivot  228  of the unidirectional coupling element  224 . 
     The unidirectional coupling element  224  has a first coupling element part  225  and a second coupling element part  226  which, when not transmitting force from either the calibration weight or the load-receiving part, embrace each other loosely without physical contact. As soon as the calibration weight  123  is freed of its support and, as a result, a tensile force needs to be transmitted from the second flexure pivot  228  to the first flexure pivot  227  and thus to the long lever arm  118  of the lever  116 , the second coupling element part  226  moves into force-transmitting engagement with the first coupling element part  225  after a relatively short displacement of the two parts along a displacement path defined by the flexure pivots  227 ,  228 . 
     It should also be noted that the tensile force generated by the calibration weight  123  and transmitted to the long lever arm  118  acts in opposition to the force that is to be measured, which is generated by a load placed on the load receiver  140 . Consequently, the calibration weight  123  can also be used for the purpose of expanding the weighing range of the weighing cell  200 . The weighing range can be as much as doubled, if the effective force acting on the measurement transducer  130  due to the calibration weight  123  corresponds to the maximum amount of force that the transducer  130  is capable of measuring. The weighing range is now subdivided into two parts, although this is invisible to the user, with a first part of the range where no force from the calibration weight  123  is acting on the measurement transducer  130 , and a second part of the range where the calibration weight  123  is coupled to the lever mechanism. 
     It is possible that releasing the calibration weight  123  from the supported state may slightly influence the measurement values, specifically the added effect of the pivot  113  of the calibration lever  220  which is thereby brought into play. To address this issue, one could select a coarser resolution or specify a lower accuracy class for the higher part of the measurement range. All of the operations that have just been described can be controlled by the electronic part of the balance responding automatically to the amount of the weighing load, so that the user need not be concerned about deciding which part of the weighing range to select. 
     As is illustrated in  FIG. 3 , arrangements are possible where the calibration weight  123  acts more directly on the load-receiving part  112 .  FIG. 3  shows a weighing cell  300  in a third embodiment of the force-transmitting device  310 . With the exception of the calibration lever  320  and the unidirectional coupling element  324 , all of the elements shown in this drawing are analogous to  FIG. 1  and will therefore not be described again in the following. 
     The lever mechanism has a lever  316  which is arranged between the load-receiving part  112  and the measurement transducer  130 . The calibration lever  320  with a calibration weight  123  is pivotally connected to the stationary part  311  and, in relation to the operating position of the weighing cell  300 , arranged below the load-receiving part  112 . The transmission of the force from the calibration lever  320  to the load-receiving part  112  takes place by way of the unidirectional coupling element  324  which is designed for the transmission of tensile forces. In this embodiment, the calibration force that is transmitted to the load-receiving part  112  through the unidirectional coupling element is related to the lengths of the first lever arm  321  and the second lever arm  322  relative to each other. Specifically, the longer the second lever arm  322  is in relation to the first lever arm  321 , the larger the calibration force that will be transmitted. 
       FIG. 4  shows a side view of a schematic representation of a weighing cell  400  with a fourth embodiment of the force-transmitting device  410  which has a lever mechanism with two levers  416 ,  417  arranged between the load-receiving part  112  and the measurement transducer  130  for the reduction of the force that is acting on the load-receiving part  112 .  FIG. 4  likewise contains components that are identical to some of the components in the preceding figures and which therefore carry the same reference symbols or are not described again. 
     The first lever  416  is force-transmitting linked to the second lever  417  through the coupling element  418 . Arranged between the coupling element  418  and a calibration lever  420  that is pivotally mounted on the stationary part  411  is a unidirectional coupling element  424  with the capability to transmit compressive force only. In the illustrated arrangement, the respective lines of action of the forces of the coupling element  418  and of the laterally connected unidirectional coupling element  424  are offset from each other, and as a result a torque is produced which creates a load on the fulcrum pivots  460 ,  461  of the levers  416 ,  417 . One should therefore aim preferably for solutions in which the two lines of action coincide. 
     To facilitate the transmission of force to the measurement transducer  130 , the stationary part  411  has an opening  412  through which the second lever  417  passes to the outside, so that the measurement transducer  130  can be arranged on the side of the stationary part  411  that faces away from the parallel guides  114 ,  115 . 
       FIG. 5  shows a schematic representation of a weighing cell  500  with a fifth embodiment of the force-transmitting device  510  in a side view. The force-transmitting device  510  has a lever mechanism with three levers  516 ,  517 ,  518  arranged between the load-receiving part  112  and the measurement transducer  130  for the reduction of the force that is acting on the load-receiving part  112 . As illustrated in  FIG. 4  and described above, the first lever  516  is force-transmitting linked through a second coupling element  519  to the second lever  517  which, in turn, is force-transmitting linked through a third coupling element  529  to the third lever  518 . A unidirectional coupling element  524  which is only capable of transmitting tensile forces is arranged between the lever mechanism and a calibration lever  520 . The introduction of the calibration force occurs at the second coupling element  519  which connects the first lever  516  to the second lever  517 . In contrast to  FIG. 4 , the unidirectional coupling element  524  is not arranged with a parallel offset from the second coupling element  519 , but is in line with the latter. This in-line arrangement avoids the possibility of introducing a torque into the second coupling element  519  during the calibration process, a problem which can occur with the second coupling element  419  shown in  FIG. 4 . 
     In  FIG. 6  a monolithically formed force-transmitting device  610  is shown in a side view. A material block  699  which is delimited by its profile contours is traversed by narrow linear cuts passing through the material block  699  at a right angle to the plane of the drawing so as to form a first lever  616 , a second lever  617 , a calibration lever  620 , a first parallel guide  614 , a second parallel guide  615 , the stationary part  611 , the load-receiving part  612  as well as a first coupling element and a second coupling element. All of these parts which are formed by means of linear cuts are appropriately connected to each other by flexure pivots that are likewise produced through linear cuts, so that the force-transmitting device  610  essentially has a load-receiving part  612  linked to the stationary part  611  for guided movement, a lever mechanism  616 ,  617 , as well as a calibration lever  620 . The second lever is connected by means of a lever arm extension (not shown in the drawing) to the measurement transducer (likewise not shown). Two holes  641  in the second lever serve for the attachment of the lever arm extension. The calibration lever also has two holes to which a calibration weight can be fastened. Arranged between the first lever  616  and the calibration lever  620  is a unidirectional coupling element  624  which has two coupling element parts  625 ,  626  connected to each other by means of a flexible hinge  648 . Although the two coupling element parts  625 ,  626  are physically connected to each other, the unidirectional coupling element  624  is nevertheless considered to be divided into two parts in accordance with the disclosed embodiments. The opening direction of the flexible hinge  648  is oriented to correspond to the central longitudinal axis of the first and second flexure pivots  627 ,  628 . For the transmission of compressive forces, a projection  643  is formed on the first coupling element part  625  and a contact surface oriented to cooperate with the projection is formed on the second coupling element part  626 . 
       FIG. 7  illustrates an embodiment of a unidirectional coupling element  724  in its operating position, which can transmit only a tensile force. A first coupling element part  725  is connected by means of a first flexure pivot  727  to the load-receiving part or the lever mechanism of a force-transmitting device (indicated in fragmentary fashion). Due to the hanging position of the first coupling element part  725 , the first flexure pivot  727  can have a very slender shape. The first coupling element part  725  further includes a first traction element  741 . 
     The second coupling element part  726  is connected through a second flexure pivot  728  to the calibration lever  720  (indicated in fragmentary fashion). The second flexure pivot  728 , which remains in an upstanding orientation, needs to have a stiffer spring constant. Accordingly, it needs to be given a sturdier design than the first flexure pivot  727 . If its design is sufficiently well matched to the operating conditions of the force-transmitting device, the second coupling element part  726  will not be deflected sideways due to the force of gravity or due to forces caused by inertia and acceleration. 
     The second coupling element part  726  includes a second traction element  742 . As soon as a force needs to be transmitted from the second flexure pivot  728  to the first flexure pivot  727 , the first traction element  741  moves into force-transmitting engagement with the second traction element  742  after a short displacement of the two traction elements relative to each other. For an even more reliable transmission of tensile forces, the first traction element  741  can have a projection  743 , and the second traction element  742  can have a V-shaped bearing  744 . As soon as the first traction element  741  and the second traction element  742  are engaged in each other, the projection  743  and the V-bearing  744 , as a result of their shapes, are in self-centering alignment with each other and the projection  743  is seated in the V-bearing  744 . 
       FIG. 8  illustrates a possible design of a unidirectional coupling element  824  in its operating position, which can transmit only a compressive force. A first coupling element part  825  is connected by means of a first flexure pivot  827  to the load-receiving part or the lever mechanism of a force-transmitting device (indicated in fragmentary fashion). 
     A second coupling element part  826  is connected through a second flexure pivot  828  to the calibration lever  820  (indicated in fragmentary fashion). Since the forces transmitted during the calibration process are compressive forces and the two coupling element parts  825 ,  826  must be absolutely prevented from buckling and breaking away sideways under a load, the unidirectional coupling element  824  needs to have intrinsic stability. The second coupling element part  826  has a surface  841  facing towards the first coupling element part  825 , with projections  843  protruding from the surface  841  which are arranged mirror-symmetrically relative to the central longitudinal axis X of the second flexure pivot  828 . Furthermore, the first coupling element part  825  has receiving surfaces  842  facing towards the coupling element part  826  so that one of the projections  843  can seat itself on each of the receiving surfaces  842 . With this configuration, two contact points arranged in a plane that extends orthogonal to the central longitudinal axes X of the two flexure pivots  827 ,  828  are lending an intrinsic stability to the unidirectional coupling element  824 , enabling the latter to transmit a compressive force. 
     As shown in  FIG. 8 , this arrangement allows the first coupling element part  825  and the second coupling element part  826  to be aligned with each other in the calibration process. As a result, displacements perpendicular to the central longitudinal axes X of the flexure pivots  827 ,  828  can be prevented. To perform this alignment function, the receiving surfaces  842  on the first coupling element part  825  are slanted at a shallow angle relative to each other. However, the slope angle of the receiving surfaces  842  is subject to certain limits due to the requirement for intrinsic stability of the unidirectional coupling element  824 . 
     These limits depend on the geometric proportions of the unidirectional coupling element  824 , wherein the factor to be considered is the angle α between a receiving surface  842  and the central longitudinal axis X of the flexure pivot  827 . In the following relationships, b stands for the perpendicular distance of a contact point, more specifically a projection  843 , from the central longitudinal axis of the flexure pivot and s stands for the distance of a contact point, more specifically of a receiving surface  842 , from the bending axis Y of the first flexure pivot  827 .
 
 X =arcsin( b/s )
 
α=(90 °−X )=arccos( b/s )
 
     The angle α has to satisfy the condition:
 
90°≧α≧arccos( b/s )
 
     According to the foregoing definition of the angle α, intrinsic stability is attained if the lines of action W of the forces transmitted at the contact points are not intersecting within the first coupling element part  825 . The borderline case where α=arccos(b/s) is shown in  FIG. 8 . 
     Although the disclosed embodiments have been described through the presentation of specific embodiments, it is evident that numerous further variant solutions could be created based on knowledge gained from the disclosed embodiments, for example by combining the features of the individual embodiments with each other and/or by interchanging individual functional units of the exemplary embodiments. Among other possibilities, one could consider alternative embodiments in which several calibration weights can be coupled independently of each other to a lever mechanism either at the same location or at different locations by means of unidirectional coupling elements as proposed by the disclosed embodiments. This makes it possible for example to expand the weighing range of a balance to almost any desired extent. Of course, the possible alternatives also include unidirectional coupling elements that are split vertically into two or more segments, calibration levers and calibration weights, which are operable independently of each other. The scope of possibilities also includes designs where the calibration lever and the calibration weight are monolithically combined with each other. 
     Further benefits of the device according to the disclosed embodiments are due to the fact that by varying the calibration lever arm lengths it is possible to cover different load ranges with the same physical components, if the calibration weight can be mounted selectively at different fulcrum distances on the calibration lever arm. It is also possible to arrange an adjustment screw between the calibration weight and the calibration lever, serving to precisely set the distance between the calibration weight and the fulcrum point of the calibration lever. This allows the calibration force to be adjusted by precisely shifting the position of the weight, so that in spite of variations caused by manufacturing tolerances and inhomogeneous materials, the calibration force will be the same from one calibration device to another. 
     Having shown and described an embodiment of the invention, those skilled in the art will realize that many variations and modifications may be made to affect the described invention and still be within the scope of the claimed invention. Additionally, many of the elements indicated above may be altered or replaced by different elements which will provide the same result and fall within the spirit of the claimed invention. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims.